WO2012101397A2 - Organic light emitting diode displays - Google Patents

Organic light emitting diode displays Download PDF

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Publication number
WO2012101397A2
WO2012101397A2 PCT/GB2012/000060 GB2012000060W WO2012101397A2 WO 2012101397 A2 WO2012101397 A2 WO 2012101397A2 GB 2012000060 W GB2012000060 W GB 2012000060W WO 2012101397 A2 WO2012101397 A2 WO 2012101397A2
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WO
WIPO (PCT)
Prior art keywords
pixel
stereoscopic
line
oled
display
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PCT/GB2012/000060
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French (fr)
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WO2012101397A3 (en
Inventor
Euan Smith
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Cambridge Display Technology Limited
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Application filed by Cambridge Display Technology Limited filed Critical Cambridge Display Technology Limited
Publication of WO2012101397A2 publication Critical patent/WO2012101397A2/en
Publication of WO2012101397A3 publication Critical patent/WO2012101397A3/en
Priority to ZA2013/04747A priority Critical patent/ZA201304747B/en

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Classifications

    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
    • G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
    • G09G3/22—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources
    • G09G3/30—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels
    • G09G3/32—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED]
    • G09G3/3208—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED] organic, e.g. using organic light-emitting diodes [OLED]
    • G09G3/3225—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED] organic, e.g. using organic light-emitting diodes [OLED] using an active matrix
    • G09G3/3233—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED] organic, e.g. using organic light-emitting diodes [OLED] using an active matrix with pixel circuitry controlling the current through the light-emitting element
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
    • G09G3/001—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes using specific devices not provided for in groups G09G3/02 - G09G3/36, e.g. using an intermediate record carrier such as a film slide; Projection systems; Display of non-alphanumerical information, solely or in combination with alphanumerical information, e.g. digital display on projected diapositive as background
    • G09G3/003—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes using specific devices not provided for in groups G09G3/02 - G09G3/36, e.g. using an intermediate record carrier such as a film slide; Projection systems; Display of non-alphanumerical information, solely or in combination with alphanumerical information, e.g. digital display on projected diapositive as background to produce spatial visual effects
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
    • G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
    • G09G3/22—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources
    • G09G3/30—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels
    • G09G3/32—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED]
    • G09G3/3208—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED] organic, e.g. using organic light-emitting diodes [OLED]
    • G09G3/3225—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED] organic, e.g. using organic light-emitting diodes [OLED] using an active matrix
    • G09G3/3233—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED] organic, e.g. using organic light-emitting diodes [OLED] using an active matrix with pixel circuitry controlling the current through the light-emitting element
    • G09G3/3241—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED] organic, e.g. using organic light-emitting diodes [OLED] using an active matrix with pixel circuitry controlling the current through the light-emitting element the current through the light-emitting element being set using a data current provided by the data driver, e.g. by using a two-transistor current mirror
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2300/00—Aspects of the constitution of display devices
    • G09G2300/08—Active matrix structure, i.e. with use of active elements, inclusive of non-linear two terminal elements, in the pixels together with light emitting or modulating elements
    • G09G2300/0809—Several active elements per pixel in active matrix panels
    • G09G2300/0819—Several active elements per pixel in active matrix panels used for counteracting undesired variations, e.g. feedback or autozeroing
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2300/00—Aspects of the constitution of display devices
    • G09G2300/08—Active matrix structure, i.e. with use of active elements, inclusive of non-linear two terminal elements, in the pixels together with light emitting or modulating elements
    • G09G2300/0809—Several active elements per pixel in active matrix panels
    • G09G2300/0842—Several active elements per pixel in active matrix panels forming a memory circuit, e.g. a dynamic memory with one capacitor
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2300/00—Aspects of the constitution of display devices
    • G09G2300/08—Active matrix structure, i.e. with use of active elements, inclusive of non-linear two terminal elements, in the pixels together with light emitting or modulating elements
    • G09G2300/0809—Several active elements per pixel in active matrix panels
    • G09G2300/0842—Several active elements per pixel in active matrix panels forming a memory circuit, e.g. a dynamic memory with one capacitor
    • G09G2300/0852—Several active elements per pixel in active matrix panels forming a memory circuit, e.g. a dynamic memory with one capacitor being a dynamic memory with more than one capacitor
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2300/00—Aspects of the constitution of display devices
    • G09G2300/08—Active matrix structure, i.e. with use of active elements, inclusive of non-linear two terminal elements, in the pixels together with light emitting or modulating elements
    • G09G2300/0809—Several active elements per pixel in active matrix panels
    • G09G2300/0842—Several active elements per pixel in active matrix panels forming a memory circuit, e.g. a dynamic memory with one capacitor
    • G09G2300/0861—Several active elements per pixel in active matrix panels forming a memory circuit, e.g. a dynamic memory with one capacitor with additional control of the display period without amending the charge stored in a pixel memory, e.g. by means of additional select electrodes

Definitions

  • This invention relates to active matrix (AM) organic light emitting diode (OLED) displays, and in particular to driving an AMOLED panel driving to provide a shuttered stereoscopic (3D) display.
  • AM active matrix
  • OLED organic light emitting diode
  • AMOLED display panels It is known to drive display panels, including AMOLED display panels, with alternating left and right eye images, using in conjunction with shutter glasses worn by the viewer(s) to give the impression of a 3-dimensional image.
  • the shutters operate so that when a left eye stereoscopic view is displayed only the left eye sees this image, and vice-versa.
  • the shutters may be wired or may automatically synchronise to the display frame rate using a photosensor.
  • a conventional display is built up line-by-line and thus to avoid displaying parts of both left and right eye images at the same time a blanking interval is interposed between left and right stereoscopic image frames. Absent this blanking interval the right eye would see part of the old left eye frame until this was refreshed.
  • Figure 1 illustrates the concept of a blanking interval with vertical screen position on the vertical axis and time on the horizontal axis. This shows that, for example, for a 60Hz (left and right) frame rate, when a blanking interval is employed the effective frame rate is 240Hz and the display is off 50% of the time.
  • LCD liquid crystal display
  • OLED organic light-emitting diode
  • OLED Organic light emitting diode
  • OLEDs include polymer OLEDs, so-called small molecule OLEDs, and organometallic OLED displays.
  • One potential difficulty with using OLEDs in a blanking scheme as describe above is that because the display is off half the time twice the drive is required, effectively giving half the lifetime.
  • a stereoscopic OLED display having a plurality of OLED pixels each driven by an active matrix stereoscopic pixel circuit, each stereoscopic pixel circuit comprising: at least one select line to select the OLED pixel; at least one programming line to program the pixel circuit with pixel data to control brightness of the pixel; at least one driver transistor to drive the pixel; left and right data storage capacitors to store left stereoscopic pixel data and right stereoscopic pixel data to control the brightness of the OLED pixel in respective left and right stereoscopic images displayed on said display, and wherein said pixel data comprises said left and right stereoscopic pixel data; a third line to enable said left and right stereoscopic pixel data to be selectively stored in said pixel circuit in said respective left and right data storage capacitors; and at least one drive control line to control said at least one driver transistor to enable said OLED pixel to be selectively driven with either said left
  • Embodiments of the above-described display enable the pixel driver circuit to be controlled using a reduced number of tracks and, in addition, enable programming of left and right pixel data independent of which data is driven to a pixel. This in turn enables further advantages as discussed below. (The skilled person will appreciate that references to left and right data storage capacitors are to their functions in storing left and right pixel data rather than to their physical layout).
  • the circuit is controlled by two power supply (V DD ) lines; preferably the first and second power supply (V DD ) lines are directly connected to the respective driver transistors.
  • V DD power supply
  • the third line may comprise either a select line or a programming line, in which case the pixel circuit has only a single one of the other lines, that is the programming line or select line, respectively.
  • the pixel circuit may comprise a voltage-programmed or a current-programmed pixel circuit.
  • the pixel circuit may comprise two pairs of select transistor one associated with each of the 'left' and 'right' driver transistors.
  • each pair of select transistors is connected for control by a respective select line and a first transistor of the pair is connected between the programming line and a first terminal of one of the data storage capacitors, and a second transistor of a select pair is coupled in series between two terminals of a driver transistor to program the current through the driver transistor.
  • the second transistor of a pair may be connected across a driver transistor to diode-connect the driver transistor, or a further transistor may be coupled in series across the driver transistor, and the two select transistors connected in series.
  • a further, current-programming transistor may be employed as a voltage-to-current converter to convert a programming voltage to a programming current for the pixel.
  • this transistor may have a control connection coupled to the programming line and an output connection coupled to a current - programming line of the pixel circuit.
  • the inventor has recognised that in embodiments of the above described arrangements because programming of the pixel circuit is independent of drive of the OLED pixel, the drive to a pixel may be switched between left and right stereoscopic pixel drive data at a rate which is faster than that at which the pixel circuit is being programmed. This is because with such an arrangement the viewer's eye only sees left or right image data at any one time, from either the left or right part of the pixel drive circuit, irrespective of whether or not the left or right frame data has been fully updated. Thus whilst under certain circumstances there may be portions of two successive left image frame or right image frames visible, there will not be portions of both a left image frames and a right image frame visible at the same time to adjust the left eye or adjust the right eye of a viewer. The inventor has recognised that, potentially, seeing portions of two successive left image frames or right image frames at the same time is visually acceptable whereas seeing portions of a left image frame and a right image frame together, viewed with just one of the left or right eyes is not.
  • the inventor has further recognised that this enables switching between the left and right image displays at essentially an arbitrarily fast rate, thus reducing the volume the effects of image flicker (which can otherwise be present at relatively low switch rates of order 60Hz).
  • the invention provides a method of providing a stereoscopic OLED display, the display having a plurality of OLED pixels each driven by an active matrix stereoscopic pixel circuit, each stereoscopic pixel circuit comprising: at least one select line to select the OLED pixel; at least one programming line to program the pixel circuit with pixel data to control brightness of the pixel; at least one driver transistor to drive the pixel; left and right data storage capacitors to store left stereoscopic pixel data and right stereoscopic pixel data to control the brightness of the OLED pixel in respective left and right stereoscopic images displayed on said display, and wherein said pixel data comprises said left and right stereoscopic pixel data; a third line to enable said left and right stereoscopic pixel data to be selectively stored in said pixel circuit in said respective left and right data storage capacitors; and at least one drive control line to control said at least one driver transistor to enable said OLED pixel to be selectively driven with either said left stereoscopic pixel data or
  • programming the pixel drive circuits may take place whenever convenient, provided the rate matches the overall input data rate from the left and right image frames of the source, with left and right drive switching at a higher rate, albeit probably synchronised to the programming, for convenience. In this way the left eye, for example, only sees the left image frames, either complete or being updated, but there is no need to program a complete left image frame before showing it.
  • the display drive rate is an integer multiple of the programming rate, but this is not essential and the two rates may, for example, be defined by a ratio of two integers (this latter may introduce banding artefacts but, apart from some special case images, these should not be readily discernible).
  • the pixel driver circuit may have two control lines and a single programming line, or two programming lines and a single control line. In either case independent control over programming and drive is provided.
  • the drive control is performed by a power supply line, for example V D D, providing left and right VDD lines which can be controlled/switched to control/switch the drive to a pixel between left and right stereoscopic images (or on and off).
  • a single power supply (V D D) line is provided together with a drive control line for switching between left and right pixel data.
  • two active matrix pixel circuits may effectively be provided per OLED pixel.
  • the pixel drive circuit may comprise a current-programmed pixel circuit, but more particularly a circuit which is programmed by pulling the power supply line (V DD ) low. This may constrain the drive since drive can then not take place at the same time as programming. To address this programming may be performed on one portion or half of the display panel whilst another portion or half of the panel is on, that is the relevant left or right stereoscopic image data is being driven to the pixels. This results in a display in which, for example, the left stereoscopic image of a first frame is being displayed in one half of the display (the bottom half) panel whilst the right stereoscopic image of a second, subsequent frame is being programmed into the other half of the display panel (say the top half).
  • V DD power supply line
  • a stereoscopic display having a plurality of OLED pixels each driven by an active matrix stereoscopic pixel circuit, each stereoscopic pixel circuit comprising: at least one select line to select the OLED pixel; at least one programming line to program the pixel circuit; with pixel data to control brightness of the pixel; at least one driver transistor to drive the pixel; left and right data storage capacitors to store left stereoscopic pixel data and right stereoscopic pixel data to control the brightness of the OLED pixel in respective left and right stereoscopic images displayed on said display, and wherein said pixel data comprises said left and right stereoscopic pixel data; a third line to enable said left and right stereoscopic pixel data to be selectively stored in said pixel circuit in said respective left and right data storage capacitors; and at least one drive control line to control said at least one driver transistor to enable said OLED pixel to be selectively driven with either said left stereoscopic pixel data or said right stereoscopic pixel data;
  • the pixel driver circuit in effect comprises a form of analogue shift register with charge sharing between the capacitors to move the data from one to the other, for example, via a controllable transistor switch between the two capacitors.
  • the programming data is preferably pre-adjusted to compensate for the affect of charge sharing between the capacitors. For example if the capacitors are of equal value then the programming data (voltage) should be doubled so that the correct data (voltage) is obtained after charge sharing.
  • the invention provides a stereoscopic pixel driver circuit for driving a pixel of a stereoscopic OLED display in accordance with left stereoscopic pixel data and right stereoscopic pixel data to control the brightness OLED pixel in respective left and right stereoscopic images displayed on said display
  • said pixel driver circuit comprising: at least one select line to select the OLED pixel; at least one programming line to program the pixel circuit with pixel data to control brightness of the pixel; at least one driver transistor to drive the pixel; left and right data storage capacitors to store left stereoscopic pixel data and right stereoscopic pixel data to control the brightness of the OLED pixel in respective left and right stereoscopic images displayed on said display, and wherein said pixel data comprises said left and right stereoscopic pixel data; a select transistor having a control terminal coupled to said select line; first and second data storage capacitors to store respective first and second charges to control driving said OLED pixel; a programming transistor having a control terminal coupled to said
  • the first and second data storage capacitors have substantially the same value.
  • the programming voltage applied to the second data storage capacitor could be selected dependent on a previously known state of the pixel, more particularly of the voltage on the first data storage capacitor, such than when charge sharing takes place a desired voltage is obtained on the first data storage capacitor (starting initially from some known, for example 0 voltage state).
  • the invention still further provides a method of programming a circuit as described above, in particular adjusting a voltage applied to the programming line when programming the pixel driver circuit to compensate for the charge sharing, for example by doubling a programming voltage applied to the programming line.
  • Figure 1 illustrates a problem with shuttered 3D display operation
  • Figures 2a to 2d show, respectively, a first example of a stereoscopic active matrix pixel circuit and examples of, respectively, a voltage-programmed OLED drive circuit, a first current-programmed OLED drive circuit, and a second current-programmed OLED drive circuit suitable for use in the pixel circuit of Figure 2a;
  • Figure 3 illustrates a second example of a stereoscopic active matrix pixel circuit, employing charge sharing
  • Figures 4a to 4c show first, second and third examples of a stereoscopic active matrix pixel drive circuit enabling independent programming and OLED drive;
  • Figure 5 shows an example of a current-programmed stereoscopic active matrix pixel drive circuit allowing independent programming and OLED drive
  • Figure 6 conceptually illustrates the operation of the circuit of Figure 5; and Figure 7 shows an OLED display system comprising a display incorporating stereoscopic active matrix pixel drive circuits according to an embodiment of the invention.
  • stereoscopic active matrix pixel circuits which provide the ability to switch an entire OLED in display to a new image by employing local pixel data storage and/or duplicate drive circuits. Because, in embodiments there is no blanking time the resulting display appears brighter and/or the OLED display panel life may be extended. Furthermore embodiments of the invention enable increased OLED drive switching speeds, more particularly faster than the programming rate of the pixels, which in turn facilitates low flicker 3D reproduction and, potentially, reduced data refresh rates whilst maintaining high left/right (L/R) image switching rates.
  • L/R left/right
  • a second scenario is where there are two alternate drive level storage areas and/or two alternate complete pixel circuits.
  • the display can rapidly alternate between these two frames, in principle at much higher switching speeds than current update times allow, thus substantially reducing flicker, and in principle completely independently of the fame update rate.
  • FIG. 2a shows an example of a stereoscopic active matrix pixel drive circuit 200 having left and right output stages 202a, b each with an associated storage capacitor, selectively couplable to drive an OLED pixel 204 via a controllable output switch 206.
  • a programming block 208 has a DATA input 210 and is controlled by a SELECT line 212; this block provides an output via a second controllable switch 214 for programming the pixel data into either the left or right output stage.
  • a FRAME select line 2 6 controls switches 214, 206 so that when one of output stages 202a, b is being programmed, the other is driving output data to OLED pixel 204, and vice versa.
  • the switches 206, 214 may be implemented as shown by the insets or alternatively, for example, using one n and one p switching transistor.
  • the FRAME line 2 6 alternates to repetitively program one stereoscopic FRAME and display the other, so that there is no need for a blanking interval.
  • the OLED display may be driven for substantially 100% of the time and the effective bandwidth required for the display is reduced by a factor of 2.
  • Figures 2b to 2d show examples of pixel drive circuits incorporating storage which may be employed with the stereoscopic pixel driver circuit of Figure 2a, storing the pixel data by, respectively, directly programming a gate voltage on the drive transistor, employing a current mirror in a current-program circuit, and employing a current-copy technique in a current-program circuit.
  • the skilled person will appreciate that for one of these circuits to be employed, for example in the programming/output stages of the stereoscopic pixel driver circuit of Figure 2a, modifications will need to be made, in particular to include switches 206, 214 as described above.
  • a gate connection 259 of driver transistor 258 is coupled to a storage capacitor 220 and a control transistor 222 couples gate 259 to column data line 226 under control of row select line 224.
  • switch 222 is on a voltage on column data line 226 can be stored on a capacitor 220, and the voltage at gate node 259 controls or programs the current through OLED 252 and hence the brightness of the OLED.
  • a current on the (column) data line set by current generator 266, programs the current through thin film transistor (TFT) 260, which in turn sets the current through OLED 252, since when transistor 222a is on (matched) transistors 260 and 258 form a current mirror.
  • TFT thin film transistor
  • FIG. 3 shows a stereoscopic active matrix pixel driver circuit 300 comprising a driver transistor 302 coupled between a V D D line 304 and an OLED pixel 306.
  • a first data storage capacitor 308 is coupled between the gate of transistor 302 and the V 0 o line 304, and a reset or blanking transistor 310 is coupled across capacitor 308 to enable the charge on capacitor 308 to be reset by applying a control signal to a pixel BLANK line 312.
  • a second data storage capacitor 314 is also coupled to V D D line 304 and a programming transistor 316 is coupled between a second connection of capacitor 314 and the terminal of capacitor 308 connected to the gate of transistor 302.
  • a programming line PROG 318 to control transistor 316 on charge is shared between capacitors 314 and 308.
  • a select transistor 320 is coupled to the junction of capactor 314 and transistor 316 to selectively connect capacitor 314 to data line 322 under the control of pixel SELECT line 324.
  • the gate voltage of transistor 302 is programmed relative to V DD and, broadly speaking, this is achieved by programming capacitor 314 with a voltage for a next display frame, and then using the blanking transistor 314 to reset the charge on capacitor 308 before operating transistor 316 to transfer charge from capacitor 314 to capacitor 318.
  • presuming capacitor 308 stores a current programming voltage for the pixel, and taking the example of equal valued capacitors 314 and 308, the procedure is as follows: select the pixel and programme capacitor 314 with twice the desired voltage for the next pixel state; (2) reset the charge on capacitor 308 by briefly operating transistor 310 (in embodiments the whole display is blanked simultaneously), to set the gate voltage of transistor 302 at V DD ; (3) operate programming transistor 316 to share charge between capacitors 314 and 308 so that each of these settles to a voltage equal to the desired gate drive voltage for the next pixel state.
  • capacitor 308 is programmed with a voltage determined by the fraction of the value of capacitor 308 compared with the total capacitance of capacitors 308 and 314, and thus in the general case capacitor 314 is programmed with the inverse of this fraction multiplying the desired programming voltage for the pixel.
  • the generalised pixel drive circuit comprises a programming block 402 having a programming data input PROG 404 and a select control line SEL 406.
  • An output of programming block 402 is coupled via a controllable switch 408 under control of a set LR (set left-right) control line 410, to respective left and right 412a, b OLED drive stages.
  • the output of drive stages 412a, b are coupled by a controllable drive select switch 414 under control of a DRIVE line 416, to an OLED pixel 418.
  • the controllable switches 408, 414 may be as previously described with reference to Figure 2a.
  • FIG. 4b shows an alternative stereoscopic active matrix pixel derive circuit 420 employing separate left and right programming data lines PROGL 422a and PROGR 422b.
  • normal and inverted DRIVE lines are employed to control the drive select switch 414; the inverted DRIVE line may be derived using a local inverter; alternatively p and n switching transistors may be employed for switch 414 to dispense with the need for an inverted DRIVE line.
  • the illustrated example employs a single select line and two programming lines; in an alternative approach two select lines and a single programming line may be employed.
  • Figures 4a and 4b provide independent control of programming the left and right pixel data and of driving the pixel with the programmed data, thus enabling the display to be switched between left and right image frames faster than the rate at which the data is programmed, thus reducing flicker.
  • the programming and drive rates may be different, in embodiments they may still be synchronised.
  • Figure 4c this shows details of an example implementation of the arrangement of Figure 4b, in which like elements are indicated by like reference numerals.
  • Figure 4c employs two active matrix pixel driver circuits per OLED pixel, but the arrangement of Figure 4c is particularly advantageous because separate left and right V 0 o lines are employed to control the drive output, which enables a reduction in the number of busbar connections to the pixel driver circuit.
  • the V D D line for one or other (left or right) portion of the stereoscopic pixel driver circuit should be actively pulled to a particular voltage, for example pulled low (more particularly to zero volts) when programming.
  • a particular voltage for example pulled low (more particularly to zero volts) when programming.
  • this requirement may be avoided.
  • a combination of a (single) drive control line and a (single) V DD (power supply) line may be employed to control the left/right data output drive to the OLED pixel.
  • the respective left right portions of the pixel driver circuit comprise a driver transistor 426a, b, a gate storage capacitor 428a, b coupled between the gate of the respective driver transistor and the respective V D D line 424a, b, and a select transistor for 30a, b coupled between the gate of the driver transistor and the respective PROG L/R line 422a, b.
  • Whichever drive transistor is driving the OLED 418 only left or right pixel data is ever applied to the OLED and thus although rapid switching of the drive between the left and right pixel data may show left data or right data for two different frames on the display simultaneously, it will not show left pixel data and right pixel data simultaneously on the display.
  • Table 1 shows an example drive scheme for the circuit of Figure 4c.
  • the left and right pixel data are simultaneously programmed into the circuit (since both transistors 430a and 430b are controlled together), but during programming the left and right Voo line states are not defined - that is in embodiments it is necessary to know the value of the voltages on these lines in order to correctly define the voltage programming levels on the programming lines (since these are defined with respect to V DD ), but the pixel circuit does not require any particular voltage level on the respective V DD lines.
  • driving the voltage levels on the select and programming lines are irrelevant because each half of a circuit is only being updated with either the left or the right data, not both.
  • the programming and driving can be independent of one another, and although in embodiments that may be convenient to synchronise one to the other, the rate at which the drive swaps between left and right image frames can be higher than the rate at which data is written to the pixel driver circuit, thus reducing flicker and producing a better 3D effect.
  • VDD-L - VL Active (High) Not defined Not defined (VDD-L - VL) (VDD-R - VR) left and right data)
  • Table 1 Referring next to Figure 5, this shows a stereoscopic active matrix pixel drive circuit 500 configured to provide independent control of programming and data drive, and employing current rather than voltage programming.
  • the pixel drive circuit 500 comprises a left current-programmed pixel drive circuit 502a and a right current- programmed pixel drive circuit 502b each coupled to drive a common OLED 504.
  • the left and right pixel drive circuits 502a, b each have a respective select line 506a, b and a power supply line (V 0D ) 508a, b.
  • the left and right current- programmed circuits share a common current programming line 510 in which a current is programmed to set the brightness of OLED pixel 504,
  • the current may be programmed by a constant current generator (current source or current sink), but in embodiments a transistor 512 may be coupled between the current programming line 510 and a second power supply line (V ss ) 514 with a control (gate) terminal connected to a voltage programming line 516, transistor 512 acting as a voltage-to-current converter.
  • V ss second power supply line
  • transistor 512 acting as a voltage-to-current converter.
  • transistor 512 is fabricated from amorphous silicon, the way in which the transistor is used reduces the effects of threshold voltage variation seen in such transistors.
  • the circuit of Figure 5 uses the V D o power supply lines 508a, b to control the drive to OLED 504, but the configuration of Figure 5 is constrained to require V DD to be low when the left or right pixel circuit 502a,b is programmed.
  • the circuit of Figure 5 is programmed in one half, for example the left, whilst the other half of the circuit can for example the right, is driving OLED pixel 504, and vice-versa.
  • this programming arrangement still effectively halves the required bandwidth for the display and removes the need for a blanking interval, but the display of left and right images of a succession of image frames is different to that previously described, as illustrated in Figure 6: the left and right (L and R) image fames still update at a rate of 60Hz, but each left (and right) image shows parts of two successive image frames.
  • the data drive may be independent from the data programming and, in principle, there is no need for the drive rate to be an integer multiple of the programming rate (although this is preferred).
  • FIG. 7 shows an example of a 3D image display system 700 including an OLED display 702 comprising a plurality of stereoscopic pixel driver circuits 704, in particular each according to an embodiment of the invention as described above.
  • OLED display 702 comprising a plurality of stereoscopic pixel driver circuits 704, in particular each according to an embodiment of the invention as described above.
  • controller 706 controls the select, power supply and programming lines as previously described to write left and right image frame data to the display in accordance with 3D colour illustrator received on line 708.

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Abstract

A stereoscopic OLED display comprises a plurality of OLED pixels each driven by an active matrix stereoscopic pixel circuit, each comprising: at least one select line; at least one programming line to control brightness of the pixel; at least one driver transistor to drive the pixel; left and right data storage capacitors to store left stereoscopic pixel data and right stereoscopic pixel data to control the brightness of the OLED pixel in respective left and right stereoscopic images; a third line to enable said left and right pixel data to be selectively stoned in said pixel circuit in said respective left and right data storage capacitors; and at least one drive control line to control said at least one driver transistor to enable said OLED pixel to be selectively driven with either said left stereoscopic pixel data or said right stereoscopic pixel data; and wherein the display further comprises: a second said driver transistor such that said pixel driver comprises two said driver transistors coupled to the same said OLED pixel, each said driver transistor being coupled to a respective one of said left and right data storage capacitors, and either i) a second said drive control line to control said second driver transistor; wherein said first and second drive control lines comprise respective first and second power supply lines each to provide a power supply to said OLED pixel via a respective said driver transistor; or ii) a single said drive control line and a single power supply line connected to both said first and second drive transistors.

Description

ORGANIC LIGHT EMITTING DIODE DISPLAYS
FIELD OF THE INVENTION
This invention relates to active matrix (AM) organic light emitting diode (OLED) displays, and in particular to driving an AMOLED panel driving to provide a shuttered stereoscopic (3D) display.
BACKGROUND TO THE INVENTION
It is known to drive display panels, including AMOLED display panels, with alternating left and right eye images, using in conjunction with shutter glasses worn by the viewer(s) to give the impression of a 3-dimensional image. The shutters operate so that when a left eye stereoscopic view is displayed only the left eye sees this image, and vice-versa. The shutters may be wired or may automatically synchronise to the display frame rate using a photosensor. A conventional display is built up line-by-line and thus to avoid displaying parts of both left and right eye images at the same time a blanking interval is interposed between left and right stereoscopic image frames. Absent this blanking interval the right eye would see part of the old left eye frame until this was refreshed.
Figure 1 illustrates the concept of a blanking interval with vertical screen position on the vertical axis and time on the horizontal axis. This shows that, for example, for a 60Hz (left and right) frame rate, when a blanking interval is employed the effective frame rate is 240Hz and the display is off 50% of the time.
Different techniques and advantages are required/provided by LCD (liquid crystal display) and OLED-based display panels. For example, LCD-based display panels are relatively slow so that an image frame can bleed in to the blanking interval. Background prior art relating to stereoscopic LCD-based displays can be found in, for example.
US2007/0035494; US2007/0035492; WO2009/069026; WO2002/059691 ;
US2008/0084512; US2007/0195163; US2007/0153380; GB2336963A; W01996; 032665; and W01994/014104. Techniques for plasma displays are described in US2009/0002482; techniques for DLP (digital light processing) displays are described in US2008/0036854. Other background can be found in JP2004/253879A (compression techniques); and WO2001/069944 (eliminating ghosting/cross images).
Organic light emitting diode (OLED) - based displays potentially have significant advantages over the above-described technologies because they are easily able to provide an increased refresh rate without image bleeding and related issues. In this specification references to OLEDs include polymer OLEDs, so-called small molecule OLEDs, and organometallic OLED displays. One potential difficulty with using OLEDs in a blanking scheme as describe above is that because the display is off half the time twice the drive is required, effectively giving half the lifetime.
One technique to address this is described in US2007/0035483. This describes an approach in which during right perspective image display periods the pixels of the array are loaded with analogue voltage potential signals corresponding to a left perspective image while displaying the preceding right perspective image, and vice-versa, to avoid cross-frame image interference and expand the left and right perspective image display periods. However, the approach described uses a pixel circuit requiring a relatively high number of connections to tracks on the panel and, more generally, there is a desire to provide improved techniques.
SUMMARY OF THE INVENTION According to a first aspect of the invention there is therefore provided a stereoscopic OLED display, the display having a plurality of OLED pixels each driven by an active matrix stereoscopic pixel circuit, each stereoscopic pixel circuit comprising: at least one select line to select the OLED pixel; at least one programming line to program the pixel circuit with pixel data to control brightness of the pixel; at least one driver transistor to drive the pixel; left and right data storage capacitors to store left stereoscopic pixel data and right stereoscopic pixel data to control the brightness of the OLED pixel in respective left and right stereoscopic images displayed on said display, and wherein said pixel data comprises said left and right stereoscopic pixel data; a third line to enable said left and right stereoscopic pixel data to be selectively stored in said pixel circuit in said respective left and right data storage capacitors; and at least one drive control line to control said at least one driver transistor to enable said OLED pixel to be selectively driven with either said left stereoscopic pixel data or said right stereoscopic pixel data; and wherein the display further comprises: a second said driver transistor such that said pixel driver comprises two said driver transistors coupled to the same said OLED pixel, each said driver transistor being coupled to a respective one of said left and right data storage capacitors, and either i) a second said drive control line to control said second driver transistor; wherein said first and second drive control lines comprise respective first and second power supply lines each to provide a power supply to said OLED pixel via a respective said driver transistor; or ii) a single said drive control line and a single power supply line connected to both said first and second drive transistors.
Embodiments of the above-described display enable the pixel driver circuit to be controlled using a reduced number of tracks and, in addition, enable programming of left and right pixel data independent of which data is driven to a pixel. This in turn enables further advantages as discussed below. (The skilled person will appreciate that references to left and right data storage capacitors are to their functions in storing left and right pixel data rather than to their physical layout).
In some preferred embodiments the circuit is controlled by two power supply (VDD) lines; preferably the first and second power supply (VDD) lines are directly connected to the respective driver transistors. (The skilled person will appreciate that the pixel driver circuit is also provided with a common or ground connection). The third line may comprise either a select line or a programming line, in which case the pixel circuit has only a single one of the other lines, that is the programming line or select line, respectively.
The pixel circuit may comprise a voltage-programmed or a current-programmed pixel circuit. In the case of a current-programmed pixel circuit the pixel circuit may comprise two pairs of select transistor one associated with each of the 'left' and 'right' driver transistors. In embodiments each pair of select transistors is connected for control by a respective select line and a first transistor of the pair is connected between the programming line and a first terminal of one of the data storage capacitors, and a second transistor of a select pair is coupled in series between two terminals of a driver transistor to program the current through the driver transistor. For example the second transistor of a pair may be connected across a driver transistor to diode-connect the driver transistor, or a further transistor may be coupled in series across the driver transistor, and the two select transistors connected in series.
In a current-programmed arrangement a further, current-programming transistor may be employed as a voltage-to-current converter to convert a programming voltage to a programming current for the pixel. Thus this transistor may have a control connection coupled to the programming line and an output connection coupled to a current - programming line of the pixel circuit. Although such an arrangement is typically not preferred with amorphous silicon, because the threshold voltage of the current programming transistor can drift over time, in embodiments of the invention the current programming transistor is used only briefly when programming the pixel circuit and therefore this threshold voltage shift is of less concern.
The inventor has recognised that in embodiments of the above described arrangements because programming of the pixel circuit is independent of drive of the OLED pixel, the drive to a pixel may be switched between left and right stereoscopic pixel drive data at a rate which is faster than that at which the pixel circuit is being programmed. This is because with such an arrangement the viewer's eye only sees left or right image data at any one time, from either the left or right part of the pixel drive circuit, irrespective of whether or not the left or right frame data has been fully updated. Thus whilst under certain circumstances there may be portions of two successive left image frame or right image frames visible, there will not be portions of both a left image frames and a right image frame visible at the same time to adjust the left eye or adjust the right eye of a viewer. The inventor has recognised that, potentially, seeing portions of two successive left image frames or right image frames at the same time is visually acceptable whereas seeing portions of a left image frame and a right image frame together, viewed with just one of the left or right eyes is not.
The inventor has further recognised that this enables switching between the left and right image displays at essentially an arbitrarily fast rate, thus reducing the volume the effects of image flicker (which can otherwise be present at relatively low switch rates of order 60Hz).
Thus in a related aspect the invention provides a method of providing a stereoscopic OLED display, the display having a plurality of OLED pixels each driven by an active matrix stereoscopic pixel circuit, each stereoscopic pixel circuit comprising: at least one select line to select the OLED pixel; at least one programming line to program the pixel circuit with pixel data to control brightness of the pixel; at least one driver transistor to drive the pixel; left and right data storage capacitors to store left stereoscopic pixel data and right stereoscopic pixel data to control the brightness of the OLED pixel in respective left and right stereoscopic images displayed on said display, and wherein said pixel data comprises said left and right stereoscopic pixel data; a third line to enable said left and right stereoscopic pixel data to be selectively stored in said pixel circuit in said respective left and right data storage capacitors; and at least one drive control line to control said at least one driver transistor to enable said OLED pixel to be selectively driven with either said left stereoscopic pixel data or said right stereoscopic pixel data; the method comprising: controlling, said at least one select line, said at least one programming line, and said third line, to write said left and right stereoscopic pixel data into OLED pixels of said display at a first rate; and controlling said at least one drive control line to control the brightness of said OLED pixels of said display to selectively display said left and right stereoscopic images, at a second rate; and wherein said second rate is greater than said first rate.
In embodiments programming the pixel drive circuits may take place whenever convenient, provided the rate matches the overall input data rate from the left and right image frames of the source, with left and right drive switching at a higher rate, albeit probably synchronised to the programming, for convenience. In this way the left eye, for example, only sees the left image frames, either complete or being updated, but there is no need to program a complete left image frame before showing it. In embodiments the display drive rate is an integer multiple of the programming rate, but this is not essential and the two rates may, for example, be defined by a ratio of two integers (this latter may introduce banding artefacts but, apart from some special case images, these should not be readily discernible).
As previously described, in embodiments the pixel driver circuit may have two control lines and a single programming line, or two programming lines and a single control line. In either case independent control over programming and drive is provided. In some preferred embodiments the drive control is performed by a power supply line, for example VDD, providing left and right VDD lines which can be controlled/switched to control/switch the drive to a pixel between left and right stereoscopic images (or on and off). In other arrangements, however, a single power supply (VDD) line is provided together with a drive control line for switching between left and right pixel data. In embodiments two active matrix pixel circuits may effectively be provided per OLED pixel.
In embodiments the pixel drive circuit may comprise a current-programmed pixel circuit, but more particularly a circuit which is programmed by pulling the power supply line (VDD) low. This may constrain the drive since drive can then not take place at the same time as programming. To address this programming may be performed on one portion or half of the display panel whilst another portion or half of the panel is on, that is the relevant left or right stereoscopic image data is being driven to the pixels. This results in a display in which, for example, the left stereoscopic image of a first frame is being displayed in one half of the display (the bottom half) panel whilst the right stereoscopic image of a second, subsequent frame is being programmed into the other half of the display panel (say the top half). Thus in embodiments in particular of a current-programmed pixel circuit approach driving the left (or right) stereoscopic image shows parts of two successive image frames, albeit both left (or right) stereoscopic images.
In a related aspect there is provided a stereoscopic display having a plurality of OLED pixels each driven by an active matrix stereoscopic pixel circuit, each stereoscopic pixel circuit comprising: at least one select line to select the OLED pixel; at least one programming line to program the pixel circuit; with pixel data to control brightness of the pixel; at least one driver transistor to drive the pixel; left and right data storage capacitors to store left stereoscopic pixel data and right stereoscopic pixel data to control the brightness of the OLED pixel in respective left and right stereoscopic images displayed on said display, and wherein said pixel data comprises said left and right stereoscopic pixel data; a third line to enable said left and right stereoscopic pixel data to be selectively stored in said pixel circuit in said respective left and right data storage capacitors; and at least one drive control line to control said at least one driver transistor to enable said OLED pixel to be selectively driven with either said left stereoscopic pixel data or said right stereoscopic pixel data; and wherein the display further comprises a controller to: control said at least one select line, said at least one programming line, and said third line to write said left and right stereoscopic pixel data into OLED pixels of said display at a first rate; and control said at least one drive control line to control the brightness of said OLED pixels of said display to selectively display said left and right stereoscopic images, at a second rate, higher than said first rate. In a still further approach two data storage capacitors are employed but only a single driver transistor is used. In such an arrangement the pixel driver circuit in effect comprises a form of analogue shift register with charge sharing between the capacitors to move the data from one to the other, for example, via a controllable transistor switch between the two capacitors. In such an approach the programming data is preferably pre-adjusted to compensate for the affect of charge sharing between the capacitors. For example if the capacitors are of equal value then the programming data (voltage) should be doubled so that the correct data (voltage) is obtained after charge sharing.
Thus in a further aspect the invention provides a stereoscopic pixel driver circuit for driving a pixel of a stereoscopic OLED display in accordance with left stereoscopic pixel data and right stereoscopic pixel data to control the brightness OLED pixel in respective left and right stereoscopic images displayed on said display, said pixel driver circuit comprising: at least one select line to select the OLED pixel; at least one programming line to program the pixel circuit with pixel data to control brightness of the pixel; at least one driver transistor to drive the pixel; left and right data storage capacitors to store left stereoscopic pixel data and right stereoscopic pixel data to control the brightness of the OLED pixel in respective left and right stereoscopic images displayed on said display, and wherein said pixel data comprises said left and right stereoscopic pixel data; a select transistor having a control terminal coupled to said select line; first and second data storage capacitors to store respective first and second charges to control driving said OLED pixel; a programming transistor having a control terminal coupled to said programming line; wherein said first data storage capacitor is coupled to a control terminal of said driver transistor; wherein said second data storage capacitor is coupled to said programming line via said select transistor; and wherein said programming transistor is coupled between said first and second data storage capacitors to, when enabled, enable charge sharing between said first and second storage capacitors; such that said pixel driver circuit is programmable with one of said left and right stereoscopic pixel data whilst driving said OLED pixel at a brightness determined by the other of said left and right stereoscopic pixel data.
In embodiments the first and second data storage capacitors have substantially the same value. In theory the programming voltage applied to the second data storage capacitor could be selected dependent on a previously known state of the pixel, more particularly of the voltage on the first data storage capacitor, such than when charge sharing takes place a desired voltage is obtained on the first data storage capacitor (starting initially from some known, for example 0 voltage state). However it is preferable to provide an additional reset transistor coupled across the first data storage capacitor to reset or blank the pixel data so that when the charge sharing takes place we charge on the first capacitor is in a known state, for example, of approximately zero stored charge/voltage.
The invention still further provides a method of programming a circuit as described above, in particular adjusting a voltage applied to the programming line when programming the pixel driver circuit to compensate for the charge sharing, for example by doubling a programming voltage applied to the programming line.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other aspects of the invention will now be further described by way of example only, with reference to the accompanying Figures in which:
Figure 1 illustrates a problem with shuttered 3D display operation;
Figures 2a to 2d show, respectively, a first example of a stereoscopic active matrix pixel circuit and examples of, respectively, a voltage-programmed OLED drive circuit, a first current-programmed OLED drive circuit, and a second current-programmed OLED drive circuit suitable for use in the pixel circuit of Figure 2a;
Figure 3 illustrates a second example of a stereoscopic active matrix pixel circuit, employing charge sharing;
Figures 4a to 4c show first, second and third examples of a stereoscopic active matrix pixel drive circuit enabling independent programming and OLED drive;
Figure 5 shows an example of a current-programmed stereoscopic active matrix pixel drive circuit allowing independent programming and OLED drive;
Figure 6 conceptually illustrates the operation of the circuit of Figure 5; and Figure 7 shows an OLED display system comprising a display incorporating stereoscopic active matrix pixel drive circuits according to an embodiment of the invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Broadly speaking we will describe stereoscopic active matrix pixel circuits which provide the ability to switch an entire OLED in display to a new image by employing local pixel data storage and/or duplicate drive circuits. Because, in embodiments there is no blanking time the resulting display appears brighter and/or the OLED display panel life may be extended. Furthermore embodiments of the invention enable increased OLED drive switching speeds, more particularly faster than the programming rate of the pixels, which in turn facilitates low flicker 3D reproduction and, potentially, reduced data refresh rates whilst maintaining high left/right (L/R) image switching rates. The techniques we describe can also, in effect provide more time to program the OLED display panel, which is particularly important as displays trend towards higher resolution. This in turn can potentially also facilitate the use of a potentially slower, but cost-saving methods of programming, such as multiplexing the column driver outputs. In outline, we will describe techniques which involve additional storage in a pixel driver circuit to enable, for example, programming of a display panel without initially changing the displayed image and then substantially instantaneous update of the whole screen (although alternative approaches are also envisaged, as described later). Two primary scenarios are envisaged. The first is a panel designed to store the programmed image merely until the update. For example for each pixel may be provided with an additional storage capacitor and switching transistors such that the new pixel drive level is programmed onto this capacitor. Then when completed all pixels on the display are controlled to reset the drive storage capacitor and then transfer the charge from the additional capacitor to the drive storage capacitor. This means that there is no retention of the original programmed data.
A second scenario is where there are two alternate drive level storage areas and/or two alternate complete pixel circuits. In this case the display can rapidly alternate between these two frames, in principle at much higher switching speeds than current update times allow, thus substantially reducing flicker, and in principle completely independently of the fame update rate.
While such drive techniques constitute a display modification to improve 3D performance they do not require any extra functional components beyond the pixel circuit (such as polarising optics or micro-lenses). Thus the techniques should not present a significantly increased cost.
Referring now to Figure 2a this shows an example of a stereoscopic active matrix pixel drive circuit 200 having left and right output stages 202a, b each with an associated storage capacitor, selectively couplable to drive an OLED pixel 204 via a controllable output switch 206. A programming block 208 has a DATA input 210 and is controlled by a SELECT line 212; this block provides an output via a second controllable switch 214 for programming the pixel data into either the left or right output stage. A FRAME select line 2 6 controls switches 214, 206 so that when one of output stages 202a, b is being programmed, the other is driving output data to OLED pixel 204, and vice versa. The switches 206, 214 may be implemented as shown by the insets or alternatively, for example, using one n and one p switching transistor. In operation the FRAME line 2 6 alternates to repetitively program one stereoscopic FRAME and display the other, so that there is no need for a blanking interval. Thus the OLED display may be driven for substantially 100% of the time and the effective bandwidth required for the display is reduced by a factor of 2. Figures 2b to 2d show examples of pixel drive circuits incorporating storage which may be employed with the stereoscopic pixel driver circuit of Figure 2a, storing the pixel data by, respectively, directly programming a gate voltage on the drive transistor, employing a current mirror in a current-program circuit, and employing a current-copy technique in a current-program circuit. The skilled person will appreciate that for one of these circuits to be employed, for example in the programming/output stages of the stereoscopic pixel driver circuit of Figure 2a, modifications will need to be made, in particular to include switches 206, 214 as described above. The examples are included to help facilitate understanding of the operation of embodiments of the invention, to give examples of the type of circuits which may be employed for OLED pixel programming and data storage. Thus in Figure 2b a gate connection 259 of driver transistor 258 is coupled to a storage capacitor 220 and a control transistor 222 couples gate 259 to column data line 226 under control of row select line 224. When switch 222 is on a voltage on column data line 226 can be stored on a capacitor 220, and the voltage at gate node 259 controls or programs the current through OLED 252 and hence the brightness of the OLED.
In Figure 2c a current on the (column) data line, set by current generator 266, programs the current through thin film transistor (TFT) 260, which in turn sets the current through OLED 252, since when transistor 222a is on (matched) transistors 260 and 258 form a current mirror.
In Figure 2d (from our WO03/038790) the current through the OLED is programmed by setting a drain source current for transistor 258 and copying/memorising the gate voltage required for this, using transistor 264 to inhibit OLED illumination during programming: To copy the programming current, switch 268 is closed and switch 264 is opened so that the programming current flows through drive transistor 258, and switch transistor 270 is closed to set Vg on drive transistor 270 for the programmed current and to store this Vg value on capacitor 220. We now describe a different approach, with reference to Figure 3, in which two switched storage capacitors are employed to provide, in effect, an analogue shift register using charge sharing.
Thus Figure 3 shows a stereoscopic active matrix pixel driver circuit 300 comprising a driver transistor 302 coupled between a VDD line 304 and an OLED pixel 306. A first data storage capacitor 308 is coupled between the gate of transistor 302 and the V0o line 304, and a reset or blanking transistor 310 is coupled across capacitor 308 to enable the charge on capacitor 308 to be reset by applying a control signal to a pixel BLANK line 312. A second data storage capacitor 314 is also coupled to VDD line 304 and a programming transistor 316 is coupled between a second connection of capacitor 314 and the terminal of capacitor 308 connected to the gate of transistor 302. Thus by activating a programming line PROG 318 to control transistor 316 on charge is shared between capacitors 314 and 308. A select transistor 320 is coupled to the junction of capactor 314 and transistor 316 to selectively connect capacitor 314 to data line 322 under the control of pixel SELECT line 324. In operation the gate voltage of transistor 302 is programmed relative to VDD and, broadly speaking, this is achieved by programming capacitor 314 with a voltage for a next display frame, and then using the blanking transistor 314 to reset the charge on capacitor 308 before operating transistor 316 to transfer charge from capacitor 314 to capacitor 318. More particularly, presuming capacitor 308 stores a current programming voltage for the pixel, and taking the example of equal valued capacitors 314 and 308, the procedure is as follows: select the pixel and programme capacitor 314 with twice the desired voltage for the next pixel state; (2) reset the charge on capacitor 308 by briefly operating transistor 310 (in embodiments the whole display is blanked simultaneously), to set the gate voltage of transistor 302 at VDD; (3) operate programming transistor 316 to share charge between capacitors 314 and 308 so that each of these settles to a voltage equal to the desired gate drive voltage for the next pixel state. The skilled person will appreciate that capacitor 308 is programmed with a voltage determined by the fraction of the value of capacitor 308 compared with the total capacitance of capacitors 308 and 314, and thus in the general case capacitor 314 is programmed with the inverse of this fraction multiplying the desired programming voltage for the pixel.
Referring now to Figure 4a, this shows a further example of a stereoscopic active matrix pixel drive circuit 400 with independent programming and drive capability. Thus the generalised pixel drive circuit comprises a programming block 402 having a programming data input PROG 404 and a select control line SEL 406. An output of programming block 402 is coupled via a controllable switch 408 under control of a set LR (set left-right) control line 410, to respective left and right 412a, b OLED drive stages. The output of drive stages 412a, b are coupled by a controllable drive select switch 414 under control of a DRIVE line 416, to an OLED pixel 418. The controllable switches 408, 414 may be as previously described with reference to Figure 2a.
Referring now to Figure 4b, in which like elements to those of Figure 4a are indicated by like reference numerals, this shows an alternative stereoscopic active matrix pixel derive circuit 420 employing separate left and right programming data lines PROGL 422a and PROGR 422b. In Figure 4b normal and inverted DRIVE lines are employed to control the drive select switch 414; the inverted DRIVE line may be derived using a local inverter; alternatively p and n switching transistors may be employed for switch 414 to dispense with the need for an inverted DRIVE line. The illustrated example employs a single select line and two programming lines; in an alternative approach two select lines and a single programming line may be employed.
The arrangements of Figures 4a and 4b provide independent control of programming the left and right pixel data and of driving the pixel with the programmed data, thus enabling the display to be switched between left and right image frames faster than the rate at which the data is programmed, thus reducing flicker. Although the programming and drive rates may be different, in embodiments they may still be synchronised. Referring now to Figure 4c this shows details of an example implementation of the arrangement of Figure 4b, in which like elements are indicated by like reference numerals. Broadly speaking the arrangement of Figure 4c employs two active matrix pixel driver circuits per OLED pixel, but the arrangement of Figure 4c is particularly advantageous because separate left and right V0o lines are employed to control the drive output, which enables a reduction in the number of busbar connections to the pixel driver circuit.
In the particular circuit configuration employed, with the storage capacitor connected between the gate of the driver transistor and a respective VDD line, the VDD line for one or other (left or right) portion of the stereoscopic pixel driver circuit should be actively pulled to a particular voltage, for example pulled low (more particularly to zero volts) when programming. This enables the programming voltage to be defined with respect to a defined, for example zero, value of VDD. However in other arrangements this requirement may be avoided. The skilled person will also appreciate that in alternative configurations a combination of a (single) drive control line and a (single) VDD (power supply) line may be employed to control the left/right data output drive to the OLED pixel. In Figure 4c the respective left right portions of the pixel driver circuit comprise a driver transistor 426a, b, a gate storage capacitor 428a, b coupled between the gate of the respective driver transistor and the respective VDD line 424a, b, and a select transistor for 30a, b coupled between the gate of the driver transistor and the respective PROG L/R line 422a, b. Whichever drive transistor is driving the OLED 418, only left or right pixel data is ever applied to the OLED and thus although rapid switching of the drive between the left and right pixel data may show left data or right data for two different frames on the display simultaneously, it will not show left pixel data and right pixel data simultaneously on the display.
Table 1 , below, shows an example drive scheme for the circuit of Figure 4c. As can be seen, the left and right pixel data are simultaneously programmed into the circuit (since both transistors 430a and 430b are controlled together), but during programming the left and right Voo line states are not defined - that is in embodiments it is necessary to know the value of the voltages on these lines in order to correctly define the voltage programming levels on the programming lines (since these are defined with respect to VDD), but the pixel circuit does not require any particular voltage level on the respective VDD lines. When driving the voltage levels on the select and programming lines are irrelevant because each half of a circuit is only being updated with either the left or the right data, not both. Thus the programming and driving can be independent of one another, and although in embodiments that may be convenient to synchronise one to the other, the rate at which the drive swaps between left and right image frames can be higher than the rate at which data is written to the pixel driver circuit, thus reducing flicker and producing a better 3D effect.
Programming:
SEL VDD-L VDD-R PROG L PROG R
(Simultaneously
Active (High) Not defined Not defined (VDD-L - VL) (VDD-R - VR) left and right data)
Driving:
SEL VDD-L VDD-R PROG L PROG R
Driving right frame: Don't care VDD GND Don't care Don't care
Driving left frame: Don't care GND VDD Don't care Don't care
Table 1 Referring next to Figure 5, this shows a stereoscopic active matrix pixel drive circuit 500 configured to provide independent control of programming and data drive, and employing current rather than voltage programming. The pixel drive circuit 500 comprises a left current-programmed pixel drive circuit 502a and a right current- programmed pixel drive circuit 502b each coupled to drive a common OLED 504. The left and right pixel drive circuits 502a, b each have a respective select line 506a, b and a power supply line (V0D ) 508a, b. As illustrated in Figure 5 the left and right current- programmed circuits share a common current programming line 510 in which a current is programmed to set the brightness of OLED pixel 504, The current may be programmed by a constant current generator (current source or current sink), but in embodiments a transistor 512 may be coupled between the current programming line 510 and a second power supply line (Vss) 514 with a control (gate) terminal connected to a voltage programming line 516, transistor 512 acting as a voltage-to-current converter. Although, in embodiments, transistor 512 is fabricated from amorphous silicon, the way in which the transistor is used reduces the effects of threshold voltage variation seen in such transistors.
The circuit of Figure 5 uses the VDo power supply lines 508a, b to control the drive to OLED 504, but the configuration of Figure 5 is constrained to require VDD to be low when the left or right pixel circuit 502a,b is programmed. Thus in a preferred mode of operation the circuit of Figure 5 is programmed in one half, for example the left, whilst the other half of the circuit can for example the right, is driving OLED pixel 504, and vice-versa. In embodiments this programming arrangement still effectively halves the required bandwidth for the display and removes the need for a blanking interval, but the display of left and right images of a succession of image frames is different to that previously described, as illustrated in Figure 6: the left and right (L and R) image fames still update at a rate of 60Hz, but each left (and right) image shows parts of two successive image frames. Again the data drive may be independent from the data programming and, in principle, there is no need for the drive rate to be an integer multiple of the programming rate (although this is preferred).
Figure 7 shows an example of a 3D image display system 700 including an OLED display 702 comprising a plurality of stereoscopic pixel driver circuits 704, in particular each according to an embodiment of the invention as described above. The skilled person will appreciate that in a colour display a stereoscopic pixel driver circuit is employed to drive a colour sub-pixel of the display. The display operates under the control of controller 706 which controls the select, power supply and programming lines as previously described to write left and right image frame data to the display in accordance with 3D colour illustrator received on line 708.
No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the scope of the claims appended hereto.

Claims

1. A stereoscopic OLED display, the display having a plurality of OLED pixels each driven by an active matrix stereoscopic pixel circuit, each stereoscopic pixel circuit comprising:
at least one select line to select the OLED pixel;
at least one programming line to program the pixel circuit with pixel data to control brightness of the pixel;
at least one driver transistor to drive the pixel;
left and right data storage capacitors to store left stereoscopic pixel data and right stereoscopic pixel data to control the brightness of the OLED pixel in respective left and right stereoscopic images displayed on said display, and wherein said pixel data comprises said left and right stereoscopic pixel data;
a third line to enable said left and right stereoscopic pixel data to be selectively stored in said pixel circuit in said respective left and right data storage capacitors; and at least one drive control line to control said at least one driver transistor to enable said OLED pixel to be selectively driven with either said left stereoscopic pixel data or said right stereoscopic pixel data; and
wherein the display further comprises:
a second said driver transistor such that said pixel driver comprises two said driver transistors coupled to the same said OLED pixel, each said driver transistor being coupled to a respective one of said left and right data storage capacitors, and either
i) a second said drive control line to control said second driver transistor;
wherein said first and second drive control lines comprise respective first and second power supply lines each to provide a power supply to said OLED pixel via a respective said driver transistor; or
ii) a single said drive control line and a single power supply line connected to both said first and second drive transistors.
2. A stereoscopic OLED display as claimed in claim 1 wherein said display comprises a second said drive control line to control said second driver transistor; and wherein said first and second drive control lines comprise respective first and second power supply lines each to provide a power supply to said OLED pixel via a respective said driver transistor.
3. A stereoscopic OLED display as claimed in claim 1 or 2 wherein said third line comprises a second said select line, and wherein said stereoscopic pixel circuit has a single said programming line.
4. A stereoscopic OLED display as claimed in claim 1 or 2 wherein said third line comprises a second said programming line, and wherein stereoscopic pixel circuit has a single said select line.
5. A stereoscopic OLED display as claimed in any one of claims 1 to 4 wherein said third line comprises a second said select line, and wherein said stereoscopic pixel circuit is a current programmed pixel circuit comprising two pairs of select transistors, one pair of select transistors associated with each said driver transistor.
6. A stereoscopic OLED display as claimed in claim 5 wherein each said pair of select transistors is connected for control by a respective said select line, wherein a first transistor of a said pair is connected between said at least one programming line and a first terminal of one of said data storage capacitors and wherein a second transistor of a said pair is coupled in series between two terminals of a said driver transistor to program a current through said driver transistor.
7. A stereoscopic OLED display as claimed in claim 5 or 6 wherein said stereoscopic pixel circuit further comprises a current programming transistor coupled between each of said driver transistors and a power supply line and having a control connection coupled to said at least one programming line.
8. A stereoscopic OLED display as claimed in any preceding claim further comprising a controller to control said drive control lines to switch between applying said left and right stereoscopic pixel drive data to a said pixel at a rate faster than a rate at which said left and right stereoscopic pixel drive data is programmed into said pixel circuit.
9. A method of providing a stereoscopic OLED display, the display having a plurality of OLED pixels each driven by an active matrix stereoscopic pixel circuit, each stereoscopic pixel circuit comprising :
at least one select line to select the OLED pixel; at least one programming line to program the pixel circuit with pixel data to control brightness of the pixel;
at least one driver transistor to drive the pixel;
left and right data storage capacitors to store left stereoscopic pixel data and right stereoscopic pixel data to control the brightness of the OLED pixel in respective left and right stereoscopic images displayed on said display, and wherein said pixel data comprises said left and right stereoscopic pixel data;
a third line to enable said left and right stereoscopic pixel data to be selectively stored in said pixel circuit in said respective left and right data storage capacitors; and at least one drive control line to control said at least one driver transistor to enable said OLED pixel to be selectively driven with either said left stereoscopic pixel data or said right stereoscopic pixel data;
the method comprising:
controlling said at least one select line, said at least one programming line, and said third line, to write said left and right stereoscopic pixel data into OLED pixels of said display at a first rate; and
controlling said at least one drive control line to control the brightness of said OLED pixels of said display to selectively display said left and right stereoscopic images, at a second rate; and
wherein said second rate is greater than said first rate.
10. A method as claimed in claim 9 wherein said at least one drive control line comprises a pair of left and right power supply lines to enable said OLED pixel to be selectively driven with either said left stereoscopic pixel data or said right stereoscopic pixel data, and wherein said selective driving of said OLED pixel comprises controlling a power supply to said left and right power supply lines.
11. A method as claimed in claim 10 comprising providing a sequence of image frames to said stereoscopic OLED display, each comprising said left and right stereoscopic images, wherein the method displays only either a left stereoscopic image or a right stereoscopic image on said display at a time, and wherein the method further comprises displaying simultaneously on said display parts of left stereoscopic images from two successive said image frames and displaying simultaneously on said display parts of right stereoscopic images from two successive said image frames.
12. A stereoscopic display having a plurality of OLED pixels each driven by an active matrix stereoscopic pixel circuit, each stereoscopic pixel circuit comprising:at least one select line to select the OLED pixel;
at least one programming line to program the pixel circuit; with pixel data to control brightness of the pixel;
at least one driver transistor to drive the pixel;
left and right data storage capacitors to store left stereoscopic pixel data and right stereoscopic pixel data to control the brightness of the OLED pixel in respective left and right stereoscopic images displayed on said display, and wherein said pixel data comprises said left and right stereoscopic pixel data,
a third line to enable said left and right stereoscopic pixel data to be selectively stored in said pixel circuit in said respective left and right data storage capacitors; and at least one drive control line to control said at least one driver transistor to enable said OLED pixel to be selectively driven with either said left stereoscopic pixel data or said right stereoscopic pixel data; and
wherein the display further comprises a controller to:
control said at least one select line, said at least one programming line, and said third line to write said left and right stereoscopic pixel data into OLED pixels of said display at a first rate, and
control said at least one drive control line to control the brightness of said OLED pixels of said display to selectively display said left and right stereoscopic images, at a second rate, higher than said first rate.
13. A stereoscopic pixel driver circuit for driving a pixel of a stereoscopic OLED display in accordance with left stereoscopic pixel data and right stereoscopic pixel data to control the brightness OLED pixel in respective left and right stereoscopic images displayed on said display, said pixel driver circuit comprising:
at least one select line to select the OLED pixel;
at least one programming line to program the pixel circuit with pixel data to control brightness of the pixel;
at least one driver transistor to drive the pixel;
left and right data storage capacitors to store left stereoscopic pixel data and right stereoscopic pixel data to control the brightness of the OLED pixel in respective left and right stereoscopic images displayed on said display, and wherein said pixel data comprises said left and right stereoscopic pixel data;
a select transistor having a control terminal coupled to said select line; first and second data storage capacitors to store respective first and second charges to control driving said OLED pixel;
a programming transistor having a control terminal coupled to said programming line;
wherein said first data storage capacitor is coupled to a control terminal of said driver transistor;
wherein said second data storage capacitor is coupled to said programming line via said select transistor; and
wherein said programming transistor is coupled between said first and second data storage capacitors to, when enabled, enable charge sharing between said first and second storage capacitors;
such that said pixel driver circuit is programmable with one of said left and right stereoscopic pixel data whilst driving said OLED pixel at a brightness determined by the other of said left and right stereoscopic pixel data.
14. A stereoscopic pixel data circuit as claimed in claim 13 further comprising: a reset transistor coupled to said first data storage capacitor to reset a charge on said first data storage capacitor; and
wherein said programming comprise resetting said charge on said first data storage capacitor and enabling said programming transistor to share a charge from said second data storage capacitor between said second and first data storage capacitors.
15. A method of programming a stereoscopic pixel driver circuit as claimed in claim 13 or 14 comprising adjusting a voltage applied to said programming line when programming the pixel driver circuit to compensate for said charge sharing.
PCT/GB2012/000060 2011-01-06 2012-01-18 Organic light emitting diode displays WO2012101397A2 (en)

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