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
  • G09G3/34—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 by control of light from an independent source
• • 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
  • G09G3/20—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]
• • 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
  • G09G2320/00—Control of display operating conditions
  • G09G2320/02—Improving the quality of display appearance
  • G09G2320/029—Improving the quality of display appearance by monitoring one or more pixels in the display panel, e.g. by monitoring a fixed reference pixel
  • G09G2320/0295—Improving the quality of display appearance by monitoring one or more pixels in the display panel, e.g. by monitoring a fixed reference pixel by monitoring each display pixel
• • G—PHYSICS
  • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
  • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
  • G09G2320/00—Control of display operating conditions
  • G09G2320/04—Maintaining the quality of display appearance
  • G09G2320/041—Temperature compensation
• • G—PHYSICS
  • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
  • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
  • G09G2320/00—Control of display operating conditions
  • G09G2320/04—Maintaining the quality of display appearance
  • G09G2320/043—Preventing or counteracting the effects of ageing
• • G—PHYSICS
  • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
  • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
  • G09G2320/00—Control of display operating conditions
  • G09G2320/04—Maintaining the quality of display appearance
  • G09G2320/043—Preventing or counteracting the effects of ageing
  • G09G2320/045—Compensation of drifts in the characteristics of light emitting or modulating elements
• • 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
  • G09G3/2007—Display of intermediate tones
  • G09G3/2018—Display of intermediate tones by time modulation using two or more time intervals
  • G09G3/2022—Display of intermediate tones by time modulation using two or more time intervals using sub-frames

Definitions

• the present invention relates to solid-state electroluminescent flat- panel displays and more particularly to such displays having ways to compensate for aging of the electroluminescent display components.
• Electroluminescent (EL) devices have been known for some years and have been recently used in commercial display devices. Such devices employ both active-matrix and passive-matrix control schemes and can employ a plurality of subpixels. hi an active-matrix control scheme, each subpixel includes an EL emitter and a drive transistor for driving current through the EL emitter. The subpixels are typically arranged in two-dimensional arrays with a row and a column address for each subpixel, and having a data value associated with the subpixel. Subpixels of different colors, such as red, green, blue, and white are grouped to form pixels.
• Active-matrix EL displays can be made from various emitter technologies, including coatable-inorganic light-emitting diode, quantum- dot, and organic light-emitting diode (OLED), and various backplane technologies, including amorphous silicon (a-Si), zinc oxide, and low-temperature polysilicon (LTPS).
• emitter technologies including coatable-inorganic light-emitting diode, quantum- dot, and organic light-emitting diode (OLED), and various backplane technologies, including amorphous silicon (a-Si), zinc oxide, and low-temperature polysilicon (LTPS).
• a-Si amorphous silicon
• LTPS low-temperature polysilicon
• FIG. 8 shows an example histogram of subpixel luminance exhibiting differences in characteristics between subpixels. All subpixels were driven at the same level, so should have had the same luminance. As FIG. 8 shows, the resulting luminances varied by 20 percent in either direction. This results in unacceptable display performance.
• a digital drive display scans the rows multiple times per frame. Each time a row is selected in a digital drive scheme, each subpixel in the row is either activated to output light at a selected level, or inactivated to emit no light. This is different from an analog drive display, in which each subpixel is caused to emit light at one of a plurality of levels corresponding to the available code values (e.g. 256).
• Kawabe in commonly-assigned U.S. Patent Application No. 2008/088561, teaches an improvement to the above method wherein a single shift register is used to track the multiple sequences for data writing, and a series of enable control lines are used to control which of the multiple sequences is written at a given time.
• This method uses a two-transistor, one-capacitor (2Tl C) subpixel circuit.
• transistor-related mura is not the only cause of nonuniformity in an EL display.
• organic light-emitting materials in the display age and become less efficient at emitting light. Aging of an OLED emitter causes a decrease in the efficiency of the emitter, the amount of light output per unit current, and an increase in the impedance of the emitter, and thus its voltage at a given current. Both effects reduce the lifetime of the display.
• the differing organic materials can age at different rates, causing differential color aging and a display whose white point varies as the display is used.
• each individual subpixel can age at a rate different from other subpixels, resulting in display nonuniformity.
• changes in the temperature of an OLED emitter can change its voltage at a given current.
• 2006/0022305 describes a six-transistor, two-capacitor subpixel circuit driven in a scan phase, a light emission phase, and a reset phase during which the threshold voltage of the drive transistor and the turn-on voltage of the OLED are stored on capacitors connected to the data voltage terminal.
• This method does not compensate for OLED efficiency loss, and it requires a very complex subpixel having a very small aperture ratio. Such a subpixel ages more quickly and has lower manufacturing yields.
• U.S. Patent Application Publication No. 2002/0167474 by Everitt describes a pulse width modulation driver for an OLED display.
• One embodiment of a video display includes a voltage driver for providing a selected voltage to drive an organic light-emitting diode in a video display.
• the voltage driver can receive voltage information from a correction table that accounts for aging, column resistance, row resistance, and other diode characteristics, hi one embodiment of the invention, the correction tables are calculated prior to or during normal circuit operation. Since the OLED output light level is assumed to be linear with respect to OLED current, the correction scheme is based on sending a known current through the OLED diode for a duration sufficiently long to permit the transients to settle out, and then measuring the corresponding voltage with an analog-to-digital converter (AfD) residing on the column driver. A calibration current source and the AJD can be switched to any column through a switching matrix.
• this technique is only applicable to passive-matrix displays, not to the higher-performance active-matrix displays which are commonly employed. Further, this technique does not include any correction for changes in OLED emitters as they age, such as OLED efficiency loss.
• Arnold et al. in U.S. Patent No. 6,995,519, teach a method of compensating for aging of an OLED device (emitter). This method relies on the drive transistor to drive current through the OLED emitter.
• drive transistors known in the art have non-idealities that are confounded with the OLED emitter aging in this method.
• Low-temperature polysilicon (LTPS) transistors can have nonuniform threshold voltages and mobilities across the surface of a display, and amorphous silicon (a-Si)transistors have a threshold voltage which changes with use.
• the method of Arnold et al. will therefore not provide complete compensation for OLED efficiency losses in circuits wherein transistors show such effects. Additionally, when methods such as reverse bias are used to mitigate a-Si transistor threshold voltage shifts, compensation of OLED efficiency loss can become unreliable without appropriate and expensive tracking and prediction of reverse bias effects.
• Naugler et al. in U.S. Patent Application Publication No. 2008/0048951 , teach measuring the current through an OLED emitter at various gate voltages of a drive transistor to locate a point on precalculated lookup tables used for compensation.
• this method requires a large number of lookup tables, consuming a significant amount of memory.
• An advantage of this invention is an electroluminescent display, such as an OLED display, that compensates for the aging of the organic materials in the display wherein circuitry or transistor aging or nonuniformities are present, without requiring extensive or complex circuitry for accumulating a continuous measurement of subpixel use or time of operation. It is a further advantage of this invention that such compensation can be performed in displays driven by pulse- width, time-modulated signals to effect desired intensity levels at each subpixel. It is a further advantage of this invention that it uses simple voltage measurement circuitry. It is a further advantage of this invention that by making all measurements of voltage, it is more sensitive to changes than methods that measure current. It is a further advantage of this invention that a single select line can be used to enable data input and data readout.
• FIG. 1 is a graph showing a representative relationship between OLED efficiency and OLED voltage change for a given OLED drive current density
• FIG. 2 is a graph showing a representative relationship between temperature and OLED voltage for a given OLED drive current density
• FIG. 3 is a schematic diagram of one embodiment of an electroluminescent (EL) display that can be used in the practice of the present invention
• FIG. 4 is a schematic diagram of one embodiment of an EL subpixel and connected components that can be used in the practice of the present invention
• FIG. 5 is a timing diagram of a digital-drive scheme according to the prior art
• FIG. 6 is a representative load-line diagram showing the effect of aging of an OLED emitter on OLED current
• FIG. 7 A is a block diagram of an embodiment of the method of the present invention
• FIG. 7B is a block diagram of an embodiment of the method of the present invention.
• FIG. 8 is a histogram of luminances of subpixels exhibiting differences in characteristics.
• Characteristics of an EL emitter include its efficiency, typically expressed in cd/A or as a percentage of a reference cd/A value, and its resistance, which relates to the voltage across the emitter for a given current.
• efficiency typically expressed in cd/A or as a percentage of a reference cd/A value
• resistance which relates to the voltage across the emitter for a given current.
• FIG. 1 there is shown a representative relationship between efficiency and ⁇ VOLED for an OLED emitter.
• variations in the characteristics of the EL emitter e.g. efficiency
• ⁇ VOLED- variations in the characteristics of the EL emitter, e.g. efficiency
• the relationship has been experimentally determined to be approximately independent of fade current density.
• a change in corrected signal necessary to cause an EL emitter to output a nominal luminance can be determined. This measurement can be done on a model system and thereafter stored in a lookup table or used as an algorithm.
• FIG. 2 there is shown an example of the relationship between OLED emitter temperature and the OLED voltage measured at a given current density.
• variations in characteristics of the EL emitter e.g. resistance and thus voltage, are caused by variations in the temperature of the EL emitter.
• FIG. 1 and FIG. 2 show two factors known to impact the OLED voltage: aging and temperature.
• aging In order to effect an accurate compensation for the effects of aging, it is necessary to differentiate between the change in OLED voltage caused by the aging process, and that caused by changes in the temperature. Note that OLED emitter temperature is affected by ambient temperature around the display and by heat generated on the display itself.
• EL display 10 includes an array of a plurality of EL subpixels 60 arranged in rows and columns.
• EL display 10 includes a plurality of row select lines 20 wherein each row of EL subpixels 60 has a row select line 20.
• EL display 10 includes a plurality of readout lines 30 wherein each column of EL subpixels 60 has a readout line 30.
• Each readout line 30 is connected to a third switch 130, which selectively connects readout line 30 to a current source 160 during the calibration process.
• connected it is meant that the elements are directly connected or connected via another component, e.g.
• each column of EL subpixels 60 also has a data line, described further below.
• the plurality of readout lines 30 is connected to one or more multiplexers 40, which permits parallel/sequential readout of signals from EL subpixels, as will become apparent.
• Multiplexer 40 can be a part of the same structure as EL display 10, or can be a separate construction that can be connected to or disconnected from EL display 10. Note that "row” and “column” do not imply any particular orientation of the display.
• Readout lines 30 are connected through third switch 130 to current source 160, as will be described below.
• the EL display 10 includes one or more temperature sensors 65 to permit measurement of the display or ambient temperature.
• the temperature sensor can be a discrete component on the driving electronics and accessed by a processing unit or integrated into a component of the driving electronics as is typical in the trade (analog-to-digital converters, microprocessors, application specific integrated circuits, etc.). Measurements of temperature can be performed and recorded during readout of signals from EL emitters in order to ascertain the impact of temperature on OLED voltage. For the description that follows, it is assumed that with this capability, we are able to then measure a signal as described, namely OLED voltage, and observe changes caused only by the aging process of the EL emitter.
• EL subpixel 60 includes an EL emitter 50, a drive transistor 70, a capacitor 75, a readout transistor 80, and a select transistor 90. Each of the transistors has a first electrode, a second electrode, and a gate electrode.
• a first voltage source 140 is selectively connected to the first electrode of drive transistor 70 by a first switch 110, which can be located on the EL display substrate or on a separate structure.
• the second electrode of drive transistor 70 is connected to EL emitter 50, and a second voltage source 150 can be selectively connected to EL emitter 50 by a second switch 120, which can also be off the EL display substrate.
• the EL emitter 50 can also be connected directly to the second voltage source 150. At least one first switch 110 and second switch 120 are provided for the EL display. Additional first and second switches can be provided if the EL display has multiple powered subgroupings of pixels.
• the drive transistor 70 can be used as the first switch 110 by operating it in reverse bias so that substantially no current flows. Methods for operating transistors in reverse bias are known in the art. In normal display mode, the first and second switches are closed, and third and fourth switches described below are open.
• the gate electrode of drive transistor 70 is connected to the second electrode of select transistor 90 to selectively provide data from data line 35 to drive transistor 70 as is well known in the art.
• the first electrode of select transistor 90 is connected to a data line 35.
• Each of the plurality of row select lines 20 is connected to the gate electrodes of the select transistors 90 in the corresponding row of EL subpixels 60.
• the gate electrode of select transistor 90 is connected to the gate electrode of readout transistor 80.
• the first electrode of readout transistor 80 is connected to the second electrode of drive transistor 70 and to EL emitter 50.
• Each of the plurality of readout lines 30 is connected to the second electrodes of the readout transistors 80 in the corresponding column of EL subpixels 60.
• Readout line 30 is connected to third switch 130.
• a respective third switch 130 (S3) is provided for each column of EL subpixels 60.
• the third switch permits current source 160 to be selectively connected to the second electrode of readout transistor 80.
• Current source 160 when connected by the third switch, provides a selected test current to EL emitter 50, causing constant current flow through the EL emitter.
• Third switch 130 and current source 160 can be provided located on or off the EL display substrate.
• the current source 160 can be used as the third switch 130 by setting it to a high-impedance (Hi-Z) mode so that substantially no current flows. Methods for setting current sources to high-impedance modes are known in the art.
• the second electrode of readout transistor 80 is also connected to a voltage measurement circuit 170, which measures voltages to provide signals representative of characteristics of EL subpixel 60.
• Voltage measurement circuit 170 includes an analog-to-digital converter 185 for converting voltage measurements into digital signals, and a processor 190. The signal from analog- to-digital converter 185 is sent to processor 190.
• Voltage measurement circuit 170 can also include a memory 195 for storing voltage measurements, and a low-pass filter 180.
• Voltage measurement circuit 170 is connected through a multiplexer output line 45 and multiplexer 40 to a plurality of readout lines 30 and readout transistors 80 for sequentially reading out the voltages from a plurality of EL subpixels 60.
• each can have its own multiplexer output line 45.
• a plurality of EL subpixels can be driven simultaneously.
• the plurality of multiplexers permits parallel reading out of the voltages from the various multiplexers 40, and each multiplexer permits sequential reading out of the readout lines 30 attached to it. This will be referred to herein as a parallel/sequential process.
• Processor 190 can also be connected to data line 35 and select line 20 by way of a control line 95 and a driver circuit 155.
• processor 190 can provide predetermined data values to data line 35, and thus to the gate electrode of drive transistor 70, during the measurement process to be described herein.
• Processor 190 can also accept display data via input signal 85 and provide compensation for changes as will be described herein, thus providing compensated data to data line 35 using driver circuit 155 during the display process.
• Driver circuit 155 is a pulse- width modulated driver circuit which can include a gate driver connected to the row select lines 20, and a source driver connected to the data lines 35, as known in the art. This permits driver circuit 155, through the source driver, to provide selected test and drive voltages through select transistor 90 to the gate electrode of drive transistor 70.
• an EL emitter 50 e.g. an OLED emitter
• its efficiency can decrease and its resistance can increase. Both of these effects can cause the amount of light emitted by an EL emitter to decrease over time. The amount of such decrease will depend upon the use of the EL emitter. Therefore, the decrease can be different for different EL emitters in a display, which effect is herein termed spatial variations in characteristics of EL emitters 50. Such spatial variations can include differences in brightness and color balance in different parts of the display, and image "burn-in" wherein an oft-displayed image (e.g. a network logo) can cause a ghost of itself to always show on the active display. It is desirable to compensate for such effects to prevent spatial variations from becoming objectionable to a viewer of the EL display.
• FIG. 5 there is shown a graphical view of an embodiment of a digital drive scanning sequence according to the prior art.
• a horizontal axis 410 shows time, and a vertical axis 430 shows horizontal scanning lines.
• FIG. 5 gives an example of four-bit (sixteen code value) digital driving for ease of description.
• one cycle or frame period 420 contains a plurality of different subframes 440, 450, 460, and 470, wherein each subframe has a respective duration which is different from the duration of at least one other subframe.
• the durations are weighted so as to correspond to code values representing display element brightness. That is, the durations of N subframes within a cycle have ratios of 1 : 2 : 4 : 8 : ... : 2N.
• FIG. 5 is not to scale.
• an code value bit is "l”
• a selected drive voltage is provided to the gate of the drive transistor 70, causing the EL subpixel 60 to be activated or illuminated for the corresponding subframe, which is herein called an activated subframe.
• an intensity bit is “0”
• a selected black voltage is provided to the gate of the drive transistor 70, to cause the EL subpixel 60 to be deactivated or extinguished for the corresponding subframe, which is herein called a deactivated subframe.
• the on-time is defined as the sum of the durations of activated subframes for a given EL subpixel circuit 60 and its EL emitter 50, and corresponds to the desired brightness of the display element of such circuit.
• the selected drive voltage causes the drive transistor to operate in the linear region during the on-time
• the selected black voltage causes the drive transistor to produce a current (e.g. ⁇ 10nA) which does not produce visible light from the EL emitter (e.g. ⁇ 0.1 nit emission).
• FIG. 7A there is shown a block diagram of one embodiment of the method of the present invention.
• first switch 110, and fourth switch 131 are opened, and second switch 120 and third switch 130 are closed (Step 340).
• Select line 20 is made active for a selected row to turn on readout transistor 80 (Step 345).
• a selected test current, I tes u u thus flows from current source 160 through EL emitter 50 to second voltage source 150.
• the value of current through current source 160 is selected to be less than the maximum current possible through EL emitter 50; a typical value will be in the range of 1 to 5 microamps and will be constant for all measurements during the lifetime of the EL subpixel. More than one measurement value can be used in this process, e.g. measurement can be performed at 1 , 2, and 3 microamps. Taking measurements at more than one measurement value permits forming a complete I- V curve of the EL subpixel 60.
• Voltage measurement circuit 170 is used to measure the voltage on readout line 30 (Step 350). This voltage is the voltage V ou t at the second electrode of readout transistor 80 and can be used to provide a first emitter- voltage signal V 2 that is representative of characteristics of EL emitter 50, including the resistance and efficiency of EL emitter 50.
• V 2 CV + V 0LED + V ⁇ (Eq. 1)
• V out the voltage at the second electrode of readout transistor 80 (V out ) to adjust to fulfill Eq. 1.
• CV is a set value and V read can be assumed to be constant as the current through the readout transistor is low and does not vary significantly over time.
• V OLED will be controlled by the value of current set by current source 160 and the current- voltage characteristics of EL emitter 50.
• V OLED can change with age-related changes in EL emitter 50.
• two separate test measurements are performed at different times.
• the first measurement is performed at a first time, e.g. when EL emitter 50 is not degraded by aging. This can be any time before EL subpixel 60 is used for display purposes.
• the value of the voltage V 2 for the first measurement is the first emitter-voltage signal (hereinafter V 2a ), and is measured and stored.
• V 2a the first emitter-voltage signal
• the measurement is repeated.
• the resulting measured V 2 is a second emitter-voltage signal (hereinafter V 2b ), and is stored.
• multiplexer 40 connected to a plurality of readout lines 30 is used to permit voltage measurement circuit 170 to sequentially measure each of a plurality of EL subpixels, e.g. every subpixel in the row (decision step 355), and provide a corresponding first and second emitter-voltage signal for each subpixel.
• Each of the plurality of EL subpixels can be driven simultaneously to advantageously reduce the time required for measurement by permitting all EL subpixels to settle simultaneously rather than sequentially. If the display is sufficiently large, it can require a plurality of multiplexers wherein the first and second emitter- voltage signals are provided in a parallel/sequential process.
• Steps 345 to 355 are repeated for each row (decision step 360).
• the EL emitter in each of the plurality of EL subpixels e.g. each EL subpixel in the row, can be provided with the selected test current simultaneously so that any settling time will have elapsed when the measurement is taken. This prevents having to wait for each subpixel to settle individually before taking a measurement.
• Changes in EL emitter 50 can cause changes to V OLED to maintain the test current I tes t su - These V OLED changes will be reflected in changes to V 2 .
• the first and second stored emitter-voltage signals (V 2a and V 2b ) for each EL subpixel 60 can therefore be compared to calculate an aging signal ⁇ V 2 representative of the characteristics, e.g. efficiency and resistance, of the EL emitter 50 (Step 370) as follows:
• the aging signal for the EL subpixel 60 can then be used to compensate for changes in characteristics of that EL subpixel.
• FIG. 6 in a p-channel non-inverted configuration wherein the drive transistor is operated in the linear region, VOLED changes cannot be compensated with V OLED measurements alone, as V OLED changes modulate Vds of the drive transistor, affecting the whole system.
• Complete compensation can be provided by calculating a drive transistor load line, which is a Vds-Ids curve, and comparing it with an EL emitter VOLED-IOLED curve.
• FIG. 6 shows Vds on the abscissa and drain current Ids on the ordinate.
• a drive transistor load line 601 can be determined by transistor characterization and stored in a nonvolatile memory when a display is manufactured, or it can be measured for each drive transistor.
• an aged current 693 is at the intersection of an aged OLED load line 603 and drive transistor load line 601.
• One advantage of operating in the linear region is indicated by equal voltage intervals 680a and 680b. In the linear region, voltage interval 680a corresponds to current interval 681a. In the saturation region, the same voltage shift (680b) corresponds to much smaller current interval 681b. Therefore, operating in the linear region advantageously improves signal-to-noise ratio. Another advantage of operating in the linear region is that the behavior of the transistor can be approximated by a straight line (640) without incurring unacceptable error.
• a current sink 165 is used to measure a drive transistor load line.
• a fourth switch 131 is provided for selectively connecting the current sink 1 15 to the second electrode of the readout transistor.
• the current sink 165 can be used as the fourth switch 131 by setting it to a high- impedance (Hi-Z) mode so that substantially no current flows.
• a selected test voltage is provided by driver circuit 155 to the gate electrode of the drive transistor. The test voltage is preferably equal to the selected drive voltage used in normal operation of the display.
• FIG. 7B there is shown a block diagram of load line measurements according to the present invention.
• the test voltage (Vdata) is provided to data line 35 (Step 310).
• the first and fourth switches are closed and the second and third switches are opened (Step 315).
• Select line 20 is made active for a selected row to provide the test voltage to the gate electrode of drive transistor 70 and to turn on readout transistor 80 (Step 320).
• a selected first current I Sk, i is provided by the current sink (Step 322) and thus flows from first voltage source 140 through the first and second electrode of drive transistor 70 and readout transistor 80 to current sink 165.
• the first current is selected to be less than the resulting current through drive transistor 70 due to the application of the test voltage; a typical value is from 1 to 5 microamps.
• the limiting value of current through drive transistor 70 will be controlled entirely by current sink 165, which will be the same as through drive transistor 70.
• the test voltage and first current can be selected based upon known or determined current-voltage and aging characteristics of drive transistor 70.
• Voltage measurement circuit 170 is used to measure the voltage on readout line 30, which is the voltage V ou t at the second electrode of readout transistor 80, providing a first transistor- voltage signal V 1 J that is representative of characteristics of drive transistor 70 (Step 325).
• the voltage at the second electrode of readout transistor 80 (V out ) will adjust to fall on the point on the drive transistor load line corresponding to I s ⁇ 1 .
• multiplexer 40 connected to a plurality of readout lines 30 can be used to permit voltage measurement circuit 170 to sequentially read out the first signals Vn from a plurality of EL subpixels, e.g. every subpixel in the row (decision step 330). If the display is sufficiently large, it can require a plurality of multiplexers wherein the first signal can be provided in a parallel/sequential process. If there are additional rows of subpixels to be measured (Step 335), a different row is selected by a different select line and the measurements are repeated.
• the drive transistor load line 601 is approximately a straight line, and so can be characterized by two points.
• the offset and slope of a linear fit 640 of the linear region of the drive transistor load line 601 are thus calculated as known in the mathematical art from the two points (V )T , I S k,i) 610 and (V 2 ⁇ , I sk , 2 ) 611.
• First current I s k,i is shown as 690; second current I Sk , 2 is shown as 691.
• the two measurements of each subpixel can be taken in either order, and the first measurement for all subpixels on all rows of the display can be taken before the second measurement of any subpixel.
• the first current can be higher or lower than the second current, so point 610 can be above point 611 instead of below it.
• EL emitter voltage can be affected by both aging effects and temperature. Measurements obtained must be adjusted for temperature variations from measurement to measurement in order to effectively compensate for both current loss and efficiency loss.
• a correlation between ambient temperature and OLED voltage can be obtained and stored as an equation or lookup table.
• FIG. 2 This relationship represents the voltage of the EL emitter over a typical operating temperature range at the current I testsu to be used for EL emitter characterization.
• the function an example of which is given by a curve fit 2, will hereinafter be denoted VbyT(T), as it provides a representative OLED voltage for each temperature T.
• the temperature in the manufacturing environment where the reference measurements are performed is likely to differ from that of the consumer environment, where subsequent measurements of the EL emitter are performed.
• ⁇ V oled _ tem p VbyT(T 2 ) - VbyT(T 1 ) (Eq. 3)
• ⁇ V o ied_tem P is the OLED voltage change caused by variations in ambient temperature
• Voled(Ti) and Voled(T 2 ) are the EL emitter voltages in the factory and consumer environments, respectively.
• First and second emitter- voltage measurements can then be adjusted according to temperature:
• V 2a - V 2a - ⁇ V o led_temp (Eq. 4a)
• V 2b . V 2b - ⁇ V oled _ temp (Eq. 4b)
• V 2a' and V 2b' can then be used in place of V 2a and V 2b wherever necessary, hi a preferred embodiment, first emitter-voltage signal V 2a is measured in the factory at temperature Tj, and only second emitter- voltage signal V 2b , measured at temperature T 2 , is adjusted for temperature.
• ⁇ V 2 ' can be used in place of ⁇ V 2 wherever necessary.
• FIG. 6 there is shown a graphical illustration of the effect of EL emitter aging, and in this example of OLED aging.
• An unaged OLED load line 602 shows the I-V behavior of an OLED emitter before aging.
• Aged OLED load line 603 shows the I-V behavior of the same OLED emitter after aging.
• Aged line 603 is approximately a percentage of unaged line 602.
• a point 621 indicates the OLED voltage V 2a 631, the first emitter- voltage signal, at test current 692 (I te stsu) before aging;
• a point 622 indicates an OLED voltage V 2 b 632, the second emitter-voltage signal, at test current 692 after aging.
• the first emitter- voltage signal can be after aging and the second emitter- voltage signal before aging.
• Unaged OLED load line 602 can be characterized or measured for each subpixel, a group containing a plurality of subpixels, or the whole display.
• the display can be divided into multiple spatial or color (e.g. red, green, blue or white) regions, each of which can have a different unaged OLED load line curve than at least one other region.
• Unaged OLED load line(s) 602 can be stored in nonvolatile memory with the display as equation coefficient(s) or in lookup table(s).
• Aged OLED load line 603 is typically a percentage of imaged load line 602. Denoting unaged load line 602 as a function O_New(V) mapping voltage to current, and aged load line 603 as an analogous function O_Aged(V),
• O_Aged(V) gamma * O_New(V) for all V. (Eq. 5)
• any point on aged load line 603 can be calculated using Eq. 5.
• the drive transistor load line and thus the first and second transistor- voltage signals and the first and second currents, can therefore be used in providing the aging signal to provide complete compensation.
• the operating point of the EL subpixel after aging is point 624, the intersection of the drive transistor 601 and aged OLED 603 load lines respectively.
• the aged OLED load line 603 can be calculated according to Eq. 5. Standard mathematical techniques such as Newton's method can then be used to find the intersection of aged OLED load line 603 and drive transistor load line 601. To use Newton's method, point 621 or 622, or another point, can be used as the starting point.
• a region of the unaged OLED load line 602 close to the typical operating voltage of the system can be selected, and a linear approximation made of that region.
• the region between points 623 and 621 can be approximated with linear fit 641.
• This selection can be made at manufacturing time or while the display is operating.
• Linear fit 641 can then be multiplied by gamma to approximate the unaged OLED load line 603.
• a linear fit can be made of a region of the unaged OLED load line 603 after multiplication by gamma.
• points 622 and 625 can define a region with linear fit 642.
• intersection point 624 between the aged OLED load line 603 and the drive transistor load line 601 can be expressed as (Vd S ,aged, Ids,age ⁇ 0-
• the original operating point, the intersection point 621 between the unaged OLED load line 602 and the drive transistor load line 601, can be expressed as (Vds,new, Ids,n e w)-
• a normalized current can be calculated using these intersection points:
• I n o rm can be the aging signal for the EL subpixel and represent characteristics of the EL emitter including resistance (forward voltage).
• I d s, n ew is shown in this example as equal to test current 692 and I ds,aged is shown as current 693. Note, however, that test current I tes t S u 692 and I ds , n ew are not required to be equal.
• the present invention does require any particular value of I test s u - AV 2 , computed in Eq. 2 above, can be the aging signal for the EL subpixel and represent characteristics of the EL emitter including efficiency, as will be described below.
• the normalized current is used, as shown above in Eq. 7a, in which I norm represents the normalized current relative to its original current.
• I norm represents the normalized current relative to its original current.
• a decrease in current can be corrected by increasing the amount of on- time for the EL emitter.
• the reciprocal of I n0 ⁇ t i is used as a scaling factor for the original on-time requested:
• ti_ comp represents the on-time of EL emitter 50 to correct for the change in current flowing through it
• t data is the on-time corresponding to a desired amount of light emission when the EL emitter was new. For example, if the aged current is found to be 0.5 (or 50%) of its original value, I nOrm would be 0.5, and hence ti_ comp would be found to be 2 times the original on-time tdata- To compensate for changes in EL emitter efficiency, EL emitter voltage change ⁇ V 2 is used.
• the EL emitter efficiency at any given time can be determined by understanding the relationship between ⁇ V 2 , adjusted for temperature if necessary so that it represents only changes caused by the aging process, and EL emitter efficiency.
• the relationship is denoted EbyV( ⁇ V).
• Normalized efficiency E nO rm can thus be computed:
• E nom EbyV( ⁇ V 2 ). (Eq. 7b) where ⁇ V 2 is as computed in Eq. 2.
• FIG. 1 shows an example of this relationship for a given OLED device.
• the relationship between current and luminance is generally linear.
• the EL emitter 50 is provided the reciprocal of the normalized efficiency in on-time. EL emitter 50 is thus activated for e.g. 1/0.77 ⁇ 1.3 times the amount of time it was before aging.
• Adjustment of the pulse-width modulated signal to obtain such increase in EL emitter 50 on-time can be performed by processor 190 using driver circuit 155.
• tE_ CO mp represents the on-time of EL emitter 50 required to correct for the change in EL efficiency
• E nOrm is the efficiency of the aged EL emitter as computed in Eq. 7b
• t data is the on-time corresponding to a desired amount of light emission when the EL emitter was new.
• the two compensations are combined to produce a single selected on- time. Note that it is shown here that the light output is returned to the original value, but this is not required. For example, when temperature shifts, the entire display can be permitted to shift, assuming that temperature will affect all EL emitters equally.
• the first step in the compensation process is to drive the EL emitter in such a way that the integrated time and current are constant over time.
• Eq. 8 provides the method by which to compute the adjustment in the original amount of time the EL emitter would have been driven when it was new and flowing a full amount of current.
• t ft ,i l comp represents the amount of time required to fully compensate for the current and efficiency loss of the EL emitter
• E nOrm represent the normalized efficiency of the EL emitter
• ti_ comp represents the on-time required to compensate for the loss in EL emitter current.
• E norm can be the aging signal for the EL subpixel, representing characteristics of the EL emitter, including the efficiency of the EL emitter.
• the aging signal for the EL emitter can include both ⁇ norm and E n0r ⁇ i to represent the resistance and efficiency of the EL emitter.
• the aging signal can thus be 2.6, 1/2.6, or the tuples (0.5, 0.77) or (2,1.3), or some combination..
• an input signal is received (Step 375) which corresponds to the amount t data of time during a given frame which the EL emitter is to be emitting light.
• the input signal can be a digital code value, a linear intensity, an analog voltage, or other forms known in the art.
• the aging signal and the input signal can then be used to calculate a selected on-time tftiiLcomp according to Eq. 10, above.
• the selected on-time can then be used to produce a corresponding compensated drive signal (Step 380).
• the input signal / and compensated drive signal D are four- bit code values b 3 b 2 bib 0 , where each b x corresponds to the duration ratio 2 x l (e.g. b 3 to 8).
• the selected on-time tfiiii_ C omp calculated from tdata using Eq. 10 is rounded to the nearest multiple of 1/15 and multiplied by 15 to form the corresponding drive signal.
• the compensated drive signal can be computed, e.g. by processor 190, using lookup tables, piecewise linear functions, or other techniques known in the art.
• t ⁇ _ comp or t E _ COm p can be used as the selected on- time if compensation is only desired for one effect.
• the selected drive voltage is provided (Step 385) to the gate electrode of the drive transistor for the selected on-time corresponding to compensated drive signal D.
• This selected on-time can be divided into a plurality of activated subframes, as described above.
• Activating the subpixel for the selected on-time compensates for variations in characteristics (e.g. voltage and efficiency) of the EL emitter according to the calculations given above.
• each subpixel is measured to provide a plurality of first and second emitter-voltage signals for respective subpixels, as described above.
• a respective aging signal for each subpixel is provided using the corresponding first and second emitter-voltage signals, also as described above.
• a corresponding input signal for each subpixel is received, and a corresponding compensated drive signal calculated as above using the corresponding aging signals.
• the compensated drive signal corresponding to each subpixel in the plurality of subpixels is provided to the gate electrode of that subpixel using driver circuit 155 as described above. This permits compensation for changes in characteristics of each EL emitter in the plurality of EL subpixels.
• respective first and second transistor-voltage signals for each transistor can be measured and used in producing the corresponding aging signals for each of the plurality of EL subpixels.
• the invention is employed in a display that includes Organic Light Emitting Diodes (OLEDs), which are composed of small molecule or polymeric OLEDs as disclosed in but not limited to U.S. Patent No. 4,769,292, by Tang et al., and U.S. Patent No. 5,061,569, by VanSlyke et al. Many combinations and variations of organic light emitting materials can be used to fabricate such a display.
• OLEDs Organic Light Emitting Diodes
• the EL emitter 50 is an OLED emitter
• the EL subpixel 60 is an OLED subpixel.
• the embodiment shown in FIG. 4 is a non-inverted, NMOS subpixel.
• the EL emitter 50 can be an OLED emitter or other emitter types known in the art.
• the drive transistor 70, and the other transistors (80, 90) can be low-temperature polysilicon (LTPS), zinc oxide (ZnO), or amorphous silicon (a- Si) transistors, or transistors of another type known in the art.
• Each transistor (70, 80, 90) can be N-channel or P-channel, and the EL emitter 50 can be connected to the drive transistor 70 in an inverted or non-inverted arrangement.
• the polarities of the first and second power supplies are reversed, and the EL emitter 50 conducts current towards the drive transistor rather than away from it.
• Current source 160 of the present invention therefore sources a negative current, that is, behaves as a current sink, to draw current through the EL emitter 50.
• current sink 165 sinks a negative current, that is, behaves as a current source, to force current through the drive transistor 70.
• the on-time of each subpixel can be continuous rather than divided into subframes, or the subframes can be in various orders. Longer subframes can be divided into multiple sub-windows, as is known in the art.

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