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
  • G09G3/36—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 using liquid crystals
• • 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
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
  • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
  • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
  • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
• • G—PHYSICS
  • G06—COMPUTING OR CALCULATING; COUNTING
  • G06F—ELECTRIC DIGITAL DATA PROCESSING
  • G06F3/00—Input arrangements for transferring data to be processed into a form capable of being handled by the computer; Output arrangements for transferring data from processing unit to output unit, e.g. interface arrangements
  • G06F3/01—Input arrangements or combined input and output arrangements for interaction between user and computer
  • G06F3/03—Arrangements for converting the position or the displacement of a member into a coded form
  • G06F3/033—Pointing devices displaced or positioned by the user, e.g. mice, trackballs, pens or joysticks; Accessories therefor
  • G06F3/038—Control and interface arrangements therefor, e.g. drivers or device-embedded control circuitry
• • 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]
  • G09G3/3275—Details of drivers for data electrodes
  • G09G3/3291—Details of drivers for data electrodes in which the data driver supplies a variable data voltage for setting the current through, or the voltage across, the light-emitting 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
  • 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
  • G09G2310/00—Command of the display device
  • G09G2310/02—Addressing, scanning or driving the display screen or processing steps related thereto
  • G09G2310/0264—Details of driving circuits
  • G09G2310/0291—Details of output amplifiers or buffers arranged for use in a driving circuit
• • 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
  • G09G2320/0233—Improving the luminance or brightness uniformity across the screen
• • 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
  • G09G2320/043—Preventing or counteracting the effects of ageing

Definitions

• the present invention relates to an active matrix display and method to drive the display.
• Active matrix displays are formed of many light emitting units called pixels. Each pixel includes an electronic circuit that controls a light emitting diode. The pixels are arranged in an array of rows and columns to form a display. In operation, each pixel of an array is sequentially programmed with an updating data value that is transformed into a light level.
• the data value that determines light intensity is provided externally in the form of a voltage.
• the voltage is transformed by the pixel circuit into a current that is directed to the organic light emitting diode (OLED).
• OLED organic light emitting diode
• the amount of current determines an amount of diode emitted light.
• a thin film transistor (TFT) transmits a data value voltage from a program line to a gate of another transistor that regulates current that flows to the OLED from a power supply.
• the current that flows through the current regulating transistor depends on the voltage at its gate. Factors such as the transistor material properties have a direct affect on current flow through a transistor. Transistor material property variations (mismatch) can result in a different current for the same programmed voltage level to two different pixels. This in turn results in a difference in light output.
• Various pixel designs have been proposed with increased number of transistor and control lines to address this problem. However, these designs are complex structures with reduced yield and aperture ratio.
• the invention provides an active matrix display with a display driver control circuit that produces high levels of uniformity without increasing display pixel complexity.
• the invention can be described as a display comprising: a plurality of pixels and a data line, a select line and a current line for the pixels, at least one pixel comprising a circuit with at least two thin film transistors, a capacitor and a light emitting diode; and a circuit external to the plurality of pixels that adjusts voltage of the data line according to drawn current from a power supply signal to the display.
• the invention is an active matrix display comprising: at least one data driver circuit comprising a column data line and a column current line; a plurality of pixels connected to both the column data line and the column current line comprising at least one pixel that is responsive to a column data line voltage to drive a pixel current to the at least one pixel; and a loopback circuit at the head of the column data line and column current line and external to the plurality of pixels and which senses a voltage of the driven pixel current and adjusts a column data line voltage to program a voltage of an adjusted pixel current to match an external reference current.
• a method to drive an active matrix display comprises: sensing a voltage difference between a voltage of a current drawn by a programmed pixel and a voltage of a first power supply current to the active matrix display; and adjusting a data programming voltage to a pixel of the display according to the difference; wherein the sensing and adjusting are conducted by a loopback control circuit at a head of a column of pixels that includes the pixel and external to the pixels of the column.
• an active matrix display comprises: a plurality of AMOLED pixels arranged in matrix columns and rows, wherein each column of pixels is connected to a common current line and to a common data voltage source and each row of pixels is connected to a common select line, wherein the at least one pixel comprises: a current drive transistor having a drain/source, gate and a source/drain connected to the column current line; an address transistor having a source/drain connected to the gate of the drive transistor and a drain source connected to the column data line; a select line connected to the gate of the address transistor; and an OLED connected to the drain/source of the current drive transistor; wherein the plurality of AMOLED pixels is connected to a loopback control circuit at the head of at least one of the columns and external to the plurality of pixels of that column and that senses a voltage of a driven pixel current and adjusts a column data line voltage to program a voltage of an adjusted pixel current to match an external reference current.
• the invention is a data driver circuit comprising: at least one column data line; at least one parallel column current line; a plurality of pixels connected in series to both the at least one column data line and a corresponding parallel column current line, comprising at least one pixel that is responsive to the column data line to drive a pixel current to the at least one pixel; and a loopback control circuit at the head of a column of a data line and a current line and external to a plurality of pixels of the column that senses a voltage difference between a voltage of a first input data current and a voltage of a load drawing on the current line and that adjusts the input data current according to the difference.
• a method to drive an active matrix display comprises: (A) sampling an initial current that represents a first program data value from a power supply to an active matrix display; (B) storing the same first program voltage data value at a second capacitor circuit and applying the first program voltage data value to the selected pixel circuit; (C) drawing a current according to a next voltage data value that is reduced from the applied first program voltage data value as a result of pixel property variations; (D) sensing a voltage of the drawn current and comparing it to a voltage of the sampled initial current signal; (E) adjusting the first program voltage data value to a new program voltage data value at the second capacitor according to the comparing; (F) applying the new program voltage data value to the selected pixel; and (G) repeating (B) through (F) until a compared stored program voltage data value is the same as a voltage of the sampled initial current.
• FIG. 1 is a schematic representation of aa display circuit
• FIG. 2 is a diagrammatic illustration of a display circuit
• FIG. 3 is a schematic representation of a display circuit
• FIG. 4 and FIG. 5 are graphs of current at a pixel and current at a driver
• FIG. 6 and FIG. 8 are graphs showing current relationship with data
• FIG. 7 and FIG. 9 are graphs of current percentage mismatch as a function of data.
• the brightness of an AMOLED display in part depends on current through the OLED elements.
• Each pixel circuit in an AMOLED element is programmed to drive a desired current by applying a voltage to circuit transistors for a voltage-programmed pixel or by applying a current to differently configured circuit transistors for a current- programmed pixel.
• voltage-to-current conversion is based on a transistor's large signal transconductance, a quantity that represents a ratio of current- output to voltage-input.
• OLED element current varies with the transconductance of a pixel circuit transistor. Transconductance depends on factors such as transistor mobility, which can vary across a display thereby creating nonuniformity, both within a display and from display to display.
• voltage programmed pixels can have sensitivity to transistor threshold voltage, which also varies across the display and from display to display.
• the invention relates to a data driver circuit that reduces active matrix backplane complexity and that improves AMOLED performance compared to more complex uniformity correction mechanisms.
• a driver is a programming circuit or sequence of instructions that control a display.
• the invention provides a data driver for a 2-TFT pixel that produces high levels of uniformity without the need for an increased number of pixel transistors or control lines.
• the data driver operates inside a feedback loop formed by the data line and the current line in each column or for a plurality of columns.
• a switching circuit discriminates an individual pixel current from the rest of the column current in the current line. Then, a current-sense circuit controls the feedback loop that charges the data line until a desired level of pixel current is reached.
• FIG. 1 is a schematic representation of the proposed AMOLED display circuit 10.
• the circuit 10 includes a plurality of pixels 12 arranged as a matrix array.
• FIG. 1 shows a 3X3 matrix that is only representative of AMOLEDs that can be formed of thousands of the light emitting pixels 12.
• the 3X3 matrix is shown as including columns A, B and C and rows R, S and T.
• Each pixel 12 is provided between a crossing of a pixel select line 26 and a pair of a column data line 16 and a column current line 18.
• Each of the pixels 12 includes an electronic circuit that controls a light emitting diode 14.
• Each column includes a data line 16 and a current line 18 and each pixel circuit includes transistor Ml 20 and transistor M2 22 and a storage capacitor 24.
• the transistors 20 and 22 are three terminal devices (gate, drain and source) that can act in two manners: as a switch that allows information to pass through in the form of a voltage, or as a variable valve that controls amount of flowing current.
• transistor Ml 20 is a current drive transistor having a drain/source connected to the column current line 18, a source/drain coupled to diode 14 and a gate coupled to the source of the transistor M2 22.
• Address transistor M2 22 has a source/drain connected to the gate of the drive transistor Ml 20, a drain/source connected to the column data line 16.
• Address transistor M2 22 functions as a switch; when the switch is ON, voltage on its drain is passed through to the source; when the switch is OFF no voltage is allowed to be transmitted.
• Transistor Ml 20 functions as a regulating valve that can control the flow of current depending on the state of its gate. As a general rule, the amount of voltage on the gate of transistor Ml 20 determines the current that flows through the device (into the drain and out from the source.)
• data line 16 supplies a data value in the form of a voltage. This is done one row at a time with each pixel 12 in a row simultaneously provided with its corresponding data value.
• the voltage is transformed into a current by transistor Ml 20 and provided by the current line 18.
• the current is directed to the light emitting diode with the amount of current determining an amount of emitted light.
• Data to an AMOLED is written one row at a time into the pixel, but diodes are operated at an essentially 100% duty cycle. This is accomplished by providing a memory circuit for each pixel provided via the combination of transistor 22 and a capacitor 24.
• a select line 26 is pulsed to select a pixel 12.
• the transistor M2 22 is activated by the select line 26 pulse and is turned to an ON position (shown in FIG. 3).
• new current I is drawn from the column current line 18. Since all other pixels 12 are non-selected, the new current I flows through the selected pixel.
• the current that flows through a transistor depends on the voltage at its gate.
• material properties of the transistors that form the array of pixels can vary significantly across a display area. These factors create non-uniform brightness levels. Hence, light output will be different for two different pixels even with the same programmed voltage level. Variations in properties of the pixels can result in mismatches throughout a device display.
• the invention provides an external control circuit for a display .
• the control produces high levels of uniformity without increasing the display pixel complexity.
• the invention can result in displays with higher aperture ratio (brighter displays), lower OLED operating voltages, lower power consumption, higher yield and lower production cost.
• the invention driver can be implemented in standard ICs or integrated on the same panel as the active backplane, further reducing display costs.
• FIG. 2 diagrammatically illustrates an external control circuit 28 of the invention poised in combination with internal pixels 12.
• the internal circuit of pixel 12 includes transistors 20 and 22 and light emitting diode 14.
• the external control circuit 28 operates inside a feedback loop formed by the data line 16 and the current line 18 at the head of each display circuit column, for example A.
• external control circuit 28 includes source/sensing module 30 in combination with data programming module 32.
• the source/sensing module 30 discriminates an individual pixel current from the rest of the column current in the current line 18 and controls the internal feedback loop to control programming module 32 to attain a target level of pixel current. Because current sensing and control are performed at the driver column head and not within the pixel 12, material mismatch characteristics of the pixel transistors 20, 22 are not adverse factors. Further, since the same external control circuit 28 is used to program all pixels in a given column, pixel current variation is minimized.
• FIG. 3 is a schematic circuit diagram of one display according to the invention, including external control circuit 28.
• external control circuit means a control circuit relating or connected outside of pixels of an array.
• the external control circuit can be located within a display circuit, at the head of a pixel column. In an array, each pixel column can be associated with its separate external control circuit.
• source/sensing module 30 includes transistor MSource 34, transistor MSense 36 and amplifier Amp 1 38.
• Transistor MSource 34 is a transistor that supplies current at low voltage; transistor MSense 36 is a transistor that senses small current change; amplifier Amp 1 38 controls both transistors 34, 36.
• FIG. 3 shows a single source/sensing module 30 with a data programming module 32 in combination with a single display column.
• a source/sensing module 30 and data programming module 32 combination is associated at the head of each of a plurality of columns of a display matrix.
• the FIG. 3 current source/sensing module 30 provides a control mechanism by means of amplifier Ampl 38, transistor MSense 36 and Msource34.
• Amplifier Ampl 38 has three terminals; two voltage input terminals 46, 48 labeled as '+' and '-' and an output terminal 50 that controls the gate of transistor MSense 36 and transistor Msource 34.
• Input terminal 46 is connected to constant externally applied voltage Vcol.
• Input terminal 48 is connected to node nc 44. Node nc 44 stays constant at voltage Vcol except for small variations during programming.
• switch MS 1 40 When switch MS 1 40 is ON, transistor MSense 36 and transistor Msource 34 gate voltages are established by a current that flows through transistors MSense 36 and Msource 34 in response to current line 18.
• amplifier Ampl 38 regulates MSense transistor 36 and Msource transistor 34 gate voltage to regulate current voltage through transistors MSense 36 and Msource 34.
• the resulting change in the voltage at output terminal 50 changes the gate of transistor MSense 36 and Msource 34 until the current supplied by both transistors matches the drawn current.
• the change in voltage at the gate of transistor MSense 36 is directly related to the size of the transistor.
• a larger transistor can produce more current with a small change in gate voltage.
• a smaller transistor can more accurately control its output current by requiring a larger voltage change at its gate (for a given small change in current).
• a larger transistor MSource 34 and the smaller transistor MSense 36 can be sized to meet specific display requirements. They are connected through a switch MSl 40 and are controlled by the amplifier Ampl 38.
• the element 'A' represents one display column.
• switch MS 1 40 When operation starts, switch MS 1 40 is ON and most of the column current flows through the large transistor MSource 34 (at this point, no current flows through a selected pixel).
• switch MSl 40 When switch MSl 40 is turned OFF, voltage in the gate of large transistor MSource 34 stays constant per capacitor CSl 42 and hence, current supplied by larger transistor MSource 34 also stays constant. This operation is referred to as column current sampling by transistor MSource 34.
• the FIG. 3 data programming module 32 is connected to the current source/sensing module 30.
• the data programming module 32 comprises amplifier Amp2 52 and a series of switches.
• the amplifier Amp2 52 has one input 54 connected to a capacitor CS2 60 and another input 56 connected to the gate of MSense transistor MSl 36.
• Another output terminal 58 is connected to the data line 16 of column A.
• switch transistor MS2 62 samples voltage at the gate of the smaller MSense transistor 36 and stores it in the capacitor CS2 60 (this sets a base level that is indicative of column current and will be used in a later comparison step).
• the current source/sensing module 30 is on standby/sensing mode and MSense 36 is sensing change in column current flowing into the node nc 44.
• Amplifier Amp2 52 can adjust voltage at the gate of MSense transistor 36 accordingly.
• data line 16 is connected to the gate of transistor Ml 20 through transistor M2 22.
• Transistor Ml 20 is always connected to node nc 44.
• This configuration provides a feedback loop comprising current source/sensing module 30, data programming module 32 and pixel transistor Ml 20 through current line 18 at node nc 44 and data line 16.
• the mobility of a transistor is a device property that quantifies the amount of current a transistor of a certain size can provide.
• the amount of current that flows is a function of its mobility (among other factors). For example, if the same voltage is applied at the gates of two transistors of the same size, but one with 20% higher mobility, then the higher mobility transistor will provide 20% more current (all other factors being equal).
• Mobility is a function of material properties and device fabrication and for the technologies used to fabricate displays it can vary throughout the display area.
• the threshold voltage of a transistor is the minimum voltage required at the transistor gate for current to flow. Threshold voltage is a function of material properties and device fabrication and as a result, threshold voltage can vary throughout a display area.
• a circuit simulation was performed with PSPICE® computer software.
• PSPICE® is computer software for analog and mixed analog/digital circuit simulation and is provided by ORCAD, Inc., 2655 Seely Avenue, San Jose, CA 95134 through EMA Design Automation, Inc., PO Box 23325, Rochester, NY 14692.
• PSPICE® software accepts user input circuit schematics and transistor models and addressing information and generates a simulated response.
• a circuit schematic was PSPICE® simulated to substantially match the FIG. 2 and FIG. 3 circuits.
• the signals represented in FIG. 4 are the control signals that activated the different switches in the display driver; in particular, the voltage signals that controlled MSl, MS2 and transistor M2 in the pixel being programmed.
• the FIG. 5 output graph represents current though the programmed pixel as a function of time as well as the data current as a function of time (Idata 64 feed to node nc of the display driver).
• Simulated system variables included the following: (1) total column current, the sum of all the pixel currents in a given column, varied from a 150 ⁇ A to 3500 ⁇ A; (2) pixel data current was varied from 0.3 ⁇ A to 20 ⁇ A; (3) pixel transistor Ml was varied in different sizes to emulate mobility changes of up to 25%; and (4) pixel transistor threshold voltage was varied by connecting voltage sources to the gate of Ml to simulate changes of up to 50% in threshold voltage.
• FIG. 5 plot shows how pixel current matches data current. This simulation was performed under several system conditions to show the display driver performing required operation for a range of conditions.
• FIG. 5 shows that the proposed display driver programmed a desired level of current into the intended pixel under all the system variables described above.
• the simulations results establishes that the circuit can perform pixel addressing with current mismatch correction as intended at required operation speeds and current demands. A qualitative representation of accuracy can also be extracted from the simulation results.
• EXAMPLE was set up to compare programming of data current in display pixels with drive transistors, Ml with different properties and to demonstrate this function at different column current levels.
• a display driver for a display column was fabricated in single crystal silicon integrated circuits (ICs).
• the column included test pixel circuits implemented in the IC as well.
• the test pixel circuits were fabricated to have identical properties except for Ml transistor size.
• Two pixels were fabricated with a size difference of 20% in the width of Ml to emulate 20% mobility differences.
• an external v voltage source was connected to the pixel. This voltage source represented a change of 25% to the threshold of transistor Ml .
• Lab VIEW® computer software was used to apply circuit voltages.
• LabVIEW® computer software is used to control and emulate scientific and engineering instruments and instrumentation systems and to perform instrumentation functions.
• a voltage level was established in a program line of each of two pixels, which was then transformed to a current by Ml.
• the following conditions were varied to prove performance: (1) total column current was varied from 150 ⁇ A to 3000 ⁇ A; (2) pixel data current was varied from 0.5 ⁇ A to 15 ⁇ A; (3) mobility of pixel transistor Ml was varied by varying size up to 20% change; and (4) threshold voltage of pixel transistor Ml was varied by introducing voltage sources of up to 25% change.
• FIG. 6 shows current flow through the two pixels (Pixl and Pix2) when they are programmed in a typical prior art manner.
• FIG. 6 shows threshold voltage raised by 25% (Pixl) and mobility raised by 20% (Pix2).
• Pixl provided about 2.5 ⁇ A; however, at the same data level, Pix2 provided about 4 ⁇ A. This difference would result in difference in brightness in the display, even though both pixels were intended to have the same intensity level (as intended by the same data levels).
• the plot of FIG. 7 represents the normalized percentage change between the two pixels as a function of data voltage.
• FIG 6 and FIG. 7 establish degree of current change with changes in Ml properties.
• FIG. 8 shows current through the same two pixels but programmed with the FIG. 3 display driver.
• FIG.8 shows the resulting two currents matched perfectly even though the transistors Ml had varying properties.
• the FIG. 9 normalized percentage plot shows only a measuring tolerance variation between the two pixel currents.
• the experimental data establishes that non-uniformity is reduced from 70% to below 3% for two pixels driven with the standard technique and adjusting driver of the invention respectively. This uniformity level is improved to an order of magnitude throughout an entire data range. Further, simulations established that programming time can be reduced below the time required by a typical prior art current-copy pixel.
• the inventive data driver can be implemented in standard ICs or on the same panel as the active backplane.
• Poly-silicon TFTs offer a good compromise between performance and cost for the proposed driver.
• the inventive circuit reduces complexity of active matrix backplanes by providing uniformity levels with lower complexity than those of typical prior art correction techniques in connection with two transistors TFT pixels. And, since the performance requirements on the transistors on a backplane have been reduced, lower cost technologies can be used for the large area array.

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• 2007
  • 2007-12-11 KR KR1020097014517A patent/KR101462695B1/ko active Active
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