CA2550102C - Method and system for driving a pixel circuit in an active matrix display - Google Patents
Method and system for driving a pixel circuit in an active matrix display Download PDFInfo
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- CA2550102C CA2550102C CA002550102A CA2550102A CA2550102C CA 2550102 C CA2550102 C CA 2550102C CA 002550102 A CA002550102 A CA 002550102A CA 2550102 A CA2550102 A CA 2550102A CA 2550102 C CA2550102 C CA 2550102C
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- 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
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- 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
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- 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
- G09G2300/0833—Several active elements per pixel in active matrix panels forming a linear amplifier or follower
-
- 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
- G09G2320/0252—Improving the response speed
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- G—PHYSICS
- G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
- G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
- G09G2360/00—Aspects of the architecture of display systems
- G09G2360/14—Detecting light within display terminals, e.g. using a single or a plurality of photosensors
- G09G2360/145—Detecting light within display terminals, e.g. using a single or a plurality of photosensors the light originating from the display screen
- G09G2360/147—Detecting light within display terminals, e.g. using a single or a plurality of photosensors the light originating from the display screen the originated light output being determined for each pixel
- G09G2360/148—Detecting light within display terminals, e.g. using a single or a plurality of photosensors the light originating from the display screen the originated light output being determined for each pixel the light being detected by light detection means within each pixel
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- Engineering & Computer Science (AREA)
- Physics & Mathematics (AREA)
- Computer Hardware Design (AREA)
- General Physics & Mathematics (AREA)
- Theoretical Computer Science (AREA)
- Control Of Indicators Other Than Cathode Ray Tubes (AREA)
Abstract
A method and system for driving a pixel circuit in an active matrix display is provided. The system implements a feedback driving scheme to enhance programming speed of the pixel circuit. The system includes a column driver for driving the pixel circuit with feedback. A controller controls a signal on a programming signal line during a programming cycle. For example, the driver may include a model for reducing the settling time of a pixel current. During the programming mode, an accelerating pulse may be provided to accelerate the programming of the pixel circuit.
Description
Method and System for Driving a Pixel Circuit in an Active Matrix Display FIELD OF INVENTION
[0001 ] The present invention relates to display technologies, more specifically a method and system for driving a pixel circuit in an active matrix display.
BACKGROUND OF THE INVENTION
[0001 ] The present invention relates to display technologies, more specifically a method and system for driving a pixel circuit in an active matrix display.
BACKGROUND OF THE INVENTION
[0002] Active-matrix organic light emitting diode (AMOLED) displays are attracting attention due to several key advantages such as high efficiency, wide viewing angle, high contrast, and low fabrication cost. Among different technologies for implementation of AMOLED pixel circuits, hydrogenated amorphous silicon (a-Si:H) thin film transistor (TFT) is gathering more attention due to well established manufacturing infrastructure and low fabrication cost. However the threshold voltage (VT) of a-Si:H TFTs shifts over time with gate bias stress. If the current in the pixels depends on the VT of TFTs, VT shift causes degradation in the OLED luminance.
This signifies the demand for pixel circuits and driving schemes that provide the OLED with a VT-independent current. Among different driving schemes, current programming has shown reasonable stability (A. Nathan et al., "Amorphous silicon thin film transistor circuit integration for organic LED displays on glass and plastic," IEEE J.
Solid-State Circuits, vol. 39, no. 9, Sept. 2004, pp. 1477-1486). However, for small currents the programming time is large due to low field-effect mobility of a-Si:H TFTs and high parasitic capacitance of the data line. VT-compensating voltage-programmed pixels have smaller programming times ( J. Goh et al., "A new a-Si:H thin-film transistor pixel circuit for active matrix organic light-emitting diodes," IEEE Electron Dev.
Letts., vol.
24, no. 9, pp. 583-585, 2003) at the cost of imperfect compensation of VT.
This signifies the demand for pixel circuits and driving schemes that provide the OLED with a VT-independent current. Among different driving schemes, current programming has shown reasonable stability (A. Nathan et al., "Amorphous silicon thin film transistor circuit integration for organic LED displays on glass and plastic," IEEE J.
Solid-State Circuits, vol. 39, no. 9, Sept. 2004, pp. 1477-1486). However, for small currents the programming time is large due to low field-effect mobility of a-Si:H TFTs and high parasitic capacitance of the data line. VT-compensating voltage-programmed pixels have smaller programming times ( J. Goh et al., "A new a-Si:H thin-film transistor pixel circuit for active matrix organic light-emitting diodes," IEEE Electron Dev.
Letts., vol.
24, no. 9, pp. 583-585, 2003) at the cost of imperfect compensation of VT.
[0003] Recently, a driving scheme based on voltage feedback has been presented (S.
Jafarabadiashtiani et al., "A New Driving Method for a-Si AMOLED Displays Based on Voltage Feedback," Dig. of Tech. Papers, SID Int. Symp., Boston, pp. 316-317,May 27, 2005). The method provides proven stability and faster programming than the current-programming scheme. However, it is not fast enough to fulfill the demands for high-resolution large displays.
Jafarabadiashtiani et al., "A New Driving Method for a-Si AMOLED Displays Based on Voltage Feedback," Dig. of Tech. Papers, SID Int. Symp., Boston, pp. 316-317,May 27, 2005). The method provides proven stability and faster programming than the current-programming scheme. However, it is not fast enough to fulfill the demands for high-resolution large displays.
[0004] It is therefore desirable to provide a method and system that enhance the programming speed of a light emitting device display.
SUMMARY OF THE INVENTION
SUMMARY OF THE INVENTION
[0005] It is an object of the invention to provide a method and system that obviates or mitigates at least one of the disadvantages of existing systems.
[0006] In accordance with an aspect of the present invention there is provided a system for driving a pixel circuit in an active matrix display. The system includes a driver for driving a data line connected to the pixel circuit. The driver includes a feedback mechanism for producing a data signal on the data line based on a feedback signal on a feedback line from the pixel circuit and a signal on a programming signal line, and a module for reducing the settling time of a pixel current. The system includes a controller for controlling the signal on the programming signal line during a programming cycle such that the signal on the programming signal line has a primary pulse for boosting the charging of a capacitance of the feedback line.
[0007] In accordance with an aspect of the present invention there is provided a method of driving a pixel circuit in an active matrix display. The pixel circuit is connected to a data line for receiving data from a driver and a feedback line for providing a feedback signal to the driver. The driver drives the data line based on the feedback signal and a signal on a programming signal line. The method includes the steps of: during a programming cycle, providing, to the programming signal line, a primary pulse for boosting the charging of a capacitance of the feedback line, and subsequently providing a pulse with programming data.
[0008] In accordance with a further aspect of the present invention, there is provided a a system for driving a pixel circuit in an active matrix display. The system includes a driver for driving a data line connected to the pixel circuit. The driver includes a feedback mechanism for producing a data signal on the data line based on a feedback signal on a feedback line from the pixel circuit and a signal on a programming signal line, and a lead compensator provided between the feedback mechanism and the data line.
[0009] This summary of the invention does not necessarily describe all features of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] These and other features of the invention will become more apparent from the following description in which reference is made to the appended drawings wherein:
[0011 ] Figure 1 illustrates a pixel system for a feedback driving scheme in accordance with an embodiment of the present invention;
[0012] Figure 2 illustrates an example of the pixel system;
[0013] Figure 3 illustrates an example of waveforms for driving a pixel circuit of Figure 2;
[0014] Figure 4 illustrates a simulation result of the effect of lead compensation on the settling time of the OLED current;
[0015] Figure 5 illustrates another example of a column driver employed at the pixel system;
[0016] Figure 6 illustrates simulation results of the lead compensation and an accelerating pulse; and [0017] Figure 7 illustrates an example of a display system which implements the feedback driving scheme.
DETAILED DESCRIPTION
[0018] Embodiments of the present invention are described using an AMOLED
display including a plurality of pixel circuits, each having an organic light emitting diode (OLED) and a plurality of thin film transistors (TFTs). However, the pixel circuit may include any light emitting device other than OLED, and the pixel circuit may include any transistors other than TFTs. The transistors in the pixel circuit may be n-type transistors or p-type transistors. The transistors in the pixel circuit may be fabricated using amorphous silicon, nano/micro crystalline silicon, poly silicon, organic semiconductors technologies (e.g., organic TFT), NMOS/PMOS technology or CMOS
technology (e.g., MOSFET). The pixel circuit may be a current-programmed pixel or a voltage-programmed pixel.
[0019] In the description, "pixel circuit" and "pixel" maybe used interchangeably. In the description, "signal", "(signal) line" and "line" may be used interchangeably.
[0020] The embodiments of the present invention involve a feedback driving scheme which enhances the programming speed of pixel circuits.
[0021 ] Figure 1 illustrates a pixel system for a feedback driving scheme in accordance with an embodiment of the present invention. The pixel system includes a pixel circuit 20, a driver 10 for driving the pixel circuit 20, and a controller 2 for controlling the operation of the pixel system. The driver 10 includes a feedback module 12 and a module 14 for reducing the settling time and overshot for programming signals.
The driver 10 may be shared by a plurality of pixel circuits in a column. The pixel circuit 20 is selected by the controller 2. The driver 10 produces a data signal based on a signal on a programming signal line and a feedback signal from the pixel circuit 20.
The feedback signal is associated with the OLED current. As described below, the programming signal has an accelerating pulse. The accelerating pulse is set so as to accelerate the programming of the pixel circuit 20. The pixel circuit 20 may, but not limited to, have a current feedback, a voltage feedback, or an optical feedback.
[0022] Figure 2 illustrates an example of the pixel system. The pixel circuit 20 of Figure 2 includes a pixel driver having a driving TFT 22, switching TFTs 24 and 26, a storage capacitor 28 and a feedback resistor 30 for driving an OLED 32. The pixel circuit 20 is fabricated with a-Si:H TFTs. The feedback resistor 30 is fabricated with a stable n+ amorphous or microcrystalline silicon layer, which is compatible with the TFT process and is used for fabrication of TFT contacts. However, in poly silicon or organic technology, the resistor can be fabricated using poly silicon and organic semiconductor/metallic material.
[0023] The anode terminal of the OLED 32 is connected to a voltage supply Vdd and the cathode terminal of the OLED 32 is connected to the first terminal of the driving TFT 22. The first terminal of the switching TFT 24 is connected to a data line 40. The second terminal of the switching TFT 24, the gate terminal of the driving TFT
22, and the first terminal of the storage capacitor 28 are connected at node Al. The first terminal of the switching TFT 26 is connected to a feedback line 42. The second terminal of the switching TFT 26, the second terminal of the driving TFT 22, and the second terminal of the storage capacitor 28 are connected to node B 1. The gate terminals of the switching TFTs 24 and 26 are connected to a select line 44.
The resistor 30 is connected between node B1 and ground. The feedback line 42 transmits to the column driver 10 a feedback signal associated with the OLED current.
[0024] In Figure 2, the feedback resistor 30 is in the pixel circuit 20.
However, the feedback resistor 30 may be in the column driver 10, and thus be shared by a plurality of pixel circuits.
[0025] During the programming cycle, the pixel circuit 20 is connected to the external driving system through the data line 40 and the feedback line 42, forming a voltage-controlled current source. After the programming cycle, the gate-source voltage VG of the driving TFT 22 is saved by the storage capacitor 28 thereby allowing the pixel circuit 20 to drive the OLED 32 with the appropriate programming current.
[0026] In Figure 2, a differential amplifier is shown as an example of the feedback module 12 of Figure 1. In Figure 2, a lead compensator is shown as an example of the module 14 of Figure 1. The column driver 10 of Figure 2 includes the differential amplifier 12 with high voltage gain in series with the lead compensator 14.
The column driver 10 maybe implemented in a high-voltage CMOS technology. The differential amplifier 12 may be an Op-Amp, such as a monolithic FET-input Op-Amp. The differential amplifier 12 receives the feedback signal on the feedback line 42 and a signal on a programming signal line Vin. The output of the differential amplifier 12 is provided to the lead compensator 14. The output of the lead compensator 14 is connected to the data line 40. The lead compensation reduces the settling time and overshot for larger programming signal.
[0027] The transfer function of the compensator 14 is, for example, in the form of H(s)= (1 +siZ)/(1 +siP) . . . (1) where tip<iZ for non-zero values of tiP and iZ. tiP and iz may be equal to zero.
[0028] The values of -Ep and TZ are designed based on, for example, the circuit parameters such as parasitic capacitance of the data and feedback, gain and unity-gain bandwidth of the differential amplifier, the mobility of the thin film transistors of the pixel circuit, or combinations thereof. The lead compensation can enhance the settling time of the current in the AMOLED pixel circuit, preferably the settling time at larger programming currents associated with higher greyscales. The lead compensation effectively reduces the settling time of the OLED current associated with medium and higher greyscale levels.
[0029] Circuit analysis and simulation results show that the smallest programming times are achieved if iZ satisfies:
1/(CFP Rs3) < TZ < 1/(Cs Rs2) ... (2) where CFP is the parasitic capacitance of the feedback line 42 and Cs is the storage capacitor 28 of the pixel circuit 20. Rs2 and Rs3 are the ON resistance of the switching TFTs 24 and 26, respectively.
[0030] The operation of the pixel circuit 20 of Figure 2 is described in detail. An accelerating pulse is provided to the pixel circuit 20 to enhance the settling as shown in Figure 3. Figure 3 illustrates an example of waveforms for driving the pixel circuit 20 of Figure 2. As shown in Figure 3, the signal on the programming signal line Vin includes (1) a primary accelerating pulse 50 between tl and t2 and (2) a pulse between t2 and 0 with the desired programming voltage Vdata (tl<t2<t3). The primary accelerating pulse 50 has a value Vpulse that is larger than the desired programming voltage Vdata. The accelerating pulse 50 increases the loop gain and boosts the charging of CFP at the beginning of programming and results in a faster programming.
[0031 ] During the programming mode tl-t3, the select line 44 goes high, turning on the switching transistors 24 and 26. Consequently, the driving transistor 22, the feedback resistor 30 and the differential amplifier 12 form a voltage-controlled current source.
The feedback resistor 30 converts the current of the driving transistor 22 to a voltage VF. The voltage VF is then compared to Vin by the differential amplifier 12.
Due to the inherent negative feedback in the circuit, the output of the column driver 10 adjusts the gate voltage of the driving transistor 22. During tl-t2, the accelerating pulse 50 increases the loop gain and boosts the charging of CFP, resulting in a faster programming. During t2-t3, Vin goes to the desired programming level. The pixel circuit 20 compensates for the shift of the threshold voltage in the driving transistor 22, as long as the voltage VG at the gate of the driving transistor 22 does not exceed the maximum output range of the differential amplifier 12, and the voltage at the select line 44 is high enough to turn on the switching transistor 24.
[0032] After t3, the select line 44 goes low, disconnecting the pixel circuit 20 from the differential amplifier 12 by turning off the switching transistors 24 and 26.
The current through the OLED 32 does not change considerably as the storage capacitor 28 stores the gate-source voltage of the driving transistor 22.
[0033] The driving signals of Figure 3 are applied, for example, to the AMOLED
display for small programming currents. For large currents, Vpulse may be equal or even smaller than Vdata. The value of Vpulse is defined, for example, based on the parameters of the pixel circuit of Figure 2 and the value of Vdata.
[0034] Figure 4 illustrates a simulation result of the effect of the lead compensation (e.g., 14 of Figure 2) on the settling time of the OLED current. Since without lead compensation the system experience lots of ripples, the settling time increases dramatically. However, using the lead compensation controls the ripples and thus improves the settling time.
[0035] Figure 5 illustrates another example of the column driver 10 of Figure 1. The column driver of Figure 5 includes a trans-conductance differential amplifier 60 with a gain of Gm, a resistor 62, a voltage gain stage 64 with a gain of A, a compensating MOS
transistor 66, and a capacitor 68. The differential amplifier 60 receives two inputs V+
and V-. The voltage amplifier 64 receives the output of the differential amplifier 60.
The transistor 66 and the capacitor 68 are connected in series between the output of the differential amplifier 60 and the output Vout of the voltage amplifier 64. The resistor 62 converts the output current of the trans-conductance amplifier 60 to a voltage for the voltage amplifier 64.
[0036] The differential amplifier 60 corresponds to the differential amplifier 12 of Figure 2. The combination of the gain stage 64, the transistor 66 and the capacitor 68 corresponds to the lead compensator 14 of Figure 2.
[0037] The transistor 66 may be a NMOS or PMOS transistor or a transmission gate.
The value of iZ is determined, for example, by the capacitance Cc of the capacitor 68 and the resistance of the transistor 66. For fine tuning of the value of iZ, the gate of the transistor 66 is connected to a controlling voltage Vc.
[0038] Figure 6 illustrates simulation results of the feedback driving scheme.
In Figure 6, a waveform 70 is a programming current of an AMOLED pixel circuit with feedback, when driven by the feedback driving scheme having the accelerating pulse (e.g., 50 of Figure 3) and the lead compensator (e.g., 14 of Figures 1 and 2). In Figure 6, a waveform 72 is a programming current of an AMOLED pixel circuit with feedback, when driven by a simple differential amplifier without the accelerating pulse and the lead compensator. As shown in Figure 6, the feedback driving scheme having the accelerating pulse and the lead compensator is able to considerably improve the programming speed.
[0039] Figure 7 illustrates an example of a display system 80 that implements the feedback driving scheme. In Figure 5, SELi (i=1, 2, ...) represents a select line, DLj (j=1, 2, ...: column number) represents a data line, and FLj represents a feedback line.
Each of SELI, SEL2, ... corresponds to the signal line 44 of Figure 1, each of DLl, DL2, ... corresponds to the data line 40 of Figure 1, and each of FL1 and FL2 ,...
corresponds to the feedback line 42 of Figure 1. The data line DLj and the feedback line FLj (j=1, 2, ...) are shared by all the pixel circuits of the jth column. The display system 80 includes a pixel array 82 in which a plurality of pixel circuits 20 are arranged in row and column. Preferably, the pixel array 82 is an AMOLED display. A data driver and an address driver 86 are provided to the pixel array 82. The data driver 84 includes a plurality of the column drivers 10, each of which is arranged in a column of the pixel array 82. The address driver 86 provides select signals SEL1, SEL2, ... The address driver 86 may drive Vc of Figure 5. The timing of each signal is controlled by a controller 88. The accelerating pulse 50 of Figure 3 is generated under the control of the controller 88.
[0040] In the description above, the pixel circuit 20 with voltage feedback is shown as an example of a pixel circuit to which the feedback driving scheme is applied.
However, the feedback driving scheme in accordance with the embodiments of the present invention is applicable to any other pixel circuits with feedback.
[0041] The driving scheme of the embodiment of the present invention, including the pulsed shaped data and the lead compensated differential op-amp, accelerates the prograrnming of AMOLED feedback pixel circuits, such as voltage feedback pixel circuits, cnirent feedback pixel circaits, and optical feedback pixel circuits. The combination of the lead compensator and the accelerating pulse improves the programming speed at both high and low OLED currents.
[0042] By sending a feedback voltage from each pixel to the column driver during the programming cycle, the driving scheme can compensate for the instability of the pixel elements, e.g., the shift in the threshold voltage of TFfs.
[0043] The present invention has been described with regard to one or more embodiments. However, it will be apparent to persons slcilled in the art that a nuinber of variations and modifications can be made without departing from the scope of the invention as defined in the claims.
[0011 ] Figure 1 illustrates a pixel system for a feedback driving scheme in accordance with an embodiment of the present invention;
[0012] Figure 2 illustrates an example of the pixel system;
[0013] Figure 3 illustrates an example of waveforms for driving a pixel circuit of Figure 2;
[0014] Figure 4 illustrates a simulation result of the effect of lead compensation on the settling time of the OLED current;
[0015] Figure 5 illustrates another example of a column driver employed at the pixel system;
[0016] Figure 6 illustrates simulation results of the lead compensation and an accelerating pulse; and [0017] Figure 7 illustrates an example of a display system which implements the feedback driving scheme.
DETAILED DESCRIPTION
[0018] Embodiments of the present invention are described using an AMOLED
display including a plurality of pixel circuits, each having an organic light emitting diode (OLED) and a plurality of thin film transistors (TFTs). However, the pixel circuit may include any light emitting device other than OLED, and the pixel circuit may include any transistors other than TFTs. The transistors in the pixel circuit may be n-type transistors or p-type transistors. The transistors in the pixel circuit may be fabricated using amorphous silicon, nano/micro crystalline silicon, poly silicon, organic semiconductors technologies (e.g., organic TFT), NMOS/PMOS technology or CMOS
technology (e.g., MOSFET). The pixel circuit may be a current-programmed pixel or a voltage-programmed pixel.
[0019] In the description, "pixel circuit" and "pixel" maybe used interchangeably. In the description, "signal", "(signal) line" and "line" may be used interchangeably.
[0020] The embodiments of the present invention involve a feedback driving scheme which enhances the programming speed of pixel circuits.
[0021 ] Figure 1 illustrates a pixel system for a feedback driving scheme in accordance with an embodiment of the present invention. The pixel system includes a pixel circuit 20, a driver 10 for driving the pixel circuit 20, and a controller 2 for controlling the operation of the pixel system. The driver 10 includes a feedback module 12 and a module 14 for reducing the settling time and overshot for programming signals.
The driver 10 may be shared by a plurality of pixel circuits in a column. The pixel circuit 20 is selected by the controller 2. The driver 10 produces a data signal based on a signal on a programming signal line and a feedback signal from the pixel circuit 20.
The feedback signal is associated with the OLED current. As described below, the programming signal has an accelerating pulse. The accelerating pulse is set so as to accelerate the programming of the pixel circuit 20. The pixel circuit 20 may, but not limited to, have a current feedback, a voltage feedback, or an optical feedback.
[0022] Figure 2 illustrates an example of the pixel system. The pixel circuit 20 of Figure 2 includes a pixel driver having a driving TFT 22, switching TFTs 24 and 26, a storage capacitor 28 and a feedback resistor 30 for driving an OLED 32. The pixel circuit 20 is fabricated with a-Si:H TFTs. The feedback resistor 30 is fabricated with a stable n+ amorphous or microcrystalline silicon layer, which is compatible with the TFT process and is used for fabrication of TFT contacts. However, in poly silicon or organic technology, the resistor can be fabricated using poly silicon and organic semiconductor/metallic material.
[0023] The anode terminal of the OLED 32 is connected to a voltage supply Vdd and the cathode terminal of the OLED 32 is connected to the first terminal of the driving TFT 22. The first terminal of the switching TFT 24 is connected to a data line 40. The second terminal of the switching TFT 24, the gate terminal of the driving TFT
22, and the first terminal of the storage capacitor 28 are connected at node Al. The first terminal of the switching TFT 26 is connected to a feedback line 42. The second terminal of the switching TFT 26, the second terminal of the driving TFT 22, and the second terminal of the storage capacitor 28 are connected to node B 1. The gate terminals of the switching TFTs 24 and 26 are connected to a select line 44.
The resistor 30 is connected between node B1 and ground. The feedback line 42 transmits to the column driver 10 a feedback signal associated with the OLED current.
[0024] In Figure 2, the feedback resistor 30 is in the pixel circuit 20.
However, the feedback resistor 30 may be in the column driver 10, and thus be shared by a plurality of pixel circuits.
[0025] During the programming cycle, the pixel circuit 20 is connected to the external driving system through the data line 40 and the feedback line 42, forming a voltage-controlled current source. After the programming cycle, the gate-source voltage VG of the driving TFT 22 is saved by the storage capacitor 28 thereby allowing the pixel circuit 20 to drive the OLED 32 with the appropriate programming current.
[0026] In Figure 2, a differential amplifier is shown as an example of the feedback module 12 of Figure 1. In Figure 2, a lead compensator is shown as an example of the module 14 of Figure 1. The column driver 10 of Figure 2 includes the differential amplifier 12 with high voltage gain in series with the lead compensator 14.
The column driver 10 maybe implemented in a high-voltage CMOS technology. The differential amplifier 12 may be an Op-Amp, such as a monolithic FET-input Op-Amp. The differential amplifier 12 receives the feedback signal on the feedback line 42 and a signal on a programming signal line Vin. The output of the differential amplifier 12 is provided to the lead compensator 14. The output of the lead compensator 14 is connected to the data line 40. The lead compensation reduces the settling time and overshot for larger programming signal.
[0027] The transfer function of the compensator 14 is, for example, in the form of H(s)= (1 +siZ)/(1 +siP) . . . (1) where tip<iZ for non-zero values of tiP and iZ. tiP and iz may be equal to zero.
[0028] The values of -Ep and TZ are designed based on, for example, the circuit parameters such as parasitic capacitance of the data and feedback, gain and unity-gain bandwidth of the differential amplifier, the mobility of the thin film transistors of the pixel circuit, or combinations thereof. The lead compensation can enhance the settling time of the current in the AMOLED pixel circuit, preferably the settling time at larger programming currents associated with higher greyscales. The lead compensation effectively reduces the settling time of the OLED current associated with medium and higher greyscale levels.
[0029] Circuit analysis and simulation results show that the smallest programming times are achieved if iZ satisfies:
1/(CFP Rs3) < TZ < 1/(Cs Rs2) ... (2) where CFP is the parasitic capacitance of the feedback line 42 and Cs is the storage capacitor 28 of the pixel circuit 20. Rs2 and Rs3 are the ON resistance of the switching TFTs 24 and 26, respectively.
[0030] The operation of the pixel circuit 20 of Figure 2 is described in detail. An accelerating pulse is provided to the pixel circuit 20 to enhance the settling as shown in Figure 3. Figure 3 illustrates an example of waveforms for driving the pixel circuit 20 of Figure 2. As shown in Figure 3, the signal on the programming signal line Vin includes (1) a primary accelerating pulse 50 between tl and t2 and (2) a pulse between t2 and 0 with the desired programming voltage Vdata (tl<t2<t3). The primary accelerating pulse 50 has a value Vpulse that is larger than the desired programming voltage Vdata. The accelerating pulse 50 increases the loop gain and boosts the charging of CFP at the beginning of programming and results in a faster programming.
[0031 ] During the programming mode tl-t3, the select line 44 goes high, turning on the switching transistors 24 and 26. Consequently, the driving transistor 22, the feedback resistor 30 and the differential amplifier 12 form a voltage-controlled current source.
The feedback resistor 30 converts the current of the driving transistor 22 to a voltage VF. The voltage VF is then compared to Vin by the differential amplifier 12.
Due to the inherent negative feedback in the circuit, the output of the column driver 10 adjusts the gate voltage of the driving transistor 22. During tl-t2, the accelerating pulse 50 increases the loop gain and boosts the charging of CFP, resulting in a faster programming. During t2-t3, Vin goes to the desired programming level. The pixel circuit 20 compensates for the shift of the threshold voltage in the driving transistor 22, as long as the voltage VG at the gate of the driving transistor 22 does not exceed the maximum output range of the differential amplifier 12, and the voltage at the select line 44 is high enough to turn on the switching transistor 24.
[0032] After t3, the select line 44 goes low, disconnecting the pixel circuit 20 from the differential amplifier 12 by turning off the switching transistors 24 and 26.
The current through the OLED 32 does not change considerably as the storage capacitor 28 stores the gate-source voltage of the driving transistor 22.
[0033] The driving signals of Figure 3 are applied, for example, to the AMOLED
display for small programming currents. For large currents, Vpulse may be equal or even smaller than Vdata. The value of Vpulse is defined, for example, based on the parameters of the pixel circuit of Figure 2 and the value of Vdata.
[0034] Figure 4 illustrates a simulation result of the effect of the lead compensation (e.g., 14 of Figure 2) on the settling time of the OLED current. Since without lead compensation the system experience lots of ripples, the settling time increases dramatically. However, using the lead compensation controls the ripples and thus improves the settling time.
[0035] Figure 5 illustrates another example of the column driver 10 of Figure 1. The column driver of Figure 5 includes a trans-conductance differential amplifier 60 with a gain of Gm, a resistor 62, a voltage gain stage 64 with a gain of A, a compensating MOS
transistor 66, and a capacitor 68. The differential amplifier 60 receives two inputs V+
and V-. The voltage amplifier 64 receives the output of the differential amplifier 60.
The transistor 66 and the capacitor 68 are connected in series between the output of the differential amplifier 60 and the output Vout of the voltage amplifier 64. The resistor 62 converts the output current of the trans-conductance amplifier 60 to a voltage for the voltage amplifier 64.
[0036] The differential amplifier 60 corresponds to the differential amplifier 12 of Figure 2. The combination of the gain stage 64, the transistor 66 and the capacitor 68 corresponds to the lead compensator 14 of Figure 2.
[0037] The transistor 66 may be a NMOS or PMOS transistor or a transmission gate.
The value of iZ is determined, for example, by the capacitance Cc of the capacitor 68 and the resistance of the transistor 66. For fine tuning of the value of iZ, the gate of the transistor 66 is connected to a controlling voltage Vc.
[0038] Figure 6 illustrates simulation results of the feedback driving scheme.
In Figure 6, a waveform 70 is a programming current of an AMOLED pixel circuit with feedback, when driven by the feedback driving scheme having the accelerating pulse (e.g., 50 of Figure 3) and the lead compensator (e.g., 14 of Figures 1 and 2). In Figure 6, a waveform 72 is a programming current of an AMOLED pixel circuit with feedback, when driven by a simple differential amplifier without the accelerating pulse and the lead compensator. As shown in Figure 6, the feedback driving scheme having the accelerating pulse and the lead compensator is able to considerably improve the programming speed.
[0039] Figure 7 illustrates an example of a display system 80 that implements the feedback driving scheme. In Figure 5, SELi (i=1, 2, ...) represents a select line, DLj (j=1, 2, ...: column number) represents a data line, and FLj represents a feedback line.
Each of SELI, SEL2, ... corresponds to the signal line 44 of Figure 1, each of DLl, DL2, ... corresponds to the data line 40 of Figure 1, and each of FL1 and FL2 ,...
corresponds to the feedback line 42 of Figure 1. The data line DLj and the feedback line FLj (j=1, 2, ...) are shared by all the pixel circuits of the jth column. The display system 80 includes a pixel array 82 in which a plurality of pixel circuits 20 are arranged in row and column. Preferably, the pixel array 82 is an AMOLED display. A data driver and an address driver 86 are provided to the pixel array 82. The data driver 84 includes a plurality of the column drivers 10, each of which is arranged in a column of the pixel array 82. The address driver 86 provides select signals SEL1, SEL2, ... The address driver 86 may drive Vc of Figure 5. The timing of each signal is controlled by a controller 88. The accelerating pulse 50 of Figure 3 is generated under the control of the controller 88.
[0040] In the description above, the pixel circuit 20 with voltage feedback is shown as an example of a pixel circuit to which the feedback driving scheme is applied.
However, the feedback driving scheme in accordance with the embodiments of the present invention is applicable to any other pixel circuits with feedback.
[0041] The driving scheme of the embodiment of the present invention, including the pulsed shaped data and the lead compensated differential op-amp, accelerates the prograrnming of AMOLED feedback pixel circuits, such as voltage feedback pixel circuits, cnirent feedback pixel circaits, and optical feedback pixel circuits. The combination of the lead compensator and the accelerating pulse improves the programming speed at both high and low OLED currents.
[0042] By sending a feedback voltage from each pixel to the column driver during the programming cycle, the driving scheme can compensate for the instability of the pixel elements, e.g., the shift in the threshold voltage of TFfs.
[0043] The present invention has been described with regard to one or more embodiments. However, it will be apparent to persons slcilled in the art that a nuinber of variations and modifications can be made without departing from the scope of the invention as defined in the claims.
Claims (31)
1. A system for driving a pixel circuit in an active matrix display, comprising:
a driver for driving a data line connected to the pixel circuit, the driver including a feedback mechanism for producing a data signal on the data line based on a feedback signal on a feedback line from the pixel circuit and a signal on a programming signal line, and a module for reducing the settling time of a pixel current;
the module comprising a lead compensator provided between the output of the feedback mechanism and the data line;
a controller for controlling the signal on the programming signal line during a programming cycle such that the signal on the programming signal line has a primary pulse for boosting the charging of a capacitance of the feedback line.
a driver for driving a data line connected to the pixel circuit, the driver including a feedback mechanism for producing a data signal on the data line based on a feedback signal on a feedback line from the pixel circuit and a signal on a programming signal line, and a module for reducing the settling time of a pixel current;
the module comprising a lead compensator provided between the output of the feedback mechanism and the data line;
a controller for controlling the signal on the programming signal line during a programming cycle such that the signal on the programming signal line has a primary pulse for boosting the charging of a capacitance of the feedback line.
2. The system as claimed in claim 1, wherein the feedback mechanism includes a differential amplifier for receiving the signal on the programming signal line at a first input and receiving the feedback signal on the feedback line at a second input.
3. The system as claimed in claim 2, wherein the differential amplifier includes an Op-Amp.
4. The system as claimed in claim 2, wherein the differential amplifier includes a trans-conductance differential amplifier.
5. The system as claimed in claim 2, wherein the lead compensator includes a voltage amplifier for amplifying the output of the differential amplifier, and a transistor and a capacitor connected in series between the output of the differential amplifier and the programming signal line.
6. The system as claimed in any one of claims 2-5, wherein the pixel circuit includes:
a first switching transistor connected to the output of the lead compensator;
and a second switching transistor connected to the second input of the differential amplifier.
a first switching transistor connected to the output of the lead compensator;
and a second switching transistor connected to the second input of the differential amplifier.
7. The system as claimed in any one of claims 1-6, wherein the pixel circuit is driven by voltage, current or optical feedback through the driver.
8. The system as claimed in any one of claims 1-6, wherein the pixel circuit is a voltage or current programmed pixel circuit.
9. The system as claimed in any one of claims 1-8, wherein the pixel circuit is arranged in row and column to form the display, the driver being arranged in each column and being shared by the pixel circuit in the column.
10. The system as claimed in any one of claims 1-9, wherein the display is an Active-Matrix Organic Light Emitting Diode (AMOLED) display.
11. The system as claimed in any one of claims 1-10, wherein the signal on the programming signal line has a subsequent pulse having the value of a programming data, after the primary pulse.
12. A method of driving a pixel circuit in an active matrix display, the pixel circuit being connected to a data line for receiving data from a driver and a feedback line for providing a feedback signal to the driver, the driver driving the data line based on the feedback signal and a signal on a programming signal line, comprising the steps of:
during a programming cycle, providing, to the programming signal line, a primary pulse for boosting the charging of a capacitance of the feedback line, and subsequently providing a pulse with programming data; and using a lead compensator connected between the output of the feedback signal and the data line to reduce a settling time of the programming signal.
during a programming cycle, providing, to the programming signal line, a primary pulse for boosting the charging of a capacitance of the feedback line, and subsequently providing a pulse with programming data; and using a lead compensator connected between the output of the feedback signal and the data line to reduce a settling time of the programming signal.
13. A method as claimed in claim 12, further comprising the step of:
during the programming cycle, setting a select signal to connect the pixel circuit and the driver.
during the programming cycle, setting a select signal to connect the pixel circuit and the driver.
14. The method as claimed in claim 12 or 13, further comprising the step of after the programming cycle, resetting the select line to disconnect the pixel circuit and the driver.
15. The method as claimed in any one of claims 12-14, wherein the pixel circuit is arranged in column and row to form a display, the driver being shared by the pixel circuit in each column.
16. The method as claimed in any one of claims 12-15, wherein the pixel circuit is driven by voltage, current or optical feedback through the driver.
17. The method as claimed in any one of claims 12-15, wherein the pixel circuit is a voltage or current programmed pixel circuit.
18. A system for driving a pixel circuit in an active matrix display, comprising:
a driver for driving a data line connected to the pixel circuit, the driver including a feedback mechanism for producing a data signal on the data line based on a feedback signal on a feedback line from the pixel circuit and a signal on a programming signal line, and a lead compensator provided between the feedback mechanism and the data line.
a driver for driving a data line connected to the pixel circuit, the driver including a feedback mechanism for producing a data signal on the data line based on a feedback signal on a feedback line from the pixel circuit and a signal on a programming signal line, and a lead compensator provided between the feedback mechanism and the data line.
19. The system as claimed in claim 18, wherein the feedback mechanism includes a differential amplifier for receiving the signal on the programming signal line at a first input and receiving the feedback signal on the feedback line at a second input.
20. The system as claimed in claim 19, wherein the differential amplifier includes an Op-Amp.
21. The system as claimed in claim 19, wherein the differential amplifier includes a trans-conductance differential amplifier.
22. The system as claimed in any one of claims 19-21, wherein the lead compensator includes a voltage amplifier for amplifying the output of the differential amplifier, and a transistor and a capacitor connected in series between the output of the differential amplifier and the programming signal line.
23. The system as claimed in any one of claims 19-22, wherein the pixel circuit includes:
a first switching transistor connected to the output of the lead compensator;
and a second switching transistor connected to the second input of the differential amplifier.
a first switching transistor connected to the output of the lead compensator;
and a second switching transistor connected to the second input of the differential amplifier.
24. The system as claimed in any one of claims 18-23, wherein the pixel circuit is driven by voltage, current or optical feedback through the driver.
25. The system as claimed in any one of claims 18-23, wherein the pixel circuit is a voltage or current programmed pixel circuit.
26. The system as claimed in any one of claims 18-25, wherein the pixel circuit is arranged in row and column to form the display, the driver being arranged in each column and being shared by the pixel circuit in the column.
27. The system as claimed in any one of claims 18-26, wherein the display is an Active-Matrix Organic Light Emitting Diode (AMOLED) display.
28. The system as claimed in any one of claims 18-27, further comprising:
a controller for controlling the signal on the programming signal line during a programming cycle such that the signal on the programming signal line has an accelerating pulse for accelerating the programming of the pixel circuit.
a controller for controlling the signal on the programming signal line during a programming cycle such that the signal on the programming signal line has an accelerating pulse for accelerating the programming of the pixel circuit.
29. The system as claimed in claim 28, wherein the accelerating pulse boosts the charging of a capacitance of the feedback line.
30. The system as claimed in claim 5 or 22, wherein the transistor includes at least one of amorphous, nano/micro crystalline, poly, organic material, n-type material, p-type material, and CMOS silicon.
31. The system as claimed in claim 1 or 18, wherein the pixel circuit includes a plurality of transistors including at least one of amorphous, nano/micro crystalline, poly, organic material, n-type material, p-type material, and CMOS silicon.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA002550102A CA2550102C (en) | 2005-07-06 | 2006-07-06 | Method and system for driving a pixel circuit in an active matrix display |
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA002510855A CA2510855A1 (en) | 2005-07-06 | 2005-07-06 | Fast driving method for amoled displays |
| CA2,510,855 | 2005-07-06 | ||
| CA002550102A CA2550102C (en) | 2005-07-06 | 2006-07-06 | Method and system for driving a pixel circuit in an active matrix display |
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| CA2550102A1 CA2550102A1 (en) | 2006-09-18 |
| CA2550102C true CA2550102C (en) | 2008-04-29 |
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| CA002550102A Expired - Fee Related CA2550102C (en) | 2005-07-06 | 2006-07-06 | Method and system for driving a pixel circuit in an active matrix display |
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