Pixel circuits including feedback capacitors and reset capacitors, and display systems therefore

• US 10,424,245 B2
• Filed: 07/27/2017
• Issued: 09/24/2019
• Est. Priority Date: 05/11/2012
• Status: Active Grant

• Current Assignee: IGNIS Innovation Incorporated
• Original Assignee: IGNIS Innovation Incorporated
• Inventors: Chaji, Gholamreza
• Primary Examiner(s): Eurice, Michael J
• Application Number:
  US15/661,777
• Publication Number:
  US 20170323599A1
• Time in Patent Office: 789 Days
• CPC Class Codes: G09G 2300/0819 used for counteracting undesired variations, e.g. feedback or autozeroing
• CPC Class Codes: G09G 2300/0852 being a dynamic memory with more than one capacitor
• CPC Class Codes: G09G 2300/0861 with additional control of the display period without amending the charge stored in a pixel memory, e.g. by means of additional select electrodes
• CPC Class Codes: G09G 2310/061 for resetting or blanking
• CPC Class Codes: G09G 2310/066 Waveforms comprising a gently increasing or decreasing portion, e.g. ramp
• CPC Class Codes: G09G 3/3208 organic, e.g. using organic light-emitting diodes [OLED]
• CPC Class Codes: G09G 3/3233 with pixel circuitry controlling the current through the light-emitting element
• CPC Class Codes: G09G 3/325 the data current flowing through the driving transistor during a setting phase, e.g. by using a switch for connecting the driving transistor to the data driver
• CPC Class Codes: G09G 3/3258 with pixel circuitry controlling the voltage across the light-emitting element
• CPC Class Codes: G09G 3/3283 in which the data driver supplies a variable data current for setting the current through, or the voltage across, the light-emitting elements
• CPC Class Codes: G09G 3/3291 in which the data driver supplies a variable data voltage for setting the current through, or the voltage across, the light-emitting elements

Abstract

A display with a pixel circuit for driving a current-driven emissive element includes a feedback capacitor in series between the emissive element and a programming node of the pixel circuit. During driving, variations in the operating voltage of the emissive element due to variations in the current conveyed through the emissive element by a driving transistor are accounted for. The feedback capacitor generates voltage adjustments at the programming node that correspond to the variations at the emissive element, and thus reduces variations in light emission. A reset capacitor connected to a select line is selectively connected to the gate terminal of the driving transistor and resets the driving transistor prior to programming. The select line adjusts the voltage on the gate terminal to reset the driving transistor by the capacitive coupling of the select line to the gate terminal created by the reset capacitor.

Claims

1. A pixel circuit, connected to a data line and an emission control line, comprising:

a drive transistor including a gate terminal and arranged to convey a drive current through a light emitting device, the drive current being conveyed according to a voltage on the gate terminal;

a storage capacitor connected to the gate terminal for storing programming voltages conveyed via the data line;

an emission control transistor, connected in series between the drive transistor and the light emitting device, and connected to the emission control line, thereby capable of preventing emission of the light emitting device while the storage capacitor is being charged; and

a feedback capacitor connected between the light emitting device and the gate terminal of the drive transistor;

wherein in response to voltage changes across the light emitting device, the feedback capacitor is capable of generating corresponding voltage changes at the gate terminal of the drive transistor based on a combined capacitance of the storage and feedback capacitors, resulting in changes in the drive current conveyed to the light emitting device; and

wherein the feedback capacitor capacitively couples the gate terminal of the drive transistor directly to the light emitting device to automatically correct for voltage instabilities at the light emitting device providing a stable drive current throughout an emission cycle.

2. The pixel circuit according to claim 1, wherein in response to a voltage increase at the light emitting device caused by an increase in current through the light emitting device, the feedback capacitor is capable of generating a corresponding voltage decrease at the gate terminal of the drive transistor to cause the current through the drive transistor to decrease.

3. The pixel circuit according to claim 1, wherein in response to a voltage decrease at the light emitting device caused by a decrease in current through the light emitting device, the feedback capacitor is capable of generating a corresponding voltage increase at the gate terminal of the drive transistor to cause the current through the drive transistor to increase.

4. The pixel circuit according to claim 1, wherein the emission control transistor is capable of turning off prior to programming the pixel circuit, such that the voltage of the light emitting device discharges to an off voltage.

5. The pixel circuit according to claim 1, wherein the voltage changes at the gate terminal of the drive transistor generated by the feedback capacitor are generated according to a voltage division relationship between the storage capacitor and the feedback capacitor.

6. The pixel circuit according to claim 1, wherein a first terminal of the storage capacitor is connected to the gate terminal of the drive transistor and a second terminal of the storage capacitor connected to a stable voltage to allow the storage capacitor to be charged according to programming information.

7. The pixel circuit according to claim 1, wherein a first terminal of the storage capacitor is connected to the gate terminal of the drive transistor and a second terminal of the storage capacitor is connected to a power supply line.

8. The pixel circuit according to claim 1, wherein the light emitting device is an organic light emitting diode, and

wherein the feedback capacitor is connected to an anode terminal of the organic light emitting diode.

9. The pixel circuit according to claim 1, wherein the drive transistor is an n-type or p-type thin film transistor.

10. The pixel circuit according to claim 1, further comprising:

a switching circuit connected to a select line capable of selectively coupling the gate terminal of the drive transistor to the data line for charging the storage capacitor and programming the pixel circuit according to programming information.

11. The pixel circuit according to claim 10, wherein the switching circuit further includes a second switch transistor connected between the gate terminal of the drive transistor and a terminal of the drive transistor other than the gate terminal, and

wherein the gate terminal of the drive transistor is capacitively coupled to the data line such that while the second switch transistor is turned on and a ramp voltage is applied to the data line, a current is conveyed through the drive transistor, the second switch transistor, and across the programming capacitor while the gate terminal of the drive transistor adjusts according to the conveyed current.

12. The pixel circuit according to claim 10, wherein the switching circuit includes a first switch transistor connected to a first select line capable of selectively connecting the gate terminal of the drive transistor to the data line.

13. The pixel circuit according to claim 12, wherein the switching circuit further includes a programming capacitor, and a second switch transistor connected to a second select line capable of selectively connecting the gate terminal of the drive transistor to a current path through the drive transistor, and wherein the first switch transistor is capable of selectively coupling the gate terminal of the drive transistor to the data line via the programming capacitor.

14. The pixel circuit according to claim 13, wherein the second switch transistor is connected to the current path through the drive transistor at a node between the drive transistor and the emission control transistor.

15. A display system comprising a plurality of pixel circuits arranged in rows and columns, each of plurality of pixel circuits including:

a drive transistor including a gate terminal and arranged to convey a drive current through a light emitting device, the drive current being conveyed according to a voltage on the gate terminal;

a storage capacitor connected to the gate terminal for storing programming voltages conveyed via a data line;

an emission control transistor, connected in series between the drive transistor and the light emitting device, and connected to an emission control line, thereby capable of preventing emission of the light emitting device while the storage capacitor is being charged; and

a feedback capacitor connected between the light emitting device and the gate terminal of the drive transistor;

wherein in response to voltage changes across the light emitting device, the feedback capacitor is capable of generating corresponding voltage changes at the gate terminal of the drive transistor based on a combined capacitance of the storage and feedback capacitors, resulting in changes in the drive current by modifying conductance of a channel region of the drive transistor; and

wherein each pixel circuit is configured such that the feedback capacitor capacitively couples the gate terminal of the drive transistor directly to the light emitting device to automatically correct for voltage instabilities at the light emitting device providing a stable drive current throughout an emission cycle.

16. A pixel circuit connectable to a data line comprising:

a drive transistor including a gate terminal and arranged to convey a drive current through a light emitting device during emission cycles, the drive current being conveyed according to a voltage on the gate terminal;

a storage capacitor connected to the gate terminal for storing programming voltages conveyed via the data line during programming and/or compensation cycles;

a first switch transistor connected between the gate terminal of the drive transistor and a first terminal of the drive transistor between the drive transistor and the light emitting device;

a select line connected to a gate of the first switch transistor for transmitting a signal to turn on the first switch transistor; and

a reset capacitor connected between the first terminal of the drive transistor and the select line such that the select line is capacitively coupled to the gate terminal of the drive transistor while the first switch transistor is turned on capable of generating a change in voltage at the gate terminal based on the storage and reset capacitors for resetting the drive transistor between programming cycles.

17. The pixel circuit according to claim 16, wherein the first switch transistor is connected to the select line such that turning on the first switch transistor by adjusting the voltage on the select line simultaneously generates a change in voltage at the gate terminal of the drive transistor.

18. The pixel circuit according to claim 16, further comprising:

a feedback capacitor connected between the light emitting device and the gate terminal of the drive transistor such that voltage changes across the light emitting device generate corresponding voltage changes at the gate terminal of the drive transistor.