OLED luminance degradation compensation

• US 8,279,143 B2
• Filed: 07/11/2011
• Issued: 10/02/2012
• Est. Priority Date: 08/15/2006
• Status: Active Grant

• Current Assignee: IGNIS Innovation Incorporated
• Original Assignee: IGNIS Innovation Incorporated
• Inventors: Nathan, Arokia, Chaji, Gholamreza
• Primary Examiner(s): Eisen, Alexander
• Assistant Examiner(s): MARINELLI, PATRICK
• Application Number:
  US13/179,963
• Publication Number:
  US 20110279488A1
• Time in Patent Office: 449 Days
• Field of Search: 345 76- 83, 345204-215, 345690-699, 3151691-1694
• US Class Current: 345/78
• CPC Class Codes: G09G 2300/0465 Improved aperture ratio, e.g. by size reduction of the pixel circuit, e.g. for improving the pixel density or the maximum displayable luminance or brightness
• CPC Class Codes: G09G 2300/0819 used for counteracting undesired variations, e.g. feedback or autozeroing
• CPC Class Codes: G09G 2300/0842 forming a memory circuit, e.g. a dynamic memory with one capacitor
• CPC Class Codes: G09G 2320/0233 Improving the luminance or brightness uniformity across the screen
• CPC Class Codes: G09G 2320/029 by monitoring one or more pixels in the display panel, e.g. by monitoring a fixed reference pixel
• CPC Class Codes: G09G 2320/0295 by monitoring each display pixel
• CPC Class Codes: G09G 2320/041 Temperature compensation
• CPC Class Codes: G09G 2320/043 Preventing or counteracting the effects of ageing
• CPC Class Codes: G09G 2320/045 Compensation of drifts in the characteristics of light emitting or modulating elements
• CPC Class Codes: G09G 3/3233 with pixel circuitry controlling the current through the light-emitting element
• CPC Class Codes: G09G 3/3241 the current through the light-emitting element being set using a data current provided by the data driver, e.g. by using a two-transistor current mirror
• 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: H05B 45/60 Circuit arrangements for operating LEDs comprising organic material, e.g. for operating organic light-emitting diodes [OLED] or polymer light-emitting diodes [PLED]

Abstract

A system and method are disclosed for determining a pixel capacitance. The pixel capacitance is correlated to a pixel age to determine a current correction factor used for compensating the pixel drive current to account for luminance degradation of the pixel that results from the pixel aging.

Claims

1. A method of compensating for luminance degradation of a pixel having an electroluminescent device, the method comprising:

determining a capacitance of the electroluminescent device;

correlating the determined capacitance of the electroluminescent device to a current correction factor for the electroluminescent device;

compensating a drive current for the electroluminescent device according to the correlated current correction factor; and

driving the electroluminescent device with the compensated drive current.

2. The method as claimed in claim 1, wherein the pixel is one of a plurality of pixels arranged in an array to form a display.

3. The method as claimed in claim 1, wherein the correlating includes:

extrapolating the current correction factor from a linear or polynomial relationship according to a plurality of established current correction factors associated with electroluminescent device capacitance values.

4. The method as claimed in claim 1, wherein the correlating includes:

extrapolating the current correction factor from a look-up table of empirically determined current correction factors associated with electroluminescent device capacitance values.

5. The method as claimed in claim 1, further comprising:

updating the determined capacitance of the electroluminescent device more than once during a lifetime of the pixel so as to account for ongoing aging degradation during the lifetime of the pixel.

6. The method as claimed in claim 5, wherein the updating is carried out at a regular interval.

7. The method as claimed in claim 1, wherein the determining the capacitance of the electroluminescent device includes measuring a voltage change on the electroluminescent device in response to a first voltage and a second voltage being applied to the electroluminescent device.

8. The method as claimed in claim 1, wherein the determining includes applying a first voltage to the electroluminescent device and measuring an amount of charge required to change the first voltage to a second voltage.

9. The method as claimed in claim 8, wherein the measuring is carried out via a read block including a read capacitor for detecting the amount of charge.

10. A display for driving an array of a plurality of pixel circuits with a current compensated for luminance degradation, each of said pixel circuits having an electroluminescent device, the display comprising:

a display panel comprising the array of pixel circuits, the pixel circuits arranged in at least one row and a plurality of columns;

a column driver for driving the electroluminescent devices in the pixel circuits with a driving current;

a read block for determining a capacitance of the electroluminescent device and correlating the determined capacitance of the electroluminescent device to a current correction factor for the electroluminescent device; and

a control block for controlling the operation of the column driver and the read block, the control block being operable to adjust the driving current based on the correlated current correction factor, and to drive the electroluminescent device with the compensated drive current.

11. The display as claimed in claim 10, wherein the array of pixel circuits includes at least two rows of pixel circuits;

and the display further comprising;

a row driver for selecting the row of pixel circuits to be driven by the column driver.

12. The display as claimed in claim 11, wherein each pixel circuit comprises:

a driving transistor for driving the electroluminescent device to emit light based on the driving current; and

a switching transistor, controlled by the row driver, to selectively connect the driving transistor to the column driver.

13. The display as claimed in claim 12, wherein the electroluminescent device is an organic light emitting diode.

14. The display as claimed in claim 12, wherein the read block comprises:

a plurality of read block elements, each read block element comprising;

a switch for electrically connecting and disconnecting the read block element to a pixel circuit of the plurality of pixel circuits;

an operational amplifier electrically connected to the switch; and

a read capacitor connected in parallel with the operational amplifier.

15. The display as claimed in claim 10, wherein each pixel circuit comprises:

a transistor for controlling the driving current from the column driver such that the electroluminescent device emits light based on the driving current.

16. The display as claimed in claim 15, wherein the electroluminescent device is an organic light emitting diode.

17. The display as claimed in claim 15, wherein the read block comprises:

a plurality of read block elements, each read block element comprising;

a switch for electrically connecting and disconnecting the read block element to a pixel circuit of the plurality pixel circuits;

an operational amplifier electrically connected to the switch; and

a read capacitor connected in parallel with the operational amplifier.

18. The display as claimed in claim 10, wherein the control block operates the display in one of at least two modes:

a display mode wherein the control block controls the current driver for driving the plurality of pixel circuits with a current based on a display signal and the current correction factor, to emit light; and

a read mode wherein the control block controls the read block to determine the electroluminescent device capacitance of a pixel circuit of the plurality of pixel circuits, the control block determining the current correction factor based on the electroluminescent device capacitance of the pixel circuit.

19. The system as claimed in claim 10, further comprising:

a base pixel including an electroluminescent device not driven by display currents,

wherein the control block operates the read block to determine the capacitance of the electroluminescent device in the base pixel and determine the current correction factor for a pixel in the array according to a difference between the capacitances of the electroluminescent devices in the pixel and the base pixel.

20. The system as claimed in claim 10, wherein the electroluminescent devices in the array of pixel circuits are organic light emitting diodes.

21. A method of determining the capacitance of an electroluminescent device comprising:

charging a capacitance of the electroluminescent device to a first voltage V1;

charging a parasitic capacitance to a second voltage V2;

electrically connecting the parasitic capacitance and the electroluminescent device capacitance in parallel; and

measuring a voltage change, Δ

V, across a read capacitor of capacitance C_read, and

evaluating the electroluminescent device capacitance according to a function including the first and second voltages, the capacitance of the read capacitor, and the measured voltage change.

22. The method as claimed in claim 21, where the evaluating includes determining that the capacitance of the electroluminescent device is equal to:

23. A method of compensating the aging degradation of a display including a plurality of pixel circuits arranged in an array, each of the pixel circuits including an electroluminescent device, the method comprising:

determining a capacitance of the electroluminescent device in each of the plurality of pixel circuits;

correlating the determined capacitance of the electroluminescent device to a current correction factor for each of the plurality of pixel circuits;

compensating a drive current for the electroluminescent device in each of the plurality of pixel circuits according to the correlated current correction factor for each of the plurality of pixel circuits; and

driving each of the electroluminescent devices with the compensated drive currents.

24. A system for determining a capacitance of electroluminescent devices in a plurality of pixel circuits, the pixel circuits arranged in an array to form a display, the system comprising:

a plurality of read block elements, each read block element comprising;

a switch for electrically connecting and disconnecting the read block element to an electroluminescent device in a pixel circuit of the plurality of pixel circuits;

an operational amplifier electrically connected to the switch; and

a read capacitor connected in parallel with the operational amplifier such that a voltage change across the read capacitor while the voltage of the electroluminescent device changes from a first voltage to a second voltage corresponds to the capacitance of the electroluminescent device in the pixel circuit; and

a controller operated to apply the first voltage and the second voltage to the electroluminescent device and determine, based on the first and second voltages and the change in voltage across the read capacitor, the capacitance of the electroluminescent device in the pixel circuit.

25. The system as claimed in claim 24, wherein the read elements have a parasitic capacitance connected in parallel to the capacitance of the electroluminescent device in the pixel circuit when the switch electrically connects the read element to the pixel circuit.

26. The system as claimed in claim 25, wherein the switch is a transistor.

27. The system as claimed in claim 24, wherein the controller is further configured to determine a current correction factor for the pixel circuit, based on the determined capacitance of the electroluminescent device, and to adjust a driving current for the pixel circuit based on the determined current correction factor.

28. The system as claimed in claim 27, wherein the current correction factor is based on an estimated degradation of the electroluminescent device that correlates with the determined capacitance of the electroluminescent device.

29. The system as claimed in claim 27, wherein the current correction factor is extrapolated from a linear or polynomial relationship according to a plurality of established current correction factors associated with electroluminescent device capacitance values.

30. The system as claimed in claim 27, wherein the current correction factor is extrapolated from a look-up table of empirically determined current correction factors associated with electroluminescent device capacitance values.

31. The system as claimed in claim 24, wherein the controller is further configured to:

determine the capacitance of the electroluminescent device in each of the plurality of pixel circuits; and

operate the plurality of pixel circuits to emit light according to desired luminance information adjusted according to feedback information based on the respective determined capacitances of the electroluminescent elements in each of the plurality of pixel circuits.

32. The system as claimed in claim 31, wherein the controller is further configured to:

update the determined capacitances of the electroluminescent devices in each of the plurality of pixel circuits more than once during a lifetime of the display so as to account for ongoing aging degradation during the lifetime of the display.

33. The system as claimed in claim 32, wherein the controller is further configured to update the determined capacitances at a regular interval.

34. The system as claimed in claim 24, wherein the electroluminescent devices are organic light emitting diodes.

35. A method of compensating for luminance degradation due to aging of a display pixel having an electroluminescent device, the method comprising:

determining a capacitance of the electroluminescent device;

determining a capacitance of an electroluminescent device in one or more base pixels not driven by display currents;

correlating a difference between the determined capacitances of the electroluminescent devices to a current correction factor for the electroluminescent device in the display pixel;

compensating a drive current for the electroluminescent device in the display pixel according to the correlated current correction factor; and

driving the electroluminescent device in the display pixel with the compensated drive current.

36. The method as claimed in claim 35, wherein the display pixel is one of a plurality of pixels arranged in an array to form a display.