Active matrix display devices

• US 20060256048A1
• Filed: 08/29/2004
• Published: 11/16/2006
• Est. Priority Date: 09/02/2003
• Status: Active Grant

• Current Assignee: Beijing Xiaomi Mobile Software Co., Ltd. (Xiaomi Corp.)
• Original Assignee: Koninklijke Philips Electronics N.V. (Koninklijke Philips N.V.)
• Inventors: Fish, David A., Hector, Jason
• Granted Patent: US 9,214,107 B2
• Time in Patent Office: Days
• US Class Current: 345/81
• CPC Class Codes: G09G 2300/0417 Special arrangements specific to the use of low carrier mobility technology
• CPC Class Codes: G09G 2300/0809 Several active elements per pixel in active matrix panels
• 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 2310/0251 Precharge or discharge of pixel before applying new pixel voltage
• CPC Class Codes: G09G 2310/066 Waveforms comprising a gently increasing or decreasing portion, e.g. ramp
• CPC Class Codes: G09G 2320/0295 by monitoring each display pixel
• 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 2360/148 the light being detected by light detection means within each pixel
• CPC Class Codes: G09G 3/2014 by modulation of the duration of a single pulse during which the logic level remains constant
• CPC Class Codes: G09G 3/3233 with pixel circuitry controlling the current through the light-emitting element

Abstract

An active matrix LED display has a light-dependent device for detecting the brightness of the display element and threshold voltage measurement circuitry for measuring a threshold voltage of a pixel the drive transistor. Compensation for ageing of the display element is thus provided by an optical feedback path, and compensation for drive transistor threshold variations is provided by measurement of the threshold voltage. This provides a reliable compensation scheme for the threshold voltage variations, whilst also providing ageing compensation.

Claims

1. An active matrix display device comprising an array of display pixels, each pixel comprising:

a current-driven light emitting display element (2);

a drive transistor (22) for driving a current through the display element (2);

a storage capacitor (24) for storing a pixel drive voltage to be used for addressing the drive transistor (22);

a light-dependent device (27) for detecting the brightness of the display element; and

compensation circuitry for generating a compensation voltage for combination with pixel data voltages to derive the pixel drive voltage, and for applying the pixel drive voltage such as to compensate for threshold voltage variations of the drive transistor (22) and ageing of the display element (2).

2. A device as claimed in claim 1, wherein the compensation circuitry comprises threshold voltage measurement circuitry (50,52,54) for measuring a threshold voltage of the drive transistor (22) for combination with a pixel data signal to derive the pixel drive voltage.

3. A device as claimed in claim 2, further comprising a discharge transistor (28) for discharging the storage capacitor (24) thereby to switch off the drive transistor.

4. A device as claimed in claim 3, wherein the light-dependent device (27) controls the timing of the operation of the discharge transistor (28) by varying the gate voltage applied to the discharge transistor (28) in dependence on the light output of the display element (2).

5. A device as claimed in claim 4, wherein the light-dependent device (27) controls the timing of the switching of the discharge transistor (28) from an off to an on state.

6. A device as claimed in claim 3, 4, or 5, wherein a timing switch (34) is provided between the gate of the discharge transistor (28) and the light dependent device (27).

7. A device as claimed in any preceding claim, wherein each pixel further comprises a sense transistor (42) connected between the source of the drive transistor (22) and a sense line (40).

8. A device as claimed in claim 1, wherein the light dependent device (27) is connected in series with a switch (62) between the gate and source of the drive transistor (22).

9. A device as claimed in claim 8, wherein the storage capacitor (24) is connected between the gate and source of the drive transistor (22).

10. A device as claimed in claim 8 or 9, wherein the compensation circuitry comprises means (52) for applying a ramped voltage input to the pixel, and means (42,50) for measuring the light dependent device output thereby to determine the voltage input of the ramp corresponding to a predetermined display element brightness.

11. A device as claimed in claim 10, wherein the determined voltage input of the ramp is used as a compensation voltage which compensates for the drive transistor threshold voltage and the display element ageing.

12. A device as claimed in any one of claims 8 to 11, wherein each pixel further comprises a sense transistor (42) connected between the source of the drive transistor (22) and a sense line (40).

13. A device as claimed in claim 1, wherein the light dependent device (27) is connected in series with a sense transistor (42) between a power supply line (26) and a sense line.

14. A device as claimed in claim 13, wherein the compensation circuitry comprises means (52) for applying a predetermined voltage as input to the pixel, and means (42,50) for measuring the light dependent device output thereby to determine the light output corresponding to the predetermined voltage input.

15. A device as claimed in claim 14, wherein the determined light output is used to derive a compensation scheme which compensates for the drive transistor threshold voltage and the display element ageing.

16. A device as claimed in any preceding claim, wherein the light dependent device (27) comprises a discharge photodiode.

17. A device as claimed in any preceding claim, wherein each pixel further comprises an address transistor (16) connected between a data signal line (6) and an input to the pixel.

18. A device as claimed in any preceding claim, wherein the drive transistor (22) is connected between a power supply line (26) and the display element (2).

19. A device as claimed in any preceding claim, wherein the transistors of each pixel comprise amorphous silicon n-type transistors.

20. A device as claimed in any preceding claim, wherein the current-driven light emitting display element (2) comprises an electroluminescent display element.

21. A method of driving an active matrix display device comprising an array of display pixels each comprising a drive transistor (22) and a current-driven light emitting display element (2), the method comprising, for each addressing of the pixel:

deriving a pixel drive voltage which takes into account at least the threshold voltage of the drive transistor;

sensing the light output of the display element (2); and

deriving a pixel drive scheme which is dependent on the threshold voltage and the light output, and applying the pixel drive scheme to the pixel.

22. A method as claimed in claim 21, wherein the pixel drive scheme comprises, during an addressing phase, deriving a pixel drive voltage which takes into account the threshold voltage, and during a driving phase, switching off the drive transistor when the integrated light output reaches a threshold.

23. A method as claimed in claim 21, wherein the pixel drive scheme comprises, during an addressing phase, deriving a pixel drive voltage which takes into account the threshold voltage and the light output characteristics of the display element.

24. A method as claimed in claim 23, wherein the light output is measured for predetermined drive conditions.

25. A method as claimed in claim 23, wherein the drive conditions are varied until a predetermined light output is obtained.