System and methods for power conservation for AMOLED pixel drivers

• US 9,093,028 B2
• Filed: 12/02/2010
• Issued: 07/28/2015
• Est. Priority Date: 12/06/2009
• Status: Active Grant

• Current Assignee: IGNIS Innovation Incorporated
• Original Assignee: IGNIS Innovation Incorporated
• Inventors: Chaji, Gholamreza
• Primary Examiner(s): Sim, Yong H
• Assistant Examiner(s): Lin, Chun-Nan
• Application Number:
  US12/958,938
• Publication Number:
  US 20110134157A1
• Time in Patent Office: 1,699 Days
• Field of Search: 345/690, 345/691, 345/692, 345/693, 345/694, 345/695, 345/696, 345/697, 345/698, 345/699, 315/169.3
• US Class Current: 1/1
• CPC Class Codes: G09G 2310/0254 Control of polarity reversal in general, other than for liquid crystal displays
• CPC Class Codes: G09G 2310/066 Waveforms comprising a gently increasing or decreasing portion, e.g. ramp
• 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 2320/043 Preventing or counteracting the effects of ageing
• CPC Class Codes: G09G 2320/0673 for control of gamma adjustment, e.g. selecting another gamma curve
• CPC Class Codes: G09G 2330/021 Power management, e.g. power saving
• CPC Class Codes: G09G 2360/16 Calculation or use of calculated indices related to luminance levels in display data
• CPC Class Codes: G09G 3/3233 with pixel circuitry controlling the current through the light-emitting element
• CPC Class Codes: G09G 3/3258 with pixel circuitry controlling the voltage across the light-emitting element
• 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 circuit and method to improve energy conservation of an AMOLED display. The display includes a matrix of pixels each having an organic light emitting device coupled to a drive transistor. The brightness of the light emitting device is controlled by a programming voltage applied to the gate of the drive transistor. The supply voltage to the drive transistor is adjusted to different levels based on the required brightness of the pixel. Since the drive transistor operates in saturation mode, especially when maximum brightness is desired, the supply voltage may be reduced while maintaining the same brightness level.

Claims

1. A circuit for a pixel of a display, the circuit comprising:

a controllable supply voltage source outputting a supply voltage;

an organic light emitting device emitting light having a brightness level as a function of current flow;

a drive transistor having one terminal coupled to the controllable supply voltage source and a second terminal coupled to the organic light emitting device, the drive transistor having a gate input controlled by a voltage input to determine the current flow through the organic light emitting device; and

wherein the supply voltage output by the controllable supply voltage source,

operate said drive transistor in a saturation region during a normal operation mode when said gate input is above a predetermined threshold voltage, and operate said drive transistor in a linear operation region by reducing the gate input voltage when the voltage input of a desired brightness is below a predetermined threshold thereby reducing an effect of a voltage drop across the drive transistor.

2. The circuit of claim 1, wherein the controllable supply voltage source maintains a substantially constant voltage when the voltage input is indicative of a desired brightness below the predetermined threshold.

3. The circuit of claim 1, further comprising:

a pair of bias transistors coupled between the gate and the source of the drive transistor, one of the pair of bias transistors being coupled to a control signal input to bias the drive transistor to compensate for voltage drop and cross talk.

4. The circuit of claim 1 in which the input voltage is modified to achieve a desired brightness when the supply voltage is reduced.

5. The circuit of claim 1, wherein the supply voltage output by the controllable supply voltage source is maintained substantially constant when the voltage input is indicative of a desired brightness below the predetermined threshold.

6. An active matrix organic light emitting device display, comprising:

an adjustable supply voltage source;

a plurality of pixels, each coupled to the adjustable supply voltage source, each pixel including;

an organic light emitting device; and

a drive transistor having one terminal coupled to the organic light emitting device and a second terminal coupled to the adjustable supply voltage source, the drive transistor having a gate controlled by a voltage input to determine current flow through the organic light emitting device;

a plurality of voltage inputs coupled to the gates of the drive transistors of the plurality of pixels, the voltage inputs outputting an input voltage indicative of a desired brightness of each of the plurality of pixels; and

a supply voltage controller coupled to the adjustable supply voltage source to regulate the level of a supply voltage supplied to each of the drive transistors, the supply voltage controller being configured to

operate said drive transistor in a saturation region during a normal operation mode when said gate input is above a predetermined threshold voltage, and operate said drive transistor in a linear operation region by reducing the gate input voltage when the voltage input of a desired brightness is below a predetermined threshold thereby reducing an effect of a voltage drop across the drive transistor.

7. The display of claim 6, wherein the plurality of pixels is a row of pixels in the display.

8. The display of claim 6, wherein the plurality of pixels is a column of pixels in the display.

9. The display of claim 6, wherein the controllable supply voltage maintains a substantially constant voltage when the voltage input is indicative of a desired brightness below the predetermined threshold.

10. A method of conserving energy in an AMOLED display having pixels including a drive transistor and an organic light emitting device that has a controllable brightness, the method comprising:

supplying a controllable supply voltage to one terminal of the drive transistor;

supplying an input voltage to the gate of the drive transistor to control current flow through the organic light emitting device and thus control brightness of the organic light emitting device;

maintaining a substantially constant supply voltage when said controllable brightness is below a predetermined threshold to operate said drive transistor in the saturated operating region in normal operation of a pixel during a display;

reducing said supply voltage when said controllable brightness is above said predetermined threshold; and

reducing the supply voltage to the drive transistor to a lower level based on the brightness corresponding to said input voltage, wherein the adjusted supply voltage output shifts the drive transistor towards a linear region of operation, thereby reducing an effect of a voltage drop across the drive transistor.

11. The method of claim 10, wherein the brightness is controlled based on video data to the display.

12. A circuit for a pixel of a display, the circuit comprising:

a controllable supply voltage source outputting a supply voltage;

an organic light emitting device emitting light having a brightness level as a function of current flow;

a drive transistor having one terminal coupled to the controllable supply voltage source and a second terminal coupled to the organic light emitting device, the drive transistor having a gate input controlled by a voltage input to determine the current flow through the organic light emitting device; and

wherein the supply voltage output by the controllable supply voltage source

operates said drive transistor in a saturation region during a normal operation mode when said gate input is above a predetermined threshold voltage, and operates said drive transistor in a linear operation region by reducing the gate input voltage when the voltage input of a desired brightness is below a predetermined threshold thereby reducing an effect of a voltage drop across the drive transistor.

13. An active matrix organic light emitting device display, comprising:

an adjustable supply voltage source;

a plurality of pixels, each coupled to the adjustable supply voltage source, each pixel including;

an organic light emitting device; and

a drive transistor having one terminal coupled to the organic light emitting device and a second terminal coupled to the adjustable supply voltage source, the drive transistor having a gate controlled by a voltage input to determine current flow through the organic light emitting device;

a plurality of voltage inputs coupled to the gates of the drive transistors of the plurality of pixels, the voltage inputs outputting an input voltage indicative of a desired brightness of each of the plurality of pixels;

a supply voltage controller coupled to the adjustable voltage source to regulate the level of a supply voltage supplied to each of the drive transistors, the supply voltage controller being configured to

operate said drive transistor in a saturation region during a normal operation mode when said gate input is above a predetermined threshold voltage, and operate said drive transistor in a linear operation region by reducing the gate input voltage when the voltage input of a desired brightness is below a predetermined threshold thereby reducing an effect of a voltage drop across the drive transistor.

14. The display of claim 13, wherein the supply voltage controller is further configured to maintain the supply voltage substantially constant when the voltage inputs are indicative of images with non-sparse grayscales.

15. A method of conserving energy in an AMOLED display having pixels including a drive transistor coupled to an organic light emitting device that has a controllable brightness, the method comprising:

supplying a controllable supply voltage to one terminal of the drive transistor,

supplying an input voltage to the gate of the drive transistor to control current flow through the organic light emitting device and thus control brightness of the organic light emitting device, and

operating said drive transistor in a saturation region during a normal operation mode when said gate input is above a predetermined threshold voltage, and operating said drive transistor in a linear operation region by reducing the gate input voltage when the voltage input of a desired brightness is below a predetermined threshold thereby reducing an effect of a voltage drop across the drive transistor.