Charged-based compensation and parameter extraction in AMOLED displays
A system reads a desired circuit parameter from a pixel circuit that includes a light emitting device, a drive device to provide a programmable drive current to the light emitting device, a programming input, and a storage device to store a programming signal. One embodiment of the extraction system turns off the drive device and supplies a predetermined voltage from an external source to the light emitting device, discharges the light emitting device until the light emitting device turns off, and then reads the voltage on the light emitting device while that device is turned off. The voltages on the light emitting devices in a plurality of pixel circuits may be read via the same external line, at different times. In-pixel, charge-based compensation schemes are also discussed, which can be used with the external parameter extraction implementations.
1. A method of driving a pixel circuit, the pixel circuit including a light emitting device, a drive transistor to provide a programmable drive current to the light emitting device, a programming input, and a storage device to store a programming signal, the method comprising the steps of:
applying a reference voltage from a first line to a storage device in the pixel circuit; and
redirecting to a second line current from a first node between the drive transistor and the light emitting device to charge the storage device based on the reference voltage to self-compensate for a variation or aging of the drive transistor or the light emitting device or both in the pixel circuit.
2. The method of claim 1 , further comprising the steps of:
extracting, using a circuit external to the pixel circuit, a circuit parameter from the pixel circuit; and
subsequently driving the pixel circuit using programming information that has been compensated based on at least the extracted circuit parameter.
3. The method of claim 2 , wherein extracting the circuit parameter comprises reading a voltage or a current of at least the drive transistor or at least the light emitting device or at least the drive transistor and the light emitting device.
4. The method of claim 1 , wherein the storage device is a capacitor and is connected across a gate and a first terminal of the drive transistor.
5. The method of claim 4 , wherein a second terminal of the drive transistor is directly connected to the light emitting device.
6. The method of claim 1 , where the pixel circuit internally compensates for variations in a threshold voltage of the drive transistor by charging a second node connected to the drive transistor to the reference voltage and discharging through the drive transistor to the first node to store a charge in the storage device indicative of the threshold voltage of the drive transistor.
7. The method of claim 1 , further comprising supplying a programming voltage to the storage device such that at least some of the programming voltage is used to cause the light emitting device to emit light according to the at least some of the programming voltage.
8. The method of claim 3 , wherein extracting the circuit parameter comprises reading a voltage or a current of at least the drive transistor, wherein a voltage of a monitor line connected to a second node connected to the drive transistor for reading the voltage or current of at least the drive transistor is held at a low enough magnitude to keep the light emitting device off.
9. The method of claim 3 , wherein extracting the circuit parameter comprises reading a voltage or a current of at least the light emitting device, wherein a voltage applied to the gate of the drive transistor is held at a high enough magnitude so that the drive transistor acts as a switch enabling the reading of the voltage or current of at least the light emitting device over a monitor line connected to a second node connected to the drive transistor.
10. The method of claim 3 , wherein extracting the circuit parameter comprises reading the voltage or the current of at least the drive transistor or at least the light emitting device or at least the drive transistor and the light emitting device over the second line.
11. A pixel circuit having a light emitting device, comprising: a drive transistor for providing driving current to the light emitting device; a storage device for storing programming information to cause the light emitting device to emit light according to the programming information via the drive transistor; a first transistor connected between the storage device and a first line for applying a reference voltage from the first line to the storage device; and a second transistor connected between a second line and a first node between the drive transistor and the light emitting device for redirecting to a second line current from the first node for charging the storage device based on the reference voltage to self-compensate for a variation or aging of the drive transistor or the light emitting device or both in the pixel circuit.
12. The pixel circuit of claim 11 , further comprising a third transistor connected between the pixel circuit and a monitor line for extracting a circuit parameter of the pixel circuit and storing the circuit parameter externally to the pixel circuit, wherein the pixel circuit is compensated externally to the pixel circuit for variations or aging of the pixel circuit with use of the extracted circuit parameter.
13. The pixel circuit of claim 12 , wherein the third transistor for extracting the circuit parameter is for reading a voltage or a current of at least the drive transistor or at least the light emitting device or at least the drive transistor and the light emitting device.
14. The pixel circuit of claim 11 , wherein the storage device comprises a capacitor and is connected across a gate and a first terminal of the drive transistor.
15. The pixel circuit of claim 14 , wherein a second terminal of the drive transistor is directly connected to the light emitting device.
16. The pixel circuit of claim 11 , wherein the pixel internally compensates for variations in a threshold voltage of the drive transistor by charging a second node connected to the drive transistor to the reference voltage and discharging through the drive transistor to the first node to store a charge in the storage device indicative of the threshold voltage of the drive transistor.
17. The pixel circuit of claim 11 , further comprising a fourth transistor connected between the storage device and a data line for supplying a programming voltage to the storage device such that at least some of the programming voltage is used to cause the light emitting device to emit light according to the at least some of the programming voltage.
18. The pixel circuit of claim 13 , wherein the third transistor for extracting the circuit parameter is for reading a voltage or a current of at least the drive transistor, wherein a voltage of a monitor line connected to a second node connected to the drive transistor for reading the voltage or current of at least the drive transistor is held at a low enough magnitude to keep the light emitting device off.
19. The pixel circuit of claim 13 , wherein the third transistor for extracting the circuit parameter is for reading a voltage or a current of at least the light emitting device, wherein a voltage applied to a gate of the drive transistor is held at a high enough magnitude so that the drive transistor acts as a switch enabling the reading of the voltage or current of at least the light emitting device over a monitor line connected to a second node connected to the drive transistor.
20. The pixel circuit of claim 13 , wherein the third transistor for extracting the circuit parameter is for reading the voltage or the current of at least the drive transistor or at least the light emitting device or at least the drive transistor and the light emitting device over the second line.