Stable driving scheme for active matrix displays
A method and system for operating a pixel array having at least one pixel circuit is provided. The method includes repeating an operation cycle defining a frame period for a pixel circuit, including at each frame period, programming the pixel circuit, driving the pixel circuit, and relaxing a stress effect on the pixel circuit, prior to a next frame period. The system includes a pixel array including a plurality of pixel circuits and a plurality of lines for operation of the plurality of pixel circuits. Each of the pixel circuits includes a light emitting device, a storage capacitor, and a drive circuit connected to the light emitting device and the storage capacitor. The system includes a drive for operating the plurality of lines to repeat an operation cycle having a frame period so that each of the operation cycle comprises a programming cycle, a driving cycle and a relaxing cycle for relaxing a stress on a pixel circuit, prior to a next frame period.
1. A method of operating a pixel array having pixel circuits arranged in rows and columns, each of the pixel circuits including a switch, a select line connected to the switch, a drive transistor coupled to a data line via the switch and to a controllable power supply line, a light emitting device coupled to the drive transistor, and a storage capacitor coupled to the drive transistor, the method comprising:
programming a first of the pixel circuits responsive to driving the select line for the first pixel circuit from a first state to a second state to select the first pixel circuit for programming, the programming including:
during a first operating cycle, adjusting the controllable power supply line for the first pixel circuit to a first voltage while applying on the data line for the first pixel circuit a second voltage of opposite polarity to that of the first voltage;
responsive to the first operating cycle, during a second operating cycle, changing the controllable power supply line to a driving voltage corresponding to a voltage used to drive the light emitting device of the first pixel circuit, the driving voltage being different from the first voltage; and
responsive to the second operating cycle, during a programming cycle, changing the controllable power supply line for the first pixel circuit to a third voltage different from the first voltage and the driving voltage while applying at least a programming voltage on the data line for the first pixel circuit.
2. The method of claim 1 , wherein the switch and the drive transistor are n-type thin-film transistors, and the first voltage is negative, and the second and third voltages are positive.
3. The method of claim 1 , wherein the first voltage is negative to develop a negative voltage at a second node between a source terminal of the drive transistor and a first terminal of the light emitting device, and wherein the second voltage is positive to charge a first node between a gate of the drive transistor and a first terminal of the storage capacitor to the second voltage during the first operating cycle.
4. The method of claim 3 , wherein during the second operating cycle the second node is charged until the drive transistor turns off, thereby causing a threshold voltage of the drive transistor to be stored in the storage capacitor.
5. The method of claim 4 , wherein during the programming cycle, a voltage stored in the storage capacitor includes a threshold voltage of the drive transistor and the programming voltage provided from the data line.
6. The method of claim 1 , wherein the second voltage is smaller than a threshold voltage of the drive transistor in an unstressed state and an ON voltage of the light emitting device in an unstressed state.
7. The method of claim 1 , further comprising:
programming a second of the pixel circuits responsive to driving the select line for the second pixel circuit from a first state to a second state to select the second pixel circuit for programming, the programming including:
during a first operating cycle, adjusting the controllable power supply line for the second pixel circuit to a first voltage while applying on the data line for the second pixel circuit a second voltage of opposite polarity to that of the first voltage;
responsive to the first operating cycle, during a second operating cycle, changing the controllable power supply line to a driving voltage corresponding to a voltage used to drive the light emitting device of the second pixel circuit, the driving voltage being different from the first voltage; and
responsive to the second operating cycle, during a programming cycle, changing the controllable power supply line for the second pixel circuit to a third voltage different from the first voltage and the driving voltage while applying at least a programming voltage on the data line for the second pixel circuit.
8. The method of claim 7 , wherein the first pixel circuit and the second pixel circuits are in different rows of the pixel array.
9. The method of claim 7 , wherein each of the controllable power supply lines for the first and second pixel circuits intersects both of the data lines for the first and second pixel circuits.