Display device and method for driving same
In a pixel circuit, during a period during which an organic EL element is not emitting light, a transistor is in an “on” state and a reverse-direction voltage determined by a reverse-direction current that depends on the degree to which degradation of the organic EL element has progressed is written to a capacitor. The transistor then turns off, another transistor turns on, and a compensating current that depends on the reverse-direction voltage flows from another capacitor towards a reverse-biasing power-supply line, causing a drive voltage maintained by the capacitor to change by a compensating voltage change. This makes it possible to minimize decreases in the emission luminance of an electro-optical element such as an organic EL element due to degradation thereof over time.
1. An active matrix display device, comprising:
a plurality of data wiring lines supplying data signals;
a plurality of scan wiring lines that are each selectively driven;
a plurality of pixel circuits provided at respective intersections between the plurality of data wiring lines and the plurality of scan wiring lines,
wherein each of the pixel circuits includes:
an electrooptical element provided between a first power source line that supplies a first power source potential and a second power source line that supplies a second power source potential; and
a driving unit that controls a current flowing through the electrooptical element, the driving unit including a driving transistor provided between the first power source line and the second power source line and connected in series to the electrooptical element, and a driving capacitance element that stores a drive voltage for controlling the driving transistor;
an input unit that supplies to the driving unit a voltage of the data signal supplied by a corresponding data wiring line in response to a corresponding scan wiring line being selected;
a first compensation unit connected to a reverse bias control line that supplies a control potential at least during a first prescribed period, the first compensation unit causing the electrooptical element to be reverse biased between the second power source line and the reverse bias control line during the first prescribed period, receiving a resultant reverse direction current flowing through the electrooptical element, and supplying a compensation signal based on the reverse direction current to the driving capacitance element; and
a light emission control transistor provided between the first power source line and the electrooptical element, the light emission control transistor being in an off state during a second prescribed period that includes the first prescribed period, and
wherein the driving unit determines a drive voltage for controlling the driving transistor in accordance with at least a voltage of the data signal and the compensation signal, the driving unit causing the electrooptical element to emit light in accordance with the determined drive voltage after the second prescribed period ends.
2. The display device according to claim 1 ,
wherein the compensation signal is a compensation current determined on the basis of the reverse direction current, the compensation current flowing between the driving capacitance element and the reverse bias control line during the first prescribed period, and
wherein the driving unit determines the drive voltage based on at least a voltage of the data signal and a first voltage based on the compensation current.
3. The display device according to claim 2 , wherein the driving capacitance element is provided between the control terminal and the first conductive terminal of the driving transistor.
4. The display device according to claim 3 ,
wherein the first compensation unit includes:
a capacitance element for controlling the compensation current provided between the electrooptical element and the reverse bias control line, said capacitance element being supplied the reverse direction current flowing through the electrooptical element between the second power source line and the reverse bias control line during a first compensation period in the first prescribed period, and storing a second voltage based on the reverse direction current; and
a first transistor for controlling the compensation current based on the second voltage stored in the capacitance element for controlling the compensation current, said first transistor being provided between the driving capacitance element and the reverse bias control line, said first transistor causing the compensation current to flow between the driving capacitance element and the reverse bias control line during a second compensation period during the first prescribed period and after the first compensation period.
5. The display device according to claim 4 , wherein the capacitance element for controlling the compensation current is provided between a control terminal of the first transistor for controlling the compensation current and a conductive terminal conductive terminal of said first transistor located towards the reverse bias control line.
6. The display device according to claim 5 , wherein the first compensation unit further includes a second transistor for controlling the compensation current, said second transistor being provided between the driving capacitance element and the first transistor for controlling the compensation current and being turned on during the second compensation period.
7. The display device according to claim 6 , wherein the first compensation unit further includes a transistor for supplying a reverse direction current provided between the electrooptical element and the capacitance element for controlling the compensation current, said transistor being turned on during the first compensation period.
8. The display device according to claim 7 , wherein the pixel circuit further includes a first compensation initializing transistor provided between terminals of the capacitance element for controlling the compensation current, the first compensation initializing transistor being turned on during the second prescribed period and before or after the first prescribed period.
9. The display device according to claim 8 , wherein the first compensation initializing transistor is turned on immediately after the first prescribed period.
10. The display device according to claim 3 ,
wherein the first compensation unit includes:
a resistor through which the reverse direction current flows; and
a compensation current control transistor that is provided between the driving capacitance element and the reverse bias control line and that allows through a compensation current based on a second voltage that occurs between terminals of the resistor.
11. The display device according to claim 10 , wherein the first compensation unit further includes a transistor for supplying a reverse direction current provided between the electrooptical element and the resistor, said transistor being turned on during the first prescribed period.
12. The display device according to claim 1 ,
wherein the pixel circuit further includes a second compensation transistor provided between a control terminal and a second conductive terminal of the driving transistor, the second compensation transistor being turned on before the compensation signal is supplied to the driving capacitance element during the first prescribed period, and
wherein the input unit includes:
an input transistor having a control terminal connected to a corresponding scan wiring line, and a first conductive terminal connected to a corresponding data wiring line; and
an input capacitance element provided between a second conductive terminal of the input transistor and the driving capacitance element.
13. The display device according to claim 12 , wherein the pixel circuit further includes a second compensation initializing transistor provided between the driving capacitance element and the reverse bias control line, the second compensation initializing transistor being turned on during the second prescribed period and before the first prescribed period.
14. The display device according to claim 3 , wherein the first conductive terminal of the driving transistor is located towards the first power source line.
15. The display device according to claim 1 ,
wherein the reverse bias control line supplies the control potential during the second prescribed period, and
wherein a control terminal of the light emission control transistor is connected to the reverse bias control line.
16. A method of driving an active matrix display device including: a plurality of data wiring lines that each supply a data signal; a plurality of scan wiring lines that are each selectively driven; and a plurality of pixel circuits provided at respective intersections between the plurality of data wiring lines and the plurality of scan wiring lines, each of the pixel circuits including: an electrooptical element provided between a first power source line that supplies a first power source potential and a second power source line that supplies a second power source potential; and a driving unit for controlling a current flowing through the electrooptical element, the driving unit having a driving transistor provided between the first power source line and the second power source line and connected in series to the electrooptical element, and a driving capacitance element that stores a drive voltage for controlling the driving transistor, the method comprising:
supplying to the driving unit a voltage of the data signal supplied by a corresponding data wiring line in response to a corresponding scan wiring line being selected;
supplying to the driving capacitance element a compensation signal based on a reverse direction current flowing through the electrooptical element between the second power source line and a reverse bias control line that supplies a control potential at least during a first prescribed period;
determining the drive voltage based on at least a voltage of the data signal and the compensation signal;
controlling a light emission timing of the electrooptical element to block current flowing between the first power source line and the electrooptical element during a second prescribed period that includes the first prescribed period; and
causing the electrooptical element to emit light in accordance with the determined drive voltage after the second prescribed period ends.