Display device that sets a value of a power supply voltage to compensate for changes in light emitting element I/V characteristics

1. A display device comprising:

a plurality of display pixels that are arranged two-dimensionally and have first light emitting elements;

at least one adjustment pixel that has a reference light emitting element;

a drive section configured to drive the plurality of display pixels to emit light at respective gradations corresponding to respective video signals input to the plurality of display pixels and to drive the at least one adjustment pixel to emit light at a gradation corresponding to an adjustment-pixel signal; and

a processing section configured to set a value of the adjustment-pixel signal during a given field period to be no larger than a largest value of the video signals input to the plurality of display pixels during the given field period,

wherein the drive section is configured to apply a display power-supply voltage to the plurality of display pixels when they emit light,

the display power-supply voltage is variably set to a value depending on a voltage variation of the reference light emitting element when the reference light emitting element emits light, and

wherein the processing section is configured to, during the given field period, update a value of the adjustment-pixel signal every horizontal scanning period such that the value of the adjustment-pixel in the horizontal scanning period in which the update is performed is no larger than a largest value of the video signals input to the plurality of display pixels from the start of the given field period to the horizontal scanning period in which the update is performed.

2. The display device of claim 1 ,

wherein the drive section is configured to determine the voltage variation of the reference light emitting element by monitoring a voltage value of the reference light emitting element only when the reference light emitting element emits light.

3. The display device of claim 2 ,

wherein a monitoring node of the adjustment pixel, which is used for monitoring a voltage value of the reference light emitting element, is connected to the drive section via a switching element, and

the drive section applies a control signal to the switching element so that the switching element is on only when the reference light emitting element emits light.

4. The display device of claim 1 ,

wherein the drive section is configured to determine the voltage variation of the reference light emitting element by comparing a voltage value of the reference light emitting element when the reference light emitting element emits light to a reference voltage value.

5. The display device of claim 4 ,

wherein the driving section is configured to generate the display power-supply voltage by adding the voltage variation of the reference light emitting element to a predetermined voltage value corresponding to the adjustment-pixel signal.

6. The display device of claim 5 ,

wherein, when the display power-supply voltage is designated as V cc , then the following equation is satisfied:

V cc =V sig — adj −V TH +V EL +ΔV,

where V sig — adj is a present value of the adjustment-pixel signal, V TH is a threshold voltage of a drive transistor of the adjustment pixel, V EL is a predetermined value equal to an initial voltage of the reference light emitting element when the adjustment pixel is driven to emit light with an adjustment-pixel signal having a value equal to V sig — adj , and ΔV is the voltage variation of the reference light emitting element.

7. The display device of claim 1 ,

wherein an adjustment power-supply voltage of a fixed value is applied to the adjustment pixel when it emits light.

8. The display device of claim 1 ,

wherein each display pixel has a first transistor controlling electric current flowing into the first light emitting element, and a second transistor writing a signal voltage corresponding to the video signal to a gate of the first transistor,

a first current electrode of the first transistor is connected to the first light emitting element, and

a second current electrode of the first transistor is connected to a power supply line.

9. The display device of claim 1 ,

wherein the display pixels are disposed in a display region and the at least one adjustment pixel is disposed in a non-display region.

10. A display device comprising:

a plurality of display pixels that are arranged two-dimensionally and each have a first light emitting element and a drive transistor; and

at least one adjustment pixel that has a reference light emitting element;

a drive section configured to drive the plurality of display pixels to emit light at respective gradations corresponding to respective video signals input to the plurality of display pixels,

wherein the drive section is configured to apply a display power-supply voltage to the plurality of display pixels when they emit light,

a processing section configured to set a value of an adjustment-pixel signal during a given field period to be no larger than a largest value of the video signals input to the plurality of display pixels during the given field period, and

a value of the display power-supply voltage is variably set such that, during a given field period, the value of the display power-supply voltage is a smallest value that will ensure that, for each of the plurality of display pixels an operation point of the light emitting element thereof during the given field period is within a saturation region of the drive transistor thereof,

wherein the drive section is configured to

drive the adjustment pixel to emit light at a gradation corresponding to the adjustment-pixel signal,

determine a voltage variation of the reference light emitting element when it emits light, and

generate the display power-supply voltage by adding the voltage variation of the reference light emitting element to a predetermined voltage value that corresponds to the adjustment-pixel signal, and

wherein the processing section is configured to,

during the given field period, update a value of the adjustment-pixel signal every horizontal scanning period such that the value of the adjustment-pixel in the horizontal scanning period in which the update is performed is no larger than a largest value of the video signals input to the plurality of display pixels from the start of the given field period to the horizontal scanning period in which the update is performed.

11. The display device of claim 10 , wherein when the display power-supply voltage is designated as V CC , then the following equation is satisfied:

V CC =V DS +V EL +ΔV,

where V DS is a predetermined value equal to a minimum potential that is within a saturation region of a drive transistor of the adjustment pixel when it is driven to emit light with an adjustment-pixel signal having a value equal to a present value of the adjustment-pixel signal, V EL is a predetermined value equal to an initial voltage of the reference light emitting element when the adjustment pixel is driven to emit light with an adjustment-pixel signal having a value equal to a present value of the adjustment-pixel signal, and ΔV is the voltage variation of the reference light emitting element.

12. The display device of claim 11 ,

wherein V DS equals a present value of the adjustment-pixel signal minus a threshold voltage of a drive transistor of the adjustment pixel.

13. A method of driving a display device comprising a plurality of display pixels that are arranged two-dimensionally and have first light emitting elements and at least one adjustment pixel that has a reference light emitting element, the method comprising:

causing the plurality of display pixels to emit light at respective gradations corresponding to respective video signals input to the plurality of display pixels and causing the at least one adjustment pixel to emit light at a gradation corresponding to an adjustment-pixel signal;

setting a value of the adjustment-pixel signal during a given field period to be no larger than a largest value of the video signals input to the plurality of display pixels during the given field period;

applying a display power-supply voltage to the plurality of display pixels when they emit light;

variably setting the display power-supply voltage to a value depending on a voltage variation of the reference light emitting element when the reference light emitting element emits light, and

wherein during the given field period, updating a value of the adjustment-pixel signal every horizontal scanning period such that the value of the adjustment-pixel in the horizontal scanning period in which the update is performed is no larger than a largest value of the video signals input to the plurality of display pixels from the start of the given field period to the horizontal scanning period in which the update is performed.