Electroluminescent display device to display low brightness uniformly
View Patent ↗An electroluminescent (EL) display device has current-driven pixels and is operable in at least two phases within each frame period. In one phase, one of a first plurality ( 31 ) of analogue drive currents can be driven through EL display element. In another longer phase, one of a second plurality ( 33 ) of analogue drive currents is independently driven through the EL display element. This device combines a time ratio method with an analogue drive scheme. A shorter phase may provide the higher resolution (smaller) increments and one longer phase may provide lower resolution (larger) increments. Low brightness outputs can be achieved with a higher drive current, but over a short duration, which reduces non-uniformity in the pixel output.
1. An electroluminescent (EL) display device comprising an array of display pixels, each display pixel comprising an EL display element and a current source circuit for driving a current through the EL display element in dependence on a data voltage, the display device being operable in at least a first and a second phase within each frame period,
the first phase having a first duration and during which a first one of a first plurality of drive currents can be driven through the EL display element, and
the second phase having a second duration, different to the first duration, and during which a second one of a second plurality of drive currents can be driven through the EL display element, wherein the first and second ones of the respective pluralities of drive currents are independently selectable and at least one of the first and second pluralities of drive currents include more than two drive current levels, and wherein the first plurality of drive currents comprises a number n of drive current levels, including a zero drive level, and wherein a duration of the second phase is approximately n times a duration of the first phase, wherein during said first phase each of the pixel display elements is sequentially driven for said first duration with a corresponding one of said first plurality of drive currents and during said second phase each of the pixel display elements is sequentially driven for said second duration with a corresponding one of said second plurality of drive currents associated with said second phase.
2. A device as claimed in claim 1 , wherein n is 8.
3. A device as claimed in claim 1 , wherein the first plurality of drive currents is the same as the second plurality of drive currents.
4. A device as claimed in claim 1 , wherein the first plurality of drive currents comprises a first number n of drive current levels for providing the lowest n non-zero brightness levels, and the second plurality of drive currents comprises a second number m of non-zero drive current levels for providing the highest m brightness levels, where n+m is the total number of brightness levels.
5. A device as claimed in claim 1 , wherein each pixel comprises a drive transistor, a storage capacitor for storing a gate voltage of the drive transistor and an address transistor for switching a data voltage to the gate of the drive transistor during an addressing phase.
6. A portable electronic device comprising a display device as claimed in claim 1 .
7. A device as claimed in claim 1 , wherein the first plurality of drive currents is the same as the second plurality of drive currents.
8. A device as claimed in claim 1 , wherein each pixel comprises a drive transistor, a storage capacitor for storing a gate voltage of the drive transistor and an address transistor for switching a data voltage to the gate of the drive transistor during an addressing phase.
9. The EL display device of claim 1 , wherein only the first phase is used to provide lowest n brightness levels.
10. The EL display device of claim 9 , wherein only the second phase is used to provide brightness levels higher than the lowest n brightness levels.
11. The EL display device of claim 1 , wherein the first phase is used for higher resolution and the second phase is used for lower resolution.
12. The EL display device of claim 1 , wherein a highest brightness level is provided by turning off drive currents during the first phase, and increasing a peak drive current in the second phase to a higher level than an allowable peak drive current in the first phase.
13. The EL display device of claim 1 , wherein all the display pixels are addressed twice within each frame period.
14. The EL display device of claim 1 , wherein all the display pixels are addressed once row by row during the first phase, and are re-addressed in a same row by row order during the second phase.
15. A method of driving an electroluminescent (EL) display device comprising an array of display pixels, each display pixel comprising an EL display element and a current source circuit for driving a current through the EL display voltage in dependence on a data voltage, the method comprising the acts of:
in a first phase having a first duration, sequentially driving each of said display pixels with a corresponding one of a first plurality of drive currents for said first duration; and
in a second phase having a second duration, different to the first duration, sequentially driving each of said display pixels with a corresponding one of a second plurality of drive currents for said second duration, wherein the first and second ones of the plurality of drive currents are selected to provide a desired combined EL display element output, and at least one of the first and second pluralities of drive currents includes more than two drive current levels, and wherein the plurality of drive currents comprises a number n of drive levels, and wherein a duration of the second phase is approximately n times a duration of the first phase.
16. A method as claimed in claim 15 , wherein n is 8.
17. A method as claimed in claim 15 , wherein the first plurality of drive currents is the same as the second plurality of drive currents.
18. A method as claimed in claim 15 , wherein the first plurality of drive currents comprises a first number n of non-zero drive current levels for providing the lowest n brightness levels excluding zero, and the second plurality of drive currents comprises a second number m of non-zero drive current levels for providing the highest m brightness levels, where n+m is the total number of non-zero brightness levels.
19. The method of claim 15 , further comprising the acts of:
using only the first phase to provide lowest n brightness levels; and
using only the second phase to provide brightness levels higher than the lowest n brightness levels.
20. The method of claim 19 , further comprising the act of providing a highest brightness level by turning off drive currents during the first phase, and increasing a peak drive current in the second phase to a higher level than an allowable peak drive current in the first phase.