Apparatus and method for driving an LED display utilizing a pre-programming period

1. A pixel circuit comprising:

a light-emitting diode;

a current-driving unit receiving a signal current on a data line only during a programming period to provide a corresponding driving current to the light-emitting diode;

a first switch coupled between the data line and the current-driving unit, and turned on only during the programming period to conduct the signal current; and

a constant current unit providing a constant current on the data line continuously from a pre-programming period to the programming period;

wherein a level of the constant current is the same during the pre-programming period and the programming period;

wherein the programming period follows the end of the pre-programming period;

wherein the constant current during the pre-programming period is lower than a sum of the signal current and the constant current during the programming period.

2. The pixel circuit of claim 1 , wherein the driving current is provided during an emission period following the programming period.

3. The pixel circuit of claim 2 , wherein the current-driving unit comprises:

a driving transistor having a source coupled to receive a supply voltage and a gate coupled to the first switch;

a second switch coupled between a drain and the gate of the driving transistor;

a capacitor coupled between the source and gate of the driving transistor; and

a third switch coupled between the drain of the driving transistor and the light-emitting diode;

wherein the second switch is turned on during the programming period and the third switch is turned on during the emission period.

4. The pixel circuit of claim 1 , wherein the constant current unit comprises:

a transistor having a source coupled to receive a supply voltage;

a capacitor coupled between the source and a gate of the transistor;

a fourth switch coupled between the gate and a drain of the transistor; and

a fifth switch coupled between the data line and the drain of the transistor;

wherein the fourth switch is turned on during the pre-programming period, and the fifth switch is turned on during the pre-programming period and the programming period.

5. The pixel circuit of claim 1 , wherein the constant current unit comprises:

a constant current source; and

a sixth switch coupled between the constant current source and the data line, and turned on during the pre-programming period and the programming period.

6. An apparatus for driving a display, comprising:

a scan-driving circuit enabling a row of pixel circuits of the display during a programming period;

a data-driving circuit providing signal currents on data lines to drive the enabled row of pixel circuits only during the programming period; and

a plurality of constant current units, each providing a constant current on one of the data lines continuously from a pre-programming period to the programming period;

wherein a level of the constant current is the same during the pre-programming period and the programming period;

wherein the programming period follows the end of the pre-programming period;

wherein the constant current during the pre-programming period is lower than a sum of the signal current and the constant current during the programming period.

7. The apparatus of claim 6 , wherein the pixel circuits selectively emit light according to the signal currents during an emission period following the programming period.

8. The apparatus of claim 7 , wherein each of the pixel circuits comprises:

a light-emitting diode;

a current-driving unit receiving a signal current on one of the data lines during the programming period to provide a corresponding driving current to the light-emitting diode; and

a first switch coupled between one of the data lines and the current-driving unit, and turned on by the scan-driving circuit during the programming period to conduct the signal current.

9. The apparatus of claim 8 , wherein the current-driving unit comprises:

a driving transistor having a source coupled to receive a supply voltage and a gate coupled to the first switch;

a second switch coupled between a drain and a gate of the driving transistor;

a capacitor coupled between the source and the gate of the driving transistor; and

a third switch coupled between the drain of the driving transistor and the light-emitting diode;

wherein the second switch is turned on during the programming period and the third switch is turned on during the emission period.

10. The apparatus of claim 9 , wherein the first switch, the driving transistor, the second switch and the third switch are PMOS transistors.

11. The apparatus of claim 6 , wherein each of the constant current unit comprises:

a transistor having a source coupled to receive a supply voltage;

a capacitor coupled between the source and a gate of the transistor;

a fourth switch coupled between the gate and a drain of the transistor; and

a fifth switch coupled between one of the data lines and the drain of the transistor;

wherein the fourth switch is turned during the pre-programming period, and the fifth switch is turned on during the pre-programming period and the programming period.

12. The apparatus of claim 6 , wherein each of the constant current units comprises:

a constant current source; and

a sixth switch coupled between the constant current source and one of the data lines, and turned on during the pre-programming period and the programming period.

13. A method for driving a pixel having a light-emitting diode, the method comprising the steps of:

receiving a signal current on a data line only during a programming period to provide a corresponding driving current to the light-emitting diode; and

providing a constant current on the data line continuously from a pre-programming period to the programming period;

wherein a level of the constant current is the same during the pre-programming period and the programming period;

wherein the programming period follows the end of the pre-programming period;

wherein the constant current during the pre-programming period is lower than a sum of the signal current and the constant current during the programming period.

14. The method of claim 13 , wherein the driving current is provided during an emission period following the programming period.