Threshold voltage compensation method for electroluminescent display devices

1. An active matrix electroluminescent display device comprising an array of display pixels, each pixel comprising:

an electroluminescent (EL) display element ( 2 );

a drive transistor ( 22 ) for driving a current through the display element ( 2 );

an address transistor ( 16 ) for providing a pixel drive signal from a data line to the gate of the drive transistor ( 22 ); and

a shorting transistor ( 30 ) connected between the gate and drain of the drive transistor,

wherein the display device further comprises means ( 42 ) for measuring a voltage on the data line ( 6 ) after the shorting transistor ( 30 ) is turned on and discharges the voltage on the drive transistor ( 22 ) until the drive transistor( 22 ) switches off; and a single storage capacitor ( 24 ) is directly connected between a power supply line ( 26 ) and the gate of the drive transistor ( 22 ); and

wherein the data input line ( 6 ) is switchable between a voltage driving mode in which it provides voltages to the pixels connected to the line and a floating mode in which the data line is placed in a high impedance state thereby allowing the data line to float to the voltage of the gate of the drive transistor of an addressed pixel.

2. A device as claimed in claim 1 , wherein the EL display element ( 2 ) and the drive transistor ( 22 ) are connected in series between first ( 26 ) and second ( 34 ) power lines.

3. A device as claimed in claim 2 , wherein the voltage on the second power line ( 34 ) is switchable between two values, one of which causes the EL display element ( 2 ) to be turned off.

4. A device as claimed in claim 1 , wherein each pixel is operable in two modes:

a first, threshold voltage measuring mode, in which the display element is disabled, the address transistor is turned on and the shorting transistor is turned on; and

a second, pixel drive mode, in which the display element is enabled, the address transistor is turned on and the shorting transistor is turned off.

5. A device as claimed in claim 4 , wherein during the first, threshold voltage measuring mode, during a first period ( 40 ) a predetermined voltage is applied to the data line so that a current is driven through the drive transistor ( 22 ) and during a second period ( 42 ) the data line is allowed to float so that the voltage on the data line ( 6 ) substantially follows the gate voltage of the drive transistor ( 22 ).

6. A device as claimed in claim 1 , wherein the drive transistor ( 22 ) is a poly-silicon TFT.

7. A device as claimed in claim 6 , wherein the drive transistor ( 22 ) is a low temperature poly-silicon TFT.

8. A device as claimed in claim 1 , further comprising a storage capacitor ( 24 ) between the gate and source of the drive transistor ( 22 ).

9. A method of addressing the pixels of an active matrix electroluminescent display device, comprising an electroluminescent (EL) display element ( 2 ) and a drive transistor ( 22 ) for driving a current through the display element ( 2 ), the method comprising:

disabling the display element ( 2 );

applying a first voltage to a data line ( 6 );

coupling a power supply line ( 26 ) directly to the gate of the drive transistor ( 22 ) by a single storage capacitor ( 24 );

driving a current through the drive transistor ( 22 ), through a shorting transistor ( 30 ) connected between the gate and drain of the drive transistor and through an address transistor ( 16 ) connected between the gate of the drive transistor and the data line ( 6 );

allowing the data line ( 6 ) electrically to float in which the data line is placed in a high impedance state thereby allowing the data line to float to the voltage of the gate of the drive transistor of an addressed pixel;

measuring a voltage on the data line ( 6 ); and

modifying a data voltage to be applied to the drive transistor ( 22 ) using the voltage measured on the data line.

10. A method as claimed in claim 9 , wherein disabling the display element comprising applying a disable voltage to a terminal of the display element.

11. A method as claimed in claim 10 , wherein disabling the display element comprising applying a disable voltage to terminal ( 34 ) of the display element ( 2 ) which is common to all display elements.

12. A method as claimed in claim 9 , further comprising enabling the display element ( 2 ), and addressing the pixel with the modified data voltage on the data line, with the shorting transistor turned off.