Electroluminescent display devices
View Patent ↗An active matrix electroluminescent display device comprises an array of display pixels. In each pixel, a first switch connects power from a power supply line to the display element and a second switch routes current from a current-measurement supply line to the display element. The gate voltage applied to a drive transistor is controlled by a feedback system, such that control voltage is controlled in closed loop manner to achieve the desired current. The control voltage can then be used for subsequent addressing of the pixel.
1. An active matrix electroluminescent display device comprising an array of display pixels, each pixel comprising:
an electroluminescent display element and a drive transistor for driving a current through the display element;
a first switch enabling power from a power supply line to be supplied to the display element;
a second switch for routing current from a current-measurement supply line to the display element, the first and second switches being operated in complementary manner;
a control line for controlling the gate voltage applied to the drive transistor, and
an address transistor connected between the control line and the gate of the drive transistor wherein a feedback system is provided between the current-measurement supply line and the control line.
2. The device of claim 1 , wherein the feedback system enables a gate voltage to be determined corresponding to a desired current flow through the drive transistor.
3. The device of claim 1 or 2 , wherein the feedback system is provided in a column driver of the display device.
4. The device of claim 1 , wherein each pixel further comprises a storage capacitor for storing a gate-source voltage of the drive transistor.
5. The device of claim 1 , wherein the address transistor, and the first and second switches are each controlled by a shared control line.
6. The device of claim 5 , wherein the address transistor and the second switch are controlled synchronously.
7. The device of claim 1 , wherein each switch comprises a transistor.
8. The device of claim 7 , wherein one of the switches is an NMOS TFT and the other is a PMOS TFT.
9. The device of claim 1 , wherein the feedback system comprises:
a current-to-voltage converter section for providing a first voltage corresponding to the current drawn from the current-measurement supply line;
a comparator section for comparing the first voltage with an input voltage representing the desired current; and
a drive section for providing a voltage on the control line, the feedback loop being in equilibrium when the control line voltage provides drive of the drive transistor giving rise to a current corresponding to the input voltage.
10. The device of claim 1 , wherein the device is operable in two modes:
a first mode in which a desired pixel drive current is drawn from the current-measurement supply line and the feedback system generates the corresponding gate voltage for the drive transistor, the corresponding gate-source voltage for the drive transistor being stored; and
a second mode in which a current is driven through the drive transistor and the EL display element using the stored gate-source voltage.