An 8-Bit Driver for OLED-on-Silicon Micro-Displays

The luminance of an OLED is proportional to its current. Therefore, current driving scheme can obtain good luminance uniformity. However, the OLED current ranges from hundreds of picoamperes to tens of nanoamperes for an OLED-on-silicon micro-display. The driving capability of the small current is weak to drive the parasitic data line. In this work, a voltage driving scheme is used to drive the micro-display. 3T1C pixel circuit is designed. One transistor is used for the current source, the other one is a switch, and another one is a dummy switch for the reduction of the charge injection and clock feedthrough errors. An image voltage is driven to the gate of the current source transistor, this device converts the image voltage to a current and sends this current to the corresponding OLED. Because the device current is not linearly proportional to the gate voltage, the characteristic of the digital-to-analog converter (DAC) should be nonlinear to fit the inverse current-voltage characteristic of CMOS device. 256 binary currents flow into 256 diode connected transistors to generate 256 gray level voltages. These 256 gray level voltages are used for all output channels. Each output channel contains an 8-bit DAC and a buffer amplifier. The DAC chooses one voltage from the 256 gray level voltages according to the corresponding image data. The buffer amplifier drives the gray voltage to the corresponding pixel.