National Chiao-Tung University

Department of Electronics Engineering & Institute of Electronics





Time Ratio Grayscale Driving Circuits for Active Matrix Organic Light Emitting Diodes






Student : Min-Sung Tseng

Advisor : Dr. Wei Hwang



Abstract

Using improved luminance efficiency for the driving circuit of Active Matrix Organic Light Emitting Diodes (AMOLED) is presented in this thesis. The driven pixel is based on the structure with two transistors and one capacitor. We propose to enhance the luminance efficiency of conventional time ratio grayscale by increased lighting time and decreased addressing time. Two practical approaches to enhance luminance will be proposed. For decreased addressing time, shift register of sampling adopt double rate serial-in. For increased lighting time, amplitude modulation coding will reduce the number of sub-frame, and increase the lighting time. The combined method could enhance 24% luminance. The higher luminance allows a small pixel. Small pixel will allow high resolution displays to be created. Moreover, the final goal is that driver circuits are integrated on the display panel. TFT by poly-silicon is always adopted. For poly-silicon thin film transistor (TFT), both mobility and threshold-voltage vary randomly across the plate. The former two variables would lead to analog circuit is hard to implement on panel. The improved time ratio grayscale also adopts the digital driving. There is no analog design in driver by this method. It is possible that the driving circuit is fully implemented to System on Panel (SOP). Another proposed scan signal gating for correct image display prevent from error-programming when adjacent scan line are toggling. A prototype of this AMOLED driver is simulated by 0.6 um 18V CMOS technology. Because lighting time is limited, it is suitable for the Quarter Video Graphic Array (QVGA) or below the resolution.