The Design Of Pixel Driving Circuit For OLED Microdisptay Based On PWM

Organic Light-emitting Device (OLED) is a promising candidate for thenext-generation Flat Panel Displays due to the advantages such as solid-state, highbrightness, short responsive time, wide operating temperature range, wide view-angle,low power dissipation and flexibility.OLED microdisplay is a new kind of microdisplay technology which combinesOLED and the mature CMOS technology for driving circuits to realize high resolutionand inch-size display on a single silicon chip. All the driving circuits including thepixel driving circuit, column driving circuit, row driving circuit and other externaldriving circuit could be integrated on the silicon chip. These circuit modules arecompatible with the CMOS process which is easy to realize. This provides asystem-on-chip solution for near-eyes display systems such as head mounted displaysand viewfinders.Currently, the OLED microdisplay technology is only in its initial stage, there isstill much work to do on the implementation of small driving current and full-color. Atpresent, there are only several companies such as eMagin and IBM to have matureproducts on the market. In China, there are only a few reports of the research onOLED pixel driving circuits. Therefore, it is of great significance to design new,high-performance OLED microdisplay pixel driving circuit.Because of the small pixel area and high luminous efficiency of OLED,traditional pixel driving circuit can not drive OLED directly with normalwidth-to-length ratio. To fit the OLED pixel, MOS should have a very large size and itis the problem of single pixel driving circuit using standard CMOS process. How torealize the required small driving current within the limited pixel area is the key issuein the design of OLED microdisplay pixel driving circuit. In this paper, for two kinds of input signal (voltage signal and current signal), two different MOS pixel drivingcircuit for OLED microdisplay based on pulse width modulation (PWM) technologyare proposed to solve the problem of small output current. The main contents of thethesis are as follows:First, in the commonly-used EDA software such as HSPICE, there are nosimulation models of TOLED, so it is difficult to design the OLED microdisplay pixeldriving circuit. To solve the problem, we introduced a equivalent circuit model forTOLED in the design of OLED microdisplay. Final fitted and simulated resultsshowed that the model meets the requirements of pixel circuits design.Then a voltage programmed pixel driving circuit and a current programmed pixeldriving circuit are designed. Both circuits are based on PWM technique to control theon-time of the switching MOS within a frame period, so as to control thelight-emitting time of OLED to solve the problem of small driving current of OLEDmicrodisplay. The driving MOS transistors are in saturation in both circuits. Thesimulation results showed that both pixel driving circuit can achieve single-colordisplay effect within15×15μm2pixel area, and the brightness of OLED is in the rangeof1.0cd/m2and214.3cd/m2with16gray levels display.