A simple pixel circuitry for OLED display

Active OLED are good challenger for becoming the new commonality technology for large displays. Current-controlled pixels that are used in several prototypes are very complex and do not correct all the problems of the use of OLED pixels. This paper presents a different approach to solve these problems. It suggests keeping the pixel circuitry as simple as possible and to put a correction mechanism in the associated VLSI driver. By using this approach, the cost of the screen will decrease and those of its driver will slightly increase. The pixel circuitry transforms the input control voltage to a non-linear mixed current and timing control of the OLED. The driver contains a pipeline circuitry to perform the on-line corrections on the input video stream. A calibration step, during the display manufacturing or maintenance, is used to measure the pixel efficiency and to compute the coefficients to be stored into its flash memories. Objective and Background Active OLED pixels are good challengers for becoming the new commonality technology for flat panel displays. Their color, luminosity and viewing angle are better than LCD, but their reliability has not yet reach a satisfactory level. Several prototypes of such displays already exist in the research environment. The emitted light of an OLED diode is directly proportional to the current flowing through it. In order to use this property of linearity, many OLED displays use complex pixel circuitry for controlling the screen matrix by current [3]. This approach presents several drawbacks: The circuitry of a current-controlled pixel needs to use 4 TFT, or more, organized in a current-mirror circuitry to compensate the drift of the threshold of TFT build on polysilicon or other deposited materials. The command lines of current-controlled pixels must carry the same current as those flowing thought their OLED. The ohmic resistance of the command lines introduces voltage decay and leads to a poor energy efficiency for this kind of display. However, the natural decay of the lightemission efficiency of the OLED itself cannot be easily compensated by such an approach.