A digital driving technique for an 8b QVGA AMOLED display using ΔΣ modulation

Active-matrix organic LED (AMOLED) is one of the most promising contenders for next-generation displays. However, the VT-shift issue in thin-film transistors (TFT) has to be addressed to enable wide deployment [1]. Voltage programming and current programming are well-known VT-shift-compensation techniques for analog driving. However, they all need more than 4 TFTs per pixel, which increases the panel complexity and decreases yield and aperture ratio. Recently, a VT-shift compensation technique that uses a 2TFT1C pixel in an analog driving AMOLED has been reported [2]. However, it requires OLED supply voltage programming, and shows a 14% variation in OLED current after VT-shift compensation, which is not enough for highdefinition applications. Digital driving has been proposed as an alternative to mitigate the VT-shift issue with a simple pixel structure and to provide flexibility to the driver design. Figure 15.4.1 shows the pixel structures for voltage programming, current programming and digital driving. While the gate of the driving TFT (M2) is in the high state, the voltage across M2 is very small due to the large current-driving capability of a TFT as compared with an OLED. Hence, the current through the OLED is dominated by the supply voltage (PVDD), and minimally affected by the variations in TFT characteristics [3]. Digital driving is also useful for true dark-level expression since the OLED can be completely turned off for black gray levels.