30.2 Digital PWM-driven AMOLED display on flex reducing static power consumption

摘要

The efficiency of small-molecule OLED devices increased substantially in recent years, creating opportunities for power-efficient displays, as only light is generated proportional to the subpixel intensity. However, current active matrix OLED (AMOLED) displays on foil do not validate this power-efficient advantage, as too much power is lost in the AM backplane. AMOLED displays use the analog voltage on the gate of a drive transistor (e.g. M1 in Fig. 30.2.1) to control the pixel current and hence the pixel brightness. Accurate and uniform pixel currents can only be obtained when transistor M1 is driven is saturation. In highresolution technologies on foil, transistor parameters W, L and the mobility μ are limited by technology, imposing a minimal VGS-VT to obtain sufficient current, i.e. VGS-VT > 4V for a-IGZO on foil [1]. Subsequently, to obtain saturation, VDS > 4V, which translates in a static backplane power loss surpassing the OLED power consumption (see red stars in Fig 30.2.1). However, when the OLED pixel impedance around a specific reference current can be matched along a display column line, the accurate pixel current control can be imposed by current DACs implemented in external silicon display column drivers. In this work, we operate M1 as a switch and pixel intensity variations are obtained using Pulse Width Modulation (PWM) of a predefined pixel current, i.e. 2μA/pixel [80∗80μm2] (which corresponds in our OLED technology to a light output of 2000Cd/m2). When, in a future implementation the external DACs are calibrated at 0.2μA/pixel, the full brightness would correspond to the typical display brightness of a portable PC, i.e. 200Cd/m2. This concept enables us to reduce the display power voltage at full brightness from 8.2V in a classical AMOLED display on foil configuration to 5V (measured) and for future imple- entations even down to 4V (see Fig. 30.2.1). As the OLED current load remains equal, a corresponding static power reduction of the display (and increased battery lifetime) is obtained. Digital driving methods of AMOLED displays have been shown before. However, ΔΣ techniques [2] still integrate charge packets on the gate of M1 and hence do not solve the power issue on foil. Other PWM techniques [3] activate only a single active line in the linedriver yielding difficulties to obtain color depths above 6 bits. When multiple independent linedrivers are implemented and their output is multiplexed to alternately drive a single select line, a higher color depth can be obtained [4]. This leads however to a bulky linedriver, which is hard to get within an e.g. 80μm pitch. The design and implementation of a compact integrated linedriver on foil enabling multiple alternating active signals through a single shift register is demonstrated here.