Control Method of High-Efficient DC-DC Converter for OLED Microdisplays

Abstract

Organic light-emitting diodes (OLEDs) are one of the prospective candidates for microdisplays in virtual reality (VR) and augmented reality (AR) applications because they have outstanding characteristics such as self-emission, high contrast ratio, and low power consumption. To drive these OLED microdisplays, DC-DC converters require to have a high efficiency for longer battery usage time and a small form factor for less system cost. The OLED microdisplays with higher resolution than SXGA (1280 x 1024) need to operate in a broad load current range from zero at black to tens of mA at full white. Thus, it is difficult to realize high efficiency over the entire load current range, especially at light-load, because of a large switching power loss caused by power switches. To solve aforementioned problems, this thesis proposes a fully digital-controlled switch width modulation (SWM) with a duty sensing method, which can be applied to an inverting buck-boost converter at light-load for microdisplay applications. The buck-boost converter using the proposed fully digital-controlled SWM, which operates in discontinuous conduction mode, accomplishes high efficiency at light-load. In addition, it achieves a small form factor by adjusting the switch width according to the detected on-duties of the main power switches using a full-digital method. It generates an output voltage of –4 V with an input voltage of 1.8 V for a maximum load current (ILOAD) of 20 mA. The proposed converter was implemented in a 0.18 µm BCDMOS process and measured to verify its performance. The measurement results show that the buck-boost converter using the proposed SWM controller achieves a maximum efficiency of 89.3% at an ILOAD of 20 mA and, in addition, a high efficiency of 81.9% at light-load, where ILOAD is less than 5 mA. Therefore, the proposed fully digital-controlled SWM is appropriate for microdisplay applications requiring a small form factor and a high efficiency at light-load.