1 OLED Driving Scheme

OLED (Organic Lighting-Emitting Diode) is made by organic materials that emit light when current is applied. Each pixel can emit light, so OLED does not have a problem of light from backlight leaking through the display. The basic structure of OLED pixel is shown in Figure 1-1. Scan line is used for pixel selection to turn on T1 to allow data to be written to T2. Vg from Data line can control the gray scale. Data storage capacitor keeps Vg constant during frame time. In general, driving OLED requires bipolar voltage rails like ELVDD(Positive), ELVSS(Negative). Once T2 is turned on, current can be driven from ELVDD(Positive) to ELVSS(Negative). The luminous brightness is adjusted by changing the voltage across OLED.

Figure 1-1 Basic Structure of OLED Pixel

Simply the ground can be used for ELVSS rather than negative output voltage. But recently, the panel makers use negative output voltage for ELVSS to minimize flickering issue. Most applications using OLED panel such as monitors support VRR (Variable Refresh Rate) feature to synchronize the display refresh rate with the video input frame rate. VRR can eliminate the stuttering or tearing of the image and enable smooth display of the source. However, as the refresh rate is changed within VRR range such as 30Hz – 140Hz, the charging speed of data line can be impacted. This changes the charging level of the data storage capacitor, which can cause the current of OLED to be changed accordingly. VRR causes the luminance to vary depending on the frequency (refresh rate) even though the same luminance is the target. This phenomenon has been considered as the flickering issue by users. To prevent and minimize this flickering issue, ELVDD voltage level or internal compensation circuit of OLED can be tuned. But it is sometimes limited and complex. So this becomes popular to simply use negative output voltage for ELVSS to minimize the flickering issue. The voltage level is decided by the characteristics of OLED and internal compensation circuit. Therefore, the requirement of panel makers is very important.

The power supply for ELVDD and ELVSS need to have sufficient current capability to drive OLED pixels. Higher current capability is required if OLED panel size is bigger which can contain more pixels. This means negative output power also need to cover high current capability as much positive output power can do. Therefore inverting buck-boost for negative voltage become popular than charge pump design which has limited current capability. Also IBB design can help designer achieve lower BOM cost and small PCB size design.