Location of flicker in LCDs

AN2686

6/11

During the manufacturing of LCDs, process variations can cause deviation in the optimal

VCOM from the ideal voltage of 1/2 AVDD. This is illustrated in Figure 4.

Figure 4.

Error of VCOM resulting in flicker

This error of VCOM results in a visible flicker in the panel screen and the flicker cannot be

eliminated since the VCOM error is due to the manufacturing process. Considering the

example of the V0 gray scale, in frame N+1, the absolute voltage between the display

electrode and VCOM is smaller than in frame N+2. This difference results in a different gray

scale in the two different frames which generates flicker. Traditionally, VCOM is adjusted by

tuning a mechanical potentiometer in a voltage divider. This method is simple but the VCOM

value cannot be adjusted very accurately. It requires too much manpower and the accuracy

also depends on the skill of each operator and even different shifts of the same operator.

Even worse, the mechanical potentiometer value can be changed inadvertently, (eg. during

transport).