The general principle of the LCD screen

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Figure 2 shows the detailed pixel schematic. The Clc is the equivalent capacitance of the

liquid crystal. The Cs is the capacitance built inside to maintain the signal between frame

updates. The light transmitted from the pixel is related to the absolute voltage (from the

source driver) applied to the pixel so that different gray scales can be obtained. The data-

receiving side of the capacitor is called the display electrode and the other side is called the

common electrode (VCOM). The VCOM of all pixels are interconnected.

Figure 2.

Basic pixel schematic

Since the molecules of the liquid crystal cannot be set at a constant voltage for a long time

without being damaged. The voltage on the pixels should be alternated in intervals to avoid

this damage. The solution is to set the VCOM to about half of the supply voltage (AVDD) in

order to divide the display voltage into two different polarities in different frames. When the

voltage on the display electrode is higher than the voltage on the VCOM, this is the positive

polarity voltage, otherwise it is the negative polarity. When the absolute differential voltage

on the display electrode is constant, regardless of the voltage polarity, the gray scale

displayed is the same. In both cases the directions of the molecules of the liquid crystal are

completely different. By using this polarity inversion, the direction of the molecules has

changed and they will not be damaged.