ST AN2686 Application note

AN2686

Application note

Using the STVM100 to fully auto adjust LCD flicker

Introduction

Liquid crystal displays (LCDs), from advanced VGA computer monitors to wide-screen TVs, are very popular with consumers. Compared to its conventional cathode-ray tube (CRT) counterpart, the LCD screen is superior as it is lightweight, thin, and consumes less power. Another advantage is that in principle the LCD screen does not flicker as much as a CRT screen and the flicker on an LCD screen can be minimized. This application note presents the principle of LCDs and the cause of flicker. We also propose a solution of automatically minimizing flicker using the STVM100 (LCD VCOM calibrator).

May 2008

Rev 1

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Contents	AN2686

Contents

1	The general principle of the LCD screen . . . . . . . . . . . . . . . . . . . . . . .	. 3
2	Location of flicker in LCDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
3	Using the STVM100 to adjust the VCOM . . . . . . . . . . . . . . . . . . . . . . . . .	7
4	Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	9
5	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	10

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AN2686	The general principle of the LCD screen

1 The general principle of the LCD screen

The LCD screen uses liquid crystal to control the passage of backlight. The basic structure of a TFT-LCD (Thin Film Transistor) screen may be thought of as two glass substrates sandwiching a layer of liquid crystal molecules. The whole screen has been divided into many columns and rows. In each cross of a column and a row is the display unit (pixel). The front glass substrate is fit with a color filter which gives each pixel its own color, while the back glass substrate has transistors fabricated on it. When a positive or negative voltage is applied through a transistor to a pixel, the liquid crystal changes direction and allows backlight to pass through the pixel as shown in Figure 1.

In Figure 1, the gate driver sends a line selection pulse to the top line and turns on the first line TFT. The data (analog voltage) is transferred from the source driver to the pixels in the first line. After the first line data is updated, the gate driver selects the second line. After the last line is scanned, the picture on the whole screen is updated which is referred to as a frame update. Then the gate driver starts from the first line again and begins the next frame update.

Figure 1. LCD screen layout

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The general principle of the LCD screen	AN2686

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 datareceiving 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.

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