LCD logic board working principle detailed explanation - android - Phones Developers

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VGL voltage generation circuit

VGH voltage generating circuit

VDD voltage generating circuit

VDA (VAA) voltage

It is the data driving voltage of the column driving circuit; the voltage is finally processed to generate a nonlinear step voltage to control different twist angles of the molecules of the liquid crystal screen. This voltage is called gray scale voltage. If there is no such voltage or the voltage is abnormal, There will be no or severe grayscale distortion in the image. Screens of different characteristics have different voltage levels, generally ranging from 14V to 20V.

Gamma correction

Since the transmittance of the liquid crystal panel and the applied control voltage are a strictly disproportionate nonlinear relationship, it is an S-shaped curve if the uncorrected pixel signal voltage is directly applied to the source driving electrode of the liquid crystal panel. The gray scale of the image will be seriously distorted. In order to make the grayscale of the reproduced image not distorted, we need to pre-distort the change of the amplitude of the added pixel signal.

The predistortion process uses a sequence of predistortion voltages that are not proportional to the amplitude variation. This series of voltages is called gray scale voltage. The grayscale voltage of this series is used to assign different brightness information carried by the pixel signals. Correct the grayscale distortion of the image on the LCD screen.

This process of predistorting pixel signal changes is called gamma correction.

The gamma voltage is a series of non-linearly varying voltages, and there are currently two ways to generate gamma voltages;

1. Using a special programmable gamma voltage generating chip, under the control of the program, a series of voltages that meet the nonlinear variation of the transmittance characteristics of the liquid crystal screen are generated;

2 , is the use of resistor divider, resulting in a series of voltages in line with the nonlinearity of the LCD transmittance characteristics.

Vcom common voltage generation method

The electrode voltage of the liquid crystal pixel is the source driving voltage, the other side is the common electrode, and the common electrode voltage is Vcom. These two voltage differences determine the voltage applied to the liquid crystal molecules, and this Vcom voltage has a great influence on the final display effect.

The Vcom voltage requirement is a stable DC voltage, and the stability of the voltage determines whether the brightness of the LCD screen is stable when the image is reproduced. General LCD screen; Vcom voltage is within the range of 6V to 7V (1/2 of the maximum gamma correction voltage) in the TCON circuit; Vcom voltage is divided by the reference voltage (VREF) through the voltage divider circuit is obtained, since the common electrode of the liquid crystal panel, the Vcom voltage obtained by dividing the change in the image content as easily fluctuate, so must the Vcom voltage buffer circuit coupled to the electrode Vcom of the liquid crystal panel, is actually a similar buffer circuit The current amplifier circuit of the emitter follower, the output voltage is stable regardless of the load change.

Such as the buffer circuit integration block: AS15.

Fault performance when each voltage is abnormal

There is no picture on the screen when VIN (12V/5V), VDD (3V3), VAA (15V) fails;

When the VGL (-5V) or VGH (30V) fails, the image is abnormal or the image is switched slowly;

The image is abnormal or flickers when VCOM (7V) fails.

The power supply system of the LCD driver circuit; mainly generates the voltage required for the four-way driver circuit:

1 , VDD: generally 3.3V, used for power supply of T-CON board integrated block;

2 , VGL: screen TFT switch MOS tube shutdown voltage, generally -5V;

3 , VGH: the opening voltage of the screen TFT, generally 20V ~ 35V;

4 , VDA: screen data drive voltage, generally 14V ~ 20V, the gray level voltage is generated by the gamma correction circuit, the gray scale voltage has about 14 different step voltages;

5 , Vcom: screen common electrode voltage (1/2 of the maximum value of gamma correction voltage);

Different screens have different voltage values ​​of VGL and VGH. If there is a problem with any of the above voltages, different image failures will occur, which is a faulty part.

Vin : PWB input voltage (12V)

VDD : ASIC, S-IC, G-IC drive voltage (3.3V)

VGH : TFT component turn-on voltage (~30V)

VGL : TFT component turn-off voltage (~-6V)

VAA : tone control voltage (~17V)

VCOM : LCD flip reference voltage (~7V)

No picture after Vin , VDD, VAA failure

Abnormal screen or slow screen change after VGH or VGL failure

The screen is light or the screen flickers after the Vcom fault

The role of VGL and VGH

To control the spot of each pixel, you must install a "switch". A liquid crystal screen with a resolution of 1920X1080 will have more than 6.2 million such "switches". These switches are the "film field effect" when manufacturing the LCD screen. Tube "TFT. Each field period, the TFT must be turned on once to discharge the capacitor once, then the voltage to turn on the TFT is VGH. The voltage to turn off the TFT is VGL. If there is a problem with the VGH and VGL voltages, voltage loss or voltage amplitude changes will cause an abnormal image failure.

VGL voltage and VGH voltage generating circuit

The VGH voltage and the VGL voltage are generated using a "charge pump" circuit.

What is a "charge pump" circuit? The charge pump circuit is a DC-DC converter circuit that uses a capacitor as an energy storage element.