VGL Voltage Generation Circuit

VGH Voltage Generation Circuit

VDD Voltage Generation Circuit

VDA (VAA) voltage

It is the data driving voltage of the column driving circuit; the voltage will eventually be processed to produce a non-linear step voltage to control the molecular distortions of the LCD screen at different angles. This voltage is called gray-scale voltage. Without this voltage or abnormal voltage, there will be no or serious gray-scale distortion in the image. Different characteristics of the screen this voltage level is different, generally in the range of 14 V to 20 V. Guangzhou Chuangliyou Machinery Equipment Co., Ltd. specializes in the production of various maintenance equipment, LCD screen press!

Gamma correction 

Because the transmittance of LCD screen is a serious disproportionate non-linear relationship with the applied control voltage, and it is a curve similar to S-shape. If the pixel signal voltage without correction is directly applied to the source driving electrode of LCD screen, the gray level of the image will be seriously distorted, so that the gray level of the reproduced image will not appear. Distortion, we need to pre-distortion the change of the amplitude of the added pixel signal.
Pre-distortion processing uses a series of pre-distortion voltage whose amplitude varies disproportionately. This series of voltage is called gray-scale voltage, which is used to assign different brightness information carried by the pixel signal to correct the image gray-scale distortion of the LCD screen.
This process of pre-distortion of pixel signal changes is called gamma correction.
Gamma voltage is a series of non-linear varying voltages. There are two ways to generate gamma voltage at present.
1. A special programmable gamma voltage generator chip is used to generate a series of non-linear varying voltages in accordance with the transmittance characteristics of liquid crystal screens under the control of the program.
2. Resistance voltage divider is used to generate a series of non-linear varying voltages which conform to the transmittance characteristics of liquid crystal screens.

Vcom Common Voltage Generation

One side of the LCD 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 molecule, and this Vcom voltage has a great influence on the final display effect. Guangzhou Chuangliyou Machinery Equipment Co., Ltd. specializes in the production of various maintenance equipment, LCD screen press!
Vcom voltage is required to be a stable DC voltage, and its voltage stability determines whether the brightness of LCD screen is stable when it reproduces images. General LCD screen; Vcom voltage is within the range of 6V to 7V (1/2 of the maximum gamma correction voltage), in TCON circuit; Vcom voltage is obtained by dividing voltage of reference voltage (VREF) through dividing circuit, because it is the common electrode of LCD screen, the voltage of Vcom after dividing voltage is easy to fluctuate because of the change of image content. Therefore, the Vcom voltage must pass through the buffer circuit and be added to the Vcom electrode of the LCD screen. The buffer circuit is actually a current amplifier circuit similar to the radio follower. No matter how the load changes, the output voltage is stable.
For example, buffer circuit integrated block: AS15.

Failure Performance of Various Voltage Abnormalities

When VIN (12V/5V), VDD (3V3) and VAA (15V) fail, there is no screen image.
When VGL (- 5V) and VGH (30V) fail, the image is abnormal or the image switch is slow.
When VCOM (7V) fails, the image is abnormal or flickers. Guangzhou Chuangliyou Machinery Equipment Co., Ltd. specializes in the production of various maintenance equipment, LCD screen press!
The power supply system of LCD driving circuit mainly generates the voltage required by four driving circuits:
VDD: Generally 3.3V, it is used for power supply of T-CON integrated board.
2. VGL: Turn-off voltage of screen TFT thin film switch MOS transistor, generally - 5V;
3. VGH: The switching voltage of screen TFT is generally 20V-35V.
4. VDA: The driving voltage of screen data is generally 14V-20V. The gray-scale voltage is generated by the gamma-ray correction circuit. The gray-scale voltage has about 14 different steps.
5. Vcom: screen common electrode voltage (1/2 of the maximum gamma correction voltage);
Different screen VGL, VGH voltage values are different. If any of the above voltage problems occur, different image faults will occur, which is the fault-prone part.

Vin: PWB input voltage (12V)
VDD: ASIC, S-IC, G-IC driving voltage (3.3V)
VGH: On Voltage of TFT Element (~30V)
VGL: Turn-off Voltage of TFT Component (~-6V)
VAA: Step Control Voltage (~17V)
VCOM: LCD flip reference voltage (~7V)
No picture after Vin, VDD and VAA malfunction
Abnormal picture or slow screen switching after VGH and VGL failures
Screen fading or flickering after Vcom failure
The role of VGL and VGH
To control the light point of each pixel, a "switch" must be installed. A LCD screen with 1920X1080 resolution requires more than 622 million such "switches", which are the "thin film field effect transistors" TFT fabricated one by one in the production of LCD screens. In each field cycle, TFT is turned on once in order to discharge the capacitor impulsively. Then the voltage to turn on TFT is VGH. The voltage to turn off TFT is VGL. If VGH and VGL voltage problems occur, voltage loss or voltage amplitude changes will cause abnormal image faults.

VGL Voltage and VGH Voltage Generation Circuit
The generation of VGH voltage and VGL voltage is accomplished by a charge pump circuit.
What is a charge pump circuit? Charge pump circuit is a DC-DC conversion circuit which uses capacitance as energy storage element.