What is VGH, VGL and VCOM and its characteristics to LCM display?

For a electronics product, normally there are VGH, VGL and VCOM on the logic board, sometimes also include VGoffL and VGoffH. Today we mainly discuss about VGH, VGL and VCOM's features.

1. VGH: Vgatehigh refers to the high potential of the gate level, which is the voltage that opens the gate level. Normally VGH is 12V~25V. The voltage value is decided by the driver IC and LCD glass.
2. VGL: Vgatelow is the low potential of the gate stage, which is the voltage that turns off the gate stage. This voltage is valid during second-level driving. During third-level driving, this voltage is only used to generate Vgoffl. Normally VGL is -6V~-15V. The voltage value is decided by the driver IC and LCD glass.
3. VCOM: is the Liquid crystal deflection base, the reference voltage for deflection of liquid crystal molecules. Normally VCOM is about 10V, sometimes it could be 0V if it's ground setting.

VGH/VGL/VCOM is very important to lcd display's performance. If the VGH and VGL voltages are unstable or vary in amplitude, it will cause image display failures, such as blurred screens, ghosting/sticking image, white screens, etc. For example, if the display have the issue of sticking image, the biggest reason is the VGH/VGL/VCOM voltage is not in the reference voltage range of driver IC and lcd glass, so engineers will need to adjust this three voltages until the sticking image is gone.

The issue about VGH/VGL/VCOM could be easily solve, since we could adjust them by hardware by connecting the display's FPC to control board, to set the voltage pin. If the display's FPC interface don't have the pinout of these three pins, then what to do? Engineers could set the voltage by software on computer, according to the display interface initialization file, this file normally provided by lcd display's supplier.

Except above three types of voltage, in a digital circuit, there is also VCC/VDD/VSS voltage. VCC is the power supply voltage of the circuit, VDD is the working voltage of the chip (usually VCC>VDD), and VSS is the ground point. Some ICs have both VDD pins and VCC pins, indicating that the device itself has a voltage conversion function. Generally speaking, VCC=analog power supply, VDD=digital power supply, VSS=digital ground, VEE=negative power supply.

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