voltage is one of the most critical technical elements. These voltages are the core factors determining whether the screen can display normally, whether the colors are accurate, and whether the image is stable.
Once the voltage is abnormal, common defects such as screen flickering, no display, image retention, horizontal lines, and screen distortion can easily occur.
This article will systematically review the most common driving voltages in TFT-LCD modules and their functions, hoping to help you find problems more quickly and improve the efficiency of handling defects encountered in LCD module production.
Basic Components of a TFT-LCD Module
A complete TFT-LCD liquid crystal display module mainly includes a liquid crystal panel and its driving circuit system, a backlight (BLU) and its driving system, while a fully laminated module will also include a protective cover plate (CG), a capacitive touch screen (CTP), and optical adhesive (OCA).
From a process stage perspective, before the polarizer is attached and the driver IC is bonded, it is called a single cell; after the polarizer is attached and the driver IC and FPC/PCBA are bonded, it is called an Open Cell (OC or FOG).
Currently, small and medium-sized products generally use GOA (Gate on Array) technology to replace traditional Gate Driver ICs, and the T-CON and source driver board are also highly integrated, but the basic principle of the driving voltage remains the same.
The general process of driving an open-cell driver circuit is as follows: Power from an external source passes through the FPCA and PCBA to the driver IC. Through the internal circuitry of the driver IC or the combined action of internal and external circuitry, various voltages can be generated and output.
These combined output voltages serve two purposes: firstly, they provide power to the driver circuitry of the IC itself, ensuring its normal operation; secondly, after adjustment, they are supplied to the LCD display panel, ensuring normal display. Therefore, these driving voltages are crucial and directly affect the display effect and stability of the LCD.
Common driving voltages and their functions
VDDI
VDDI is the power supply voltage for the driver IC's I/O interface, directly provided by the motherboard or test fixture. It is primarily used for the driver IC's logic circuitry and data communication.
- Voltage range: 1.65V～3.6V
- Common value: 1.8V is commonly used in mobile phones and tablets.
The stability of VDDI directly affects the communication between the motherboard and the driver IC. If VDDI is abnormal, problems such as the device failing to power on or communication failures often occur.
VSP And VSN
This is the core analog power supply voltage for the source driver circuit, with one positive and one negative voltage supplied symmetrically.
- VSP (Positive Voltage): Typical range 4.5V～6.5V, commonly used value approximately 5.5V
- VSN (Negative Voltage): Typical range -4.5V～-6.5V, commonly used value approximately -5.5V
VSP and VSN are responsible for providing precise voltages to the liquid crystal pixel electrodes, directly affecting grayscale performance, contrast, and image uniformity.
Common abnormal phenomena: screen flicker, no display, high current, abnormal image, etc.
Vcom
Vcom is the reference voltage for the common electrode of an LCD panel. Its value needs to be finely adjusted according to the specific panel characteristics. Vcom tuning is a crucial step in mass production module calibration,Inappropriate Vcom is a major cause of the following problems:
- Flickering
- Image retention/image ghosting
- Color distortion/whitening
- Poor low grayscale performance
VGH And VGL
- VGH (High-level turn-on voltage): Typical range 7V～20V, responsible for turning on a row of TFTs, allowing the source data voltage to charge the pixel capacitors.
- VGL (Low-level turn-off voltage): Typical range -7V～-15V, ensures the TFTs are completely turned off, maintaining stable pixel charge.
Abnormal effects: No display, entire screen whitening, horizontal lines, ghosting, unstable image, etc.
VSS
VSS, also known as VSSA, VSSD, etc., provides a reference ground for various circuits. It is usually 0V. Although it is ground, the grounding quality directly affects the overall noise level and voltage stability. It must be clean and have low impedance.
Gamma voltage (GVDDP / GVDDN)
- GVDDP (Positive Gamma): Typically ranges from 3.0V to 6.0V
- GVDDN (Negative Gamma): Typically ranges from -3.0V to -6.0V
The gamma voltage is used to generate the grayscale voltage curve inside the driver IC. It is usually fine-tuned in steps of 10mV or 20mV, and has a significant impact on white balance, gamma curve, and image gradation.
Common power supply modes for driver ICs
Based on the number of input voltages and the design of the internal boost circuit, driver ICs can be divided into several different power supply modes.
2 Power Mode
Input Voltage: VDDI + VCI
An external power IC is required to generate high voltages such as VSP, VSN, Vcom, VGH, and VGL.
Characteristics: This solution has the most complex external circuitry and the highest cost, but it places lower demands on the internal circuitry of the driver IC. It is rarely used in small-to-medium-sized products and is considered a non-mainstream solution, primarily appearing in some early or specific low-cost designs.
3 Power Mode
Input Voltage: VDDI + VSP + VSN
The driver IC integrates a boost circuit to generate VGH, VGL, Vcom, and Gamma voltages, which is currently the most mainstream solution for small and medium-sized TFT-LCD products.
Advantages: Simple peripheral circuitry, low BOM cost, good overall power consumption, and relatively low motherboard design difficulty, making it widely used in mobile phones, tablets, automotive, and industrial control products.
4 Power Mode
Input Voltage: VDDI + VSP + VSN + VDD
This adds VDD compared to the 3Power mode (typically for powering the driver IC's internal digital circuitry or specific modules).
Features: Most modern driver ICs support compatibility with both 3Power and 4Power modes, allowing for flexible switching based on actual needs.
Advantages: Provides better power isolation and stability when higher performance or specific functions are required, making it suitable for mid-to-high-end products.
5 Power Mode
The input voltage typically includes: VDD + AVDD + VSP + VSN + Vcom.
This is the most integrated mode, requiring the driver IC to handle more voltage domains internally.
Suitable scenarios: For products with extremely high display quality requirements, needing precise and independent control of Vcom, or specific large-screen/high refresh rate products.
Advantages: More flexible power management, greater room for display quality optimization; Disadvantages: Relatively complex motherboard power supply design.
Conclusion
TFT-LCD driving voltages connect the motherboard and the LCD panel. Each voltage group has a specific function. A deep understanding of the power supply mode and the role of each voltage can not only solve current problems but also help you avoid potential risks during the product design phase.
