A complete TFT-LCD display module mainly consists of a TFT-LCD panel and its driver circuit system, together with a backlight unit (BLU) and its corresponding driver circuit system.
For a fully laminated TFT-LCD display module, the structure may also include a cover glass (CG), a capacitive touch panel (CTP), and optical clear adhesive (OCA).
Throughout the manufacturing process of a TFT-LCD display module, there is an intermediate stage after the mother glass has been cut but before the polarizer is attached and the driver IC, FPCA/PCBA, and other driver circuit components have been bonded. This state is generally referred to in the industry as a single Cell.
Once the polarizer attachment and bonding of the driver IC, FPCA/PCBA, and other driver circuit components have been completed, the product is generally referred to as an Open Cell (OC). Some manufacturers also broadly describe products at this stage as FOG. However, FOG, or Film on Glass, more specifically refers to a bonding structure or process involving a flexible circuit or film bonded to the glass, so the two terms should not always be treated as exact synonyms.
Open Cell Driver Circuit System
A conventional Open Cell driver circuit system generally includes a Gate Driver IC, a Source Driver IC, a printed circuit board assembly (PCBA) connected to the source driver circuit, a Timing Controller (T-CON) board, and a flexible printed circuit (FPC) connecting the PCBA and T-CON board. This represents a relatively traditional Open Cell driver architecture.
However, the architecture used in small- and medium-sized display products is now often more highly integrated. In many applications, particularly smartphones, tablets, and other compact display products, Gate on Array (GOA) technology is used instead of a separate Gate Driver IC. At the same time, the functions traditionally distributed between the PCBA and T-CON board can be integrated into a single board, reducing the number of separate components and simplifying the overall structure of the Open Cell driver system.
How Does the TFT-LCD Driving Voltage System Work?
The basic driving process of an Open Cell can be understood as a process of power input, voltage conversion, and voltage distribution.
External power supplied by the mainboard or test fixture enters the driver system through the FPCA and PCBA and is then supplied to the driver IC. Depending on the architecture, the driver IC may use its internal circuits, or a combination of internal and external circuits, to generate and output multiple supply, bias, and driving voltages.
These generated voltages serve two main purposes. First, they provide power to the internal circuits of the driver IC itself, allowing the IC to operate normally. Second, after being converted or adjusted as required, they are supplied to the LCD panel and its driving circuits to ensure normal display operation.
For this reason, the driving voltages of a TFT-LCD module are extremely important. Their voltage levels, stability, and relationship with one another can directly affect display quality, panel operation, and overall module stability. In practical failure analysis, abnormal driving voltages are often associated with problems such as flicker, no display, image sticking, horizontal lines, abnormal colors, and excessive current consumption.
The exact voltage architecture varies according to the LCD panel, driver IC, product size, and circuit design, but several key driving voltages are commonly encountered in TFT-LCD modules.
VDDI: I/O Supply and Logic Voltage
VDDI, also referred to as IOVCC or VCC, is the I/O supply voltage, also commonly called the logic voltage.
It is supplied by the test fixture or mainboard to the driver IC and is mainly used to power the input/output interface circuits. The typical VDDI range is approximately 1.65 V to 3.6 V, although the exact allowable range depends on the driver IC specification.
In many smartphone and tablet applications, 1.8 V is a commonly used VDDI voltage.
Because VDDI supplies the logic and interface circuitry, an abnormal VDDI level can affect IC initialization, communication with the host system, and other basic operating functions of the display module.
VSP: Positive Analog Supply Voltage
VSP, also known as AVDD, is a positive voltage supplied by the mainboard or test fixture and is one of the core supply voltages of the Source Driver.
VSP directly affects the operating range and output accuracy of the Source Driver and therefore has a direct influence on the voltage applied to the LCD pixel electrodes and on overall image quality.
The typical VSP range is approximately 4.5 V to 6.5 V, although the actual required value should always be determined according to the driver IC datasheet and LCD panel specification. In many small- and medium-sized display products, including smartphones and tablets, a typical VSP value is around 5.5 V.
Together with VSN, VSP provides the positive and negative analog supply rails required by the Source Driver to generate the appropriate source output voltages during pixel addressing.
When VSP is abnormal, the Source Driver may fail to generate the required output levels correctly, resulting in display problems such as flicker, no display, abnormal image quality, or excessive current consumption.
VSN: Negative Analog Supply Voltage
VSN, also known as AVEE, is the negative supply voltage corresponding to VSP. The two voltages form a positive-and-negative supply pair for the Source Driver.
VSN is also a core Source Driver supply and directly affects the accuracy of the analog voltages used to drive the LCD pixel electrodes.
The typical VSN range is approximately -4.5 V to -6.5 V, with -5.5 V being a common value in many small- and medium-sized display products. As with VSP, the actual voltage should be checked against the specific IC and panel specifications.
VSP and VSN work together to establish the analog voltage rails needed by the Source Driver. Through the source-driving process, the corresponding pixel electrodes and pixel capacitors are charged to the required voltage levels, allowing the LCD panel to produce the intended image.
If VSN is abnormal, the display may exhibit symptoms including flicker, no display, abnormal brightness, or excessive current consumption, depending on the failure mode and circuit design.
VCOM: LCD Common Electrode Voltage
VCOM is the common electrode voltage of the LCD panel and serves as an important reference voltage for LCD driving.
VCOM has a significant influence on the overall optical performance of the display. An inappropriate VCOM setting can contribute to problems such as image sticking or ghosting, color distortion with a whitish appearance, and flicker.
Unlike some driver supply voltages, the appropriate VCOM level depends strongly on the LCD panel itself. Therefore, the actual VCOM voltage range and target value should be determined from the panel datasheet and validated as part of the panel’s electrical and optical characterization.
In practice, even when the main supply rails are within their specified ranges, an incorrect VCOM level can still produce visible image-quality problems.
VGH: Positive Gate-On Voltage
VGH, also known as VGHO, is the positive voltage used to turn on the TFT gate electrodes.
When one row of the LCD panel is being scanned, the corresponding Gate line is driven to the VGH level. This turns on the TFTs in that row and allows the data voltage from the Source line to charge the corresponding pixel capacitors.
The typical VGH range is approximately 7 V to 20 V, although the exact value depends on the LCD panel and driver IC.
Because VGH controls TFT turn-on, an abnormal VGH voltage can prevent the TFTs from turning on correctly or from reaching the required operating condition. Depending on the nature of the fault, the display may show symptoms such as no display, an overall whitish image, horizontal lines, abnormal images, or other display defects.
VGL: Negative Gate-Off Voltage
VGL, also known as VGLO, is the negative voltage used to turn off the TFT gate electrodes.
After the scanning operation for a row is completed, the Gate line returns to the VGL level, ensuring that the TFTs are fully turned off. This helps isolate the pixel capacitors so that their stored charge remains stable until the next frame refresh.
The typical VGL range is approximately -15 V to -7 V, but the actual target voltage must be determined according to the LCD panel and driver IC specifications.
VGH and VGL work together to control the turn-on and turn-off states of the TFT gate electrodes. Therefore, abnormalities in either voltage can significantly affect the scanning and pixel-charging process. Common symptoms may include no display, an overall whitish image, image sticking, horizontal lines, and corrupted or garbled images.
When these symptoms occur during module debugging or failure analysis, VGH and VGL are important voltage rails to check.
VSS: Ground or Ground-Reference Voltage
VSS may also appear under names such as VSSA, VSSREF, VSSD, VSSP, and VSSH, depending on the driver IC architecture and pin naming convention.
In general, VSS-related signals represent a ground or ground-reference rail and are typically around 0 V. However, engineers should still verify the exact function of each VSS pin in the relevant IC datasheet rather than assuming that every VSS designation represents an electrically identical node.
GVDDP and GVDDN: Positive and Negative Gamma Voltages
GVDDP, also known as GVMP, represents the positive Gamma voltage used by the LCD display. The naming can vary between different driver ICs.
Its typical range is approximately 3.0 V to 6.0 V. During Gamma adjustment, the voltage may be adjusted in relatively small increments, such as 10 mV or 20 mV per step, although the actual adjustment resolution depends on the driver IC.
The corresponding negative Gamma voltage is GVDDN, also known as GVMN. Its typical range is approximately -3.0 V to -6.0 V, and its adjustment step may also be around 10 mV or 20 mV, depending on the IC specification.
Gamma voltages influence the analog voltage levels associated with different grayscale outputs. As a result, improper Gamma settings can affect grayscale reproduction, contrast, color performance, and the overall visual appearance of the LCD display.
Four Common Driver IC Input Power Modes
Some driver IC manufacturers classify their input power architectures as 2Power, 3Power, 4Power, and 5Power modes. The exact definition of these modes can vary between ICs, so the corresponding IC datasheet should always be used as the final reference.
In the 2Power mode, the main input voltages are VDDI + VCI. An external boost IC, also called a Power IC, is required to generate the positive and negative analog supply voltages such as VSP and VSN. Compared with the other architectures described here, relatively fewer products use the 2Power configuration.
In the 3Power mode, the input voltages are VDDI + VSP + VSN. No separate external boost IC is required for generating these main analog rails because the driver IC can perform the required voltage conversion internally. This type of architecture is widely used in integrated display driver designs.
The 4Power mode adds VDD to the architecture, giving VDDI + VSP + VSN + VDD. As with the 3Power configuration, no additional external boost IC is necessarily required. Many driver ICs are designed to support both 3Power and 4Power configurations.
The 5Power mode generally refers to an architecture involving five major voltage domains, typically VDD + AVDD + VSP + VSN + VCOM. The relevant voltage-generation and conversion functions can likewise be integrated into the driver IC, depending on the specific design.
Because terminology and power-domain definitions are not completely standardized across all LCD driver IC vendors, engineers should confirm the exact meaning of 2Power, 3Power, 4Power, and 5Power in the specific IC datasheet.
Why Are TFT-LCD Driving Voltages Important in Failure Analysis?
The driving voltages discussed above are among the key electrical parameters encountered in TFT-LCD display modules.
VDDI is mainly associated with logic and interface operation, while VSP and VSN provide the positive and negative analog supply rails for the Source Driver. VCOM acts as an important panel reference voltage, while VGH and VGL control the turn-on and turn-off of the TFT gate electrodes. GVDDP and GVDDN are associated with Gamma voltage generation and adjustment, which in turn affects grayscale and image characteristics.
When an LCD module shows symptoms such as flicker, no display, an overall whitish image, image sticking, horizontal lines, corrupted images, or abnormal current consumption, checking these voltage rails can help engineers narrow down the possible cause more efficiently.
However, the voltage values discussed in this article are typical ranges rather than universal specifications. The actual requirements can vary according to the LCD panel, driver IC, display size, operating mode, and overall circuit architecture. For accurate troubleshooting and circuit analysis, the LCD panel datasheet and driver IC datasheet should always be treated as the final authority.
The driving voltages covered here represent some of the most common and important voltage rails in TFT-LCD display modules, but they are not the only ones that matter. Different panels and driver ICs may involve additional bias, reference, pumping, or timing-related voltages. For engineers involved in TFT-LCD development, module manufacturing, testing, or failure analysis, studying the relevant LCD and IC datasheets in detail is essential to understanding the complete voltage-generation and driving system.