TFT LCD Basic Driving Methods

Introduction to TFT LCD Technology

To drive an m×n matrix-type TFT LCD (Figure 5-17), address signals from the scanning circuit (also called sequential scanning signals) are supplied to the gate lines [row lines] Y₁, Y₂, Y₃, ..., Yₘ in sequence. Simultaneously, data signals from the hold circuit (also called display data signals or analog input signals) are supplied to the data lines [called column lines or source lines] X₁, X₂, X₃, ..., Xₙ in sequence. Understanding this process is key to answering what is an lcd screen and how it operates.

TFT LCD pixels are arranged at the intersections of gate lines and data lines on the TFT substrate. Rows are selected sequentially by gate lines, specific pixels are chosen by data lines, and corresponding analog signals are written to them. This fundamental structure is essential to understanding what is an lcd screen and how it creates images through the manipulation of light.

Because TFT LCDs use "line sequential scanning," once the pixels corresponding to a gate line Yᵢ are selected, the pixels corresponding to gate line Yᵢ₊₁ are selected next, and so on, proceeding from Y₁ to Yₘ. When all gate lines are selected once from top to bottom, a single画面 (frame) is formed. If we divide the time of one frame by the number of gate lines, each line has a corresponding selection time t₁, t₂, ..., tₘ. This scanning method is crucial to understanding what is an lcd screen and how it refreshes images rapidly enough to create the illusion of continuous motion.

TFT LCD Operational Principles

Pixel Selection and Data Writing

During period t₁: An address signal is applied to gate line Y₁, turning on all TFTs connected to Y₁. As a result, data signals supplied to data lines X₁, X₂, X₃, ..., Xₙ pass through their corresponding TFTs and connect to the pixel capacitor Cₚₓₗ and storage capacitor Cₛₜ, charging both to the data signal voltage. This process is fundamental to understanding what is an lcd screen and how it maintains image integrity between refreshes.

During period t₂: All TFTs connected to gate line Y₁ turn off. This electrically isolates the pixels selected by Y₁ from the data lines. However, the pixel capacitor Cₚₓₗ and storage capacitor Cₛₜ maintain the data signal charged during t₁. This charge is maintained through subsequent periods t₃, ..., tₘ until the next frame's selection time. Meanwhile, during t₂, all TFTs connected to gate line Y₂ turn on, electrically connecting their pixels to the data lines, which then supply data signals through the TFTs to these pixels, charging Cₚₓₗ and Cₛₜ to the appropriate voltages. This sequence repeats to complete one frame's drive cycle.

The data signals supplied in this manner write to the pixel capacitor Cₚₓₗ and storage capacitor Cₛₜ through TFTs controlled by address signals, creating pixel voltages. However, it's the difference between this pixel voltage and the voltage applied to the counter electrode that actually drives the liquid crystal for image display. It's important to note that written data signals remain until the next address signal arrives—in other words, previously written data is maintained until overwritten by the next address signal, a process called refresh. Because the applied voltage corresponding to the data signal remains on the liquid crystal, from the liquid crystal's perspective, it operates in a fundamentally "static" state. This persistence is a key aspect of what is an lcd screen and how it differs from other display technologies.

AC Driving of Liquid Crystals

If liquid crystals were driven with DC voltage, their lifespan would be reduced, making AC voltage driving necessary. For this reason, the polarity of the voltage applied to the liquid crystal must reverse every frame. While reversing the polarity of the entire画面 simultaneously is simple and convenient, it's not actually used because it tends to cause flickering and brightness gradients across the画面. This alternating current approach is fundamental to understanding what is an lcd screen and how it maintains image quality over time.

The driving method where画面 polarity reverses simultaneously is called ① frame inversion driving. To reduce the various adverse effects of frame inversion driving on画面 quality, driving methods that reverse the polarity of each pixel with a slight phase difference are generally used. These include: ② a method with a slight phase difference in polarity inversion for each horizontal line (called H-line inversion driving); ③ a method with a slight phase difference in polarity inversion for each vertical line (called V-line inversion driving); and ④ a method with alternating polarity inversion for each pixel with a slight phase difference (called H/V-line inversion driving or pixel inversion driving). Additionally, the power supply to the common electrode can either be a DC voltage or an AC voltage with level inversion for each scanning line. Table 5-2 summarizes these various driving methods and helps explain what is an lcd screen's underlying complexity.

A frame refers to the time required to complete one画面 display when all gate lines are scanned sequentially from top to bottom. Considering human eye responsiveness, the frame rate (frame repetition frequency) is generally desired to be above 60Hz (with a period of 16.7ms). A frame rate of 60Hz means the画面 is rewritten 60 times per second. Among these 60 frames, there are naturally odd and even frames. The common potential is set so that the voltages applied to the liquid crystal layer in these odd and even frames are equivalently equal. However, for reasons described later, the voltage applied to the liquid crystal is generally asymmetric. This voltage asymmetry can produce 30Hz AC harmonics and DC components, which adversely affect display画面 quality, including "afterimages" (persistence of displayed images) and "flicker" (画面 blinking). Understanding these factors is crucial to comprehensively answering what is an lcd screen and how manufacturers optimize display performance.

Line Sequential vs. Point Sequential Scanning

TFT LCDs utilize line sequential scanning, which differs fundamentally from the point sequential scanning used in CRT displays. In line sequential scanning, an entire row is addressed at once, allowing all pixels in that row to receive their data simultaneously during their selection period. This approach is more efficient for matrix displays and contributes to the thin form factor that characterizes what is an lcd screen's physical appearance compared to older technologies.

The key advantage of line sequential scanning is that it allows for simpler drive electronics and lower power consumption compared to point sequential scanning. By addressing entire lines at once, the data can be latched and held, reducing the required data rate and simplifying the timing requirements. This efficiency is part of what makes TFT LCD technology so versatile and helps explain what is an lcd screen's widespread adoption in various devices from smartphones to large displays.

Another important aspect of line sequential scanning is how it interacts with the human visual system. By refreshing the entire画面 sufficiently quickly (typically 60 times per second or more), the brain perceives a continuous, flicker-free image even though the display is actually updating line by line. This persistence of vision effect is central to understanding what is an lcd screen's ability to display smooth motion and video content.

Advanced Technical Considerations

Capacitance and Signal Integrity

The pixel capacitor (Cₚₓₗ) and storage capacitor (Cₛₜ) play critical roles in maintaining signal integrity between refreshes. The storage capacitor is particularly important as it helps mitigate charge leakage, ensuring that the voltage across the liquid crystal remains stable during the frame period. The ratio between these capacitors is carefully engineered to balance response time and stability, which are key factors in determining what is an lcd screen's overall performance characteristics.

Voltage Asymmetry and Compensation

While efforts are made to maintain symmetric AC driving, practical implementations often result in some degree of voltage asymmetry. This asymmetry can lead to DC components that degrade liquid crystal performance over time. To address this, sophisticated compensation techniques are employed, including specialized common voltage generation and data signal calibration. These methods help ensure long-term reliability and consistent performance, which are essential aspects of what is an lcd screen's quality and durability.

Response Time and Refresh Rate

The relationship between response time (how quickly a pixel can change state) and refresh rate (how often the entire画面 is updated) is crucial for motion handling. Higher refresh rates (such as 120Hz or 240Hz) reduce motion blur but require faster response times from the liquid crystal material and more efficient driving circuitry. Balancing these factors is a key engineering challenge in modern TFT LCD design and contributes significantly to what is an lcd screen's suitability for different applications, from static text displays to fast-action gaming.