Driving principle of TFT LCD liquid crystal display
The core of TFT-LCD driving is the relationship between voltage and transmittance. The driving voltage determines the brightness of the sub-pixels. Different RGB sub-pixel brightness combinations form various colors, and different colors on different pixels finally form a colorful picture.
A brief introduction to the driving principle
Gamma adjustment and correction
By VT curve, it is the relationship curve between transmittance and driving voltage. The VT curve is a nonlinear response, and the exponential curve produced by this nonlinearity is called gamma. The most important work of driving the LCD display is to perform gamma adjustment and gamma correction.
Gamma correction (Gamma correction), also known as gamma nonlinearity and gamma encoding, is used to target the luminance or trichromatic stimulation of light in a film or imaging system. Values (tristimulus values) performed non-linear operation or inverse operation.
The main reasons for TFT-LCD gamma adjustment : ① Utilize the instinctive response of human eyes to brightness; ② All video signals in history have been corrected by CRT gamma; ③ Unify the visual effects of TFT-LCD produced by different liquid crystal materials and different manufacturers .
Note: Why does the monitor need Gamma correction ? Because the human eye's perception of brightness is not directly proportional to the physical power, but is a power function. The exponent of this function is usually 2.2, which is called the Gamma value.
Explanation : The inherent distortion of TFT-LCD will make the middle gray scale of the picture darker. If the input is a color picture signal, this distortion will not only cause the displayed picture to be darker but also shift the tone of the picture. After gamma correction can effectively remove these distortions.
drive mode
In the application of AC drive, if the liquid crystal molecules remain unchanged under a fixed voltage, the characteristics of the liquid crystal molecules will be polarized (cured). After canceling the fixed voltage, the liquid crystal molecules will no longer respond to changes in the applied voltage. In order to avoid this situation, an AC voltage driving method is adopted . However, the liquid crystal molecules do not rotate continuously with the AC voltage, but the AC driving changes the direction of the electrical dipole moment of the liquid crystal molecules.
Due to the flicker problem of AC drive, usually in TFT-LCD, the space fusion method is used to eliminate the flicker component, so that adjacent pixels maintain opposite driving polarities, so that the optical response waveforms of adjacent pixels have a phase difference of 180° , which requires that the driving voltages of adjacent pixels remain opposite in polarity.
There are several ways to realize polarity inversion of adjacent pixels: dot inversion, row inversion, column inversion, column 2 dot inversion, row 2 dot inversion.
The dot inversion driving method is the most delicate in flicker space fusion, and it is refined to each sub-pixel, which has the best flicker suppression effect and the largest power consumption.
Column inversion There is no phase difference between the sub-pixel flicker waveforms on all columns, and vertical line flicker is easy to appear. It is a low-frequency inversion with the lowest power consumption. There is no phase difference between pixel flicker waveforms on all sub-rows in line
inversion , and horizontal line flicker is easy to occur, and the power consumption is as large as dot inversion.
Gray scale enhancement technology
TFT-LCD basically uses digital signals for display transmission, and increasing the number of gray scales displayed by TFT-LCD can increase the smooth and delicate display of realistic images. Since the larger the data bit to be transmitted, the higher the cost of the driving circuit , an algorithm is generally used to increase the number of gray scales. The ways to realize the grayscale enhancement technology include: frame rate control (Frame Rate Control, RFC) and pixel dithering (Pixel Dithering, PD).
The key to FRC technology is to control the number of occurrences of two real gray scales, each corresponding to one frame time. There are fine picture disturbances, called FRC noise. The premise of improvement is to eliminate the different brightness differences of FRC pixels, and gray scale correction technology is generally used to eliminate brightness differences.
TFT-LCD Circuit Technology
TFT-LCD drive circuit includes power supply circuit (Power IC), timing control circuit (TCON IC) , gray scale circuit, data drive circuit (Source Driver IC) , scan drive circuit (Gate Driver IC) and system interface (System I/F ).
The system interface provides various display data and timing control signals to the TFT-LCD drive circuit.
① After part of these data and signals are transmitted to the Power IC, the power supply voltage required for other circuit operations and the liquid crystal deflection reference voltage Vcom are generated.
② A part is transmitted to TCON to generate the working timing of the data driving circuit (SD) and scanning driving circuit (Gate), as well as the overall timing of TFT-LCD.
③ The data drive circuit (SD) converts the signal from TCON related to the display data into an analog voltage, and outputs it to the pixel electrode to form the voltage required for liquid crystal deflection.
④Gate IC generates high and low level digital voltages, which are output to the gate of the TFT switch to control the switching status of each row of pixels.
⑤The grayscale circuit generates the reference voltage (Gamma reference voltage) required by the DAC (digital-to-analog conversion) part of the data drive circuit.
