Functions of vcom in TFT

The specific voltage value is determined based on the input data and the com voltage. It is used to display different gray scales, that is to say, the realization of the Color Display gamma is simply to divide the change process from white to black into 2 N (6, 8) idempotence.

Gamma voltage is used to control the brightness of the display, which is generally divided into G0 ~ G14, the difference between different gamma voltage and vcom voltage causes different LCD rotation angles to form brightness differences. The best condition of vcom voltage is that it is located in the center of G0 and G14, in this way, the LCD screen will flash better, which is basically hard to achieve. Liquid Crystal requires dynamic voltage control, otherwise it is easy to form inertia. G0 ~ The pressure difference between G14 and vcom forms a voltage mode similar to a sine wave, which avoids the inertia of the liquid crystal at the same brightness. I do not know whether to make it clear.

The clamping pressure of source and vcom is the actual pressurization of liquid crystal, which can also be understood as gamma voltage. You can adjust the DC value of vcom to adjust the flicker. Taking sourceic 6-bit as an example, 14 gamma power supplies will be sent to sourceic, and these 14 power supplies will be connected to the electrical resistance string of sourceic, this group of electronic resistance strings can be divided into 128 (because there are two computers in the gray market), and the corresponding power score is 64 gray lines (because it is 6 bit, so there are only 64 gray charts), and then the FRC technology later converts it into a 256 mirror (which is the same for all big data display devices ), when a gray image is to be written, the corresponding computer will be generated. The role of Gamma is to determine the approximate shape of gammacurve, in order to provide the accurate location of the corresponding electronic signal, it is actually a/d digital signal. Vcom is a computer test. It only has data. How can it form a power gap? Two LCD terminals need electricity, and they are switching between power grids. Therefore, vcom will be in the first gamma TV and the last TV. (The first gamma TV and the last Gamma TV both refer to the same gray TV, but they only represent a positive TV) whether vcom is DC or DC + AC is determined by the nature of the screen. for dotinversion screens, only DC vcom can be used. For lineinversion screens, DC vcom can be used. However, to save power, DC + AC vcom is often used. for DC + AC vcom, the general practice is to output vcomac from tcon, And Then zoom in after op, then couple the vcomdc, and finally send the vcom signal to the screen. the GAMMA value is related to the brightness of different gray scales. The brightness of different views is certainly different, so gamma will change. This is the so-called gammashift. "vcomac is designed to prevent the appearance of flick." ==> vcomac is designed to reduce power consumption. "No vcomac signal is displayed on the digital screen." ==> the screen does not use otinversion instead of vcomac. vcomac signal, similar to lineinversion digital screens Instead of using a digital Pol signal, the vcomac signal is used to prevent flick from "=> Pol signal is used to determine whether the output is a positive voltage or a negative voltage for driveric, that is to say, Pol determines the positive and negative performance of the dot. No matter which type of inversion method, you must ensure that the polarity of this field is the opposite to that of the next one for a fixed point. in fact, you do not express it as an odd or even number of fields. It is neither this field nor the next one... but not from the implementation perspective... in addition, can you tell me the 26pin Analog Screen oev and Oeh what is the phase difference... the Calling headers are all big... in addition, the pol of the digital-to-analog hybrid screen is sampled by the rising edge of Oeh (some manuals also write LD). Assume that the AC part of vcom is directly generated using the logic of generating Pol, don't you need to modify the phase? In terms of logic implementation, can the AC of vcom be considered as being sampled by Oeh?
Vcomac should also be able to improve the contrast "the pol signal is used to determine whether the output is a positive voltage or a negative voltage for driveric, that is to say, Pol determines the positive and negative performance of the dot. "This refers to the driver inside the LCD screen ??
What you said is capable of adjusting the comparison because it changes the black and white voltage of the liquid crystal. The avdd of the vcomac screen is generally about 5 V, and the avdd needs about 10 V if the DC vcom is used. Vcomac is used to save power.
I am engaged in the application of small size TFT-LCD screen for almost two years, now I have summarized some experience in the LCD screen application in the past two years to tell you, some new people who have just entered this industry may have their own opinions. If you are not a beginner, you will inevitably say something wrong in some places. I also hope that you can give a generous correction!

The first thing to get a panel is to look at its specifications. The important parameters include: lamp operating frequency, lamp current, operating voltage, data/time series interface, etc, now I am using analog screens (Digital screens are similar ).

1, TFT-LCD LCD screen I do not light, mainly rely on the screen after the tube to light, we saw on the Panel Brightness, contrast, etc, to put it bluntly, it is displayed based on the light of the lamp tube. Why is the brightness of the Panel very dark? In fact, the light transmittance of the lamp is controlled by changing the deflection of the LCD itself, therefore, we can intuitively adjust its brightness. However, for the control of brightness and contrast parameters, we can not only control the brightness by the lamp, you can also adjust the video signal y/C or RGB parameters generated by video decoding. The driving Panel itself needs to comply with the various sequence of panel, we do this part of the work of the circuit called T-CON, T-CON can control the image zoom ratio, left and right up and down reversal and other functions, to present an image on a panel, you need to decode the video to obtain the RGB color. Similarly, we call this part of the circuit decoder, it decodes the input video signal into RGB that meets the needs of the Panel, and involves some TV principles. If you do not know, you can find the relevant information.

2. The lamp we mentioned just now is the CCFL we often call (of course there are also LED light-emitting). Its lighting principle is somewhat the same as that of the fluorescent lamp and requires a high start voltage, it is hundreds of volts or even thousands of volts, which should be used according to the screen size. However, the important part of the screen is its lamp current, the lamp current affects the brightness of the lamp, and more importantly, its service life. If the lamp current is too high, the service life of CCFL is a challenge. In the early days, the lighting circuit was based on Royer, and there were multiple methods such as semi-bridge and full-bridge. The above methods had their own advantages and disadvantages. The Royer circuit is used to enable and disconnect the base current of the switch transistor by starting the base resistance of the transistor and taking advantage of the saturation characteristics of the transformer. The circuit structure of the transformer is simple, but the loss of the switch tube is high, easy to break. The advantage is that the sine wave is relatively standard, which has a good effect on the service life of the LCD panel. Full bridge circuit is the use of four toggle transistor bridge circuit, because it adopts the zero voltage switching mode, so the power loss of the switch tube is very small, with the increase of TFT-LCD application, now there are many integrated IC lighting circuits. Chips such as bit3107 and oz960

3, CCFL can light up, then we will start to say T-CON, view Panel specifications you can find there are a lot of training number, what stvd, stvu, sthr, sthl, Oeh, CPH and so on. STV is related to the field, and something is related to the line. What is reflected in the image is related to the reversal, in fact, such a variety of time series for beginners can first do not need to know so much, now a lot of T-CON manufacturers have made these signal interfaces, you only need to connect the panel according to their solution. Of course, it is also necessary to understand the timing above. However, one of the most important signals is vcom, which is the public electrode of the liquid crystal in the panel, which has a significant impact on the display effect. It can be found in the specifications, which is a square wave, the frequency is about half-line frequency. Its AC feature has a relationship with the brightness of the display, and the DC feature has a relationship with the jitter of the display screen. You need to adjust this parameter for each panel.

4. the decoding circuit is actually an RGB three-color Circuit Used to decode the input video signal into a panel. However, a good decoding scheme can bring an ideal effect to the image, in the past, the most common problems were the jitters of character edges in some images, the color was not bright enough, and the image quality was not clear enough,

Summary, the above description is actually a liquid crystal driving circuit constitute the idea, but now there are a lot of Single Chip Solutions, decoder and T-CON and other technology, in this way, most books need to use software to set parameters. The circuit is also simple, and the OSD technology is also an important part of the LCD driver. For people who have just entered this industry, I personally think that it is a good way to understand the Single-Chip Solution by doing a good job of separate circuits first.