1. Liquid crystal display principle

1.1 Formation and types of Liquid Crystal

We generally think of objects as having three states: solid, liquid, and gaseous, but that's just for water, with some organization and matter. There is also the intermediate state between the solid and liquid states which is the liquid crystal state.




Based on their composition and the physical conditions under which they occur, liquid crystals can be divided into two categories, thermally-induced liquid crystals, and solvothermal liquid crystals.

Thermoluminescence: Liquid crystal formed when certain organic substances are dissolved by heating, and the liquid crystal lattice is destroyed by heating.

Solvation liquid crystal: The liquid crystal is formed when certain organic substances are put in a certain solvent, which destroys the liquid crystal lattice.

The liquid crystal materials used for display are thermal liquid crystals, and there are a large number of soluble liquid crystals in biological systems. Liquid crystal molecules formed by rod-shaped molecules (thermochromic liquid crystals) have three types of liquid crystal phases.

Near-phase liquid crystals (smectic liquid crystals)

Nematic liquid crystals (nematic liquid crystals)

Cholesteric liquid crystals

Near-crystalline phase liquid crystal molecules were two-dimensional orderly, molecular arrangement into layers, the viscosity and surface tension of this liquid crystal is relatively large, not sensitive to external electricity, magnetism, temperature, and other changes.


The molecules of centrifugal liquid crystals are one-dimensional and orderly, they can slide up and down, left and right, back and forth, the molecules are arranged and move more freely, and they are sensitive to an external magnetic field, temperature, and stress, they are the main materials of the current display.


Cholesterol liquid crystal is very much like a linear liquid crystal, but from the Z-axis will find his pointing vector with layers of different like a spiral distribution, certain electric and magnetic fields also make it into a liquid crystal.


1.2 Optical and electrical properties of liquid crystals



When it comes to optical electronics will be followed by complex principles and calculation formulas, which simply means that liquid crystal molecules have anisotropic properties in terms of dielectric coefficient and refractive index, etc. Therefore, we can use these properties to change the intensity of the incident light, in order to form a grayscale, which is used in display components. Liquid crystals are used in displays to cause a change in the vectorial direction of light. Light is also an electromagnetic wave that has vectorial properties. Natural light is directed in all directions, but after being filtered by a polarizer, only light in the same direction remains.






1.3 Structure of the liquid crystal screen

Two polarized fence angles perpendicular to each other prevent light from passing through at all, a test done with polarized sunglasses.




Liquid crystal display is to use the polarizing plate to complete this feature, using the upper and lower two fences perpendicular to each other between the polarizing plate filled with liquid crystal, in the use of electric field control of liquid crystal branch rotation, to change the direction of travel of light, so that different electric field size, will form different color degrees.





When the incoming light passes through the lower polarizer, the light wave is polarized in one direction, and when it passes through the liquid crystal molecules, the liquid crystal molecules rotate a total of 90 degrees. The angle difference between the lower polarizer and the upper polarizer is also exactly 90 degrees. This allows light to pass through, and if the light hits the red filter, it appears red. If the liquid crystal molecules are standing, and the light path is not changed, the light cannot pass through the upper polarizer, and the liquid crystal under the blue filter cannot be displayed.

Structure of the LCD section.




Structure of the LCD section.



As we have mentioned from the beginning, a liquid crystal display needs voltage control to generate grayscale. A TFT LCD uses a thin-film transistor to generate a voltage to control the direction of the liquid crystal. In the cross-sectional structure shown in Fig. 8, the liquid crystal is sandwiched between the upper and lower glass layers to form a parallel-plate capacitor, which is called a CLC (capacitor of liquid crystal). The size of this capacitor is about 0.1pF, but in practice, it does not hold the voltage until the next time the screen is updated. This means that when the TFT charges this capacitor, it does not hold the voltage until the next time the TFT charges this point. (With a normal 60Hz screen update frequency, it takes about 16ms to hold.) As a result, when the voltage changes, the grayscale displayed will not be correct. For this reason, the panel is usually designed with a storage capacitor (CS) of about 0.5pF (0.5pF), so that the charged voltage can be maintained until the next screen update. However, it is correct to say that the TFT itself, which grows on the glass, is only a switch made using a transistor. Its main job is to determine whether or not the voltage on the LCD source driver should be charged to this point. It is up to the external LCD source driver to determine how high the voltage should be charged to this point in order to display the grayscale. This is determined by the external LCD source driver.


We know that red, blue, and green are the so-called primary colors. This means that these three colors can be used to create a variety of different color combinations. Many flat panel displays use this principle to display colors. We divide the three RGB colors into three independent points, each with its own grayscale variation, and then use the three adjacent RGB display points as a basic unit of display, which is the pixel. The pixel, then, can have a different color variation. For a screen that requires a resolution of 1024*768, we can display the screen correctly as long as there are 1024*768 pixels on the flat panel display. In Figure 9, the black part between each RGB point is called the black matrix. If we look back at Figure 9, we can see that the black matrix is mainly used to cover the part that is not intended to transmit light. For example, the ITO lines, or the Cr/Al lines, or the TFT parts. This is why in figure (ix), each RGB highlight does not appear to be a rectangle, but rather a black matrix in the upper left corner, where the TFT is located.

Common filter arrangement.




The bar arrangement is most commonly used on laptops or desktop computers. The reason for this is that with today's software, most of which is windowed, we see the content of the screen as a bunch of boxes of varying sizes, and the stripes are just enough to make the edges of the boxes look straighter, without looking frayed or jagged.

In AV products, because most of the TV signals are characters, and most of their outlines are irregular curves, the AV products are

Was using a mosaic arrangement, which has now been improved to use a triangular arrangement.



Each pixel on the LCD panel is divided into three colors, red, green and blue, RGB is the so-called three primary colors, using these three colors can be mixed in a variety of different colors, we divide the three RGB colors into three independent points, each with a different grayscale variation, and then we use the three adjacent RGB display points as a display of the basic unit, which is the pixel, this pixel can have a different color variation.


Color depth.

256 Color 8(R)*8(G)*4(B) = 256 Color

High Color  

32 (R) * 64 (G) * 32 (B) = 65536 Color

Full Color  

64 (R) * 64 (G) * 64 (B) = 262144 Color

True Color

256 (R) * 256 (G) * 256 (B) = 167




Therefore, only the area is shown on the right side of Fig. 11 is left as an effective transmittance area. The ratio of this effective light-transmitting area to the total area is called the aperture ratio. The light emitted from the backlight panel will sequentially pass through the polarizer, glass, liquid crystal, color filters, and so on. Assuming the light transmission rate of each component is as follows:


Polarizer: 50% (because it only allows the passage of light waves in one direction).

Glass: 95% (need to calculate upper and lower pieces)

Liquid Crystal: 95%

Aperture ratio: 50% (only half of the effective light transmission area)

Color filter: 27% (set the material itself is 80% penetration because the filter itself is painted with color, only allow the color of the light waves through, so only one-third of the remaining brightness, a total of 80% * 33% = 27%.)

To calculate the penetration rate mentioned above, the light from the backlight plate will only leave 6%, which is really very little. This is why in the design of TFT LCD, to try to increase the aperture rate of the reason, as long as the aperture rate is increased, you can increase the brightness, while the brightness of the backlight plate is not so high, can save power consumption and costs.


2. LCD internal circuit



The main parts that drive TFT work are the following. 

1.source driver, responsible for power supply. 

2.gate driver, responsible for turning on and off.The Gate driver is responsible for opening and closing the gate.

3. The timing control circuit, responsible for controlling the gate driver.

4.Gamma Control Circuit




From Figure (XIII) we can see the equivalent circuit of the whole panel, where each TFT is connected in parallel with two capacitors, representing a display point. For a basic display unit pixel, we need three such display points, representing the RGB primary colors respectively. For a 1024*768 resolution TFT LCD, a total of 1024*768*3 such dots are needed. The general structure of the whole panel is like this, then by the waveform from the gate driver in the figure, each TFT row is opened in sequence, so that the source driver of the whole row can simultaneously charge a whole row of display points to the required voltage to display different grey levels. When one row is charged, the gate driver will turn off the voltage, and then the next row will turn on the voltage, and then the same source driver will charge and discharge the next row's display points. In this order, when the display in the last line is charged, it will start charging again from the beginning of the first line. For a 1024*768 SVGA resolution LCD, there will be a total of 768 gate lines, and the source lines will be 1024*3=3072. For an LCD monitor with a 60Hz refresh rate, the display time of each screen is about 1/60=16.67ms. Since the screen is composed of 768 lines of gate lines, the switching time for each gate line is 16.67ms/768=21.7us. Therefore, in Figure (XIII) As we can see from the waveforms sent out by the gate driver, these waveforms are one pulse after another with a width of 21.7us, opening each TFT line in sequence. The sourced driver will charge and discharge the display electrodes to the required voltage during the 21.7us period via the source routing. , so as to display the corresponding grayscale. 

Open the first line first, close the others.


Then close the first row, with the voltage already fixed and the solid color fixed, then open the second class, close the rest, and so on.




Another characteristic of liquid crystal molecules is that they cannot be fixed at a certain voltage for a long period of time, otherwise, even if you cancel the voltage, the liquid crystal molecules will no longer be able to rotate in response to the changes in the electric field and form different gray levels. For this reason, it is necessary to restore the voltage to its original state every once in a while to avoid damaging the characteristics of the liquid crystal molecules. But what happens if the screen does not move, i.e., if the screen always shows the same grayscale? So the display voltage in an LCD is divided into two polarities, one positive and the other negative. When the voltage of the display electrode is higher than the common electrode voltage, it is called positive polarity. When the voltage at the display electrode is lower than the voltage at the common electrode, the polarity is called negative. Whether the polarity is positive or negative, there is a set of gray levels of equal brightness. Therefore, when the absolute value of the pressure difference between the upper and lower glass layers is fixed, the gray levels are identical regardless of whether the voltage of the display electrode is high or the common electrode is high. In both cases, however, the liquid crystal molecules are turned in opposite directions, which prevents the characteristic damage that occurs when the liquid crystal molecules are turned in one direction. In other words, when the display remains unchanged, we can still achieve the result of the display remaining unchanged and the characteristics of the liquid crystal molecules not being destroyed by the constant alternation of positive and negative polarity. So when you see an LCD screen that is not moving, the voltage inside the screen is constantly changing, and the liquid crystal molecules are constantly rotating in one direction and the other in the opposite direction.

The following diagram shows how the panel reverses.



Combined display performance and circuit design cost Most LCDs use the common constant dot inversion method.

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TFT liquid crystal display principle