TFT-LCD display principle

What is LCD

We generally think that objects have three states: solid, liquid, and gas. In fact, this is only for water. There are some organic compounds and the state between solid and liquid. When the two polarized barriers of liquid crystal are perpendicular to each other, the light It is completely impossible to pass. The liquid crystal display is completed by using the characteristics of the polarizing plate. The polarizing plate between the upper and lower fences is filled with liquid crystal, and the electric field is used to control the rotation of the liquid crystal branch to change the direction of light. , In this way, different electric fields will form different colors.

When the electrode is not added, when the incident light passes through the polarizer below, a unidirectional light wave will remain. When passing through the liquid crystal molecules, the liquid crystal molecules rotate 90 degrees in total, so when the light wave When it reaches the upper polarizer, the polarization direction of the light wave turns by exactly 90 degrees. The angle between the lower polarizer and the upper polarizer is exactly 90 degrees different. Therefore, the light can pass smoothly, and if the light hits the red filter, it will appear red. When the electrode is added (the largest electrode), the liquid crystal molecules are all standing under the influence of the electric field, and the light path does not change, the light cannot pass through the upper polarizer, and it cannot be displayed.

TFT-LCD driver circuit

In order to display arbitrary graphics, TFT-LCD uses m × n dots arranged in a progressive scan matrix display. The driving modes that have been formed include: voltage selection mode, ramp mode, DAC mode and analog mode. Since TFT-LCD is mainly used in notebook computers, the drive circuit is roughly divided into: signal control circuit, power supply circuit, gray voltage circuit, common electrode drive circuit, data line drive circuit and address line drive circuit (gate drive IC). The main function of the above-mentioned drive circuit is: the signal control circuit supplies the digital signal, control signal and clock signal to the digital IC, and supplies the control signal and clock signal to the gate drive IC; the power supply circuit supplies the required power voltage to the digital IC and the gate Driver IC; gray voltage circuit supplies each of the 10 gray voltages generated by the digital driver circuit for data driving; common electrode driver circuit supplies common voltage to the common electrode relative to the pixel electrode; data line driver circuit sends the signal control circuit The 6-bit display data and clock signal of each of the RGB signal are latched in a timing sequence and continue to enter the internal. Then the display data is converted into an analog signal by a 6-bit DA converter, and then converted into an impedance by the output circuit, and supplied to the LCD screen. Data line; the gate drive circuit sends the clock signal from the signal control circuit, through the shift register conversion action, the output circuit is switched to ON/OFF voltage, and sequentially added to the LCD screen. Finally, assemble the drive circuit on the TAB (Automatic Welding Flexible Circuit Board), and use ACF (Anisotropic Conductive Adhesive Film) and TCP (Drive Circuit Flexible Lead Tape) to connect with the LCD screen.

TFT-LCD working principle

The principle of liquid crystal display is based on the characteristic that the light transmittance of liquid crystal changes with the magnitude of the applied voltage. When the light passes through the upper polarizer, it becomes linearly polarized light, and the polarization direction is consistent with the vibration direction of the polarizer, and the order of the liquid crystal molecules on the upper and lower glass substrates is consistent. When light passes through the liquid crystal layer, linearly polarized light is split into two beams due to being refracted by the liquid crystal. And because the two beams of light have different propagation speeds (the same phase), when the two beams of light are combined, the vibration direction of the vibrating light will inevitably change. The light passing through the liquid crystal layer is gradually distorted. When the light reaches the lower polarizer, its optical axis vibration direction is twisted by 90 degrees and is consistent with the vibration direction of the lower polarizer. In this way, the light passes through the lower polarizer to form a bright field. After the voltage is applied, the liquid crystal is oriented under the action of the electric field, and the distortion disappears. At this time, the linearly polarized light passing through the upper polarizer no longer rotates in the liquid crystal layer and cannot pass through the lower polarizer to form a dark field. It can be seen that the liquid crystal itself does not emit light and can only be displayed under the modulation of an external light source. During the entire display process, the liquid crystal acts as a voltage-controlled light valve. The working principle of TFT-LCD can be summarized as follows: when the gate positive voltage is greater than the applied voltage, the drain-source electrode is turned on, and when the gate positive voltage is equal to 0 or negative voltage, the drain-source electrode is disconnected. The drain electrode is connected to the ITO pixel electrode, the source electrode is connected to the source line (column electrode), and the gate is connected to the gate line (row electrode). This is the simple working principle of TFT-LCD.

One of the most important parameters in liquid crystal displays is brightness, and the most important factor that determines brightness is the aperture ratio. The aperture ratio is the ratio of the effective area through which light can pass. Each pixel on the LCD panel is divided into three colors: red, green, and blue. RGB is the so-called three primary colors. Various colors can be mixed by using these three colors. We divide the three colors of RGB into three independent points. Each has a different grayscale change, and then the three adjacent RGB display points are regarded as the basic unit of a display, which is a pixel, and this pixel can have different color changes.