How the TFT LCD pixels are maintained

Because of the capacitors!

TFT-LCD displays work in many ways that are different from TN-LCDs. The imaging principle of TFT-LCD liquid crystal display is to use the "back-transmitted" irradiation method. When the light source is illuminated, it is first transmitted upwards through the lower polarizer, and the light is conducted with the help of liquid crystal molecules.

Since the electrodes of the upper and lower layers are changed to FET electrodes and common electrodes, when the FET electrodes are turned on, the arrangement of liquid crystal molecules will also change, and the purpose of display is also achieved by shading and transmitting light. However, the difference is that because FET transistors have a capacitive effect, they are able to maintain a potential state, and the liquid crystal molecules that previously transmitted light will remain in this state until the next time the FET electrodes are energized and change their arrangement.

In the LCD structure, there is a pixel capacitor, which can be used to form an electric field by turning on and discharging, so that the angle of the liquid crystal molecule can be controlled.

Generally, the design of this capacitor will affect the problem you mentioned: when the capacitor does not receive the next electrical signal, it will remain in the same state, so that the electric field formed by its line will not change, and then the angle of the liquid crystal will not change, and the macroscopic image will be maintained.

For more information, please refer to the schematic diagram of the relevant TFT-LCD structure.

This is controlled by the TFT circuit.

To put it simply, when the data is written, the TFT is turned on, and the charging is accepted, and the angle of the liquid crystal molecule is controlled when it is completed, and then the TFT is turned off, and the TFT behind is turned on and charged again.

When the TFT is turned off, the voltage is maintained, which ensures that the voltage changes very little until the next refresh, i.e. the deflection angle of the liquid crystal analysis remains the same. This will keep the original content.

Actually when the voltage is input, the TFT is turned on, the first step is to discharge, and then it will be recharged, the process of writing.

You can take a look at the charging and discharging principle of the triode, and the LCD uses the triode principle for charging and holding.

Liquid crystals are a state of matter that is somewhere between liquid and crystalline. In addition to having some properties of both liquid and crystal (such as fluidity, anisotropy, etc.), it also has its own unique properties. The liquid crystal controls the arrangement of the liquid crystal under different electric fields to obtain the amount of light with different transmittance, so as to achieve the display principle.

Therefore, as long as the electric field of this point does not change, the content displayed at this point does not change.

Organic electroluminescent devices (OLEDs) are all-solid-state devices that directly convert electrical energy into light energy, which have attracted great attention because of their advantages of thinness and lightness, high contrast, fast response, wide viewing angle, and wide operating temperature range, and are considered to be a new generation of display devices. In order to truly realize its large-scale industrialization, it is necessary to improve the luminous efficiency and stability of the device, and design an effective image display driver circuit. Recently, with the deepening of research, the luminous efficiency and stability of OLED have met the requirements of some applications, and its dedicated drive circuit technology is not very mature.

At present, all flat panel displays are driven by matrix drives, which are matrix displays composed of X and Y electrodes. According to the introduced and unintroduced switching components in each pixel, the matrix display is divided into active matrix (AM) display and passive matrix (PM) display. Starting from the TFT-OLED active matrix pixel cell circuit, the voltage-controlled and current-controlled pixel cell circuits are analyzed, and the influence of the control driver IC on the TFT-OLED active driving circuit is briefly discussed.

Its working principle is as follows: when the scan line is selected, the switch T1 is turned on, the data voltage is charged to the storage capacitor CS through the T1 tube, and the voltage of the CS controls the drain current of the drive tube T2; When the scan line is not selected, T1 cuts off, the charge stored on CS continues to maintain the gate voltage of T2, and T2 remains on, so the OLED is under constant current control throughout the frame period. The constant current source structure and the source follow structure, the former OLED is at the drain end of the drive tube T2, which overcomes the influence of the change of OLED opening voltage on the current of the T2 tube. The latter is more process-wisely achievable.

The disadvantage of the two-tube circuit structure is that the inconsistency of the threshold voltage of the T2 driver tube will lead to the uneven brightness of each display, and the current and data voltage of the OLED are non-linear, which is not conducive to the adjustment of gray scale. The source voltage of the T1 tube should be lower than the turning on voltage of the OLED to prevent the OLED from turning on.

Gohjc et al. proposed a voltage control circuit that uses subthreshold current to compensate for threshold voltage changes, adding a compensation stage to the drive sequence, so that the driver works in the subthreshold region, and the gate source voltage of the drive tube, that is, the threshold voltage bending VTH, is stored in the storage capacitor, and the voltage can compensate for the drift of the TFT threshold voltage in the data input stage. They also proposed a voltage-controlled drive circuit that uses the discharge type to compensate for the change of threshold voltage, which is different from the former circuit in that the drive tube enters the subthreshold region by means of discharge, and obtains the superposition value of the data voltage and the threshold voltage, so as to effectively compensate for the change of threshold voltage. In addition to effectively compensating for threshold voltage changes, the voltage control drive circuit also has the advantage of fast response characteristics, because the voltage is directly applied to both ends of the storage capacitor CS, and the charging current will have a momentary high current to charge the capacitor at the beginning, which greatly reduces the charging time.