LCD backlight adjustment and drive

LCD backlight adjustment and drive

Principle of backlight adjustment based on PWM

In small and medium-sized LCD screens, white LEDs are generally used as the backlight source of the screen. PWM is pulse width modulation, PWM dimming is to use the human eye's visual pause principle to control the conduction of the LED with a square wave of a certain frequency and duty cycle. The forward current of the LED switches back and forth from zero current to the rated operating current. Through high-speed switching of the backlight, square waves with different duty cycles are provided periodically to achieve brightness adjustment. As long as the LED forward current is constant when it is turned on, the white light emitted will not have a color shift, and as long as the frequency is higher than 100 Hz, the human eye will see a continuous light source. Assuming that the high level represents turning on the backlight, and the low level represents turning off the backlight, different ratios of the backlight on and off time will result in square waves with different duty cycles. From the output waveform, the average power of the wave is different, so that different brightness is obtained and the backlight adjustment is realized.

Pulse width modulation (PWM) is the use of the digital output of the microprocessor to control the analog circuit. In short, PWM is a method of digitally encoding the level of the analog signal. This way, the analog circuit is controlled digitally. The method can greatly reduce the cost and power consumption of the system, so it is widely used in many fields from measurement and communication to power control and conversion.

The working principle of TFT-LCD

TFT-LCD (Thin Film Transistor Liquid Crystal Display), a liquid crystal display requires voltage control to generate grayscale. TFT uses a thin film transistor to generate voltage to control the display of liquid crystal turning. The liquid crystal is sandwiched between the upper and lower layers of glass to form a parallel plate capacitor, which we call CLC (capacitor of liquid crystal). Its size is about 0.1pF, but in practical applications, this capacitor cannot keep the voltage down. When the image data is updated again. That is to say, when the TFT charges the capacitor, it cannot hold the voltage until the next time the TFT recharges this point. (At the general image update frequency of 60Hz, It needs to be maintained for about 16ms.) In this way, if the voltage changes, the displayed grayscale will be incorrect. Therefore, generally in the panel design, a storage capacitor CS (storage capacitor about 0.5pF) is added , So that the charged voltage can be maintained until the next time the screen is updated. But to be correct, the TFT itself growing on the glass is just a switch made of transistors. Its main job is to determine the LCD source driver Is it necessary to charge the voltage to this point? As for how high the voltage should be charged at this point, in order to display what kind of grayscale. It is determined by the external LCD source driver.