# Introduction of PWM dimming principle

- Aug 03, 2018-

With LED backlight, energy saving, environmental protection, high performance advantages highlighted, LCD backlight gradually switch from CCFL to LED, at present, the company's LCD screen also gradually from CCFL backlight model to the LED backlight model switching. The difference from two kinds of backlight model drive way, CCFL backlight screen backlight drive needs to provide high voltage inverter, and LED backlight driving method is relatively simple, you just need to constant low voltage dc power supply. But whether it is the inverter or the LED backlight drive circuit, a led pwm dimmer technique is used to adjust the brightness of backlighting. Here's a summary of the principles of PWM regulation.

1. Take the Duty Cycle or Duty Ratio to understand the principle of PWM dimming, first to understand the concept of space Ratio. The explanation of the space ratio can be summarized as follows: 1) in a series of ideal pulse sequences (e.g., square wave), the duration of the positive pulse is the ratio of the total period of the pulse. For example, the pulse width of 1 mu s and the pulse sequence of the signal period 4 mu s are 0.25. 2) the ratio of time to total time during a continuous working time. 3) in the phenomenon of periodicity, the ratio of time to total time. In fact, it is the ratio of the effective time of the release energy of the circuit to the total release time.

2. The dimmer ratio is calculated according to the following method: Foper= working frequency; Fpwm= dimming frequency; Adjustment ratio = Foper/Fpwm, (in fact, the lowest effective ratio of dimming) such as Foper=100khz; Fpwm=200Hz, the dimming ratio is: 100k/200=500; This indicator will be explained in the specification of many driver chips.