LCD display brightness & PWM

Pulse Width Modulation (PWM) is a technique of creating modulated electronic pulses or the electronic pulses of the desired width. LCD backlight control is one of the PWM applications.

Topway's LCD displays use LED as backlight source. By using a PWM scheme, several advantages are realized over a simple DC voltage method. The main advantage lies in efficiency.

The LEDs are fed with pulsed high current for a short period of time, in stead of a constant DC current. For example, a normal LED driving current is 120mA. Now we apply 5 times the current, 600mA; but only apply for 1/5 of the time. The LCD display doesn't consume more electricity. Yet its perceived brightness is a lot brighter.

Human eye has certain degree of persistence. It remembers previous light for a short period of time. This enables us to view a movie as steady stream of images. When in fact it flickers at 24-30 times per second. Similarly, our eyes remember LED backlight at brightness level. Resulting the perceived brightness of backlight  is close to the high pulsed brightness.

Benefits 

This effect brings several advantages to LCD display backlight:

If a LCD display needs the brightest possible result, the display can be pulsed at 1:4 ration with 5x typical current. The pulse frequency repetition should be higher than 100Hz but lower than 1000Hz. So human eye can't detect flickering.

This technique can also be applied to provide a normal looking brightness level to LCD display and save at least 50% energy consumption at the same time. This is a big benefit for battery operated product.

Another usage is to facilitate brightness control for LCD display. By adjusting the high/low current ratio,  a wide range of brightness can be achieved while maintaining a very even backlight appearance.