Sinrace Power Supply Technology (Shenzhen) Co., Ltd.

As LED production costs fall, they're being used more frequently in applications ranging from handheld devices, to automotive and architectural lighting. Their high reliability (operational lifetimes of greater than 50,000hrs), good efficiency (175lm/W), and nearly instantaneous response make them a very attractive light source. However, driving LEDs is not without its challenges.

A controlled brightness requires driving the LED with a constant current, which must be maintained regardless of input voltage. Quite often, LEDs have a dimming requirement. For instance, it may be desirable to dim a display or architectural lights. There are two ways to accomplish this: either vary the LED current, or use pulse width modulation (PWM). The least effective way is varying the current because the light output isn't completely linear with current and the LED colour spectrum tends to shift at currents below full rating.

It's important to remember that human perception of brightness is exponential, requiring a large percentage change in current for full dimming. This has a profound impact on the circuit design as a 3 per cent regulation error at full current can become a 30 per cent or more error at 10 per cent load due to circuit tolerances. Dimming current waveform through PWM is more accurate, although the response speed becomes an issue. In lighting and displays, it's desirable to PWM above 100Hz so the human eye doesn't perceive flicker.