1 Introduction

LED applications are spreading in consumer and automotive markets worldwide as a replacement for fluorescent lighting. There always has dimming requirement in LED lighting to save energy and to provide comfortable for human eyes and so on. Currently, there are two methods to dim LEDs: one is called analog dimming and the other is called PWM dimming. In analog dimming methods, the current going through the LED is adjusted by an input controlling voltage. PWM dimming leverages the slow response of human eyes by quickly turning the LED on and off (at a frequency above 100Hz) without changing the LED current flowing through during turn-on time. Since human eyes are not responsive to any frequency above 30Hz, brightness seems to change linearly according to the duty cycle of the PWM signal.

The drawbacks of PWM dimming are in the quality of light and the effect on different objects. PWM dimming involves frequency. Low-frequency dimming is in the 100Hz to 2kH range, a range that is perceptible to humans (subtly) and this dimming introduces eye strain or fatigue. A banding effect can occur when taking photos or recording videos if the dimming frequency is in such a range. PWM dimming can also create the stroboscopic effect, which is when moving objects or rotational objects look stationary. In short, to use PWM dimming and avoid any drawbacks, set the PWM dimming frequency higher than 2kHz.

To achieve high-frequency dimming, most LED drivers have a PWM dimming input. However, the bandwidth of the LED driver limits the dimming frequency and contrast ratio. For a fixed-frequency switched mode power supply type LED driver using a DC-to-DC conversion architecture, the loop bandwidth is typically designed at or below 50kHz. That imposes a limit on the contrast ratio to about 25-to-1 with a 2kHz PWM dimming frequency. To achieve a better contrast ratio, either use a lower PWM dimming frequency or try to further increase the loop bandwidth.