LM3405A

SNVS508C – OCTOBER 2007 – REVISED MAY 2013

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Time

Delayed

LED PWM DIMMING

The LED brightness can be controlled by applying a periodic pulse signal to the EN/DIM pin and varying its

frequency and/or duty cycle. This so-called PWM dimming method controls the average light output by pulsing

the LED current between the set value and zero. A logic high level at the EN/DIM pin turns on the LED current

whereas a logic low level turns off the LED current. Figure 21 shows a typical LED current waveform in PWM

dimming mode. As explained in the previous section, there is approximately a 100µs delay from the EN/DIM

signal going high to fully establishing the LED current as shown in Figure 22. This 100µs delay sets a maximum

frequency limit for the driving signal that can be applied to the EN/DIM pin for PWM dimming. Figure 23 shows

the average LED current versus duty cycle of PWM dimming signal for various frequencies. The applicable

frequency range to drive LM3405A for PWM dimming is from 100Hz to 5kHz. The dimming ratio reduces

drastically when the applied PWM dimming frequency is greater than 5kHz.

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