STLD20D datasheet(19/31 Pages) STMICROELECTRONICS

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STLD20D

PWM dimming control

19/31

PWM dimming control

By applying a PWM signal on the shutdown pin SHDN, the average current and the

brightness of the LED can be adjusted.

Figure 21. shows ILED current and the other typical

waveform during this dimming control mode.

Note that the Load Disconnect Switch LDS turns ON/OFF at the same frequency and with

the same duty cycle as the PWM signal. Thus, the LED current is a perfect square wave

phased with the dimming signal. This leads to a good correlation between the real average

current of the LED and the theoretical current given by:

Where:

●

ILED: is the nominal current programmed by the RLED resistance

●

DC: is the duty cycle of the dimming signal.

Figure 14. shows that the correlation between the real average current and the theoretical

value is given for a minimum duty cycle of 5% when the dimming frequency is 300Hz and

20% for a 10kHz dimming signal.

Figure 21.

Typical waveform when the PWM dimming is used at 300Hz

δ = 0.38

Vshutdown

ILED

VOUT

ILIM

Ireg

VOUT

ILED

Vshutdown

LED

Theo

–

LED

STLD20D Datasheet(PDF) 19 Page - STMicroelectronics