STLDC08

Detailed description

Doc ID 18476 Rev 1

15/29

7.5

Dimming

When PWMOUT goes to zero, the LED current immediately goes to zero and the energy

stored in the coil is discharged on the output capacitor, causing an increase in the output

voltage. As soon as the PWM goes back to high value, there is a big spike current on the

LED. This could damage the LED itself. To avoid this, as soon as the input PWM signal goes

to zero the controller immediately turns off the main switch (in order to discharge the coil

current on the LED branch). In this way the PWM power is turned off with a delay in order to

guarantee that FB goes high after PowerMOS turn off. After this delay, the flip-flop is ready

to be set and the PWM power is turned off. In this condition the output voltage is slightly

lower than the regulated value, but a current spike on the LED is avoided.