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LM3409HV-Q1: PWM question in different LED

KJ Liu
Intellectual 530 points
Part Number: LM3409HV-Q1
Other Parts Discussed in Thread: LM3409

Hi Sir,

      We used LM3429HV and found if different number of LED series, the current is different in same dimming ratio as attached file,would you please help to check whether the schematic is problem or give us a direction.Thanks you! 

dimming curve.xlsx

  • 0
    TI__Mastermind 28655 points
    Hello,

    PWM dimming, via the PWM pin, requires the inductor current to rise and fall. If Vled changes the rise and fall times will change. To get more accurate dimming you would have to go to Shunt FET dimming. The on and off times for 20 KHz dimming will be affected by PWM dimming via the LM3409 PWM pin, the LED current set-point, Vin, Vled and the inductor value. It will work, it just won't be very accurate. Inductor current slew is V = L * (di/dt) where V applied (during Ton), is Vin - Vled and V off is Vled.

    Try PWM dimming at 200 Hz and see the difference.

    Best Regards,