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LM3410: Fast PWM dimming

tateo yamashiro
Guru 10890 points
Part Number: LM3410

Hi,

We got a question from the customer about LM3410.
Could you help us?

[Question]
They are considering fast PWM dimming such as 20kHz(0~100%, 1% step) under the following condition. The IC has approximately 20usec delay time between rising edge of PWM signal and led current rising. Therefore they can't achieve this control. Are there any recommended schematics for their requirement?

Vin=3~5.5V
Vout=12V
Iled=120mA

They assume the PWM dimming circuit on figure 29 in datasheet might achive fast PWM dimming. Are there any concerns on this?

Best Regards,
tateo

  • 0
    TI__Mastermind 28655 points
    Hello tateo,

    That is a SEPIC meant for camera strobe application, not 20 KHz dimming, it won't be faster. R1 and R2 are to allow C2 to charge for the strobe.

    There is also the inductor rise time, this will not be accurate at 20 KHz dimming down to low dim levels. It would be better to use a part that can SHUNT fet dim is high resolution high frequency dimming is needed.

    7.3.2 DIM Pin and Shutdown Mode
    The average LED current can be controlled using a PWM signal on the DIM pin. The duty cycle can be varied
    from 0 to 100%, to either increase or decrease LED brightness. PWM frequencies from 1 Hz to 25 kHz can be
    used. For controlling LED currents down to the μA levels, it is best to use a PWM signal frequency from 200 to
    1 kHz. The maximum LED current would be achieved using a 100% duty cycle, that is the DIM pin always high.

    Best Regards,
  • 0 in reply to Irwin Nederbragt
    TI__Mastermind 28655 points
    Hello Tateo,

    I haven’t heard back from you, I’m assuming you were able to resolve your issue.
    If not, just post a reply below (or create a new thread if the thread has locked due to time-out)

    Best Regards,