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LM3409HV: Using IADJ and PWM for current limiting and dimming

Expert 1280 points

Part Number: LM3409HV

I've been looking at the evaluation kit documentation and the datasheet for the LM3409HV LED driver. The datasheet says that I can connect an external power source between 0-1.243V to limit the current. I was wondering if it is possible to use the IADJ pin to do just this and then also use PWM on the EN pin to limit the final current to the LEDs. For e.g. if my design is set for max LED current of 4A then can I use the IADJ to set this max current to say 3A by varying the VADJ and then use a PWM signal to vary the end output current between 0 to 3A?

  • Hello,

    Yes, that is a common way to operate the device. You can use any combination of IADJ voltage and EN pin duty cycle to adjust your average LED current.

    Regards,

    Clint

  • Expert 1280 points

    In reply to Clinton Jensen:

    Thanks. Also, is there any way to adjust the soft start time?
  • In reply to HPB:

    Hello,

    This device is designed to start very quickly which is how you get fast PWM dimming. However if you apply the IADJ voltage through an RC after you have reached your UVLO level it will implement a soft start as the peak current will start low and increase as the cap charges.

    Regards,

    Clint

  • Expert 1280 points

    In reply to Clinton Jensen:

    Awesome. Is there a way to calculate the soft start time? Rather is it possible to calculate the RC values to match a specific soft start time?

  • In reply to HPB:

    Hello,

    You can approximate it as tss = VIADJ*R*C where VIADJ is the max voltage applied to IADJ. You can get even closer if you want by using a natural log equation for charging a capacitor, but a simple RC time constant will be close.

    Regards,

    Clint

  • Expert 1280 points

    In reply to Clinton Jensen:

    Thank you.

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