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LM3409: Can I drive a single LED?

David Naviaux
Intellectual 590 points
Part Number: LM3409

Hi,

I would like to use the LM3409 to drive a single LED with a forward voltage as low as 1.5V but as high as 6.2V with in input voltage of 24V.  The output LED current will range from 250mA to 4A.  Can I use a fixed off-time by using a fixed voltage at Roff, Coff network instead of Vo?

Thank you,

David 

  • 0
    TI__Mastermind 28655 points

    Hello David,

    Yes you can drive that load, you just have to make sure the minimum on-time doesn't get reached.  This means the switching frequency cannot be on the high end.  Ton of 211 ns is the maximum Tonmin.  (Vout/Vin)*1/Fsw needs to stay above this where Vout is the lowest voltage.

    The off time can be generated from an external voltage however the off time compensation for keeping the current ripple constant will not be functioning.  To mitigate this as much as possible is to minimize the current ripple (higher inductor value) since the LM3409 regulates peak current and the current ripple will change with change in output voltage.

    If your input voltage is fixed at 24 volts you can even use that for the Roff.

    Another option is figure 27, page 17  of  the datasheet which is used for shunt FET dimming.  This also works for low output voltages and start-up issues with low output voltage and high output capacitance.

    Since your output voltage range is very low the off-time compensation will not be that accurate regardless since it's based on charging an RC to 1.243V.

    Best Regards,

  • 0 in reply to Irwin Nederbragt
    Intellectual 590 points

    Hi Irwin,

    Thank you for your quick response.

    Regards,

    David