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Constant Current LED PSU for a 5V supply suitable for shunt FET PWM

Martin Walton97
Prodigy 190 points
Other Parts Discussed in Thread: TPS92510, LM3405, LM3401

Hello

I'm looking for an LED PSU device with either internal or external FETs to power a 3.5V 700mA LED from a nominal 5V supply (so working down to 4.5V).  It must be suitable for external shunt FET PWM for high resolution dimming.  I've found parts like TPS92510 and LM3405 which meet my voltage/current requirements, but it's not clear to me if they'll cope with shunt FET dimming which would be 14 or 15 bit resolution at about 1KHz.  Is there a clear way to identify which devices will do this?

Thanks

Martin

  • 0
    TI__Mastermind 28655 points

    Hello Martin,

    At 14 bit resolution and 1 KHz pwm the resolution will be 61 ns steps.  What are you trying to accomplish with your design?  The LM3405 isn't the correct part to shunt fet dim.  Other parts that are suited for shunt FET dimming generally have a higher minimum input voltage above your range.  The only part I find that may work down to 4.5V while shunt FET dimming is the LM3401.

    Best Regards,

  • 0 in reply to Irwin Nederbragt
    Prodigy 190 points

    Hello

    Thanks for the suggestion of the LM3401 - that looks like it could work in my application.  I need to achieve smooth fades, especially at the bottom end of the dimming range, which requires high resolution.

    Thanks

    Martin