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LM3409: LM3409 PWM Dimming Question

Zhiang Hu
Prodigy 200 points
Part Number: LM3409

Drive LED(OSRAM SFH4725S) with LM3409EVM. When dimming frequency is 60Hz and duty cycle<20%, LM3409 can not work, in Ton time, wave is such as above picture. If increase frequency to 80Hz(don't change Ton  ) or increase duty to 24%(don't change frequency), LM3409 will normally work.

  • 0
    TI__Mastermind 25500 points

    Hello Zhiang,

    If the EN pin is pulled low long enough for VCC to discharge the LM3409 will go into low power shutdown mode. This is almost certainly what you are seeing.

    But there is a very easy fix. Use the UVLO pin for PWM dimming rather than the EN pin. They behave identically for dimming. The only difference is that VCC will stay charged with UVLO low and the device will not enter low power shutdown. This will enable you to PWM dim to very low frequency and duty cycle combinations without any issues.

    This is a common issue/question so feel free to search around on E2E for previous answers if you are interested.

    Regards,

    Clint