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LM3401 DIM issues

orien walker
Prodigy 145 points

I am using the LM3401 for a lighting project and I've had the board up and running for a while. Before now I have had the DIM pin tied to my Vin until i began to work on my MCU to power the PWM signal. Now that i am at that phase i have tried to input a PWM signal to DIM pin to dim the LED but no dimming occurs. I began troubleshooting and have found out that the LED lights up regardless of any signal tied to the DIM PIN even if i just have it disconnected and floating the circuit is still operating normally. According to the data sheet if the DIM pin is not receiving a voltage signal higher than 2V HG pin should go low and cut off the mosfet. I need some guidance as to what could be going wrong and why the circuit isn't operating as it should...

  • 0
    TI__Mastermind 25500 points

    If the DIM pin is floating it is possible it could pull high so I don't see an issue with that, but you might want to have some sort of pullup/pulldown if your MCU is an open drain output or something of that nature. In any case, you are applying a square wave to DIM and there is no light difference or stoppage in switching? Have you probed the DIM pin to make sure the signal is what you expect? If so is there any way you could send some dimming waveforms (Vsw, Vdim, and Iled) and perhaps a schematic and operating parameters (Vin, Vout, LED current, BOM if possible)? This should be relatively easy to solve one way or another. These circuits have been performing well for years in many applications. Thanks.

  • 0 in reply to Clinton Jensen
    Prodigy 145 points

    What i meant by floating is that i disconnected DIM from any signal and the circuit was still operating, where as the data sheet says if DIM isnt above 2V that HG will shut down and deactivate the mosfet. I have probed the pin and tested all my signals. Its acting as if DIM has no effect on the circuit. Let me get my information together and i will post it here as well.