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LM3409MY: high frequency noise can be heard

Eric Chen27
Intellectual 470 points

Hi TI,

I use LM3409MY to output 10V/1A from 12V input.

I use 3.3uH inductor and  1uF output capacitor, but I can hear high frequency noise.

Even if I remove 1uF capacitor, I still can hear the noise.

Could you please tell me how to do next step?

Do I need to change inductor value or rating current?

Best regards.

Eric

  • 0
    TI__Mastermind 25500 points
    Are you PWM dimming? What is your switching frequency? If the switching frequency or PWM dimming frequency is in audible range the inductor may buzz. If you could provide a schematic and BOM I can take a look at them and check for possible causes.
  • 0 in reply to Clinton Jensen
    Intellectual 470 points

    Hi Clinton,

    Not PWM dimming, but I use a external mosfet to control LED connect to GND. The mosfet switch frequency is 360Hz.

    I measure the switching frequency is about 430KHz.

    Please refer to attached circuit and let me know your comment.

    Thanks.

    Eric

     

    LM3409_circuit_BOM.xlsx

    LM3409_circuit.pdf

  • 0 in reply to Eric Chen27
    TI__Mastermind 25500 points

    Ok, you are shunt FET PWM dimming then. That puts a lot of stress on components as well, mainly the input capacitor. So the buzzing is likely either in the input capacitor or the inductor. Some comments:

    1. When PWM dimming by either method the input will take a lot more stress than in steady state operation. You generally want to use more input capacitance if you can. I also notice the 0805 case size. It is quite possible you are exceeding the RMS current rating of such a small capacitor when PWM dimming. I would at least double the capacitance and would also consider using a larger case size if possible. Perhaps two 4.7uF caps in parallel and in a 1206 case? Changing case sizes and/or number of caps in parallel is a good way to stop audible noise even if the RMS currents are not being exceeded. All ceramics can buzz if the frequency is right.

    2. I can't find any information on the inductor you are using, but it could be the culprit as well. If so changing to a different case size and/or different manufacturer and/or different core type can usually get rid of the noise. Lower quality inductors are also more likely to exhibit audible noise. I'm not sure what type or quality you are using however.

  • 0 in reply to Clinton Jensen
    Intellectual 470 points

    Hi Clinton,

    Thanks for your quickly response.

    The inductor value 3.3uH is ok or not?

    Do you have recommend parts? (manufacturer part number)

    Thanks

    Best regards.

    Eric

  • 0 in reply to Eric Chen27
    TI__Mastermind 25500 points
    3.3uH is fine as long as your LED current ripple isn't too high. Without an output cap the LED current ripple will be the same as the inductor current ripple, in this case about 1.8A peak-to-peak, which is pretty large. But if your LEDs can handle it then it's fine. I would personally increase the value to get closer to 20% peak-to-peak ripple so maybe a 22uH or 33uH. Some good shielded inductors that would have enough current capability up to those values would be the Coilcraft MSS1038 series.