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LM3409HV-Q1: The LM3409 Chip generate frequency multiplication audible ringing of PWM input

Part Number: LM3409HV-Q1
Other Parts Discussed in Thread: LM3409HV, LM3409

Dear Sir:

I make my own board of led driver wtih LM3409HV,  is very similar to the circuit in AN-1953 LM3409HV Evaluation Board, but different layout.

When set PWM duty cycle lower 95%, the Board generate noisy ringing. I record it and do pectrum analysis to get the follow image:

input PWM frequency is 2KHz。

Then I try to find where is the noise from. I isolate all Capacitor/Inductor/PMOS by wiring these devices to board with thin wire, and use my headphone microphone to record sound of device one by one and findout that noise is generated mainly by LM3409, which supprise me. All I found on web say Capacitor or Inductor generate noise. I'm stuck..

What changes do I need to eliminate the noise?





INPUT voltage: 52


  • Hello,

    Is R9 0.33 ohm or 0.18 ohm?  At 0.18 ohm the current will be higher than the inductor rating.

    Audible noise is common when PWM dimming.  Most of the time it is the inductor and can also be the ceramic capacitors.  You don't have one across the LEDs though the input capacitors could still cause audible noise.  It is unlikely that the LM3409 is causing audible noise though it could be generated in the PCB from other components.  I would try different inductors to start with.  You can run the inductor away from the board with short wires to see if it is the main culprit.

    The frequency multiplication is due to the PWM being a squarewave.  These are the harmonics of the fundamental frequency of 2 KHz.

    What is the voltage of your LED load and expected current?

    Best Regards,

  • Thank you Irwin!

    R9 is 0.33 ohm, and I expect the current of the 28W LED module to be 700mA.

    Refer to, I run all the capacitors and inductors away from the PCB with short wires and tried the paper tube to find that  C1/C2 will produce the greatest noise, when the inductor remain silent,

    But I still hear the sound of the PCB board. Is it the MOSFIT or the doide?

    It was decided to change the PWM frequency to 20KHz in order to generate excessive human-perceived noise. Then the peak of spectrum moves to 20KHz.

  • Hello,

    Capacitors can be bad.  I don't think the MOSFET or diode is causing noise though it is possible.  This is a very common issue.  Going to 20 KHz is a common way around it.  Note that your dimming linearity may not be as good.  This depends on your input voltage, inductor value and LED voltage.

    Best Regards,