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LM2621: high-frequency audible noise

KIM Andy
Expert 2530 points
Part Number: LM2621

HI

My client is currently in circuit configuration with the LM2621.
Circuit specifications are as follows.
VIN: 3.7V to 4.2V (BAT)
VOUT: 5V
IOUT: 300mA
The circuits are circuit based on the webench tool.

The problem is that in no-load condition, high-frequency audible noise is generated in the inductor.
If a load of about 50mA is applied, no audible noise will occur.
However, since the battery is used, the load resistance can not be applied to the output stage.
We changed the frequency by changing the RT resistance, and applied the inductor to 4.7uH ~ 15uH. But I could not solve the problem.
Is there a solution to the problem?

  • 0
    TI__Mastermind 34231 points
    Hi Andy,

    Could you have a test on the SW pin to get the waveform? It will be helpful for us to analyze the reason.
    If the device is not working steady at no-load condition, from the SW waveform or inductor current wavefrom we could see something.
  • 0 in reply to Zack Liu
    Expert 2530 points

    HI Zack Liu,

    I attached the switching waveform. The sw pin voltage were measured.

    I do not understand the waveform at all.

    I want to know why it is on time and how to solve it.

    Regards,
    Andy Kim

  • 0 in reply to KIM Andy
    TI__Mastermind 34231 points
    Hi Andy,

    Please refer to the 7.3.1 of LM2621 datasheet, When the output voltage is below the upper threshold of the window, the LM2621 switches continuously with a fixed duty cycle of 70% at the switching frequency selected by the user. When the output voltage hits the upper threshold of the window, the LM2621 stops switching completely.

    What does the Vo connect to? Can you accept add a dummy load at the output?
  • 0 in reply to Zack Liu
    Expert 2530 points
    Hi Zack Liu,
    Thank you for your quick reply.
    The application runs on batteries.
    I think the dummy resistor consumes that much current.
    So the customer will not use dummy resistance.
    Is there any other way to improve it?
    I would like to know precisely what exactly causes this audible noise to occur.
    The contents of "7.3.1 Gated Oscillator Control Scheme" of LM2621 datasheet have also been found in other ICs. If so, can I assume that other ICs also generate audible noise at very low loads?
  • 0 in reply to KIM Andy
    TI__Mastermind 34231 points
    Hi Andy,

    A lot of IC has a similar pulse skipping mode in low load condition in order to increase the efficiency. But most of times, we could not hear the noise created by the inductor very clearly.
    Are you sure the noise is created by inductor? Is the noise very obvious?