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LMZ21700: LMZ21700 audible noise

Part Number: LMZ21700

I have a PCB designed with the LMZ21700SILT device. I like it for the small package due to size constraints on my board. However there is considerable audible noise associated with the 22uf input & output MLCC SMT caps (22uf, X7R). I was able to minimize the audible noise by going with more expensive through hole caps. I now have a new design that I am working on in which I would like to use the same LMZ21700SILT part but not if I have the same audible noise issue. I want to go back to the SMT caps. I have researched the methods to resolve the MLCC cap & audible noise issue but they are not practical. I think the part itself may exhibit audible noise due to its physical interface with the PCB & vibration & that's one of the reasons I can't get rid of the noise altogether.

I've followed the PCB layout recommendations in the data sheet.

What is your experience with audible noise with this part & what suggestions do you have to eliminate it?

My schematic & BOM are revealed in the image below which is extracted from the data sheet.

Thanks.

  • Hi Ken,

    Happy to help. Do you have any filter on the input or output of the LMZ? Also are your Vout traces from the module to the Caps wide and thick? The singing of caps usually is a parasitic resonance set up between the inductance of the traces or incorrectly sized filter inductance and the output caps.

    Unfortunately the schematic and BOM didn't upload correctly, can you repost them with the Layout?

    Thanks,
    Anston
  • Anston,

    Thanks for getting back to me. I used 2 layers for this application. 4 layers were considered but deemed too pricey. I've inserted the schematic, & top & bottom layer images.

    Thanks,

    Ken

  • Hi Ken,

    Images didn't load, can you email them to me. anston [*at*] ti [*dot*] com.

    Thanks,

    Anston