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LM3150: LM3150 - squeaking noise

Petr Zkrat
Intellectual 410 points
Part Number: LM3150

Dear team,

we see some issues with LM3150 buck producing noise. 

Vin=24V, Vout=12V, snapshots taken at light load (0.7A)

-issue seams to develop over time. Produced PCBAs are usually ok, after some time this becomes audible

-more audible at higher loads

-heareable frequency does not change when touching the output MLCCs, so unlikely a piezoelectric effect issue?

Many thanks for your help and ideas. 

Petr

-SW node:

Overshoot detail:

Undershoot detail:

Schematics:

SW node spectrum analysis (highest peak 14.21kHz)

Edit: added SW node @50us

SW node with persistence

  • 0
    TI__Mastermind 22301 points

    Hi Petr.

    Is your SW node pulsing at 14 kHz? (Zoom out to ~50us/div)

    The LM3150 uses Constant ON-time (COT) architecture, at light-load COT converters enter a discontinuous mode(DCM) and folds-back operating frequency naturally.

    The boundary between DCM and continuous conduction (CCM) is

    DCM: IOUT < ΔIL/2
    CCM: IOUT > ΔIL/2

    You can reduce foldback point by decreasing inductor ripple current ΔIL, however too low ripple current will reduce performance.

    Hope this helps,

    -Orlando

  • 0 in reply to Orlando Murray
    Intellectual 410 points

    Hi Orlando,

    thanks for suggestions. I did check the SW for DCM but did not observe that.

    What came to my mind was to check SW node with persistence turned on, and it can be seen that the switching frequency slightly changes (the delta is about 14kHz).

    (added scrennshots to original post as for some reason I am not able to add a snapshot to a reply).

  • 0 in reply to Petr Zkrat
    TI__Mastermind 22301 points

    Hi Petr,

    Yes I see, the jitter on your SW node is very large.

    This usually means you should adjust your type-3 ripple generation network, I think C17 130pF is low for 200kHz, try 470pF or 1nF.

    You can reference the appnote: https://www.ti.com/lit/an/snva776a/snva776a.pdf

    Hope this helps,

    -Orlando