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LM27403: Unwanted 13kHz Oscillation When Under Load

John Fagerlund38
Intellectual 260 points
Part Number: LM27403
Other Parts Discussed in Thread: LM5145

I have an LM27403 configured for +12Vin with 0.9Vout operating up to 30A.  Circuit was designed using Webench tool.  I have Cin using 3 x  22uF and Cout using 1 x 0.1uF, 2 x 100uF and 1 x 470uF.   I configured an evaluation board with similarly and performed load testing over temperature without issue.  I even stepped the load to mimic the dynamic current draw required by our TDD system.

On my PCB what I'm seeing is, at low current draw ~1.5A, the circuit is behaving; no oscillation.  When the ARM in our SoC starts to bring up the TDD link and the current draw bounces between 5A and 8A, the 200mVpp, 13kHz oscillation is present on Vout.  The processor reacts negatively to this unwanted oscillation.

I disabled the remote sensing loop, added a 22uF Tantilum cap to Vin and 200uF to Cout with no change in amplitude or frequency.

I see the oscillation on Vout, at the switching node and at Vin.  I bypassed our 12V LM5145 supply with a bench supply and oscillation remained.

Any ideas where to look?

Thanks,

-j

  • 0
    TI__Genius 17905 points

    Hi John,

    Would you be able to share your WEBENCH design or schematic with me? I am curious if the bode plot in WEBENCH crosses at around 13kHz.

    It may be good to review your PCB layout, too. If you do not want to share the schematic/layout on the forum, you can send to me at kristoffer.flores _at_ ti.com

    Can you please also share an oscilloscope capture of the oscillation?

    Lastly, is it possible to test with DC 5A and 8A loads on your PCB and confirm if is is oscillating with the DC loads applied?

    Regards,
    Kris

  • 0 in reply to Kris F
    Intellectual 260 points

    Hi Kris,

    Here is the LM27403 portion of the schematic for you to look over.  I'm putting the board back to original configuration, when done I'll get some scope screen shots.  Unfortunately I can't disassemble a board to tack on loads.  Since we do have an NDA in place, I'll send you the Allegro layout.

    Thanks,

    -j

  • 0 in reply to John Fagerlund38
    TI__Genius 17905 points

    Hi John,

    As we have taken this offline to email, I am marking this closed on the forum. I am reviewing the files you sent and will reply via email.

    Kris