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Original question:

LM5176: LM5176

LM5176: LM5176 OUTPUT oscillation during low transient test in boost mode.

li zhaobo
Prodigy 50 points
Part Number: LM5176


Hi TI support team,

 

We are designing solution with LM5176RHF, for buck-boost wide input range scenario.

During our load transient test, we met some oscillation at output when in 9V boost mode. So could you help to check the COMP and slope circuit if properly set?

 

 

**************************************

Design parameter:

INPUT: 9V~36V.

OUTPUT: 12V

MAX current(avg.): 12A

OCP SETTING: 17A

*************************************

 

 

yellow: input 9V

green: output.

blue: load current.

schematic:

 

 

 

Thanks in advance

Zack

  • 0
    TI__Guru 59761 points

    HI Zack,

    can you please take the design calculator from the product page and fill in the the design parameters and attached it here.

    https://www.ti.com/tool/download/SNVC208

    Can you also provide a scope lot which shows the output voltage for several fsw cycles for the 9V input.

    Can you also describe where and how you have connected the scope probe for measuring VOUT

    Best regards,

     Stefan

  • 0 in reply to Stefan Schauer
    Prodigy 50 points

    Very appreciate you response.

    1.  waveform capture for output for a few Fsw(551kHz) cycles.

    2. the measure point is at the output cap C158 of vout, close to MOSFET Q30. using coaxial cable.

    using the tool you have shared, we got configuration as below:

    the crossover frequency should be 5~20% Fsw right?

    thanks.

    zack

  • 0 in reply to li zhaobo
    TI__Mastermind 31210 points

    Hi Zack,

    Thanks for the update.
    Yes, a rule of thumb for the crossover frequency is either 1/10 of the switching frequency, or 1/5 of the RHPZ, whichever is lower.
    You can also attach the calculator you filled in here in this thread.

    I will then make a review of your design and the waveforms and get back to you with my feedback within 1-2 days.

    Thanks and best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    Prodigy 50 points

    Hi Niklas,

    thanks a lot for the checking.

    i attached the re-calculated parameters here, and re-do the test, and the oscillation disappeared. plz help to review the calculation as well.

    thanks in advance.

    zack

    LM5176_2ND_ROUND_CALCULATION1213.xlsm

  • +1 in reply to li zhaobo
    TI__Mastermind 31210 points

    Hi Zack,

    Thanks for the update and recalculation.
    I am glad to hear that the oscillation disappeared.

    The bode plots of the calculator looks okay.
    We generally recommend to aim for 60 degree phase margin or higher. The calculator returns 55 degree, which is still a good amount.
    For crossover frequency you set 19kHz.
    A rule of thumb for the maximum crossover frequency is either 1/10 of the switching frequency, or 1/5 of the RHPZ, whichever is lower.
    The calculator estimates a RHPZ of 81kHz, so a crossover frequency of 16kHz would be preferable, but these are just minimal improvements.

    The rest of your schematic and the calculator looks good to me.

    Best regards,
    Niklas