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LM5176: SW1 work on DCM mode with no load or light load

Juin Chen
Prodigy 40 points
Part Number: LM5176

Tool/software:

Dears,

LM5176 should be already CCM mode. but input voltage sets on Vin=18.5V~25V and Vout=12V, SW1 works on DCM mode. However, Vin=18.4V~9V and Vout=12V, SW1 or SW2 works on CCM mode.

On DCM mode, PG has pulse issue.

please help to review what happen. Thanks.

Fig.1: CH3: FB pin, CH4: SW1

Fig.2: CH1: PG pin, CH2: COMP pin, CH3:Vout, CH4:SW1

also attached schematics.

PEB-99B0_LM5176_VIN_24V_VOUT_12V_10A_2.2uH.pdf

  • 0
    TI__Prodigy 540 points

    Hi Juin,

    Thanks for using E2E.

    Please expect a reply by early next week.

    Best regards

    Yezi

  • 0 in reply to Yezi Yang
    TI__Mastermind 47941 points

    Hi Juin,

    what you see here is not DCM but overvoltage protection of the LM5176.
    In CCM mode the device would need to push energy from the output back to the input in very low load condition.

    If the input side is not able to consume (or sink) this energy the overvoltage protection gets triggered as the output voltage increases and the controller stops till the output voltage is back into the expected range.

    Best regards, 

     Stefan

  • 0 in reply to Stefan Schauer
    Prodigy 40 points

    Hi Stefan,

    Thanks for your response, it seems like OVP protection. Do you think about inductor selection? I try to change from 2.2uH to 3.3uH, this issue is gone. how to know delta I across inductor to meet requirement when design LM5176? From our test, Vin=18.4V~9V can keep CCM mode but Vin=18.5V~25Vcan't, the result let me considers Delta I of Inductor and than change inductor value.

  • +1 in reply to Juin Chen
    TI__Mastermind 47941 points

    HI Juin,

    i have never evaluated how the inductor does impact this behavior and also do not now a reference where this is discussed.

    So with an large inductor you get less ripple on the inductor but i do no think that you can solve this with the inductor in the full range.

    Maybe a combination of inductor and larger input cap.

    You can try with the power stage designer - maybe this will help here further.

    Best regards,

     Stefan