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LM25118: buck boost peak inductor current

Kai Nomiyama
Expert 5285 points
Part Number: LM25118

Hi Team,

could you share how to induce equation (16) of LM25118 datasheet?

especially I'd like to know the purpose of VIN(MIN) in equation (16).

in SLVA535B, Peak inductor current describes as below and when I apply Dboost=1-(Vin-η)/Vout to equation(24), Isw max becomes Isw max=(Iout*Vout)/(Vinmax *η) + Iripple/2.

2*(1-Ltol) is coming from inductor tolerance but I'm not sure where Vin(min) comes.

Basic Calculations of a 4 Switch Buck-Boost Power Stage (Rev. B)

Regards,

Kai

  • 0
    TI__Genius 15890 points

    Hi Kai,

    Thanks for using the e2e forum.

    The equations you took from the Basic Power Stage Calculations Apps Note are completely right and I understand that the equation from the datasheet seems confusing with the additional Vin(min) term.

    I would assume, that the main difference between the LM25118 datasheet and application report calculations is a difference in topologies:
    The application report refers to a Boost topology, but the LM25118 is running in Buck-Boost mode, which is slightly different than pure Boost mode.
    This also affect the maximum current ripple.

    When extracting the Vin(min) term from the I2(peak) equation from the datasheet, it becomes clear that the maximum current ripple in Buck-Boost mode is a combination of the max current ripple for a Boost mode plus the max current ripple for Buck mode:

    I hope this explanation is understandable.
    Please let me know if you have additional questions.

    Best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    TI__Genius 15890 points

    Hi Kai,

    I hope you made progress with your design.
    As there has been no further comments, I will close this thread for now.
    If you have additional questions, please reply below to re-open the thread.
    Otherwise feel free to click the resolved button.

    Thank you and best regards,
    Niklas