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LM51551-Q1: question about choosing inductor

Jay wang
Intellectual 1961 points
Part Number: LM51551-Q1
Other Parts Discussed in Thread: LM51551

hi expert

i saw this APP note Basic Calculation of a Coupled Inductor SEPIC Power Stage. i have two questions.

Q1: how to choose D value? Dmax or Dmin?

Q2: if using coupled inductor, how to understand the meaning of L in SEPIC? primary side inductor value or others?

  • 0
    TI__Intellectual 1961 points

    hi expert 

    i use LM51551 quick calculation tool and AN-1484 app note to calculate the same parameter, L. but get the different L value.

    according quick calculation tool, L value is 6.4uH. according AN-1484, L value is 3.2uH(not the same core inductor)

    what are your valuable comments?

  • 0 in reply to Jay wang
    TI__Mastermind 20940 points

    Hi Jay,

    Thanks for using the e2e forum.

    Q1: The design should run stable under all operation conditions, so if an equation does not specify min or max value, I would recommend to calculate with both min and max value and take the higher (for L) value.
    The inductance defines the value of load, where the device switches from DCM to CCM operation, as well as the overall ripple current. As these two parameters can be specified depending on application requirements, the equations have different default parameters for this. (e.g. if you increase the "desired current ripple ratio', the recommendation for L will decrease in the calculator)

    Q2: In general, the primary side inductance is calculated.

    Best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    TI__Intellectual 1961 points

    hi Niklas

    thanks for your reply.

    additional question.

    Q1：in our quick calculation tool, whether do quick calculation tool use the same core to calculate L value?

  • +1 in reply to Jay wang
    TI__Mastermind 20940 points

    Hi Jay,

    The value the SEPIC quickstart calculator uses for CCM operation is:

    (VIN_nom^2)/(2*ILrip*IOUT*Fsw*(VIN_nom+VOUT))

    I assume the difference in calculation results comes from the calculator using the nominal input voltage.

    Best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    TI__Intellectual 1961 points

    hi Niklas 

    could you share me the password of LM51551 SEPIC quick calculation excel tool in order to see the calculation formula?

  • +1 in reply to Jay wang
    TI__Mastermind 20940 points

    Hi Jay,

    Unfortunately we are not allowed to share the quickstart calculator password itself, not even internally.
    If there are any specific calculations you are interested in, please let me know and I will dig up the formula behind them.

    In general, most calculations should match directly with the datasheet and app note formula.

    Best regards,
    Niklas