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LM3100: Inductor calculation cancels out

Matt Staniszewski
Prodigy 50 points
Part Number: LM3100


Hi,


I'm trying to calulate the inductor (L) value for my LM3100 regulators.  I'm running into an issue with Equation 11 on p. 13 of the spec sheet.  It refers to using Equation 1 to get the switching frequency (Fsw).  However, Equation 1 contains L, so it would appear that the two equations would cancel out the inductor value.

Could someone help explain how to calculate out L with these equations?

Best,

Matt

  • +1
    TI__Guru* 78645 points

    Hi Matt,

    Equation (1) is for DCM, which is the light load condition.  In DCM the inductor current can drops to zero before the end of a switching cycle.  In designing, you may choose the inductor by applying Equation (2) for CCM--Continuous Conduction Mode (please google on line if you have questions about the term CCM mode) for your full load operation. 

    The equations (9) through (11) is to check the inductor selection not to exceed the IC's current limit, otherwise you would not be able to deliver full power because it can hit the current limit.

    You may refer to the Webench to help you to get a good start point in your design.

    Hope this clarifies, and good luck in your project. 

    Thanks,

    Youhao