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TPS61178: Cout with Webench Power Designer does not match Data Sheet

Elder Costa
Prodigy 100 points
Part Number: TPS61178

Tool/software:

I am working with the design below:

webench.ti.com/.../SDP.cgi

I am applying the equations of the data sheet to it to check if I am in the right path. When I get to the equation to calculate Cout:

The result does not match with the total capacitance in the design (38uF, ceramic capacitores derated).With delta Istep of 1,44, deltaVtran of ~0,2V (per the load transient simulation) and fBW=21.55kHz (Cross Frequency @ 16V Vin, which I assume is fBW), I get 53uF.

What am I missing?

TIA.

  • 0
    TI__Genius 16225 points

    Hi TIA,

    Sorry I'm not very clear of what how Webench get the parameters.

    Usually, we choose the capacitor mainly due to the ripple calculation.

    The result is ceramic derated capacitance because most of the ripple current flew through ceramic capacitor.

    To meet the load transient demand, the Cout can be estimated by:

    The Cout is total capacitance.

    The two calculations are not conflicted and please choose the larger one for your design.

    Best Regards,

    Fergus