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TPS54JA20: Bulk Capacitance

Richard Hebel
Prodigy 10 points
Part Number: TPS54JA20


I'm using the referenced switching regulator in a design.  The Web bench simulation shows no bulk capacitance. I did calculations based on a TI app note that shows the bulk capacitance value is lower than the ceramic input capacitor. This leads to me believe bulk input capacitance is not needed.  I would like to confirm this is accurate. Web bench sims are attached for reference.

  • 0
    TI__Guru 56985 points

     

    I presume from comparison between the bulk capacitance and the ceramic input capacitance, that you are referring to bulk input capacitance and not bulk output capacitance.

    The requirement for total input capacitance (Ceramic plus Bulk) depends heavily on the source impedance of the supply voltage powering the input and the dynamic loading from the output plus the allowable voltage drop or rise on the input voltage.  If that calculation shows that the total input capacitance is less then or equal to the ceramic input capacitance, then no bulk capacitance is required.

    It is important however, to make sure you have estimated the source impedance relatively accurately, including distribution resistance and inductance from the supply regulation point, or the drop on VIN at the TPS54JA20 could be larger than expected.

    Note:  If an input filter inductor is used to prevent switching noise from the TPS54JA20 from propagating back to the source supply, a lack of higher ESR bulk capacitance at the input could create stability issues at the input voltage and induce L-C tank resonance.  Using a bulk capacitor with higher ESR, or a damping circuit can help mitigate that risk.