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TPS7A25: Effects of output capacitance on inrush current

Sehaz Dayal
Prodigy 10 points
Part Number: TPS7A25

Hi, 

I am planning on using TPS7A25 in an application which imposes a limit on total capacitance allowed during initial turn-on for the first 20ms to avoid tripping of the upstream inrush current limiter. How can I compute the total equivalent capacitance seen by the input (Vin) in the first 20ms for this component? 

I am assuming both the input and output capacitance of TPS7A25 has an effect on the inrush current. Is the following method accurate: 

C_equivalent_total = Cin + [(Cout x Vout^2)/Vin^2]

The output capacitance of TPS7A25 is scaled from the output voltage rail to the input voltage rail using the capacitor energy formula. This would give the total capacitance seen by upstream inrush limiting circuitry for the first 20ms.  

I would appreciate any support with the above. 

Thank you.

  • +1
    TI__Mastermind 38195 points

    Hi Sehaz,

    I think what you've done seems reasonable, but I'm not sure how accurate it is because the equivalent capacitance you calculated assumes that all of the capacitance begins charging at the initial turn on at the same time, but the LDO does not begin charging its output capacitors until ~800us after the input comes up. This will affect the effective capacitance because the inrush current for the input caps and output caps will peak at different points in time instead of simultaneously so the peak inrush current seen will be less than if it is assumed that they do charge simultaneously. Furthermore, once the input caps are charged, they will help to supplement the inrush current to charge the output caps as well, thereby reducing the peak inrush current to charge the output caps. 

    I suppose this could be used as a worst-case condition. If that is satisfactory then I think this is reasonable enough. I haven't seen this kind of calculation before so I cannot speak definitively to its accuracy or validity. 

    Regards,

    Nick

  • 0 in reply to Nick Butts
    Prodigy 10 points

    Yes a worst-case estimate of total effective capacitance is what I was looking for. Thanks Nick.