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TPS7A05: input/output capacitor use 0.47uF cap

Will Wang47
Expert 5510 points
Part Number: TPS7A05

Hi team,

My customer want to check with about the TPS7A05 input and output cap. Does it OK for them to use 0.47uF with 01005 very small cap? What the effective input and output cap requirement?

Thanks and Best Regards,

Will

  • 0
    TI__Mastermind 29260 points

    Hi Will,

    The capacitance on the output is needed for stability.  The requirement is no less than 0.47uF.  Using a 01005 size capacitor will most likely not provide enough capacitance due to applied voltage bias and tolerance.  To confirm you will want to review the capacitor manufacturer data on the capacitor you are considering.  For example, here is a Yageo capacitor in this package size and value.  The effective capacitance of the capacitor really goes down with applied voltage, without considering of other tolerances, so this would not meet the datasheet spec.

    https://www.yageo.com/en/ProductSearch/PartNumberSearch?part_number=CC0100MRX5R5BB474&page=1&page_size=10

    A 4.7uF, 0201 package size, Murata capacitor is GRM033R60J475ME05D.  Even at 3.3V output this will probably work, however you need to take temperature into consideration as this is an X5R capacitor.

    Thanks,

    Stephen

  • 0 in reply to Stephen Ziel
    TI__Expert 5510 points

    Hi Stephen,

    Thanks for your feedback. I want to confirm with you about the cap, I think our datasheet capacitor value doesn't consider DC bias, since the recommended value is the nominal value. Does my understanding correct?

    If follow the following guide, it will get the 50% effective cap of nominal value, then we could let choose 0.47uF cap, but need to get at least 50% effective cap.

    As for the input cap, does it OK for the to use 01005 0.47uF? 

    They need to use small package to save space. Thanks.

     

    Best regards,

    Will

  • 0 in reply to Will Wang47
    TI__Mastermind 29260 points

    Hi Will,

    What the guidance is saying is that the 0.47uF already accounts for the 50% derating.  Section 8.1 does not specify where that 50% comes from, but in practice some of it will come from voltage bias, initial tolerance, environmental conditions, AC ripple voltage, etc.  The absolute minimum effective capacitance is 0.47uF and from my previous review, I do not see a single 01005 capacitor meeting this spec.  They would need to use multiple parallel 01005 capacitors, or something like the 0201 capacitor I suggested if the output voltage is low.

    Thanks,

    Stephen