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TPS7B84-Q1: Question for output capacitor

Kazuki Kuramochi
Expert 8370 points
Part Number: TPS7B84-Q1

Hello expert,

I have question for TPS7B94-Q1's output capacitor.
This device's datasheet required they need to place 2.2uF output from the point of view of stability but  they can use 0.1uF if they add electrolytic capacitor.
However, at this case, customer cannot place 220uF electrolytic capacitor near by  output pin.
Then, would you tell me your opinion about whether they should use 2.2uF or 0.1uF?

I'm looking forward to hearing back from you.


Thank you and best regards,
Kazuki Kuramochi

  • 0
    TI__Genius 12040 points

    Hello Kazuki,

                          TPS7B84 device requires a minimum of 1uF capacitance at the OUT pin. I recommend that a 10uF ceramic capacitor be used at the OUT pin. Thanks!

    Regards,

    Srikanth 

  • 0 in reply to Srikanth Lolla Venkata
    TI__Expert 8370 points

    Hello Srikanth,

    As far as I confirmed 1uF requirement is effective capacitor from the point of view of stability and your recommended 10uF is for improving transient response.
    At that time, I'd like to discuss about this based on stability.

    As I explained in previous post, datasheet said 0.1uF capacitor is acceptable when they add electrolytic  capacitor.
    At that time, How long is distance between pin to electrolytic capacitor acceptable to use 0.1uF capacitor?

    I understand it is difficult question to answer precisely so general theory is acceptable.

    Best regards,
    Kazuki Kuramochi

  • 0 in reply to Kazuki Kuramochi
    TI__Genius 12040 points

    Hello Kazuki,

                         Please note that the Datasheet does not say that a 0.1uF Electrolytic capacitor is sufficient for stability. It says that if an Electrolytic/High ESR cap is used (which must still be at least 2.2uF) at the OUT pin, a 100nF ceramic (low ESR) cap must be placed in parallel to this high ESR cap to reduce the effective ESR and ensure stability. We recommend that the output cap be placed as close as possible to the OUT pin (perhaps within an inch). Thanks!

    Regards,

    Srikanth  

  • 0 in reply to Srikanth Lolla Venkata
    TI__Expert 8370 points

    Hello Srikanth,

    I understood 0.1uF must be used with electrolytic capacitor.
    At this time, customer's layout may be that electrolytic capacitor place away from LDO.(But, they don't have detailed distance.)
    So I'm asking about how far is acceptable  when they want to use electrolytic capacitor with 0.1uF.
    Anyway, I understand that less then 1inch is required to electrolytic capacitor when they want to use 0.1uF.


    Thank you and best regards,
    Kazuki Kuramochi

  • +1 in reply to Kazuki Kuramochi
    TI__Genius 12040 points

    Hello Kazuki,

                         Yes, if the capacitor is placed within an inch of the pin, then typically the parasitic's would would be too small to have an impact on stability. Thanks!

    Regards,

    Srikanth