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TPS7A91: Output Cap

Part Number: TPS7A91
Other Parts Discussed in Thread: TPS7A88

Hi

My customer has question.

Customer plan to use TPS 7A91 as attached, but there are not any places that are considered to be particularly problem?

TPS7A91_output cap.pdf

Regard 

T Kishi

  • Hi T,

    building a pi-filter with ultra low ESR caps can be problematic because of resonances. Eventually you might want to add some damping resistance.

    Kai
  • Hi T Kishi,

    I have seen many customers use TPS7A91 in this way successfully. As TPS7A91 is stable with high output capacitance, you should be able to successfully use it in your application.

    Since it looks like you are splitting the output of TPS7A91 into several rails, you may want to consider using a dual LDO such as TPS7A88. The two independent channels of TPS7A88 will provide more isolation between the output rails than simply using pi filters and may help simplify your design.

    Very Respectfully,
    Ryan
  • Hi kai ,Ryan

    Thank you for reply.
    I understood that the capacity is satisfactory.
    As kai says, when implementing large capacity should we use a large ESR capacitor together?Alternatively, Implementation of damping resistance.
    I will be saved if you give me advice.

    Regard
    T Kishi
  • Hi T Kishi,

    Using a damping resistor will be much more flexible for debugging.

    Very Respectfully,
    Ryan
  • Hi T,

    I agree with Ryan. Replacing a ceramic with a low ESR by a electrolytic with higher ESR only to avoid ringing is no good idea, because this will impair the decoupling performance. Another method of putting an electrolytic cap (snubber) in parallel to the ceramic cap can be helpful in situations where a resistance in series to the choke cannot be allowed because the load current would create a too high voltage drop. But in this case the snubber must be designed very carefully, because only a well dosed ESR will do the trick.

    But in your case it's much easier to add a series damping resistance. An example: The BLM18AG601SN1D has a low frequency inductance of about 3.2µH. In your circuit it forms a low pass filter with a 10µF ceramic. As a rule of thumb a series resistance of R>=SQRT(2L/C) = 0.8R would be needed to fully dampen the resonance:

    But your BLM18AG601SN1D has already a DC resistance of about 0.22R This allows for certain damping:

    If your application can live with this resonance you must not add any series resistance.

    Kai