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TPS7A54: Soft start spec

Hailiang Zhang
Expert 7535 points
Part Number: TPS7A54
Other Parts Discussed in Thread: TPS7A91,

Tool/software:

Hi Team,

The TPS7A91 datasheet shows that when VFB reaches 97% VREF, INR/SS is turned off and VNR/SS is charged through RNR. For TPS7A54, what is the switching voltage? Is it also 97%? Furthermore, what are the benefits of this mechanism? Are there any P2P devices with the TPS7A54 that do not use this mechanism?

Regards，
Hailiang

  • 0
    TI__Mastermind 36525 points

    Hi Hailiang,

    Yes, use 97%.  Common values for modern LDO's are anywhere from 95% - 98% but many (like the TPS7A54) are designed at 97%.

    The NR resistor and capacitor form a low pass RC filter to improve the noise and PSRR performance of the reference voltage before it is gained up by the high gain internal error amplifier.  Unfortunately this NR filter significantly slows down the turn on time of the LDO since the feedback loop will follow the long time constant of the NR filter.  The fast charge current source speeds up the turn on time of the LDO while maintaining the benefits of the NR filter during steady state.

    Is there a reason why you are looking for a P2P device without this mechanism?  Are you looking for an LDO that can turn on faster (or something else)?

    Thanks,

    Stephen