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TPS7A91: Maximum soft-start capacitor

Hirotsugu Kobayashi
Mastermind 9530 points
Part Number: TPS7A91
Other Parts Discussed in Thread: TPS74401,

Hi, team.

Please tell me the maximum recommended soft-start capacitor(Cnr/ss).

In TPS74401, 0.015uF is the maximum recommendation value because of a internal discharge circuit.

In TPS7A91, are there similar limit?

Best Regards,

  • 0
    TI__Guru 50030 points

    Hi Kobayashi-san,

    The only concern with very large CNR/SS capacitors is without respect to a power-down then sudden restart. If this capacitor is too large, it may not have time to fully discharge, and if the device is suddenly re-enabled, there could be higher inrush into the load.

    The only other possible concern with very large CNR/SS capacitors could be with respect to leakage.Larger Capacitors may have higher leakage and the 0.73uA typical SS current of the TPS744 may not be enough to properly swamp out severe leakage. This is where the TPS7A91 is nice because the SS_CTRL pin can increase the charging current to 100uA which both swamps out potential leakage and reduces startup time.

    I need to confirm our designers as to the typical pull-down strength of each device.

    I hope this helps.

    Regards,

  • 0 in reply to JCHK
    Mastermind 9530 points
    Hi, John,
    My customer want to reduce inrush current and a load is very sensitive, so TPS7A91 is very suitable.
    Please check the no problem maximum Cnr/ss when re-enabled.

    Best Regards,
  • 0 in reply to Hirotsugu Kobayashi
    TI__Guru 50030 points
    HI Kobayashi-san,

    The pull-down strength is typically a 100-300Ohm resistor. So the issue is, how quickly would the device be enabled after being disabled? If you do not see the application disabling and suddenly re-enabling the LDO, this is not an issue.

    Does this answer your question?

    Regards,
  • 0 in reply to JCHK
    Mastermind 9530 points

    Hi, John,
    When does a discharge circuit become active?
    Does it become active once after Enable?
    Or is it always active in Disable?

    Best Regards,

  • 0 in reply to Hirotsugu Kobayashi
    TI__Guru 50030 points
    HI Kobayashi-san,

    I checked the design database and confirmed that the discharge circuit becomes active when the device is disabled (i.e. EN=Low) or if VIN falls below our UVLO. It is not active during normal operation.

    Regards,