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TPS7H1101A-SP: Input capacitor effects on stability

1243558
Prodigy 190 points
Part Number: TPS7H1101A-SP

I have a design which uses multiple channels of TPS7H1101A-SP to create several supplies at relatively low current (mA to tens of mA) from a single input supply.  We have a restriction from the upstream regulator to limit the upstream regulator's load capacitance.  The existing individual 7H1101 regulator channel schematics each have a 0.1uF || 22 uF input cap network, which causes my design to violate the loading requirements of the upstream regulator.  We'd like to reduce the input capacitance or possibly combine the capacitance from multiple 7H1101 channels together to reduce the load on the upstream regulator.  The 7H1101 data sheet recommends a minimum 10 uF input cap in parallel with a ceramic for stability reasons (from the datasheet):

TI recommends to use a tantalum capacitor along with a 0.1-μF ceramic capacitor. The device is stable for input and output tantalum capacitor values of 10 μF to 220 μF with the ESR range of 10 mΩ to 2 Ω. However, the dynamic performance of the device varies based on load conditions and the capacitor values used.

Can TI comment on the stability impacts of operating the 7H1101 with less than 10 uF of input capacitance, or on the sharing of input capacitance across multiple regulator circuits so long as they are reasonably closely located to each other?

  • 0
    TI__Mastermind 28645 points
    Hey,

    The capacitance requirement, although it doesn't explicitly say, is assuming you have a larger load.
    Something to notice about the TPS7H1101A-SP is that the gain of the loop decreases as the output load decreases.

    While I have not personally tested it, I would expect you can go lower in capacitance if you limit how much load the device is supplying.
    I do have some data to at least tell you how much of a decrease in gain you can expect if you are willing to provide how much current the LDO is supplying.

    While possible, using a lower capacitance would need to be tested.

    Thanks,
    Daniel
  • 0 in reply to Daniel Hartung
    Prodigy 190 points

    Thanks for the information.  After some thought we've decided to build our prototype with the footprints to allow capacitance to be installed at each regulator, but we'll depopulate and test a minimal input capacitance initially.