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LP5900: Larger bulk capacitance at the output

Joel Kindem
Prodigy 30 points
Part Number: LP5900

Hi,

I'm using several of the LP5900 in my design as they have low leakage when disabled which is perfect for my battery powered application. The question is I would like to cascade  (add one more level of regulation 3.1V) ahead of the current regulators I'm using (2.7V and 3.0V), so I can isolate my leaky bulk capacitor (100uf tantalum polymer) from the battery so I don't drain it while in storage.

The connection is Battery -> 3.1V regulator -> large bulk capacitor and other system regulators.

One issue I have had with this regulator is that with more output capacitance (the datasheet says max is 10uf), the regulator tends to blow out, so I'm forced to use a small series resistor to prevent this. The problem is that I then have the resistor in my power path. The question is what is the smallest theoretical value by design I can use for this resistor to prevent it from blowing out? I would prefer not to have to experiment with this and would rather like to know how to get around the 10uF max output capacitance.

Thanks

  • 0
    TI__Mastermind 22610 points

    "... the regulator tends to blow out ..."

    Hello Joel,

    Not obvious to me why the LP5900 would blow out based on the Cout value.

    At power-up the current through the LP5900, and into Cout, would be limited by the internal current limiting. The time to charge Cout would be extended, but the device should not be damaged.

    A possibility at power-down is that the input voltage goes down quickly (i.e. not floating). In this case Vout>Vin and there will be reverse current through the LP5900 internal PMOS pass element body diode. This condition ~might~ damage the LP5900. The fix for this condition would be to add a Schottky diode from Vout to Vin to redirect the reverse current around the LP5900.

    .

  • 0 in reply to Donald Jones
    Prodigy 30 points
    Can you shed some light on why is there an upper limit on the datasheet for max capacitance?

    Typically the part that blows out has the output stage connected to 1uF x7r in parallel with 47uf 0805 x5r which per the mfg is really more like 17uf due to the voltage characteristic for that part, by adding a resistor (5ohm) in the path of the 47uf cap, I have not had any issues. The problem with the schottky solution for me may be that the leakage in the schottky diode may increase the passive drain I'm trying to avoid (see original post).
  • 0 in reply to Joel Kindem
    TI__Mastermind 22610 points

    The reason for the 10uF max Cout limit is that the original design team did not expect there to be any applications for the LP5900 device that would need a larger Cout.