how to discharge caps and inductors at powerdown?

Hmm, what about a parallel reverse diode?

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Good idea, but I would like to know why the regulator oscillates.

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Enrico,

What is the frequency of the oscillation we taking about here? and is the

Normally you would also add some smaller caps (e.g. 1* 220n and 1*22p) for the higher frequencies. (This is also the case at the output side ). Large C's have a relatively large internal inductance.

Furthermore, do you need such large C on the input side if it's powering the system for a while? The system is rather "high impedance" (no much current draining) otherwise it couldn't be powered by the C for a while. You may consider a smaller buffer cap.

gerard.

You may want to use a regulator with SHTDN input. Many of this kind are available from any major semiconductor vendor. Once the input power is low, assert the SHTDN signal to switch the regulator off instantly.

Vladimir Vassilevsky, Ph.D.

DSP and Mixed Signal Design Consultant

Enrico Migliore wrote:

There's one more thing I should say about this:

Many voltage regulators do not like having the output voltage being higher than the input voltage. The circuit I described above can cause this situation. So the solution is to add a diode in parallel with the input and output terminals of the voltage regulator. For a positive voltage regulator, the diode's cathode would go to the input terminal of the voltage regulator.