how to discharge caps and inductors at powerdown?

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Hi guys,
 
 I noticed a problem on a board I designed:
 when I power off the board, the inductor and
 the capacitor of the LC filter (470 uH and 1000uF)
 keeps powering the voltage regulator for a certain
 amount of time. The bad thing is that during power
 down I noticed an oscillatio of the regulated +5V

 connector --> LC filter --> voltage regulator --> ICs

 How can I get rid of all the reactive enrgy stored
 in the LC filter, at power down? I don't want this
 energy go to re-power the voltage regulator.

 In principle I need a resistor that is active just
 for the time of power down.

 thanks
  Enrico

Re: how to discharge caps and inductors at powerdown?
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Hmm, what about a parallel reverse diode?

--
Bernhard Roessmann
Don't Fear The Penguins!


Re: how to discharge caps and inductors at powerdown?
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Good idea, but I would like to know why the regulator oscillates.


--
Thanks,
Frank Bemelman
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Re: how to discharge caps and inductors at powerdown?

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Enrico,

What is the frequency of the oscillation we taking about here? and is the
1000uF the only C on the input side?

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.

www.stacktools.com



Re: how to discharge caps and inductors at powerdown?

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

http://www.abvolt.com


Enrico Migliore wrote:
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Re: how to discharge caps and inductors at powerdown?
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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.

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