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From: "Graham Holloway"
Subject: Re: Unusual latchup problem
Date: Wed, 6 Nov 2002 08:45:54 -0000
NNTP-Posting-Date: Wed, 6 Nov 2002 08:45:11 +0000 (UTC)
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"Linear" wrote in message
> "Jean-Rene David" wrote in message
> This is a circuit I didn't design but which I'm troubleshooting.
> >Also, when the -12V *doesn't* latchup, it actually goes up to 0.7V for
> >50-400ms after the +12V rise before coming back down again to -12V. The
> >delay seems random.
> >If anyone has any idea on 1) the possible cause of this problem or 2) a
> >to troubleshoot the problem or otherwise find the faulty device, I would
> >greatly appreciate.
> It sounds like power supply sequencing may be causing the problem.
> Depending on the design of the op-amp it will prefer to have one supply
> powered up first. If the supply sequencing is wrong it will frequently
> cause latchup. To test this out try sequencing the supplies to see if
> turning on the -12V supply first then the +12V will eliminate the problem.
> If so then you may be able to fix the problem by tailoring the supply
All you should need to do is to put reverse-biased 1N4001's across each PSU
output. The regulators latch when their outputs are taken outside the normal
operating range by current flow directly between the positive and negative
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