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From: John Larkin
Subject: Re: Unusual latchup problem
Date: Tue, 05 Nov 2002 21:54:49 -0800
Organization: Posted via Supernews, http://www.supernews.com
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On Wed, 06 Nov 2002 02:10:15 -0500, Jean-Rene David
>This is a circuit I didn't design but which I'm troubleshooting.
>It seems I have a latchup problem on a circuit with no connection to the
>exernal world. There are four supplies (+-12, +-5) but no part is connected
>to both (I removed them all) and no +-12V part is connected to any +-5V
>part. All the +-12V parts are opamps or instrumentation amplifiers.
>Now the symptoms are strange in the following way: when I fire up the power
>supply with the circuit plugged in, the -12V latches up 90% of the time.
>When I turn the supply on and THEN connect it to the circuit, the -12V
>latches-up about 10% of the time.
>Since the circuit is not connected to the external world, I thought ringing
>from a steep rise on the P/S might cause the latchup. However the rise time
>when turning the P/S on with the circuit plugged in (the case that latches
>up) has SLOWER rise time (about 3ms) than the other case (a few
>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.
>There are more than a dozen opamps on the thing and though I will do it if
>I have to, I don't really feel like adding protection diodes or current
>limiting resistors on each one.
>If anyone has any idea on 1) the possible cause of this problem or 2) a way
>to troubleshoot the problem or otherwise find the faulty device, I would
Since there are ICs connected between +12 and -12, at powerup it is
possible for one supply to be pulled 'through ground' to the wrong
polarity, and then all sorts of weird things might happen, either in
the opamps or in the supply itself. Another possibility is that one of
the supplies gets stuck in a current-foldback state; foldback supplies
can always (assuming sensible supplies) pull up a resistive load but
sometimes can't pull up a constant-current or similar load that pulls
a lot of current at low voltages.
Try putting schottky power diodes across all supplies to ground to
keep them from going (much) through ground the wrong way. Also try
measuring the currents in the hangup states to see if any supplies are
in foldback. Over-zealous foldback designs can have weird startup
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