At 09:44 PM 1/24/2006 -0800, you wrote:
>I apologize in advance if I've missed something that should be obvious, 
>but this one's making me crazy.  I have an LM1972 uPOT on +/-5V rails 
>feeding a TL072 on +/-12V rails wired as a unity gain buffer as per the 
>LM1972 datasheet.  I've used this circuit for years with a linear power 
>supply without any problems, but when the circuit was ported to a system 
>with a (noisier) switching power supply, the TL072 would latch up after 
>only a few hours of operation .  First problem: I had put the TL072 on 
>+/-5V rails without consulting its spec sheet (too low, as it turns out), 
>so I substituted the more forgiving LF353.  The latch-up condition would 
>still occur after several hours of operation.  I then replaced the 353 and 
>raised the rails to +/-12V as they had been in the functioning legacy 
>system.  No dice: the latch-up still occurs.  Besides power supply noise, 
>the only other difference between the functioning legacy circuit and the 
>faulty new one is the impedance they're each loo!
>  king into: 2.7k for the former; 22k for the latter.  Does anyone have 
> any ideas about what might be causing the op amp to latch up?


Are you sure the op-amp itself is the problem, and not, say, the LM1972?

Usually latch-up problems are caused by exceeding the power supply range or
input voltage range of a part. This causes parasitic devices to trigger, or
puts the part in a mode what is unusual and may self-latch because of
internal or external feedback paths. So, we'd tend to be less suspicious
of a device that is running off the highest supply voltage unless the
input(s) can go outside of the normal range of operation (for example, the
common mode range on an op-amp). If the latter is happening, like banging
your head against the wall, the solution is simply not to do that (eg.
clamp the voltages into the inputs so they stay within permissible limits).

Perhaps you can get a full set of measurements in the failure state and
record the voltages at all relevant nodes (perhaps including some that
don't initially appear to be relevant) and put them on a schematic.
I suspect the real cause will become abundantly clear at that point.

BTW, the TI TL072 advertises that its operation is "latch up free".

Is  it possible that your switching supply is so horrible that it is
exceeding the absolute maximum input voltage or supply voltage ratings
under some conditions? Or maybe the particular system you're checking it
on has a problem such as an intermittent in the feedback loop that causes
the SMPS regulator to go "balls to the wall" intermittently.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
->> Inexpensive test equipment & parts http://search.ebay.com/_W0QQsassZspeff


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