Spehro's got it. I can count on one finger the number of latchup's I've ever had- and it was caused by a faulty ground in a PCB layout. You will need to rent a FAST scope, fast enough to catch those noise spikes and locate 'em. The TL072 is a very rugged chip; to kill a TL072 it will need to be FAST and HIGH (at least 5V above the VCC rail). That's what those rental companies are for- special tests like this. Use 'em. Hint: if your design has a switching supply, and you are not using a ground plane (PCB of 4 layers or more, with a layer reserved for GND) that's the reason. Switchers operate in the Mhz+ region, and if the ground is poor, RF noise will swamp the other circuits. --Bob Spehro Pefhany wrote: >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 > > > > -- Note: To protect our network, attachments must be sent to attach@engineer.cotse.net . 1-520-850-1673 USA/Canada http://beam.to/azengineer -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist