Op-Amp output spike on power down - Page 1
Forum section: electronics / projects

author: m4rv
author_rank: Newbie
timestamp: April 17, 2015, 11:14:09 pm
content: I'm having issues with the TLV2374 op-amp (Datasheet:http://www.ti.com/lit/gpn/tlv2374)The specific issue is that the output pulses slightly as the voltage drops below about 1.3 volts, which happens as the mains PSU I'm using in the final circuit slowly looses its charge. This happens even if the inputs to the amp are in a state where the output should be at 0v.For experimentation, I set up one of the amps on a breadboard as a non-inverting buffer with no external components at all. I connected the positive input directly to ground to try and keep the output at 0v.I can lower the supply voltage all the way from 6V (the voltage I'm using in the finished circuit) down to about 1.3v and the output will stay at 0V. As soon as i go below 1.3V the output voltage jumps up to the input voltage and then tracks it down to near 0v. As soon as I raise the supply voltage above 1.3V again, the output jumps back to 0v.The really weird thing is that this only happens on one of the amps in the quad package. I went through 4 different ICs and in each one, amp 3 would behave the same way. The other amps in the IC didn't have this issue, their output would stay at 0v regardless of the supply voltage.I bought the IC's in packets of 5, and the first 4 chips I tried came from the same pack. When I opened another packet to test one more, it was op-amp 1, rather than 3 which had the same issue. A one-off DIP version of the IC I had or testing exhibits the issue on amps 3 and 4.Unfortunately, this is a fairly serious problem for my application. I can't have the output of the op-amp pulse on power down. The power supply I'm using in the finished project ramps its voltage down over a second or so when mains power is removed. So for the fraction of a second where the supply voltage is between 1.3V and 0V, the output of the op-amp is tracking this voltage.I appreciate that this op-amp isn't designed to run below 2.7v and I wouldn't expect it to work properly below this voltage, but I wouldn't have expected behaviour like this.Is this something that occurs often? Is it something that a more experienced designer than myself would just know about and expect to happen, and therefore design the rest of the circuitry to accommodate the issue?Is there a way to overcome the issue of the power supply capacitance and have the supply voltage drop to 0v much more quickly when mains power is removed? (I have already tried adding a dummy load to the power supply hoping that more current draw would cause the output to fall fast enough, but there is still a significant ramp down period).Is there anything else that I can do to stop the op-amps behaving like this?Unfortunately I didn't see the issue during design and testing of the circuit and I've had PCBs produced already so I'm hoping that I can find a drop in replacement op-amp that doesn't exhibit this behaviour (I've got a few different IC's on order), or find a fairly simple bodge solution that doesn't require new PCBs to be manufactured.Any thoughts or advice would be much appreciated!

author: moffy
author_rank: Super Contributor
timestamp: April 18, 2015, 12:47:01 am
content: When outside the recommended voltage range of not just op amps but all ICs their behaviour is indeterminate, i.e. you cannot depend on a certain behaviour. You could use an UVLO (under voltage lock out) circuit to remove power below 2.7v and clamp it to 0v, it just needs a little hysteresis.