To whomever reviews this post,
I had a question in regards to TI's LMV774 series op-amp. I cannot provide schematics, subsystems, etc because all of this information is disclosed.
I am trying to diagnose why an intermittent failure occurred in a 4-20mA transmitter. The output of the failed unit was pinned at a very low mA output. At the time of the failure, severe thunderstorms were present in the area. The unit was unplugged and then powered back on and the unit did not recover to it's normal operation. After opening up the device (a welded housing), I noticed the LMV774 Op-Amp on the PCB had a scorch mark on it's surface. This brown discoloration indicates that this part was overheated. Given the nature of the weather, I suppose the unit saw an ESD, Surge, etc. Since the unit operates through a current loop, it is not grounded. When I probed the LMV774 op-amp, I noticed the pin out voltages were correct.
**Is it possible for an LMV774 op-amp's CMOS transistors to latch-up in the event of a transient or surge and not be damaged? Note: As soon as I probed the OP-AMP and discovered it was still functioning, I checked the overall output of the unit and it began functioning again. The output was no longer pinned and the unit resumed normal operation. Since the device is not grounded, perhaps by touching the PCB I grounded the Op-Amp's terminals and this dissipated the potential energy stored in the latched transistors?
I am trying to provide an explanation as to what caused the unit to fail. Since the failure was intermittent and not permanent, it is really hard to provide an explanation. Since the output was pinned at a specific output independent of the devices input, it leads me to believe that the OP-AMP was indeed latched.
My main question is whether these theories regarding why the unit failed are plausible. Essentially, is it possible for the LMV774 to latch up during the event of a transient or surge? If it can latch, does this mean that all of the pins are shorted to the negative terminal of the unit since no bias is applied to the transistors to change their state? When latched, can the Op-Amp self recover or does the unit need to be grounded to PE? Does this brown scorch mark indicate overheating do to a transient event or could this simply be from heat wave soldering?
It is very important whether this scenario is plausible so that a proper Engineering Report regarding this failure can be written for my customer. Although I cannot disclose information publicly, I would be more than willing to answer questions over the phone. I would actually prefer to discuss this issue over the phone with an engineer representing TI. The customer support can only answer questions regarding the datasheet. I was told the only way I can contact an engineer was through the E2E forum. Again this matter is urgent and any help or opinions would be greatly appreciated.
Keith Oldano
(Electrical Engineering)
973-616-6938
