We are having a problem with our Analog Devices ADN8831 TEC driver circuit. The symptom is we lose TEC lock.
1. We have been using this circuit successfully for 4 years on 3 different line cards.
2. Recently we have 4 failures of TEC lock.
3. They all trace back to a destroyed linear P-FET. We use FDS9934C dual N/P FETs. Only the P-FET is bad. There are two other TEC drivers with the same circuit on the board and they are fine. The other FET in the package is probably OK. In all instances, the P-FET gate and source are shorted or low resistance. In two instances, there are burn marks on the top of the FET package.
4. These TEC controllers drive TECs in lasers. The lasers have been moved to other boards and they are OK. Also the problem happened with two different laser vendors. Ie, the problem is believed to reside with the electronics, not with the laser TEC.
5. I swapped the burned out FET chip and the TEC driver works fine.
6. We designed this based on Rev 0 of the datasheet. On the current datasheet Rev A, there are two changed recommendations:
a: The bypass cap on the linear side is moved to the downstream side of the sense resistor.
b: The CMPSW network is more elaborate, no just a single cap as before.
Could the changed recommendations have any effect on the problem.
What could be causing my burned out FET?