Tool/software:
Hi,
I'm characterising an industrial design at +85C Tambient and the linear regulator we're using is (LM78M05CDT/NOPB) getting excessively hot, near its Tj limits.
I'm measuring 119°C at the TAB, and by my calculations the Tj is 123°C, which is 2°C shy of the maximum limit.
I've looked into other similar parts and they all seems to have Tj = 150°C.
My question is, what's the long term implications in terms of device reliability at these temperatures?
Thanks,
Best,
OA
Hi OA,
Operating at high junction temperatures produces an accelerated rate of common failures. However, another concern with operating close to the thermal limit is the possibility of cycling in and out of thermal shutdown:
Thermal cycling produces high inrush current at frequent intervals. Essentially the device overheats, triggers TSD, cools off to overcome thermal hysteresis, turns back on and starts drawing a large initial current before current limiting circuit kicks in. And then heats up and repeats. Normally this isn’t a problem but if it’s cycling frequently, it may be more of a concern. Some more information on managing inrush current here.
In this case it's best to design the board to maximize RθJA as much as practical.
If you're interested in other parts with better thermals, let me know your requirements and I can give you a list of everything that matches.
Best,
Gregory Thompson
