Thanks for the answer, we use the LPC1758 datasheet Rev. 8.7.
We did a worst-case temperature test in a climate chamber at 50°C ambient temperature. The chip is located inside an enclosure. The board also contains power electronics. We measured a case temperature of 93°C. The board is slighlty above 85°C, the air is cooler (but hard to measure).
The 85°C ambient is valid for the maximum power draw of the chip. But we dont use maximum clock speed, maximum operating voltage and not all peripherals. So even if the ambient temperautre is slightly above 85°C, the chip may be a lot cooler then the maximum recommended junction temperature.
Using the power draw of the chip < 300mW and Rth(j-c), I can estimate the junction temperature at Tc+4°C (97°C). I need to determine, if this is inside the recommended ratings or not.
150°C junction temperature as stated in the maximum ratings is a stress rating only. Operation is not guaranteed.
