MMi203: Operating temperature ratings

The maximum allowable operating temperature of the board is governed by several factors.To determine the highest permissible operating temperature you need to evaluate each factor separately and then use the worst (lowest temperature) answer.

The factors that must be taken into account are:

1. The semiconductors on the board have a maximum ambient temperature rating of 70°C.
2. The LCD, if you are using one, may suffer from reduced readability at elevated temperatures. If you are supplying your own, make sure to select one with a suitable temperature rating. The ones we supply are rated to 70°C but may suffer a reversible loss of readability over 60°C.
3. The 5V power supply regulator will shut down and self-protect if its junction temperature exceeds 150°C. How hot it actually gets depends heavily on the supply voltage. The table below will help you with this factor.
4. The chip driving the outputs will also self-heat, and eventually fail if run too hot for too long. Evaluating this factor is discussed under MMi201 External digital outputs
5. Hot components fail faster than cool components. For semiconductors, lowering the temperature by 10°C doubles the life time.

5V regulator

Allowing 3mA for an LCD and 5mA for a SPice board, the worst case load on the LM317 regulator is 64mA. With a maximum permissible operating junction temperature of 125°C and a junction to ambient thermal resistance of 50°C/W, this translates into the following maximum ambient temperatures versus supply voltage:

Note: Figures at 28V and 32V are given for reference only. We do not recommend operating the board at voltages over 24V for sustained periods of time. The board can be damaged if the power supply voltage goes over 32V, even momentarily.