So you're saying that heat testing with my finger on the board after building the prototype is not scientific enough? :-)  I'll run these numbers for my app to see where it stands, but I know in actual practice I can detect any heat rise using MMBF170's (finger method).  Only other thing is the there is a 7805 in the case generating some heat, so the air in there is already 20-30 deg C above ambient.... or more?  I'll have to get a temp sensor in there some time to find out for sure.

Cheers,
-Neil.



> I think you have to work out the max die temperature from the
> transient thermal impedance plot for the max temperature,  timing &
> load you're using.
> Then, as long as it's less than 150C, you should be OK. (preferably
> significantly less).
> 
> For your frequency & pulse rate the transient thermal impedance looks
> to be about 40K / Watt.
> At 250mA & 10 ohms the max power is 625mW which correspnds to a
> temperature rise of 0.625 * 40 = 25K (or C)
> Therefore at an operating ambient temperature of 50C the die
> temperature will be 50+25 = 75C which is much less than the 150C
> maximum. You should be OK unless I've stuffed up the calculation
> somewhere.
> 
> Note also the plot is for a device mounted on a metal clad substate -
> For a pcb the continuous typical value is 350k/W cp 150 for the metal
> clad.
> There should be enough safety marging to cover this however.
> 
> RP
> 
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