Kai, I'm looking at your example as I'd REALLY like to be able to understand how your calculating this in the future.

For thermal resistance, the datasheet says:
Oja = 220 deg/W

For junction temperature range:
-40 deg C to +125 deg C

And the formula for Maximum Power Dissipation is:

P(max) = (Tj-Ta)/Oja

My product will be used mainly outdoors in the US in most cities. I'd say that the hottest temperature it will be used in would maybe be 48 degrees celcius. The coldest maybe -28 degrees celcius.

In your calculation you calculate the power given the voltage drop and current usage. Then you multiply the power by the thermal resistance. This gives you the 72k number.

So then I assume, using the max power dissipation formula, you took an ambient temperature of 40 and arrived at a Tj = 112. Kinda confused how you got that number...

Thanks in advance.
James