ADuM3470: thermal derating

Q 

I have some doubts about “thermal derating curve” of iCoupler products on the
datasheet, these two curves confused me, could you please explain to me the
meaning of these two curves?

 

A 

The thermal derating curve is developed like this:

At 25C the worst case current at VCC is 1.25A for a worst case VCC = 5.5V, for
input power of 6.87W, but this power is not all consumed by the ADuM347x. Per
the TPC’s for the 5V to 5V configuration which can deliver the most power of
2W, the worst case efficiency is 65% for 400mA guaranteed load, so 35% of the
power is assumed dissipated by  the ADuM347x, which is a worst case since the
external components like transformer and diodes actually consume a lot of the
power. So, 35% of 6.87W is 2.4W and the theta JA is 50.5C, so temp rise worst
case is 121C at 25C or 146C, actually less than the 150C maximum junction
temperature.

For 105C, the safe operating current is 0.45A which is 2.48W at 5.5V VCC, and
35% of which is 0.87W, which won’t all be consumed by the ADuM347x, but if it
were the 50.5C/W at worst could cause a temp rise of 43.8C, for a total
junction temp of 148.8C.

In practice, to avoid achieving maximum junction temperature when at 105C, you
would need to limit the input current, but since the ADuM347x has a thermal
shutdown, you do not need this precaution.