Meaning of Rth sink-ambient negative

Thread Starter


Joined Jan 17, 2017
Hi there !

I would need use the LM338 TO-220 with 2A.
Rthja= 50 C/W
Rth jc= 4 C/W
Tj=100 degC - Ta= 50 Deg C
Pawer supply ; 12 v @1.5 A

PD= (12 V - 5 V) x 2 A= 14 W

Heatsink thermal Resistance
Rthsa= (Tj-Ta) / PD - RthJc- Rthcs = (100-50) C/ 14W - 4C/w-0.1C/W= -1.43C/W
Which is the meaning of negative Rthsa ???

Thanks fore any help.


Joined Aug 21, 2008
It means that because the thermal resistance from junction to case is too high you would have to use a refrigerated heatsink.

Thermal resistance junction to case 4°C/W, dissipation 7V x 2A = 14W.
Thermal rise between case and junction is 14V x 4°C/W = 56°C°.

To avoid using a chiller you would need to run at a lower ambient temperature, split the regulation chores among more than one chip to lower total thermal resistance junction to ambient, lower your input voltage or shunt some of the current around the regulator.