John THX.

But I was wondering if Anybody can calculate the heatsink from these given

information.

Heatsink info is in format xxx °C/W.

So The question is CAN U CALC THE HEATSINK or NOT?

Rok said:

Hi.

I'm wondering if anybody can help me. I made a voltage regulator with LM338T

and everything works OK. The problem is in heatsink. In have to calc it and

I don't know how. Can somebody help me. Information from datasheet of LM338

are:

Rjc=4°C/W

Rja=50°C/W

Rcs=0,5°C/W

Ta=40°C

Pd=25W

Now can somebody give me the whole calc of heatsink.

THX to anybody.

Rja is thermal resistance junction to ambient, when the device is used

without a heat sink, so you can forget that value.

Rjc is thermal resistance junction to the mounting surface of the

case. Rcs is thermal resistance between case and heat sink (under

some specified mounting setup, probably just a bit of zinc oxide paste

between case and sink and a specified clamping pressure). You still

need Rsa, the thermal resistance of your sink to ambient to add up the

total thermal resistance between junction and ambient. That would be

the sum of Rjc, Rcs, and Rsa. The total thermal rise above ambient is

the power being dissipated by the junction (difference of input to

output voltage times load current, for a regulator) times the total

thermal resistance.