uA78xx vs. other 78xx thermal spec

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How can TI have such a low R_thetaJA? Are they measuring it differently? I am looking at the specs for the TO-220 package. Usually I see the TO-220 having about 55 to 65 degC/W. When they say JESD 51-5 are they saying that they mount it on the horizontally? Isn't this just for getting the junct-case number?

http://focus.ti.com/lit/ds/symli...

vs.

http://www.national.com/ds/LM/LM... (note 2)
http://www.fairchildsemi.com/ds/...

Any insight would be appreciated.

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That board that they are spec'ing it with is a high thermal conductivity circuit board. That is why its low.

But, it looks more strange when you compare junction-case thermal resistance. As I read it, National is 4C/W while TI is 17C/W.

For any design I would do, though I generally do not use 7805/LM340, I would scan a variety of data sheets and choose worst case conditions over the set of sources so that my manufacturing processes could use any that are currently available.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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I am stuck with the 7805, it is already in the existing design.

Quote:
But, it looks more strange when you compare junction-case thermal resistance. As I read it, National is 4C/W while TI is 17C/W.
This is why I think they must be mounting it directly to the board. The junction-pad looks more like the typical junction-case number. I have an email in to TI asking about this.

I am trying to find a pin compatible substitute that is also specified down to at least -20C.

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ka7ehk wrote:
But, it looks more strange when you compare junction-case thermal resistance. As I read it, National is 4C/W while TI is 17C/W.

The difference is in the terminology. What National calls J-C (from the die to the metal tab) is what TI calls J-P ("Junction to Pad"). The TI doc shows J-P at 3 C/W.