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LM317 thermal resistance values: Junction to case and junction to pad?

Abhay Mohan
Prodigy 10 points
Other Parts Discussed in Thread: LM317, LM317HV, LM338

I'm using a TO-220 package of LM317 at 700 mA current and Vi-Vo = 12 V. This dissipates 8.4 W of power. Page 2 of the LM317 datasheet by TI gives the following values:

Junction to case thermal resistance: 17 degrees C/W

Junction to pad thermal resistance: 3 degrees C/W.

The junction to case thermal resistance seems a bit too high for me. For LM317HV, the Rjc is just 5 degrees C/W maximum. When I use attach a heat sink to the back of the LM317 IC, which one of these thermal resistance values should I use? Junction to case or junction to pad? Why is the Junction to case so high in this particular IC while its pretty low in LM317HV, LM338 etc? The same LM317 TO-220 of National Semiconductor is just 4 degrees C per watt maximum...