Which pins to dissipate heat for SO-8 regulator?

Bust one open and see. I'd guess that the GND pin is the chip paddle, which is where most of the heat comes out. The in/out pins are probably wire bonds, which don't conduct heat very well.

In 317 type parts, Vout is the paddle, and the heat comes out there, on 4 pins for SO-8. On 337's it's Vin, again 4 pins.

How much power do you plan to dissipate? If it's marginal, put a power pad on the pcb below the chip and epoxy it to that. Or use a chip that has better thermals. Their value of 150 K/W for the SO-8 sounds optimistic.

John

The MCP1726 has a DFN package version with an exposed power pad (EP), to remove heat. The exposed pad is connected to the substrate, which is at ground potential. You get a low RthJA = 41 deg-C/W thermal resistance by soldering to that, as opposed 150C/W (3.6x worse) with the soic package. With a 150C maximum junction temperature, and 25C ambient temp., this means you can dissipate 3W vs. 0.8W, without any added safety margin. Call it 2W vs 0.5W, or 4x more.

A lot of times on power devices they use a heavier bond wire. I don't have specs handy, but I recall 1mil standard and 1.3mil heavy. Anyway, the heat carrying ability of the bond is proportional to the cross sectional area, so a slight increase in width is a big increase in heat transfer via bonds.