LM317 TO-220 heat sinking: Junction to case or junction to pad?

Question

I have some LM317 ICs (TO-220) that I'm using on a power supply. I'm trying to figure out the heat sink values necessary to keep the IC under safe operation.

All of my requirements are with 700 mA current, however Vi-Vo varies. For an example, the maximum Vi-Vo I have is 12 V. So the power dissipated is 8.4 W.

So I got the following thermal resistance values from the data sheet:

Junction to ambient: 19 ℃\W
Junction to case: 17 ℃\W
Junction to pad: 3 ℃\W

I'm using a thermal tape to fasten it to a heatsink. I can do the calculations myself, but my problem is that I'm confused if I should consider the junction to case or junction to pad thermal resistance value? The junction to case resistance seems too high. If I use that for heatsinking calculations, I find that no heatsink can save me from overheating it. But junction to pad gives me hope.

Which one of the following is correct?

• Junction temperature = Ambient temperature + Pd * (Junction to case + thermal interface material + heatsink thermal resistance)

• Junction temperature = Ambient temperature + Pd * (Junction to pad + thermal interface material + heatsink thermal resistance)**

I'd like to avoid swithcing regulators as much as possible, I already have these ICs and switching regulators will add unwanted complexity here.

Answer 1

According to the note just below the thermal data table from that datasheet:

For packages with exposed thermal pads, such as QFN, PowerPAD™, or PowerFLEX™, \$\theta_{JP}\$ is defined as the thermal resistance between the die junction and the bottom of the exposed pad.

The way I interpret that:

So if you are attaching the heatsink in the normal way, on the back, against the pad, use the junction to pad thermal resistance.