3
\$\begingroup\$

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.

\$\endgroup\$
  • 1
    \$\begingroup\$ If you don't solder the LM317 into the PCB, then use junction to case thermal resistance value. btw the heatsink you need is quite big. If you want to use a 2mm aluminium plate it will be not less than 16x16 cm. I suggest to reduce Vi-Vo or use a series resistor. \$\endgroup\$ – GR Tech Feb 1 '14 at 3:45
  • \$\begingroup\$ @GRTech, I'm going to use the LM317 like this, except with a heatsink. Is this what you mean by soldering it into PCB?nabilkarami.com/projects/images/lm317%201.JPG \$\endgroup\$ – Analon Feb 1 '14 at 4:10
  • \$\begingroup\$ θjp usually refered when soldering pad directly to pcb copper. However the θjc given as 17 ℃\W it is unusual for TO-220 packages, which ia typically 5 ℃\W. Anyway standard PCB copper can provide thermal resistance not less than about 20 ℃\W \$\endgroup\$ – GR Tech Feb 1 '14 at 4:44
3
\$\begingroup\$

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:

TO-220 with case and pad labeled

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

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.