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LM317: Significance of Junction to case (Top & Bottom) thermal resistance in TO-220 packages that are mounted vertically on a PCB

Sivasai Praneeth Nanduri1
Prodigy 20 points
Part Number: LM317

Hi All,

I have a generic doubt about Junction to case (Top & Bottom) thermal resistances specified in the data sheet of LM317 (http://www.ti.com/lit/ds/symlink/lm317.pdf). If I am mounting this component vertically on a PCB and have a heatsink that touch bottom pad (the dotted lined in the datasheet, page 3) and also the remainder of the package on the backside should I consider RθJC(bot) or RθJC(top) for my heatsink calculations? 

Typical heat sinks for vertically mounted TO-220 packages

I think it should be the RθJC(bot) since the heatsink touches the bottom surface of LM317 and hence directly taking away heat from the closest point on the die but as http://www.ti.com/lit/an/spra953c/spra953c.pdf describes in page 7, section 2.4, typically RθJC(bot) is used for the ICs that are soldered right onto the PCB with metal slugs to take away heat from a point closest to the die. (As seen below)

So I am a little bit confused as to should I use the RθJC(bot) or RθJC(top) for heatsink calculations because the vertically mounted LM317 is not exactly same as the heat management process used in above picture (Used copper plate on PCB as a heatsink). Also, when in doubt, and if there is no limitation on the size of heatsink it is ok to choose a heatsink based on worst possible scenario calculation? 

Thanks,

Siva.

  • 0
    TI__Guru 55940 points

    Hi Siva,

    Since you are mounting the heatsink to the bottom of the package, you should use RθJC(bot).  This will be most similar to how when a IC is mounted using a thermal pad soldered to the PCB where the PCB is the primary heatsink.

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    Prodigy 20 points

    Ryan,

    Thank you very much for the confirmation. Just wondering, what are the cases where RθJC(top) will be used for LM317 or any TO220 package? All the common heat sinks that I tried searching looked like the one I have attached in my question.

    Regards,

    Siva.

  • 0 in reply to Sivasai Praneeth Nanduri1
    TI__Guru 55940 points

    Hi Siva,

    I agree that the best location for a heatsink will be attached to the primary path for heat to exit an IC (such as a thermal pad); however, some heat will also exit through the top of the case as well.  As such, a heatsink on the top of the case will have less of an impact, but it will still help pull heat out of the regulator.  Ultimately the selection an application of heatsinks will be determined by application specific parameters including clearance constraints for the end product.  

    Very Respectfully,

    Ryan