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LM317 Can work at +85C celsius

Hector Lopez
Prodigy 20 points
Other Parts Discussed in Thread: LM317

I'm wondering if the LM317T (TO-220) can work at +85C (temperature) with a heatsink? and which heatsink can handle such temperature? or perhaps even using a heatsink I need to attach more than one LM317T in parallel to have a current of 1.0A at the output. My most critical case is when the Vin=13.7V and the Vout=1.3V and the Iout=1.5A (or 1.0A)

  • 0
    TI__Mastermind 22610 points

    I presume that the 85°C is the ambient temperature.

    In the case where Vin=13.7V, Vout=1.30V, and Iout=1.5A the dissipation in the device will be ((13.7V-1.3V) x 1.5A) 18.6W

    In order to keep the junction temperature below 125°C the device temperature can only rise (125°C - 85°C) 40°C above ambient.

    Therefore, the required Theta(j-a) (junction to ambient) thermal resistance would then need to be (40°C/18.6W) 2.2°C/W.

    The LM317 data-sheet (SNVS774L.pdf) says the typical Theta(j-c) (junction to case) for LM317 in TO-220 is 4°C/W. So, adding an infinite heat-sink to the TO-220, the Theta(j-a) will not be better than 4°C/W.

    If you go with parallel LM317's you will need to add a ballast resistor on each LM317 output to prevent one regulator from hogging 100% of the current. 

    Better option might be series IxR dropping resistor(s) on the LM317 input. Dissipation is then shared between the series resistor(s) and the LM317.

    Best option would be to use SMPS where the much higher efficiency makes this an easy conversion.

  • 0 in reply to Donald Jones
    Prodigy 20 points

    Thanks Mr. Jones. Now is clear that I'm going to use a SMPS.