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  1. #1
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    Junction Temperature calculation of LM317 using surface temperature rise

    Hi,
    I am using LM317MQDCYR from TI as a current limiter in my circuit. I want to estimate the Junction temperature of LM317 when it is limiting the current. I have measured the top surface temperature rise above ambient of LM317. I have Θja from the datasheet of LM317 and maximum power being dissipated in LM317.
    I can use the formula Pd = (Tj - Tamb) / Θja to theoretically estimate the junction temperature, but how can I estimate the junction temperature practically if I have the top surface temperature rise figure available.

    Any pointers to the solution would be appreciated.

    Regards
    Nikhil

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  2. #2
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    Re: Junction Temperature calculation of LM317 using surface temperature rise

    Rj-amb = 50 deg C/W, so P watts, Tj = 50 X P + Tamb (not Tc). ( http://pdf1.alldatasheet.com/datashe.../LM317MDT.html)
    Frank



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    Re: Junction Temperature calculation of LM317 using surface temperature rise

    The top surface is a poor place to measure, it has the
    most convection-variable temperature. The base or
    tab is better by far, especially on a plastic package.
    If you get a case measurement then you can use the
    thetaJC number without as much uncertainty.



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  4. #4
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    Re: Junction Temperature calculation of LM317 using surface temperature rise

    The thermal metric Psi-jt (Junction to top of Package) has been adopted by the industry (EIA/JESD 51-2) which helps to estimate the junction temperature of component based on the surface temperature and power dissipated.
    The actual junction temperature can be very easily estimated using the equation;
    Tjun = Tcase + (Psi-jt x Power) where Psi-jt is in oC/W and can be found in datasheets or from the IC manufacturer.

    Regards
    Nikhil



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    Re: Junction Temperature calculation of LM317 using surface temperature rise

    Means you are using datasheet θjc value as ψjt estimation?
    Last edited by FvM; 5th November 2013 at 11:46.



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    Re: Junction Temperature calculation of LM317 using surface temperature rise

    Quote Originally Posted by FvM View Post
    Means you are using datasheet θjc value as ψjt estimation?
    No. The value of ψjt was not given in the datasheet. I found it on the TI E2E community forum. Somebody had already asked for the details and it was posted on TI forum.
    As I said earlier, if its not given in datasheet, contact the manufacturer for the values.



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  7. #7
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    Re: Junction Temperature calculation of LM317 using surface temperature rise

    Just out of curiosity, at which value did the arrive?



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    Re: Junction Temperature calculation of LM317 using surface temperature rise

    Refer below link:
    http://e2e.ti.com/support/power_mana...01/847118.aspx

    Psi JT (standard datasheet value) = 1.7 oC/W


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