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24th July 2013, 06:12 #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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25th July 2013, 17:41 #2
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Re: Junction Temperature calculation of LM317 using surface temperature rise
Rjamb = 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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25th July 2013, 17:53 #3
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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 convectionvariable 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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5th November 2013, 11:18 #4
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Re: Junction Temperature calculation of LM317 using surface temperature rise
The thermal metric Psijt (Junction to top of Package) has been adopted by the industry (EIA/JESD 512) 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 + (Psijt x Power) where Psijt is in oC/W and can be found in datasheets or from the IC manufacturer.
Regards
Nikhil

5th November 2013, 11:37 #5
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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.

6th November 2013, 06:32 #6
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Re: Junction Temperature calculation of LM317 using surface temperature rise

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6th November 2013, 09:35 #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?

6th November 2013, 11:08 #8
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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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