# [SOLVED]Junction Temperature calculation of LM317 using surface temperature rise

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#### knikhil271083

##### Member level 2
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

#### dick_freebird

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.

#### knikhil271083

##### Member level 2
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

#### FvM

##### Super Moderator
Staff member
Means you are using datasheet θjc value as ψjt estimation?

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#### knikhil271083

##### Member level 2
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.

#### FvM

##### Super Moderator
Staff member
Just out of curiosity, at which value did the arrive?

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