# Junction Temperature calculation of LM317 using surface temperature rise

1. ## 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 •

2. ## 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 •

3. ## 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. •

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

Means you are using datasheet θjc value as ψjt estimation? 6. ## Re: Junction Temperature calculation of LM317 using surface temperature rise Originally Posted by FvM 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. •

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

Just out of curiosity, at which value did the arrive? 8. ## 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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