LM340 thermal resistance

We caclulated junction temperature at 1 W power dissapation and case temperature 69.6 C and it is 134.8 C. I am a quality engineer and stated that it will reduce product reliability to operate near the maximum temperature limit. Our design engineers think that it is not a problem. Moreover, some devices are going to thermul shutdown, if power consumption increases for any reason. We would like to setup conference call  with TI to discuss this device applicaiton.

I have sent a request to the Thermal lab for modeling of the Psi(jt) value for the LM340MP (SOT-223-4).

This would be proper benchmark to use to estimate the internal junction temperature (Tj) from the measured surface temperature (Tc).

   Tj = Tc + ( Psi(jt) x Pdiss )

http://www.ti.com/lit/an/spra953b/spra953b.pdf

http://www.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=sprabi3&fileType=pdf

The declared standard turn-time for a new request for thermal modeling is stated to be two (2) weeks, but typically it takes one week. Although, the up-coming US holiday (27-May) could add a day or two,

Results are back from the thermal modeling group for the LM340MP ...

Theta JA- (High K)  = 60.2 °C/W
Theta JC, top   = 42.1 °C/W
Theta JB   = 8.9 °C/W
Psi JT  = 2.7 °C/W
Psi JB  = 8.8 °C/W