I was wondering if you could help us out, or point us in the right direction in regards to Junction Temp. Estimation for the TPS74401RGW.
We originally measured the case of the device to be ~ 93*C and ~2.53W (3.3v to 1v @ 1.1A).
Using the original “Theta Jc” metric; We get a Junction Temp. of approx. 172*C
The TI online calculator seems to agree. (Reports a Junction Temp of ~ 180*C) 
Therfore, by using the “Theta Jc” ϴ metric, we will need to use the DDPAK part moving forward, as this smaller package is causing the Junction temperature to exceed the recommended operating condition temp of 150*C (172*C vs 150*C (~22*C Exceedance))
Then, ”Psi ψ” is brought to our attention.
According to this, an estimation of Tj (Junction Temp.) can be made using the following:
The ψ JT is….. 0.4
http://www.ti.com/lit/ds/symlink/tps74401.pdf?ts=1590772500775
Therefore, our updated Junction Temperature using ψ JT is: ~94*C
Summary:
Using this updated ψ method (vs using ϴ) we see a considerable improvement in our Junction Temperature.
ψ = 172*C vs ϴ = 94*C (a 78*C delta)
Due to this significant temp disparity, we would like to get TI’s feedback on if we are calculating this correctly, or if we could do anything additional to better estimate our Junction Temp, so ensure we have high reliability of our part.
