Original question:
Hi Team:
There is a Pd calculation issue of TPS7B6950 and condition as below
Vin=30.5V , Vout=5V , Iout=0.009 , Tc=90.9 °C , RθJC(top)=126.1 °C/W
PD=(30.5-5)*0.009=0.2295 , (0.2295*126.1)+90.91=119.8°C
As end customer rule , they will have 80% derating margin Tj=150*0.8=120°C so 119.8°C to close
Please help to confirm formula is right ? or any suggestion about formula or another better solution for this application , thanks.
Regards / Mark
Hi Team:
AS datasheet package 4 SPRA953 formula (8) which one is more better to present to end customer ?
Please help to double confirm , thanks a lot.
Regards / Mark
Hi Mark,
The best parameters to use for thermals are RθJA and PsiJT :
For predicting the thermal rise of the die, TJA is best. So in the application above:
(30.5-5)*0.009= 0.2295W, 0.2295W*210.4C/W=48.29C. Now, this is the rise above the PCB ambient. So if the PCB ambient temperature is around 60C, then the die junction temperature would be 108.29C.
As the application from the SPRA953 correctly points out, PsiJT is used to confirm the thermal performance. This requires a thermal couple to measure the case of the device while dissipating power in the application. Then knowing TC, you can estimate that die temperature with:
TJ=TC+(PsiJT*Power)
For example if you measure TC to be 80C, then
TJ= 80C+(16*0.2295W)=83.67
Does this answer your question?
