TPS54560: the difference between ψJT=5.9℃/W and θJCtop=45.8℃/W

Hirotaka Matsumoto

Part Number: TPS54560

Hi team

Would you mind if we ask TPS54560?
We are sorry for asking the similar question many times.

The datasheet shows ψJT=5.9℃/W and θJCtop=45.8℃/W.
Why does TPS54560 have the difference between ψJT=5.9℃/W and θJCtop=45.8℃/W?
Because, Ψjb=23.4℃/W and θjb=23.4℃/W are the same value.

We appreciate your help.

Kind regards,

Hirotaka Matsumoto

over 8 years ago

0 JohnTucker over 8 years ago

TI__Guru**** 191445 points

Was my explanation not clear? I recommend you study this document in detail:
www.ti.com/.../spra953c.pdf

It explains the thermal metrics in detail.

0 JohnTucker over 8 years ago in reply to JohnTucker

TI__Guru**** 191445 points

Actually it looks like this may be a duplicate post:

"Why are theta JB and psi JB equal but theta JC and psi JT greatly different?"

It is because the thermal resistance between junction and top of case is large. For packages with thermal pads, it is assumed that almost all the heat is transferred from the die to the thermal pad thru the low resistance theta JC bottom path. So even though top case temperature is very close to the junction temperature (low psi JT), the actual thermal resistance is still high.

0 Hirotaka Matsumoto over 8 years ago in reply to JohnTucker

Guru 27585 points

John san

We are sorry that we have some mistake.
So, the reply is enough.

Kind regards,

Hirotaka Matsumoto

Power management

Power management forum

TPS54560: the difference between ψJT=5.9℃/W and θJCtop=45.8℃/W