TPS745-Q1: Question for the RθJC(top) and RθJA of TPS745-Q1.

Hi Zhaocai,

There can be a lot of variation in how RθJC(top) compares with RθJA for a specific devices. This is because there are several variables which can significantly alter the RθJC(top) value, such as the thermal properties of the encapsulation/mold compound even how close the die is to the top of the package (die can be thicker/thinner and the height at which the die is placed within a specific leadframe can result in the die being closer or farther away from the top of the package). Below is an example device with 4 packages, half have RθJC(top)>RθJA  and the other half do not. 

As a general comment, RθJC(top) can be a difficult Thermal Metric to apply to most designs as it assumes all of the heat is transferred through the top of the package, in real applications heat is generally transmitted through the bottom of the package/pins to the PCB which acts as the primary heat sink. So if you want to estimate the junction temperature based on measuring the temperature at the top of the package it is better to use the ψJT thermal metric which accounts for the heat transmitting through all possible routes.