CSD95375Q4M: Junction-to-ambient Thermal Resistance Theta-ja

Henry,

Thanks for posting. 

RthJA of a device is a function of both packageand thermal environment which is why whenever you see it specified on a datasheet, it is specified for a given board thickness and size. 

A 5x6 QFN will have a RthJA of ~40deg C / W typically on a 1x1" 2oz Cu board. A 3x3 QFN will be closer to 50deg C / W on the same board. So this 3.5 x 4.5 would be somewhere in between, around 45deg C / W on a 1x1" 2oz Cu board. 

With thicker PCB, or other means of pulling heat out from the package, be it airflow or a heatsink, you might be able to get that down to 20deg C/W or less but again, that would depend on the design.