I am TSR for NEC. I got the following question from NEC. Please answer it.
For TPS54540DDAR at Input 24V, Output 5V & 3.3V,
Customer is considering using it in a power supply circuit.
Looking at the data sheet, the thermal resistance is higher than the values of θjctop: 45.8℃/W and θja: 42.0℃/W.
It appears to be large.
As shown in the figure below, θjctop is the thermal resistance from the circuit to the exterior surface, and θja is the thermal resistance from the circuit to the atmosphere.
It seems that θja is likely to have a larger value, but why is θjctop larger?
