Thermal resistance in LDO

[engr_joni_ee]

I have found in a LDO voltage regulator the following information.

Junction-to-ambient thermal resistance = 28 degC

Junction-to-case (top) thermal resistance = 43 degC

This look like strange to me. How the junction-to-ambient is lower then the junction-to-case (top) thermal resistance ?

 

[danadakk]

The thermal vias, lowest Rth, on leadframe, closest to bottom of case....

Top of case is a sandwich, plastic covering die, so much higher Rth ?


Regards, Dana.

 

[engr_joni_ee]

Is it not like that, junction to ambient thermal resistance is the sum of junction to case thermal resistance and case to ambient thermal resistances ?

If thermal resistance junction t ambient is smaller compared to thermal resistance junction to case then which way the heat is going faster to the ambient, side ways and not through the top case ?

 

[danadakk]

Tj-a is all paths from j-a, eg. conduction, convection.....?

 

[barry]

obviously, junction-to-ambient path is not JUST through the top. Junction-to-case (bottom) Rth is only 0.3 degrees.