SNVS579L February 2009 – May 2018 LM26420
To accurately measure the silicon temperature for a given application, two methods can be used. The first method requires the user to know the thermal impedance of the silicon junction to top case temperature.
Some clarification needs to be made before we go any further.
RθJC is the thermal impedance from silicon junction to the exposed pad.
RθJT is the thermal impedance from top case to the silicon junction.
In this data sheet RθJT is used so that it allows the user to measure top case temperature with a small thermocouple attached to the top case.
RθJT is approximately 20°C/W for the 16-pin WQFN package with the exposed pad. Knowing the internal dissipation from the efficiency calculation given previously, and the case temperature, which can be empirically measured on the bench we have:
From the previous example: