TLV766-Q1: Rjc ，Rjb and Rja value

Hi Steven,

 The junction to ambient thermal resistance, Rja, should always be lower than the junction to top of case thermal resistance, Rjc(top). This because Rja is the thermal resistance through all thermal paths which include the top of the case as well as the bottom of the case to the PCB. Rcj(top) is only the thermal resistance through the top of the case, it does not include any other thermal paths. So, since you have two paths for Rja that lowers the thermal resistance. 

Regarding the values listed in the Thermal Information table in TI's datasheets, it is an industry standard to provide this data using simulations. We use a very larger amount of data collected over decades from actual measurements to generate robust thermal models.

Note, that Rja will be PCB specific and we provide the our results using the standard JEDEC High-K board. We have an App Note which shows how the thermal dissipation (Rja) can be affected by board layout and it shows that with an optimized PCB layout the Rja listed in our datasheets can be reduced by 35%-55%. 

https://www.ti.com/lit/an/slvae85/slvae85.pdf

Hi  Kyle Van ，

 Thank you for your answer. Actually thermal resistance through all thermal paths， if Rjc(top) is only the thermal resistance through the top of the case， ,junction temperature≠ case temperature +TDP*Rjc， how can i get the junction temperature  if i get the  case temperature from thermocouple, thank you!