The thermal resistance of LP2951-Q1

I have one question for thermal resistance from datasheet of LP2951,

From datasheet Rthja=55.7C/W<Rthjc=66.5C/W,I think Rthja should be bigger than Rthjc,so can you help me confirm it,thanks!


Johnsin Tao:

HiFor exposed pad packages, it is common for Theta Jc (top) to be higher than Theta Ja since the preferred thermal path is through the bottom of the device and into the PCB.Theta Jc (top) is measured (per JEDEC specifications) by putting a liquid cooled heat sink on top of the package and forcing all of the heat to the top of the device–not very 'real' for most systems, but it is a common and historic spec, so we include it in our datasheets.
If you are planning to do lab measurements on this device and want to estimate die temperature by using a case temperature measurement, the proper metric to use is Psi Jt.Psi Jt is measured at the top of the device in a normal board mounted condition.You can read more about the thermal specifications in our application note here:www.ti.com/…/spra953a

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