TPA3110R热阻问题

看了你的问题，对于theta ja比theta jc还要小，我也觉得奇怪，查了些资料，以下是美国工程师对于这个问题的回复：

For 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:  http://www.ti.com/lit/pdf/spra953a