TPS54394 power dissipation rating

Power dissipation is limited by two factors.  First is junction temperature.  Maximum operating junction temperature is 150 C. for TPS54394.  So PD max = (150 - Ta max) / Theta ja.  This is valid for a JEDEC standard PCB.  Your PCB may have better or worse thermal performance.  If you know the PCB operating temperature you can use

PD max = (150 - T board) /Theta jb.

The other limiting factor is the internal current limit.

I would recommend using the Webench design tool to estimate the power dissipation for your application.