"I copied the heatsink design from the EVM and I am using a high quality (high K) thermal pad instead of heatsink paste. I am having thermal shutdown issues. What is the explanition and mitigation?"
ANSWER: Due to the high power density, a very good thermal connection must be be made between the TAS5xxx power stage and its heatsink. While some thermal pads have very low thermal resistance, they are too thick and the thermal connection is a combination of the thermal resistance and its thickness.
Use a good quality heatsink compound (heatsink grease) and try to limit the thickness of the thermal connection between the power stage and the heatsink to <.002".