How do I estimate the junction temperature?

The temperature of the heatsink of the audio power IC, Ttab, is measured using a thermocouple as shown in Fig. 1.

Insert the thermocouple between the heatsink of the audio power IC and the external heat sink. Tighten the screws securely.
(Use a thermocouple as thin as possible for measurement, and ensure adhesion to the external heat sink by applying silicone grease, etc.)

The thermal resistance of IC, Rth(j-t), is shown in the datasheet. And refer to another FAQ “How do I calculate the current consumption and power dissipation?” for the calculation of the power dissipation Pd.