Derating Temperature Curve for TPS84620

There is not really a derating curve per se.  The thermal performance is almost completely determined by the PCB design and other external operating conditions.  The limiting factor is maximum junction temperature.  The thermal parameters are in teh data sheet as well as SOA in figs. 4, 9 and 14.  That implies that for a properly designed PCB, you can get full output current at 85 C (slightly less for PVIN = 12 V, Vin = 5 V). Ambient  based on the PCB design of note 2.  You can also estimate your power dissipation from figs 3, 8 and 13, then use the thermal parameters and you PCB information to calculate the junction temperature using an appropriate software simulation such as Thermoflow.