The thermal characteristics of any IC are determined by four parameters. Maximum allowable IC chip junction temperature TJ and thermal resistance Rθ are specified by the IC manufacturer. Ambient temperature TA and the power dissipation PD are determined by the:
PDISS - power dissipated by a MOSFET
IO - load current
RDSON - Dran to Source ON resistance of a MOSFET
DC - duty cycle. DC is 1 for continuous conduction.
Junction-to-ambient = Junction-to-case + case-to-ambient
Junction-to-board thermal resistance or Junction-to-ambient thermal resistance:
Thermal resistance Rθ is the basic thermal characteristic for ICs. It is usually
expressed in terms of OC/W and represents the rise in junction temperature with a unit of
power applied in still air. Total IC power to be dissipated depends on input current, output
current, voltage drop, and duty cycle.
Required thermal resistivity of a heatsink (maximum):