This "virtual test circuit" can help designers predict the temperatures of a linear regulator, based on readily available information from the manufacturer's datasheet. Users can simply make a copy of this circuit, then adjust the regulator model's parameters to match the electrical and thermal characteristics for their particular part.
These parameters include the output voltage, VDO and current limit, as well as the junction-to-case and junction-to-ambient or heat-sink thermal resistance values. In this example, an L78S05 with direct case to ambient heat transfer is modeled (i.e. no heat-sink). The datasheet specifies the junction-to-case resistance is 5degC/Watt, and the junction-to-ambient resistance for a T0-220 package is 50 degC/Watt. Therefore, the difference of 45 degC/Watt is assumed to be the case-to-ambient thermal resistance. This value is assigned as the "heat-sink" resistance.
If an actual heat sink is being used, its published thermal resistance would be used instead. If the heat sink heat capacitance is also provided, that value can be applied to the thermal capacitor, and then the simulation will predict not only the steady-state operating temperature, but also temperature transients.
The input voltage function generator can also be adjusted to apply any time-varying input voltage profile, and the test circuit will show the corresponding time-varying temperature profile. This can be used to identify peak as well as average operating temperatures.
The user can also change the load current level and/or the load type using any relevant models in the Component Library, or create custom load models.