Most data sheets include a value for θ_JA, along with other information, in the thermal characteristics table. Figure 3-1 shows an example from the automotive qualified LM636x5-Q1 3.5-V to 36-V, 1.5-A, and 2.5-A Automotive Step-down Voltage Converter Data Sheet.

The value of θ_JA (or R_θJA) given in this table is taken under very specific conditions, that do not necessarily apply to the real application. Typically, the value of θ_JA will be quite a bit larger than what can be achieved with proper PCB layout. Therefore the table value cannot be used for design purposes; its main use is in comparing between different regulators and different packages. As will be shown, the other metrics in the table can be very helpful. For packages with a Die Attach Paddle (DAP) the value of θ_JC (or R_θJC(bott)) for the bottom of the device is also important. Equation 4 shows how to use this parameter.

In this equation, θ_SA is the thermal resistance from the heat sink to the ambient air. Usually, the value of θ_SA is not known and θ_JA is the true value of interest (Equation 1). However, some calculator tools require θ_JC when estimating the overall thermal performance. Also, a smaller value of θ_JC, will help to reduce the overall θ_JA.

You will notice in the table that there are those metrics termed as "thermal resistance" and those termed as "thermal parameters". The thermal resistance values are defined and measured assuming that all of the power flows in the path indicated by the metric name. As an example, with θ_JC (or R_θJC(bott)) it is assumed that all the power is flowing from the junction to the bottom DAP. These metrics are most important when designing the thermal management of the overall system from the junction to the ambient environment. Thermal parameters are defined and measured assuming that only some of the power is flowing in the path indicated by the metric name. As an example, the parameter Ψ_JT is used to calculate the junction temperature by measuring the top case temperature with a thermocouple or thermal camera. This calculation is discussed in Section 8.1. Essentially the "Ψ" parameters are used when evaluating and testing the system based on measurements. The "θ" resistances are used when designing or calculating the thermal performance of the system. Suppose a heatsink is used on the top of the package, rather than using the PCB through the bottom DAP connection. A safe assumption would be that most, if not all, of the heat is going through the top heatsink. In this case use the resistance, R_θJC(top), rather than the Ψ_JT parameter to calculate the overall performance. Even though both the metrics indicate heat flow from "junction" to "top", the correct value to use would be the
R_θJC(top). The parameter Ψ_JB can also be useful. A measurement of the board temperature, T_B, could be used to estimate T_J using Equation 6. See Semiconductor and IC Package Thermal Metrics to get a much more detailed explanation of thermal metrics and how they are measured and used.