The junction temperature of the UCC21540UCC2154x can be estimated with:

Equation 18.

where

• T_J is the junction temperature.
• T_C is the UCC2154x case-top temperature measured with a thermocouple or some other instrument.
• ψ_JT is the junction-to-top characterization parameter from the Section 6.4 table.

Using the junction-to-top characterization parameter (Ψ_JT) instead of the junction-to-case thermal resistance (R_ΘJC) can greatly improve the accuracy of the junction temperature estimation. The majority of the thermal energy of most ICs is released into the PCB through the package leads, whereas only a small percentage of the total energy is released through the top of the case (where thermocouple measurements are usually conducted). R_ΘJC can only be used effectively when most of the thermal energy is released through the case, such as with metal packages or when a heatsink is applied to an IC package. In all other cases, use of R_ΘJC will inaccurately estimate the true junction temperature. Ψ_JT is experimentally derived by assuming that the amount of energy leaving through the top of the IC will be similar in both the testing environment and the application environment. As long as the recommended layout guidelines are observed, junction temperature estimates can be made accurately to within a few degrees Celsius. For more information, see the Section 11.1 and Semiconductor and IC Package Thermal Metrics application report.