SBVS067R January 2006 – December 2019 TPS737
Using the thermal metrics ΨJT and ΨJB, as shown in the Thermal Information table, the junction temperature can be estimated with corresponding formulas (given in Equation 8). For backward compatibility, an older θJC,Top parameter is listed as well.
Where PD is the power dissipation shown by Equation 6, TT is the temperature at the center-top of the IC package, and TB is the PCB temperature measured 1-mm away from the IC package on the PCB surface (as Figure 38 shows).
Both TT and TB can be measured on actual application boards using a thermo‐gun (an infrared thermometer).
By looking at Figure 37, the new thermal metrics (ΨJT and ΨJB) have very little dependency on board size. That is, using ΨJT or ΨJB with Equation 8 is a good way to estimate TJ by simply measuring TT or TB, regardless of the application board size.
For a more detailed discussion of why TI does not recommend using θJC(top) to determine thermal characteristics, refer to application report, Using New Thermal Metrics (SBVA025), available for download at www.ti.com. For further information, refer to application report, IC Package Thermal Metrics (SPRA953), also available on the TI website.