Using the thermal metrics Ψ_JT and Ψ_JB, as listed in the Thermal Information table, the junction temperature can be estimated with corresponding formulas (given in Equation 3). For backwards compatibility, an older θ_JC,Top parameter is listed as well.

Equation 3.

where:

• P_D is the power dissipation shown by Equation 2
• T_T is the temperature at the center-top of the device package
• T_B is the PCB temperature measured 1 mm away from the device package on the PCB surface (see Figure 7-14)

For more information about measuring T_T and T_B, see the Using New Thermal Metrics application note.

Figure 7-14 Measuring Points For T_T and T_B

From Figure 7-15, the new thermal metrics (Ψ_JT and Ψ_JB) are shown to have very little dependency on board size. That is, using Ψ_JT or Ψ_JB with Equation 3 is a good way to estimate T_J by simply measuring T_T or T_B, regardless of the application board size.

Figure 7-15 Ψ_JT And Ψ_JB vs Board Size

For a more detailed discussion of why TI does not recommend using θ_JC(top) to determine thermal characteristics, see the Using New Thermal Metrics application note. For further information, see the Semiconductor and IC Package Thermal Metrics application note.