Using the thermal metrics Ψ_JT and Ψ_JB, as shown in the Thermal Information table, the junction temperature can be estimated with corresponding formulas (given in Equation 7). For backwards compatibility, an older R_θJC,Top parameter is listed as well.

Equation 7.

Where P_D is the power dissipation shown by Equation 7, T_T is the temperature at the center-top of the device package, and T_B is the PCB temperature measured 1mm away from the device package on the PCB surface (see Figure 7-6).

For more information about measuring T_T and T_B, see the Using New Thermal Metrics application note, available for download at www.ti.com.

In reference to Figure 7-5, the new thermal metrics (Ψ_JT and Ψ_JB) have very little dependency on board size. That is, using Ψ_JT or Ψ_JB with Equation 7 is a good way to estimate T_J by simply measuring T_T or T_B, regardless of the application board size.

Figure 7-5 Ψ_JT and Ψ_JB vs Board Size

For a more detailed discussion of why TI does not recommend using R_θJC(top) to determine thermal characteristics, see the Using New Thermal Metrics application note, available for download at www.ti.com. For further information, see the Semiconductor and IC Package Thermal Metrics application note, also available on the TI website.

A. T_T is measured at the center of both the X- and Y-dimensional axes.

B. T_B is measured below the package lead on the PCB surface.

Figure 7-6 Measuring Points for T_T and T_B