Using the thermal metrics Ψ_JT and Ψ_JB, shown in Section 5.4, the junction temperature can be estimated with corresponding formulas (given in Equation 6). For backwards compatibility, an older θ_JC,Top parameter is listed as well.

Equation 6.

where

• P_D is the power dissipation given by P_D = (V_IN – V_OUT) × I_OUT
• T_T is the temperature at the center-top of the IC package
• T_B is the PCB temperature measured 1mm away from the IC package on the PCB surface (as Figure 7-10 shows).

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.

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 7-10 Measuring Points for T_T and T_B

Compared with θ_JA, the new thermal metrics Ψ_JT and Ψ_JB are less independent of board size, but the metrics do have a small dependency. Figure 7-11 shows characteristic performance of Ψ_JT and Ψ_JB versus board size.

Looking at Figure 7-11, the RGW package thermal performance has negligible dependency on board size. The KTW package, however, does have a measurable dependency on board size. This dependency exists because the package shape is not point‐symmetric to an IC center. In the KTW package, for example (see Figure 7-10), silicon is not beneath the measuring point of T_T, which is the center of the X and Y dimension, so that Ψ_JT has a dependency. Also, because of that non-point‐symmetry, device heat distribution on the PCB is not point‐symmetric, either, so that Ψ_JB has a dependency.

Figure 7-11 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, refer to the Using New Thermal Metrics application note, available for download at www.ti.com. Also, refer to the IC Package Thermal Metrics application note (also available on the TI website) for further information.