A better method of estimating the thermal measure comes from using the thermal metrics Ψ_JT and Ψ_JB, as shown in Equation 6. These metrics are a more accurate representation of the heat transfer characteristics of the die and the package than R_θJA. The junction temperature can be estimated with the corresponding formulas given in Equation 6.

Equation 6.

where:

• P_D is the power dissipation shown by Equation 4
• T_T is the temperature at the center-top of the device package
• T_B is the PCB temperature measured 1mm away from the device package on the PCB surface (see Figure 7-8)

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 7-8 Measuring Points for T_T and T_B

Compared with R_θJA, the thermal metrics Ψ_JT and Ψ_JB are less independent of board size but do have a small dependency on board size and layout. Figure 7-9 shows characteristic performance of Ψ_JT and Ψ_JB versus board size.

Referring to Figure 7-9, 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 the center of a device. In the KTW package, for example (see Figure 7-8), silicon is not beneath the measuring point of T_T that 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 greater dependency on board size and layout.

Figure 7-9 Ψ_JT and Ψ_JB versus 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. Also, see the IC Package Thermal Metrics application note (also available on the TI website) for further information.