10.1.4 Estimating Junction Temperature

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

Equation 6.

where

• P_D is the power dissipation shown by Equation 5
• T_T is the temperature at the center-top of the IC package
• T_B is the PCB temperature measured 1 mm away from the IC package on the PCB surface (see Figure 31)

For more information about measuring T_T and T_B, see the application note SBVA025, Using New Thermal Metrics, available for download at www.ti.com.

As shown in Figure 30, the new thermal metrics (Ψ_JT and Ψ_JB) have little dependency on board size. That is, using Ψ_JT or Ψ_JB with Equation 6 is a good way to estimate T_J by simply measuring T_T or T_B, regardless of the application board size.

Figure 30. 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 application report SBVA025, Using New Thermal Metrics, available at www.ti.com.

For further information, see the application report SPRA953, IC Package Thermal Metrics, also available on the TI website.

Figure 31. Measuring Point for T_T and T_B