JAJS204S December 2005 – November 2024 TPS74401
PRODUCTION DATA
A better method of estimating the thermal measure comes from using the thermal metrics ΨJT and ΨJB, as shown in the Thermal Information table. These metrics are a more accurate representation of the heat transfer characteristics of the die and the package than RθJA. The junction temperature can be estimated with the corresponding formulas given in Equation 8.
where
Both TT and TB can be measured on actual application boards using a thermo‐gun (an infrared thermometer).
For more information about measuring TT and TB, see the Using New Thermal Metrics application note.
Compared with θJA, the thermal metrics ΨJT and ΨJB are less independent of board size, but do have a small dependency on board size and layout. Figure 7-16 shows characteristic performance of ΨJT and ΨJB versus board size.
Referring to Figure 7-16, 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 7-15), silicon is not beneath the measuring point of TT which is the center of the X and Y dimension, so that ΨJT has a dependency. Also, because of that non-point symmetry, device heat distribution on the PCB is not point symmetric either, so that ΨJB has a greater dependency on board size and layout.
For a more detailed description of why TI does not recommend using θJC(top) to determine thermal characteristics, see the Using New Thermal Metrics application note. Also, see the IC Package Thermal Metrics application note for further information.