The useful range of a driver is greatly affected by the drive power requirements of the load and the thermal characteristics of the IC package. In order for a power driver to be useful over a particular temperature range, the package must allow for the efficient removal of the heat produced while keeping the junction temperature within rated limits. The UCC2742x family of drivers is available in three different packages to cover a range of application requirements.

As shown in the power dissipation rating table, the SOIC-8 (D) and PDIP-8 (P) packages have a power rating of around 0.5 W with T_A = 70°C. This limit is imposed in conjunction with the power derating factor also given in Section 6.6. Note that the power dissipation in our earlier example is 0.432W with a 10nF load, 12 V_DD, switched at 300kHz. Thus, only one load of this size could be driven using the D or P package, even if the two onboard drivers are paralleled. The difficulties with heat removal limit the drive available in the older packages.

The MSOP PowerPAD-8 (DGN) package significantly relieves this concern by offering an effective means of removing the heat from the semiconductor junction. As illustrated in Reference 3 of Section 11.2, the PowerPAD packages offer a leadframe die pad that is exposed at the base of the package. This pad is soldered to the copper on the PC board directly underneath the IC package, reducing the R_θJC(bot) down to 5.9°C/W. Data is presented in Reference 3 of Section 11.2 to show that the power dissipation can be quadrupled in the PowerPAD configuration when compared to the standard packages. The PC board must be designed with thermal lands and thermal vias to complete the heat removal subsystem, as summarized in Reference 4 of Section 11.2. This allows a significant improvement in heatsinking over that available in the D or P packages, and is shown to more than double the power capability of the D and P packages. Note that the PowerPAD? is not directly connected to any leads of the package. However, it is electrically and thermally connected to the substrate which is the ground of the device.