The useful range of a driver is greatly affected by the drive power requirements of the load and the thermal characteristics of the device package. 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 UCC3732x 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 8-pin SOIC (D) and 8-pin PDIP (P) packages each 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 the table. The power dissipation in our earlier example is 0.432 W with a 10-nF load, 12 VDD, switched at 300 kHz. Thus, only one load of this size could be driven using the D or P package. The difficulties with heat removal limit the drive available in the D or P packages.

The 8-pin MSOP PowerPAD (DGN) package significantly relieves this concern by offering an effective means of removing the heat from the semiconductor junction. As illustrated in Reference 3, 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 device package, reducing the θjc down to 4.7°C/W. Data is presented in Reference 3 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 .This allows a significant improvement in heatsinking over that available in theDor P packages, and is shown to more than double the power capability of the D and P packages.

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.