The THS3470 comes in a top-side cooled REB package which gives thermal performance benefits over traditional bottom-side cooled QFN devices. Traditional packages, as shown in Figure 7-14, must interface with the PCB material before entering the heat sink. Without a heat sink, bottom-side cooled devices rely on convection across a smaller surface area of the PCB to remove heat. A helpful conceptualization in these circumstances is to think about the different interfaces as “thermal resistances” that prevent the heat generated by the silicon die from moving to the ambient air. The result of these bottom-side cooled packages often results in a best-case θ_JA of above 10°C/W for the total system level performance, even with a forced air system.

The THS3470 top-side cooled package, however, does not rely on heat to travel through the PCB before interfacing with the air or heat sink. This more direct contact method essentially removes the PCB “thermal resistance”, as shown in Figure 7-15, and can enable a full system level θ_JA performance of approximately 4°C/W. This configuration also enables the potential use of a cold plate or other direct contact methods of cooling that have better mechanisms for removing heat from the system.

Due to the high voltage nature of the THS3470, mechanically mounting the heat-sink can require special considerations depending on the application. To provide the best thermal performance possible, the THS3470 uses a conductive die attach that internally connects the thermal pad to VEE. In single ended applications where VEE is connected to −60V and VCC is connected to 0V, the high voltage potential on the thermal pad creates the possibility of a high voltage hazard for the system. To circumvent potential high voltage hazards, a non-conductive thermal interface material can be placed in between a copper or graphite heat spreader and the heat sink as shown in Figure 7-16. The copper heat spreader helps to distribute the heat to a wider surface area before the thermal interface, resulting in a lower overall thermal resistance. Additionally, using an anodized heat sink can help to provide electrical isolation while not degrading the thermal performance significantly. Designers need to keep in mind that anodization on heat sinks can become scratched or damaged, so using a thermal interface material is preferable for situations where the heat-sink has to be regularly removed from the device. Lastly, use alumina or ceramic screws to mechanically secure the fixture while maintaining electrical isolation between the heat spreader and heat sink.