For a DC/DC converter 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 LM64440-Q1 and LM64460-Q1 converter are available in a small 3.5mm × 4mm 22-pin Enhanced HotRod QFN (RYF) package to cover a range of application requirements. The Thermal Information table summarizes the thermal metrics of this package, with related detail provided by the Semiconductor and IC Package Thermal Metrics Application Report.

The 22-pin Enhanced HotRod QFN package offers a means of removing heat from the semiconductor die through the exposed thermal pad at the base of the package. The exposed pad of the package is thermally connected to the substrate of the LM64440-Q1 or LM64460-Q1 device (ground). This allows a significant improvement in heatsinking, and it becomes imperative that the PCB is designed with thermal lands, thermal vias, and one or more ground planes to complete the heat removal subsystem. The exposed pad of the LM64440-Q1 or LM64460-Q1 is soldered to the ground-connected copper land on the PCB directly underneath the device package, reducing the IC thermal resistance to a very low value.

Preferably, use a four-layer board with 2oz copper thickness for all layers to provide low impedance, proper shielding and lower thermal resistance. Numerous vias with a 0.3mm diameter connected from the thermal land (and from the area around the PGND1 and PGND2 pins) to the internal and solder-side ground planes are vital to promote heat transfer. In a multi-layer PCB design, a solid ground plane is typically placed on the PCB layer below the power-stage components. Not only does this provide a plane for the power-stage currents to flow, but it also represents a thermally conductive path away from the heat-generating devices.