The useful operating temperature range of a PWM controller with integrated gate drivers and bias supply LDO regulator is greatly affected by:

• average gate drive current requirements of the power MOSFETs;
• switching frequency;
• operating input voltage (affecting bias regulator LDO voltage drop and hence its power dissipation);
• thermal characteristics of the package and operating environment.

For a PWM controller 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 LM5145 controller is available in a small 3.5-mm × 4.5-mm 20-pin VQFN (RGY) PowerPAD™ package to cover a range of application requirements. The thermal metrics of this package are summarized in Section 7.4. The application report Semiconductor and IC Package Thermal Metrics provides detailed information regarding the thermal information table.

The 20-pin VQFN package offers a means of removing heat from the semiconductor die through the exposed thermal pad at the base of the package. While the exposed pad of the package is not directly connected to any leads of the package, it is thermally connected to the substrate of the LM5145 device (ground). This allows a significant improvement in heat sinking, and it becomes imperative that the PCB is designed with thermal lands, thermal vias, and a ground plane to complete the heat removal subsystem. The exposed pad of the LM5145 is soldered to the ground-connected copper land on the PCB directly underneath the device package, reducing the thermal resistance to a very low value. Wide traces of the copper tying in the no-connect pins of the LM5145 (pins 9 and 16) and connection to this thermal land helps to dissipate heat.

Numerous vias with a 0.3-mm diameter connected from the thermal land to the internal and solder-side ground plane(s) are vital to help dissipation. In a multi-layer PCB design, a solid ground plane is typically placed on the PCB layer below the power 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.

The thermal characteristics of the MOSFETs also are significant. The drain pad of the high-side MOSFET is normally connected to a VIN plane for heat sinking. The drain pad of the low-side MOSFET is tied to the SW plane, but the SW plane area is purposely kept relatively small to mitigate EMI concerns.