As mentioned above, TI recommends to use one of the middle layers as a solid ground plane. A ground plane provides shielding for sensitive circuits and traces. A ground plane also provides a quiet reference potential for the control circuitry. The PGND pin is connected to the source of the internal low side MOSFET switch. This pin must be connected directly to the ground of the input and output capacitors. The PGND net contains noise at the switching frequency and can bounce due to load variations.

TI recommends to provide adequate device heat sinking by using the PGND of the IC as the primary thermal path. Thermal vias must be evenly distributed under the PGND pin. Use as much copper as possible for system ground plane on the top and bottom layers for the best heat dissipation. TI recommends to use a four-layer board with the copper thickness, starting from the top, as: 2oz / 1oz / 1oz / 2oz. A four-layer board with enough copper thickness and proper layout provides low current conduction impedance, proper shielding and lower thermal resistance.

Resources for thermal PCB design:

• AN-2020 Thermal Design By Insight, Not Hindsightapplication report
• A Guide to Board Layout for Best Thermal Resistance for Exposed Pad Packages application report
• Semiconductor and IC Package Thermal Metrics application note
• Thermal Design made Simple with LM43603 and LM43602 application report
• PowerPAD™ Thermally Enhanced Package application report
• PowerPAD Made Easy application report
• Using New Thermal Metrics application report