11.1 Layout Guidelines
Layout is a critical portion of good power supply design. The following guidelines will help users design a PCB with the best power conversion performance, thermal performance, and minimized generation of unwanted EMI.
- The input bypass capacitor CIN must be placed as close as possible to the VIN and PGND pins. Grounding for both the input and output capacitors must consist of localized top side planes that connect to the PGND pin and PAD.
- Place bypass capacitors for VCC close to the VCC pin and ground the bypass capacitor to device ground.
- Minimize trace length to the FB pin net. Locate both feedback resistors, RFBT and RFBB close to the FB pin. Place CFF directly in parallel with RFBT. If VOUT accuracy at the load is important, make sure VOUT sense is made at the load. Route VOUT sense path away from noisy nodes and preferably through a layer on the other side of a shielded layer.
- Use ground plane in one of the middle layers as noise shielding and heat-dissipation path.
- Have a single point ground connection to the plane. Route the ground connections for the feedback and enable components to the ground plane. This prevents any switched or load currents from flowing in the analog ground traces. If not properly handled, poor grounding can result in degraded load regulation or erratic output voltage ripple behavior.
- Make VIN, VOUT and ground bus connections as wide as possible. This reduces any voltage drops on the input or output paths of the converter and maximizes efficiency.
- Provide adequate device heat-sinking. Use an array of heat-sinking vias to connect the exposed pad to the ground plane on the bottom PCB layer. If the PCB has multiple copper layers, these thermal vias can also be connected to inner layer heat-spreading ground planes. Ensure enough copper area is used for heat sinking to keep the junction temperature below 125°C.