10.1 Layout Guidelines

PC board layout is an important part of DC-DC converter design. Poor board layout can disrupt the performance of a DC-DC converter and surrounding circuitry by contributing to EMI, ground bounce and resistive voltage drop in the traces. These can send erroneous signals to the DC-DC converter resulting in poor regulation or instability. Good layout can be implemented by following a few simple design rules.

1. Minimize area of switched current loops.

There are two loops where currents are switched at high di/dt slew rates in a buck regulator. The first loop represents the path taken by AC current flowing during the high side PFET on time. This current flows from the input capacitor to the regulator PVIN pins, through the high side FET to the regulator SW pin, filter inductor, output capacitor and returning via the PCB ground plane to the input capacitor.
The second loop represents the path taken by AC current flowing during the low side NFET on time. This current flows from the output capacitor ground to the regulator PGND pins, through the NFET to the inductor and output capacitor. From an EMI reduction standpoint, it is imperative to minimize this loop area during PC board layout by physically locating the input capacitor close to the LM2854. Specifically, it is advantageous to place C_IN as close as possible to the LM2854 PVIN and PGND pins. Grounding for both the input and output capacitor should consist of a localized top side plane that connects to PGND and the exposed die attach pad (DAP). The inductor should be placed close to the SW pin and output capacitor.

2. Minimize the copper area of the switch node.

The LM2854 has two SW pins optimally located on one side of the package. In general the SW pins should be connected to the filter inductor on the top PCB layer. The inductor should be placed close to the SW pins to minimize the copper area of the switch node.

3. Have a single point ground for all device analog grounds located under the DAP.

The ground connections for the Feedback, Soft-start, Enable and AVIN components should be routed to the AGND pin of the device. The AGND pin should connect to PGND under the DAP. 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 switching behavior.

4. Minimize trace length to the FB pin.

Since the feedback (FB) node is high impedance, the trace from the output voltage setpoint resistor divider to FB pin should be as short as possible. This is most important as relatively high value resistors are used to set the output voltage. The FB trace should be routed away from the SW pin and inductor to avoid noise pickup from the SW pin. Both feedback resistors, R_FB1 and R_FB2, and the compensation components, R_COMP and C_COMP, should be located close to the FB pin.

5. Make input and output bus connections as wide as possible.

This reduces any voltage drops on the input or output of the converter and maximizes efficiency. To optimize voltage accuracy at the load, ensure that a separate feedback voltage sense trace is made to the load. Doing so will correct for voltage drops and provide optimum output accuracy.

6. Provide adequate device heat-sinking.

Use an array of heat-sinking vias to connect the DAP to the ground plane on the bottom PCB layer. If the PCB has a plurality of copper layers, these thermal vias can also be employed to make connection to inner layer heat-spreading ground planes. For best results use a 5 x 3 via array with minimum via diameter of 10 mils. Ensure enough copper area is used to keep the junction temperature below 125°C.

7. Keep sensitive system signals away from SW node.

SW is a high-voltage, rapidly changing signal which can couple to adjacent signal lines. Signal integrity of lines that have high impedances or are very sensitive to noise can be compromised by capacitive coupling to SW node.