11.1 Layout Guidelines

PCB 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.

From an EMI reduction standpoint, it is imperative to minimize the high di/dt current paths. The high current that does not overlap contains high di/dt, see Figure 29. Therefore physically place input capacitor (C_in1) as close as possible to the LMZ10503EXT VIN pin and GND exposed pad to avoid observable high-frequency noise on the output pin. This will minimize the high di/dt area and reduce radiated EMI. Additionally, grounding for both the input and output capacitor should consist of a localized top side plane that connects to the GND exposed pad (EP).

2. Have a single point ground.

The ground connections for the feedback, soft-start, and enable components should be routed only to the GND pin of the device. This prevents any switched or load currents from flowing in the analog ground traces. If not properly placed, poor grounding can result in degraded load regulation or erratic output voltage ripple behavior. Provide the single point ground connection from pin 4 to EP.

3. Minimize trace length to the FB pin.

Both feedback resistors, R_fbt and R_fbb, and the compensation components, R_comp and C_comp, should be located close to the FB pin. Because the FB node is high impedance, keep the copper area as small as possible. This is most important as relatively high-value resistors are used to set the output voltage.

4. 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 at the load. Doing so will correct for voltage drops and provide optimum output accuracy.

5. 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, thermal vias can also be employed to make connection to inner layer heat-spreading ground planes. For best results use a 6 × 6 via array with minimum via diameter of 8 mils thermal vias spaced 59 mils (1.5 mm). Ensure enough copper area is used for heat-sinking to keep the junction temperature below 125°C.