11.1 Layout Guidelines

Layout is a critical portion of good power supply design. There are several signal paths that conduct fast changing currents or voltages that can interact with stray inductance or parasitic capacitance to generate noise or degrade the power supplies performance.

• Minimize the loop area formed by the bypass capacitor connections and the VIN pins. See Figure 48 for a PCB layout example.
• Tie the GND pins and AGND pin directly to the thermal pad under the TPS54678 device. Connect the thermal pad to any internal PCB ground planes using multiple vias directly under the device. Additional vias can be used to connect the top-side ground area to the internal planes near the input and output capacitors. For operation at full rated load, the top-side ground area along with any additional internal ground planes must provide adequate heat dissipating area.
• Place the input bypass capacitor as close as possible to the device.
• Route the PH pin to the output inductor. Because the PH connection is the switching node, place the output inductor close to the PH pins. Minimize the area of the PCB conductor to prevent excessive capacitive coupling.
• The boot capacitor must also be located close to the device.
• The sensitive analog ground connections for the feedback voltage divider, compensation components, soft-start capacitor and frequency set resistor must be connected to a separate analog ground trace as shown in Figure 48.
• The RT/CLK pin is particularly sensitive to noise so the RT resistor should be located as close as possible to the device and routed with minimal trace lengths.
• The additional external components can be placed approximately as shown. It is possible to obtain acceptable performance with alternate PCB layouts, however, this layout has been shown to produce good results and can be used as a guide.