10.1 Layout Guidelines

Layout is a critical portion of good power supply design. See Figure 56 for a PCB layout example. The top layer contains the main power traces for VIN, VOUT, and the PH node. Also on the top layer are connections for the remaining pins of the TPS54320 and a large top-side area filled with ground. The top layer ground area should be connected to the internal ground layer or layers using vias at the input bypass capacitor, the output filter capacitor, and directly under the TPS54320 device to provide a thermal path from the exposed thermal pad land to ground. The GND pin should be tied directly to the exposed thermal pad under the IC.

For operation at full-rated load, the top-side ground area together with the internal ground plane must provide adequate heat dissipating area. Several signals paths conduct fast-changing currents or voltages that can interact with stray inductance or parasitic capacitance to generate noise or degrade the power supplies performance. To help eliminate these problems, the PVIN pin should be bypassed to ground with a low-ESR ceramic bypass capacitor with X5R or X7R dielectric. Take care to minimize the loop area formed by the bypass capacitor connections, the PVIN pins, and the ground connections. The VIN pin must also be bypassed to ground using a low-ESR ceramic capacitor with X5R or X7R dielectric. Make sure to connect this capacitor to the quiet analog ground trace rather than the power ground trace of the PVIN bypass capacitor.

Because the PH connection is the switching node, the output inductor should be located close to the PH pins, and the area of the PCB conductor minimized to prevent excessive capacitive coupling. The output filter capacitor ground should use the same power ground trace as the PVIN input bypass capacitor. Try to minimize this conductor length while maintaining adequate width. The small signal components should be grounded to the analog ground path as shown. The RT/CLK pin is sensitive to noise so the RT resistor should be located as close as possible to the IC and routed with minimal lengths of trace. The additional external components can be placed approximately as shown. It may be possible to obtain acceptable performance with alternate PCB layouts. However, this layout has been shown to produce good results and is meant as a guideline.

The estimated PCB area for the components used in the design of Figure 34 is 0.35 in² (227 mm²). This area does not include test points or connectors.