10.1 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-supply performance. To help eliminate these problems, the VIN pin should be bypassed to ground with a low-ESR ceramic bypass capacitor with X5R or X7R dielectric. Care should be taken to minimize the loop area formed by the bypass capacitor connections, the VIN pin, and the anode of the catch diode. See Figure 54 for a PCB layout example. The GND pin should be tied directly to the thermal pad under the IC.

The thermal pad should be connected to any internal PCB ground planes using multiple vias directly under the IC. The PH pin should be routed to the cathode of the catch diode and to the output inductor. Because the PH connection is the switching node, the catch diode and output inductor should be located close to the PH pins, and the area of the PCB conductor minimized to prevent excessive capacitive coupling. For operation at full rated load, the top-side ground area must provide adequate heat dissipating area. 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.

10.2 Layout Example

Figure 54. PCB Layout Example

10.3 Estimated Circuit Area

The estimated printed circuit board area for the components used in the design of Figure 40 is 0.55 in² (3.55 cm²). This area does not include test points or connectors.