SLVS839F July 2008 – October 2014 TPS54331
The VIN pin should be bypassed to ground with a low-ESR ceramic bypass capacitor. 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. The typical recommended bypass capacitor is 10-μF ceramic with a X5R or X7R dielectric and the optimum placement is closest to the VIN pins and the source of the anode of the catch diode. Figure 25 shows a PCB layout example. The GND pin should be tied to the PCB ground plane at the pin of the device. The source of the low-side MOSFET should be connected directly to the top side PCB ground area used to tie together the ground sides of the input and output capacitors as well as the anode of the catch diode. 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 very 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 TPS54331 device uses a fused lead frame so that the GND pin acts as a conductive path for heat dissipation from the die. Many applications have larger areas of internal or back-side ground plane available, and the top-side ground area can be connected to these areas using multiple vias under or adjacent to the device to help dissipate heat. The additional external components can be placed approximately as shown. Obtaining acceptable performance with alternate layout schemes may be possible, however this layout has been shown to produce good results and is intended as a guideline.