8.9 PCB Layout Information

Good printed circuit board layout techniques are important to any switching circuitry, and switch mode power supplies are no different. When wiring the switching high-current paths, short and wide traces must be used. Therefore, it is important that the input and output capacitors be placed as close as possible to the MCP16367/8/9 to minimize the loop area.

The feedback resistors and feedback signal must be routed away from the switching node and the switching current loop. When possible, using ground planes and traces is recommended to help shield the feedback signal and minimize noise and magnetic interference.

A good MCP16367/8/9 layout starts with C_IN placement. C_IN supplies current to the input of the circuit when the switch is turned on. In addition to supplying high-frequency switch current, C_IN also provides a stable voltage source for the internal MCP16367/8/9 circuitry. Unstable PWM operation can result if there are excessive transients or ringing on the VIN pin of the MCP16367/8/9 device. A ground plane on the bottom of the board provides a low resistive and inductive path for the return current. The next priority in placement is the freewheeling current loop formed by the Schottky Diode, C_OUT and L, while strategically placing C_OUT return close to C_IN return. Next, the bootstrap capacitor must be placed between the boost pin and the switch node pin, SW. R_TOP and R_BOT are routed away from the switch node, so noise is not coupled into the high-impedance FB input.