SNVS480J January 2007 – July 2020 LM5022
The diagram of Figure 29 is also useful for analyzing the flow of continuous current versus the flow of pulsating currents. The circuit paths with current flow during both the on-time and off-time are considered to be continuous current, while those that carry current during the on-time or off-time only are pulsating currents. Preference in routing must be given to the pulsating current paths, as these are the portions of the circuit most likely to emit EMI. The ground plane of a PCB is a conductor and return path, and it is susceptible to noise injection just as any other circuit path. The continuous current paths on the ground net can be routed on the system ground plane with less risk of injecting noise into other circuits. The path between the input source, input capacitor and the MOSFET and the path between the output capacitor and the load are examples of continuous current paths. In contrast, the path between the grounded side of the power switch and the negative output capacitor terminal carries a large pulsating current. This path must be routed with a short, thick shape, preferably on the component side of the PCB. Multiple vias in parallel must be used right at the negative pads of the input and output capacitors to connect the component side shapes to the ground plane. Vias must not be placed directly at the grounded side of the MOSFET (or RSNS) as they tend to inject noise into the ground plane. A second pulsating current loop that is often ignored but must be kept small is the gate drive loop formed by the OUT and VCC pins, Q1, RSNS, and capacitor CF.