SM74203 Datasheet, PDF (20/24 Pages) Texas Instruments – SM74203 60V Low Side Controller for Boost and SEPIC

The dark grey, inner loops represents the high current paths
during the MOSFET on-time. The light grey, outer loop rep-
resents the high current path during the off-time.
GROUND PLANE AND SHAPE ROUTING
The diagram of Figure 11 is also useful for analyzing the flow
of continuous current vs. 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 should be given to the pulsat-
ing 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 be-
tween the input source, input capacitor and the MOSFET and
the path between the output capacitor and the load are ex-
amples of continuous current paths. In contrast, the path
between the grounded side of the power switch and the neg-
ative output capacitor terminal carries a large pulsating cur-
rent. This path should be routed with a short, thick shape,
preferably on the component side of the PCB. Multiple vias in
parallel should be used right at the negative pads of the input
and output capacitors to connect the component side shapes
to the ground plane. Vias should not be placed directly at the
grounded side of the MOSFET (or R_SNS) 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, R_SNS and capac-
itor C_F.
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