ACT8740
Rev PrB, 25-Feb-08
STEP-DOWN DC/DC CONVERTER
simplify external component selection and optimize
transient performance over its full operating range.
REG1 was optimized for operation with a 3.3µH in-
ductor, although inductors in the 2.2µH to 4.7µH
range can be used. Choose an inductor with a low
DC-resistance, and avoid inductor saturation by
choosing inductors with DC ratings that exceed the
maximum output current of the application by at least
30%.
PCB Layout Considerations
High switching frequencies and large peak currents
make PC board layout an important part of step-
down DC/DC converter design. A good design mini-
mizes excessive EMI on the feedback paths and
voltage gradients in the ground plane, both of which
can result in instability or regulation errors. Step-
down DC/DCs exhibit discontinuous input current,
so the input capacitors should be placed as close
as possible to the IC, and avoiding the use of vias if
possible. The inductor, input filter capacitor, and
output filter capacitor should be connected as close
together as possible, with short, direct, and wide
traces. The ground nodes for each regulator's
power loop should be connected at a single point in
a star-ground configuration, and this point should
be connected to the backside ground plane with
multiple vias. The output node should be connected
to the OUT1 pin through the shortest possible route,
while keeping sufficient distance from switching
nodes to prevent noise injection. Finally, the ex-
posed pad should be directly connected to the
backside ground plane using multiple vias to
achieve low electrical and thermal resistance.
Innovative Products. Active Solutions.
- 19 -
www.active-semi.com
ActivePMUTM is a trademark of Active-Semi.
Copyright © 2008 Active-Semi, Inc.
I2CTM is a trademark of Philips Electronics.
ACTIVE-SEMI [ ACTIVE-SEMI, INC ]
