ACT8325
Active- Semi
Rev0, 13-May-08
STEP-DOWN DC/DC CONVERTER
output voltage option. output voltage option. Select
components as follows: Set RFB2 = 51kΩ, then
calculate RFB1 using the following equation:
PCB Layout Considerations
High switching frequencies and large peak current
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. For fixed output voltage options, con-
nect the output node directly to the FBx pin. For
adjustable output voltage options, connect the feed-
back resistors and feed-forward capacitor to the
FBx pin through the shortest possible route. In both
cases, the feedback path should be routed to main-
tain sufficient distance from switching nodes to pre-
vent noise injection.
⎛
⎞
VOUTX
VFBX
⎜
⎜
⎟
⎟
RFB1 = RFB2
−1
(1)
⎝
⎠
where VFBX is 0.625V. Finally choose CFF using the
following equation:
2.2 ×10 −6
CFF
=
(2)
RFB1
Where RFB1 = 47kΩ, use 47pF.
Inductor Selection
REG1 and REG2 utilize current-mode control and a
proprietary internal compensation scheme to simul-
taneously simplify external component selection
and optimize transient performance over their full
operating range. These devices were optimized for
operation with 3.3µH inductors, 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%.
Innovative PowerTM
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ACTIVE-SEMI [ ACTIVE-SEMI, INC ]
