ACT8325

Rev0, 13-May-08

STEP-DOWN DC/DC CONVERTER

Active-Semi

Innovative Power

TM

- 10 -

www.active-semi.com

Copyright © 2008 Active-Semi, Inc.

ActivePMU

(1)

output voltage option. output voltage option. Select

following equation:

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%.

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.

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

=

1

V

V

R

R

FBX

OUTX

2

FB

1

FB

1

FB

6

FF

R

10

2

.

2

C

−

×

=

(2)