ACT8342 datasheet(12/17 Pages) ACTIVE-SEMI

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ACT8342

Rev0, 26-May-08

STEP-DOWN DC/DC CONVERTER

Innovative Power

- 12 -

www.active-semi.com

ActivePMU

TM is a trademark of Active-Semi.

RFB1

RFB2

OUT1

ACT8342

CFF

Figure 4:

Output Voltage Programming

Finally choose CFF using the following equation:

where RFB1 = 47kΩ, use 47pF.

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 loops 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 FB1 pin. For

adjustable output voltage options, connect the feed-

back resistors and feed-forward capacitor to the

FB1 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. Finally, the exposed pad

should be directly connected to the backside

ground plane using multiple vias to achieve low

electrical and thermal resistance.

(2)

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