ACT8342 datasheet(12/17 Pages) ACTIVE-SEMI

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ACT8342

Rev 3, 11-Sep-09

STEP-DOWN DC/DC CONVERTER

Innovative PowerTM

- 12 -

www.active-semi.com

ActivePMUTM is a trademark of Active-Semi.

RFB1

RFB2

OUT1

ACT8342

OUT1

CFF

VFB

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

minimizes 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,

connect the output node directly to the OUT1 pin.

For adjustable output voltage options, connect the

feedback resistors and feed-forward capacitor to

the OUT1 pin through the shortest possible route. In

both cases, the feedback path should be routed to

maintain sufficient distance from switching nodes to

prevent 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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