ACT8740 datasheet(19/40 Pages) ACTIVE-SEMI

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STEP-DOWN DC/DC CONVERTER

ACT8740

Rev PrB, 25-Feb-08

Innovative Products. Active Solutions.

- 19 -

www.active-semi.com

ActivePMU

TM is a trademark of Active-Semi.

2CTM is a trademark of Philips Electronics.

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.

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