ACT8740 datasheet(25/40 Pages) ACTIVE-SEMI

25 / 40 page

WLED BIAS DC/DC CONVERTER

ACT8740

Rev PrB, 25-Feb-08

Innovative Products. Active Solutions.

- 25 -

www.active-semi.com

ActivePMU

TM is a trademark of Active-Semi.

2CTM is a trademark of Philips Electronics.

adjusted using the ACT8740’s Direct-PWM fea-

ture. REG2 is also compatible with a variety of

well-known LED dimming circuits, such as with a

DC control voltage and a filtered PWM signal.

PCB Layout Considerations

High switching frequencies and large peak currents

make PC board layout a very important part of the

design. 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-up DC/DCs exhibit con-

tinuous input current, so there is some amount of

flexibility in placing vias in the input capacitor circuit.

The inductor, input filter capacitor, rectifier, and out-

put filter capacitor should be connected as close

together as possible, with short, direct, and wide

traces. Connect the ground nodes together in a star

configuration, with a direct connection to the ex-

posed pad. Finally, the exposed pad should be di-

rectly connected to the backside ground plane us-

ing multiple vias to achieve low electrical and ther-

mal resistance. Note that since the LED string is a

low, DC-current path, it does not generally require

special layout consideration.

ACT8740 数据表(PDF) 25 Page - Active-Semi, Inc