ACT8828 datasheet(36/50 Pages) ACTIVE-SEMI

36 / 50 page

STEP-UP DC/DC CONVERTER

ACT8828

Rev 7, 15-Nov-12

Innovative Power

- 36 -

www.active-semi.com

ActivePMU

TM and ActivePathTM are trademarks of Active-Semi.

2CTM is a trademark of NXP.

FUNCTIONAL DESCRIPTION CONT’D

Setting the LED Bias Current

The LED bias current is set by a resistor

connected from FB4 and ground, and the regulator

is satisfied when the LED current is sufficient to

generate 250mV across this resistor. Once the

bias current is programmed, the LED current can

be adjusted using the ACT8828’s Direct-PWM

feature. REG4 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

continuous input current, so there is some amount

of flexibility in placing vias in the input capacitor

circuit. The inductor, input filter capacitor, rectifier,

and output 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

exposed pad. Finally, the exposed pad should be

directly connected to the backside ground plane

using multiple vias to achieve low electrical and

thermal resistance. Note that since the LED string is

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

special layout consideration.

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