ACT8712 datasheet(28/36 Pages) ACTIVE-SEMI

28 / 36 page

WLED BIAS DC/DC CONVERTER

Rev0, 25-Feb-08

ACT8712

Innovative Products. Active Solutions.

- 28 -

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ActivePMU

TM is a trademark of Active-Semi.

2CTM is a trademark of Philips Electronics.

applications, although larger output capacitors may

be used to minimize output voltage ripple, if

needed. Ceramic capacitors are recommended for

most applications.

Rectifier Selection

REG3 requires a Schottky diode to rectify the induc-

tor current. Select a low forward voltage drop

Schottky diode with a forward current (IF) rating that

is sufficient to support the maximum switch current

and a sufficient peak repetitive reverse voltage

(VRRM) to support the output voltage.

Setting the LED Bias Current

The LED bias current is set by a resistor connected

from FB3 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 us-

ing the ACT8712’s Direct-PWM feature. REG3 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.

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