ACT8740
Rev PrB, 25-Feb-08
WLED BIAS DC/DC CONVERTER
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.
Innovative Products. Active Solutions.
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ActivePMUTM is a trademark of Active-Semi.
Copyright © 2008 Active-Semi, Inc.
I2CTM is a trademark of Philips Electronics.
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