ACT6357/ACT6358

Rev 1, 15-Nov-12

- 6 -

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Copyright © 2012 Active-Semi, Inc.

Setting the LED Current

The LED current is programmed by appropriate

between FB and G. To set the LED current, choose

the resistor according to the equation:

mum LED current. Once the LED current is selected

a simple means of LED dimming. The BC pin sup-

ports both analog as well as PWM dimming control.

Analog Dimming Control

To implement analog dimming, apply a voltage be-

tween 0.1V to 1.25V to BC. The resulting LED cur-

1.8V produces a constant LED current given by:

Direct PWM Dimming Control

The ACT6357 and ACT6358 support direct PWM

dimming control, allowing LED current to be ad-

justed via a PWM signal without the need for an

external RC network. For PWM dimming, drive BC

with a logic-level PWM signal to scale the LED cur-

rent proportionally with the PWM duty cycle, with

resulting LED current given by:

For best results, use PWM frequencies in the

100Hz to 10kHz range.

Inductor Selection

The ACT6357 and ACT6358 were designed for op-

eration with inductors in the 4.7µH to 47µH range,

and achieve best results under most operating con-

ditions when using 22µH to 33µH. Keep in mind

that larger-valued inductors generally result in con-

tinuous conduction mode operation (CCM) and

yield higher efficiency due to lower peak currents,

while smaller inductors typically yield a smaller foot-

print but at the cost of lower efficiency, resulting

losses). For best results, choose an inductor with a

low DC-Resistance (DCR) and be sure to choose

an inductor with a saturation current that exceeds

the current limit (500mA for the ACT6357 and 1A

for the ACT6358).

Capacitor Selection

The ACT6357 and ACT6358 only require a tiny

0.47µF output capacitor for most applications. For

circuits driving 6 or fewer LEDs, a 4.7µF input ca-

pacitor is generally suitable. For circuits driving

more than 6 LEDs, a 10µF input capacitor may be

required.

When choosing a larger inductor which results in

CCM operation, stability and ripple can be improved

by adding a small feed-forward capacitor from OUT

to FB. About 3000pF is a good starting point for

most applications, although a larger value can be

used to achieve best result in applications with 6 or

fewer LEDs

Ceramic capacitors are recommended for most ap-

plications. For best performance, use X5R and X7R

type ceramic capacitors, which possess less degra-

dation in capacitance over voltage and temperature.

Diode Selection

The ACT6357 and ACT6358 require a Schottky

diode as the rectifier. Select a low forward voltage

that exceeds the peak current limit (500mA for the

ACT6357 and 1A for the ACT6358) and a peak re-

the maximum output voltage, typically set by the OV

threshold.

Shutdown

The ACT6357 and ACT6358 feature low-current

shutdown modes. In shutdown mode, the control

circuitry is disabled and the quiescent supply cur-

rent drops to less than 1µA. To disable the

ACT6357 and ACT6358, simply drive EN to a logic

low. To enable the ICs, drive EN to a logic high or

connect it to the input supply.

Low Input Voltage Applications

In applications that have low input voltage range,

such as those powered from 2-3 AA cells, the

ACT6357 and ACT6358 may still be used if there is

a suitable system supply (such as 3.3V) available to

power the controller. In such an application, the in-

ductor may be connected directly to the battery,

while the IC power is supplied by the system sup-

ply.

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DUTY

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