ACT6357/ACT6358
Rev0, 03-Feb-08
Setting the LED Current
The LED current is programmed by appropriate
selection of the feedback resistor R
FB
connected
between FB and G. To set the LED current, choose
the resistor according to the equation:
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).
R
FB
V
=
FB
I
LED
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.
where V
FB
is the FB feedback voltage (typically
207mV at V
BC
= 1.25V) and I
LED
is the desired maxi-
mum LED current. Once the LED current is selected
via R
FB
, it may be adjusted via the BC pin to provide
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-
rent as a function of V
BC
is given by:
⎛
V
I
LED
=
0.16
× ⎜
BC
⎜
R
⎝
FB
⎞
⎟
⎟
⎠
BC may be overdriven, but driving V
BC
higher than
1.8V produces a constant LED current given by:
I
LED
=
290 mV
R
FB
Diode Selection
The ACT6357 and ACT6358 require a Schottky
diode as the rectifier. Select a low forward voltage
drop Schottky diode with forward current (I
F
) rating
that exceeds the peak current limit (500mA for the
ACT6357 and 1A for the ACT6358) and a peak re-
petitive reverse voltage (V
RRM
) rating that exceeds
the maximum output voltage, typically set by the OV
threshold.
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:
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.
⎛
V
⎞
I
LED
= ⎜
FB
⎟ ×
DUTY
⎜
R
⎟
⎝
FB
⎠
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
from higher peak currents (and their associated I
2
R
losses). For best results, choose an inductor with a
Innovative Products.
Active Solutions.
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.
-6-
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