AT1314
Preliminary Product Information
High-Efficiency Step-Down
Current Regulator For LEDs
Applications Information
External components of step-down converter can be designed by performing simple
calculations. It must be to follow regulation by the output voltage and the maximum
load current, as well as maximum and minimum input voltages. Begin by selecting an
inductor value. Once L is know, choose the diode and capacitors.
PWM Dimming Control
EN is also used as a digital input, allowing LED brightness control with a
logic-level PWM signal applied directly to EN. The frequency range is from 100Hz
to 1kHz, while 0% duty cycle corresponds to zero current and 100% duty cycle
corresponds to full current. The error amplifier and compensation Capacitor form a
lowpass filter, so PWM dimming results in DC current to the LEDs without any
additional RC filters required
Inductor
Inductor selection depends on input voltage, output voltage, maximum current,
switching frequency and availability of inductor values. The following buck circuit
equations are useful in choosing the inductor values based on the application. Choose
an inductor that does not saturate under the maximum rating load conditions. The
magnitude of inductance is selected to maintain a peak to peak ripple current of 30%
of the maximum load current.
The peak inductor current is given by:
I
Lpeak
=
I
LAVG
+
I
LAVG
=
D
=
V
O
V
in
I
in
D
∆
I
L
2
D is the MOSFET turn on ratio
where:
△
I
L
is the inductor peak-to-peak current ripple and is decided by:
(
V
−
V
O
)
×
D
V
−
V
O
D
∆
I
L
=
in
×
⇒
L
=
in
L
f
OSC
∆
I
L
×
f
OSC
f
OSC
is the switching frequency.
The inductor should be chosen to be able to handle this current and inductor saturation
current rating should be greater than I
PEAK
.
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1/17/2006 REV:1.0
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