al8805 Diodes, Inc., al8805 Datasheet - Page 9

• Manufacturer Part Number: al8805
• Description: High Efficiency 30v 1a Buck Led Driver
• Manufacturer: Diodes, Inc.

Applications Information
AL8805 Operation
In normal operation, when voltage is applied at +V
AL8805 internal switch is turned on. Current starts to flow
through sense resistor R
current ramps up linearly, and the ramp rate is determined
by the input voltage +Vin and the inductor L1.
This rising current produces a voltage ramp across R
internal circuit of the AL8805 senses the voltage across
R
internal comparator.
When this voltage reaches an internally set upper threshold,
the internal switch is turned off. The inductor current
continues to flow through R
diode D1, and back to the supply rail, but it decays, with the
rate of decay determined by the forward voltage drop of the
LEDs and the schottky diode.
This decaying current produces a falling voltage at R
is sensed by the AL8805. A voltage proportional to the
sense voltage across R
internal comparator. When this voltage falls to the internally
set lower threshold, the internal switch is turned on again.
This switch-on-and-off cycle continues to provide the
average LED current set by the sense resistor R
LED Current Control
The LED current is controlled by the resistor R
Connected between V
output current in the LED(s) is defined as:
If the CTRL pin is driven by an external voltage (higher than
0.4V and lower than 2.5V), the average LED current is:
For example for a desired LED current of 660mA and a
default voltage V
AL8805
Document number: DS35030 Rev. 1 - 2
1
and applies a proportional voltage to the input of the
R
SET
=
CTRL
V
I
LED
THD
I
LED
=2.5V the resulting resistor is:
V
IN
I
V
LED
CTRL
=
1
REF
1
, inductor L1, and the LEDs. The
and SET the nominal average
V
1
V
is applied at the input of the
, L1, the LEDs and the schottky
CTRL
=
REF
R
V
=
THD
SET
. 0
0
R
V
66
1 .
THD
SET
2
2
5 .
5 .
≈
150
1
m
in Figure 30.
1
.
Ω
1
, which
IN
1
. The
www.diodes.com
, the
9 of 16
HIGH EFFICIENCY 30V 1A BUCK LED DRIVER
DC Dimming
The CTRL pin can be driven by an external DC voltage
(V
nominal average value defined by RSET. The LED current
decreases
0.5V ≤ V
levels.
When the CTRL voltage falls below the threshold, 0.4V, the
output switch is turned off which allows PWM dimming.
Note
V
CTRL pin that is higher than V
overdrive the LED current and will still set the current
according to the equation V
PWM Dimming
LED current can be adjusted digitally, by applying a low
frequency Pulse Width Modulated (PWM) logic signal to the
CTRL pin to turn the device on and off. This will produce an
average output current proportional to the duty cycle of the
control signal.
resolution of 10bit can be applied to the CTRL pin to change
the output current to a value below the nominal average
value set by resistor R
PWM frequency has to be lower than 500Hz, however
higher dimming frequencies can be used, at the expense of
dimming dynamic range and accuracy.
CTRL
CTRL
C
1
= V
), to adjust the output current to a value below the
that
CTRL
REF
Figure 30: Typical Application Curcuit
, nominally 2.5V. For any voltage applied on the
≤ 2.5V, as in figure 2 for 4 different current
linearly
100%
CTRL
GND
In particular, a PWM signal with a max
AL8805
brightness
with
SET
SET
SW
V
. To achieve this resolution the
CTRL
IN
the
= V
setting
REF
REF
CTRL
, the device will not
.
AL8805
© Diodes Incorporated
September 2010
R
corresponds
L
1
1
voltage
C
2
D
1
when
1
to