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AL8805W5-7 Datasheet(PDF) 9 Page - Diodes Incorporated

Part # AL8805W5-7
Description  HIGH EFFICIENCY 30V 1A BUCK LED DRIVER
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Manufacturer  DIODES [Diodes Incorporated]
Direct Link  http://www.diodes.com
Logo DIODES - Diodes Incorporated

AL8805W5-7 Datasheet(HTML) 9 Page - Diodes Incorporated

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AL8805
HIGH EFFICIENCY 30V 1A BUCK LED DRIVER
AL8805
Document number: DS35030 Rev. 1 - 2
9 of 16
www.diodes.com
September 2010
© Diodes Incorporated
Applications Information
AL8805 Operation
In normal operation, when voltage is applied at +VIN, the
AL8805 internal switch is turned on. Current starts to flow
through sense resistor R1, inductor L1, and the LEDs. The
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 R1. The
internal circuit of the AL8805 senses the voltage across
R1 and applies a proportional voltage to the input of the
internal comparator.
When this voltage reaches an internally set upper threshold,
the internal switch is turned off. The inductor current
continues to flow through R1, L1, the LEDs and the schottky
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 R1, which
is sensed by the AL8805. A voltage proportional to the
sense voltage across R1 is applied at the input of the
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 R1.
LED Current Control
The LED current is controlled by the resistor R1 in Figure 30.
Connected between VIN and SET the nominal average
output current in the LED(s) is defined as:
SET
THD
LED
R
V
I
=
If the CTRL pin is driven by an external voltage (higher than
0.4V and lower than 2.5V), the average LED current is:
SET
THD
REF
CTRL
LED
R
V
V
V
I
=
For example for a desired LED current of 660mA and a
default voltage VCTRL=2.5V the resulting resistor is:
Ω
=
=
m
150
5
.
2
5
.
2
66
.
0
1
.
0
V
V
I
V
R
REF
CTRL
LED
THD
SET
R1
AL8805
L1
VIN
SET
SW
CTRL
GND
C1
C2
D1
Figure 30: Typical Application Curcuit
DC Dimming
The CTRL pin can be driven by an external DC voltage
(VCTRL), to adjust the output current to a value below the
nominal average value defined by RSET. The LED current
decreases
linearly
with
the
CTRL
voltage
when
0.5V
≤ VCTRL ≤ 2.5V, as in figure 2 for 4 different current
levels.
When the CTRL voltage falls below the threshold, 0.4V, the
output switch is turned off which allows PWM dimming.
Note
that
100%
brightness
setting
corresponds
to
VCTRL= VREF, nominally 2.5V. For any voltage applied on the
CTRL pin that is higher than VREF, the device will not
overdrive the LED current and will still set the current
according to the equation VCTRL = VREF.
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.
In particular, a PWM signal with a max
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 RSET. To achieve this resolution the
PWM frequency has to be lower than 500Hz, however
higher dimming frequencies can be used, at the expense of
dimming dynamic range and accuracy.


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