Application information, Al8805 – Diodes AL8805 User Manual
Page 9
AL8805
Document number: DS35030 Rev. 4 - 2
9 of 16
July 2012
© Diodes Incorporated
AL8805
Application Information
AL8805 Operation
In normal operation, when voltage is applied at +V
IN
, the AL8805 internal switch is turned on. Current starts to flow through sense resistor R
1
,
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 R
1
. The internal circuit of the AL8805 senses the voltage across R
1
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 R
1
,
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 R
1
, which is sensed by the AL8805. A voltage proportional to the sense voltage across R
1
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 R
1
.
LED Current Control
The LED current is controlled by the resistor R
1
in Figure 30.
Connected between V
IN
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 V
CTRL
=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
Figure 30 Typical Application Circuit
DC Dimming
The CTRL pin can be driven by an external DC voltage (V
CTRL
), 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
≤ V
CTRL
≤ 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 V
CTRL
= V
REF
, nominally 2.5V. For any voltage applied on the CTRL pin that is higher than V
REF
,
the device will not overdrive the LED current and will still set the current according to the equation V
CTRL
= V
REF
.
R
1
AL8805
1
L
1
V
IN
SET
SW
CTRL
GND
C
1
C
2
D
1