Diodes AL8807 - Manual (Page 9)

AL8807
Document number: DS35281 Rev. 5 - 2
9 of 20
www.diodes.com
March 2013
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AL8807
Application Information
The AL8807 is a hysteretic (also known as equal ripple) LED driver with integrated power switch. It is available in two packages that provide a PCB
area-power dissipation capability compromise. It is recommended that at higher LED currents/smaller PCBs that the MSOP-8EP version is used to
maximize the allowable LED current over a wider ambient temperature range.
AL8807 Operation
In normal operation, when voltage is applied at +V
IN
, the AL8807 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 AL8807 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 AL8807. 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 R1 in Figure 30.
Figure 30 Typical Application Circuit
Connected between V IN and SET the nominal average output current in the LED(s) is defined as:
1 R
V
I THD
LED

For example for a desired LED current of 660mA and a default voltage V
CTRL
=2.5V the resulting resistor is:
   
m 150
66 . 0
1 . 0
I
V
1 R
LED
THD
DC Dimming
Further control of the LED current can be achieved by driving the CTRL pin with an external voltage (between 0.4V and 2.5V); the average LED
current becomes:
SET
THD
REF
CTRL
LED
R
V
V
V
I

With 0.5V ≤ V
CTRL
≤ 2.5V the LED current varies linearly with V
CTRL , as in figure 2. If the CTRL pin is brought higher than 2.5V, the LED current will
be clamped to approximately 100% and follows
SET
THD
LED
R
V
I

.
When the CTRL voltage falls below the threshold, 0.4V, the output switch is turned off which allows PWM dimming.