AL8807EV3
Issue 1 – December 2011 www.diodes.com
© Diodes Incorporated 2011
4
AL8807 Operation
In normal operation, when a voltage is applied at +Vin, the AL8807 internal NDMOS switch is turned
on. Current starts to flow through sense resistor R1, inductor L1, and the LED. The current ramps up
linearly, the ramp rate being determined by the input voltage +Vin and the inductor L1. This rising
current produces a voltage ramp across R1. The internal circuit of the AL8807 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 NDMOS switch is turned off. The inductor
current continues to flow through R1, L1, the LED 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
AL8807. 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 NDMOS switch is turned
on again. This switch-on-and-off cycle continues to provide the average LED current set by the sense
resistor R1. Please refer to the datasheets for the threshold limits, AL8807 internal circuits, electrical
characteristics and parameters.
AL8807EV3 Component list
rer
U1 AL8807 MSOP AL8807MP-13 Diodes
Zetex
DC-DC converter
D1 60V, 2A DFLS260 Diodes
Zetex
Schottky diode
R1 0R1 1206 Generic 1% 250mW
R2 NOT FITTED
R3 1k0 0805 generic Generic 1% 125mW
C1, C5 4.7uF 50V 1210
C1210X475K5RAC
Generic
KEMET
X7R
C2, C3
100nF, 100V 0805
NMC0805X7R104K100 PF
GRM21BR71H104KA01L
Generic
NIC Comps
MURATA
X7R
C4 1uF 100V 1206
Generic
NIC Comps
X7R
C6, C7,
C8, C9
C12
0805
NOT FITTED
Generic Fit to improve
EMC
performance if
required
C10, C11 1n5, 100V 0805 Generic
L1 68uH MSS1038-683ML
NPIS24H680MTRF
NPIS104F680MTRF
Coilcraft
NIC Comps
Note: The component part numbers are correct at the time of publication. Diodes Inc reserves the
right to substitute other parts where necessary, without further notification.
