AL8805EV2

Issue 1 – August 2010

www.diodes.com

© Diodes Incorporated 2010

4

AL8805 Operation

In normal operation, when voltage is applied at +Vin, the AL8805 internal NDMOS switch is turned on.

Current starts to flow through sense resistor R1, inductor L1, and the LED. 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 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

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 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, AL8805 internal circuits, electrical

characteristics and parameters.

AL8805EV2 Component list

QUANTITY

PCB

IDENT

VALUE

DESCRIPTION

SUGGESTED

SOURCE

1

U1

AL8805

LED Driver IC

Diodes Inc.

1

D1, D2,

D3, D4,

D5

DFLS240L

Input bridge & freewheeling diode

Diodes Inc.

1

R1

0R15

Resistor, 0805, +/-1% <+/-300ppm

Generic

KOA

SR732ATTDR150F

Kemet

1

C2

150uF 20v

SMD tantalum Kemet D case,

T491X157K020AT

Kemet

1

C1

---

Not Fitted

1

C3

100nF >=25v

X7R 0805 Generic

Kemet C0805C104K5RAC (50v)

NIC NMC0805X7R104K50TRPF (50v)

Kemet

NIC Components

1

C4

1uF >=25v

X7R 1206 Generic

Kemet C1206105K5RAC7800 (50v)

NIC NMC1206X7R105K50F (50v)

Kemet

NIC Components

1

C5

---

Not Fitted

1

C6

---

Not Fitted

1

L1

33uH

LPS6235 - 333MLB

Coilcraft

1

L2

-----

Not Fitted

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.