AL8805EV1

Issue 3 – 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.

AL8805EV1 Component list

Ref

Value

Package

Part Number

Manufactu

rer

Notes

U1

AL8805

SOT25

AL8805E5TA

Diodes

Zetex

DC-DC converter

D1

40V, 3A

B240A

Diodes

Zetex

Schottky diode

R1

0R15

0805

Generic

5%

R2,

1K

0805

Generic

1%

C1

4.7uF 50V

1210

C1210X475K5RAC

Generic

KEMET

X7R

C2

Not Fitted

Optional soft

start capacitor

C3

100nF, 100V

0805

NMC0805X7R104K100 PF

GRM21BR71H104KA01L

Generic

NIC Comps

MURATA

X7R

C4

100nF 100V

1206

NMC1206X7R104K100

Generic

NIC Comps

X7R

L1

68uH

MSS1038-683ML

NPIS24H680MTRF

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.

AL8805EV1 Basic operation at full voltage

1.

Connect Vin and GND

Warning: The board does not feature reverse battery/supply protection.

2.

Set the PSU to 30V

3.

Turn on the PSU and the LED will illuminate and the current should be approximately 680mA.

Warning: Do not stare at the LED directly.