AL8805 datasheet(9/16 Pages) DIODES

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AL8805

HIGH EFFICIENCY 30V 1A BUCK LED DRIVER

AL8805

Document number: DS35030 Rev. 1 - 2

9 of 16

www.diodes.com

September 2010

Applications Information

AL8805 Operation

In normal operation, when voltage is applied at +VIN, the

AL8805 internal switch is turned on. Current starts to flow

through sense resistor R1, 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 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 internal switch is turned off. The inductor current

continues to flow through R1, 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 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 internal switch is turned on again.

This switch-on-and-off cycle continues to provide the

average LED current set by the sense resistor R1.

LED Current Control

The LED current is controlled by the resistor R1 in Figure 30.

Connected between VIN and SET the nominal average

output current in the LED(s) is defined as:

SET

THD

LED

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

For example for a desired LED current of 660mA and a

default voltage VCTRL=2.5V the resulting resistor is:

≈

150

REF

CTRL

LED

THD

SET

AL8805

VIN

SET

CTRL

GND

Figure 30: Typical Application Curcuit

DC Dimming

The CTRL pin can be driven by an external DC voltage

(VCTRL), 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

≤ VCTRL ≤ 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

VCTRL= VREF, nominally 2.5V. For any voltage applied on the

CTRL pin that is higher than VREF, the device will not

overdrive the LED current and will still set the current

according to the equation VCTRL = VREF.

PWM Dimming

LED current can be adjusted digitally, by applying a low

frequency Pulse Width Modulated (PWM) logic signal to the

CTRL pin to turn the device on and off. This will produce an

average output current proportional to the duty cycle of the

control signal.

In particular, a PWM signal with a max

resolution of 10bit can be applied to the CTRL pin to change

the output current to a value below the nominal average

value set by resistor RSET. To achieve this resolution the

PWM frequency has to be lower than 500Hz, however

higher dimming frequencies can be used, at the expense of

dimming dynamic range and accuracy.

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