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AL8807A

HIGH EFFICIENCY LOW EMI WIDE ANALOG DIMMING RANGE 36V 1A/1.3A BUCK LED DRIVER

Diodes Incorporated. 

AL8807A

Application Information

AL8807A Operation

The AL8807A is a hysteretic LED current switching regulator sometimes known as an equal ripple switching regulator. In normal operation, when

voltage is applied at +VIN (See Figure 31), the AL8807A 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 (See Figure 32).

This rising current produces a voltage ramp across R1. The internal circuit of the AL8807A 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 AL8807A. 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, with a switching current determined by

the input voltage and LED chain voltage.

In normal operation the off time is relatively constant (determined mainly by the LED chain voltage) with only the on-time varying as the input

voltage changes. At duty cycles up to around 80% the ramp of the LED/switch current is very linear; however, as the duty cycle approaches 95%

the LED current ramp starts to become more exponential. This has two effects:

1. The overall on time starts to increase lowering the overall switching frequency.

2. The average LED current starts to increase – which may impact accuracy.

Ch4: LED Current

VIN = 12V

TA =25ºC

2 LEDs

20ns/div

No C2

Ch2: 2V/div

Ch4: 100mA/div

Ch2: SW Pin

Figure 31 Typical Application Circuit

Figure 32 Typical Operating Waveform (C2 not fitted)

LED Current Control

With the CTRL pin open circuit, the LED current is determined by the resistor, R1, (see Figure 31), connected between VIN and SET. The

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

ILED



VTH

R1

where VTH is nominally 100mV

For example for a desired LED current of 660mA the resulting resistor is:

R1  VTH  0.1  150m

ILED 0.66

AL8807A

Document number: DS35990 Rev. 2 - 2

10 of 20

www.diodes.com

March 2013

© Diodes Incorporated

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