AL8821 datasheet(7/14 Pages) DIODES

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AL8821

Document number: DS37671 Rev. 1 - 2

7 of 14

www.diodes.com

January 2015

AL8821

Application Information

AL8821 Operation

The AL8821 is a boost converter that delivers an accurate constant current for driving LEDS. With hysteretic control scheme, the LED driver is

compatible with most of commonly used electronic transformers. The driver can be compatible with leading-edge dimmer and trailing-edge dimmer.

RSET

RHYS

VCC

COMP

VIN

(Exposed Pad)

Vac

RFB

Figure 1. Typical Application Circuit

LED Current Control

The LED current is controlled by the resistor RFB in Figure 1.

Connected between FB pin and Ground, the nominal average output current in the LED(s) is defined as:

0.444

LED



Ｉ

RSET and RHYS Setting

The Boost converter of the AL8821 operates at continuous conduction mode and is based on hysteresis schematic which has lower threshold and

upper threshold. Refer to Figure 2 depicting the inductor current waveform.

IL(ave)

IL(val)

Time

tON

tOFF

Peak or Upper Threshold

Valley or Lower Threshold

IL(peak)

Figure 2. Inductor Current

When switch SW is turned on, the inductor current flows through RSET and ramps up linearly. The rising current produces a voltage ramp across

RSET. When the voltage across RSET reaches the upper threshold, switch SW is turned off. The inductor current continues to flow through RSET but

decays. The decaying current produces a falling voltage at RSET. When the voltage across RSET falls to the lower threshold, switch SW is turned

on again.

The lower threshold voltage VLT depends on the voltage VCOMP at COMP pin that varies with the input voltage and output load. The equation is

shown as below.

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