7

Worcester, Massachusetts 01615-0036 (508) 853-5000

115 Northeast Cutoff, Box 15036

www.allegromicro.com

A8430

White LED Driver Constant Current Step-up Converter

Schematic 3. Dimming control with filtered PWM

A843

VIN

SW

EN GND

FB

Li-ion

2.5 V to

4.2 V

C1

1 µF

L1

22 µH

D1

C2

0.22 µF

R1

6.3 Ω

R3

90 kΩ

R2

5 kΩ

Enable

R4

10 kΩ

C3

100 nF

Schematic 4. Soft start operation

A8430

VIN

SW

EN GND

FB

Li-ion

2.5V to

4.2V

C1

1µF

L1

22µH

D1

C2

0.22µF

R1

R3

5kΩ

R2

1kΩ

Enable

C3

2.2 nF

6.3Ω

By applying a PWM signal directly to the EN pin, the A8430

R1) or zero. By varying the duty cycle of the PWM signal,

the LED brightness can be controlled from off (0% duty

cycle) to full (100% duty cycle). The PWM frequency should

be in the range from 1 kHz to 10 kHz.

Several other schemes are possible, for example, digitally

In this case, R1 would be selected for the minimum desired

brightness.

Soft Start-Up

To provide fast start-up operation, no soft start is

implemented in the control circuit. At power-on, the bypass

capacitor (C1) is discharged, which means that the supply

must provide the in-rush current through the inductor.

This can be reduced by modulating the feedback with a soft-

start circuit as shown in schematic 4. When power is first

applied, the capacitor C3 is discharged and pulls the FB pin

high, reducing the output drive to minimum. As C3 charges,

when the bottom drops below about 0.8 V, the feedback from

the sense resistor (R1) takes over full control of the output

current.

Overvoltage Protection

An overvoltage event can occur when the LEDs become

disconnected or fail in an open state. In these cases, the

current flow through the sense resistor becomes zero, thus

the feedback voltage becomes zero. The A8430 compensates

by increasing the on time of the switch, which increases the

output voltage.