a8431 Allegro MicroSystems, Inc., a8431 Datasheet - Page 8

• Manufacturer Part Number: a8431
• Description: White Led Driver Constant Current Step-up Converter
• Manufacturer: Allegro MicroSystems, Inc.

A8431
PWM Control
generated by a filtered PWM signal, as shown in schematic
3. In this case, a 0% duty cycle (PWM = 0 V) corresponds
to full brightness and a 100% duty cycle causes the LED
current, I
By applying a PWM signal directly to the EN pin, the A8431
is turned on or off, and I
R1) or 0 A. 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
switching additional resistors across R1 to increase I
In this case, R1 would be selected for the minimum desired
brightness.
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, L1.
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.
LOAD
. The control voltage, V
, to go to 0 A.
LOAD
is either full (as defined by
C
, also can be
LOAD
White LED Driver Constant Current
.
Schematic 3. Dimming control with filtered PWM
Schematic 4. Soft start operation
Li-ion
2.5V to
4.2V
Li-ion
2.5 V to
4.2 V
C1
1.0 µF
C1
1.0 µF
V
VIN
C
Enable
(PMW)
6
EN
4
VIN
Enable
A843
6
GND
EN
L1
22 µH
R4
10 kΩ
4
2
1
A843
GND
L1
22 µH
115 Northeast Cutoff, Box 15036
FB
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 [phone]
www.allegromicro.com
Step-up Converter
2
3
1
SW
OVP
1
FB
C3
100 nF
3
D1
OVP
SW
5
R3
90 kΩ
1
D1
R2
5 kΩ
5
R2
1 kΩ
C3
2.2 nF
R3
5 kΩ
R1
6.3 Ω
R1
6.3 Ω
C2
0.22 µF
C2
0.22 µF
8