a8431 Allegro MicroSystems, Inc., a8431 Datasheet - Page 5

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

A8431
Typical Application
A typical application circuit for the A8431 is provided in
schematic diagram 1. This illustrates a method of driving
three white LEDs in series. The conversion efficiency of this
configuration is shown in chart 1.
Pin Functions
The diagram also shows a method of connecting the individ-
ual pins, whos functions are described as follows:
VIN. Supply to the control circuit. A bypass capacitor, C1, must
be connected from close to this pin to GND.
SW. Low-side switch connection between the inductor, L1,
and ground. Because rapid changes of current occur at this
pin, the traces on the PCB that are connected to this pin should
Schematic 1. Typical application
Li-ion
2.5V to
4.2V
C1
1.0 μF
VIN
Enable
6
EN
4
A8431
GND
L1
22 μH
2
FB
3
OVP
SW
1
D1
5
Functional Description
R1
6.3Ω
C2
0.22 μF
White LED Driver Constant Current
be minimized. In addition, L1 and the diode D1 should be
connected as close to this pin as possible.
OVP. Overvoltage Protection sense pin to protect the A8431
from excessive voltage on the SW pin. This pin should be
connected to the output capacitor, C2. To disable this feature
connect the pin to ground.
EN. Setting lower than 0.4 V disables the A8431 and puts the
control circuit into the low-power Sleep mode. Greater than
1.5 V fully enables the A8431.
GND. Ground reference connected directly to the ground plane.
The sense resistor, R1, should have a separate connection directly
to this point.
FB. Feedback pin for LED current control. The reference voltage
is 95 mV. The top of R1 is typically connected here.
Chart 1. Conversion efficiency when driving three LEDs
in the typical application circuit.
90
85
80
75
70
65
60
Conversion Efficiency versus Current
0
5
LED Current (mA)
115 Northeast Cutoff, Box 15036
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 [phone]
www.allegromicro.com
Step-up Converter
10
V
V
15
IN
IN
= 3 V
= 4 V
20
5