a8431 Allegro MicroSystems, Inc., a8431 Datasheet - Page 7

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

A8431
Component Selection
The component values shown in schematic 1 are sufficient
for most applications. To reduce the output ripple, L1 may be
increased, but in most cases this results in excessive board
area and cost.
Inductor Selection
1.2 MHz, the optimal L1 value for most cases is 22 μH.
For worst case conditions (high output voltage and current
and low input voltage), the inductor should be rated at the
switch current limit, I
is required, a derating factor will have to be considered. In
some cases, where lower inductor currents are expected,
the current rating can be decreased. Several inductor
manufacturers, including: Coilcraft, Murata, Panasonic,
Sumida, Taiyo Yuden, and TDK, have and are developing
suitable small-size inductors.
Diode Selection
voltage to reduce conduction losses. In addition, it should
have a low capacitance to reduce switching losses. Schottky
diodes can provide both these features, if carefully
selected. The forward voltage drop is a natural advantage
for Schottky diodes, and it reduces as the current rating
increases. However, as the current rating increases, the
diode capacitance also increases. As a result, the optimal
selection is usually the lowest current rating above the circuit
maximum. In this application, a current rating in the range
from 100 mA to 200 mA is usually sufficient.
Capacitor Selection
ceramic capacitors are the best choice for this application.
To reduce performance variation as temperature changes,
low- drift capacitor types, such as X7R and X5R, should
be used. A 1.0 μF capacitor on the VIN pin is suitable for
most applications. In cases where large inductor currents
are switched, a larger capacitor may be needed. The
output capacitor, C2, can be as small as 0.22 μF for most
applications and most input/output voltage ranges. Increasing
the capacitor value on the output aids in increasing the
efficiency of low input voltage/high output voltage
. The diode should have a low forward
. With an internal PWM frequency of
. Because the capacitor values are low,
SWLIM
. If high temperature operation
Application Information
White LED Driver Constant Current
conditions. Suitable capacitors are available from TDK,
Taiyo Yuden, Murata, Kemet, and AVX.
Dimming Control
LED brightness can be controlled either: (a) by modify-
ing the voltage at the top of R1 to control the LED current,
I
to chop the output.
Feedback Modulation
the FB pin and R1, as shown in schematic 2, the LED
current, I
voltage) increases, the voltage drop across R2 also increases.
This causes the voltage at FB to increase, and the A8431
reduces I
current drops to 0 A, and R2 maintains the full 95 mV on FB.
Reducing V
V
level is defined by R1. Reducing V
I
in the reverse direction. This continues until, when V
0 V, there is approximately 5 mV across R2. At that point,
I
where R1 is the resistance of the sense resistor (Ω).
LOAD
LOAD
LOAD
C
is at 95 mV, there is no drop across R2 and the current
, directly, or (b) by using a PWM signal on the EN pin
to increase further, due to the voltage drop across R2
(mA), is defined as:
Li-ion
2.5 V to
4.2 V
Schematic 2. Dimming control with dc voltage
LOAD
LOAD
C
C1
1.0 µF
diminishes the voltage across R2 until, when
, can be made to decrease. As V
to compensate. As V
VIN
Enable
I
LOAD
6
EN
. By adding a voltage drop between
4
A8431
V
GND
L1
22 µH
C
= 100 mV ⁄ R1
115 Northeast Cutoff, Box 15036
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 [phone]
www.allegromicro.com
2
Step-up Converter
FB
3
OVP
SW
R3
90 kΩ
1
C
C
D1
R2
5 kΩ
below 95 mV causes
5
increases further, the
R1
6.3 Ω
C
(control
C2
0.22 µF
C
is at
7