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A8431 Fiches technique(PDF) 6 Page - Allegro MicroSystems

No de pièce A8431
Description  White LED Driver Constant Current Step-up Converter
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Fabricant  ALLEGRO [Allegro MicroSystems]
Site Internet  http://www.allegromicro.com
Logo ALLEGRO - Allegro MicroSystems

A8431 Fiches technique(HTML) 6 Page - Allegro MicroSystems

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26185.301
Worcester, Massachusetts 01615-0036 (508) 853-5000
115 Northeast Cutoff, Box 15036
www.allegromicro.com
Allegro MicroSystems, Inc.
A8431
White LED Driver Constant Current Step-up Converter
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
. With an internal PWM frequency of
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, ISWLIM. If high temperature operation
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
. The diode should have a low forward
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
. Because the capacitor values are low,
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
Application Information
Schematic 2. Dimming control with dc voltage
A8431
VIN
SW
EN GND
FB
Li-ion
2.5 V to
4.2 V
C1
1.0 µF
L1
22 µH
D1
C2
0.22 µF
R1
V
C
R3
90 kΩ
R2
5 kΩ
Enable
6.3 Ω
1
23
5
6
4
OVP
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,
ILOAD, directly, or (b) by using a PWM signal on the EN pin
to chop the output.
Feedback Modulation
. By adding a voltage drop between
the FB pin and R1, as shown in schematic 2, the LED
current, ILOAD, can be made to decrease. As VC (control
voltage) increases, the voltage drop across R2 also increases.
This causes the voltage at FB to increase, and the A8431
reduces ILOAD to compensate. As VC increases further, the
current drops to 0 A, and R2 maintains the full 95 mV on FB.
Reducing VC diminishes the voltage across R2 until, when
VC is at 95 mV, there is no drop across R2 and the current
level is defined by R1. Reducing VC below 95 mV causes
ILOAD to increase further, due to the voltage drop across R2
in the reverse direction. This continues until, when VC is at
0 V, there is approximately 5 mV across R2. At that point,
ILOAD (mA), isdefinedas:
ILOAD = 100 mV ⁄ R1
where R1 is the resistance of the sense resistor (Ω).
Preliminary Data Sheet
Subject to Change Without Notice
April 5, 2005


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