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A8430 Fiches technique(PDF) 6 Page - Allegro MicroSystems

No de pièce A8430
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

A8430 Fiches technique(HTML) 6 Page - Allegro MicroSystems

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Worcester, Massachusetts 01615-0036 (508) 853-5000
115 Northeast Cutoff, Box 15036
www.allegromicro.com
A8430
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 the
inductor 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 inductor value for most cases is 22 µH.
The inductor should have low winding resistance, typically
< 1
Ω, and the core should have low losses when operating
at 1.2 MHz. 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 have and are developing suitable
small-size inductors, including: Murata, Panasonic, Sumida,
Taiyo Yuden, and TDK.
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. With the A8430, 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 use with the
A8430. To reduce performance variation as temperature
changes, low drift capacitor types, such as X7R and X5R,
should be used. Suitable capacitors are available from: Taiyo
Yuden, Murata, Kemet, and AVX.
Dimming Control
LED brightness can be controlled either by modifying the
voltage at the top of the sense resistor (R1) to control the
LED current, ILOAD, directly, or by using a PWM signal on
the EN pin to chop the output.
Application Information
Schematic 2. Dimming control with dc voltage
feedback modulation
A8430
VIN
SW
EN GND
FB
Li-ion
2.5V to
4.2V
C1
1µF
L1
22µH
D1
C2
0.22µF
R1
V
C
R3
90kΩ
R2
5kΩ
Enable
6.3Ω
Feedback modulation
. By adding a voltage drop
between the FB pin and R1 (the sense resistor), 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 A8430 reduces ILOAD to compensate. As VC
increases further, the current drops to zero, and R2 maintains
the full 95 mV on FB. Reducing VC diminishes the voltage
across R2 until, at 95 mV on VC, 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,
at zero volts on VC, 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 resister (Ω).
PWM Control
. LED dimming control can also be 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,
ILOAD, to go to zero.


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