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A8430 Schematic ( PDF Datasheet ) - Allegro MicroSystems

Teilenummer A8430
Beschreibung White LED Driver Constant Current Step-up Converter
Hersteller Allegro MicroSystems
Logo Allegro MicroSystems Logo 




Gesamt 10 Seiten
A8430 Datasheet, Funktion
www.DataSheet4U.com
A8430
White LED Driver Constant Current Step-up Converter
A8430 MLPD
Approximate actual size
SW
GND
FB
1
2
3
5 VIN
4 EN
Same pad footprint as SOT-23-5
RθJA = 50 °C/W, see note 1, page 2
ABSOLUTE MAXIMUM RATINGS
The A8430 is a noninverting boost converter that steps-up the input
voltage, to provide a programmable constant current output at up to
36 V for driving white LEDs in series. Driving LEDs in series ensures
identical currents and uniform brightness. Up to four white LEDs can
be driven at 20 mA from a single cell Li-ion or a multicell NiMH power
source. Up to seven white LEDs can be driven by increasing the supply
voltage up to 10 V.
The A8430 incorporates a power switch and feedback sense amplifier
to provide a solution with minimum external components. The output
current can be set by adjusting a single external sense resistor and can
be varied with a voltage or filtered PWM signal when dimming control
is required. The high switching frequency of 1.2 MHz allows the use of
small inductor and capacitor values.
The A8430 is provided in a 5-pin 3 mm x 3 mm MLP package (part
number suffix EK), that has a nominal height of only 0.75 mm. The
lead-free version (part number suffix EK-T) has 100% lead-free matte
tin leadframe plating.
FEATURES
Output voltage up to 36 V
2.5 V to 10 V input
Drives up to 4 LEDs at 20 mA from a 2.5 V supply
Drives up to 5 LEDs at 20 mA from a 3 V supply
1.2 MHz switching frequency
300 mA switch current limit
1 µA shutdown current
SW Pin ................................................–0.3 V to 36 V
Remaining Pins .................................. –0.3 V to 10 V
Ambient Operating Temperature, TA....... –40°C to 85°C
Junction Temperature, TJ(max)............................... 150°C
Storage Temperature, TS .................... –55°C to 150°C
APPLICATIONS
LED backlights
Portable battery-powered equipment
Cellular phones
PDAs (Personal Digital Assistant)
Camcorders, personal stereos, MP3 players, cameras
Mobile GPS systems
Use the following complete part number when ordering:
Part Number
Package
Description
A8430EEK
5-pin, MLPD
Surface Mount
A8430EEK-T
5-pin, MLPD
Lead-Free, Surface Mount






A8430 Datasheet, Funktion
A8430
White LED Driver Constant Current Step-up Converter
Application Information
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 , 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.
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), is defined as:
ILOAD = 100 mV/ R1
where R1 is the resistance of the sense resister ().
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.
L1
22µH
D1
VIN
C1
1µF
A8430
SW
EN GND FB
Li-ion
2.5V to
4.2V
Enable
VC
R2
5k
R3
90k
C2
0.22µF
R1
6.3
Schematic 2. Dimming control with dc voltage
feedback modulation
www.allegromicro.com
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
6

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