Data Sheet
26185.300b
A8430
Constant Current LED Driver Boost Converter
Use the following complete part number when ordering:
AB SO LUTE MAX I MUM RAT INGS
SW Pin ................................................–0.3 V to 36 V
Remaining Pins .................................. –0.3 V to 10 V
Package Thermal Impedance:
Two-sided PCB1...........................θJA tbaºC / W
“High-K” PCB2............................θJA tbaºC / W
Operating Temperature Range
Ambient Temperature, TA.............–40°C to +85°C
Junction Temperature, TJ..................+150°C Max.
Storage Temperature, T
S
..........–55°C to +150°C
1Measured on a typical two-sided PCB with 2 in.2 copper
ground plane.
3Measured on a JEDEC-standard “High-K” 4-layer PCB.
A8430EEK MLPD
Advance Information Data Sheet
Subject to Change without Notice
January 20, 2004
Output voltage up to 36 V
2.5 V to 10 V input
Drives up to 4 LEDs from a 3.2 V supply
Drives up to 6 LEDs from a 5 V supply
1.2 MHz switching frequency
200 mA switch current limit
1 µA shutdown current
FEATURES
LED backlights
Portable battery-powered equipment
Cellular phones
PDAs (Personal Digital Assistant)
Camcorders, personal stereos, MP3 players, cameras
Mobile GPS systems
APPLICATIONS
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 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 amplifi er
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 fi ltered 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 available in a low profi le 5-lead MLP package (part num-
ber suffi x EK).
51
2
3 4
SW
GND
FB
VIN
EN
Part Number Package Description
A8430EEK 5-pin, MLPD Surface Mount
Same pad footprint as SOT-23-5.
2
Worcester, Massachusetts 01615-0036 (508) 853-5000
115 Northeast Cutoff, Box 15036
www.allegromicro.com
Data Sheet
Advance Information
26185.300b
A8430
Constant Current LED Driver Boost Converter
Functional Block Diagram
VIN FB
Enable
SW
VREF
95 mV
Switch
Control
OSC
EN
GND
ELECTRICAL CHARACTERISTICS at TA = 25°C, VIN = 3 V (unless otherwise noted)
Characteristics Symbol Test Conditions Min. Typ. Max. Units
Input Voltage Range VIN – 2.5 – 10 V
Supply Current ISUP
Active: ILOAD = 15 mA, VLOAD = 12 V – 2.5 3.5 mA
Shutdown (EN = 0 V) – 0.1 1 µA
Feedback Reference Voltage VREF ISW = 100 mA, D = 66% 86 95 104 mV
Feedback Input Current IFB – – 45 100 nA
Switch Current Limit ISWLIM – – 200 – mA
Switch Frequency FSW – 0.8 1.2 1.6 MHz
Switch Maximum Duty Cycle D – 85 90 – %
Switch Saturation voltage VCESAT – – 350 – mV
Switch Leakage Current ISL –––5µA
Enable Input
Input Threshold Low VIL – – – 0.4 V
Input Threshold High VIH – 1.5 – – V
Input Leakage IIL – – 65 – µA
3
Worcester, Massachusetts 01615-0036 (508) 853-5000
115 Northeast Cutoff, Box 15036
www.allegromicro.com
Data Sheet
Advance Information
26185.300b
A8430
Constant Current LED Driver Boost Converter
PIN Functions
(Refer to schematic diagram 1.)
VIN. Supply to the control circuit. A bypass capacitor must be
connected from close to this pin to GND.
GND. Ground reference connected directly to the ground plane.
The sense resistor (R1) should have a separate connection
directly to this point.
EN. Setting lower than 0.4 V disables the A8430 and puts the
control circuit into the low-power Sleep mode. Greater than
1.5 V fully enables the A8430.
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 be
minimized. In addition, the inductor (L1) and diode (D1) should
be connected as close to this pin as possible.
FB. Feedback pin for LED current control. The reference
voltage is 95 mV. The top of the sense resistor (R1) is typically
connected to this pin.
Device Operation
The A8430 uses a constant-frequency, current-mode control
scheme to regulate the current through the load. The load
current produces a voltage across the external sense resistor
(R1) and the input at the FB pin. This voltage is then compared
to the internal 95 mV reference to produce an error signal.
The switch current is sensed by the internal sense resistor and
compared to the load current error signal. As the load current
increases, the error signal diminishes, reducing the maximum
switch current and thus the current delivered to the load. As
the load current decreases, the error signal rises, increasing
the maximum switch current and thus increasing the current
delivered to the load.
To set the load current, ensure that the required internal
reference value of 95 mV is produced at the desired load. To
do so, select a resistance value for the sense resistor, R1 (Ω),
such that:
R1 = 95 mV / ILOAD
where ILOAD is the target load current (mA).
The following table shows typical values for R1. Note that the
resistance value is from the standard E96 series.
Target Load Current (ILOAD)
(mA)
Sense Resistor (R1)
(Ω)
5 19.1
10 9.53
12 7.87
15 6.34
20 4.75
As load current is reduced, the energy required in the inductor
diminishes, resulting in the inductor current dropping to zero for
low load current levels. This is known as Discontinuous mode
operation, and results in some low-frequency ripple. The average
load current, however, remains regulated down to zero.
In Discontinuous mode, when the inductor current drops to zero,
the voltage at the SW pin rings, due to the capacitance in the
resonant LC circuit formed by the inductor and the capacitance
of the switch and the diode. This ringing is low-frequency and is
not harmful. It can be damped with a resistor across the inductor,
but this reduces effi ciency and is not recommended.
Functional Description
Schematic 1. Typical application
A8430
VIN SW
EN GND FB
Li-ion
2.5V to
4.2V
C1
1µF
L1
22µH D1
C2
0.22µF
R1
6R3
Enable
Ω
4
Worcester, Massachusetts 01615-0036 (508) 853-5000
115 Northeast Cutoff, Box 15036
www.allegromicro.com
Data Sheet
Advance Information
26185.300b
A8430
Constant Current LED Driver Boost Converter
Component Selection
The component values shown in schematic 1 are suffi cient 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 suffi cient.
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
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
6R3
VC
R3
90k
R2
5k
Enable Ω
Ω
Ω
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 sche-
matic 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 cur-
rent level is defi ned 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 defi ned as:
ILOAD = 100 mV / R1
where R1 is the resistance of the sense resister (Ω).
PWM Control. LED dimming control can also be gener-
ated by a fi ltered 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.
5
Worcester, Massachusetts 01615-0036 (508) 853-5000
115 Northeast Cutoff, Box 15036
www.allegromicro.com
Data Sheet
Advance Information
26185.300b
A8430
Constant Current LED Driver Boost Converter
Schematic 3. Dimming control with fi ltered PWM
A8430
VIN SW
EN GND FB
Li-ion
2.5V to
4.2V
C1
1µF
L1
22µH D1
C2
0.22µF
R1
6R3
VC(PMW)
R3
90k
R2
5k
Enable
R4
10k
C3
100 nF
Ω
Ω
Ω
Ω
Schematic 4. Soft start operation
A8430
VIN SW
EN GND FB
Li-ion
2.5V to
4.2V
C1
1µF
L1
22µH D1
C2
0.22µF
R1
6R3
R3
5k
R2
1k
Enable
C3
2.2 nF
Ω
ΩΩ
By applying a PWM signal directly to the EN pin, the A8430
is turned on or off, and ILOAD is either full (as defi ned by R1) or
zero. By varying the duty cycle of the PWM signal, the LED
brightness can be controlled from off (0% duty cycle) to full
(100% duty cycle). The PWM frequency should be in the range
from 1 kHz to 10 kHz.
Several other schemes are possible, for example, digitally switching
additional resistors across R1 to increase ILOAD . In this case, R1
would be selected for the minimum desired brightness.
Soft Start-Up
To provide fast start-up operation, no soft start is
implemented in the control circuit. At power-on, the input
capacitor (C1) is discharged, which means that the supply
must provide the in-rush current through the inductor.
This can be reduced by modulating the feedback with a soft-start
circuit as shown in schematic 4. When power is fi rst applied, the
capacitor C3 is discharged and pulls the FB pin high, reducing
the output drive to minimum. As C3 charges, when the bottom
drops below about 0.8 V, the feedback from the sense resistor
(R1) takes over full control of the output current.
6
Worcester, Massachusetts 01615-0036 (508) 853-5000
115 Northeast Cutoff, Box 15036
www.allegromicro.com
Data Sheet
Advance Information
26185.300b
A8430
Constant Current LED Driver Boost Converter
Package EK
Terminal List Table
Pin Name Function
1 SW Internal power FET
2 GND Ground
3 FB Feedback input
4 EN Enable input
5 VIN Input supply
3.15
2.85
0.124
0.112
.50
.30
0.020
0.012
.50
.30
0.020
0.012
.95
BSC
0.037
.80
.70
0.031
0.028
.05
MAX
0.002
.20
REF
0.008
2
5
1
2
5
1
Dimensions in millimeters
U.S. Customary dimensions (in.)
in brackets, for reference only
AExposed thermal pad
A
7
Worcester, Massachusetts 01615-0036 (508) 853-5000
115 Northeast Cutoff, Box 15036
www.allegromicro.com
Data Sheet
Advance Information
26185.300b
A8430
Constant Current LED Driver Boost Converter
The products described here are manufactured under one or
more U.S. patents or U.S. patents pending.
Allegro MicroSystems, Inc. reserves the right to make, from time
to time, such de par tures from the detail spec i fi ca tions as may be
required to permit improvements in the per for mance, reliability,
or manufacturability of its products. Before placing an order, the
user is cautioned to verify that the information being relied upon is
current.
Allegro products are not authorized for use as critical compo-
nents in life-support devices or sys tems without express written
approval.
The in for ma tion in clud ed herein is believed to be ac cu rate and
reliable. How ev er, Allegro MicroSystems, Inc. assumes no re spon -
si bil i ty for its use; nor for any in fringe ment of patents or other
rights of third parties which may result from its use.
Copyright©2003, 2004 AllegroMicrosystems, Inc.
