Preliminary Data Sheet
Subject to Change without Notice
December 7, 2004
A8431
The A8431 is a noninverting boost dc-dc converter that provides a
programmable constant current output up to 32 V for driving white
LEDs in series.The A8431 also offers an OVP (overvoltage protection)
pin. Driving the 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 two parallel strings
of eight white LEDs can be driven at 20 mA by increasing the supply
voltage up to 10 V.
The A8431 incorporates a power switch and a 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 a filtered PWM signal when dimming con-
trol is required. The high switching frequency of 1.2 MHz allows the
use of small inductor and capacitor values.
The A8431 is provided in a 0.75 mm high, 6-pin, 2 x 3 mm MLP pack-
age. It is lead (Pb) free, with 100% matte tin leadframe plating.
White LED Driver Constant Current Step-up Converter
A8431 MLPD
Approximate actual size
SW
GND
FB
1
2
3
6
5
4
VIN
OVP
EN
R
θJA
= 50 °C/W, see note 1, page 2
FEATURES
Output voltage up to 32 V (OVP level)
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
OVP pin eliminates the need for an external Zener diode on the output
ABSOLUTE MAXIMUM RATINGS
SW Pin ................................................–0.3
V to 36 V
OVP Pin ..............................................–0.3
V to 36 V
Remaining Pins ..................................
–0.3 V to 10 V
Ambient Operating Temperature, T
A
.......
–40°C to 85°C
Junction Temperature, T
J(max)
...............................
150°C
Storage Temperature, T
S
....................
–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 numbers when ordering:
Part Number
A8431EEH-T
Package
6-pin, MLPD
Description
Pb-Free, Surface Mount
26185.301
Preliminary Data Sheet
Subject to Change Without Notice
December 7, 2004
White LED Driver Constant Current Step-up Converter
Functional Block Diagram
FB
SW
A8431
VIN
V
REF
1.25 V
95 mV
A1
R
C
C
C
A2
R
S
Q
Driver
Σ
OVP
Ramp
Generator
OVP
1.2 MHz
Oscillator
EN
Enable
GND
ELECTRICAL CHARACTERISTICS
at T
A
= 25°C, V
IN
= 3 V (unless otherwise noted)
Characteristics
Input Voltage Range
Supply Current
Feedback Reference Voltage
Feedback Input Current
Switch Current Limit
Switch Frequency
Switch Maximum Duty Cycle
Switch Saturation Voltage
Switch Leakage Current
Enable Input
Input Threshold Low
Input Threshold High
Input Leakage
Overvoltage Protection
Output Overvoltage Rising Limit
Output Overvoltage Falling Limit
Output Overvoltage Hysteresis
OVP Pin Resistance
V
OVPR
V
OVPF
V
OVPHYS
R
OVP
30
29.5
–
–
32
31.5
0.5
1.0
34
33.5
–
–
V
V
V
MΩ
V
IL
V
IH
I
IL
–
1.5
–
–
–
65
0.4
–
–
V
V
µA
Symbol
V
IN
I
SUP
V
FB
I
FB
I
SWLIM
F
SW
D
V
CE(SAT)
I
SL
V
SW
= 5 V
Active
Shutdown (EN = 0 V)
Test Conditions
Min.
2.5
–
–
86
–
–
0.8
85
–
–
Typ.
–
2.5
0.1
95
20
300
1.2
90
350
–
Max.
10
3.5
1
104
50
–
1.6
–
–
5
Units
V
mA
µA
mV
nA
mA
MHz
%
mV
µA
Note 1. Measured with 4-layer PCB. Please refer to application note “Package Thermal Characteristics,“ for thermal perfor-
mance measurement for 2 x 3 mm MLP package for additional information.
26185.301
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
2
Preliminary Data Sheet
Subject to Change Without Notice
December 7, 2004
White LED Driver Constant Current Step-up Converter
Operating Characteristics
Using Typical Application Circuit (Schematic 1)
Quiescent Current versus Input Voltage
2.5
Quiescent Current (mA)
Quiescent Current (mA)
A8431
Quiescent Current versus Temperature
2.15
2.10
2.05
2.00
1.95
1.90
2.0
1.5
1.0
0.5
0
0
2
4
V
IN
(V)
6
8
10
–50
0
50
Temperature (°C)
100
150
Feedback Bias Current versus Temperature
20
Feedback Bias Current (nA)
Switching Frequency (MHz)
0
50
Temperature (°C)
100
150
Switching Frequency versus Temperature
1.25
1.20
1.15
1.10
1.05
1.00
15
10
5
0
–50
–50
0
50
Temperature (°C)
100
150
Switch Pin Voltage versus Temperature
300
250
V
CE(SAT)
(mV)
Conversion Efficiency (%)
Conversion Efficiency versus Input Voltage
95
90
85
80
75
70
65
2
3
4
5
6
V
IN
(V)
7
200
150
100
50
0
–50
0
50
Temperature (°C)
100
150
3 LEDs
4 LEDs
5 LEDs
8
9
10
26185.301
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
3
Preliminary Data Sheet
Subject to Change Without Notice
December 7, 2004
White LED Driver Constant Current Step-up Converter
Functional Description
A8431
Typical Application
A typical application circuit for the A8431 is provided in
schematic diagram 1. This illustrates a method of driving
three white LEDs in series. The conversion efficiency of this
configuration is shown in chart 1.
be minimized. In addition, L1 and the diode D1 should be
connected as close to this pin as possible.
OVP.
Overvoltage Protection sense pin to protect the A8431
from excessive voltage on the SW pin. This pin should be
connected to the output capacitor, C2. To disable this feature
connect the pin to ground.
EN.
Setting lower than 0.4 V disables the A8431 and puts the
control circuit into the low-power Sleep mode. Greater than
1.5 V fully enables the A8431.
GND.
Ground reference connected directly to the ground plane.
The sense resistor, R1, should have a separate connection directly
to this point.
FB.
Feedback pin for LED current control. The reference voltage
is 95 mV. The top of R1 is typically connected here.
Pin Functions
The diagram also shows a method of connecting the individ-
ual pins, whos functions are described as follows:
VIN.
Supply to the control circuit. A bypass capacitor, C1, must
be connected from close to this pin to GND.
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
L1
22 µH
6
VIN
C1
1.0 µF
EN
Li-ion
2.5V to
4.2V
4
Enable
A8431
GND
2
1
SW
Conversion Efficiency versus Current
D1
90
85
Efficiency (%)
OVP
FB
3
5
C2
0.22 µF
80
75
70
65
60
0
5
10
LED Current (mA)
15
20
R1
6.3Ω
V
IN
= 3 V
V
IN
= 4 V
Schematic 1. Typical application
Chart 1. Conversion efficiency when driving three LEDs
in the typical application circuit.
26185.301
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
4
Preliminary Data Sheet
Subject to Change Without Notice
December 7, 2004
White LED Driver Constant Current Step-up Converter
Device Operation
The A8431 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 in schematic 1) 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
⁄
I
LOAD
where
I
LOAD
is the target load current (mA).
The table below shows typical values for R1. Note that the
resistance value is from the standard E96 series.
As load current is reduced, the energy required in the
inductor, L1, diminishes, resulting in the inductor current
dropping to 0 A 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 0 A.
In Discontinuous mode, when the inductor current drops to
0 A, 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 efficiency and is
not recommended.
Overvoltage Protection
An overvoltage event can occur when the LEDs become
disconnected or fail in an open state. In these cases, the
current flow through the sense resistor, R1, becomes 0 A and
thus the feedback voltage, V
FB
becomes 0 V. The A8431
compensates by increasing the on time of the switch, which
increases the output voltage.
The A8431 has built-in protection to guard against excessive
voltage on the SW pin. If the output voltage exceeds the
typical level of the Output Overvoltage Rising Limit, V
OVPR
,
then the overvoltage protection circuitry shuts off the internal
switch until the output voltage falls below the Output
Overvoltage Falling Limit, V
OVPF
. At this point, the A8431
operates normally. There is no need for an external Zener
diode for the A8431.
A8431
Power Dissipation versus I
OUT
120
110
100
90
80
70
60
50
40
30
20
10
0
5
10
Vin = 3V, 3 LED
15
I
OUT
(mA)
Vin = 5V, 3 LED
20
Vin = 3V, 4 LED
25
Vin = 5V, 4 LED
Target Load Current
(
I
LOAD
)
(mA)
5
10
12
15
20
Sense Resistor (R1)
(Ω)
19.1
9.53
7.87
6.34
4.75
PD (mW)
26185.301
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
5
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