Allegro A8431 White led driver constant current step-up converter Datasheet

Preliminary Data Sheet
Subject to Change without Notice
April 5, 2005
A8431
White LED Driver Constant Current Step-up Converter
A8431 MLPD
Approximate actual size
SW
1
6
VIN
GND
2
5
OVP
FB
3
4
EN
RθJA = 50 °C/W, see note 1, page 2
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, TA....... –40°C to 85°C
Junction Temperature, TJ(max)............................... 150°C
Storage Temperature, TS .................... –55°C to 150°C
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 control 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 package. It is lead (Pb) free, with 100% matte tin leadframe plating.
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
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:
26185.301
Part Number
Package
Description
A8431EEH-T
6-pin, MLPD
Pb-Free, Surface Mount
Preliminary Data Sheet
Subject to Change Without Notice
April 5, 2005
A8431
White LED Driver Constant Current Step-up Converter
Functional Block Diagram
FB
VIN
VREF
1.25 V
95 mV
SW
A1
A2
RC
R
Q
Driver
S
CC
Σ
OVP
Ramp
Generator
OVP
1.2 MHz
Oscillator
EN
Enable
GND
ELECTRICAL CHARACTERISTICS at TA = 25°C, VIN = 3 V (unless otherwise noted)
Characteristics
Input Voltage Range
Symbol
Test Conditions
Min.
Typ.
Max.
Units
2.5
–
10
V
Active
–
2.5
3.5
mA
Shutdown (EN = 0 V)
–
0.1
1
µA
VIN
Supply Current
ISUP
Feedback Reference Voltage
VFB
86
95
104
mV
Feedback Input Current
IFB
–
20
75
nA
ISWLIM
–
300
–
mA
FSW
0.8
1.2
1.6
MHz
D
85
90
–
%
VCE(SAT)
–
350
–
mV
–
–
5
µA
Switch Current Limit
Switch Frequency
Switch Maximum Duty Cycle
Switch Saturation Voltage
Switch Leakage Current
ISL
VSW = 5 V
Enable Input
Input Threshold Low
VIL
–
–
0.4
V
Input Threshold High
VIH
1.5
–
–
V
–
–
1
µA
Input Leakage
Leakage
IIL
Overvoltage Protection
Output Overvoltage Rising Limit
VOVPR
28
32
35
V
Output Overvoltage Falling Limit
VOVPF
27.5
31.5
34.5
V
VOVPHYS
–
0.5
–
V
ROVP
–
1.0
–
MΩ
Output Overvoltage Hysteresis
OVP Pin Resistance
Note 1. Measured with 4-layer PCB. Please refer to application note “Package Thermal Characteristics,“ for thermal performance measurement for 2 x 3 mm MLP package for additional information.
2
26185.301
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
Preliminary Data Sheet
Subject to Change Without Notice
April 5, 2005
A8431
White LED Driver Constant Current Step-up Converter
Operating Characteristics
Using Typical Application Circuit (Schematic 1)
Quiescent Current versus Temperature
2.15
2.0
2.10
Quiescent Current (mA)
Quiescent Current (mA)
Quiescent Current versus Input Voltage
2.5
1.5
1.0
0.5
0
0
2
4
6
8
2.05
2.00
1.95
1.90
10
–50
0
VIN (V)
Feedback Bias Current versus Temperature
10
5
0
0
50
100
1.20
1.15
1.10
1.05
1.00
150
–50
0
Temperature (°C)
Switch Pin Voltage versus Temperature
100
150
Conversion Efficiency versus Input Voltage
Conversion Efficiency (%)
95
250
VCE(SAT) (mV)
50
Temperature (°C)
300
200
150
100
50
–50
150
Switching Frequency versus Temperature
Switching Frequency (MHz)
Feedback Bias Current (nA)
15
0
100
1.25
20
–50
50
Temperature (°C)
0
50
Temperature (°C)
100
150
90
85
80
3 LEDs
75
4 LEDs
5 LEDs
70
65
2
3
4
5
6
7
8
9
10
VIN (V)
3
26185.301
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
Preliminary Data Sheet
Subject to Change Without Notice
April 5, 2005
A8431
White LED Driver Constant Current Step-up Converter
Functional Description
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.
Pin Functions
The diagram also shows a method of connecting the individual 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
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.
Conversion Efficiency versus Current
L1
22 µH
D1
90
1
SW
A8431
C1
1.0 µF
OVP
EN
GND
FB
4
2
3
Enable
85
5
C2
0.22 µF
R1
6.3Ω
Efficiency (%)
6
VIN
Li-ion
2.5V to
4.2V
be minimized. In addition, L1 and the diode D1 should be
connected as close to this pin as possible.
80
75
VIN = 3 V
70
VIN = 4 V
65
60
0
5
10
15
20
LED Current (mA)
Schematic 1. Typical application
Chart 1. Conversion efficiency when driving three LEDs
in the typical application circuit.
4
26185.301
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
Preliminary Data Sheet
Subject to Change Without Notice
April 5, 2005
A8431
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 ⁄ ILOAD
where ILOAD 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 lowfrequency 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, VFB 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, VOVPR ,
then the overvoltage protection circuitry shuts off the internal
switch until the output voltage falls below the Output
Overvoltage Falling Limit, VOVPF . At this point, the A8431
operates normally. There is no need for an external Zener
diode for the A8431.
Power Dissipation versus IOUT
PD (mW)
120
110
100
90
80
70
60
50
40
30
20
10
0
5
10
Vin = 3V, 3 LED
15
IOUT (mA)
Vin = 5V, 3 LED
20
Vin = 3V, 4 LED
25
Target Load Current
(ILOAD)
(mA)
Sense Resistor (R1)
(Ω)
5
19.1
10
9.53
12
7.87
15
6.34
20
4.75
Vin = 5V, 4 LED
5
26185.301
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
Preliminary Data Sheet
Subject to Change Without Notice
April 5, 2005
A8431
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, 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.
conditions. Suitable capacitors are available from TDK,
Taiyo Yuden, Murata, Kemet, and AVX.
Dimming Control
LED brightness can be controlled either: (a) by modifying 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), is defined as:
ILOAD = 100 mV ⁄ R1
where R1 is the resistance of the sense resistor (Ω).
L1
22 µH
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
1
SW
6
VIN
C1
1.0 µF
Li-ion
2.5 V to
4.2 V
D1
A8431
OVP
EN
GND
FB
4
2
3
Enable
VC
5
C2
0.22 µF
R2
5 kΩ
R3
90 kΩ
R1
6.3 Ω
Schematic 2. Dimming control with dc voltage
6
26185.301
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
Preliminary Data Sheet
Subject to Change Without Notice
April 5, 2005
A8431
White LED Driver Constant Current Step-up Converter
PWM Control. The control voltage, VC , also can be
L1
22 µH
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 0 A.
By applying a PWM signal directly to the EN pin, the A8431
is turned on or off, and ILOAD is either full (as defined by
R1) or 0 A. 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.
D1
1
SW
6
VIN
A8431
C1
1.0 µF
OVP
EN
GND
FB
4
2
3
Enable
Li-ion
2.5 V to
4.2 V
VC(PMW)
5
R3
90 kΩ
R4
10 kΩ
C2
0.22 µF
R2
5 kΩ
R1
6.3 Ω
C3
100 nF
Schematic 3. Dimming control with filtered PWM
Start-Up
L1
22 µH
To provide fast start-up operation, no soft start is
implemented in the control circuit. At power-on, the bypass
capacitor, C1, is discharged, which means that the supply
must provide the in-rush current through the inductor, L1.
This can be reduced by modulating the feedback with a soft-start
circuit as shown in schematic 4. When power is first 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.
Li-ion
2.5V to
4.2V
C3
2.2 nF
1
6
VIN
C1
1.0 µF
D1
SW
A843 1
EN GND
4
2
OVP
FB
3
Enable
5
C2
0.22 µF
R2
1 kΩ
R3
5 kΩ
R1
6.3 Ω
Schematic 4. Soft start operation
7
26185.301
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
Preliminary Data Sheet
Subject to Change Without Notice
April 5, 2005
A8431
White LED Driver Constant Current Step-up Converter
Parallel LED Strings
The A8431 can be used to power parallel strings of LEDs,
which have the same number of LEDs on each string. It is
important that the voltage drop is the same across all of the
parallel strings, to ensure that all of the LEDs are illuminated
and that the current though each string is equal.
A typical circuit with two parallel strings is shown in
schematic 5. The coversion efficiency of this configuration is
shown in chart 2.
L1
22 µH
1
6
VIN
SW
A8431
C1
1.0 µF
EN GND
4
2
Li-ion
2.5 V to
4.2 V
D1
OVP
5
C2
0.22 µF
FB
3
Enable
R1
6.3 Ω
R2
6.3 Ω
Schematic 5. Parallel strings of LEDs
Conversion Efficiency for Two Parallel Strings
95
Efficiency (%)
90
85
80
75
Two 3-LED strings
70
Two 4-LED strings
Two 7-LED strings
65
2
3
4
5
6
7
8
9
10
Input Voltage (V)
Chart 2. Conversion efficiency when driving two parallel strings
of varying lengths
8
26185.301
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
Preliminary Data Sheet
Subject to Change Without Notice
April 5, 2005
A8431
White LED Driver Constant Current Step-up Converter
Terminal List Table
Pin
Name
Function
1
SW
Internal power FET
2
GND
Ground
3
FB
Feedback input
4
EN
Enable input
5
OVP
Overvoltage protection
6
VIN
Input supply
Package EH
2.00 .079
BSC
6
3.00 .118
BSC
A
1
0.30 .012
0.20 .008
0.50 .020
BSC
0.30 .012
BSC
1
2
C
0.15 .006
MIN
1.25 .049
BSC
B
0.225 .009
REF
1.250 .049
1.000 .039
Dimensions in millimeters
U.S. Customary dimensions (in.) in brackets, for reference only
A Pin index area
B Exposed thermal pad
C ø0.3 thermal via (optional)
R0.100 .004
REF
0.225 .009
BSC
D Typical pad layout; adjust as necessary to
meet application process requirements
6
1
D
0.20 .008
REF
0.05 .002
0.00 .000
0.65 .026
0.45 .018
6
0.90 .035
BSC
3.40 .134
BSC
0.80 .031
0.70 .028
0.50 .020
BSC
1.25 .049
BSC
1.250 .049
1.000 .039
9
26185.301
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
Preliminary Data Sheet
Subject to Change Without Notice
April 5, 2005
A8431
White LED Driver Constant Current Step-up 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 departures from the detail specifications as may be required to permit improvements in the performance, 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 components in life-support devices or systems without
express written approval.
The information included herein is believed to be accurate and reliable. However, Allegro MicroSystems, Inc.
assumes no responsibility for its use; nor for any infringement of patents or other rights of third parties which
may result from its use.
Copyright©2003, 2004, 2005 AllegroMicrosystems, Inc.
10
26185.301
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com