A8430: White LED Driver - Constant Current Step-Up Converter

A8430
White LED Driver Constant Current Step-up Converter
Discontinued Product
These parts are no longer in production The device should not be
purchased for new design applications. Samples are no longer available.
Date of status change: May 3, 2010
Recommended Substitutions:
NOTE: For detailed information on purchasing options, contact your
local Allegro field applications engineer or sales representative.
Allegro MicroSystems, Inc. reserves the right to make, from time to time, revisions to the anticipated product life cycle plan
for a product to accommodate changes in production capabilities, alternative product availabilities, or market demand. The
information included herein is believed to be accurate and reliable. However, Allegro MicroSystems, Inc. assumes no responsibility for its use; nor for any infringements of patents or other rights of third parties which may result from its use.
26185.300C
White LED Driver Constant Current Step-up Converter
A8430 MLPD
Approximate actual size
SW
1
GND
2
FB
3
5
VIN
4
EN
Same pad footprint as SOT-23-5
RθJA = 50 °C/W, see note 1, page 2
ABSOLUTE MAXIMUM RATINGS
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
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
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
Data Sheet
A8430
26185.300C
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
Ramp
Generator
EN
Enable
1.2 MHz
Oscillator
GND
ELECTRICAL CHARACTERISTICS at TA = 25°C, VIN = 3 V (unless otherwise noted)
Characteristics
Symbol
Test Conditions
Min.
Typ.
Max.
Units
2.5
–
10
V
Input Voltage Range
VIN
–
Supply Current
ISUP
Active: ILOAD = 15 mA,
VLOAD = 12 V
–
2.5
3.5
mA
Shutdown (EN = 0 V)
–
0.1
1
µA
95
104
mV
Feedback Reference Voltage
Feedback Input Current
VREF
–
86
IFB
–
–
20
75
nA
ISWLIM
–
–
300
–
mA
FSW
–
0.8
1.2
1.6
MHz
D
–
85
90
–
%
VCE(SAT)
–
–
350
–
mV
ISL
–
–
–
5
µA
Input Threshold Low
VIL
–
–
–
0.4
V
Input Threshold High
VIH
–
1.5
–
–
V
Leakage
IIL
–
–
1
µA
Switch Current Limit
Switch Frequency
Switch Maximum Duty Cycle
Switch Saturation voltage
Switch Leakage Current
Enable Input
Input Leakage
Note 1. Measured with 4-layer PCB. Please refer to application note “Package Thermal Characteristics,“ for thermal performance measurement for 3 mm x 3 mm MLP package for additional information.
www.allegromicro.com
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
2
Data Sheet
A8430
26185.300C
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
150
1.20
1.15
1.10
1.05
1.00
–50
0
Temperature (°C)
100
150
Conversion Efficiency versus Current
300
90
250
85
Efficiency (%)
VCE(SAT) (mV)
50
Temperature (°C)
Switch Pin Voltage versus Temperature
200
150
100
80
75
VIN = 3 V
70
VIN = 4 V
65
50
–50
150
Switching Frequency versus Temperature
Switching Frequency (MHz)
Feedback Bias Current (nA)
15
0
100
1.25
20
–50
50
Temperature (°C)
60
0
50
100
150
0
5
10
15
20
LED Current (mA)
Temperature (°C)
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115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
3
Data Sheet
A8430
26185.300C
White LED Driver Constant Current Step-up Converter
Functional Description
Typical Application
A typical application circuit for the A8430 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 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,
L1
22µH
Li-ion
2.5V to
4.2V
GND
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.
95
C2
0.22µF
FB
Enable
R1
6.3Ω
Conversion Efficiency (%)
EN
GND. Ground reference connected directly to the ground plane.
The sense resistor (R1) should have a separate connection
directly to this point.
Conversion Efficiency versus Input Voltage
A8430
C1
1µF
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.
D1
SW
VIN
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.
90
85
80
3 LEDs
75
4 LEDs
5 LEDs
70
65
2
3
4
5
6
7
8
9
10
VIN (V)
Schematic 1. Typical application
Chart 1. Conversion efficiency when driving various
quantities of LEDs in the typical application circuit
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115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
4
Data Sheet
A8430
26185.300C
White LED Driver Constant Current Step-up Converter
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:
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 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 lowfrequency 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 efficiency and is
not recommended.
R1 = 95 mV/ ILOAD
where ILOAD is the target load current (mA).
Power Dissipation versus IOUT
PD (mW)
120
110
100
90
80
70
60
50
40
Target Load Current
(ILOAD)
(mA)
Sense Resistor (R1)
(Ω)
5
19.1
10
9.53
12
7.87
15
6.34
20
4.75
30
20
10
0
5
10
Vin = 3V, 3 LED
15
IOUT (mA)
Vin = 5V, 3 LED
20
Vin = 3V, 4 LED
25
Vin = 5V, 4 LED
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115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
5
Data Sheet
A8430
26185.300C
White LED Driver Constant Current Step-up Converter
Application Information
Component Selection
Feedback modulation. By adding a voltage drop
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.
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 (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.
L1
22µH
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.
A8430
EN
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.
SW
VIN
C1
1µF
Li-ion
2.5V to
4.2V
D1
GND
C2
0.22µF
FB
R2
5kΩ
Enable
VC
R3
90kΩ
R1
6.3Ω
Schematic 2. Dimming control with dc voltage
feedback modulation
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Worcester, Massachusetts 01615-0036 (508) 853-5000
6
Data Sheet
A8430
26185.300C
White LED Driver Constant Current Step-up Converter
By applying a PWM signal directly to the EN pin, the A8430
is turned on or off, and ILOAD is either full (as defined 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.
L1
22 µH
SW
VIN
A843
C1
1 µF
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
EN
GND
FB
Li-ion
2.5 V to
4.2 V
VC(PMW)
R3
90 kΩ
R4
10 kΩ
R1
6.3 Ω
C3
100 nF
Soft Start-Up
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.
Schematic 3. Dimming control with filtered PWM
This can be reduced by modulating the feedback with a softstart 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.
L1
22µH
C1
1µF
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 becomes zero, thus
the feedback voltage becomes zero. The A8430 compensates
by increasing the on time of the switch, which increases the
output voltage.
D1
SW
VIN
Li-ion
2.5V to
4.2V
Overvoltage Protection
C2
0.22 µF
R2
5 kΩ
Enable
C3
2.2 nF
A8430
EN
GND
FB
C2
0.22µF
R2
1kΩ
Enable
R3
5kΩ
R1
6.3Ω
Schematic 4. Soft start operation
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Data Sheet
A8430
26185.300C
White LED Driver Constant Current Step-up Converter
Overvoltage protection for the A8430 requires a Zener diode
to clamp the output voltage, as shown in schematic 5. The
Zener voltage should be greater than the maximum output
voltage of the LED string. The Zener diode also should be
able to sink more than 0.1 mA of current.
L1
22µH
The A8430 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.
SW
VIN
C2
0.22µF
A8430
C1
1µF
EN
Li-ion
2.5V to
4.2V
Parallel LED Strings
D1
GND
FB
R2
1kΩ
Enable
R1
6.3Ω
Schematic 5. Overvoltage protection with Zener clamp
L1
22µH
A typical circuit with two parallel strings is shown in
schematic 6. The coversion efficiency of this configuration is
shown in chart 2.
D1
SW
VIN
A8430
C1
1µF
EN
Li-ion
2.5V to
4.2V
GND
C2
0.22µF
FB
Enable
R1
6.3Ω
R2
6.3Ω
Schematic 6. 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
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115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
8
Data Sheet
A8430
26185.300C
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
VIN
Input supply
Package EK
5
3.00 .118
BSC
A
1
2
0.45 .018
0.30 .012
0.80 .031
0.70 .028
0.95 .037
BSC
0.50 .020
0.30 .012
1
R0.20 .008
REF
0.20 .008
REF
0.05
0.80 .002
.031
0.00
0.70 .000
.028
2
1.10
0.85
B
.043
.033
Dimensions in millimeters
U.S. Customary dimensions (in.)
in brackets, for reference only
A Pin index area
B Exposed thermal pad
C Optional thermal vias, ∅0.30 [.012], pitch 1.2 [.047]
5
2.10
1.85
0.75 .030
NOM
5
0.50 .020
MIN
D Typical pad layout including solder pad for exposed
thermal pad; adjust as necessary to meet
application process requirements
.083
.073
E Typical pad layout with contact pads only; adjust as
necessary to meet application process requirements
0.75 .030
NOM
0.50 .020
MIN
5
0.15 .006
MIN
3.40 .134
REF
C
0.20
REF
.043
3.40 .134
REF
.008
1
E
1.10
MAX
1
0.95 .037
BSC
0.95 .037
BSC
D
2.10
MAX
.083
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115 Northeast Cutoff, Box 15036
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9
Data Sheet
A8430
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.
www.allegromicro.com
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
10
26185.300C
White LED Driver Constant Current Step-up Converter
Data Sheet
A8430
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