A8430EEKTR-T - Allegro Microsystems

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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 respon-

sibility for its use; nor for any infringements of patents or other rights of third parties which may result from its use.

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White LED Driver Constant Current Step-up Converter

A8430

Date of status change: November 2, 2009

Deadline for receipt of LAST TIME BUY orders: April 30, 2010

These parts are in production but have been determined to be

LAST TIME BUY. This classification indicates that the product is

obsolete and notice has been given. Sale of this device is currently

restricted to existing customer applications. The device should not be

purchased for new design applications because of obsolescence in the

near future. Samples are no longer available.

Last T ime Buy

Data Sheet

26185.300C

A8430

White LED Driver Constant Current Step-up 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

Ambient Operating Temperature, TA....... –40°C to 85°C

Junction Temperature, TJ(max)............................... 150°C

Storage Temperature, TS

.................... –55°C to 150°C

A8430 MLPD

 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

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 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.

3 4

GND

VIN

Part Number Package Description

A8430EEK 5-pin, MLPD Surface Mount

A8430EEK-T 5-pin, MLPD Lead-Free, Surface Mount

RθJA = 50 °C/W, see note 1, page 2

Approximate actual size

Same pad footprint as SOT-23-5

Worcester, Massachusetts 01615-0036 (508) 853-5000

115 Northeast Cutoff, Box 15036

www.allegromicro.com

Data Sheet

26185.300C

A8430

White LED Driver Constant Current Step-up Converter

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 – 86 95 104 mV

Feedback Input Current IFB – – 20 75 nA

Switch Current Limit ISWLIM – – 300 – mA

Switch Frequency FSW – 0.8 1.2 1.6 MHz

Switch Maximum Duty Cycle D – 85 90 – %

Switch Saturation voltage VCE(SAT) – – 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 Leakage IIL ––1µA

Note 1. Measured with 4-layer PCB. Please refer to application note “Package Thermal Characteristics,“ for thermal perfor-

mance measurement for 3 mm x 3 mm MLP package for additional information.

Functional Block Diagram

VIN

FB SW

VREF

1.25 V

GND

DriverA2

Ramp

Generator

1.2 MHz

Oscillator

95 mV

Enable

ELECTRICAL CHARACTERISTICS at TA = 25°C, VIN = 3 V (unless otherwise noted)

Worcester, Massachusetts 01615-0036 (508) 853-5000

115 Northeast Cutoff, Box 15036

www.allegromicro.com

Data Sheet

26185.300C

A8430

White LED Driver Constant Current Step-up Converter

Operating Characteristics

Using Typical Application Circuit (Schematic 1)

0 5 10 15 20

LED Current (mA)

Efficiency (%)

= 3 V

= 4 V

Quiescent Current versus Input Voltage

0.5

1.0

1.5

2.0

2.5

0246810

(V)

Quiescent Current (mA)

Feedback Bias Current versus Temperature

Temperature (°C)

Feedback Bias Current (nA)

Switch Pin Voltage versus Temperature Conversion Efficiency versus Current

100

150

200

250

300

Temperature (°C)

VCE(SAT) (mV)

Quiescent Current versus Temperature

1.90

1.95

2.00

2.05

2.10

2.15

–50 0 50 100 150

Temperature (°C)

Quiescent Current (mA)

Switching Frequency versus Temperature

1.00

1.05

1.10

1.15

1.20

1.25

Temperature (°C)

Switching Frequency (MHz)

–50 0 50 100 150

Worcester, Massachusetts 01615-0036 (508) 853-5000

115 Northeast Cutoff, Box 15036

www.allegromicro.com

Data Sheet

26185.300C

A8430

White LED Driver Constant Current Step-up Converter

Functional Description

Schematic 1. Typical application

A8430

VIN SW

EN GND FB

Li-ion

2.5V to

4.2V

1µF

22µH D1

0.22µF

6.3Ω

Enable

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 individ-

ual 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,

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.

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.

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 the sense resistor (R1) is typically

connected to this pin.

Conversion Efficiency versus Input Voltage

2345678910

(V)

Conversion Efficiency (%)

3 LEDs

4 LEDs

5 LEDs

Chart 1. Conversion efficiency when driving various

quantities of LEDs in the typical application circuit

Worcester, Massachusetts 01615-0036 (508) 853-5000

115 Northeast Cutoff, Box 15036

www.allegromicro.com

Data Sheet

26185.300C

A8430

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:

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 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 efficiency and is

not recommended.

Target Load Current

(ILOAD)

(mA)

Sense Resistor (R1)

(Ω)

5 19.1

10 9.53

12 7.87

15 6.34

20 4.75

100

110

120

510152025

IOUT (mA)

PD (mW)

Vin = 3V, 3 LED Vin = 5V, 3 LED Vin = 3V, 4 LED Vin = 5V, 4 LED

Power Dissipation versus I

OUT

Worcester, Massachusetts 01615-0036 (508) 853-5000

115 Northeast Cutoff, Box 15036

www.allegromicro.com

Data Sheet

26185.300C

A8430

White LED Driver Constant Current Step-up Converter

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 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.

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.

Application Information

Schematic 2. Dimming control with dc voltage

feedback modulation

A8430

VIN SW

EN GND FB

Li-ion

2.5V to

4.2V

1µF

22µH D1

0.22µF

90kΩ

5kΩ

Enable

6.3Ω

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 (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.

Worcester, Massachusetts 01615-0036 (508) 853-5000

115 Northeast Cutoff, Box 15036

www.allegromicro.com

Data Sheet

26185.300C

A8430

White LED Driver Constant Current Step-up Converter

Schematic 3. Dimming control with filtered PWM

A843

VIN SW

EN GND FB

Li-ion

2.5 V to

4.2 V

1 µF

22 µH D1

0.22 µF

6.3 Ω

VC(PMW)

90 kΩ

5 kΩ

Enable

10 kΩ

100 nF

Schematic 4. Soft start operation

A8430

VIN SW

EN GND FB

Li-ion

2.5V to

4.2V

1µF

22µH D1

0.22µF

5kΩ

1kΩ

Enable

2.2 nF

6.3Ω

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.

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 bypass

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 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.

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 becomes zero, thus

the feedback voltage becomes zero. The A8430 compensates

by increasing the on time of the switch, which increases the

output voltage.

Worcester, Massachusetts 01615-0036 (508) 853-5000

115 Northeast Cutoff, Box 15036

www.allegromicro.com

Data Sheet

26185.300C

A8430

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.

Parallel LED Strings

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.

A typical circuit with two parallel strings is shown in

schematic 6. The coversion efficiency of this configuration is

shown in chart 2.

A8430

VIN SW

EN GND FB

Li-ion

2.5V to

4.2V

1µF

22µH D1

0.22µF

6.3Ω

1kΩ

Enable

Schematic 5. Overvoltage protection with Zener clamp

A8430

VIN SW

EN GND FB

Li-ion

2.5V to

4.2V

1µF

22µH D1

0.22µF

6.3Ω6.3Ω

Enable

Conversion Efficiency for Two Parallel Strings

2436589710

Input Voltage (V)

Efficiency (%)

Two 3-LED strings

Two 4-LED strings

Two 7-LED strings

Chart 2. Conversion efficiency when driving two parallel strings

of varying lengths

Schematic 6. Parallel strings of LEDs

Worcester, Massachusetts 01615-0036 (508) 853-5000

115 Northeast Cutoff, Box 15036

www.allegromicro.com

Data Sheet

26185.300C

A8430

White LED Driver Constant Current Step-up Converter

Package EK

1.10

0.85

.043

.033

0.45

0.30

.018

.012

2.10

1.85

.083

.073

2.10

MAX

.083

0.50

0.30

.020

.012

0.95

BSC

.037

3.00

BSC

.118

0.80

0.70

.031

.028

0.80

0.70

.031

.028

0.05

0.00

.002

.000

0.20

REF

.008

R0.20

REF

.008

0.75

NOM

.030

0.15

MIN

.006

3.40

REF

.134

0.20

REF

.008

0.75

NOM

.030

3.40

REF

.134

Dimensions in millimeters

U.S. Customary dimensions (in.)

in brackets, for reference only

APin index area

BExposed thermal pad

Optional thermal vias, ∅0.30 [.012], pitch 1.2 [.047]

Typical pad layout including solder pad for exposed

thermal pad; adjust as necessary to meet

application process requirements

Typical pad layout with contact pads only; adjust as

necessary to meet application process requirements

1.10

MAX

.043

0.50

MIN

.020

0.95

BSC

.037

0.50

MIN

.020

0.95

BSC

.037

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

Worcester, Massachusetts 01615-0036 (508) 853-5000

115 Northeast Cutoff, Box 15036

www.allegromicro.com

Data Sheet

26185.300C

A8430

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 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 components 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.