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Preliminary Data Sheet Subject to Change without Notice April 5, 2005
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 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.
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
RJA = 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 A 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, TA....... -40C to 85C Junction Temperature, TJ(max)............................... 150C Storage Temperature, TS .................... -55C to 150C
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 April 5, 2005
White LED Driver Constant Current Step-up Converter
Functional Block Diagram
FB SW
A8431
VIN
VREF 1.25 V
95 mV
A1 RC CC A2 R S Q Driver
OVP Ramp Generator
OVP
1.2 MHz Oscillator EN Enable
GND
ELECTRICAL CHARACTERISTICS at TA = 25C, VIN = 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 VOVPR VOVPF VOVPHYS ROVP 28 27.5 - - 32 31.5 0.5 1.0 35 34.5 - - V V V M VIL VIH Leakage IIL - 1.5 - - - - 0.4 - 1 V V A Symbol VIN ISUP VFB IFB ISWLIM FSW D VCE(SAT) ISL VSW = 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 75 - 1.6 - - 5 Units V mA A mV nA mA MHz % mV A
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.
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 April 5, 2005
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 VIN (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
VCE(SAT) (mV) Conversion Efficiency (%)
Conversion Efficiency versus Input Voltage
95 90 85 80 75 70 65
2 3 4 5 6 VIN (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 April 5, 2005
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 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
L1 22 H 6 VIN C1 1.0 F 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 F
80 75 70 65 60
0 5 10 LED Current (mA) 15 20
R1 6.3
VIN = 3 V VIN = 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 April 5, 2005
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 (), 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.
A8431
Power Dissipation versus IOUT
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 Vin = 5V, 4 LED
Target Load Current (ILOAD) (mA) 5 10 12 15 20
Sense Resistor (R1) () 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
Preliminary Data Sheet Subject to Change Without Notice April 5, 2005
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
A8431
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
1.2 MHz, the optimal L1 value for most cases is 22 H. 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.
Capacitor Selection. Because the capacitor values are low,
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 ().
L1 22 H 6 VIN C1 1.0 F EN 4 Li-ion 2.5 V to 4.2 V Enable A8431 GND 2 1 SW OVP FB 3
D1
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 F 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 F 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
5 C2 0.22 F R2 5 k
VC
R3 90 k
R1 6.3
Schematic 2. Dimming control with dc voltage
26185.301
Allegro MicroSystems, Inc. 115 Northeast Cutoff, Box 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000 www.allegromicro.com
6
Preliminary Data Sheet Subject to Change Without Notice April 5, 2005
White LED Driver Constant Current Step-up Converter
A8431
PWM Control. The control voltage, VC , also can 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 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.
L1 22 H 6 VIN C1 1.0 F A8431 EN 4 Enable Li-ion 2.5 V to 4.2 V GND 2 1 SW OVP FB 3
D1
5
R2 5 k
C2 0.22 F
VC(PMW)
R4 10 k
R3 90 k C3 100 nF
R1 6.3
Schematic 3. Dimming control with filtered PWM
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, 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 L1 22 H 6 VIN C1 1.0 F A843 1 EN GND 4 2 Enable 1 SW OVP FB 3 5 R2 1 k R3 5 k R1 6.3 D1 C3 2.2 nF C2 0.22 F
Schematic 4. Soft start operation
26185.301
Allegro MicroSystems, Inc. 115 Northeast Cutoff, Box 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000 www.allegromicro.com
7
Preliminary Data Sheet Subject to Change Without Notice April 5, 2005
White LED Driver Constant Current Step-up Converter
A8431
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.
6 VIN C1 1.0 F Li-ion 2.5 V to 4.2 V
L1 22 H 1 SW A8431 EN GND 4 2 Enable OVP FB 3
D1
5
C2 0.22 F
R1 6.3
R2 6.3
Schematic 5. Parallel strings of LEDs
Conversion Efficiency for Two Parallel Strings
95 90
Efficiency (%)
85 80 75 70 65
2 3 4 5 6
Two 3-LED strings Two 4-LED strings Two 7-LED strings
7 8 9 10
Input Voltage (V)
Chart 2. Conversion efficiency when driving two parallel strings of varying lengths
26185.301
Allegro MicroSystems, Inc. 115 Northeast Cutoff, Box 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000 www.allegromicro.com
8
Preliminary Data Sheet Subject to Change Without Notice April 5, 2005
White LED Driver Constant Current Step-up Converter
Terminal List Table
Pin 1 2 3 4 5 6 Name SW GND FB EN OVP VIN Function Internal power FET Ground Feedback input Enable input Overvoltage protection Input supply
A8431
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 0.90 .035 BSC 3.40 .134 BSC 0.15 .006 MIN 6 0.65 .026 0.45 .018 C 1.25 .049 BSC B 0.225 .009 REF R0.100 .004 REF 6 1 D 1.25 .049 BSC 1.250 .049 1.000 .039 1.250 .049 1.000 .039 0.50 .020 BSC
0.80 .031 0.70 .028 0.20 .008 REF 0.05 .002 0.00 .000
1 2
Dimensions in millimeters U.S. Customary dimensions (in.) in brackets, for reference only A Pin index area B Exposed thermal pad C o0.3 thermal via (optional) D Typical pad layout; adjust as necessary to meet application process requirements
0.225 .009 BSC
26185.301
Allegro MicroSystems, Inc. 115 Northeast Cutoff, Box 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000 www.allegromicro.com
9
Preliminary Data Sheet Subject to Change Without Notice April 5, 2005
White LED Driver Constant Current Step-up Converter
A8431
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(c)2003, 2004, 2005 AllegroMicrosystems, Inc.
26185.301
Allegro MicroSystems, Inc. 115 Northeast Cutoff, Box 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000 www.allegromicro.com
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