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A8430
White LED Driver Constant Current Step-up Converter
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.
Data Sheet
26185.300C
A8430 MLPD
Approximate actual size
SW GND FB
1 2 3
5
VIN
4
EN
Same pad footprint as SOT-23-5 RJA = 50 C/W, see note 1, page 2
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 A shutdown current
ABSOLUTE MAXIMUM RATINGS
SW 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 number when ordering:
Part Number A8430EEK A8430EEK-T Package 5-pin, MLPD 5-pin, MLPD Description Surface Mount Lead-Free, Surface Mount
Data Sheet
26185.300C
White LED Driver Constant Current Step-up Converter
Functional Block Diagram
FB SW
A8430
VIN
VREF 1.25 V
95 mV
A1 RC CC A2 R S Q Driver
Ramp Generator EN
Enable
1.2 MHz Oscillator
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 VIL VIH Leakage - - IIL - 1.5 - - - - 0.4 - 1 V V A Symbol VIN ISUP VREF IFB ISWLIM FSW D VCE(SAT) ISL - Active: ILOAD = 15 mA, VLOAD = 12 V 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 3 mm x 3 mm MLP package for additional information.
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Data Sheet
26185.300C
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)
A8430
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) Efficiency (%)
Conversion Efficiency versus Current
90 85 80 75 70 65 60
200 150 100 50 0
-50 0 50 Temperature (C) 100 150
VIN = 3 V VIN = 4 V
0
5
10 LED Current (mA)
15
20
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Data Sheet
26185.300C
White LED Driver Constant Current Step-up Converter
Functional Description
A8430
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.
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.
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 22H
D1
Conversion Efficiency versus Input Voltage
95
Conversion Efficiency (%)
VIN C1 1F EN Li-ion 2.5V to 4.2V Enable A8430 GND
SW C2 0.22F FB
90 85 80 75 70 65
2 3 4 5 6 VIN (V) 7
3 LEDs 4 LEDs 5 LEDs
R1 6.3
8
9
10
Schematic 1. Typical application
Chart 1. Conversion efficiency when driving various quantities of LEDs in the typical application circuit
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Data Sheet
26185.300C
White LED Driver Constant Current Step-up Converter
A8430
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 (), such that: R1 = 95 mV/ ILOAD where ILOAD is the target load current (mA).
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
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.
PD (mW)
Target Load Current (ILOAD) (mA) 5 10 12 15 20
Sense Resistor (R1) () 19.1 9.53 7.87 6.34 4.75
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Data Sheet
26185.300C
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 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 Feedback modulation. By adding a voltage drop
A8430
1.2 MHz, the optimal inductor value for most cases is 22 H. The inductor should have low winding resistance, typically < 1 , 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
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 ().
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.
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
L1 22H
D1
VIN C1 1F EN Enable A8430 GND
SW
FB R2 5k R3 90k
C2 0.22F
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.
Li-ion 2.5V to 4.2V
VC
R1 6.3
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.
Schematic 2. Dimming control with dc voltage feedback modulation
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Data Sheet
26185.300C
White LED Driver Constant Current Step-up Converter
A8430
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.
L1 22 H
D1
VIN C1 1 F A843 EN Enable Li-ion 2.5 V to 4.2 V GND
SW
FB R2 5 k R3 90 k C3 100 nF
C2 0.22 F
VC(PMW)
R4 10 k
R1 6.3
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 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.
Li-ion 2.5V to 4.2V
Schematic 3. Dimming control with filtered PWM
L1 22H
D1 C3 2.2 nF C2 0.22F
VIN C1 1F A8430 EN Enable GND
SW
FB
R2 1k R3 5k R1 6.3
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.
Schematic 4. Soft start operation
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Data Sheet
26185.300C
White LED Driver Constant Current Step-up Converter
A8430
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.
Li-ion 2.5V to 4.2V
L1 22H
D1
VIN C1 1F A8430 EN Enable GND
SW C2 0.22F FB R2 1k R1 6.3
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.
Schematic 5. Overvoltage protection with Zener clamp
L1 22H
D1
VIN C1 1F Li-ion 2.5V to 4.2V A8430 EN Enable GND
SW C2 0.22F
FB
R1 6.3
R2 6.3
Schematic 6. 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
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Data Sheet
26185.300C
White LED Driver Constant Current Step-up Converter
Terminal List Table
Pin 1 2 3 4 5 Name SW GND FB EN VIN Function Internal power FET Ground Feedback input Enable input Input supply
A8430
Package EK
5
3.00 .118 BSC A
1
2
0.45 .018 0.30 .012 0.95 .037 BSC 0.50 .020 0.30 .012
0.80 .031 0.70 .028 0.20 .008 REF 0.05 .002 0.80 .031 0.00 .000 0.70 .028
1
2
R0.20 .008 REF
B
1.10 0.85
.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 0.75 .030 NOM 5 0.15 .006 MIN .083 .073
D Typical pad layout including solder pad for exposed thermal pad; adjust as necessary to meet application process requirements E Typical pad layout with contact pads only; adjust as necessary to meet application process requirements 0.50 .020 MIN
0.50 .020 MIN
3.40 .134 REF 0.20 REF 1 E 0.95 .037 BSC D 2.10 MAX .008 1
C
1.10 MAX
.043 3.40 .134 REF
0.95 .037 BSC .083
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9
Data Sheet
26185.300C
White LED Driver Constant Current Step-up Converter
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(c)2003, 2004, 2005 AllegroMicrosystems, Inc.
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