A8430 ALLEGRO
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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 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. SW GND FB VIN EN Part Number Package
Description
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 V Supply Current ISUP Active: ILOAD = 15 mA, VLOAD = 12 V 2.5 3.5 mA Shutdown (EN = 0 V) 0.1 µA Feedback Reference Voltage VREF 104 mV Feedback Input Current IFB nA Switch Current Limit ISWLIM 300 mA Switch Frequency FSW 0.8 1.2 1.6 MHz Switch Maximum Duty Cycle D Switch Saturation voltage VCE(SAT) 350 mV Switch Leakage Current ISL µA Enable Input Input Threshold Low VIL 0.4 V Input Threshold High VIH 1.5 V Input Leakage Leakage IIL µA 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 Driver S Ramp Generator
1.2 MHz
Σ R Q ELECTRICAL CHARACTERISTICS at TA = 25°C, VIN = 3 V (unless otherwise noted)
Worcester, Massachusetts 01615-0036 (508) 853-5000 www.allegromicro.com Data Sheet 26185.300C A8430 White LED Driver Constant Current Step-up Converter Operating Characteristics Using Typical Application Circuit (Schematic 1) LED Current (mA) Efficiency (%) VIN = 3 V VIN = 4 V Quiescent Current versus Input Voltage 0.5 1.0 1.5 2.0 2.5 VIN (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 –50 100 150 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) –50 100 150 –50 100 150 –50 100 150
Worcester, Massachusetts 01615-0036 (508) 853-5000 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 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 VIN (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 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) 19.1 9.53 7.87 6.34 4.75 100 110 120 IOUT (mA) PD (mW) Vin = 3V, 3 LED Vin = 5V, 3 LED Vin = 3V, 4 LED Vin = 5V, 4 LED Power Dissipation versus IOUT
Worcester, Massachusetts 01615-0036 (508) 853-5000 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 0.22µF VC 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 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 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 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 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 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 0.22µF 6.3Ω 6.3Ω Enable Conversion Efficiency for Two Parallel Strings 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 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 A Dimensions in millimeters U.S. Customary dimensions (in.) in brackets, for reference only A Pin index area B Exposed thermal pad C D E C D E 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 B 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 SW Internal power FET GND Ground FB Feedback input EN Enable input VIN Input supply
Worcester, Massachusetts 01615-0036 (508) 853-5000 www.allegromicro.com Data Sheet 26185.300C A8430 White LED Driver Constant Current Step-up Converter 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. Copyright©2003, 2004, 2005 AllegroMicrosystems, Inc.