LT3466 Dual Full Function White LED Step-Up Converter with Built-In Schottky Diodes U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO Drives Up to 20 White LEDs (10 in Series per Converter) from a 3.6V Supply Two Independent Step-Up Converters Capable of Driving Asymmetric LED Strings Independent Dimming and Shutdown Control of the Two LED Strings Internal Schottky Diodes Internal Soft-Start Eliminates Inrush Current Open LED Protection (42V Max VOUT) Fixed Frequency Operation Up to 2MHz 81% Efficiency Driving 16 White LEDs at 15mA (Eight per Driver) from a 3.6V Supply Wide Input Voltage Range: 2.7V to 24V Tiny (3mm × 3mm) 10-Lead DFN Package U APPLICATIO S ■ ■ ■ Main/Sub Displays Digital Cameras, Sub-Notebook PCs PDAs, Handheld Computers Automotive The two independent converters are capable of driving asymmetric LED strings. The dimming of the two LED strings can also be controlled independently. The LT3466 is ideal for providing backlight for main and sub-displays in cell phones and other handheld devices. The LT3466 operating frequency can be set with an external resistor over a 200kHz to 2MHz range. A low 200mV feedback voltage minimizes power loss in the current setting resistor for better efficiency. Additional features include output voltage limiting when LEDs are disconnected and internal soft-start. The LT3466 is available in a low profile, small footprint (3mm × 3mm × 0.75mm) DFN package. , LTC and LT are registered trademarks of Linear Technology Corporation. U ■ LT®3466 is a dual full function step-up DC/DC converter specifically designed to drive up to 20 White LEDs (10 in series per converter) with a constant current. Series connection of the LEDs provides identical LED currents resulting in uniform brightness and eliminating the need for ballast resistors and expensive factory calibration. TYPICAL APPLICATIO 3V TO 5V Conversion Efficiency 85 1µF VIN = 3.6V 8/8 LEDs 80 47µH 47µH LED1 VIN SW2 LED2 VOUT2 VOUT1 2.2µF 2.2µF LT3466 FB1 10Ω SHUTDOWN AND DIMMING CONTROL 1 RT GND CTRL2 63.4k SHUTDOWN AND DIMMING CONTROL 2 70 65 60 FB2 CTRL1 EFFICIENCY (%) 75 SW1 55 10Ω 3466 F01a 50 0 5 10 15 20 LED CURRENT (mA) 3466 F01b Figure 1. Li-Ion Powered Driver for 8/8 White LEDs 3466f 1 LT3466 W W W AXI U U ABSOLUTE RATI GS U U W PACKAGE/ORDER I FOR ATIO (Note 1) Input Voltage (VIN) ................................................... 24V SW1, SW2 Voltages ................................................ 44V VOUT1, VOUT2 Voltages ............................................. 44V CTRL1, CTRL2 Voltages ........................................... 24V FB1, FB2, RT Voltages ................................................ 2V Operating Temperature Range ................ –40°C to 85°C Storage Temperature Range .................. –65°C to 125°C Junction Temperature .......................................... 125°C ORDER PART NUMBER TOP VIEW 10 FB1 VOUT1 1 SW1 2 VIN 3 SW2 4 7 CTRL2 VOUT2 5 6 FB2 LT3466EDD 9 CTRL1 11 8 RT DD PART MARKING DD PACKAGE 10-LEAD (3mm × 3mm) PLASTIC DFN LBBH TJMAX = 125°C, θJA = 43°C/W, θJC = 2.96°C/W EXPOSED PAD (PIN 11) IS GND MUST BE SOLDERED TO PCB Consult LTC Marketing for parts specified with wider operating temperature ranges. ELECTRICAL CHARACTERISTICS The ● denotes specifications that apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 3V, VCTRL1 = 3V, VCTRL2 = 3V, unless otherwise specified. PARAMETER CONDITIONS MIN Minimum Operating Voltage TYP MAX 2.7 UNITS V Maximum Operating Voltage 24 V FB1 Voltage ● 192 200 208 mV FB2 Voltage ● 192 200 208 mV FB1 Pin Bias Current VFB1 = 0.2V (Note 3) 10 50 nA FB2 Pin Bias Current VFB2 = 0.2V (Note 3) 10 50 nA Quiescent Current VFB1 = VFB2 = 0.3V CTRL1 = CTRL2 = 0V 5 16 6 25 mA µA Switching Frequency RT = 48.7k 1 1.2 MHz 2000 kHz 0.8 Oscillator Frequency Range 200 Nominal RT Pin Voltage RT = 48.7k Maximum Duty Cycle RT = 48.7k RT = 20.5k RT = 267k 0.54 V ● 90 96 92 99 % % % Converter 1 Current Limit ● 320 400 mA Converter 2 Current Limit ● 320 400 mA Converter 1 VCESAT ISW1 = 300mA 360 mV Converter 2 VCESAT ISW2 = 300mA 360 mV Switch 1 Leakage Current VSW1 = 10V 0.01 5 µA Switch 2 Leakage Current VSW2 = 10V 0.01 5 µA CTRL1 Voltage for Full LED Current 1.8 V CTRL2 Voltage for Full LED Current 1.8 V CTRL1 and CTRL2 Voltage to Shut Down Chip CTRL1, CTRL2 Pin Bias Current VCTRL1 = VCTRL2 = 1V ● 8 10 50 mV 12 µA VOUT1 Overvoltage Threshold 42 V VOUT2 Overvoltage Threshold 42 V 3466f 2 LT3466 ELECTRICAL CHARACTERISTICS The ● denotes specifications that apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 3V, VCTRL1 = 3V, VCTRL2 = 3V, unless otherwise specified. PARAMETER CONDITIONS Schottky 1 Forward Drop ISCHOTTKY1 = 300mA MIN 0.85 TYP MAX UNITS V Schottky 2 Forward Drop ISCHOTTKY2 = 300mA 0.85 V Schottky 1 Reverse Leakage VOUT1 = 20V 5 µA Schottky 2 Reverse Leakage VOUT2 = 20V 5 µA Soft-Start Time (Switcher 1) 600 µs Soft-Start Time (Switcher 2) 600 µs Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: The LTC3466E is guaranteed to meet specified performance from 0°C to 70°C. Specifications over the –40°C to 85°C operating range are assured by design, characterization and correlation with statistical process controls. Note 3: Current flows out of the pin. U W TYPICAL PERFOR A CE CHARACTERISTICS Switching Waveforms Transient Response VFB1,2 vs VCTRL1,2 VOUT1 0.5V/DIV VSW1 20V/DIV VCTRL1 2V/DIV 200 IL1 100mA/DIV IL1 200mA/DIV 0.5µs/DIV VIN = 3.6V CIRCUIT OF FIGURE 1 3466 G01 5µs/DIV VIN = 3.6V ILED1 = 20mA TO 10mA CIRCUIT OF FIGURE 1 3466 G02 FEEDBACK VOLTAGE (mV) VOUT1 50mV/DIV 250 VIN = 3V TA = 25°C 150 100 50 0 0 1 0.5 1.5 CONTROL VOLTAGE (V) 2 3466 G03 3466f 3 LT3466 U W TYPICAL PERFOR A CE CHARACTERISTICS Switch Saturation Voltage (VCESAT) 500 TA = –50°C 450 400 350 300 250 200 150 300 250 200 150 100 50 50 0 0 100 150 200 250 300 350 400 SWITCH CURRENT (mA) 50 0 20 60 40 DUTY CYCLE (%) 80 50 40 30 20 0 100 12 43 6 8 10 12 14 16 18 20 22 24 VIN (V) 42 41 TA = 25°C RT = 48.7k 10 VOUT2 INPUT CURRENT (mA) OUTPUT CLAMP VOLTAGE (V) 41 4 Input Current with Output 1 and Output 2 Open Circuit VIN = 3.6V 44 RT = 48.7k VOUT1 2 3466 G06 45 TA = 25°C RT = 48.7k 42 TA = 100°C 60 Open-Circuit Output Clamp Voltage VOUT2 TA = 25°C 70 3466 G05 Open-Circuit Output Clamp Voltage 43 TA = –50°C 80 10 3466 G04 44 90 TA = 85°C 350 100 0 100 TA = 25°C SHUTDOWN CURRENT (µA) TA = 25°C VCE1, VCE2 400 CURRENT LIMIT (mA) SWITCH SATURATION VOLTAGE (mV) 450 OUTPUT CLAMP VOLTAGE (V) Shutdown Quiescent Current (CTRL1 = CTRL2 = 0V) Switch Current Limit vs Duty Cycle VOUT1 40 39 38 37 8 6 4 2 36 40 2 4 6 8 10 12 14 16 18 20 22 24 VIN (V) 35 –50 0 50 0 TEMPERATURE (°C) 100 6 8 10 12 14 16 18 20 22 24 VIN (V) 3466 G09 Oscillator Frequency vs VIN RT vs Oscillator Frequency 1000 1200 OSCILLATOR FREQUENCY (kHz) RT (kΩ) 4 3466 G08 3466 G07 100 10 200 2 600 1000 1400 1800 OSCILLATOR FREQUENCY (kHz) 3466 G10 RT = 48.7k 1100 1000 900 800 2 4 6 8 10 12 14 16 18 20 22 24 VIN (V) 3466 G11 3466f 4 LT3466 U W TYPICAL PERFOR A CE CHARACTERISTICS Oscillator Frequency vs Temperature Quiescent Current (CTRL1 = CTRL2 = 3V) 6 2500 2250 5 RT = 20.5k 2000 QUIESCENT CURRENT (mA) OSCILLATOR FREQUENCY (kHz) TA = 25°C VIN = 3.6V 1750 1500 1250 RT = 48.7k 1000 4 3 2 1 750 0 500 –50 0 50 100 0 TEMPERATURE (°C) 4 8 12 VIN (V) 20 16 3466 G13 3466 G12 Schottky Forward Voltage Drop TA = 25°C SCHOTTKY LEAKAGE CURRENT (µA) SCHOTTKY FORWARD CURRENT (mA) Schottky Leakage Current 6 400 350 300 250 200 150 100 50 0 24 0 200 400 800 600 SCHOTTKY FORWARD DROP (mV) 1000 3466 G14 5 4 3 VR = 40V 2 VR = 20V 1 0 –50 0 50 TEMPERATURE (°C) 100 3466 G15 3466f 5 LT3466 U U U PI FU CTIO S VOUT1 (Pin 1): Output of Converter 1. This pin is connected to the cathode of the internal Schottky diode. Connect an output capacitor from this pin to ground. SW1 (Pin 2): Switch Pin for Converter 1. Connect the inductor at this pin. VIN (Pin 3): Input Supply Pin. Must be locally bypassed with a 1µF, X5R or X7R type ceramic capacitor. SW2 (Pin 4): Switch Pin for Converter 2. Connect the inductor at this pin. CTRL2 (Pin 7): Dimming and Shutdown Pin for Converter␣ 2. Connect this pin to ground to disable the converter. As the pin voltage is ramped from 0V to 1.6V, the LED current ramps from 0 to ILED2 (= 200mV/RFB2). Any voltage above 1.6V does not affect the LED current. RT (Pin 8): Timing Resistor to Program the Switching Frequency. The switching frequency can be programmed from 200KHz to 2MHz. VOUT2 (Pin 5): Output of Converter 2. This pin is connected to the cathode of the internal Schottky diode. Connect an output capacitor from this pin to ground. CTRL1 (Pin 9): Dimming and Shutdown Pin for Converter␣ 1. Connect this pin to ground to disable the converter. As the pin voltage is ramped from 0V to 1.6V, the LED current ramps from 0 to ILED1 (= 200mV/RFB1). Any voltage above 1.6V does not affect the LED current. FB2 (Pin 6): Feedback Pin for Converter 2. The nominal voltage at this pin is 200mV. Connect cathode of the lowest LED and the feedback resisitor at this pin. The LED current can be programmed by : FB1 (Pin 10): Feedback Pin for Converter 1. The nominal voltage at this pin is 200mV. Connect cathode of the lowest LED and the feedback resistor at this pin. The LED current can be programmed by : ILED2 ≈ (200mV/RFB2), when VCTRL2 > 1.6V ILED1 ≈ (200mV/RFB1), when VCTRL1 > 1.6V ILED2 ≈ (VCTRL2/5 • RFB2), when VCTRL2 < 1V ILED1 ≈ (VCTRL1/5 • RFB1), when VCTRL1 < 1V Exposed Pad (Pin 11): The Exposed Pad must be soldered to the PCB system ground. 3466f 6 C2 RFB1 10 1 FB1 PWM LOGIC PWM COMP A2 RSNS1 DRIVER CONVERTER 1 OSC OVERVOLT DETECTION SW1 Q1 2 – + VOUT1 L1 – + A3 EA A1 Σ 20k + + – C1 RT 9 VIN START-UP CONTROL REF 1.25V CTRL1 SHDN OSC 3 7 CTRL2 80k 0.2V Figure 2. LT3466 Block Diagram 80k 0.2V RAMP GEN OSC 8 RT 20k + + – Σ A1 EA A3 11 EXPOSED PAD – + Q2 4 A2 OSC PWM LOGIC VOUT2 FB2 OVERVOLT DETECTION CONVERTER 2 PWM COMP DRIVER RSNS2 SW2 L2 – + VIN 6 5 3466 F02 RFB2 C3 LT3466 BLOCK DIAGRA 3466f 7 W LT3466 U OPERATIO Main Control Loop Minimum Output Current The LT3466 uses a constant frequency, current mode control scheme to provide excellent line and load regulation. It incorporates two identical, but fully independent PWM converters. Operation can be best understood by referring to the Block Diagram in Figure 2. The oscillator, start-up bias and the bandgap reference are shared between the two converters. The control circuitry, power switch, Schottky diode etc., are all identical for both the converters. The LT3466 can drive an 8-LED string at 2.5mA LED current without pulse skipping. As current is further reduced, the device may begin skipping pulses. This will result in some low frequency ripple, although the LED current remains regulated on an average basis down to zero. The photo in Figure␣ 3 shows circuit operation with 16 white LEDs (eight per converter) at 2.5mA current driven from 3.6V supply. Peak inductor current is less than 50mA and the regulator operates in discontinuous mode implying that the inductor current reached zero during the discharge phase. After the inductor current reaches zero, the switch pin exhibits ringing due to the LC tank circuit formed by the inductor in combination with switch and diode capacitance. This ringing is not harmful; far less spectral energy is contained in the ringing than in the switch transitions. The ringing can be damped by application of a 300Ω resistor across the inductors, although this will degrade efficiency. At power-up, the output voltages VOUT1 and VOUT2 are charged up to VIN (input supply voltage) via their respective inductor and the internal Schottky diode. If either CTRL1 and CTRL2 or both are pulled high, the bandgap reference, start-up bias and the oscillator are turned on. Working of the main control loop can be understood by following the operation of converter 1. At the start of each oscillator cycle, the power switch Q1 is turned on. A voltage proportional to the switch current is added to a stabilizing ramp and the resulting sum is fed into the positive terminal of the PWM comparator A2. When this voltage exceeds the level at the negative input of A2, the PWM logic turns off the power switch. The level at the negative input of A2 is set by the error amplifier A1, and is simply an amplified version of the difference between the feedback voltage and the 200mV reference voltage. In this manner, the error amplifier A1 regulates the feedback voltage to 200mV reference voltage. The output of the error amplifier A1 sets the correct peak current level in inductor L1 to keep the output in regulation. The CTRL1 pin voltage is used to adjust the reference voltage. If only one of the converters is turned on, the other converter will stay off and its output will remain charged up to VIN (input supply voltage). The LT3466 enters into shutdown, when both CTRL1 and CTRL2 are pulled lower than 50mV. The CTRL1 and CTRL2 pins perform independent dimming and shutdown control for the two converters. VOUT1 10mV/DIV VSW1 20V/DIV IL1 50mA/DIV 0.5µs/DIV VIN = 3.6V ILED1 = 2.5mA CIRCUIT OF FIGURE 1 3466 F03 Figure 3. Switching Waveforms Open-Circuit Protection The LT3466 has internal open-circuit protection for both the converters. When the LEDs are disconnected from the circuit or fail open, the converter output voltage is clamped at 42V. The converter will then switch at a very low frequency to minimize the input current. Output voltage and input current during output open circuit are shown in the Typical Performance Characteristics graphs. 3466f 8 LT3466 U OPERATIO In the event one of the converters has an output opencircuit, its output voltage will be clamped at 42V. However, the other converter will continue functioning properly. The photo in Figure 4 shows circuit operation with converter 1 output open-circuit and converter 2 driving eight LEDs at 20mA. Converter 1 switches at a lower frequency, reducing its input current. The converter enters into soft-start mode whenever the respective CTRL pin is pulled from low to high. Figure 5 shows the start-up waveforms with converter 1 driving four LEDs at 20mA. The filtered input current, as shown in Figure 5, is well controlled. The soft-start circuit is less effective when driving a higher number of LEDs. Undervoltage Lockout Soft-Start The LT3466 has a separate internal soft-start circuitry for each converter. Soft-start helps to limit the inrush current during start-up. Soft-start is achieved by clamping the output of the error amplifier during the soft-start period. This limits the peak inductor current and ramps up the output voltage in a controlled manner. VSW1 50V/DIV The LT3466 has an undervoltage lockout circuit which shuts down both the converters when the input voltage drops below 2.1V (typ). This prevents the converter to operate in an erratic mode when powered from low supply voltages. IIN 100mA/DIV IL1 500mA/DIV VOUT1 5V/DIV VSW2 50V/DIV VFB1 200mV/DIV IL2 200mA/DIV CRTL1 2V/DIV VIN = 3.6V CIRCUIT OF FIGURE 1 (8/8 LEDs) 1µs/DIV 3466 F04 Figure 4. Output 1 Open-Circuit Waveforms VIN = 3.6V 4 LEDs, 20mA L = 15µH C = 0.47µF 100µs/DIV 3466 F05 Figure 5. Start-Up Waveforms 3466f 9 LT3466 U W U U APPLICATIO S I FOR ATIO DUTY CYCLE The duty cycle for a step-up converter is given by: D= VOUT + VD – VIN VOUT + VD – VCESAT current that flows into the timing resistor is used to charge and discharge an internal oscillator capacitor. A graph for selecting the value of RT for a given operating frequency is shown in the Figure 6. OPERATING FREQUENCY SELECTION where: VOUT = Output voltage VD = Schottky forward voltage drop VCESAT = Saturation voltage of the switch VIN = Input battery voltage The maximum duty cycle achievable for LT3466 is 96% (typ) when running at 1MHz switching frequency. It increases to 99% (typ) when run at 200kHz and drops to 92% (typ) at 2MHz. Always ensure that the converter is not duty-cycle limited when powering the LEDs at a given switching frequency. SETTING THE SWITCHING FREQUENCY The LT3466 uses a constant frequency architecture that can be programmed over a 200KHz to 2MHz range with a single external timing resistor from the RT pin to ground. The nominal voltage on the RT pin is 0.54V, and the The choice of operating frequency is determined by several factors. There is a tradeoff between efficiency and component size. Higher switching frequency allows the use of smaller inductors albeit at the cost of increased switching losses and decreased efficiency. Another consideration is the maximum duty cycle achievable. In certain applications, the converter needs to operate at the maximum duty cycle in order to light up the maximum number of LEDs. The LT3466 has a fixed oscillator off-time and a variable on-time. As a result, the maximum duty cycle increases as the switching frequency is decreased. The circuit of Figure 1 is operated with different values of timing resistor (RT). RT is chosen so as to run the converters at 800kHz (RT = 63.4k), 1.25MHz (RT = 39.1k) and 2MHz (RT = 20.5k). The efficiency comparison for different RT values is shown in Figure 7. 1000 85 CIRCUIT OF FIGURE 1 VIN = 3.6V 80 8/8 LEDs RT = 63.4k RT = 39.1k EFFICIENCY (%) RT (kΩ) 75 100 70 RT = 20.5k 65 60 55 10 200 600 1000 1400 1800 OSCILLATOR FREQUENCY (kHz) 3466 F06 Figure 6. Timing Resistor (RT) Value 50 0 5 10 15 20 LED CURRENT (mA) 3466 F07 Figure 7. Efficiency Comparison for Different RT Resistors 3466f 10 LT3466 U W U U APPLICATIO S I FOR ATIO INDUCTOR SELECTION CAPACITOR SELECTION The choice of the inductor will depend on the selection of switching frequency of LT3466. The switching frequency can be programmed from 200kHz to 2MHz. Higher switching frequency allows the use of smaller inductors albeit at the cost of increased switching losses. The small size of ceramic capacitors make them ideal for LT3466 applications. Use only X5R and X7R types because they retain their capacitance over wider voltage and temperature ranges than other types such as Y5V or Z5U. A 1µF input capacitor is sufficient for most applications. Always use a capacitor with sufficient voltage rating. The inductor current ripple (∆IL), neglecting the drop across the Schottky diode and the switch, is given by : ∆IL = ( VIN(MIN) • VOUT(MAX) – VIN(MIN) ) VOUT(MAX) • f • L Table 2 shows a list of several ceramic capacitor manufacturers. Consult the manufacturers for detailed information on their entire selection of ceramic parts. Table 2. Ceramic Capacitor Manufacturers where: L = Inductor f = Operating frequency Taiyo Yuden (408) 573-4150 www.t-yuden.com AVX (803) 448-9411 www.avxcorp.com Murata (714) 852-2001 www.murata.com VIN(MIN) = Minimum input voltage VOUT(MAX) = Maximum output voltage The ∆IL is typically set to 20% to 40% of the maximum inductor current. The inductor should have a saturation current rating greater than the peak inductor current required for the application. Also, ensure that the inductor has a low DCR (copper wire resistance) to minimize I2R power losses. Recommended inductor values range from 10µH to 68µH. Several inductors that work well with the LT3466 are listed in Table 1. Consult each manufacturer for more detailed information and for their entire selection of related parts. Table 1. Recommended Inductors L (µH) MAX DCR (Ω) CURRENT RATING (mA) LQH32CN100 LQH32CN150 LQH43CN330 10 15 33 0.44 0.58 1.00 300 300 310 Murata (814) 237-1431 www.murata.com ELL6RH330M ELL6SH680M 33 68 0.38 0.52 600 500 Panasonic (714) 373-7939 www.panasonic.com A914BYW330M A914BYW470M A920CY680M 33 47 68 0.45 0.73 0.40 440 360 400 Toko www.toko.com CDRH2D18150NC CDRH4D18-330 CDRH5D18-680 15 33 68 0.22 0.51 0.84 0.35A 0.31A 0.43A PART VENDOR INRUSH CURRENT The LT3466 has built-in Schottky diodes. When supply voltage is applied to the VIN pin, an inrush current flows through the inductor and the Schottky diode and charges up the output voltage. Both the Schottky diodes in the LT3466 can sustain a maximum of 1A current. The selection of inductor and capacitor value should ensure the peak of the inrush current to be below 1A. For low DCR inductors, which is usually the case for this application, the peak inrush current can be simplified as follows: IPK = VIN – 0.6 ωL where: ω= 1 LCOUT Table 3 gives inrush peak current for some component selections. Sumida (847) 956-0666 www.sumida.com 3466f 11 LT3466 U W U U APPLICATIO S I FOR ATIO Table 3. Inrush Peak Current Using a DC Voltage VIN (V) L (µH) COUT (µF) IP (A) 5 15 0.47 0.78 5 33 1.00 0.77 5 47 2.2 0.95 5 68 1.00 0.53 9 47 0.47 0.84 12 33 0.22 0.93 Typically peak inrush current will be less than the value calculated above. This is due to the fact that the DC resistance in the inductor provides some damping resulting in a lower peak inrush current. PROGRAMMING LED CURRENT The LED current of each LED string can set independently by the choice of resistors RFB1 and RFB2 respectively (Figure 2). The feedback reference is 200mV. In order to have accurate LED current, precision resistors are preferred (1% is recommended). 200mV ILED1 200mV = ILED2 RFB1 = RFB2 For some applications, the preferred method of brightness control is a variable DC voltage to adjust the LED current. The CTRL1, CTRL2 pin voltage can be modulated to set the dimming of the respective LED string. As the voltage on the CTRL1, CTRL2 pin increases from 0V to 1.6V, the LED current increases from 0 to ILED1,2. As the CTRL1, CTRL2 pin voltage increases beyond 1.6V, it has no effect on the LED current. The LED current can be set by: ILED1,2 ≈ (200mV/RFB1,2), when VCTRL1,2 > 1.6V ILED1,2 ≈ (VCTRL1,2/5 • RFB1,2), when VCTRL1,2 < 1V Feedback voltage variation versus control voltage is given in the Typical Performance Characteristics graphs. Using a Filtered PWM Signal A variable duty cycle PWM can be used to control the brightness of the LED string. The PWM signal is filtered (Figure 8) by an RC network and fed to the CTRL1, CTRL2 pins. The corner frequency of R1, C1 should be much lower than the frequency of the PWM signal. R1 needs to be much smaller than the internal impedance in the CTRL pins, which is 100kΩ. Table 4. RFB1,2 Value Selection ILED1,2 (mA) RFB1,2 (Ω) 5 40.2 10 20.0 15 13.3 20 10.0 25 8.06 Most White LEDs are driven at maximum currents of 15mA to 20mA. DIMMING CONTROL There are two different types of dimming control circuits. The LED current in the two drivers can be set independently by modulating the CTRL1 and CTRL2 pins respectively. PWM 10kHz TYP LT3466 R1 10k CTRL1,2 C1 1µF 3466 F08 Figure 8. Dimming Control Using a Filtered PWM Signal LOW INPUT VOLTAGE APPLICATIONS The LT3466 can be used in low input voltage applications. The input supply voltage to LT3466 must be 2.7V or higher. However, the inductors can be run off a lower battery voltage. This technique allows the LEDs to be powered off two alkaline cells. Most portable devices have a 3.3V logic supply voltage which can be used to power the LT3466. The LEDs can be driven straight from the battery, resulting in higher efficiency. 3466f 12 LT3466 U W U U APPLICATIO S I FOR ATIO Figure 9 shows four LEDs being run off two AA cells. The battery is connected to the inductors and the chip is powered off 3.3V logic supply voltage. 3.3V 2 AA CELLS 1.8V to 3V 0.1µF 1µF L2 15µH L1 15µH SW1 VIN SW2 VOUT1 1µF VOUT2 1µF LT3466 FB1 CTRL1 FB2 RT CTRL2 10Ω 10Ω 63.4k 1% 3466 F09 BOARD LAYOUT CONSIDERATION As with all switching regulators, careful attention must be paid to the PCB board layout and component placement. To prevent electromagnetic interference (EMI) problems, proper layout of high frequency switching paths is essential. Minimize the length and area of all traces connected to the switching node pins (SW1 and SW2). Keep the feedback pins (FB1 and FB2) away from the switching nodes. The DFN package has an exposed paddle that must be connected to the system ground. The ground connection for the feedback resistors should be tied directly to the ground plane and not shared with any other component, except the RT resistor, ensuring a clean, noise-free connection. Recommended component placement is shown in the Figure 10. Figure 9. 2 AA Cells to Four White LEDs HIGH INPUT VOLTAGE APPLICATIONS The input voltage to LT3466 can be as high as 24V. This gives it the flexibility of driving a large number of LEDs when being run off a higher voltage. The maximum number of LEDs that can be driven is constrained by the converter output voltages being clamped at 42V. The LT3466 can be used to power 20 White LEDs (10 per converter) at 20mA when running off two Li-Ion cells in series. GND COUT1 RFB1 CIN 10 1 L1 9 2 VIN 3 L2 CTRL1 RT 11 8 4 7 5 6 CTRL2 RFB2 COUT2 3466 F10 GND Figure 10. Recommended Component Placement 3466f 13 LT3466 U TYPICAL APPLICATIO S Li-Ion to 2/4 White LEDs Conversion Efficiency 3V TO 5V 85 CIN 1µF VIN = 3.6V 2/4 LEDs 80 L2 15µH SW1 COUT1 1µF VIN SW2 VOUT2 VOUT1 LT3466 FB1 RFB1 10Ω EFFICIENCY (%) 75 L1 15µH COUT2 0.47µF RT 65 60 55 FB2 CTRL1 70 RFB2 10Ω CTRL2 38.3k 1% 50 0 5 10 15 20 LED CURRENT (mA) 3466 TA01a 3466 TA01b CIN: TAIYO YUDEN JMK107BJ105 COUT1: TAIYO YUDEN LMK212BJ105 COUT2: TAIYO YUDEN EMK212BJ474 L1, L2: MURATA LQH32CN150 Li-Ion to 5/5 White LEDs Conversion Efficiency 3V TO 5V 85 VIN = 3.6V 5/5 LEDs 80 CIN 1µF SW1 COUT1 0.47µF VIN SW2 VOUT2 VOUT1 LT3466 FB1 RFB1 10Ω CTRL1 EFFICIENCY (%) 75 L2 15µH L1 15µH COUT2 0.47µF CTRL2 38.3k 1% 65 60 55 FB2 RT 70 RFB2 10Ω 3466 TA02a 50 0 5 10 15 20 LED CURRENT (mA) 3466 TA02b CIN: TAIYO YUDEN JMK107BJ105 COUT1, COUT2: TAIYO YUDEN GMK212BJ474 L1, L2: MURATA LQH32CN150 3466f 14 LT3466 U TYPICAL APPLICATIO S Li-Ion to 6/6 White LEDs Conversion Efficiency 3V TO 5V 85 VIN = 3.6V 6/6 LEDs 80 CIN 1µF L2 33µH SW1 COUT1 1µF EFFICIENCY (%) 75 L1 33µH VIN SW2 VOUT1 VOUT2 LT3466 FB1 COUT2 1µF CTRL1 RT 65 60 55 FB2 RFB1 10Ω 70 CTRL2 50 0 RFB2 10Ω 63.4k 1% 5 10 15 20 LED CURRENT (mA) 3466 TA03b 3466 TA03a CIN: TAIYO YUDEN JMK107BJ105 COUT1, COUT2: TAIYO YUDEN GMK316BJ105 L1, L2: TOKO A914BYW-330M Li-Ion to 7/7 White LEDs Conversion Efficiency 3V TO 5V 85 CIN 1µF VIN = 3.6V 7/7 LEDs 80 SW1 COUT1 1µF VIN SW2 VOUT2 VOUT1 LT3466 FB1 CTRL1 EFFICIENCY (%) 75 L2 33µH L1 33µH COUT2 1µF 65 60 55 FB2 RT 70 CTRL2 50 0 RFB1 10Ω 63.4k 1% 5 10 15 20 LED CURRENT (mA) RFB2 10Ω 3466 TA04b 3466 TA04a CIN: TAIYO YUDEN JMK107BJ105 COUT1, COUT2: TAIYO YUDEN GMK316BJ105 L1, L2: TOKO A914BYW-330M 3466f 15 LT3466 U TYPICAL APPLICATIO S Li-Ion to 8/8 White LEDs Conversion Efficiency 3V TO 5V 85 VIN = 3.6V 8/8 LEDs 80 CIN 1µF SW1 COUT1 2.2µF EFFICIENCY (%) 75 L2 47µH L1 47µH VIN SW2 VOUT2 VOUT1 LT3466 FB1 COUT2 2.2µF RT 65 60 55 FB2 CTRL1 70 CTRL2 50 5 0 RFB1 10Ω 3466 TA05a Conversion Efficiency 3V TO 5V 90 CIN 1µF EFFICIENCY (%) COUT1 1µF SW2 VOUT1 VIN = 3.6V 9/9 LEDs 85 L2 68µH VIN 20 RFB2 10Ω Li-Ion to 9/9 White LEDs SW1 15 3466 TA05b CIN: TAIYO YUDEN JMK107BJ105 COUT1, COUT2: TAIYO YUDEN GMK325BJ225 L1, L2: TOKO A918CE-470M L1 68µH 10 LED CURRENT (mA) 63.4k 1% VOUT2 LT3466 COUT2 1µF 80 75 70 65 FB1 CTRL1 FB2 RT CTRL2 60 0 147k 1% RFB1 16.5Ω CIN: TAIYO YUDEN JMK107BJ105 COUT1, COUT2: TAIYO YUDEN UMK325BJ105 L1, L2: TOKO A920CY-680M 4 8 12 LED CURRENT (mA) 3466 TA06b RFB2 16.5Ω 3466 TA06a 3466f 16 LT3466 U TYPICAL APPLICATIO S Li-Ion to 10/10 White LEDs Conversion Efficiency 3V TO 5V 90 CIN 1µF L2 68µH VIN SW2 VOUT2 VOUT1 COUT1 1µF EFFICIENCY (%) 85 L1 68µH SW1 VIN = 3.6V 10/10 LEDs COUT2 1µF LT3466 80 75 70 65 FB1 CTRL1 FB2 RT 60 CTRL2 4 0 RFB1 16.5Ω RFB2 16.5Ω CIN: TAIYO YUDEN JMK107BJ105 COUT1, COUT2: TAIYO YUDEN UMK325BJ105 L1, L2: TOKO A920CY-680M 3466 TA07a Conversion Efficiency 3.3V CIN1 0.1µF L1 15µH SW1 VIN SW2 VOUT2 COUT2 1µF LT3466 FB1 RFB1 10Ω CTRL1 FB2 RT CTRL2 63.4k 1% CIN1: TAIYO YUDEN EMK107BJ104 CIN2: TAIYO YUDEN JMK107BJ105 COUT1, COUT2: TAIYO YUDEN GMK316BJ105 L1, L2: MURATA LQH32CN150 VIN = 2.4V 2/2 LEDs 70 L2 15µH VOUT1 COUT1 1µF 75 EFFICIENCY (%) CIN2 1µF 12 3466 TA07b 2 AA Cells to 2/2 White LEDs VCC 1.8V TO 3V 8 LED CURRENT (mA) 147k 1% 65 60 55 RFB2 10Ω 3466 TA08a 50 0 5 10 15 20 LED CURRENT (mA) 3466 TA08b 3466f 17 LT3466 U TYPICAL APPLICATIO S 2 Li-Ion Cells to 10/10 White LEDs Conversion Efficiency 90 6V TO 9V CIN 1µF SW1 COUT1 0.47µF 80 L2 47µH VIN SW2 VOUT1 VOUT2 LT3466 FB1 CTRL1 EFFICIENCY (%) L1 47µH COUT2 0.47µF 65 50 CTRL2 CIN: TAIYO YUDEN LMK212BJ105 COUT1, COUT2: TAIYO YUDEN UMK316BJ474 L1, L2: TOKO A914BYW-470M 70 55 0 5 10 15 20 LED CURRENT (mA) 63.4k 1% RFB1 10Ω 75 60 FB2 RT VIN = 7V 10/10 LEDs 85 3466 TA09b RFB2 10Ω 3466 TA09a 3466f 18 LT3466 U PACKAGE DESCRIPTIO DD Package 10-Lead Plastic DFN (3mm × 3mm) (Reference LTC DWG # 05-08-1699) 0.675 ±0.05 3.50 ±0.05 1.65 ±0.05 2.15 ±0.05 (2 SIDES) PACKAGE OUTLINE 0.25 ± 0.05 0.50 BSC 2.38 ±0.05 (2 SIDES) R = 0.115 TYP 6 RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 3.00 ±0.10 (4 SIDES) 0.38 ± 0.10 10 1.65 ± 0.10 (2 SIDES) PIN 1 TOP MARK (SEE NOTE 5) (DD10) DFN 0403 5 0.200 REF 1 0.75 ±0.05 0.00 – 0.05 0.25 ± 0.05 0.50 BSC 2.38 ±0.10 (2 SIDES) BOTTOM VIEW—EXPOSED PAD NOTE: 1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-2). CHECK THE LTC WEBSITE DATA SHEET FOR CURRENT STATUS OF VARIATION ASSIGNMENT 2. ALL DIMENSIONS ARE IN MILLIMETERS 3. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 4. EXPOSED PAD SHALL BE SOLDER PLATED 5. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE 3466f Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 19 LT3466 U TYPICAL APPLICATIO Conversion Efficiency 12V to 25/25 White LEDs VIN 12V C2 0.1µF VLED1 L1 33µH D6 C4 0.1µF D7 25 LEDs C5 0.1µF C7 0.1µF D2 C8 0.1µF C3 0.1µF C9 0.1µF D3 D4 D8 SW1 C6 0.22µF VIN SW2 VOUT1 VOUT2 LT3466 C10 0.1µF 25 LEDs RT 70 65 60 55 C11 0.22µF 50 FB2 CTRL1 75 0 FB1 RFB1 13.3Ω D1 VLED2 L2 33µH VIN = 12V 25/25 LEDs 80 EFFICIENCY (%) D5 85 CIN 1µF CTRL2 10 5 LED CURRENT (mA) RFB2 13.3Ω 15 3466 TA10b 3466 TA10a CIN: TAIYO YUDEN EMK316BJ105 C3-C5, C8-C10: TAIYO YUDEN UMK212BJ104 C2, C7: TAIYO YUDEN HMK316BJ104 C6, C11: TAIYO YUDEN UMK316BJ224 D1-D8: PHILIPS BAV99 L1, L2: MURATA LQH32CN330 20.5k 1% RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1618 Constant Current, Constant Voltage 1.24MHz, High Efficiency Boost Regulator Up to 16 White LEDs, VIN: 1.6V to 18V, VOUT(MAX) = 34V, IQ = 1.8mA, ISD < 1µA, MS Package LT1932 Constant Current, 1.2MHz, High Efficiency White LED Boost Regulator Up to 8 White LEDs, VIN: 1V to 10V, VOUT(MAX) = 34V, IQ = 1.2mA, ISD < 1µA, ThinSOTTM Package LT1937 Constant Current, 1.2MHz, High Efficiency White LED Boost Regulator Up to 4 White LEDs, VIN: 2.5V to 10V, VOUT(MAX) = 34V, IQ = 1.9mA, ISD < 1µA, ThinSOT, SC70 Packages LTC3200 Low Noise, 2MHz, Regulated Charge Pump White LED Driver Up to 6 White LEDs, VIN: 2.7V to 4.5V, IQ = 8mA, ISD < 1µA, MS Package LTC3200-5 Low Noise, 2MHz, Regulated Charge Pump White LED Driver Up to 6 White LEDs, VIN: 2.7V to 4.5V, IQ = 8mA, ISD < 1µA, ThinSOT Package LTC3201 Low Noise, 1.7MHz, Regulated Charge Pump White LED Driver Up to 6 White LEDs, VIN: 2.7V to 4.5V, IQ = 6.5mA, ISD < 1µA, MS Package LTC3202 Low Noise, 1.5MHz, Regulated Charge Pump White LED Driver Up to 8 White LEDs, VIN: 2.7V to 4.5V, IQ = 5mA, ISD < 1µA, MS Package LTC3205 High Efficiency, Multidisplay LED Controller Up to 4 (Main), 2 (Sub) and RGB, VIN: 2.8V to 4.5V, IQ = 50µA, ISD < 1µA, QFN-24 Package LT3465/LT3465A Constant Current, 1.2MHz/2.7MHz, High Efficiency White LED Boost Regulator with Integrated Schottky Diode Up to Six White LEDs, VIN: 2.7V to 16V, VOUT(MAX) = 34V, IQ = 1.9mA, ISD < 1µA, ThinSOT Package ThinSOT is a trademark of Linear Technology Corporation. 3466f 20 Linear Technology Corporation LT/TP 0104 1K • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com LINEAR TECHNOLOGY CORPORATION 2004