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LT3466 Dual Full Function White LED Step-Up Converter with Built-In Schottky Diodes
FEATURES
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DESCRIPTIO
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s s s s s
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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 x 3mm) 10-Lead DFN Package
LT(R)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. 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 x 3mm x 0.75mm) DFN package.
, LTC and LT are registered trademarks of Linear Technology Corporation.
APPLICATIO S
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Main/Sub Displays Digital Cameras, Sub-Notebook PCs PDAs, Handheld Computers Automotive
TYPICAL APPLICATIO
3V TO 5V 1F 47H SW1 LED1 2.2F LT3466 FB1 CTRL1 10 SHUTDOWN AND DIMMING CONTROL 1 FB2 RT GND CTRL2 SHUTDOWN AND DIMMING CONTROL 2 10
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85 80
47H
75
VIN
SW2 VOUT2 2.2F LED2
EFFICIENCY (%)
VOUT1
70 65 60 55 50
63.4k
0
Figure 1. Li-Ion Powered Driver for 8/8 White LEDs
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Conversion Efficiency
VIN = 3.6V 8/8 LEDs 5 10 LED CURRENT (mA)
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15
20
1
LT3466
ABSOLUTE
(Note 1)
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RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW VOUT1 SW1 VIN SW2 VOUT2 1 2 3 4 5 11 10 FB1 9 CTRL1 8 RT 7 CTRL2 6 FB2
Input Voltage (VIN) ................................................... 24V SW1, SW2 Voltages ................................................ 44V VOUT1, VOUT2 Voltages ............................................. 44V CTRL1, CTRL2 Voltages ........................................... 24V FB1, FB2, RT Voltages ................................................ 2V Operating Temperature Range ................ -40C to 85C Storage Temperature Range .................. -65C to 125C Junction Temperature .......................................... 125C
ORDER PART NUMBER LT3466EDD
DD PACKAGE 10-LEAD (3mm x 3mm) PLASTIC DFN
TJMAX = 125C, JA = 43C/W, JC = 2.96C/W EXPOSED PAD (PIN 11) IS GND MUST BE SOLDERED TO PCB
DD PART MARKING LBBH
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
PARAMETER Minimum Operating Voltage Maximum Operating Voltage FB1 Voltage FB2 Voltage FB1 Pin Bias Current FB2 Pin Bias Current Quiescent Current Switching Frequency Oscillator Frequency Range Nominal RT Pin Voltage Maximum Duty Cycle
The q denotes specifications that apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 3V, VCTRL1 = 3V, VCTRL2 = 3V, unless otherwise specified.
CONDITIONS MIN 2.7 24
q q
TYP
MAX
UNITS V V mV mV nA nA mA A MHz kHz V % % % mA mA mV mV
192 192
200 200 10 10 5 16
208 208 50 50 6 25 1.2 2000
VFB1 = 0.2V (Note 3) VFB2 = 0.2V (Note 3) VFB1 = VFB2 = 0.3V CTRL1 = CTRL2 = 0V RT = 48.7k RT = 48.7k RT = 48.7k RT = 20.5k RT = 267k
q
0.8 200
1 0.54
90
96 92 99 400 400 360 360 0.01 0.01 5 5
Converter 1 Current Limit Converter 2 Current Limit Converter 1 VCESAT Converter 2 VCESAT Switch 1 Leakage Current Switch 2 Leakage Current CTRL1 Voltage for Full LED Current CTRL2 Voltage for Full LED Current CTRL1 and CTRL2 Voltage to Shut Down Chip CTRL1, CTRL2 Pin Bias Current VOUT1 Overvoltage Threshold VOUT2 Overvoltage Threshold VCTRL1 = VCTRL2 = 1V ISW1 = 300mA ISW2 = 300mA VSW1 = 10V VSW2 = 10V
q q
320 320
1.8 1.8 50
q
8
10 42 42
12
2
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A A V V mV A V V
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LT3466
ELECTRICAL CHARACTERISTICS
PARAMETER Schottky 1 Forward Drop Schottky 2 Forward Drop Schottky 1 Reverse Leakage Schottky 2 Reverse Leakage Soft-Start Time (Switcher 1) Soft-Start Time (Switcher 2)
The q denotes specifications that apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 3V, VCTRL1 = 3V, VCTRL2 = 3V, unless otherwise specified.
CONDITIONS ISCHOTTKY1 = 300mA ISCHOTTKY2 = 300mA VOUT1 = 20V VOUT2 = 20V 600 600 Note 3: Current flows out of the pin. MIN TYP 0.85 0.85 5 5 MAX UNITS V V A A s s
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 0C to 70C. Specifications over the -40C to 85C operating range are assured by design, characterization and correlation with statistical process controls.
TYPICAL PERFOR A CE CHARACTERISTICS
Switching Waveforms
VOUT1 50mV/DIV VSW1 20V/DIV IL1 100mA/DIV VOUT1 0.5V/DIV FEEDBACK VOLTAGE (mV) VCTRL1 2V/DIV IL1 200mA/DIV
0.5s/DIV VIN = 3.6V CIRCUIT OF FIGURE 1
UW
Transient Response
250
VFB1,2 vs VCTRL1,2
VIN = 3V TA = 25C
200
150
100
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5s/DIV VIN = 3.6V ILED1 = 20mA TO 10mA CIRCUIT OF FIGURE 1
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50
0
0
1 0.5 1.5 CONTROL VOLTAGE (V)
2
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LT3466 TYPICAL PERFOR A CE CHARACTERISTICS
Switch Saturation Voltage (VCESAT)
450
SWITCH SATURATION VOLTAGE (mV)
400 350
TA = 25C VCE1, VCE2
CURRENT LIMIT (mA)
300 250 200 150 100 50 0 0 50 100 150 200 250 300 350 400 SWITCH CURRENT (mA)
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350 300 250 200 150 100 50 0 0 20
TA = 85C
SHUTDOWN CURRENT (A)
Open-Circuit Output Clamp Voltage
44 TA = 25C RT = 48.7k
OUTPUT CLAMP VOLTAGE (V)
OUTPUT CLAMP VOLTAGE (V)
42 41 40 39 38 37 36
INPUT CURRENT (mA)
43
42
VOUT2 VOUT1
41
40
2
4
6
8 10 12 14 16 18 20 22 24 VIN (V)
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RT vs Oscillator Frequency
1000 OSCILLATOR FREQUENCY (kHz)
1200
RT (k)
100
10 200
600 1000 1400 1800 OSCILLATOR FREQUENCY (kHz)
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4
UW
Switch Current Limit vs Duty Cycle
500 450 400 TA = -50C TA = 25C 100 90 80 70 60 50 40 30 20 10 60 40 DUTY CYCLE (%) 80 100
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Shutdown Quiescent Current (CTRL1 = CTRL2 = 0V)
TA = -50C TA = 25C TA = 100C
0
2
4
6
8 10 12 14 16 18 20 22 24 VIN (V)
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Open-Circuit Output Clamp Voltage
45 VIN = 3.6V 44 RT = 48.7k VOUT2 VOUT1
Input Current with Output 1 and Output 2 Open Circuit
12 10 8 6 4 2 0 TA = 25C RT = 48.7k
43
35 -50
50 0 TEMPERATURE (C)
100
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2
4
6
8 10 12 14 16 18 20 22 24 VIN (V)
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Oscillator Frequency vs VIN
RT = 48.7k
1100
1000
900
800
2
4
6
8 10 12 14 16 18 20 22 24 VIN (V)
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LT3466 TYPICAL PERFOR A CE CHARACTERISTICS
Oscillator Frequency vs Temperature
2500 2250
OSCILLATOR FREQUENCY (kHz) 6
VIN = 3.6V
QUIESCENT CURRENT (mA)
2000 1750 1500 1250 1000 750 500 -50
Schottky Forward Voltage Drop
400
TA = 25C
SCHOTTKY FORWARD CURRENT (mA)
350 300 250 200 150 100 50 0 0 800 600 SCHOTTKY FORWARD DROP (mV) 200 400 1000
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SCHOTTKY LEAKAGE CURRENT (A)
UW
0
Quiescent Current (CTRL1 = CTRL2 = 3V)
TA = 25C
RT = 20.5k
5 4 3 2 1 0
RT = 48.7k
50
100
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0
4
8
TEMPERATURE (C)
12 VIN (V)
16
20
24
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Schottky Leakage Current
6 5 4 3 VR = 40V 2 VR = 20V 1 0 -50
0 50 TEMPERATURE (C)
100
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LT3466
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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 1F, X5R or X7R type ceramic capacitor. SW2 (Pin 4): Switch Pin for Converter 2. Connect the inductor at this pin. 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. 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 : ILED2 (200mV/RFB2), when VCTRL2 > 1.6V ILED2 (VCTRL2/5 * RFB2), when VCTRL2 < 1V 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. 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. 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 : ILED1 (200mV/RFB1), when VCTRL1 > 1.6V ILED1 (VCTRL1/5 * RFB1), when VCTRL1 < 1V Exposed Pad (Pin 11): The Exposed Pad must be soldered to the PCB system ground.
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VIN C1 L1 RT L2
2 4 VIN SW2 SW1 RT
8 3
1
VOUT1
VOUT2 OVERVOLT DETECTION DRIVER
5 C3
C2 OSC DRIVER OSC Q1 Q2 RAMP GEN PWM LOGIC
OVERVOLT DETECTION
PWM LOGIC
+ +
A3 RSNS1 A3
OSC
RSNS2
OSC
- -
PWM COMP A2 EA REF 1.25V A1 SHDN 0.2V 0.2V A1

PWM COMP
A2
CONVERTER 1
+ + -
+ + -
10 20k 80k
FB1 START-UP CONTROL CTRL1 9 80k 20k EXPOSED PAD 7 CTRL2 11
RFB1
Figure 2. LT3466 Block Diagram
-
EA
CONVERTER 2
FB2 6 RFB2
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LT3466
BLOCK DIAGRA
+
-
+
7
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LT3466
OPERATIO
Main Control Loop 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. 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.
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Minimum Output Current 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 300 resistor across the inductors, although this will degrade efficiency.
VOUT1 10mV/DIV VSW1 20V/DIV IL1 50mA/DIV 0.5s/DIV VIN = 3.6V ILED1 = 2.5mA CIRCUIT OF FIGURE 1
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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.
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LT3466
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. 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 IL1 500mA/DIV VSW2 50V/DIV IL2 200mA/DIV VIN = 3.6V CIRCUIT OF FIGURE 1 (8/8 LEDs) 1s/DIV
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Figure 4. Output 1 Open-Circuit Waveforms
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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 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 VOUT1 5V/DIV VFB1 200mV/DIV CRTL1 2V/DIV VIN = 3.6V 4 LEDs, 20mA L = 15H C = 0.47F 100s/DIV
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Figure 5. Start-Up Waveforms
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LT3466
APPLICATIO S I FOR ATIO
DUTY CYCLE
The duty cycle for a step-up converter is given by:
D=
where:
VOUT + VD - VIN VOUT + VD - VCESAT
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
1000
EFFICIENCY (%)
RT (k)
100
10 200
600 1000 1400 1800 OSCILLATOR FREQUENCY (kHz)
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Figure 6. Timing Resistor (RT) Value
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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 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.
85 CIRCUIT OF FIGURE 1 VIN = 3.6V 80 8/8 LEDs 75 70 RT = 20.5k 65 60 55 50 0 5 10 LED CURRENT (mA)
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RT = 63.4k RT = 39.1k
15
20
Figure 7. Efficiency Comparison for Different RT Resistors
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LT3466
APPLICATIO S I FOR ATIO
INDUCTOR 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 inductor current ripple (IL), neglecting the drop across the Schottky diode and the switch, is given by :
IL =
where:
VIN(MIN) * VOUT(MAX) - VIN(MIN) VOUT(MAX) * f * L
(
)
L = Inductor f = Operating frequency 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 10H to 68H. 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 (H) 10 15 33 33 68 33 47 68 15 33 68 MAX DCR () 0.44 0.58 1.00 0.38 0.52 0.45 0.73 0.40 0.22 0.51 0.84 CURRENT RATING (mA) 300 300 310 600 500 440 360 400 0.35A 0.31A 0.43A
PART LQH32CN100 LQH32CN150 LQH43CN330 ELL6RH330M ELL6SH680M A914BYW330M A914BYW470M A920CY680M CDRH2D18150NC CDRH4D18-330 CDRH5D18-680
VENDOR Murata (814) 237-1431 www.murata.com Panasonic (714) 373-7939 www.panasonic.com Toko www.toko.com Sumida (847) 956-0666 www.sumida.com
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CAPACITOR SELECTION 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 1F input capacitor is sufficient for most applications. Always use a capacitor with sufficient voltage rating. 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
Taiyo Yuden AVX Murata (408) 573-4150 www.t-yuden.com (803) 448-9411 www.avxcorp.com (714) 852-2001 www.murata.com
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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 = where: =
VIN - 0.6 L 1 LCOUT
Table 3 gives inrush peak current for some component selections.
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LT3466
APPLICATIO S I FOR ATIO
Table 3. Inrush Peak Current
VIN (V) 5 5 5 5 9 12 L (H) 15 33 47 68 47 33 COUT (F) 0.47 1.00 2.2 1.00 0.47 0.22
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).
RFB1 = RFB2
200mV ILED1 200mV = ILED2
ILED1,2 (mA) 5 10 15 20 25 RFB1,2 () 40.2 20.0 13.3 10.0 8.06
Table 4. RFB1,2 Value Selection
PWM 10kHz TYP R1 10k C1 1F LT3466 CTRL1,2
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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.
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Using a DC Voltage
IP (A) 0.78 0.77 0.95 0.53 0.84 0.93
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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 100k.
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.
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LT3466
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.
2 AA CELLS 1.8V to 3V 3.3V 1F 0.1F L2 15H VIN SW2 VOUT2 LT3466 FB1 CTRL1 10 63.4k 1% RT FB2 CTRL2 10
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L1 15H SW1 VOUT1
1F
1F
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 L1 VIN 1 2 L2 3 4 5 11 10 9 8 7 6 RFB2 CTRL2 RT CTRL1
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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.
COUT2 GND
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Figure 10. Recommended Component Placement
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LT3466
TYPICAL APPLICATIO S
Li-Ion to 2/4 White LEDs
3V TO 5V CIN 1F L1 15H SW1 COUT1 1F VOUT1 LT3466 FB1 RFB1 10 CTRL1 RT FB2 CTRL2 38.3k 1% CIN: TAIYO YUDEN JMK107BJ105 COUT1: TAIYO YUDEN LMK212BJ105 COUT2: TAIYO YUDEN EMK212BJ474 L1, L2: MURATA LQH32CN150 RFB2 10
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L2 15H VIN SW2 VOUT2 COUT2 0.47F
EFFICIENCY (%)
Li-Ion to 5/5 White LEDs
3V TO 5V CIN 1F L1 15H SW1 COUT1 0.47F VOUT1 LT3466 FB1 RFB1 10 CTRL1 RT FB2 CTRL2 38.3k 1% CIN: TAIYO YUDEN JMK107BJ105 COUT1, COUT2: TAIYO YUDEN GMK212BJ474 L1, L2: MURATA LQH32CN150 RFB2 10
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L2 15H VIN SW2 VOUT2 COUT2 0.47F
EFFICIENCY (%)
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Conversion Efficiency
85 80 75 70 65 60 55 50 0 5 10 LED CURRENT (mA)
3466 TA01b
VIN = 3.6V 2/4 LEDs
15
20
Conversion Efficiency
85 80 75 70 65 60 55 50 0 5 10 LED CURRENT (mA)
3466 TA02b
VIN = 3.6V 5/5 LEDs
15
20
3466f
LT3466
TYPICAL APPLICATIO S
Li-Ion to 6/6 White LEDs
3V TO 5V CIN 1F L1 33H SW1 COUT1 1F VOUT1 LT3466 FB1 CTRL1 RFB1 10 RT FB2 CTRL2 63.4k 1% RFB2 10
3466 TA03a
L2 33H VIN SW2 VOUT2 COUT2 1F
EFFICIENCY (%)
CIN: TAIYO YUDEN JMK107BJ105 COUT1, COUT2: TAIYO YUDEN GMK316BJ105 L1, L2: TOKO A914BYW-330M
Li-Ion to 7/7 White LEDs
3V TO 5V CIN 1F L1 33H SW1 COUT1 1F VOUT1 LT3466 FB1 CTRL1 RT FB2 CTRL2 63.4k 1% VIN L2 33H SW2 VOUT2 COUT2 1F
85 80 75
EFFICIENCY (%)
RFB1 10
CIN: TAIYO YUDEN JMK107BJ105 COUT1, COUT2: TAIYO YUDEN GMK316BJ105 L1, L2: TOKO A914BYW-330M
U
Conversion Efficiency
85 80 75 70 65 60 55 50 0 5 10 LED CURRENT (mA)
3466 TA03b
VIN = 3.6V 6/6 LEDs
15
20
Conversion Efficiency
VIN = 3.6V 7/7 LEDs
70 65 60 55 50 0 5 10 LED CURRENT (mA) 15 20
3466 TA04b
RFB2 10
3466 TA04a
3466f
15
LT3466
TYPICAL APPLICATIO S
Li-Ion to 8/8 White LEDs
3V TO 5V CIN 1F L1 47H SW1 COUT1 2.2F VOUT1 LT3466 FB1 CTRL1 RT FB2 CTRL2 63.4k 1% RFB1 10 RFB2 10
3466 TA05a
L2 47H VIN SW2 VOUT2 COUT2 2.2F
EFFICIENCY (%)
CIN: TAIYO YUDEN JMK107BJ105 COUT1, COUT2: TAIYO YUDEN GMK325BJ225 L1, L2: TOKO A918CE-470M
Li-Ion to 9/9 White LEDs
3V TO 5V CIN 1F L1 68H SW1 COUT1 1F VOUT1 LT3466 FB1 CTRL1 RT FB2 CTRL2 147k 1%
60 0 90 85
EFFICIENCY (%)
VIN
SW2 VOUT2 COUT2 1F
RFB1 16.5
CIN: TAIYO YUDEN JMK107BJ105 COUT1, COUT2: TAIYO YUDEN UMK325BJ105 L1, L2: TOKO A920CY-680M
16
U
Conversion Efficiency
85
80 VIN = 3.6V 8/8 LEDs
75 70
65 60 55 50
0
5
10
15
20
3466 TA05b
LED CURRENT (mA)
Conversion Efficiency
VIN = 3.6V 9/9 LEDs
L2 68H
80 75 70 65
4
8
12
3466 TA06b
LED CURRENT (mA)
RFB2 16.5
3466 TA06a
3466f
LT3466
TYPICAL APPLICATIO S
Li-Ion to 10/10 White LEDs
3V TO 5V CIN 1F L1 68H SW1 COUT1 1F VOUT1 LT3466 FB1 CTRL1 RT FB2 CTRL2 147k 1%
60 0 4 8 12
3466 TA07b
L2 68H VIN SW2 VOUT2 COUT2 1F
EFFICIENCY (%)
RFB1 16.5
CIN: TAIYO YUDEN JMK107BJ105 COUT1, COUT2: TAIYO YUDEN UMK325BJ105 L1, L2: TOKO A920CY-680M
2 AA Cells to 2/2 White LEDs
3.3V VCC 1.8V TO 3V CIN1 0.1F
75
CIN2 1F
EFFICIENCY (%)
L1 15H SW1 VIN
SW2 VOUT2 COUT2 1F
COUT1 1F
VOUT1 LT3466 FB1 RFB1 10 CTRL1 RT
CTRL2 63.4k 1%
CIN1: TAIYO YUDEN EMK107BJ104 CIN2: TAIYO YUDEN JMK107BJ105 COUT1, COUT2: TAIYO YUDEN GMK316BJ105 L1, L2: MURATA LQH32CN150
U
Conversion Efficiency
90 85 80 75 70 65 VIN = 3.6V 10/10 LEDs
LED CURRENT (mA)
RFB2 16.5
3466 TA07a
Conversion Efficiency
VIN = 2.4V 2/2 LEDs
70
L2 15H
65
60
FB2 RFB2 10
3466 TA08a
55
50 0 5 10 LED CURRENT (mA)
3466 TA08b
15
20
3466f
17
LT3466
TYPICAL APPLICATIO S
2 Li-Ion Cells to 10/10 White LEDs
6V TO 9V CIN 1F 90 85 80
EFFICIENCY (%)
L1 47H SW1 COUT1 0.47F VOUT1 LT3466 FB1 CTRL1 RT VIN
SW2 VOUT2 COUT2 0.47F
FB2 CTRL2 63.4k 1%
RFB1 10
CIN: TAIYO YUDEN LMK212BJ105 COUT1, COUT2: TAIYO YUDEN UMK316BJ474 L1, L2: TOKO A914BYW-470M
18
U
Conversion Efficiency
VIN = 7V 10/10 LEDs
L2 47H
75 70 65 60 55 50 0 5 10 LED CURRENT (mA)
3466 TA09b
15
20
RFB2 10
3466 TA09a
3466f
LT3466
PACKAGE DESCRIPTIO
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) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS R = 0.115 TYP 6 0.38 0.10 10
PIN 1 TOP MARK (SEE NOTE 5) 5 0.200 REF 0.75 0.05 2.38 0.10 (2 SIDES) 1
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
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.
U
DD Package 10-Lead Plastic DFN (3mm x 3mm)
(Reference LTC DWG # 05-08-1699)
0.675 0.05 3.00 0.10 (4 SIDES) 1.65 0.10 (2 SIDES)
(DD10) DFN 0403
0.25 0.05 0.50 BSC
0.00 - 0.05
BOTTOM VIEW--EXPOSED PAD
3466f
19
LT3466
TYPICAL APPLICATIO
C2 0.1F L1 33H C3 0.1F
12V to 25/25 White LEDs
VIN 12V
85
D5 VLED1 D6 C4 0.1F D7 25 LEDs C5 0.1F D8
L2 33H C8 0.1F
D2 C9 0.1F D3 D4 C10 0.1F 25 LEDs
EFFICIENCY (%)
C6 0.22F
RFB1 13.3
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
RELATED PARTS
PART NUMBER LT1618 LT1932 LT1937 LTC3200 LTC3200-5 LTC3201 LTC3202 LTC3205 LT3465/LT3465A DESCRIPTION Constant Current, Constant Voltage 1.24MHz, High Efficiency Boost Regulator Constant Current, 1.2MHz, High Efficiency White LED Boost Regulator Constant Current, 1.2MHz, High Efficiency White LED Boost Regulator Low Noise, 2MHz, Regulated Charge Pump White LED Driver Low Noise, 2MHz, Regulated Charge Pump White LED Driver Low Noise, 1.7MHz, Regulated Charge Pump White LED Driver Low Noise, 1.5MHz, Regulated Charge Pump White LED Driver High Efficiency, Multidisplay LED Controller Constant Current, 1.2MHz/2.7MHz, High Efficiency White LED Boost Regulator with Integrated Schottky Diode COMMENTS Up to 16 White LEDs, VIN: 1.6V to 18V, VOUT(MAX) = 34V, IQ = 1.8mA, ISD < 1A, MS Package Up to 8 White LEDs, VIN: 1V to 10V, VOUT(MAX) = 34V, IQ = 1.2mA, ISD < 1A, ThinSOTTM Package Up to 4 White LEDs, VIN: 2.5V to 10V, VOUT(MAX) = 34V, IQ = 1.9mA, ISD < 1A, ThinSOT, SC70 Packages Up to 6 White LEDs, VIN: 2.7V to 4.5V, IQ = 8mA, ISD < 1A, MS Package Up to 6 White LEDs, VIN: 2.7V to 4.5V, IQ = 8mA, ISD < 1A, ThinSOT Package Up to 6 White LEDs, VIN: 2.7V to 4.5V, IQ = 6.5mA, ISD < 1A, MS Package Up to 8 White LEDs, VIN: 2.7V to 4.5V, IQ = 5mA, ISD < 1A, MS Package Up to 4 (Main), 2 (Sub) and RGB, VIN: 2.8V to 4.5V, IQ = 50A, ISD < 1A, QFN-24 Package Up to Six White LEDs, VIN: 2.7V to 16V, VOUT(MAX) = 34V, IQ = 1.9mA, ISD < 1A, ThinSOT Package
ThinSOT is a trademark of Linear Technology Corporation.
20
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 q FAX: (408) 434-0507
q
U
Conversion Efficiency
VIN = 12V 25/25 LEDs
CIN 1F
C7 0.1F
80
D1 VLED2
75 70 65 60 55
SW1 VOUT1
VIN
SW2 VOUT2 C11 0.22F
LT3466 FB1 CTRL1 RT FB2 CTRL2 20.5k 1%
50 0 10 5 LED CURRENT (mA) 15
3466 TA10b
RFB2 13.3
3466 TA10a
3466f LT/TP 0104 1K * PRINTED IN USA
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2004

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