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LT3466-1 White LED Driver and Boost Converter in 3mm x 3mm DFN Package
FEATURES

DESCRIPTIO
Drives Up to 10 White LEDs from a 3.6V Supply Two Independent Step-Up DC/DC Converters Independent Dimming and Shutdown Control of the Outputs 1.5% Output Voltage Accuracy (Boost Converter) 4% LED Current Programming Accuracy Internal Schottky Diodes Internal Soft-Start Eliminates Inrush Current Output Overvoltage Protection (39.5V Max VOUT) Fixed Frequency Operation Up to 2MHz 83% Efficiency Driving 8 White LEDs at 15mA from a 3.6V Supply Wide Input Voltage Range: 2.7V to 24V Tiny (3mm x 3mm) 10-Lead DFN Package
LT(R)3466-1 is a dual switching regulator that combines a white LED driver and a boost converter in a low profile, small footprint (3mm x 3mm x 0.75mm) DFN package. The LED driver can be configured to drive up to 10 White LEDs in series and the boost converter can be used for generating the LCD bias voltages or driving a secondary OLED display. 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 LT3466-1 provides independent dimming and shutdown control of the two converters. The operating frequency can be set with an external resistor over a 200kHz to 2MHz range. The white LED driver features a low 200mV reference, thereby minimizing power loss in the current setting resistor for better efficiency. The boost converter achieves 1.5% output voltage accuracy by the use of a precision 0.8V reference. Protection features include output overvoltage protection and internal soft-start. Wide input supply range allows operation from 2.7V to 24V.
, LTC and LT are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners.
APPLICATIO S

White LED and OLED Displays Digital Cameras, Sub-Notebook PCs PDAs, Handheld Computers TFT - LCD Bias Supply Automotive
TYPICAL APPLICATIO
33H
3V TO 5V
33H
1F
90 85
SW1
VIN
SW2
80
EFFICIENCY (%)
75 70 65 60 55
6 LEDs 1F
VOUT1
LT3466-1
VOUT2 1F
16V 30mA 475k
FB1 CTRL1 10 SHUTDOWN AND DIMMING CONTROL 1 63.4k RT GND
FB2 CTRL2 SHUTDOWN AND DIMMING CONTROL 2
24.9k
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50
Figure 1. Li-Ion Powered Driver for 6 White LEDs and OLED Display
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Conversion Efficiency
VIN = 3.6V LED DRIVER BOOST CONVERTER 0 5 10 15 20 25 30 OUTPUT CURRENT (mA)
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1
LT3466-1
ABSOLUTE
(Note 1)
AXI U
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 Voltages ...................................................... 2V Operating Temperature Range (Note 2) ... -40C to 85C Storage Temperature Range .................. -65C to 125C Junction Temperature .......................................... 125C
ORDER PART NUMBER LT3466EDD-1
DD PACKAGE 10-LEAD (3mm x 3mm) PLASTIC DFN
DD PART MARKING LBRX
TJMAX = 125C, JA = 43C/W, JC = 3C/W EXPOSED PAD (PIN 11) IS GND MUST BE SOLDERED TO PCB
Order Options Tape and Reel: Add #TR Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF Lead Free Part Marking: http://www.linear.com/leadfree/ 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 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 22

TYP
MAX
UNITS V V mV mV nA nA mA A MHz kHz V % % % mA mA mV mV
192 788
200 800 10 10 5 16
208 812 50 50 7.5 25 1.25 2000
VFB1 = 0.2V (Note 3) VFB2 = 0.8V (Note 3) VFB1 = VFB2 = 1V CTRL1 = CTRL2 = 0V RT = 48.7k (Note 4) RT = 48.7k RT = 48.7k RT = 20.5k RT = 267k
0.75 200
1 0.54
90
96 92 99 400 400 320 320 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 Feedback Voltage CTRL1 or CTRL2 Voltage to Turn On the IC CTRL1 and CTRL2 Voltages to Shut Down Chip CTRL1 Pin Bias Current CTRL2 Pin Bias Current VCTRL1 = 1V VCTRL2 = 1V (Note 3) ISW1 = 300mA ISW2 = 300mA VSW1 = 10V VSW2 = 10V

310 310

1.8 1 150 70

6
9 10
12.5 120
2
U
A A V V mV mV A nA
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LT3466-1
ELECTRICAL CHARACTERISTICS
PARAMETER VOUT1 Overvoltage Threshold VOUT2 Overvoltage Threshold 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 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 TYP 39.5 39.5 ISCHOTTKY1 = 300mA ISCHOTTKY2 = 300mA VOUT1 = 20V VOUT2 = 20V 600 600 0.85 0.85 5 5 MAX UNITS V V 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 LTC3466-1E 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.
Note 3: Current flows out of the pin. Note 4: Guaranteed by design and test correlation, not production tested.
TYPICAL PERFOR A CE CHARACTERISTICS
Switching Waveforms (LED Driver)
VOUT1 100mV/DIV (AC-COUPLED) VSW1 20V/DIV IL1 100mA/DIV VIN = 3.6V 0.5s/DIV 6 LEDs AT 20mA CIRCUIT OF FIGURE 1
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VFB1 vs VCTRL1
250
VIN = 3.6V 6 LEDs 900
200
VFB1 (mV)
VFB2 (mV)
150
100
50
0
0
0.5
UW
1
TA = 25C unless otherwise specified
Switching Waveforms (Boost Converter)
VOUT2 100mV/DIV (AC-COUPLED) VSW2 20V/DIV IL2 100mA/DIV VIN = 3.6V 0.5s/DIV VOUT2 = 16V/30mA CIRCUIT OF FIGURE 1
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VFB2 vs VCTRL2
VIN = 3.6V 800 VOUT2 = 16V 700 600 500 400 300 200 100
1.5
2
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0
0
0.5
1 VCTRL2 (V)
1.5
2
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VCTRL1 (V)
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LT3466-1 TYPICAL PERFOR A CE CHARACTERISTICS TA = 25C unless otherwise specified
Switch Current Limit vs Duty Cycle
500 450 400
CURRENT LIMIT (mA)
TA = -50C
QUIESCENT CURRENT (mA)
350 300 250 200 150 100 50 0 0 20
TA = 85C
5 4 3 2 1 0
SHUTDOWN CURRENT (A)
60 40 DUTY CYCLE (%)
Open-Circuit Output Clamp Voltage
41.00 40.50 40.00 VOUT2 39.50 VOUT1 39.00 38.50 38.00 2 4 6 8 10 12 14 16 18 20 22 24 VIN (V)
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RT = 63.4k
OUTPUT CLAMP VOLTAGE (V)
OUTPUT CLAMP VOLTAGE (V)
40 VOUT1 39
INPUT CURRENT (mA)
RT vs Oscillator Frequency
1000
OSCILLATOR FREQUENCY (kHz)
RT (k)
100
10 200
600 1000 1400 1800 OSCILLATOR FREQUENCY (kHz)
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4
UW
TA = 25C 80
Quiescent Current (CTRL1 = CTRL2 = 3V)
7 6 UVLO
70 60 50 40
Shutdown Current (CTRL1 = CTRL2 = 0V)
TA = -50C
TA = 25C
TA = 100C 30 20 10 0 2 4 6 8 10 12 14 16 18 20 22 24 VIN (V)
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100
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0
4
8
12 VIN (V)
16
20
24
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Open-Circuit Output Clamp Voltage
42 RT = 63.4k
Input Current with Output 1 and Output 2 Open Circuit
20 RT = 63.4k
41 VOUT2
16
12
8
38
4
37 -50
-25
50 25 0 75 TEMPERATURE (C)
100
125
0
2
4
6
8 10 12 14 16 18 20 22 24 VIN (V)
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Oscillator Frequency vs VIN
1100 RT = 48.7k
1000
900
800 2 4 6 8 10 12 14 16 18 20 22 24 VIN (V)
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LT3466-1 TYPICAL PERFOR A CE CHARACTERISTICS TA = 25C unless otherwise specified
Oscillator Frequency vs Temperature
1100 VIN = 3.6V RT = 48.7k
OSCILLATOR FREQUENCY (kHz)
1000
CTRL VOLTAGE (mV)
900
800 -50
-25
Schottky Forward Voltage Drop
400
SCHOTTKY FORWARD CURRENT (mA)
350 300 250 200 150 100 50 0 0 800 600 SCHOTTKY FORWARD DROP (mV) 200 400 1000
SCHOTTKY LEAKAGE CURRENT (A)
FB2 Pin Voltage vs Temperature
0.810 0.805 0.800 0.795 0.790 0.785 0.780 -50 VIN = 3V VOUT2 = 16V/30mA 0
VOUT2/VOUT2 (%)
FB2 VOLTAGE (V)
-25
UW
CTRL Voltages to Shut Down the IC
150 125 CTRL1 100 CTRL2 75 50 25 0 -50 VIN = 3.6V
0 25 50 TEMPERATURE (C)
75
100
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-25
0 25 50 TEMPERATURE (C)
75
100
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Schottky Leakage Current
8
6
4 VR = 36V VR = 20V
2
0 -50
-25
0 25 50 TEMPERATURE (C)
75
100
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FB2 Pin Load Regulation
VIN = 3V VOUT2 = 16V
-0.20
-0.40
-0.60
-0.80
-1.00 75 0 25 50 TEMPERATURE (C) 100 125
0
10 20 LOAD CURRENT (mA)
30
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34661 G17
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LT3466-1
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 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 800mV. Connect the resistor divider to this pin. The feedback voltage can be programmed as: VFB2 VCTRL2, when VCTRL2 < 0.8V VFB2 = 0.8V, when VCTRL2 > 1V CTRL2 (Pin 7): Dimming and Shutdown Pin for Converter 2. As the pin voltage is ramped from 0V to 1V, the FB2 pin voltage tracks the CTRL2 voltage and ramps up to 0.8V. Any voltage above 1V does not affect the feedback voltage. Do not leave the pin floating. It must be connected to ground to disable converter 2. 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 1.8V, the LED current ramps from 0 to ILED1 (= 200mV/RFB1). Any voltage above 1.8V 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 (VCTRL1/5 * RFB1), when VCTRL1 < 1V ILED1 (200mV/RFB1), when VCTRL1 > 1.8V 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 0.8V A1
R1 PWM COMP

A2
CONVERTER 1
+ + -
+ + -
10 20k 80k
FB1 START-UP CONTROL CTRL1 9 CTRL2 7
RFB1
EXPOSED PAD 11
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Figure 2. Block Diagram
-
EA
CONVERTER 2
FB2 6 R2
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LT3466-1
BLOCK DIAGRA
+
-
+
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LT3466-1
OPERATIO
Main Control Loop The LT3466-1 uses a constant frequency, current mode control scheme to provide excellent line and load regulation. It incorporates two similar, 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 similar for both 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 voltage at the FB1 pin to 200mV. 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 feedback voltage. The working of converter 2 is similar to converter 1 with the exception that the feedback 2 reference voltage is 800mV. The error amplifier A1 in converter 2 regulates the voltage at the FB2 pin to 800mV. 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 LT3466-1 enters into shutdown, when both CTRL1 and CTRL2 are pulled lower than 70mV. The CTRL1 and CTRL2 pins perform independent dimming and shutdown control for the two converters.
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Minimum Output Current The LT3466-1 can drive a 6-LED string at 3mA 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 6 white LEDs at 3mA 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 20mV/DIV (AC-COUPLED) VSW1 20V/DIV IL1 50mA/DIV 0.5s/DIV VIN = 3.6V ILED1 = 3mA CIRCUIT OF FIGURE 1
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Figure 3. Switching Waveforms
Overvoltage Protection The LT3466-1 has internal overvoltage protection for both converters. In the event the white LEDs are disconnected from the circuit or fail open, the converter 1 output voltage is clamped at 39.5V (typ). Figure 4(a) shows the transient response of the circuit in Figure 1 with LED1 disconnected. With the white LEDs disconnected, the converter 1 starts switching at the peak current limit. The output of converter 1 starts ramping up and finally gets clamped at 39.5V (typ). The converter 1 will then switch at low inductor current to regulate the output voltage. Output voltage and input current during output open circuit are shown in the Typical Performance Characteristics graphs.
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LT3466-1
OPERATIO
In the event one of the converters has an output open-circuit, its output voltage will be clamped at 39.5V. However, the other converter will continue functioning properly. The photo in Figure 4b shows circuit operation with converter 1 output open-circuit and converter 2 driving the OLED display. Converter 1 starts switching at a lower inductor current and begins skipping pulses, thereby reducing its input current. Converter 2 continues functioning properly.
VOUT1 10V/DIV
IL1 200mA/DIV
LED1 DISCONNECTED AT THIS POINT VIN = 3.3V CIRCUIT OF FIGURE 1
Figure 4a. Transient Response of Switcher 1 with LED1 Disconnected from the Output
VSW1 50V/DIV IL1 100mA/DIV VSW2 50V/DIV IL2 100mA/DIV VIN = 3.6V 1s/DIV CIRCUIT OF FIGURE 1
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Figure 4b. Output 1 Open-Circuit Waveforms
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Soft-Start The LT3466-1 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. 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 six LEDs at 20mA. The filtered input current, as shown in Figure 5, is well controlled. The soft-start circuitry is less effective when driving a higher number of LEDs. Undervoltage Lockout
200s/DIV
34661 F04a
The LT3466-1 has an undervoltage lockout circuit which shuts down both converters when the input voltage drops below 2.1V (typ). This prevents the converter from switching in an erratic mode when powered from low supply voltages.
IIN 200mA/DIV VOUT1 20V/DIV VFB1 200mV/DIV CTRL1 5V/DIV VIN = 3.6V 200s/DIV 6 LEDs, 20mA CIRCUIT OF FIGURE 1
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Figure 5. Start-Up Waveforms
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LT3466-1
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 LT3466-1 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 or OLED at a given switching frequency. SETTING THE SWITCHING FREQUENCY The LT3466-1 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 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.
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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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 LT3466-1 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 = 38.3k) and 2MHz (RT = 20.5k). The efficiency comparison for different RT values is shown in Figure 7. INDUCTOR SELECTION The choice of the inductor will depend on the selection of switching frequency of LT3466-1. 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.
90 80
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CIRCUIT OF FIGURE 1 VIN = 3.6V 6 LEDs
RT = 63.4k
70
RT = 20.5k
RT = 38.3k
60
50
40
0
5
10
LED CURRENT (mA)
15
20
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Figure 7. Efficiency Comparison for Different RT Resistors
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LT3466-1
APPLICATIO S I FOR ATIO
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 LT3466-1 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
CAPACITOR SELECTION The small size of ceramic capacitors make them ideal for LT3466-1 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
U
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 LT3466-1 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 capacitor. Both Schottky diodes in the LT3466-1 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.
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 IP (A) 0.78 0.77 0.95 0.53 0.84 0.93
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LT3466-1
APPLICATIO S I FOR ATIO
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. SETTING THE LED CURRENT The current in the LED string can be set by the choice of the resistor RFB1 (Figure 1). The feedback reference is 200mV. In order to have accurate LED current, precision resistors are preferred (1% is recommended).
RFB1 = 200mV ILED1
ILED1 (mA) 5 10 15 20 25 RFB1 () 40.2 20.0 13.3 10.0 8.06
Table 4. RFB1 Value Selection
Most White LEDs are driven at maximum currents of 15mA to 20mA. DIMMING WHITE LEDS The LED current in the driver can be set by modulating the CTRL1 pin. There are two different ways to control the intensity of white LEDs. Using a DC Voltage For some applications, the preferred method of brightness control is a variable DC voltage to adjust the LED current. The CTRL1 pin voltage can be modulated to set the dimming of the LED string. As the voltage on the CTRL1 pin increases from 0V to 1.8V, the LED current increases from 0 to ILED1. As the CTRL1 pin voltage increases beyond 1.8V, it has no effect on the LED current. The LED current can be set by: ILED1 (VCTRL1/5 * RFB1), when VCTRL1 < 1V ILED1 (200mV/RFB1), when VCTRL1 > 1.8V
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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 pin. 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 pin, which is 100k.
R1 10k C1 1F LT3466-1 CTRL1
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PWM 10kHz TYP
Figure 8. Dimming Control Using a Filtered PWM Signal
SETTING THE BOOST OUTPUT VOLTAGE The LT3466-1 regulates the voltage at the FB2 pin to 0.8V. The output voltage of the boost converter (VOUT2) is set by a resistor divider according to the formula: R1 VOUT2 = 0.8V 1+ R2 Choose 1% resistors for better accuracy. The FB2 input bias current is quite low, on the order of 10nA (typ). Large resistor values (R1 ~ 1M) can be used in the divider network maximizing efficiency. PROGRAMMING THE BOOST OUTPUT VOLTAGE The output voltage of the boost converter can be modulated by applying a variable DC voltage at the CTRL2 pin The nominal voltage at the FB2 pin is 800mV. As the voltage on the CTRL2 pin is ramped from 0V to 1V, the FB2 pin voltage ramps up to 0.8V. The feedback voltage can be programmed as: VFB2 VCTRL2, when VCTRL2 < 0.8V VFB2 0.8V, when VCTRL2 > 1V
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LT3466-1
APPLICATIO S I FOR ATIO
Figure 9 shows the feedback voltage variation versus the control voltage. As seen in Figure 9, the linearity of the graph allows the feedback voltage to be set accurately via the control voltage. The boost converter output voltage (VOUT2) is given by: VOUT 2 R1 = VFB2 1+ R2
RFB1 10 OFF ON COUT1 1F L1 33H SW1 VOUT1
Thus a linear change in the feedback (FB2) voltage results in a linear change in the boost output voltage (VOUT2). Connect the CTRL2 pin to ground to disable converter 2. Do not leave the pin floating. Unlike the CTRL1 pin, which has an internal 100k pull-down resistor, the CTRL2 pin input impedance is very high (>100M). A small amount of board leakage current is sufficient to turn on the converter 2.
900 VIN = 3.6V 800 VOUT2 = 16V 700 600 500 400 300 200 100 0 0 0.4 0.8 1.2 VCTRL2 (V) 1.6 2
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VFB2 (mV)
Figure 9. VFB2 vs VCTRL2
OUTPUT DISCONNECT The LT3466-1 can be used for powering white LEDs (Channel 1) and an OLED display or, LCD bias (Channel 2). Some OLED displays require load isolation in order to reduce the current drained from the battery in shutdown. The LT3466-1 output can be configured to provide output disconnect by the use of only one resistor, RBASE, and a PNP transistor, Q1, as shown in Figure 10. As a design example, we target a Li-Ion powered driver for 6 white LEDs and an OLED display (16V at 30mA). We can choose a general purpose PNP switching transistor like Philips BC807 (Q1) to provide isolation.
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3V TO 5V CIN 1F L2 33H VIN SW2 VOUT2 LT3466-1 FB1 CTRL1 RT FB2 CTRL2 63.4k 1% OFF ON R2 24.9k
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IBASE
RBASE 16V 30mA COUT3 0.47F
Q1 + - VCE(SAT) R1 475k COUT2 0.47F
CIN: TAIYO YUDEN JMK107BJ105 COUT1: TAIYO YUDEN GMK316BJ105 COUT2, COUT3: TAIYO YUDEN TMK316BJ474 L1, L2: TOKO D52LC Q1: PHILIPS BC807
Figure 10. Li-Ion Powered Driver for 6 White LEDs and a Secondary OLED Display with Output Disconnect
The RBASE resistor can be calculated as: ILOAD = 30mA
IBASE =
ILOAD 0 . 4hFE(MIN)
IBASE must be chosen such that Q1 is in saturation under all conditions. The hFE(MIN) can be obtained from the Philips BC807 data sheet as: hFE(MIN) 100 This yields worst case IBASE as:
IBASE = 30mA 0 . 75mA 0 . 4(100)
RBASE is given by:
VIN(MAX ) + IBASE * RBASE + VBE(Q1) = VOUT 2 + VCE(Q1)
Thus; RBASE =
VOUT 2 - VIN(MAX ) + VCE(Q1) - VBE(Q1) IBASE
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LT3466-1
APPLICATIO S I FOR ATIO
The VCE(SAT) and VBE(SAT) values for the transistor Q1 can be obtained from the Philips BC807 data sheet:
RBASE =
16 V - 5V + 0 . 1 - 0 . 9 0 . 75mA
RBASE = 13.6k Picking the closest 1% resistor value yields: RBASE = 14k 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
GND
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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 11.
COUT1 RFB1
CIN L1 VIN
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1
2
10
9
11
RT
CTRL1
L2
3
4
5
8
7
6
R1
R2
CTRL2
COUT2 GND
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Figure 11. Recommended Component Placement
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LT3466-1
TYPICAL APPLICATIO S
Li-Ion Powered 4 White LEDs Driver and 12V Boost Converter
3V TO 5V CIN 1F L1 15H 4 LEDs COUT1 0.47F SW1 VOUT1 LT3466-1 FB1 RFB1 10 OFF ON CIN: TAIYO YUDEN JMK107BJ105 COUT1: TAIYO YUDEN EMK212BJ474 COUT2: TAIYO YUDEN EMK212BJ105 L1, L2: MURATA LQH32CN150K53 CTRL1 RT FB2 CTRL2 38.3k 1% OFF ON R2 64.9k
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EFFICIENCY (%)
VIN
SW2 VOUT2
Li-Ion Powered Driver for 6 White LEDs and OLED Display
3V TO 5V L1 33H L2 33H 1F 90 SW2 85 LED DRIVER 80 BOOST CONVERTER 75 70 65 60 55 50 0 VIN = 3.6V 6 LEDs VOUT2 = 16V 5 10 15 20 25 30 OUTPUT CURRENT (mA)
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SW1
VIN
6 LEDs COUT1 1F
VOUT1
LT3466-1
VOUT2 COUT2 1F
16V 30mA R1 475k
FB1 CTRL1 RFB1 10 SHUTDOWN AND DIMMING CONTROL 1 63.4k RT GND
FB2 CTRL2 SHUTDOWN AND DIMMING CONTROL 2
R2 24.9k
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CIN: TAIYO YUDEN JMK107BJ105 COUT1, COUT2: TAIYO YUDEN GMK316BJ105 L1, L2: 33H TOKO D52LC
EFFICIENCY (%)
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Efficiency vs Load Current
90 L2 15H R1 909k COUT2 1F 12V 30mA AT VIN = 3V 60mA AT VIN = 5V 85 80 VIN = 3V 75 70 65 60 4 LEDs/20mA VOUT2 = 12V VIN = 5V
0
10
20 30 40 LOAD CURRENT (mA)
50
60
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Conversion Efficiency
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LT3466-1
TYPICAL APPLICATIO S
Li-Ion Powered Driver for 6 White LEDs and OLED with Output Disconnect
3V TO 5V CIN 1F L1 33H 6 LEDs SW1 COUT1 1F VOUT1 LT3466-1 FB1 RFB1 10 OFF ON CTRL1 RT FB2 CTRL2 63.4k 1% OFF ON R2 24.9k
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VOUT2 20V/DIV IL2 200mA/DIV
EFFICIENCY (%)
CTRL2 5V/DIV VIN = 3.6V VOUT2 = 16V 2ms/DIV
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14k Q1 L2 33H COUT3 0.47F R1 475k COUT2 0.47F
16V 30mA
VIN
SW2 VOUT2
CIN: TAIYO YUDEN JMK107BJ105 COUT1: TAIYO YUDEN GMK316BJ105 COUT2, COUT3: TAIYO YUDEN TMK316BJ474 L1, L2: 33H TOKO D52LC Q1: PHILIPS BC807
Conversion Efficiency
90 VIN = 3.6V VOUT2 = 16V
80
70
60
50
40
0
5
10 15 20 LOAD CURRENT (mA)
25
30
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LT3466-1
TYPICAL APPLICATIO S
Li-Ion Powered Driver for 6 White LEDs and OLED with Output Disconnect
3V TO 5V CIN 1F L1 33H SW1 COUT1 1F VOUT1 LT3466-1 FB1 RFB1 10 OFF ON CTRL1 RT FB2 CTRL2 63.4k 1% OFF ON R2 24.9k
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6 LEDs
CIN: TAIYO YUDEN JMK107BJ105 COUT1: TAIYO YUDEN GMK316BJ105 COUT2, COUT3: TAIYO YUDEN TMK316BJ474 L1, L2: 33H TOKO D52LC Q1: SILICONIX TPO610
VOUT2 20V/DIV
EFFICIENCY (%)
IL2 200mA/DIV
CTRL2 5V/DIV VIN = 3.6V VOUT2 = 16V 2ms/DIV
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L2 33H VIN SW2 VOUT2
Q1 COUT3 0.47F
16V 30mA
R1 475k COUT2 0.47F
NOTE: ENSURE THAT VOUT2 > VIN(MAX) + 5V
Conversion Efficiency
VIN = 3.6V 85 VOUT2 = 16V 80 75 70 65 60 55 50 0 5 10 20 15 LOAD CURRENT (mA) 25 30 90
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LT3466-1
TYPICAL APPLICATIO S
Li-Ion to 10 White LEDs and LCD Bias (8V) with Output Disconnect
3V TO 5V CIN 1F L1 68H 10 LEDs SW1 COUT1 1F VOUT1 LT3466-1 909k FB1 RFB1 16.5 OFF ON CTRL1 RT FB2 CTRL2 147k OFF ON 10k COUT3 1F VIN SW2 VOUT2 L2 33H C2 0.1F D2
CIN: TAIYO YUDEN JMK107BJ105 COUT1: TAIYO YUDEN UMK325BJ105 COUT2, COUT3: TAIYO YUDEN GMK316BJ105 C1, C2: TAIYO YUDEN UMK212BJ104 D1, D2: PHILIPS BAT54S L1: 68H TOKO D52LC L2: 33F TOKO D52LC
+8V OUTPUT 10V/DIV
EFFICIENCY (%)
-8V OUTPUT 10V/DIV
CTRL2 5V/DIV VIN = 3.6V +8V/10mA -8V/10mA 2ms/DIV
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C1 0.1F
D1 -8V 10mA COUT2 1F 8V 10mA
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Conversion Efficiency
84 82 80 78 76 74 72 VIN = 3.6V 10 LEDs +8V/10mA -8V/10mA
0
2
4 6 8 LED CURRENT (mA)
10
12
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LT3466-1
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 6) 5 0.200 REF 0.75 0.05 2.38 0.10 (2 SIDES) 1
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.
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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 1103
0.25 0.05 0.50 BSC
0.00 - 0.05
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. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. 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 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE
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LT3466-1
TYPICAL APPLICATIO U
CIN 1F L1 33H L2 33H C1 0.1F
Li-Ion to 8 White LEDs and 15V TFT LCD Bias Supply
3V TO 5V
Conversion Efficiency
D1 -15V 10mA COUT3 1F EFFICIENCY (%) 82 80 78 76 74
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86 84
VIN = 3.6V 8 LEDs +15V/10mA -15V/10mA
SW1 8 LEDs COUT1 1F VOUT1
VIN
SW2 VOUT2 15V 10mA 475k FB2 CTRL2 26.7k COUT2 1F
LT3466-1 FB1 CTRL1 RT
RFB1 13.3
OFF ON
63.4k
OFF ON
0
2.5
5 7.5 10 LED CURRENT (mA)
12.5
15
CIN: TAIYO YUDEN JMK107BJ105 COUT1, COUT2, COUT3: TAIYO YUDEN GMK316BJ105 C1: TAIYO YUDEN UMK212BJ104 L1, L2: 33H TOKO D52LC D1: PHILIPS BAT54S
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RELATED PARTS
PART NUMBER LT1618 LT1932 LT1937 LTC(R)3200-5 LTC3202 LTC3205 LTC3216 LTC3453 LT3465/LT3465A LT3466 LT3479 DESCRIPTION Constant Current, Constant Voltage 1.4MHz, 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, 1.5MHz, Regulated Charge Pump White LED Driver High Efficiency, Multidisplay LED Controller 1A Low Noise High Current LED Charge Pump with Independent Flash/Torch Current Control 500mA Synchronous Buck-Boost High Current LED Driver in Q FN Constant Current, 1.2MHz/2.7MHz, High Efficiency White LED Boost Regulator with Integrated Schottky Diode Dual Constant Current, 2MHz High Efficiency White LED Boost Regulator with Integrated Schottky Diode 3A, Full Featured DC/DC Converter with Soft-Start and Inrush Current Protection COMMENTS VIN: 1.6V to 18V, VOUT(MAX) = 34V, IQ = 1.8mA, ISD < 1A, MS/EDD Packages VIN: 1V to 10V, VOUT(MAX) = 34V, IQ = 1.2mA, ISD < 1A, ThinSOTTM Package VIN: 2.5V to 10V, VOUT(MAX) = 34V, IQ = 1.9mA, ISD < 1A, ThinSOT, SC70 Packages VIN: 2.7V to 4.5V, VOUT(MAX) = 5V, IQ = 8mA, ISD < 1A, ThinSOT Package VIN: 2.7V to 4.5V, VOUT(MAX) = 5.5V, IQ = 5mA, ISD < 1A, MS/EDD Packages VIN: 2.8V to 4.5V, VOUT(MAX) = 6V, IQ = 50A, ISD < 1A, QFN-24 Package VIN: 2.9V to 4.4V, VOUT(MAX) = 5.5V, IQ = 300A, ISD < 2.5A, DFN Package VIN: 2.7V to 5.5V, VOUT(MAX) = 5.5V, IQ = 0.6mA, ISD < 6A, QFN Package VIN: 2.7V to 16V, VOUT(MAX) = 34V, IQ = 1.9mA, ISD < 1A, ThinSOT Package VIN: 2.7V to 24V, VOUT(MAX) = 40V, IQ = 5mA, ISD < 16A, DFN Package VIN: 2.5V to 24V, VOUT(MAX) = 40V, IQ = 6.5mA, ISD < 1A, DFN/TSSOP Packages
ThinSOT is a trademark of Linear Technology Corporation.
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 FAX: (408) 434-0507
LT/TP 0705 500 * PRINTED IN USA
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2005

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