LINER LT3466 Dual full function white led step-up converter with built-in schottky diode Datasheet

LT3466
Dual Full Function White LED
Step-Up Converter with
Built-In Schottky Diodes
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FEATURES
DESCRIPTIO
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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.
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■
■
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■
■
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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 (39.5V 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
Available in 10-Pin DFN and 16-Pin Thermally
Enhanced TSSOP Packages
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.
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APPLICATIO S
■
■
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Main/Sub Displays
Digital Cameras, Sub-Notebook PCs
PDAs, Handheld Computers
Automotive
The LT3466 is available in the 10-pin (3mm × 3mm ×
0.75mm) DFN and 16-pin thermally enhanced TSSOP
packages.
, LTC and LT are registered trademarks of Linear Technology Corporation.
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TYPICAL APPLICATIO
3V TO 5V
Conversion Efficiency
85
1µF
80
L2
47µH
L1
47µH
VIN = 3.6V
8/8 LEDs
LED1
VIN
SW2
LED2
VOUT2
VOUT1
2.2µF
2.2µF
LT3466
FB1
FB2
CTRL1
10Ω
OFF ON
SHUTDOWN
AND DIMMING
CONTROL 1
OFF ON
SHUTDOWN
AND DIMMING
CONTROL 2
70
65
60
RT GND CTRL2
63.4k
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
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LT3466
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ABSOLUTE
RATI GS
(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 (Note 2) ... –40°C to 85°C
Maximum Junction Temperature ......................... 125°C
Storage Temperature Range
DFN .................................................. –65°C to 125°C
TSSOP .............................................. –65°C to 150°C
Lead Temperature (Soldering, 10 sec, TSSOP) ..... 300°C
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PACKAGE/ORDER I FOR ATIO
ORDER PART
NUMBER
TOP VIEW
VOUT1
1
10 FB1
SW1
2
9 CTRL1
VIN
3
SW2
4
VOUT2
11
5
LT3466EDD
8 RT
7 CTRL2
6 FB2
DD PACKAGE
10-LEAD (3mm × 3mm) PLASTIC DFN
DD PART MARKING
TJMAX = 125°C, θJA = 43°C/W, θJC = 2.96°C/W
EXPOSED PAD (PIN 11) IS GND
MUST BE SOLDERED TO PCB
LBBH
ORDER PART
NUMBER
TOP VIEW
GND
1
16 GND
NC
2
15 FB1
VOUT1
3
14 CTRL1
SW1
4
VIN
5
12 RT
SW2
6
11 CTRL2
VOUT2
7
10 FB2
GND
8
9
17
LT3466EFE
13 NC
FE PART MARKING
GND
3466EFE
FE PACKAGE
16-LEAD PLASTIC TSSOP
TJMAX = 125°C, θJA = 38°C/W, θJC(PAD) = 10°C/W
EXPOSED PAD (PIN 17) 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
V
Maximum Operating Voltage
FB1 Voltage
FB2 Voltage
UNITS
24
V
●
192
200
208
mV
●
192
200
208
mV
0
1.5
7.5
mV
50
nA
Offset Voltage (VOS) Between FB1 and FB2 Voltages
VOS = |FB1 – FB2|
FB1 Pin Bias Current
VFB1 = 0.2V (Note 3)
10
FB2 Pin Bias Current
VFB2 = 0.2V (Note 3)
10
50
nA
Quiescent Current
VFB1 = VFB2 = 0.3V
CTRL1 = CTRL2 = 0V
5
16
7.5
25
mA
µA
Switching Frequency
RT = 48.7k
0.8
1.2
MHz
Oscillator Frequency Range
(Note 4)
200
2000
kHz
Nominal RT Pin Voltage
RT = 48.7k
Maximum Duty Cycle
RT = 48.7k
RT = 20.5k
RT = 267k
●
90
1
0.54
V
96
92
99
%
%
%
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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
MIN
TYP
Converter 1 Current Limit
(Note 5)
●
320
400
mA
Converter 2 Current Limit
(Note 5)
●
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
●
CTRL2 Voltage for Full LED Current
●
CTRL1 or CTRL2 Voltage to Turn-On the IC
MAX
UNITS
1.8
V
1.8
V
150
mV
CTRL1 and CTRL2 Voltages to Shut Down the IC
●
8
10
50
mV
12
µA
CTRL1, CTRL2 Pin Bias Current
VCTRL1 = VCTRL2 = 1V
VOUT1 Overvoltage-Lockout Threshold
VOUT1 Rising
39.5
V
VOUT2 Overvoltage-Lockout Threshold
VOUT2 Rising
39.5
V
Schottky 1 Forward Drop
ISCHOTTKY1 = 300mA
0.85
V
Schottky 2 Forward Drop
ISCHOTTKY2 = 300mA
0.85
V
Schottky 1 Reverse Leakage
VOUT1 = 20V
Schottky 2 Reverse Leakage
VOUT2 = 20V
5
µA
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 LT3466E 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.
Note 4: Guaranteed by design and test correlation, not production tested.
Note 5: Current limit is guaranteed by design and/or correlation to static
test. Slope compensation reduces current limit at high duty cycle.
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TYPICAL PERFOR A CE CHARACTERISTICS
Switching Waveforms
Transient Response
VOUT1
50mV/DIV
VOUT1
0.5V/DIV
VSW1
20V/DIV
VCTRL1
2V/DIV
IL1
100mA/DIV
IL1
200mA/DIV
0.5µs/DIV
VIN = 3.6V
CIRCUIT OF FIGURE 1
3466 G01
50µs/DIV
VIN = 3.6V
ILED1 = 20mA TO 10mA
CIRCUIT OF FIGURE 1
3466 G02
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LT3466
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TYPICAL PERFOR A CE CHARACTERISTICS
VFB vs VCTRL
(Temperature Variation)
VFB vs VCTRL
250
200
150
100
200
150
100
0
1
0.5
1.5
CONTROL VOLTAGE (V)
0
2
350
150
200
150
50
0
0
100 150 200 250 300 350 400
SWITCH CURRENT (mA)
20
0
60
40
DUTY CYCLE (%)
80
VOUT1
39.0
2
4
6
8 10 12 14 16 18 20 22 24
VIN (V)
3466 G07
TA = 25°C
70
TA = 100°C
60
50
40
30
20
0
100
2
4
6
Input Current with Output 1 and
Output 2 Open Circuit
25
TA = 25°C
RT = 63.4k
VIN = 3.6V
RT = 63.4k
20
41
40
39
VOUT2
VOUT1
15
10
5
38
37
–50
8 10 12 14 16 18 20 22 24
VIN (V)
3466 G06
INPUT CURRENT (mA)
OUTPUT CLAMP VOLTAGE (V)
OUTPUT CLAMP VOLTAGE (V)
42
TA = 25°C
RT = 63.4k
39.5
TA = –50°C
80
Open-Circuit Clamp Voltage
vs Temperature
VOUT2
38.5
Shutdown Quiescent Current
(CTRL1 = CTRL2 = 0V)
3466 G05
Open-Circuit Clamp Voltage
vs VIN
2
10
3466 G04
40.0
1
0.5
1.5
CONTROL VOLTAGE (V)
0
90
250
50
40.5
±4mV
100
300
100
50
±4mV
3466 G16
TA = 25°C
TA = 85°C
350
100
0
100
2
SHUTDOWN CURRENT (µA)
400
CURRENT LIMIT (mA)
SWITCH SATURATION VOLTAGE (mV)
TA = –50°C
450
200
MIN
0
1
0.5
1.5
CONTROL VOLTAGE (V)
0
500
250
MAX
150
Switch Current Limit vs Duty Cycle
TA = 25°C
VCE1, VCE2
300
TYP
3466 G17
Switch Saturation Voltage (VCESAT)
400
200
50
3466 G03
450
VIN = 3V
TA = 25°C
±5mV
50
50
0
TA = –45°C
TA = 25°C
TA = 85°C
Distribution of VFB vs VCTRL
250
FEEDBACK VOLTAGE (mV)
VIN = 3V
TA = 25°C
FEEDBACK VOLTAGE (mV)
FEEDBACK VOLTAGE (mV)
250
0
50
0
TEMPERATURE (°C)
100
3466 G08
2
4
6
8 10 12 14 16 18 20 22 24
VIN (V)
3466 G09
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LT3466
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TYPICAL PERFOR A CE CHARACTERISTICS
Oscillator Frequency vs VIN
RT vs Oscillator Frequency
1200
OSCILLATOR FREQUENCY (kHz)
RT (kΩ)
1000
100
10
200
RT = 48.7k
1100
1000
900
800
600
1000
1400
1800
OSCILLATOR FREQUENCY (kHz)
4
2
6
8 10 12 14 16 18 20 22 24
VIN (V)
3466 G10
3466 G11
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 = 36V
2
VR = 20V
1
0
–50
0
50
TEMPERATURE (°C)
100
3466 G15
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LT3466
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PI FU CTIO S
(DD/TSSOP)
VOUT1 (Pin 1/Pin 3): 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.
RT (Pin 8/Pin 12): Timing Resistor to Program the Switching Frequency. The switching frequency can be programmed from 200KHz to 2MHz.
SW1 (Pin 2/Pin 4): Switch Pin for Converter 1. Connect the
inductor at this pin.
CTRL1 (Pin 9/Pin 14): 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.
VIN (Pin 3/Pin 5): Input Supply Pin. Must be locally
bypassed with a 1µF, X5R or X7R type ceramic capacitor.
SW2 (Pin 4/Pin 6): Switch Pin for Converter 2. Connect the
inductor at this pin.
VOUT2 (Pin 5/Pin 7): 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/Pin 10): 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
FB1 (Pin 10/Pin 15): 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/Pin 17): The Exposed Pad must be
soldered to the PCB system ground.
GND (NA/Pins 1, 8, 9, 16): These pins are internally fused
to the Exposed Pad (TSSOP package only). Connect these
GND pins and the Exposed Pad to the PCB system ground.
CTRL2 (Pin 7/Pin 11): 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.
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C2
RFB1
FB1
PWM
LOGIC
PWM
COMP
A2
RSNS1
DRIVER
CONVERTER 1
OSC
OVERVOLT
DETECTION
Q1
SW1
–
+
A3
EA
PIN NUMBERS CORRESPOND TO THE 10-PIN DFN PACKAGE
10
1
2
–
+
VOUT1
L1
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
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LT3466
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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.
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 300Ω resistor across the inductors, although this
will degrade efficiency.
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 39.5V (typ). Figure 4a shows the transient response of
Figure 1’s step-up converter with LED1 disconnected.
With LED1 disconnected, the converter starts switching at
the peak inductor current limit. The converter output starts
ramping up and finally gets clamped at 39.5V (typ). The
converter will then switch at low inductor current to
regulate the converter output at the clamp voltage. Output
voltage and input current during output open circuit are
shown in the Typical Performance Characteristics graphs.
In the event one of the converters has an output opencircuit, its output voltage will be clamped at 39.5V.
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LT3466
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OPERATIO
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 eight LEDs at 20mA. Converter 1 starts switching at a
lower peak inductor current and begins skipping pulses,
thereby 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.
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.
VOUT1
10V/DIV
IL1
200mA/DIV
200µs/DIV
3466 F04a
Undervoltage Lockout
LED1 DISCONNECTED AT THIS INSTANT
VIN = 3.3V
CIRCUIT OF FIGURE 1
Figure 4a. Transient Response of Switcher 1 with LED1
Disconnected from the Output
IL1
50mA/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 from
operating in an erratic mode when powered from low
supply voltages.
IIN
100mA/DIV
VSW1
50V/DIV
IL2
100mA/DIV
VOUT1
5V/DIV
VFB1
200mV/DIV
VSW2
50V/DIV
CRTL1
2V/DIV
VIN = 3.3V
CIRCUIT OF FIGURE 1
LED1 DISCONNECTED
1µs/DIV
3466 F04b
Figure 4b. Switching Waveforms with Output 1 Open-Circuit
VIN = 3.6V
4 LEDs, 20mA
L = 15µH
C = 0.47µF
100µs/DIV
3466 F05
Figure 5. Start-Up Waveforms
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LT3466
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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 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
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LT3466
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APPLICATIO S I FOR ATIO
INDUCTOR SELECTION
CAPACITOR SELECTION
The choice of the inductor will depend on the selection of
the switching frequency of the 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
350
310
430
Sumida
(847) 956-0666
www.sumida.com
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.
3466fa
11
LT3466
U
W
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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.
The LED current can be set by:
ILED ≈ (200mV/RFB), when VCTRL > 1.6V
ILED ≈ (VCTRL/5 • RFB), when VCTRL < 1V
Feedback voltage variation versus control voltage is given
in the Typical Performance Characteristics graphs.
PROGRAMMING LED CURRENT
The LED current of each LED string can be 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).
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.
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 CTRL pin voltage can be modulated to set the dimming
of the respective LED string. As the voltage on the CTRL
pin increases from 0V to 1.6V, the LED current increases
from 0 to ILED. As the CTRL pin voltage increases beyond
1.6V, it has no effect on the LED current.
Table 4. RFB Value Selection
ILED (mA)
RFB (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 the 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.
3466fa
12
LT3466
U
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APPLICATIO S I FOR ATIO
Figure 9 shows four LEDs being powered 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
VOUT2
VOUT1
1µF
1µF
LT3466
FB1
CTRL1
FB2
RT
CTRL2
10Ω
10Ω
OFF ON
63.4k
1%
OFF ON
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 exposed paddle for both DFN and TSSOP packages
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 for the DFN package is shown in Figure 10.
Figure 9. 2 AA Cells to Four White LEDs
HIGH INPUT VOLTAGE APPLICATIONS
The input voltage to the LT3466 can be as high as 24V. This
gives it the flexibility of driving a large number of LEDs
when being powered off a higher voltage. The maximum
number of LEDs that can be driven is constrained by the
converter output voltages being clamped at 39.5V (typ).
The LT3466 can be used to drive 20 White LEDs (10 per
converter) at 20mA when powered off two Li-Ion cells in
series.
GND
COUT1
RFB1
CIN
10
1
L1
3
L2
CTRL1
9
2
VIN
RT
11
8
4
7
5
6
RFB2
CTRL2
COUT2
GND
3466 F10
Figure 10. Recommended Component Placement (DFN Package)
3466fa
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
VIN
SW2
VOUT2
VOUT1
COUT1
1µF
COUT2
0.47µF
LT3466
FB1
RFB1
10Ω
EFFICIENCY (%)
75
L1
15µH
RT
OFF ON
55
RFB2
10Ω
CTRL2
OFF ON
38.3k
1%
65
60
FB2
CTRL1
70
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
OFF ON
EFFICIENCY (%)
75
L2
15µH
L1
15µH
COUT2
0.47µF
CTRL2
38.3k
1%
65
60
55
FB2
RT
70
RFB2
10Ω
OFF ON
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
3466fa
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
VIN
SW2
VOUT1
COUT1
1µF
VOUT2
LT3466
FB1
COUT2
1µF
RT
OFF ON
65
55
CTRL2
50
0
RFB2
10Ω
OFF ON
63.4k
1%
70
60
FB2
CTRL1
RFB1
10Ω
EFFICIENCY (%)
75
L1
33µH
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
COUT1
1µF
VIN
SW2
VOUT2
VOUT1
LT3466
FB1
CTRL1
EFFICIENCY (%)
SW1
75
L2
33µH
L1
33µH
COUT2
1µF
70
65
60
55
FB2
RT
VIN = 3.6V
7/7 LEDs
80
CTRL2
50
0
RFB1
10Ω
OFF ON
63.4k
1%
OFF ON
RFB2
10Ω
5
10
15
20
LED CURRENT (mA)
3466 TA04b
3466 TA04a
CIN: TAIYO YUDEN JMK107BJ105
COUT1, COUT2: TAIYO YUDEN GMK316BJ105
L1, L2: TOKO A914BYW-330M
3466fa
15
LT3466
U
TYPICAL APPLICATIO S
Li-Ion to 8/8 White LEDs
Conversion Efficiency
3V TO 5V
85
CIN
1µF
VIN = 3.6V
8/8 LEDs
80
SW1
COUT1
2.2µF
EFFICIENCY (%)
75
L2
47µH
L1
47µH
VIN
SW2
VOUT2
VOUT1
LT3466
RT
65
60
FB2
55
CTRL2
50
FB1
CTRL1
COUT2
2.2µF
70
5
0
OFF ON
OFF ON
63.4k
1%
RFB1
10Ω
3466 TA05a
Conversion Efficiency
3V TO 5V
90
CIN
1µF
EFFICIENCY (%)
COUT1
1µF
SW2
VOUT2
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)
LT3466
COUT2
1µF
80
75
70
65
FB1
CTRL1
FB2
RT
CTRL2
60
0
OFF ON
RFB1
16.5Ω
147k
1%
OFF ON
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
3466fa
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
OFF ON
RFB1
16.5Ω
FB2
RT
60
CTRL2
RFB2
16.5Ω
3466 TA07a
Conversion Efficiency
3.3V
75
CIN1
0.1µF
VIN
SW2
VOUT2
VOUT1
COUT1
1µF
COUT2
1µF
LT3466
FB1
RFB1
10Ω
CTRL1
OFF ON
FB2
RT
CTRL2
63.4k
1%
CIN1: TAIYO YUDEN EMK107BJ104
CIN2: TAIYO YUDEN JMK107BJ105
COUT1, COUT2: TAIYO YUDEN GMK316BJ105
L1, L2: MURATA LQH32CN150
OFF ON
VIN = 2.4V
2/2 LEDs
70
L2
15µH
EFFICIENCY (%)
L1
15µH
SW1
12
3466 TA07b
CIN: TAIYO YUDEN JMK107BJ105
COUT1, COUT2: TAIYO YUDEN UMK325BJ105
L1, L2: TOKO A920CY-680M
CIN2
1µF
8
LED CURRENT (mA)
2 AA Cells to 2/2 White LEDs
VCC
1.8V TO 3V
4
0
OFF ON
147k
1%
65
60
55
RFB2
10Ω
3466 TA08a
50
0
5
10
15
20
LED CURRENT (mA)
3466 TA08b
3466fa
17
LT3466
U
TYPICAL APPLICATIO S
2 Li-Ion Cells to 10/10 White LEDs
Conversion Efficiency
90
6V TO 9V
L2
47µH
SW1
VIN
VOUT2
COUT2
0.47µF
LT3466
FB1
CTRL1
OFF ON
RFB1
10Ω
SW2
VOUT1
COUT1
0.47µF
80
EFFICIENCY (%)
L1
47µH
70
65
55
50
CTRL2
63.4k
1%
75
60
FB2
RT
VIN = 7V
10/10 LEDs
85
CIN
1µF
0
10
5
RFB2
10Ω
3466 TA09a
Conversion Efficiency
2 Li-Ion Cells to 16/16 White LEDs
6V TO 9V
90
D3
80
VLED1
VLED2
D2
D4
L1
L2
47µH 47µH
C1
0.1µF
16
LEDs
SW1
VIN
C4
0.1µF
16
LEDs
SW2
VOUT2
VOUT1
LT3466
FB1
RFB1
10Ω
CTRL1
OFF ON
CTRL2
38.3k
1%
OFF ON
75
70
65
60
C6
0.22µF
55
50
FB2
RT
VIN = 7V
16/16 LEDs
85
EFFICIENCY (%)
CIN C5
1µF 0.1µF
C2
0.1µF
C3
0.22µF
20
3466 TA09b
CIN: TAIYO YUDEN LMK212BJ105
COUT1, COUT2: TAIYO YUDEN UMK316BJ474
L1, L2: TOKO A914BYW-470M
D1
15
LED CURRENT (mA)
OFF ON
RFB2
10Ω
3466 TA10a
0
5
10
15
20
LED CURRENT (mA)
3466 TA11b
CIN: TAIYO YUDEN LMK212BJ105
C1, C2, C4, C5: TAIYO YUDEN UMK212BJ104
C3, C6: TAIYO YUDEN UMK316BJ224
D1-D4: PHILIPS BAV99
L1, L2: TOKO A914BYW-470M
3466fa
18
LT3466
U
PACKAGE DESCRIPTIO
DD Package
10-Lead Plastic DFN (3mm × 3mm)
(Reference LTC DWG # 05-08-1699)
R = 0.115
TYP
6
0.38 ± 0.10
10
0.675 ±0.05
3.50 ±0.05
1.65 ±0.05
2.15 ±0.05 (2 SIDES)
3.00 ±0.10
(4 SIDES)
1.65 ± 0.10
(2 SIDES)
PIN 1
TOP MARK
(SEE NOTE 6)
PACKAGE
OUTLINE
(DD10) DFN 1103
5
0.25 ± 0.05
1
0.75 ±0.05
0.200 REF
0.50
BSC
2.38 ±0.05
(2 SIDES)
0.25 ± 0.05
0.50 BSC
2.38 ±0.10
(2 SIDES)
0.00 – 0.05
BOTTOM VIEW—EXPOSED PAD
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
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
FE Package
16-Lead Plastic TSSOP (4.4mm)
(Reference LTC DWG # 05-08-1663)
Exposed Pad Variation BB
4.90 – 5.10*
(.193 – .201)
3.58
(.141)
3.58
(.141)
16 1514 13 12 1110
6.60 ±0.10
9
2.94
(.116)
4.50 ±0.10
2.94 6.40
(.116) (.252)
BSC
SEE NOTE 4
0.45 ±0.05
1.05 ±0.10
0.65 BSC
1 2 3 4 5 6 7 8
RECOMMENDED SOLDER PAD LAYOUT
4.30 – 4.50*
(.169 – .177)
0.09 – 0.20
(.0035 – .0079)
0.50 – 0.75
(.020 – .030)
NOTE:
1. CONTROLLING DIMENSION: MILLIMETERS
MILLIMETERS
2. DIMENSIONS ARE IN
(INCHES)
3. DRAWING NOT TO SCALE
0.25
REF
1.10
(.0433)
MAX
0° – 8°
0.65
(.0256)
BSC
0.195 – 0.30
(.0077 – .0118)
TYP
0.05 – 0.15
(.002 – .006)
FE16 (BB) TSSOP 0204
4. RECOMMENDED MINIMUM PCB METAL SIZE
FOR EXPOSED PAD ATTACHMENT
*DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.150mm (.006") PER SIDE
3466fa
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
CAR BATTERY
12V (TYP)
9V TO 18V
85
D9
C4
0.1µF
D7
25
LEDs
C5
0.1µF
L1
33µH
C2
0.1µF
L2
33µH
D1
C7
0.1µF
D2
VLED2
3.3V
C8
0.1µF
C3
0.1µF
C9
0.1µF
D3
CIN
D4
D8
SW1 VIN
C6
0.22µF
SW2
VOUT2
VOUT1
LT3466
C10
0.1µF
25
LEDs
RT
70
65
60
55
C11
0.22µF
50
FB2
CIN: TAIYO YUDEN JMK107BJ105
CTRL1
C2-C5, C7-C10: TAIYO YUDEN UMK212BJ104
C6, C11: TAIYO YUDEN UMK316BJ224
D1-D8: PHILIPS BAV99
OFF ON
D9, D10: PHILIPS BAS16
75
0
FB1
RFB1
13.3Ω
D10
EFFICIENCY (%)
D5
VLED1
D6
VIN = 12V
25/25 LEDs
80
CTRL2
20.5k
1%
10
5
LED CURRENT (mA)
RFB2
13.3Ω
15
3466 TA10b
3466 TA10a
OFF ON
L1, L2: TOKO A914BYW-330M
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LT3465/LT3465A
Constant Current, 1.2MHz/2.7MHz, High Efficiency White LED
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ThinSOT is a trademark of Linear Technology Corporation.
3466fa
20
Linear Technology Corporation
LT/LT 0305 REV A • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2004