LTC3426
1.2MHz Step-Up DC/DC
Converter in SOT-23
FEATURES
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DESCRIPTION
The
LTC
®
3426
step-up switching regulator generates an
output voltage of up to 5.5V from an input voltage as low
as 1.6V. Ideal for applications where space is limited, it
switches at 1.2MHz, allowing the use of tiny, low cost
and low profile external components. Its internal 2A,
100mΩ NMOS switch provides high efficiency even at
heavy load, while the constant frequency, current mode
architecture results in low, predictable output noise that
is easy to filter.
Antiringing circuitry reduces EMI concerns by damping
the inductor while in discontinuous mode, and internal
soft-start eases inrush current worries. Internal frequency
compensation is designed to accommodate ceramic output
capacitors, further reducing noise. The device features
very low shutdown current of 0.5µA.
The LTC3426 is available in the 6-lead SOT-23 package.
L,
LT, LTC and LTM are registered trademarks and ThinSOT is a trademark of Analog Devices,
Inc. All other trademarks are the property of their respective owners. Protected by U.S. Patents
including 6498466, 6611131.
Internal 2A MOSFET Switch
1.2MHz Switching Frequency
Integrated Soft-Start
Low 1.6V V
IN
Operation
Low R
DS(ON)
Switch: 100mΩ at 5V Output
Delivers 5V at 800mA from a 3.3V Input
Delivers 3.3V at 800mA from a 2.5V Input
Uses Small, Low Profile External Components
Low Profile (1mm) SOT-23 (ThinSOT™) Package
APPLICATIONS
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White LED Driver Supply
Local 3.3V or 5V Supply
Battery Back-Up
TYPICAL APPLICATION
3.3V to 5V Boost Converter
100
V
IN
3.3V
2.2µH
95
90
V
IN
10µF
OFF ON
V
OUT
EFFICIENCY (%)
SW
V
OUT
5V
800mA
22µF
85
80
75
70
65
60
3426 TA01
Efficiency
V
IN
= 3.3V
V
OUT
= 5V
LTC3426
SHDN
GND
FB
55
50
1
10
100
LOAD CURRENT (mA)
1000
3426 TA01b
3426fb
For more information
www.linear.com/LTC3426
1
LTC3426
ABSOLUTE MAXIMUM RATINGS
(Note 1)
PACKAGE/ORDER INFORMATION
TOP VIEW
SW 1
GND 2
FB 3
6 V
IN
5 V
OUT
4
SHDN
V
IN
Voltage .................................................. –0.3V to 6V
SW Voltage .................................................. –0.3V to 6V
SHDN,
FB Voltage ....................................... –0.3V to 6V
V
OUT
........................................................... –0.3V to 6V
Operating Temperature Range (Note 2)... – 40°C to 85°C
Storage Temperature Range...................–65°C to 125°C
Lead Temperature (Soldering, 10 sec) ................. 300°C
S6 PACKAGE
6-LEAD PLASTIC TSOT-23
T
JMAX
= 125°C,
θ
JA
= 165°C/W,
θ
JC
= 102°C/W
ORDER INFORMATION
LEAD FREE FINISH
LTC3426ES6#PBF
TAPE AND REEL
LTC3426ES6#TRPBF
http://www.linear.com/product/LTC3426#orderinfo
PART MARKING
LTAJT
PACKAGE DESCRIPTION
6-Lead Plastic TSOT-23
TEMPERATURE RANGE
–40°C to 85°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
For more information on lead free part marking, go to:
http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to:
http://www.linear.com/tapeandreel/.
Some packages are available in 500 unit reels through
designated sales channels with #TRMPBF suffix.
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 1.8V, V
OUT
= 3.3V, unless otherwise specified.
PARAMETER
Input Voltage Range
Output Voltage Adjust Range
Feedback Voltage
Feedback Input Current
Quiescent Current (Shutdown)
Quiescent Current
Switch Leakage
Switch On Resistance
Current Limit
Maximum Duty Cycle
Switching Frequency
SHDN
Input High
SHDN
Input Low
SHDN
Input Current
SHDN
= 5.5V
Note 1:
Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2:
The LTC3426 is guaranteed to meet performance specifications
from 0°C to 85°C. Specifications over the –40°C to 85°C operating
temperature are assured by design, characterization and correlation with
statistical process controls.
V
FB
= 1.15V
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CONVERTER CHARACTERISTICS
The
l
denotes the specifications which apply over the full operating
CONDITIONS
SHDN
= V
IN
MIN
1.6
2.25
1.173
1.22
V
FB
= 1.23V
V
SHDN
= 0V, Not Including Switch Leakage
SHDN
= V
IN
, Not Switching
V
SW
= 5V
V
OUT
= 3.3V
V
OUT
= 5V
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TYP
MAX
5
1.247
0.1
1
UNITS
V
V
V
µA
µA
µA
µA
Ω
Ω
A
%
600
0.2
0.11
0.10
2
80
0.85
1
2.3
85
1.2
1000
10
1.5
0.4
1
MHz
V
V
µA
Note 3:
This IC includes overtemperature protection that is intended
to protect the device during momentary overload conditions. Junction
temperature will exceed 125°C when overtemperature protection is active.
Continuous operation above the specified maximum operating junction
temperature may impair device reliability.
3426fb
2
For more information
www.linear.com/LTC3426
LTC3426
TYPICAL PERFORMANCE CHARACTERISTICS
Oscillator Frequency
vs Temperature
1.40
100
Efficiency vs V
IN
1.3
FREQUENCY (MHz)
EFFICIENCY (%)
I
OUT(MAX)
(A)
3
V
IN
(V)
3.4
4.2
FIGURE 1 CIRCUIT
98 T
A
= 25°C
I
LOAD
= 150mA
96 V
OUT
= 5V
94 C
OUT
= 22µF
L = 2.2µH
92
90
88
86
84
82
80
1.3
I
OUT(MAX)
vs V
IN
FIGURE 1 CIRCUIT
T
A
= 25°C
V
OUT
= 5V
1.1 C
OUT
= 22µF
L = 2.2µH
0.9
1.2
0.7
1.1
0.5
0.3
1.0
–50
–30
–10 10
30
50
TEMPERATURE (°C)
70
90
3426 G01
1.8
2.2
2.6
3.8
1.8
2.2
2.6
3
V
IN
(V)
3.4
3.8
4.2
3426 G03
LT1108 • TPC12
R
DS(ON)
vs Temperature
0.15
0.14
0.13
FB VOLTAGE (V)
0.12
R
DS(ON)
(Ω)
0.11
0.10
0.09
0.08
0.07
0.06
0.05
–50
–25
25
50
0
TEMPERATURE (°C)
75
100
3426 G04
FB Pin Voltage
1.25
1.24
V
OUT
500mV/DIV
SW
2V/DIV
I
L
200mA/DIV
Switching Waveforms
V
OUT
= 2.5V
V
OUT
= 5V
V
OUT
= 3.3V
1.23
1.22
1.21
1.20
1.19
–50
–25
0
25
50
TEMPERATURE (°C)
75
100
3426 G05
V
IN
= 1.8V
V
OUT
= 3.3V
C
OUT
= 22µF
L = 2.5µH
100ns/DIV
3426 G06
SW Pin Antiringing Operation
V
OUT
500mV/DIV
I
OUT
200mA/DIV
V
OUT
Transient Response
SW
1V/DIV
500mA
250mA
I
L
50mA/DIV
I
L
500mA/DIV
V
IN
= 1.8V
V
OUT
= 3.3V
100ns/DIV
3426 G07
V
IN
= 1.8V
V
OUT
= 3.3V
C
OUT
= 22µF
L = 2.5µH
40µs/DIV
3426 G08
3426fb
For more information
www.linear.com/LTC3426
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LTC3426
PIN FUNCTIONS
SW (Pin 1):
Switch Pin. Connect inductor between SW
and V
IN
. A Schottky diode is connected between SW and
V
OUT
. Keep these PCB trace lengths as short and wide
as possible to reduce EMI and voltage overshoot. If the
inductor current falls to zero, an internal 100Ω antiringing
switch is connected from SW to V
IN
to minimize EMI.
GND (Pin 2):
Signal and Power Ground. Provide a short
direct PCB path between GND and the (–) side of the
output capacitor(s).
FB (Pin 3):
Feedback Input to the g
m
Error Amplifier. Con-
nect resistor divider tap to this pin. The output voltage can
be adjusted from 2.5V to 5V by:
R1
V
OUT
=
1.22 •
1+
R2
SHDN
(Pin 4):
Logic Controlled Shutdown Input.
SHDN
= High: Normal free running operation
SHDN
= Low: Shutdown, quiescent current < 1µA
Typically,
SHDN
should be connected to V
IN
through a
1M pull-up resistor.
V
OUT
(Pin 5):
Output Voltage Sense Input. The NMOS switch
gate drive is derived from the greater of V
OUT
and V
IN
.
V
IN
(Pin 6):
Input Supply. Must be locally bypassed.
BLOCK DIAGRAM
V
OUT
1.22V
REFERENCE
V
IN
6
SW
1
+
A1
COMPARATOR
5
–
R
C
C
C
A2
–
3
+
PWM LOGIC
AND DRIVER
V
OUT
R1 (EXTERNAL)
FB
R2 (EXTERNAL)
Σ
RAMP
GENERATOR
SHDN
4
SHUTDOWN AND
SOFT-START
1.2MHz
OSCILLATOR
4
For more information
www.linear.com/LTC3426
–
2
GND
3426 F01
+
0.02
3426fb
FB
LTC3426
OPERATION
The LTC3426 is a monolithic 1.2MHz boost converter
housed in a 6-lead SOT-23 package. The device features
fixed frequency, current mode PWM control for excellent
line and load regulation. The low R
DS(ON)
NMOS switch
enables the device to maintain high efficiency over a wide
range of load current. Operation of the feedback loop
which sets the peak inductor current to keep the output in
regulation can be best understood by referring to the Block
Diagram in Figure 1. At the start of each clock cycle a latch
in the PWM logic is set and the NMOS switch is turned on.
The sum of a voltage proportional to the switch current and
a slope compensating voltage ramp is fed to the positive
input to the PWM comparator. When this voltage exceeds
either a voltage proportional to the 2A current limit or the
PWM control voltage, the latch in the PWM logic is reset
and NMOS switch is turned off. The PWM control voltage
at the output of the error amplifier is the amplified and
compensated difference between the feedback voltage on
the FB pin and the internal reference voltage of 1.22V. If
the control voltage increases, more current is delivered
to the output. When the control voltage exceeds the I
LIMIT
reference voltage, the peak current is limited to a mini-
mum of 2A. The current limit helps protect the LTC3426
internal switch and external components connected to it.
If the control voltage decreases, less current is delivered
to the output. During load transients control voltage may
decrease to the point where no switching occurs until the
feedback voltage drops below the reference. The LTC3426
has an integrated soft-start feature which slowly ramps
up the feedback control node from 0V. The soft-start is
initiated when
SHDN
is pulled high.
APPLICATIONS INFORMATION
Setting the Output Voltage
The output voltage, V
OUT
, is set by a resistive divider from
V
OUT
to ground. The divider tap is tied to the FB pin. V
OUT
is set by the formula:
R1
V
OUT
=
1.22 •
1+
R2
Inductor Selection
The LTC3426 can utilize small surface mount inductors
due to its 1.2MHz switching frequency. A 1.5µH or 2.2µH
inductor will be the best choice for most LTC3426 ap-
plications. Larger values of inductance will allow greater
output current capability by reducing the inductor ripple
current. Increasing the inductance above 3.3µH will in-
crease component size while providing little improvement
in output current capability. The inductor current ripple
is typically set for 20% to 40% of the maximum inductor
current (I
P
). High frequency ferrite core inductor materials
reduce frequency dependent power losses compared to
cheaper powdered iron types, improving efficiency. The
inductor should have low DCR (DC resistance) to reduce
the I
2
R power losses, and must be able to handle the peak
inductor current without saturating.
Several inductor manufacturers are listed in Table 1.
Table 1. Inductor Manufacturers
TDK
Sumida
Murata
www.tdk.com
www.sumida.com
www.murata.com
Output and Input Capacitor Selection
Low ESR (equivalent series resistance) capacitors should
be used to minimize the output voltage ripple. Multilayer
ceramic capacitors are an excellent choice as they have
extremely low ESR and are available in small footprints.
A 15µF to 30µF output capacitor is sufficient for most ap-
plications. X5R and X7R dielectric materials are preferred
for their ability to maintain capacitance over wide voltage
and temperature ranges.
Low ESR input capacitors reduce input switching noise
and reduce the peak current drawn from the input supply.
It follows that ceramic capacitors are also a good choice
for input decoupling and should be located as close as
3426fb
For more information
www.linear.com/LTC3426
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