LTC3426
1.2MHz Step-Up DC/DC
Converter in SOT-23
FEATURES
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DESCRIPTIO
Internal 2A MOSFET Switch
1.2MHz Switching Frequency
Integrated Soft-Start
V
IN
Range: 1.6V to 4.3V
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
TM
) Package
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 out-
put 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.
, LTC and LT are registered trademarks of Linear Technology Corporation.
ThinSOT is a trademark of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
Protected by U.S. Patents, including 6498466, 6611131
APPLICATIO S
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White LED Driver Supply
Local 3.3V or 5V Supply
Battery Back-Up
TYPICAL APPLICATIO
V
IN
3.3V
2.2µH
3.3V to 5V Boost Converter
100
95
90
SW
V
IN
10µF
OFF ON
V
OUT
V
OUT
5V
800mA
22µF
V
IN
= 3.3V
V
OUT
= 5V
EFFICIENCY (%)
85
80
75
70
65
60
LTC3426
SHDN
GND
FB
3426 TA01
55
50
1
10
100
LOAD CURRENT (mA)
1000
3426 TA01b
U
Efficiency
3426f
U
U
1
LTC3426
ABSOLUTE
(Note 1)
AXI U
RATI GS
PACKAGE/ORDER I FOR ATIO
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
ORDER PART
NUMBER
LTC3426ES6
S6 PART
MARKING
LTAJT
S6 PACKAGE
6-LEAD PLASTIC TSOT-23
T
JMAX
= 125°C,
θ
JA
= 165°C/W,
θ
JC
= 102°C/W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
CO VERTER CHARACTERISTICS
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
V
FB
= 1.15V
V
FB
= 1.23V
CONDITIONS
SHDN = V
IN
The
●
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 1.8V, V
OUT
= 3.3V, unless otherwise specified.
MIN
1.6
2.25
●
TYP
MAX
4.3
5
UNITS
V
V
V
µA
µA
µA
µA
Ω
Ω
A
%
1.173
1.22
1.247
0.1
1
V
SHDN
= 0V, Not Including Switch Leakage
SHDN = V
IN
, Not Switching
V
SW
= 5V
V
OUT
= 3.3V
V
OUT
= 5V
●
●
600
0.2
0.11
0.10
2
80
0.85
1
2.3
85
1.2
1000
10
1.5
0.4
1
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
The LTC3426 is guaranteed to meet performance specifications
from 0°C to 70°C. Specifications over the –40°C to 85°C operating
temperature are assured by design, characterization and correlation with
statistical process controls.
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.
2
U
MHz
V
V
µA
3426f
W
U
U
W W
W
U
LTC3426
TYPICAL PERFOR A CE CHARACTERISTICS
Oscillator Frequency
vs Temperature
1.40
100
1.3
FREQUENCY (MHz)
EFFICIENCY (%)
I
OUT(MAX)
(A)
3
V
IN
(V)
3.4
3.8
1.2
1.1
1.0
–50
–30
–10 10
30
50
TEMPERATURE (°C)
R
DS(ON)
vs Temperature
0.15
0.14
0.13
V
OUT
= 2.5V
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
V
OUT
= 5V
V
OUT
= 3.3V
25
50
0
TEMPERATURE (°C)
SW Pin Antiringing Operation
V
OUT
500mV/DIV
I
OUT
200mA/DIV
SW
1V/DIV
I
L
50mA/DIV
V
IN
= 1.8V
V
OUT
= 3.3V
U W
70
90
3426 G01
Efficiency vs V
IN
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
1.8
2.2
2.6
4.2
0.3
0.5
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
0.7
1.8
2.2
2.6
3
V
IN
(V)
3.4
3.8
4.2
3426 G03
LT1108 • TPC12
FB Pin Voltage
1.25
1.24
1.23
1.22
1.21
1.20
1.19
–50
I
L
200mA/DIV
V
OUT
500mV/DIV
Switching Waveforms
SW
2V/DIV
75
100
3426 G04
–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
V
OUT
Transient Response
500mA
250mA
I
L
500mA/DIV
100ns/DIV
3426 G07
V
IN
= 1.8V
V
OUT
= 3.3V
C
OUT
= 22µF
L = 2.5µH
40µs/DIV
3426 G08
3426f
3
LTC3426
PI FU CTIO S
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.
Connect 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 DIAGRA
1.22V
REFERENCE
V
OUT
R1 (EXTERNAL)
FB
R2 (EXTERNAL)
Σ
–
RAMP
GENERATOR
SHDN
4
SHUTDOWN AND
SOFT-START
1.2MHz
OSCILLATOR
2
GND
3426 F01
Figure 1
4
+
W
U
U
U
V
OUT
V
IN
6
SW
1
+
A1
COMPARATOR
5
–
R
C
C
C
A2
PWM LOGIC
AND DRIVER
–
3
FB
+
0.02Ω
3426f
LTC3426
OPERATIO
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
APPLICATIO S I FOR ATIO
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:
V
OUT
⎛
R1
⎞
=
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 appli-
cations. 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 improve-
ment 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 com-
pared to cheaper powdered iron types, improving effi-
ciency. The inductor should have low DCR (DC resistance)
U
W
U U
U
control voltage at the output of the error amplifier is the
amplified and compensated difference between the feed-
back 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 minimum 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.
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
applications. X5R and X7R dielectric materials are pre-
ferred 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
3426f
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