LT3461/LT3461A
1.3MHz/3MHz Step-Up
DC/DC Converters with
Integrated Schottky in ThinSOT
FEATURES
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DESCRIPTION
The
LT
®
3461/LT3461A
are general purpose fixed frequency
current mode step-up DC/DC converters. Both devices
feature an integrated Schottky and a low V
CESAT
switch al-
lowing a small converter footprint and lower parts cost. The
LT3461 switches at 1.3MHz while the LT3461A switches
at 3MHz. These high switching frequencies enable the use
of tiny, low cost and low height capacitors and inductors.
The constant switching frequency results in predictable
output noise that is easy to filter, and the inductor based
topology ensures an input free from switching noise typi-
cally present with charge pump solutions. The high voltage
switch in the LT3461/LT3461A is rated at 40V making the
device ideal for boost converters up to 38V.
The LT3461/LT3461A are available in a low profile (1mm)
SOT-23 package.
L,
LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and
ThinSOT is a trademark of Linear Technology Corporation.
Integrated Schottky Rectifier
Fixed Frequency 1.3MHz/3MHz Operation
High Output Voltage: Up to 38V
Low V
CESAT
Switch: 260mV at 250mA
12V at 70mA from 5V Input
5V at 115mA from 3.3V Input
Wide Input Range: 2.5V to 16V
Uses Small Surface Mount Components
Low Shutdown Current: <1µA
Soft-Start
Low Profile (1mm) SOT-23 (ThinSOT™) Package
APPLICATIONS
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Digital Cameras
CCD Bias Supply
XDSL Power Supply
TFT-LCD Bias Supply
Local 5V or 12V Supply
Medical Diagnostic Equipment
Battery Backup
TYPICAL APPLICATION
5V to 12V, 70mA Step-Up DC/DC Converter
V
IN
5V
C1
1µF
OFF ON
L1
10µH
1
SW
6
5
V
IN
V
OUT
LT3461A
4
3
SHDN
FB
GND
2
V
OUT
12V
70mA
C2
1µF
3461 TA01a
Efficiency
85
80
EFFICIENCY (%)
V
IN
= 5V
V
IN
= 3.3V
261k
15pF
75
30.1k
70
65
60
0
20
60
40
LOAD CURRENT (mA)
80
3461 TAO1b
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1
LT3461/LT3461A
ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW
SW 1
GND 2
FB 3
6 V
IN
5 V
OUT
4
SHDN
Input Voltage (V
IN
) ....................................................16V
V
OUT
, SW Voltage......................................................40V
FB Voltage ...................................................................5V
SHDN
Voltage ...........................................................16V
Operating Ambient
Temperature Range (Note 2) ....................–40°C to 85°C
Maximum Junction Temperature .......................... 125°C
Storage Temperature Range .................. –65°C to 150°C
Lead Temperature (Soldering, 10 sec)................... 300°C
S6 PACKAGE
6-LEAD PLASTIC TSOT-23
T
JMAX
= 125°C,
θ
JA
= 150°C ON BOARD OVER
GROUND PLANE,
θ
JC
= 120°C/W
ORDER INFORMATION
LEAD FREE FINISH
LT3461AES6#PBF
LT3461ES6#PBF
TAPE AND REEL
LT3461AES6#TRPBF
LT3461ES6#TRPBF
PART MARKING
LTAHG
LTAEB
PACKAGE DESCRIPTION
8-Lead Plastic TSOT-23
8-Lead Plastic TSOT-23
TEMPERATURE RANGE
–40°C to 85°C
–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.
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For more information
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LT3461/LT3461A
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C, V
IN
= 3V, V
SHDN
= 3V, unless otherwise noted.
PARAMETER
Minimum Operating Voltage
Maximum Operating Voltage
Feedback Voltage
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ELECTRICAL CHARACTERISTICS
CONDITIONS
MIN
2.5
TYP
MAX
16
UNITS
V
V
V
V
nA
mA
µA
MHz
MHz
%
%
1.235
1.225
0.005
1.255
1.275
1.280
%/V
100
3.6
0.5
3.9
1.7
Feedback Line Regulation
FB Pin Bias Current
Supply Current
Switching Frequency (LT3461A)
Switching Frequency (LT3461)
Maximum Duty Cycle (LT3461A)
Maximum Duty Cycle (LT3461)
Switch Current Limit
Switch V
CESAT
Switch Leakage Current
Schottky Forward Voltage
Schottky Reverse Leakage
SHDN
Voltage High
SHDN
Voltage Low
SHDN
Pin Bias Current
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.
I
SW
= 250mA
V
SW
= 5V
I
SCHOTTKY
= 250mA
V
OUT
– SW = 40V
FB = 1.3V, Not Switching
SHDN
= 0V
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40
2.8
0.1
2.1
1.0
82
92
300
420
260
0.01
800
0.03
1.5
3.0
1.3
600
350
1
1100
4
0.4
mA
mV
µA
mV
µA
V
V
µA
35
50
Note 2:
The LT3461E/LT3461AE is guaranteed to meet specifications from
0°C to 70°C. Specifications over the –40°C to 85°C operating temperature
range are assured by design, characterization and correlation with
statistical process controls.
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LT3461/LT3461A
TYPICAL PERFORMANCE CHARACTERISTICS
Oscillator Frequency (LT3461)
1.6
1.5
FB VOLTAGE (V)
1.4
1.3
1.2
1.1
1.0
–40 –20
0
10
CURRENT LIMIT (mA)
360
1.26
1.25
1.24
1.23
1.22
–40 –20
FREQUENCY (MHz)
480
Current Limit
T
A
= 25°C
1.28
1.27
FB Pin Voltage
240
120
40
20
60
0
TEMPERATURE (°C)
80
100
20
30
40 50 60 70
DUTY CYCLE (%)
80
90
40
20
60
0
TEMPERATURE (°C)
80
100
3461a G01
3461a G02
3461a G03
Oscillator Frequency (LT3461A)
3.9
3.6
3.3
3.0
2.7
2.4
2.1
–60 –40 –20 0
20 40 60
TEMPERATURE (°C)
CURRENT LIMIT (mA)
FREQUENCY (MHz)
480
420
360
300
240
180
120
60
80 100
3461a G04
Current Limit in Soft-Start Mode
T
A
= 25°C
SHDN
PIN CURRENT (µA)
320
280
240
200
160
120
80
40
1.5
2.1
1.9
SHDN
PIN VOLTAGE (V)
1.7
2.3
3461a G05
SHDN
Pin Current
T
A
= 25°C
0
1.3
0
0
4
8
12
16
3461a G06
SHDN
PIN VOLTAGE (V)
Switching Waveform
Circuit of Figure 4
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LOAD
V
SW
5V/DIV
70mA
35mA
Load Transient Response
Circuit of Figure 4
V
OUT
50mV/DIV
V
OUT
100mV/DIV
I
LOAD
= 60mA
0.2µs/DIV
3461a G08
50µs/DIV
3461a G09
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LT3461/LT3461A
PIN FUNCTIONS
SW (Pin 1):
Switch Pin. Connect inductor here. Minimize
trace at this pin to reduce EMI.
GND (Pin 2):
Ground Pin. Tie directly to local ground plane.
FB (Pin 3):
Feedback Pin. Reference voltage is 1.255V.
Connect resistor divider tap here. Minimize trace area at
FB. Set V
OUT
according to V
OUT
= 1.255V (1 + R1/R2).
SHDN
(Pin 4):
Shutdown Pin. Tie to 1.5V or higher to enable
device; 0.4V or less to disable device. Also functions as
soft-start. Use RC filter (47k, 47nF typ) as shown in Figure 1.
V
OUT
(Pin 5):
Output Pin. Connect to resistor divider. Put
capacitor close to pin and close to GND plane.
V
IN
(Pin 6):
Input Supply Pin. Must be locally bypassed.
BLOCK DIAGRAM
V
IN
6
V
OUT
R1 (EXTERNAL)
FB
R2 (EXTERNAL)
3 FB
1.255V
REFERENCE
+
A1
COMPARATOR
1
DRIVER
SW
5 V
OUT
–
R
C
C
C
A2
R
–
+
S
Q
Q1
+
0.1
∑
–
SHUTDOWN
R
S
(EXTERNAL)
4
SHDN
C
S
(EXTERNAL)
R
S
, C
S
OPTIONAL SOFT-START COMPONENTS
RAMP
GENERATOR
3MHz*
OSCILLATOR
*LT3461 IS 1.3MHz
2
GND
3461a F01
Figure 1. Block Diagram
OPERATION
The LT3461/LT3461A uses a constant frequency, current
mode control scheme to provide excellent line and load
regulation. Operation can be best understood by referring to
the block diagram in Figure 1. At the start of each oscillator
cycle, the SR latch is set, which turns on the power switch
Q1. 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
SR latch is reset turning 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 reference voltage of 1.255V.
In this manner, the error amplifier sets the correct peak
current level to keep the output in regulation. If the error
amplifier’s output increases, more current is delivered to
the output; if it decreases, less current is delivered.
Layout Hints
The high speed operation of the LT3461/LT3461A demands
careful attention to board layout. You will not get adver-
tised performance with careless layout. Figure 2 shows
the recommended component placement.
C1
+
GND
L1
V
IN
R2
+
R1
C2
V
OUT
SHUTDOWN
3461a F02
C3
Figure 2. Suggested Layout
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