LT3461/LT3461A
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TYPICAL APPLICATION
FEATURES DESCRIPTION
1.3MHz/3MHz Step-Up
DC/DC Converters with
Integrated Schottky in ThinSOT
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 VCESAT 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.
APPLICATIONS
n Integrated Schottky Rectifier
n Fixed Frequency 1.3MHz/3MHz Operation
n High Output Voltage: Up to 38V
n Low VCESAT Switch: 260mV at 250mA
n 12V at 70mA from 5V Input
n 5V at 115mA from 3.3V Input
n Wide Input Range: 2.5V to 16V
n Uses Small Surface Mount Components
n Low Shutdown Current: <1µA
n Soft-Start
n Low Profile (1mm) SOT-23 (ThinSOT™) Package
n Digital Cameras
n CCD Bias Supply
n XDSL Power Supply
n TFT-LCD Bias Supply
n Local 5V or 12V Supply
n Medical Diagnostic Equipment
n Battery Backup
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and
ThinSOT is a trademark of Linear Technology Corporation.
5V to 12V, 70mA Step-Up DC/DC Converter Efficiency
V
IN
5V
L1
10µH
261k
30.1k
C2
1µF
15pF
C1
1µF
VOUT
12V
70mA
1
2
VIN VOUT
SW
3
4
5
6
FB
SHDN
GND
LT3461A
3461 TA01a
OFF ON
LOAD CURRENT (mA)
0
EFFICIENCY (%)
85
80
75
70
65
60
3461 TAO1b
20 40 60 80
V
IN
= 5V
V
IN
= 3.3V
LT3461/LT3461A
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PIN CONFIGURATIONABSOLUTE MAXIMUM RATINGS
Input Voltage (VIN) ....................................................16V
VOUT, 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
(Note 1)
6 VIN
5 VOUT
4
SHDN
SW 1
TOP VIEW
S6 PACKAGE
6-LEAD PLASTIC TSOT-23
GND 2
FB 3
TJMAX = 125°C, θJA = 150°C ON BOARD OVER
GROUND PLANE, θJC = 120°C/W
ORDER INFORMATION
LEAD FREE FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE
LT3461AES6#PBF LT3461AES6#TRPBF LTAHG 8-Lead Plastic TSOT-23 –40°C to 85°C
LT3461ES6#PBF LT3461ES6#TRPBF LTAEB 8-Lead Plastic TSOT-23 –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.
LT3461/LT3461A
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ELECTRICAL CHARACTERISTICS
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 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.
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C, VIN = 3V, VSHDN = 3V, unless otherwise noted.
PARAMETER CONDITIONS MIN TYP MAX UNITS
Minimum Operating Voltage 2.5 V
Maximum Operating Voltage 16 V
Feedback Voltage
l
1.235
1.225
1.255 1.275
1.280
V
V
Feedback Line Regulation 0.005 %/V
FB Pin Bias Current l40 100 nA
Supply Current FB = 1.3V, Not Switching
SHDN = 0V
2.8
0.1
3.6
0.5
mA
µA
Switching Frequency (LT3461A) l2.1 3.0 3.9 MHz
Switching Frequency (LT3461) l1.0 1.3 1.7 MHz
Maximum Duty Cycle (LT3461A) l82 %
Maximum Duty Cycle (LT3461) l92 %
Switch Current Limit 300 420 600 mA
Switch VCESAT ISW = 250mA 260 350 mV
Switch Leakage Current VSW = 5V 0.01 1 µA
Schottky Forward Voltage ISCHOTTKY = 250mA 800 1100 mV
Schottky Reverse Leakage VOUT – SW = 40V 0.03 4 µA
SHDN Voltage High 1.5 V
SHDN Voltage Low 0.4 V
SHDN Pin Bias Current 35 50 µA
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TYPICAL PERFORMANCE CHARACTERISTICS
Oscillator Frequency (LT3461) Current Limit FB Pin Voltage
Oscillator Frequency (LT3461A) Current Limit in Soft-Start Mode SHDN Pin Current
Switching Waveform
Circuit of Figure 4
Load Transient Response
Circuit of Figure 4
TEMPERATURE (°C)
FREQUENCY (MHz)
1.6
1.5
1.4
1.3
1.2
1.1
1.0
3461a G01
–40 20 60
–20 0 40 80
100
TEMPERATURE (°C)
–60 –40
FREQUENCY (MHz)
3.9
3.6
3.3
3.0
2.7
2.4
2.1 20 60
3461a G04
–20 0 40 80
100
DUTY CYCLE (%)
10
CURRENT LIMIT (mA)
9080604020
3461a G02
30 50 70
480
360
240
120
0
TA = 25°C
SHDN PIN VOLTAGE (V)
1.3
CURRENT LIMIT (mA)
1.9
2.3
3461a G05
1.5 1.7 2.1
480
420
360
300
240
180
120
60
0
TA = 25°C
TEMPERATURE (°C)
FB VOLTAGE (V)
1.28
1.27
1.26
1.25
1.24
1.23
1.22
3461a G03
–40 20 60
–20 0 40 80
100
3461a G06
SHDN PIN VOLTAGE (V)
0
SHDN PIN CURRENT (µA)
16
4 8 12
320
280
240
200
160
120
80
40
0
TA = 25°C
VSW
5V/DIV
VOUT
0.2µs/DIV
I
= 60mA
70mA
VOUT
100mV/DIV
50µs/DIV 3461a G09
I
LOAD 35mA
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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 VOUT according to VOUT = 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.
VOUT (Pin 5): Output Pin. Connect to resistor divider. Put
capacitor close to pin and close to GND plane.
VIN (Pin 6): Input Supply Pin. Must be locally bypassed.
BLOCK DIAGRAM
OPERATION
–
+
–
+
–
+
6
1
3
2
∑
VOUT
VOUT
R1 (EXTERNAL)
R2 (EXTERNAL)
RS (EXTERNAL)
CS (EXTERNAL)
FB
SHUTDOWN
SHDN
RAMP
GENERATOR
1.255V
REFERENCE
3MHz*
OSCILLATOR
R
S
Q
4
A1
A2
COMPARATOR
DRIVER
RC
CC
SW
Q1
0.1Ω
GND
VIN
FB
3461a F01
RS
, C
S
OPTIONAL SOFT-START COMPONENTS
5
*LT3461 IS 1.3MHz
Figure 2. Suggested Layout
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.
Figure 1. Block Diagram
R2
R1
GND
C3
L1
C1
VOUT
VIN
SHUTDOWN
+
C2
+
3461a F02
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APPLICATIONS INFORMATION
Inrush Current
The LT3461 has a built-in Schottky diode. When supply
voltage is applied to the VIN pin, the voltage difference
between VIN and VOUT generates inrush current flowing
from input through the inductor and the Schottky diode to
charge the output capacitor. The maximum nonrepetitive
surge current the Schottky diode in the LT3461 can sus-
tain is 1.5A. The selection of inductor and capacitor value
should ensure the peak of the inrush current to be below
1.5A. In addition, turn-on of the LT3461 should be delayed
until the inrush current is less than the maximum current
limit. The peak inrush current can be calculated as follows:
IP=V
IN – 0.6
L
C–1Ω2
exp – π•Ω
2L
C–1Ω2
where L is the inductance, r is the resistance of the induc-
tor and C is the output capacitance.
Table 3 gives inrush peak currents for some component
selections.
Table 3. Inrush Peak Current
VIN (V) L (µH) C (µF) IP (A)
5 4.7 1 1.1
5 10 1 0.9
Thermal Considerations
Significant power dissipation can occur on the LT3461 and
LT3461A, particularly at high input voltage. Device load,
voltage drops in the power path components, and switching
losses are the major contributors. It is important to measure
device power dissipation in an application to ensure that the
LT3461 does not exceed the absolute maximum operating
junction temperature of 125°C over the operating ambient
temperature range. Generally, for supply voltages below 5V the
integrated current limit function provides adequate protection
for nonfault conditions. For supply voltages above 5V, Figures
3a and 3b show the recommended operating region of the
LT3461 and LT3461A, respectively. These graphs are based
on 250mW on-chip dissipation. Improvement of these
numbers can be expected if the LT3461 is supplied from a
separate low voltage rail.
Switching Frequency
The key difference between the LT3461 and LT3461A is
the faster switching frequency of the LT3461A. At 3MHz,
the LT3461A switches at twice the rate of the LT3461. The
higher switching frequency of the LT3461A allows physi-
cally smaller inductors and capacitors to be used in a given
application, but with a slight decrease in efficiency and
maximum output current when compared to the LT3461.
Generally if efficiency and maximum output current are
crucial, or a high output voltage is being generated, the
LT3461 should be used. If application size and cost are
more important, the LT3461A will be the better choice.
Figure 3a. LT3461 Operating Region
Figure 3b. LT3461A Operating Region
VOUT (V)
6
I
OUT
(mA)
80
120
38
3461 F03a
40
014 22 30
160
VIN >15V
VIN = 12V
VIN = 8V
VIN = 5V
VOUT (V)
6
I
OUT
(mA)
80
120
38
3461 F03b
40
014 22 30
160
VIN >15V
VIN = 12V
VIN = 8V
VIN = 5V
LT3461/LT3461A
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APPLICATIONS INFORMATION
Inductor Selection
The inductors used with the LT3461/LT3461A should
havea saturation current rating of 0.3A or greater. If the
device is used in an application where the input supply will
be hot-plugged, then the saturation current rating should
be equal to or greater than the peak inrush current. For
the LT3461, an inductor value between 10µH and 47µH,
depending upon output voltage, will usually be the best
choice for most designs. For the LT3461A, inductor values
between 4.7µH and 15µH inductor will suffice for most
applications. For best loop stability results, the inductor
value selected should provide a ripple current of 70mA or
more. For a given VIN and VOUT the inductor value to use
with LT3461A is estimated by the formula:
L (in microhenries) =
where D
Use twice this value for the LT3461.
Capacitor Selection
Low ESR capacitors should be used at the output to
minimize the output voltage ripple. Multilayer ceramic
capacitors using X5R/X7R dielectrics are preferred as
D• V
IN • VOUT •1sec
1A • 1V
=
V
OUT
+1V – V
IN
V
OUT
+1V
they have a low ESR and maintain capacitance over wide
voltage and temperature range. A 2.2µF output capacitor
is sufficient for most applications using the LT3461, while
a 1µF capacitor is sufficient for most applications using
the LT3461A. High output voltages typically require less
capacitance for loop stability. Always use a capacitor with
sufficient voltage rating.
Either ceramic or solid tantalum capacitors may be used for
the input decoupling capacitor, which should be placed as
close as possible to the LT3461/LT3461A. A 1µF capacitor
is sufficient for most applications.
Phase Lead Capacitor
A small value capacitor can be added across resistor
R1 between the output and the FB pin to reduce output
perturbation due to a load step and to improve transient
response. This phase lead capacitor introduces a pole-zero
pair to the feedback that boosts phase margin near the
cross-over frequency. The following formula is useful to
estimate the capacitor value needed:
CPL =
500kΩ
R2
•1pF
For an application running 50µA in the feedback divider,
capacitor values from 10pF to 22pF work well.
Figure 4. 5V to 12V with Soft-Start Circuit (LT3461A)
Input Current and Output Voltage
TYPICAL APPLICATIONS
VIN
5V
L1
10µH
R1
261k
R2
30.1k
C2
1µF
15pF
C1
1µF
VOUT
12V
70mA
1
2
VIN VOUT
SW
3
4
5
6
FB
SHDN
GND
LT3461A
3461a TA02a
CONTROL
SIGNAL
47nF
47k
C1, C2: TAIYO YUDEN EMK212BJ105
L1: MURATA LQH32CN100K53
IIN
50mV/DIV
VOUT
5V/DIV
CONTROL
SIGNAL
5V/DIV
1ms/DIV 3461 TA02b
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TYPICAL APPLICATIONS
PACKAGE DESCRIPTION
Please refer to http://www.linear.com/product/LT3461#packaging for the most recent package drawings.
1.50 – 1.75
(NOTE 4)
2.80 BSC
0.30 – 0.45
6 PLCS (NOTE 3)
DATUM ‘A’
0.09 – 0.20
(NOTE 3) S6 TSOT-23 0302
2.90 BSC
(NOTE 4)
0.95 BSC
1.90 BSC
0.80 – 0.90
1.00 MAX 0.01 – 0.10
0.20 BSC
0.30 – 0.50 REF
PIN ONE ID
NOTE:
1. DIMENSIONS ARE IN MILLIMETERS
2. DRAWING NOT TO SCALE
3. DIMENSIONS ARE INCLUSIVE OF PLATING
4. DIMENSIONS ARE EXCLUSIVE OF MOLD FLASH AND METAL BURR
5. MOLD FLASH SHALL NOT EXCEED 0.254mm
6. JEDEC PACKAGE REFERENCE IS MO-193
3.85 MA
X
0.62
MAX
0.95
REF
RECOMMENDED SOLDER PAD LAYOUT
PER IPC CALCULATOR
1.4 MIN
2.62 REF
1.22 REF
S6 Package
6-Lead Plastic TSOT-23
(Reference LTC DWG # 05-08-1636)
VIN
3.3V
L1
4.7µH
R1
45.3k
R2
15k
C2
1µF
15pF
C1
1µF
VOUT
5V
115mA
1
2
VIN VOUT
SW
3
4
5
6
FB
SHDN
GND
LT3461A
C1, C2: TAIYO YUDEN X7R LMK212BJ105
L1: MURATA LQH32CN4R7M33 OR EQUIVALENT
3461a TA03a
OFF ON
LOAD CURRENT (mA)
0
EFFICIENCY (%)
80
75
70
65
60
3461a TA03b
6030
120
90
3.3V to 5V Step-Up Converter (LT3461A)
3.3V to 5V Step-Up Converter Efficiency
LT3461/LT3461A
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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 representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
REVISION HISTORY
REV DATE DESCRIPTION PAGE NUMBER
A 01/16 Modified inrush current IP equation. 6
(Revision history begins at Rev B)
LT3461/LT3461A
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LINEAR TECHNOLOGY CORPORATION 2003
LT 0116 REV A • PRINTED IN USA
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com/LT3461
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3.3V to ±5V Dual Output Converter
VIN
3.3V
L1
10µH
332k
30.1k C2
2.2µF
22pF
C1
1µF
VOUT
15V
25mA
1
2
VIN VOUT
SW
3
4
5
6
FB
SHDN
GND
LT3461A
C1: TAIYO YUDEN LMK107BJ105KA
C2: TAIYO YUDEN EMK316BJ225KD (X5R)
L1: MURATA LQH2MCN100K02
3461a TA04a
OFF ON
LOAD CURRENT (mA)
0
EFFICIENCY (%)
75
70
65
60
55
50
3461a TA04b
105 20
30
15 25
V
IN
5V
L1
47µH
280k
10k
C2
0.47µF
50V
22pF
C1
1µF
VOUT
36V
18mA
1
2
VIN VOUT
SW
3
4
5
6
FB
SHDN
GND
LT3461
C1: TAIYO YUDEN X7R LMK212BJ105
C2: MURATA GRM42-6X7R474K50
L1: MURATA LQH32CN470
3461 TA05a
OFF ON
LOAD CURRENT (mA)
0
EFFICIENCY (%)
16
3461 TA05b
8 12 1462 4 10
80
75
70
65
60
55
50
18
VIN
3.3V
L1
4.7µH
45.3k
15k
15pF
C1
1µF
1
2
VIN VOUT
SW
3
4
5
6
FB
SHDN
GND
LT3461A
OFF ON
3461 TA06
C4
1µF
C2
1µF
C3
1µF
–5V
15mA
D1
D2
VOUT
5V
100mA
C1, C2, C3, C4: TAIYO YUDEN JMK107BJ105
D1, D2: PHILIPS PMEG2005EB
L1: MURATA LQH2MCN4R7M02
10Ω