LT3461/LT3461A
1.3MHz/3MHz Step-Up
DC/DC Converters with
Integrated Schottky in ThinSOT
FEATURES
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DESCRIPTIO
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
TM
) Package
APPLICATIO S
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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
The LT
®
3461/LT3461A are general purpose fixed fre-
quency current mode step-up DC/DC converters. Both
devices feature an integrated Schottky and a low V
CESAT
switch allowing a small converter footprint and lower parts
cost. The LT3461 switches at 1.3MHz while the LT3461A
switches at 3MHz. These high switching frequencies en-
able 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 switch-
ing noise typically 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.
, LTC and LT are registered trademarks of Linear Technology Corporation.
ThinSOT is a trademark of Linear Technology Corporation.
TYPICAL APPLICATIO
L1
10µH
1
SW
6
5
V
IN
V
OUT
LT3461A
4
3
SHDN
FB
GND
2
5V to 12V, 70mA Step-Up DC/DC Converter
85
V
IN
5V
C1
1µF
OFF ON
80
EFFICIENCY (%)
261k
15pF
V
OUT
12V
70mA
75
30.1k
C2
1µF
3461 TA01a
70
65
60
0
20
60
40
LOAD CURRENT (mA)
80
3461 TAO1b
U
Efficiency
V
IN
= 5V
V
IN
= 3.3V
3461af
U
U
1
LT3461/LT3461A
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
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
ORDER PART
NUMBER
LT3461AES6
LT3461ES6
S6 PART MARKING
LTAHG
LTAEB
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
Consult LTC Marketing for parts specified with wider operating temperature ranges.
The
●
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
●
ELECTRICAL CHARACTERISTICS
CONDITIONS
MIN
2.5
TYP
MAX
16
UNITS
V
V
V
V
%/V
nA
mA
µA
MHz
MHz
%
%
1.235
1.225
1.255
0.005
1.275
1.280
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:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
I
SW
= 250mA
V
SW
= 5V
I
SCHOTTKY
= 250mA
V
OUT
– SW = 40V
1.5
FB = 1.3V, Not Switching
SHDN = 0V
●
●
●
●
●
40
2.8
0.1
2.1
1.0
82
92
300
420
260
0.01
800
0.03
3.0
1.3
600
350
1
1100
4
0.4
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.
3461af
2
U
mA
mV
µA
mV
µA
V
V
µA
W
U
U
W W
W
LT3461/LT3461A
TYPICAL PERFOR A CE CHARACTERISTICS
Oscillator Frequency (LT3461)
1.6
1.5
480
T
A
= 25°C
1.27
1.3
1.2
1.1
1.0
–40 –20
FB VOLTAGE (V)
1.4
CURRENT LIMIT (mA)
FREQUENCY (MHz)
40
20
60
0
TEMPERATURE (°C)
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)
480
420
CURRENT LIMIT (mA)
360
300
240
180
120
60
SHDN PIN CURRENT (µA)
FREQUENCY (MHz)
Switching Waveform
Circuit of Figure 4
I
LOAD
V
SW
5V/DIV
70mA
35mA
V
OUT
50mV/DIV
I
LOAD
= 60mA
0.2µs/DIV
U W
80
3461a G01
Current Limit
1.28
FB Pin Voltage
360
1.26
1.25
1.24
1.23
0
10
1.22
–40 –20
240
120
100
20
30
40 50 60 70
DUTY CYCLE (%)
80
90
40
20
60
0
TEMPERATURE (°C)
80
100
3461a G02
3461a G03
Current Limit in Soft-Start Mode
320
T
A
= 25°C
280
240
200
160
120
80
40
1.5
2.1
1.9
SHDN PIN VOLTAGE (V)
1.7
0
2.3
3461a G05
SHDN Pin Current
T
A
= 25°C
80 100
3461a G04
0
1.3
0
4
8
12
16
3461a G06
SHDN PIN VOLTAGE (V)
Load Transient Response
Circuit of Figure 4
V
OUT
100mV/DIV
3461a G08
50µs/DIV
3461a G09
3461af
3
LT3461/LT3461A
PI FU CTIO S
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 func-
tions 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 DIAGRA
V
OUT
R1 (EXTERNAL)
FB
R2 (EXTERNAL)
3 FB
C
C
SHUTDOWN
R
S
(EXTERNAL)
R
S
, C
S
OPTIONAL SOFT-START COMPONENTS
OPERATIO
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 compara-
tor 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 in-
creases, more current is delivered to the output; if it
decreases, less current is delivered.
4
W
U
U
U
U
V
IN
6
1.255V
REFERENCE
+
A1
COMPARATOR
1
DRIVER
SW
5 V
OUT
–
R
C
A2
R
S
Q
–
Q1
+
+
∑
0.1Ω
–
4 SHDN
C
S
(EXTERNAL)
3MHz*
OSCILLATOR
*LT3461 IS 1.3MHz
2
GND
3461a F02
RAMP
GENERATOR
Figure 1. Block Diagram
Layout Hints
The high speed operation of the LT3461/LT3461A de-
mands careful attention to board layout. You will not get
advertised performance with careless layout. Figure 2
shows the recommended component placement.
C1
+
GND
L1
V
IN
R2
+
R1
C2
V
OUT
SHUTDOWN
3461a F03
C3
Figure 2. Suggested Layout
3461af
LT3461/LT3461A
APPLICATIO S I FOR ATIO
Inrush Current
The LT3461 has a built-in Schottky diode. When supply
voltage is applied to the V
IN
pin, the voltage difference
between V
IN
and V
OUT
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
sustain 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:
I
OUT
(mA)
⎞
⎛
⎜
π ⎟
V
IN
– 0.6
⎟
• exp
⎜ −
I
P
=
⎟
⎜
L
L
–1
−
1
⎟
⎜
2
⎠
⎝
C
C
I
OUT
(mA)
where L is the inductance, r is the resistance of the
inductor and C is the output capacitance.
Table 3 gives inrush peak currents for some component
selections.
Table 3. Inrush Peak Current
V
IN
(V)
5
5
L (µH)
4.7
10
C (µF)
1
1
I
P
(A)
1.1
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 impor-
tant 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 condi-
tions. 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
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.
3461af
U
numbers can be expected if the LT3461 is supplied from a
separate low voltage rail.
160
V
IN
= 5V
V
IN
= 8V
V
IN
= 12V
V
IN
>15V
80
120
40
0
6
14
22
V
OUT
(V)
3461 F01a
W
U U
30
38
Figure 3a. LT3461 Operating Region
160
V
IN
= 5V
120
V
IN
= 8V
V
IN
= 12V
V
IN
>15V
80
40
0
6
14
22
V
OUT
(V)
30
38
3461 F01b
Figure 3b. LT3461A Operating Region
5
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