LT3481EDD#PBF [Linear]
LT3481 - 36V, 2A, 2.8MHz Step-Down Switching Regulator with 50µA Quiescent Current; Package: DFN; Pins: 10; Temperature Range: -40°C to 85°C;
| 型号： | LT3481EDD#PBF |
| 厂家： | 
Linear |
| 描述： | LT3481 - 36V, 2A, 2.8MHz Step-Down Switching Regulator with 50µA Quiescent Current; Package: DFN; Pins: 10; Temperature Range: -40°C to 85°C 开关 光电二极管 |
| 文件： | 总24页 (文件大小：305K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LT3481
36V, 2A, 2.8MHz Step-Down
Switching Regulator with
50µA Quiescent Current
FEATURES
DESCRIPTION
TheLT®3481isanadjustablefrequency(300kHzto2.8MHz)
monolithic buck switching regulator that accepts input
voltages up to 34V (36V maximum). A high efficiency
0.18Ω switch is included on the die along with a boost
Schottky diode and the necessary oscillator, control, and
logic circuitry. Current mode topology is used for fast
transient response and good loop stability. Low ripple
Burst Mode operation maintains high efficiency at low
output currents while keeping output ripple below 15mV
in a typical application. In addition, the LT3481 can fur-
ther enhance low output current efficiency by drawing
■
Wide Input Range: 3.6V to 34V Operating,
36V Maximum
■
2A Maximum Output Current
Low Ripple Burst Mode® Operation
■
50μA I at 12V to 3.3V
Q
IN
OUT
Output Ripple < 15mV
■
■
■
■
■
■
■
■
■
■
Adjustable Switching Frequency: 300kHz to 2.8MHz
Low Shutdown Current: I < 1μA
Q
Integrated Boost Diode
Power Good Flag
Saturating Switch Design: 0.18Ω On-Resistance
1.265V Feedback Reference Voltage
Output Voltage: 1.265V to 20V
Soft-Start Capability
Synchronizable Between 275kHz to 475kHz
Small 10-Pin Thermally Enhanced MSOP and
(3mm x 3mm) DFN Packages
bias current from the output when V
is above 3V.
OUT
Shutdown reduces input supply current to less than 1μA
while a resistor and capacitor on the RUN/SS pin provide a
controlled output voltage ramp (soft-start). A power good
flag signals when V
reaches 90% of the programmed
OUT
output voltage. The LT3481 is available in 10-Pin MSOP
and 3mm x 3mm DFN packages with exposed pads for
low thermal resistance.
APPLICATIONS
L, LT, LTC, LTM, Burst Mode, Linear Technology and the Linear logo are registered
trademarks of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
■
Automotive Battery Regulation
■
Power for Portable Products
■
Distributed Supply Regulation
■
Industrial Supplies
■
Wall Transformer Regulation
TYPICAL APPLICATION
3.3V Step-Down Converter
Efficiency
V
V
3.3V
2A
90
80
70
60
50
10000.0
1000.0
100.0
10.0
IN
OUT
4.5V TO
34V
V
BD
IN
RUN/SS
BOOST
OFF ON
16.2k
0.47μF
4.7μH
V
C
SW
LT3481
GND
4.7μF
RT
PG
330pF
1.0
BIAS
FB
V
V
= 12V
IN
OUT
324k
= 3.3V
40
30
0.1
60.4k
L = 4.7μ
F = 800 kHz
200k
22μF
0.01
0.0001 0.001
0.01
0.1
1
10
LOAD CURRENT (A)
3481 TA01
3481 TA01b
3481fc
1
LT3481
(Note 1)
ABSOLUTE MAXIMUM RATINGS
Operating Temperature Range (Note 2)
V , RUN/SS Voltage.................................................36V
IN
LT3481E............................................... –40°C to 85°C
LT3481I.............................................. –40°C to 125°C
LT3481H ............................................ –40°C to 150°C
Storage Temperature Range................... –65°C to 150°C
Lead Temperature (Soldering, 10 sec)
BOOST Pin Voltage ...................................................56V
BOOST Pin Above SW Pin.........................................30V
FB, RT, V Voltage.......................................................5V
C
BIAS, PG, BD Voltage................................................30V
Maximum Junction Temperature........................... 125°C
LT3481E, LT3481I............................................. 125°C
LT3481H ........................................................... 150°C
MSE Only.......................................................... 300°C
PIN CONFIGURATION
TOP VIEW
TOP VIEW
BD
BOOST
SW
1
2
3
4
5
10 RT
BD
BOOST
SW
1
2
3
4
5
10 RT
9
8
7
6
V
C
9
8
7
6
V
C
11
FB
11
FB
V
BIAS
PG
IN
V
BIAS
PG
IN
RUN/SS
RUN/SS
MSE PACKAGE
10-LEAD PLASTIC MSOP
DD PACKAGE
10-LEAD (3mm s 3mm) PLASTIC DFN
EXPOSED PAD (PIN 11) IS GND
MUST BE CONNECTED TO GND
EXPOSED PAD (PIN 11) IS GND
MUST BE CONNECTED TO GND
θ
= 40°C/W
JA
θ
= 43°C/W
JA
ORDER INFORMATION
LEAD FREE FINISH
LT3481EDD#PBF
LT3481IDD#PBF
LT3481HDD#PBF
LT3481EMSE#PBF
LT3481IMSE#PBF
LT3481HMSE#PBF
LEAD BASED FINISH
LT3481EDD
TAPE AND REEL
PART MARKING*
LBVS
PACKAGE DESCRIPTION
TEMPERATURE RANGE
–40°C to 85°C
LT3481EDD#TRPBF
LT3481IDD#TRPBF
LT3481HDD#TRPBF
LT3481EMSE#TRPBF
LT3481IMSE#TRPBF
LT3481HMSE#TRPBF
TAPE AND REEL
10-Lead (3mm × 3mm) Plastic DFN
10-Lead (3mm × 3mm) Plastic DFN
10-Lead (3mm × 3mm) Plastic DFN
10-Lead Plastic MSOP
LBVV
–40°C to 125°C
–40°C to 150°C
–40°C to 85°C
LBVS
LTBVT
LTBVW
LTBVT
10-Lead Plastic MSOP
–40°C to 125°C
–40°C to 150°C
TEMPERATURE RANGE
–40°C to 85°C
10-Lead Plastic MSOP
PART MARKING*
LBVS
PACKAGE DESCRIPTION
LT3481EDD#TR
10-Lead (3mm × 3mm) Plastic DFN
10-Lead (3mm × 3mm) Plastic DFN
10-Lead (3mm × 3mm) Plastic DFN
10-Lead Plastic MSOP
LT3481IDD
LT3481IDD#TR
LBVV
–40°C to 125°C
–40°C to 150°C
–40°C to 85°C
LT3481HDD
LT3481HDD#TR
LBVS
LT3481EMSE
LT3481EMSE#TR
LT3481IMSE#TR
LT3481HMSE#TR
LTBVT
LT3481IMSE
LTBVW
LTBVT
10-Lead Plastic MSOP
–40°C to 125°C
–40°C to 150°C
LT3481HMSE
10-Lead Plastic MSOP
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
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/
3481fc
2
LT3481
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 10V, VRUNS/SS = 10V VBOOST = 15V, VBIAS = 3.3V unless otherwise
noted. (Note 2)
PARAMETER
CONDITIONS
MIN
TYP
3
MAX
3.6
0.5
60
UNITS
V
●
●
Minimum Input Voltage
Quiescent Current from V
V
V
V
V
V
V
= 0.2V
0.01
22
μA
μA
μA
μA
μA
μA
V
IN
RUN/SS
= 3V, Not Switching
= 0, Not Switching
BIAS
75
120
0.5
120
5
BIAS
Quiescent Current from BIAS
= 0.2V
0.01
50
RUN/SS
●
= 3V, Not Switching
= 0, Not Switching
BIAS
BIAS
0
Minimum Bias Voltage
Feedback Voltage
2.7
3
1.25
1.24
1.265
1.265
1.29
1.3
V
V
●
●
FB Pin Bias Current (Note 3)
FB Voltage Line Regulation
V
FB
= 1.25V, V = 0.4V
30
0.002
330
800
65
100
nA
%/V
C
4V < V < 34V
0.02
IN
Error Amp g
μMho
m
Error Amp Gain
V Source Current
μA
μA
A/V
V
C
V Sink Current
C
85
V Pin to Switch Current Gain
C
3.5
2
V Clamp Voltage
C
Switching Frequency
R = 8.66k
2.5
1.25
250
2.8
1.4
300
3.1
1.55
350
MHz
MHz
kHz
T
R = 29.4k
T
R = 187k
T
●
●
Minimum Switch Off-Time
Switch Current Limit
130
3.8
360
0.02
1.5
18
200
4.4
nS
A
Duty Cycle = 5%
3.2
Switch V
I
= 2A
SW
mV
μA
V
CESAT
Boost Schottky Reverse Leakage
Minimum Boost Voltage (Note 4)
BOOST Pin Current
V
SW
= 10V, V
= 0V
BIAS
2
2.1
35
10
I
= 1A
mA
μA
V
SW
RUN/SS Pin Current
V
= 2.5V
5
RUN/SS
RUN/SS Input Voltage High
RUN/SS Input Voltage Low
PG Threshold Offset from Feedback Voltage
PG Hysteresis
2.5
0.2
1
V
V
FB
Rising
122
5
mV
mV
μA
μA
PG Leakage
V
V
= 5V
0.1
600
PG
●
PG Sink Current
= 3V
100
PG
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 LT3481E is guaranteed to meet performance specifications
from 0°C to 85°C. Specifications over the –40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls. The LT3481I specifications are
guaranteed over the –40°C to 125°C temperature range. The LT3481H
specifications are guaranteed over the –40°C to 150°C operating
temperature range. High junction temperatures degrade operating
lifetimes. Operating lifetime is derated at junction temperatures greater
than 125°C.
Note 3: Bias current flows into the FB pin.
Note 4: This is the minimum voltage across the boost capacitor needed
to guarantee full saturation of the switch.
3481fc
3
LT3481
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency (VOUT = 3.3V)
Efficiency
Efficiency (VOUT = 5.0V)
100
90
80
70
60
50
40
30
20
100
90
80
70
60
50
40
30
20
90
V
= 12V
IN
85
80
V
= 12V
IN
V
= 7V
IN
V
= 24V
IN
V
= 24V
IN
V
= 12V
IN
75
70
65
60
55
V
= 24V
IN
V
= 3.3V
OUT
L: NEC PLC-0745-4R7
f: 800kHz
L: NEC PLC-0745-4R7
f: 800kHz
L = 10μH
10
0
10
0
LOAD = 1A
50
0.0001 0.001
0.01
0.1
1
10
0.0001 0.001
0.01
0.1
1
10
0.5
1
2
2.5
3
0
1.5
LOAD CURRENT (A)
LOAD CURRENT (A)
SWITCHING FREQUENCY (MHz)
3481 G02
3481 G01
3481 G03
No Load Supply Current
No Load Supply Current
Maximum Load Current
80
400
4.0
3.5
3.0
T
= 25°C
CATCH DIODE: DIODES, INC. PDS360
A
70
60
350
300
V
V
= 12V
TYPICAL
IN
OUT
= 3.3V
50
40
30
20
10
250
200
150
100
50
2.5
2.0
INCREASED SUPPLY
MINIMUM
CURRENT DUE TO CATCH
DIODE LEAKAGE AT
HIGH TEMPERATURE
V
A
= 3.3V
OUT
T
= 25°C
1.5
1.0
L = 4.7μH
f = 800kHz
FRONT PAGE APPLICATION
0
0
5
10
20
25
30
35
0
15
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3481 G05
10
20
15
INPUT VOLTAGE (V)
25
30
5
INPUT VOLTAGE (V)
3481 G04
3481 G06
Maximum Load Current
Switch Current Limit
Switch Current Limit
4.0
3.5
3.0
4.0
3.5
3.0
4.5
4.0
3.0
2.5
2.0
1.5
1.0
0.5
0
DUTY CYCLE = 10 %
TYPICAL
DUTY CYCLE = 90 %
2.5
2.0
2.5
2.0
MINIMUM
V
T
= 5.0V
OUT
A
= 25°C
1.5
1.0
1.5
1.0
L = 4.7μH
f = 800kHz
–50 –25
0
25
50
75 100 125
10
20
15
INPUT VOLTAGE (V)
25
30
20
60
40
DUTY CYCLE (%)
80
100
5
0
TEMPERATURE (°C)
3481 G09
3481 G07
3481 G08
3481fc
4
LT3481
TYPICAL PERFORMANCE CHARACTERISTICS
Switch Voltage Drop
Feedback Voltage
BOOST Pin Current
90
80
70
60
50
40
30
20
10
1.290
1.285
1.280
1.275
1.270
1.265
1.260
1.255
1.250
700
600
500
400
300
200
100
0
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
4381 G12
0
500 1000 1500
3500
2000 3000 3500
2500
2000 2500 3000
0
500 1000 1500
SWITCH CURRENT (mA)
SWITCH CURRENT (mA)
3481 G11
3481 G10
Frequency Foldback
Minimum Switch On-Time
Switching Frequency
1.20
1.15
1.10
1.05
1.00
0.95
0.90
0.85
0.80
1200
1000
800
140
120
R
T
= 45.3k
R
= 45.3k
T
100
80
60
40
20
600
400
200
0
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
4381 G13
800 1200 1400
1000
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (˚C)
3481 G15
0
200 400 600
FB PIN VOLTAGE (mV)
3481 G14
Soft-Start
RUN/SS Pin Current
Boost Diode
12
10
8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
6
4
2
0
0
20
RUN/SS PIN VOLTAGE (V)
30
35
0
1.0
BOOST DIODE CURRENT (A)
2.0
0
5
10
15
25
0.5
1.5
0.5
1
2
2.5
3
3.5
0
1.5
RUN/SS PIN VOLTAGE (V)
3481 G17
3481 G18
3481 G16
3481fc
5
LT3481
TYPICAL PERFORMANCE CHARACTERISTICS
Error Amp Output Current
Minimum Input Voltage
Minimum Input Voltage
100
80
4.5
4.0
3.5
3.0
6.5
6.0
5.5
5.0
60
40
20
0
–20
–40
–60
V
T
= 3.3V
V
T
= 5.0V
OUT
A
OUT
A
2.5
2.0
4.5
4.0
= 25°C
= 25°C
L = 4.7μH
f = 800kHz
L = 4.7μH
f = 800kHz
–80
1.065
0.001
0.01
0.1
LOAD CURRENT (A)
1
0.1
LOAD CURRENT (A)
10
0.001
0.01
1
10
1.165
1?.265
1.365
1.465
FB PIN VOLTAGE (V)
3481 G20
3481 G21
3481 G19
Switching Waveforms;
Burst Mode Operation
Power Good Threshold
VC Voltages
2.50
1.200
V
= 12V; FRONT PAGE APPLICATION
= 10mA
IN
I
LOAD
I
L
2.00
1.50
1.180
1.160
0.5A/DIV
CURRENT LIMIT CLAMP
SWITCHING THRESHOLD
V
SW
5V/DIV
1.00
0.50
0
1.140
1.120
1.100
V
OUT
10mV/DIV
PG RISING
–50 –25
0
25
50 75 100 125 150
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3481 G23
3481 G24
5μs/DIV
TEMPERATURE (°C)
3481 G22
Switching Waveforms; Transition
from Burst Mode Operation to
Full Frequency
Switching Waveforms; Full
Frequency Continuous Operation
I
L
0.5A/DIV
I
L
0.5A/DIV
V
RUN/SS
5V/DIV
V
RUN/SS
5V/DIV
V
V
OUT
OUT
10mV/DIV
10mV/DIV
V
I
= 12V; FRONT PAGE APPLICATION
= 1A
V
I
= 12V; FRONT PAGE APPLICATION
= 140mA
IN
LOAD
IN
LOAD
3481 G26
3481 G25
1μs/DIV
1μs/DIV
3481fc
6
LT3481
PIN FUNCTIONS
BD (Pin 1): This pin connects to the anode of the boost
Schottky diode.
PG (Pin 6): The PG pin is the open-collector output of an
internal comparator. PG remains low until the FB pin is
within 10% of the final regulation voltage. PG output is
BOOST (Pin 2): This pin is used to provide a drive
voltage,higherthantheinputvoltage,totheinternalbipolar
NPN power switch.
valid when V is above 3.5V and RUN/SS is high.
IN
BIAS (Pin 7): The BIAS pin supplies the current to the
LT3481’s internal regulator. Tie this pin to the lowest
SW (Pin 3): The SW pin is the output of the internal power
switch. Connect this pin to the inductor, catch diode and
boost capacitor.
available voltage source above 3V (typically V ). This
OUT
architectureincreasesefficiencyespeciallywhentheinput
voltage is much higher than the output.
V (Pin 4): The V pin supplies current to the LT3481’s
IN
IN
FB (Pin 8): The LT3481 regulates the FB pin to 1.265V.
Connect the feedback resistor divider tap to this pin.
internal regulator and to the internal power switch. This
pin must be locally bypassed.
V (Pin 9): The V pin is the output of the internal error
C
C
RUN/SS (Pin 5): The RUN/SS pin is used to put the
LT3481 in shutdown mode. Tie to ground to shut down
the LT3481. Tie to 2.3V or more for normal operation. If
amplifier. The voltage on this pin controls the peak switch
current. Tie an RC network from this pin to ground to
compensate the control loop.
the shutdown feature is not used, tie this pin to the V
IN
pin. RUN/SS also provides a soft-start function; see the
RT(Pin10):OscillatorResistorInput.Connectingaresistor
to ground from this pin sets the switching frequency.
Applications Information section.
Exposed Pad (Pin 11): Ground. The Exposed Pad must
be soldered to PCB.
BLOCK DIAGRAM
V
IN
V
IN
4
7
C1
BIAS
–
+
INTERNAL 1.265V REF
BD
1
RUN/SS
RT
5
SLOPE COMP
3
SWITCH
BOOST
2
3
LATCH
C3
D1
R
OSCILLATOR
300kHz TO 2.8MHz
Q
10
S
L1
SW
V
R
OUT
T
DISABLE
C2
SOFT-START
Burst Mode
DETECT
PG
6
V
C
CLAMP
ERROR AMP
+ 1.12V
–
+
–
V
C
9
C
C
C
F
R
C
GND
11
FB
8
R2
R1
3481 BD
3481fc
7
LT3481
OPERATION
The LT3481 is a constant frequency, current mode step-
down regulator. An oscillator, with frequency set by RT,
enables an RS flip-flop, turning on the internal power
switch. An amplifier and comparator monitor the current
The switch driver operates from either the input or from
the BOOST pin. An external capacitor and diode are used
to generate a voltage at the BOOST pin that is higher than
the input supply. This allows the driver to fully saturate
the internal bipolar NPN power switch for efficient
operation.
flowing between the V and SW pins, turning the switch
IN
off when this current reaches a level determined by the
voltage at V . An error amplifier measures the output
C
To further optimize efficiency, the LT3481 automatically
switches to Burst Mode operation in light load situations.
Between bursts, all circuitry associated with controlling
the output switch is shut down reducing the input supply
current to 50μA in a typical application.
voltage through an external resistor divider tied to the FB
pin and servos the V pin. If the error amplifier’s output
C
increases, more current is delivered to the output; if it
decreases,lesscurrentisdelivered.Anactiveclamponthe
V pinprovidescurrentlimit. TheV pinisalsoclampedto
C
C
The oscillator reduces the LT3481’s operating frequency
when the voltage at the FB pin is low. This frequency
foldbackhelpstocontroltheoutputcurrentduringstartup
and overload.
the voltage on the RUN/SS pin; soft-start is implemented
by generating a voltage ramp at the RUN/SS pin using an
external resistor and capacitor.
An internal regulator provides power to the control cir-
cuitry. The bias regulator normally draws power from the
TheLT3481containsapowergoodcomparatorwhichtrips
when the FB pin is at 91% of its regulated value. The PG
output is an open-collector transistor that is off when the
output is in regulation, allowing an external resistor to pull
the PG pin high. Power good is valid when the LT3481 is
V
IN
pin, but if the BIAS pin is connected to an external
voltage higher than 3V bias power will be drawn from the
external source (typically the regulated output voltage).
This improves efficiency. The RUN/SS pin is used to place
the LT3481 in shutdown, disconnecting the output and
reducing the input current to less than 1μA.
enabled and V is above 3.6V.
IN
3481fc
8
LT3481
APPLICATIONS INFORMATION
FB Resistor Network
where V is the typical input voltage, V
is the output
IN
OUT
SW
voltage,isthecatchdiodedrop(~0.5V),V istheinternal
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the 1% resis-
tors according to:
switch drop (~0.5V at max load). This equation shows
that slower switching frequency is necessary to safely
accommodate high V /V
ratio. Also, as shown in
IN OUT
V
⎛
⎞
⎠
the next section, lower frequency allows a lower dropout
voltage. The reason input voltage range depends on the
switching frequency is because the LT3481 switch has
finite minimum on and off times. The switch can turn on
for a minimum of ~150ns and turn off for a minimum of
~150ns. This means that the minimum and maximum
duty cycles are:
OUT
R1=R2
–1
⎜
⎝
⎟
1.265
Reference designators refer to the Block Diagram.
Setting the Switching Frequency
The LT3481 uses a constant frequency PWM architecture
thatcanbeprogrammedtoswitchfrom300kHzto2.8MHz
by using a resistor tied from the RT pin to ground. A table
DCMIN = fSWtON MIN
(
)
DCMAX =1– fSWtOFF MIN
showing the necessary R value for a desired switching
(
)
T
frequency is in Figure 1.
where f is the switching frequency, the t
is the
ON(MIN)
SW
SWITCHING FREQUENCY (MHz)
R VALUE (kΩ)
T
minimum switch on-time (~150ns), and the t
is
OFF(MIN)
0.2
0.3
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
2.8
267
187
the minimum switch off-time (~150ns). These equations
show that duty cycle range increases when switching
frequency is decreased.
133
84.5
60.4
45.3
36.5
29.4
23.7
20.5
16.9
14.3
12.1
10.2
8.66
A good choice of switching frequency should allow ad-
equate input voltage range (see next section) and keep
the inductor and capacitor values small.
Input Voltage Range
The maximum input voltage for LT3481 applications
depends on switching frequency, the absolute maximum
ratings on V and BOOST pins, and on operating mode.
IN
Figure 1. Switching Frequency vs RT Value
Iftheoutputisinstart-uporshort-circuitoperatingmodes,
Operating Frequency Trade-Offs
then V must be below 34V and below the result of the
IN
Selectionoftheoperatingfrequencyisatrade-offbetween
efficiency,componentsize,minimumdropoutvoltage,and
maximum input voltage. The advantage of high frequency
operationisthatsmallerinductorandcapacitorvaluesmay
be used. The disadvantages are lower efficiency, lower
maximum input voltage, and higher dropout voltage. The
following equation:
VOUT + VD
VIN MAX
=
) – VD + VSW
(
)
fSWtON MIN
(
where V
OUT D
is the maximum operating input voltage,
IN(MAX)
highest acceptable switching frequency (f
given application can be calculated as follows:
) for a
V
is the output voltage, V is the catch diode drop
SW(MAX)
(~0.5V), V is the internal switch drop (~0.5V at max
SW
load), f is the switching frequency (set by R ), and
SW
T
VD + VOUT
t
istheminimumswitchon-time(~150ns).Notethat
fSW MAX
=
ON(MIN)
(
)
tON MIN V + V – V
(
)
D
IN
SW
a higher switching frequency will depress the maximum
(
)
operating input voltage. Conversely, a lower switching
3481fc
9
LT3481
APPLICATIONS INFORMATION
frequency will be necessary to achieve safe operation at
high input voltages.
at least 3.5A at low duty cycles and decreases linearly to
2.5A at DC = 0.8. The maximum output current is a func-
tion of the inductor ripple current:
Iftheoutputisinregulationandnoshort-circuitorstart-up
events are expected, then input voltage transients of up to
36V are acceptable regardless of the switching frequency.
In this mode, the LT3481 may enter pulse-skipping opera-
tion where some switching pulses are skipped to maintain
output regulation. In this mode the output voltage ripple
and inductor current ripple will be higher than in normal
operation.
I
= I – ΔI /2
LIM L
OUT(MAX)
Be sure to pick an inductor ripple current that provides
sufficient maximum output current (I ).
OUT(MAX)
The largest inductor ripple current occurs at the highest
V . To guarantee that the ripple current stays below the
IN
specified maximum, the inductor value should be chosen
according to the following equation:
The minimum input voltage is determined by either the
LT3481’s minimum operating voltage of ~3.6V or by its
maximum duty cycle (see equation in previous section).
The minimum input voltage due to duty cycle is:
⎛
⎜
⎞
⎟
⎛
⎞
VOUT + VD
fΔIL
VOUT + VD
L =
1–
⎜
⎟
⎜
⎝
⎟
⎠
V
⎝
⎠
IN MAX
(
)
VOUT + VD
where V is the voltage drop of the catch diode (~0.4V),
D
V
=
) – VD + VSW
IN MIN
(
)
1– fSWtOFF MIN
V
is the maximum input voltage, V
is the output
IN(MAX)
OUT
(
voltage, f is the switching frequency (set by RT), and
SW
L is in the inductor value.
whereV
istheminimuminputvoltage,andt
OFF(MIN)
IN(MIN)
is the minimum switch off-time (150ns). Note that higher
switching frequency will increase the minimum input
voltage. If a lower dropout voltage is desired, a lower
switching frequency should be used.
Theinductor’sRMScurrentratingmustbegreaterthanthe
maximumloadcurrentanditssaturationcurrentshouldbe
about 30% higher. For robust operation in fault conditions
(start-up or short circuit) and high input voltage (>30V),
the saturation current should be above 3.5A. To keep the
efficiency high, the series resistance (DCR) should be less
than 0.1Ω, and the core material should be intended for
high frequency applications. Table 1 lists several vendors
and suitable types.
Inductor Selection
For a given input and output voltage, the inductor value
and switching frequency will determine the ripple current.
The ripple current ΔI increases with higher V or V
L
IN
OUT
and decreases with higher inductance and faster switch-
ing frequency. A reasonable starting point for selecting
the ripple current is:
Table 1. Inductor Vendors
VENDOR URL
PART SERIES
TYPE
Murata
TDK
www.murata.com
LQH55D
Open
www.componenttdk.com SLF7045
Shielded
Shielded
ΔI = 0.4( I
)
L
( OUT(MAX)
SLF10145
D62CB
D63CB
D75C
where I
is the maximum output load current. To
OUT(MAX)
Toko
www.toko.com
Shielded
Shielded
Shielded
Open
guarantee sufficient output current, peak inductor current
mustbelowerthantheLT3481’sswitchcurrentlimit(I ).
The peak inductor current is:
LIM
D75F
Sumida
www.sumida.com
CR54
Open
I
= I
+ ΔI /2
OUT(MAX) L
L(PEAK)
CDRH74
CDRH6D38
CR75
Shielded
Shielded
Open
where I
is the peak inductor current, I
is
L(PEAK)
OUT(MAX)
the maximum output load current, and ΔI is the inductor
L
ripple current. The LT3481’s switch current limit (I ) is
LIM
3481fc
10
LT3481
APPLICATIONS INFORMATION
Of course, such a simple design guide will not always
resultintheoptimuminductorforyourapplication.Alarger
value inductor provides a slightly higher maximum load
current and will reduce the output voltage ripple. If your
load is lower than 2A, then you can decrease the value of
the inductor and operate with higher ripple current. This
allows you to use a physically smaller inductor, or one
with a lower DCR resulting in higher efficiency. There are
several graphs in the Typical Performance Characteristics
section of this data sheet that show the maximum load
current as a function of input voltage and inductor value
for several popular output voltages. Low inductance may
result in discontinuous mode operation, which is okay
but further reduces maximum load current. For details
of maximum output current and discontinuous mode
operation, see Linear Technology Application Note 44.
PCB Layout section). A second precaution regarding the
ceramic input capacitor concerns the maximum input
voltage rating of the LT3481. A ceramic input capacitor
combined with trace or cable inductance forms a high
quality (under damped) tank circuit. If the LT3481 circuit
is plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT3481’s
voltage rating. This situation is easily avoided (see the Hot
Plugging Safety section).
For space sensitive applications, a 2.2μF ceramic capaci-
tor can be used for local bypassing of the LT3481 input.
However, the lower input capacitance will result in in-
creased input current ripple and input voltage ripple, and
may couple noise into other circuitry. Also, the increased
voltage ripple will raise the minimum operating voltage
of the LT3481 to ~3.7V.
Finally, for duty cycles greater than 50% (V /V
>
OUT IN
0.5), there is a minimum inductance required to avoid
subharmonic oscillations. See AN19.
Output Capacitor and Output Ripple
The output capacitor has two essential functions. Along
withtheinductor,itfiltersthesquarewavegeneratedbythe
LT3481toproducetheDCoutput. Inthisroleitdetermines
the output ripple, and low impedance at the switching
frequency is important. The second function is to store
energy in order to satisfy transient loads and stabilize the
LT3481’s control loop. Ceramic capacitors have very low
equivalent series resistance (ESR) and provide the best
ripple performance. A good starting value is:
Input Capacitor
BypasstheinputoftheLT3481circuitwithaceramiccapaci-
tor of X7R or X5R type. Y5V types have poor performance
over temperature and applied voltage, and should not be
used. A 4.7μF to 10μF ceramic capacitor is adequate to
bypasstheLT3481andwilleasilyhandletheripplecurrent.
Notethatlargerinputcapacitanceisrequiredwhenalower
switching frequency is used. If the input power source has
high impedance, or there is significant inductance due to
long wires or cables, additional bulk capacitance may be
necessary. This can be provided with a low performance
electrolytic capacitor.
100
COUT
=
VOUT SW
f
where f
is in MHz, and C is the recommended
OUT
SW
output capacitance in μF. Use X5R or X7R types. This
choice will provide low output ripple and good transient
response. Transient performance can be improved with
a higher value capacitor if the compensation network is
also adjusted to maintain the loop bandwidth. A lower
value of output capacitor can be used to save space
and cost but transient performance will suffer. See the
FrequencyCompensationsectiontochooseanappropriate
compensation network.
Step-down regulators draw current from the input supply
in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage
ripple at the LT3481 and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
A 4.7μF capacitor is capable of this task, but only if it is
placed close to the LT3481 and the catch diode (see the
3481fc
11
LT3481
APPLICATIONS INFORMATION
Table 2. Capacitor Vendors
VENDOR
PHONE
URL
PART SERIES
Ceramic,
Polymer,
Tantalum
Ceramic,
Tantalum
Ceramic,
Polymer,
Tantalum
Ceramic
COMMANDS
Panasonic
(714) 373-7366
www.panasonic.com
EEF Series
Kemet
Sanyo
(864) 963-6300
(408) 749-9714
www.kemet.com
T494, T495
POSCAP
www.sanyovideo.com
Murata
AVX
(408) 436-1300
(864) 963-6300
www.murata.com
www.avxcorp.com
Ceramic,
Tantalum
Ceramic
TPS Series
Taiyo Yuden
www.taiyo-yuden.com
Table 3. Diode Vendors
PART NUMBER
When choosing a capacitor, look carefully through the
data sheet to find out what the actual capacitance is under
operating conditions (applied voltage and temperature).
A physically larger capacitor, or one with a higher voltage
rating, may be required. High performance tantalum or
electrolyticcapacitorscanbeusedfortheoutputcapacitor.
Low ESR is important, so choose one that is intended for
use in switching regulators. The ESR should be specified
by the supplier, and should be 0.05Ω or less. Such a
capacitor will be larger than a ceramic capacitor and will
have a larger capacitance, because the capacitor must be
large to achieve low ESR. Table 2 lists several capacitor
vendors.
V
I
V AT 1A
V AT 2A
R
AVE
F
F
(V)
(A)
(mV)
(mV)
On Semicnductor
MBRM120E
MBRM140
20
40
1
1
530
550
595
Diodes Inc.
B120
20
30
20
30
40
1
1
2
2
2
500
500
B130
B220
500
500
500
B230
DFLS240L
International Rectifier
10BQ030
20BQ030
30
30
1
2
420
470
470
Ceramic Capacitors
Catch Diode
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
whenusedwiththeLT3481duetotheirpiezoelectricnature.
When in Burst Mode operation, the LT3481’s switching
frequency depends on the load current, and at very light
loads the LT3481 can excite the ceramic capacitor at audio
frequencies, generating audible noise. Since the LT3481
operates at a lower current limit during Burst Mode op-
eration, the noise is typically very quiet to a casual ear.
If this is unacceptable, use a high performance tantalum
or electrolytic capacitor at the output.
The catch diode conducts current only during switch off
time. Average forward current in normal operation can
be calculated from:
I
= I (V – V )/V
OUT IN OUT IN
D(AVG)
where I
is the output load current. The only reason to
OUT
consideradiodewithalargercurrentratingthannecessary
for nominal operation is for the worst-case condition of
shorted output. The diode current will then increase to the
typical peak switch current. Peak reverse voltage is equal
to the regulator input voltage. Use a diode with a reverse
voltage rating greater than the input voltage. Table 3 lists
several Schottky diodes and their manufacturers.
3481fc
12
LT3481
APPLICATIONS INFORMATION
A final precaution regarding ceramic capacitors concerns
themaximuminputvoltageratingoftheLT3481.Aceramic
input capacitor combined with trace or cable inductance
forms a high quality (under damped) tank circuit. If the
LT3481 circuit is plugged into a live supply, the input volt-
agecanringtotwiceitsnominalvalue, possiblyexceeding
the LT3481’s rating. This situation is easily avoided (see
the Hot Plugging Safely section).
capacitor on the V pin (C ) integrates the error amplifier
C C
output current, resulting in two poles in the loop. In most
cases a zero is required and comes from either the output
capacitor ESR or from a resistor R in series with C .
C
C
This simple model works well as long as the value of the
inductor is not too high and the loop crossover frequency
is much lower than the switching frequency. A phase lead
capacitor (C ) across the feedback divider may improve
PL
the transient response. Figure 3 shows the transient
response when the load current is stepped from 500mA
to 1500mA and back to 500mA.
Frequency Compensation
The LT3481 uses current mode control to regulate the
output.Thissimplifiesloopcompensation.Inparticular,the
LT3481 does not require the ESR of the output capacitor
for stability, so you are free to use ceramic capacitors to
achieve low output ripple and small circuit size. Frequency
compensation is provided by the components tied to the
V pin, as shown in Figure 2. Generally a capacitor (C )
LT3481
CURRENT MODE
POWER STAGE
m
SW
OUTPUT
ERROR
AMPLIFIER
g
= 3.5mho
C
PL
R1
FB
–
g
=
C
C
m
330μmho
ESR
and a resistor (R ) in series to ground are used. In addi-
+
1.265V
C
C1
+
3Meg
tion, there may be lower value capacitor in parallel. This
C1
capacitor (C ) is not part of the loop compensation but
F
POLYMER
OR
TANTALUM
CERAMIC
is used to filter noise at the switching frequency, and is
required only if a phase-lead capacitor is used or if the
output capacitor has high ESR.
V
C
GND
R
C
R2
C
F
Loop compensation determines the stability and transient
performance. Designing the compensation network is
a bit complicated and the best values depend on the
application and in particular the type of output capacitor.
A practical approach is to start with one of the circuits in
this data sheet that is similar to your application and tune
the compensation network to optimize the performance.
Stability should then be checked across all operating
conditions, including load current, input voltage and
temperature. The LT1375 data sheet contains a more
thorough discussion of loop compensation and describes
how to test the stability using a transient load. Figure 2
shows an equivalent circuit for the LT3481 control loop.
The error amplifier is a transconductance amplifier with
finite output impedance. The power section, consisting
of the modulator, power switch and inductor, is modeled
as a transconductance amplifier generating an output
C
C
3481 F02
Figure 2. Model for Loop Response
V
= 12V; FRONT PAGE APPLICATION
OUT
I
L
1A/DIV
V
OUT
100mV/DIV
10μs/DIV
3481 F03
Figure 3. Transient Load Response of the LT3481 Front Page
Application as the Load Current is Stepped from 500mA to
1500mA. VOUT = 3.3V
current proportional to the voltage at the V pin. Note that
C
the output capacitor integrates this current, and that the
3481fc
13
LT3481
APPLICATIONS INFORMATION
Burst Mode Operation
boost diode can be tied to the input (Figure 5c), or to
another supply greater than 2.8V. The circuit in Figure 5a
is more efficient because the BOOST pin current and BIAS
pin quiescent current comes from a lower voltage source.
You must also be sure that the maximum voltage ratings
of the BOOST and BIAS pins are not exceeded.
To enhance efficiency at light loads, the LT3481 auto-
matically switches to Burst Mode operation which keeps
the output capacitor charged to the proper voltage while
minimizingtheinputquiescentcurrent.DuringBurstMode
operation,theLT3481deliverssinglecycleburstsofcurrent
to the output capacitor followed by sleep periods where
the output power is delivered to the load by the output
V
OUT
BD
BOOST
capacitor. Inaddition, V andBIASquiescentcurrentsare
IN
V
V
IN
LT3481
GND
IN
C3
reduced to typically 20μA and 50μA respectively during
the sleep time. As the load current decreases towards a
no load condition, the percentage of time that the LT3481
operates in sleep mode increases and the average input
current is greatly reduced resulting in higher efficiency.
See Figure 4.
SW
4.7μF
(5a) For V
> 2.8V
OUT
V
OUT
V
I
= 12V; FRONT PAGE APPLICATION
= 10mA
IN
LOAD
D2
BD
I
L
BOOST
0.5A/DIV
V
V
IN
LT3481
IN
C3
SW
GND
4.7μF
V
SW
5V/DIV
V
OUT
10mV/DIV
(5b) For 2.5V < V
< 2.8V
OUT
V
OUT
3481 F04
5μs/DIV
BD
BOOST
V
Figure 4. Burst Mode Operation
V
IN
LT3481
IN
C3
BOOST and BIAS Pin Considerations
SW
GND
4.7μF
Capacitor C3 and the internal boost Schottky diode (see
the Block Diagram) are used to generate a boost volt-
age that is higher than the input voltage. In most cases
a 0.22μF capacitor will work well. Figure 2 shows three
ways to arrange the boost circuit. The BOOST pin must be
more than 2.3V above the SW pin for best efficiency. For
outputs of 3V and above, the standard circuit (Figure 5a)
is best. For outputs between 2.8V and 3V, use a 1μF boost
capacitor. A 2.5V output presents a special case because it
is marginally adequate to support the boosted drive stage
whileusingtheinternalboostdiode.ForreliableBOOSTpin
operation with 2.5V outputs use a good external Schottky
diode (such as the ON Semi MBR0540), and a 1μF boost
capacitor (see Figure 5b). For lower output voltages the
(5c) For V
< 2.5V
OUT
3481 FO5
Figure 5. Three Circuits For Generating The Boost Voltage
The minimum operating voltage of an LT3481 application
is limited by the minimum input voltage (3.6V) and by the
maximum duty cycle as outlined in a previous section. For
proper startup, the minimum input voltage is also limited
by the boost circuit. If the input voltage is ramped slowly,
or the LT3481 is turned on with its RUN/SS pin when the
output is already in regulation, then the boost capacitor
3481fc
14
LT3481
APPLICATIONS INFORMATION
may not be fully charged. Because the boost capacitor is
charged with the energy stored in the inductor, the circuit
will rely on some minimum load current to get the boost
circuit running properly. This minimum load will depend
on input and output voltages, and on the arrangement of
the boost circuit. The minimum load generally goes to
zero once the circuit has started. Figure 6 shows a plot
of minimum load to start and to run as a function of input
voltage. In many cases the discharged output capacitor
will present a load to the switcher, which will allow it to
Soft-Start
The RUN/SS pin can be used to soft-start the LT3481,
reducing the maximum input current during start-up.
The RUN/SS pin is driven through an external RC filter to
create a voltage ramp at this pin. Figure 7 shows the start-
up and shutdown waveforms with the soft-start circuit.
By choosing a large RC time constant, the peak start-up
current can be reduced to the current that is required to
regulate the output, with no overshoot. Choose the value
oftheresistorsothatitcansupply20μAwhentheRUN/SS
pin reaches 2.3V.
start. The plots show the worst-case situation where V
IN
is ramping very slowly. For lower start-up voltage, the
boost diode can be tied to V ; however, this restricts the
IN
input range to one-half of the absolute maximum rating
I
of the BOOST pin.
L
RUN
15k
1A/DIV
At light loads, the inductor current becomes discontinu-
ous and the effective duty cycle can be very high. This
reduces the minimum input voltage to approximately
RUN/SS
GND
V
RUN/SS
2V/DIV
0.22μF
V
OUT
2V/DIV
300mV above V . At higher load currents, the inductor
OUT
current is continuous and the duty cycle is limited by the
maximum duty cycle of the LT3481, requiring a higher
input voltage to maintain regulation.
3481 F07
2ms/DIV
Figure 7. To Soft-Start the LT3481, Add a Resisitor
and Capacitor to the RUN/SS Pin
6.0
TO START
Synchronization
5.5
5.0
4.5
4.0
The internal oscillator of the LT3481 can be synchronized
to an external 275kHz to 475kHz clock by using a 5pF
to 20pF capacitor to connect the clock signal to the RT
pin. The resistor tying the RT pin to ground should be
chosen such that the LT3481 oscillates 20% lower than
the intended synchronization frequency (see Setting the
Switching Frequency section).
TO RUN
3.5
V
A
= 3.3V
3.0
OUT
T
= 25°C
L = 4.7μ
2.5
2.0
f = 800 kHz
0.001
0.01
0.1
1
10
LOAD CURRENT (A)
8.0
7.0
6.0
5.0
4.0
3.0
2.0
The LT3481 should not be synchronized until its output
is near regulation as indicated by the PG flag. This can be
done with the system microcontroller/microprocessor or
with a discrete circuit by using the PG output. If a sync
signal is applied while the PG is low, the LT3481 may
exhibit erratic operation. See Typical Applications.
TO START
TO RUN
V
A
= 5.0V
= 25°C
OUT
T
L = 4.7μ
When applying a sync signal, positive clock transitions
reset LT3481’s internal clock and negative transitions
initiate a switch cycle. The amplitude of the sync signal
must be at least 2V. The sync signal duty cycle can range
f = 800 kHz
0.001
0.01
0.1
1
10
LOAD CURRENT (A)
3481 F06
Figure 6. The Minimum Input Voltage Depends on
Output Voltage, Load Current and Boost Circuit
3481fc
15
LT3481
APPLICATIONS INFORMATION
from 5% up to a maximum value given by the following
your system can tolerate a few mA in this state. If you
ground the RUN/SS pin, the SW pin current will drop to
equation:
essentially zero. However, if the V pin is grounded while
IN
⎛
⎞
VOUT + VD
the output is held high, then parasitic diodes inside the
DCSYNC MAX = 1–
– fSW • 600ns
⎜
⎟
(
)
V – VSW + VD
⎝
⎠
LT3481 can pull large currents from the output through
IN
the SW pin and the V pin. Figure 8 shows a circuit that
IN
where V
is the programmed output voltage, V is the
D
OUT
will run only when the input voltage is present and that
diode forward drop, V is the typical input voltage, V
IN
SW
protects against a shorted or reversed input.
is the switch drop, and f is the desired switching fre-
SW
quency. For example, a 24V input to 5V output at 300kHz
D4
MBRS140
can be synchronized to a square wave with a maximum
V
V
BOOST
SW
IN
IN
duty cycle of 60%. For some applications, such as 12V
IN
LT3481
to 5V
at 350kHz, the maximum allowable sync duty
OUT
V
OUT
RUN/SS
cycle will be less than 50%. If a low duty cycle clock can-
not be obtained from the system, then a one-shot should
be used between the sync signal and the LT3481. See
Typical Applications.
V
C
GND FB
BACKUP
The value of the coupling capacitor which connects the
clock signal to the RT pin should be chosen based on the
clock signal amplitude. Good starting values for 3.3V and
5V clock signals are 10pF and 5pF, respectively. These
values should be tested and adjusted for each individual
application to assure reliable operation.
3481 F08
Figure 8. Diode D4 Prevents a Shorted Input from
Discharging a Backup Battery Tied to the Output. It Also
Protects the Circuit from a Reversed Input. The LT3481
Runs Only When the Input is Present
PCB Layout
Caution should be used when synchronizing more than
50% above the initial switching frequency (as set by
For proper operation and minimum EMI, care must be
taken during printed circuit board layout. Figure 9 shows
the recommended component placement with trace,
ground plane and via locations. Note that large, switched
the R resistor) because at higher clock frequencies the
T
amplitude of the internal slope compensation used to
prevent subharmonic switching is reduced. This type of
subharmonic switching only occurs at input voltages less
than twice output voltage. Higher inductor values will tend
to reduce this problem.
currents flow in the LT3481’s V and SW pins, the catch
IN
diode (D1) and the input capacitor (C1). The loop formed
bythesecomponentsshouldbeassmallaspossible.These
components,alongwiththeinductorandoutputcapacitor,
should be placed on the same side of the circuit board,
and their connections should be made on that layer. Place
a local, unbroken ground plane below these components.
The SW and BOOST nodes should be as small as possible.
Shorted and Reversed Input Protection
If the inductor is chosen so that it won’t saturate exces-
sively, an LT3481 buck regulator will tolerate a shorted
output. There is another situation to consider in systems
where the output will be held high when the input to the
LT3481 is absent. This may occur in battery charging ap-
plications or in battery backup systems where a battery
or some other supply is diode ORed with the LT3481’s
Finally, keep the FB and V nodes small so that the ground
C
traces will shield them from the SW and BOOST nodes.
The Exposed Pad on the bottom of the package must be
soldered to ground so that the pad acts as a heat sink. To
keep thermal resistance low, extend the ground plane as
much as possible, and add thermal vias under and near
the LT3481 to additional ground planes within the circuit
output. If the V pin is allowed to float and the RUN/SS
IN
pin is held high (either by a logic signal or because it is
tied to V ), then the LT3481’s internal circuitry will pull
IN
board and on the bottom side.
its quiescent current through its SW pin. This is fine if
3481fc
16
LT3481
APPLICATIONS INFORMATION
Hot Plugging Safely
L1
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypasscapacitorofLT3481circuits.However,thesecapaci-
tors can cause problems if the LT3481 is plugged into a
live supply (see Linear Technology Application Note 88 for
a complete discussion). The low loss ceramic capacitor,
combined with stray inductance in series with the power
source, forms an under damped tank circuit, and the
C2
V
OUT
C
C
R
RT
R
C
R2
R1
voltage at the V pin of the LT3481 can ring to twice the
IN
nominal input voltage, possibly exceeding the LT3481’s
rating and damaging the part. If the input supply is poorly
controlled or the user will be plugging the LT3481 into an
energizedsupply, theinputnetworkshouldbedesignedto
prevent this overshoot. Figure 10 shows the waveforms
that result when an LT3481 circuit is connected to a 24V
supply through six feet of 24-gauge twisted pair. The
C1
D1
R
PG
GND
3481 F09
VIAS TO V
VIAS TO LOCAL GROUND PLANE
VIAS TO V
VIAS TO RUN/SS
VIAS TO PG
IN
OUTLINE OF LOCAL
GROUND PLANE
OUT
Figure 9. A Good PCB Layout Ensures Proper, Low EMI Operation
CLOSING SWITCH
DANGER
SIMULATES HOT PLUG
V
IN
20V/DIV
I
IN
V
IN
RINGING V MAY EXCEED
IN
ABSOLUTE MAXIMUM RATING
LT3481
4.7μF
+
I
IN
10A/DIV
LOW
STRAY
IMPEDANCE
ENERGIZED
24V SUPPLY
INDUCTANCE
DUE TO 6 FEET
(2 METERS) OF
TWISTED PAIR
20μs/DIV
(10a)
0.7Ω
V
IN
20V/DIV
LT3481
4.7μF
+
0.1μF
I
IN
10A/DIV
20μs/DIV
(10b)
V
IN
20V/DIV
LT3481
4.7μF
+
+
22μF
35V
AI.EI.
I
IN
10A/DIV
3481 F10
20μs/DIV
(10c)
Figure 10. A Well Chosen Input Network Prevents Input Voltage Overshoot and
Ensures Reliable Operation when the LT3481 is Connected to a Live Supply
3481fc
17
LT3481
APPLICATIONS INFORMATION
first plot is the response with a 4.7μF ceramic capacitor
at the input. The input voltage rings as high as 50V and
the input current peaks at 26A. A good solution is shown
in Figure 10b. A 0.7Ω resistor is added in series with the
input to eliminate the voltage overshoot (it also reduces
the peak input current). A 0.1μF capacitor improves high
frequency filtering. For high input voltages its impact on
efficiency is minor, reducing efficiency by 1.5 percent for
a 5V output at full load operating from 24V.
the LT3481, it is possible to dissipate enough heat to raise
the junction temperature beyond the absolute maximum
of 125°C (150°C for the H grade). When operating at high
ambient temperatures, the maximum load current should
be derated as the ambient temperature approaches 125°C
(150°C for the H grade).
Power dissipation within the LT3481 can be estimated
by calculating the total power loss from an efficiency
measurement and subtracting the catch diode loss. The
die temperature is calculated by multiplying the LT3481
power dissipation by the thermal resistance from junction
to ambient.
High Temperature Considerations
The PCB must provide heat sinking to keep the LT3481
cool. The Exposed Pad on the bottom of the package must
be soldered to a ground plane. This ground should be tied
to large copper layers below with thermal vias; these lay-
ers will spread the heat dissipated by the LT3481. Place
additionalviascanreducethermalresistancefurther. With
these steps, the thermal resistance from die (or junction)
Other Linear Technology Publications
Application Notes 19, 35 and 44 contain more detailed
descriptions and design information for buck regulators
and other switching regulators. The LT1376 data sheet
has a more extensive discussion of output ripple, loop
compensation and stability testing. Design Note 100
shows how to generate a bipolar output supply using a
buck regulator.
to ambient can be reduced to θ = 35°C/W or less. With
JA
100LFPM airflow, this resistance can fall by another 25%.
Further increases in airflow will lead to lower thermal re-
sistance. Because of the large output current capability of
TYPICAL APPLICATIONS
5V Step-Down Converter
V
5V
2A
OUT
V
IN
6.3V TO 34V
V
BD
IN
RUN/SS
BOOST
ON OFF
L
0.47μF
D
6.8μH
V
SW
C
LT3481
GND
4.7μF
RT
PG
20k
BIAS
FB
590k
60.4k
330pF
22μF
200k
f = 800kHz
3481 TA02
D: DIODES INC. DFLS240L
L: TAIYO YUDEN NP06DZB6R8M
3481fc
18
LT3481
TYPICAL APPLICATIONS
3.3V Step-Down Converter
V
3.3V
2A
OUT
V
IN
4.4V TO 34V
V
BD
IN
RUN/SS
BOOST
ON OFF
L
0.47μF
D
4.7μH
V
SW
C
LT3481
GND
4.7μF
RT
PG
16.2k
BIAS
FB
324k
60.4k
330pF
22μF
200k
f = 800kHz
3481 TA03
D: DIODES INC. DFLS240L
L: TAIYO YUDEN NP06DZB4R7M
2.5V Step-Down Converter
V
OUT
V
IN
2.5V
4V TO 34V
2A
V
BD
D2
IN
RUN/SS
BOOST
ON OFF
L
1μF
D1
4.7μH
V
SW
C
4.7μF
LT3481
GND
RT
PG
22.1k
BIAS
FB
196k
84.5k
220pF
47μF
200k
f = 600kHz
3481 TA04
D1: DIODES INC. DFLS240L
D2: MBR0540
L: TAIYO YUDEN NP06DZB4R7M
5V, 2MHz Step-Down Converter
V
5V
2A
OUT
V
IN
8.6V TO 22V
TRANSIENT TO 36V
V
BD
IN
RUN/SS
BOOST
ON OFF
L
0.47μF
D
2.2μH
V
SW
C
LT3481
GND
2.2μF
RT
PG
20k
BIAS
FB
590k
16.9k
330pF
10μF
200k
f = 2MHz
3481 TA05
D: DIODES INC. DFLS240L
L: SUMIDA CDRM4D22/HP-2R2
3481fc
19
LT3481
TYPICAL APPLICATIONS
12V Step-Down Converter
V
12V
2A
OUT
V
IN
15V TO 34V
V
BD
IN
RUN/SS
BOOST
ON OFF
L
0.47μF
D
10μH
V
SW
C
LT3481
GND
10μF
RT
PG
30k
BIAS
FB
845k
60.4k
330pF
22μF
100k
f = 800kHz
3481 TA06
D: DIODES INC. DFLS240L
L: NEC/TOKIN PLC-0755-100
5V Step-Down Converter with Sync Input
V
5V
2A
OUT
V
IN
20V TO 34V
V
BD
IN
4.7μF
RUN/SS
BOOST
ON OFF
L
0.47μF
8.2μH
NOTE: DO NOT APPLY SYNC
SIGNAL UNTIL PGOOD GOES HIGH
V
C
8.2pF
SW
RT
SYNC IN
LT3481
D
3.3V SQ WAVE 300kHz TO 375kHz
BIAS
PGOOD
PG
75pF
11.8k
100k
29.4k
10k
226k
FB
GND
V
OUT
1000pF
47μF
f = 250kHz
3481 TA07
D: DIODES INC. DFLS240L
L: NEC/TOKIN PLC-0755-8R2
3481fc
20
LT3481
TYPICAL APPLICATIONS
5V Step-Down Converter with Sync and One-Shot
V
5V
2A
OUT
V
IN
8V TO 34V
V
BD
IN
4.7μF
RUN/SS
BOOST
ON OFF
L
0.47μF
15μH
V
C
15pF
1k
SW
SYNC IN
RT
LT3481
GND
D
3V SQ WAVE
Hz TO 450kHz
AND
BIAS
PG
75pF
25k
11.8k
100k
Q1
29.4k
10k
133k
FB
V
OUT
25k
50pF
1000pF
47μF
f = 300kHz
3481 TA08
D: DIODES INC. DFLS240L
L: NEC/TOKIN PLC-0755-150
Q1: ON SEMI MMBT3904
1.8V Step-Down Converter
V
IN
3.5V TO 27V
V
BD
IN
RUN/SS
BOOST
ON OFF
L
3.3μH
0.47μF
D
V
1.8V
2A
OUT
V
C
SW
LT3481
GND
4.7μF
RT
PG
15.4k
BIAS
FB
84.5k
105k
330pF
47μF
200k
f = 500kHz
3481 TA09
D: DIODES INC. DFLS240L
L: TAIYO YUDEN NP06DZB3R3M
3481fc
21
LT3481
PACKAGE DESCRIPTION
DD Package
10-Lead Plastic DFN (3mm × 3mm)
(Reference LTC DWG # 05-08-1699 Rev B)
0.70 p0.05
3.55 p0.05
2.15 p0.05 (2 SIDES)
1.65 p0.05
PACKAGE
OUTLINE
0.25 p 0.05
0.50
BSC
2.38 p0.05
(2 SIDES)
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
R = 0.125
0.40 p 0.10
TYP
6
10
3.00 p0.10
(4 SIDES)
1.65 p 0.10
(2 SIDES)
PIN 1
TOP MARK
(SEE NOTE 6)
(DD) DFN REV B 0309
5
1
0.25 p 0.05
0.50 BSC
0.75 p0.05
0.200 REF
2.38 p0.10
(2 SIDES)
0.00 – 0.05
BOTTOM VIEW—EXPOSED PAD
NOTE:
1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-2).
CHECK THE LTC WEBSITE DATA SHEET FOR CURRENT STATUS OF VARIATION ASSIGNMENT
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE
TOP AND BOTTOM OF PACKAGE
3481fc
22
LT3481
PACKAGE DESCRIPTION
MSE Package
10-Lead Plastic MSOP, Exposed Die Pad
(Reference LTC DWG # 05-08-1664 Rev C)
BOTTOM VIEW OF
EXPOSED PAD OPTION
2.06 p 0.102
(.081 p .004)
1.83 p 0.102
(.072 p .004)
2.794 p 0.102
(.110 p .004)
0.889 p 0.127
(.035 p .005)
1
0.29
REF
0.05 REF
5.23
(.206)
MIN
2.083 p 0.102 3.20 – 3.45
(.082 p .004) (.126 – .136)
DETAIL “B”
CORNER TAIL IS PART OF
THE LEADFRAME FEATURE.
FOR REFERENCE ONLY
NO MEASUREMENT PURPOSE
DETAIL “B”
10
0.50
(.0197)
BSC
0.305 p 0.038
(.0120 p .0015)
TYP
3.00 p 0.102
(.118 p .004)
(NOTE 3)
0.497 p 0.076
(.0196 p .003)
10 9
8
7 6
RECOMMENDED SOLDER PAD LAYOUT
REF
3.00 p 0.102
(.118 p .004)
(NOTE 4)
4.90 p 0.152
(.193 p .006)
DETAIL “A”
0.254
(.010)
0o – 6o TYP
1
2
3
4 5
GAUGE PLANE
0.53 p 0.152
(.021 p .006)
0.86
(.034)
REF
1.10
(.043)
MAX
DETAIL “A”
0.18
(.007)
SEATING
PLANE
0.17 – 0.27
(.007 – .011)
TYP
0.1016 p 0.0508
(.004 p .002)
0.50
(.0197)
BSC
MSOP (MSE) 0908 REV C
NOTE:
1. DIMENSIONS IN MILLIMETER/(INCH)
2. DRAWING NOT TO SCALE
3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS.
MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.
INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
3481fc
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.
23
LT3481
TYPICAL APPLICATION
1.265V Step-Down Converter
V
IN
3.6V TO 27V
V
BD
IN
RUN/SS
BOOST
ON OFF
L
0.47μF
D
3.3μH
V
OUT
V
SW
1.265V
2A
C
LT3481
GND
4.7μF
RT
PG
13k
BIAS
FB
105k
330pF
47μF
f = 500kHz
3481 TA10
D: DIODES INC. DFLS240L
L: TAIYO YUDEN NP06DZB3R3M
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
V : 3.6V to 36V, V
LT1933
500mA (I ), 500kHz Step-Down Switching
= 1.2V, I = 1.6mA, I <1μA, ThinSOT™ Package
Q SD
OUT
IN
OUT(MIN)
OUT(MIN)
OUT(MIN)
OUT(MIN)
OUT(MIN)
Regulator in SOT-23
LT3437
60V, 400mA (I ), MicroPower Step-Down
V : 3.3V to 80V, V
IN
= 1.25V, I = 100μA, I <1μA, DFN Package
Q SD
OUT
DC/DC Converter with Burst Mode Operation
LT1936
36V, 1.4A (I ), 500kHz High Efficiency
V : 3.6V to 36V, V
IN
= 1.2V, I = 1.9mA, I <1μA, MS8E Package
Q SD
OUT
Step-Down DC/DC Converter
LT3493
36V, 1.2A (I ), 750kHz High Efficiency
V : 3.6V to 40V, V
IN
= 0.8V, I = 1.9mA, I <1μA, DFN Package
Q SD
OUT
Step-Down DC/DC Converter
LT1976/LT1977
60V, 1.2A (I ), 200kHz/500kHz, High Efficiency
V : 3.3V to 60V, V
IN
= 1.20V, I = 100μA, I <1μA, TSSOP16E Package
Q SD
OUT
Step-Down DC/DC Converter with Burst Mode
Operation
LT1767
25V, 1.2A (I ), 1.1MHz, High Efficiency
V : 3.0V to 25V, V
= 1.20V, I = 1mA, I <6μA, MS8E Package
Q SD
OUT
IN
OUT(MIN)
OUT(MIN)
OUT(MIN)
OUT(MIN)
Step-Down DC/DC Converter
LT1940
Dual 25V, 1.4A (I ), 1.1MHz, High Efficiency
V : 3.6V to 25V, V
IN
= 1.20V, I = 3.8mA, I <30μA, TSSOP16E Package
Q SD
OUT
Step-Down DC/DC Converter
LT1766
60V, 1.2A (I ), 200kHz, High Efficiency
V : 5.5V to 60V, V
IN
= 1.20V, I = 2.5mA, I = 25μA, TSSOP16E Package
Q SD
OUT
Step-Down DC/DC Converter
LT3434/LT3435
60V, 2.4A (I ), 200/500kHz, High Efficiency
V : 3.3V to 60V, V
IN
= 1.20V, I = 100μA, I <1μA, TSSOP16E Package
Q SD
OUT
Step-Down DC/DC Converter with Burst Mode
Operation
ThinSOT is a trademark of Linear Technology Corporation.
3481fc
LT 0809 REV C • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
24
●
●
© LINEAR TECHNOLOGY CORPORATION 2006
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