LT3480IMSE [Linear]
36V, 2A, 2.4MHz Step-Down Switching Regulator with 70μA Quiescent Current; 36V ，2A ， 2.4MHz是降压型开关稳压器具有70μA静态电流
| 型号： | LT3480IMSE |
| 厂家： | 
Linear |
| 描述： | 36V, 2A, 2.4MHz Step-Down Switching Regulator with 70μA Quiescent Current |
| 文件： | 总24页 (文件大小：293K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LT3480
36V, 2A, 2.4MHz Step-Down
Switching Regulator with
70µA Quiescent Current
DESCRIPTION
FEATURES
The LT®3480 is an adjustable frequency (200kHz to
2.4MHz) monolithic buck switching regulator that ac-
cepts input voltages up to 36V (60V maximum). A high
efficiency 0.25 switch is included on the die along with
a boost Schottky diode and the necessary oscillator, con-
trol, 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 LT3480 can
further enhance low output current efficiency by draw-
n
Wide Input Range:
Operation from 3.6V to 36V
Over-Voltage Lockout Protects Circuits
through 60V Transients
n
2A Maximum Output Current
Low Ripple Burst Mode® Operation
n
70μA IQ at 12VIN to 3.3VOUT
Output Ripple < 15mV
n
Adjustable Switching Frequency: 200kHz to 2.4MHz
n
Low Shutdown Current: IQ < 1μA
Integrated Boost Diode
n
n
Synchronizable Between 250kHz to 2MHz
ing bias current from the output when V
is above 3V.
OUT
n
Power Good Flag
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
n
Saturating Switch Design: 0.25 On-Resistance
n
0.790V Feedback Reference Voltage
Output Voltage: 0.79V to 20V
Soft-Start Capability
n
flag signals when V
reaches 86% of the programmed
OUT
n
output voltage. The LT3480 is available in 10-Pin MSOP
and 3mm × 3mm DFN packages with exposed pads for
low thermal resistance.
n
Small 10-Pin Thermally Enhanced MSOP and
(3mm × 3mm) DFN Packages
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
Burst Mode is a registered trademark of Linear Technology Corporation. All other
trademarks are the property of their respective owners.
APPLICATIONS
n
Automotive Battery Regulation
n
Power for Portable Products
n
Distributed Supply Regulation
n
Industrial Supplies
TYPICAL APPLICATION
3.3V Step-Down Converter
Efficiency
V
V
3.3V
2A
100
IN
OUT
4.5V TO 36V
TRANSIENT
V = 5V
OUT
V
BD
IN
TO 60V
90
80
70
60
50
RUN/SS
BOOST
OFF ON
14k
V
= 3.3V
OUT
0.47μF
4.7μH
V
C
SW
LT3480
GND
4.7μF
RT
470pF
PG
316k
V
= 12V
40.2k
SYNC
FB
IN
L = 5.6μH
F = 800 kHz
100k
22μF
0
0.5
1.0
LOAD CURRENT (A)
1.5
2
3480 TA01
3480 TA01b
3480fb
1
LT3480
(Note 1)
ABSOLUTE MAXIMUM RATINGS
V , RUN/SS Voltage (Note 5)...................................60V
Operating Temperature Range (Note 2)
IN
BOOST Pin Voltage ...................................................56V
LT3480E............................................... –40°C to 85°C
LT3480I.............................................. –40°C to 125°C
Storage Temperature Range................... –65°C to 150°C
Lead Temperature (Soldering, 10 sec)
BOOST Pin Above SW Pin.........................................30V
FB, RT, V Voltage .......................................................5V
C
PG, BD, SYNC Voltage ..............................................30V
Maximum Junction Temperature........................... 125°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
PG
SYNC
IN
V
IN
PG
RUN/SS
RUN/SS
SYNC
MSE PACKAGE
10-LEAD PLASTIC MSOP
DD PACKAGE
= 45°C/W,
= 10°C/W
JC
JA
10-LEAD (3mm s 3mm) PLASTIC DFN
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
= 45°C/W,
= 10°C/W
JC
JA
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
LT3480EDD#PBF
LT3480IDD#PBF
LT3480EMSE#PBF
LT3480IMSE#PBF
TAPE AND REEL
PART MARKING*
LCTP
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3480EDD#TRPBF
LT3480IDD#TRPBF
LT3480EMSE#TRPBF
LT3480IMSE#TRPBF
–40°C to 85°C
–40°C to 125°C
–40°C to 85°C
–40°C to 125°C
10-Lead (3mm × 3mm) Plastic DFN
10-Lead (3mm × 3mm) Plastic DFN
10-Lead Plastic MSOP
LCTP
LTCTM
LTCTM
10-Lead Plastic MSOP
LEAD BASED FINISH
LT3480EDD
TAPE AND REEL
LT3480EDD#TR
LT3480IDD#TR
LT3480EMSE#TR
LT3480IMSE#TR
PART MARKING*
LCTP
PACKAGE DESCRIPTION
TEMPERATURE RANGE
–40°C to 85°C
10-Lead (3mm × 3mm) Plastic DFN
10-Lead (3mm × 3mm) Plastic DFN
10-Lead Plastic MSOP
LT3480IDD
LCTP
–40°C to 125°C
–40°C to 85°C
LT3480EMSE
LT3480IMSE
LTCTM
LTCTM
10-Lead Plastic MSOP
–40°C to 125°C
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/
ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature
range, otherwise specifications are at TA = 25°C. VIN = 10V, VRUN/SS = 10V, VBOOST = 15V, VBD = 3.3V unless otherwise noted. (Note 2)
PARAMETER
CONDITIONS
MIN
TYP
3
MAX
3.6
UNITS
l
l
Minimum Input Voltage
V
V
V
IN
Overvoltage Lockout
36
38
40
3480fb
2
LT3480
ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature
range, otherwise specifications are at TA = 25°C. VIN = 10V, VRUN/SS = 10V, VBOOST = 15V, VBD = 3.3V unless otherwise noted. (Note 2)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Quiescent Current from V
V
V
V
= 0.2V
0.01
30
0.5
100
160
μA
μA
μA
IN
RUN/SS
l
l
= 3V, Not Switching
= 0, Not Switching
BD
BD
105
Quiescent Current from BD
V
V
V
= 0.2V
= 3V, Not Switching
= 0, Not Switching
0.01
80
1
0.5
120
5
μA
μA
μA
RUN/SS
BD
BD
Minimum Bias Voltage (BD Pin)
Feedback Voltage
2.7
3
V
780
775
790
790
800
805
mV
mV
l
l
FB Pin Bias Current (Note 3)
FB Voltage Line Regulation
V
= 0.8V, V = 0.4V
7
0.002
400
1000
45
30
nA
%/V
FB
C
4V < V < 36V
0.01
IN
Error Amp g
μMho
m
Error Amp Gain
V Source Current
μA
μA
A/V
V
C
V Sink Current
C
45
V Pin to Switch Current Gain
C
3.5
2
V Clamp Voltage
C
Switching Frequency
R = 8.66k
2.1
0.9
160
2.4
1
200
2.7
1.15
240
MHz
MHz
kHz
T
R = 29.4k
T
R = 187k
T
l
Minimum Switch Off-Time
Switch Current Limit
60
3.5
500
0.02
1.5
22
150
4
nS
A
Duty Cycle = 5%
3
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
2
BD
l
2.1
35
10
2.5
I
SW
= 1A
mA
μA
V
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
0.2
V
V
FB
Rising
100
12
mV
mV
μA
μA
V
PG Leakage
V
V
= 5V
0.1
600
1
PG
l
PG Sink Current
= 0.4V
100
0.5
PG
SYNC Low Threshold
SYNC High Threshold
SYNC Pin Bias Current
0.7
V
V
SYNC
= 0V
0.1
μA
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 3: Bias current flows out of the FB pin.
Note 4: This is the minimum voltage across the boost capacitor needed to
guarantee full saturation of the switch.
Note 2: The LT3480E 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 LT3480I specifications are
guaranteed over the –40°C to 125°C temperature range.
Note 5: Absolute Maximum Voltage at V and RUN/SS pins is 60V for
nonrepetitive 1 second transients, and 40V for continious operation.
IN
3480fb
3
LT3480
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency
Efficiency
Efficiency
100
90
80
70
60
50
90
85
80
75
70
65
60
55
50
90
80
70
60
50
10
1
V
= 7V
IN
V
= 12V
V
= 12V
= 24V
IN
IN
V
= 34V
IN
V
= 34V
IN
V
IN
V
IN
= 24V
0.1
V
V
= 12V
IN
OUT
= 3.3V
40
30
L = 5.6μH
L: NEC PLC-0745-5R6
f: 800kHz
L: NEC PLC-0745-5R6
f: 800kHz
F = 800 kHz
V
= 3.3V
V
= 5V
OUT
OUT
0.01
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
0
0.5
1.0
LOAD CURRENT (A)
1.5
2
LOAD CURRENT (A)
LOAD CURRENT (A)
3480 G27
3480 G01
3480 G02
No Load Supply Current
No Load Supply Current
Maximum Load Current
120
100
80
60
40
20
0
4.0
3.5
3.0
400
V
= 3.3V
OUT
CATCH DIODE: DIODES, INC. PDS360
350
300
V
V
= 12V
IN
OUT
TYPICAL
= 3.3V
INCREASED SUPPLY
250
200
150
100
50
CURRENT DUE TO CATCH
DIODE LEAKAGE AT
2.5
2.0
HIGH TEMPERATURE
MINIMUM
V
A
= 3.3V
OUT
T
= 25 °C
1.5
1.0
L = 4.7μH
f = 800 kHz
0
0
5
10
15
20
25
30
35
5
10
15
20
25
30
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3480 G05
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
3480 G04
3480 G06
Maximum Load Current
Switch Current Limit
Switch Current Limit
4.0
3.5
3.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
3.5
3.0
2.5
2.0
1.5
1.0
TYPICAL
DUTY CYCLE = 10 %
DUTY CYCLE = 90 %
2.5
2.0
MINIMUM
V
A
= 5V
OUT
T
= 25 °C
1.5
1.0
L = 4.7μH
f = 800kHz
20
60
40
DUTY CYCLE (%)
80
100
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3480 G09
0
10
20
15
INPUT VOLTAGE (V)
25
30
5
3480 G08
3480 G07
3480fb
4
LT3480
TYPICAL PERFORMANCE CHARACTERISTICS
Switch Voltage Drop
Boost Pin Current
Feedback Voltage
80
70
60
50
40
30
20
10
0
840
820
800
780
760
700
600
500
400
300
200
100
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
4380 G12
0
500
1000
1500
2000
2500
0
500
1000
1500
2000
2500
SWITCH CURRENT (mA)
SWITCH CURRENT (mA)
3480 G10
3480 G11
Switching Frequency
Frequency Foldback
Minimum Switch On-Time
1200
1000
800
1.20
1.15
1.10
1.05
1.00
0.95
0.90
0.85
0.80
140
120
100
80
60
40
20
600
400
200
0
0
700 800 900
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
4380 G13
0
100 200 300 400 500 600
FB PIN VOLTAGE (mV)
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (˚C)
3480 G14
3480 G15
Soft-Start
RUN/SS Pin Current
Boost Diode
4.0
12
10
8
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
6
4
2
0
0
0
0.5
1
2
2.5
3
3.5
20
RUN/SS PIN VOLTAGE (V)
30
35
0
1.5
15
25
0
0.5
1.0
1.5
2.0
5
10
RUN/SS PIN VOLTAGE (V)
BOOST DIODE CURRENT (A)
3480 G16
3480 G17
3480 G18
3480fb
5
LT3480
TYPICAL PERFORMANCE CHARACTERISTICS
Error Amp Output Current
Minimum Input Voltage
Minimum Input Voltage
5.0
4.5
4.0
3.5
3.0
2.5
2.0
6.5
6.0
5.5
5.0
50
40
30
20
10
0
–10
–20
–30
–40
–50
V
A
= 5V
OUT
V
A
= 3.3V
OUT
4.5
4.0
T
= 25 °C
T
= 25°C
L = 4.7μH
f = 800kHz
L = 4.7μH
f = 800kHz
1
10
100
1000
10000
1
10
100
1000
10000
–200
–100
0
100
200
LOAD CURRENT (A)
LOAD CURRENT (A)
FB PIN ERROR VOLTAGE (V)
3480 G20
3480 G21
3480 G19
VC Voltages
Power Good Threshold
Switching Waveforms; Burst Mode
2.50
95
90
85
80
75
V
SW
2.00
1.50
5V/DIV
CURRENT LIMIT CLAMP
SWITCHING THRESHOLD
I
L
0.2A/DIV
1.00
0.50
0
V
OUT
10mV/DIV
3480 G24
5μs/DIV
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3480 G22
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3480 G23
V
LOAD
= 12V; FRONT PAGE APPLICATION
IN
I
= 10mA
Switching Waveforms; Transition
from Burst Mode to Full Frequency
Switching Waveforms; Full
Frequency Continuous Operation
V
SW
5V/DIV
V
SW
5V/DIV
I
I
L
L
0.5A/DIV
0.2A/DIV
V
OUT
V
OUT
10mV/DIV
10mV/DIV
3480 G25
3480 G26
1μs/DIV
1μs/DIV
V
LOAD
= 12V; FRONT PAGE APPLICATION
V
LOAD
= 12V; FRONT PAGE APPLICATION
IN
IN
I
= 110mA
I
= 1A
3480fb
6
LT3480
PIN FUNCTIONS
BD (Pin 1): This pin connects to the anode of the boost
Schottky diode. BD also supplies current to the internal
regulator.
SYNC (Pin 6): This is the external clock synchronization
input.GroundthispinforlowrippleBurstModeoperationat
lowoutputloads.Tietoaclocksourceforsynchronization.
Clockedgesshouldhaveriseandfalltimesfasterthan1μs.
See synchronizing section in Applications Information.
BOOST (Pin 2): This pin is used to provide a drive
voltage,higherthantheinputvoltage,totheinternalbipolar
NPN power switch.
PG (Pin 7): The PG pin is the open collector output of an
internal comparator. PG remains low until the FB pin is
within 14% of the final regulation voltage. PG output is
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.
valid when V is above 3.6V and RUN/SS is high.
IN
FB (Pin 8): The LT3480 regulates the FB pin to 0.790V.
Connect the feedback resistor divider tap to this pin.
V (Pin 4): The V pin supplies current to the LT3480’s
IN
IN
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
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.
RUN/SS (Pin 5): The RUN/SS pin is used to put the
LT3480 in shutdown mode. Tie to ground to shut down
the LT3480. Tie to 2.5V or more for normal operation. If
the shutdown feature is not used, tie this pin to the V
RT(Pin10):OscillatorResistorInput.Connectingaresistor
to ground from this pin sets the switching frequency.
IN
pin. RUN/SS also provides a soft-start function; see the
Applications Information section.
Exposed Pad (Pin 11): Ground. The Exposed Pad must
be soldered to PCB.
BLOCK DIAGRAM
V
IN
V
4
IN
C1
–
+
INTERNAL 0.79V REF
BD
1
RUN/SS
5
SLOPE COMP
3
SWITCH
BOOST
2
3
LATCH
C3
R
RT
OSCILLATOR
200kHz–2.4MHz
Q
10
6
S
L1
SW
V
R
OUT
T
DISABLE
SYNC
C2
D1
BurstMode
DETECT
SOFT-START
PG
7
V
CLAMP
ERROR AMP
C
+ 0.7V
–
+
–
V
C
9
C
C
C
F
R
C
GND
11
FB
8
R2
R1
3480 BD
3480fb
7
LT3480
OPERATION
The LT3480 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
To further optimize efficiency, the LT3480 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 70μA in a typical application.
voltage at V . An error amplifier measures the output
C
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
TheoscillatorreducestheLT3480’soperatingfrequencywhen
the voltage at the FB pin is low. This frequency foldback helps
to control the output current during startup and overload.
V pinprovidescurrentlimit. TheV pinisalsoclampedto
C
C
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.
TheLT3480containsapowergoodcomparatorwhichtrips
when the FB pin is at 86% 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 LT3480 is
Aninternalregulatorprovidespowertothecontrolcircuitry.
The bias regulator normally draws power from the V pin,
IN
but if the BD 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 LT3480
in shutdown, disconnecting the output and reducing the
input current to less than 1μA.
enabled and V is above 3.6V.
IN
The LT3480 has an overvoltage protection feature which
disables switching action when the V goes above 38V
IN
typical (36V minimum). When switching is disabled, the
LT3480 can safely sustain input voltages up to 60V.
3480fb
8
LT3480
APPLICATIONS INFORMATION
FB Resistor Network
where V is the typical input voltage, V
is the output
IN
OUT
voltage, V is the catch diode drop (~0.5V) and V is the
D
SW
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the 1% resis-
tors according to:
internal 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 OUT
VOUT
0.79V
⎛
⎞
⎠
inthenextsection,lowerfrequencyallowsalowerdropout
voltage. The reason input voltage range depends on the
switchingfrequencyisbecausetheLT3480switchhasfinite
minimum on and off times. The switch can turn on for a
minimumof~150nsandturnoffforaminimumof~150ns.
Typical minimum on time at 25°C is 80ns. This means that
the minimum and maximum duty cycles are:
R1=R2
−1
⎜
⎝
⎟
Reference designators refer to the Block Diagram.
Setting the Switching Frequency
The LT3480 uses a constant frequency PWM architecture
thatcanbeprogrammedtoswitchfrom200kHzto2.4MHz
by using a resistor tied from the RT pin to ground. A table
showing the necessary RT value for a desired switching
frequency is in Figure 1.
DCMIN = fSW ON(MIN)
t
DCMAX =1– fSW OFF(MIN)
t
where f is the switching frequency, the t
is the
is
SW
ON(MIN)
SWITCHING FREQUENCY (MHz)
R VALUE (kΩ)
T
minimum switch on time (~150ns), and the t
OFF(MIN)
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
187
121
the minimum switch off time (~150ns). These equations
show that duty cycle range increases when switching
frequency is decreased.
88.7
68.1
56.2
46.4
40.2
34
A good choice of switching frequency should allow ad-
equate input voltage range (see next section) and keep
the inductor and capacitor values small.
29.4
23.7
19.1
16.2
13.3
11.5
9.76
8.66
Input Voltage Range
ThemaximuminputvoltageforLT3480applicationsdepends
on switching frequency, the Absolute Maximum Ratings of
the V and BOOST pins, and the operating mode.
IN
The LT3480 can operate from input voltages up to 38V,
andsafelywithstandinputvoltagesup60V. Notethatwhile
IN
Figure 1. Switching Frequency vs. RT Value
Operating Frequency Tradeoffs
V >38V(typical),theLT3480willstopswitching,allowing
the output to fall out of regulation.
Selection of the operating frequency is a tradeoff between
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
While the output is in start-up, short-circuit, or other
overload conditions, the switching frequency should be
chosen according to the following discussion.
For safe operation at inputs up to 60V the switching fre-
quency must be set low enough to satisfy V
≥ 40V
IN(MAX)
IN(MAX)
highest acceptable switching frequency (f
) for a
SW(MAX)
according to the following equation. If lower V
is
given application can be calculated as follows:
desired, this equation can be used directly.
VD + VOUT
fSW(MAX)
=
tON(MIN) V + V – V
(
)
D
IN
SW
3480fb
9
LT3480
APPLICATIONS INFORMATION
frequency. A reasonable starting point for selecting the
ripple current is:
VOUT + VD
V
=
– VD + VSW
IN(MAX)
fSW ON(MIN)
t
ΔI = 0.4(I
)
L
OUT(MAX)
where V
OUT
is the maximum operating input voltage,
IN(MAX)
where I
is the maximum output load current. To
OUT(MAX)
V
is the output voltage, V is the catch diode drop
D
guarantee sufficient output current, peak inductor current
(~0.5V), V is the internal switch drop (~0.5V at max
SW
mustbelowerthantheLT3480’sswitchcurrentlimit(I ).
The peak inductor current is:
LIM
load), f is the switching frequency (set by R ), and
SW
ON(MIN)
T
t
istheminimumswitchontime(~150ns).Notethat
I
= I
+ ΔI /2
OUT(MAX) L
L(PEAK)
a higher switching frequency will depress the maximum
operating input voltage. Conversely, a lower switching
frequency will be necessary to achieve safe operation at
high input voltages.
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 LT3480’s switch current limit (I ) is
LIM
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:
If the output is in regulation and no short-circuit, start-
up, or overload events are expected, then input voltage
transients of up to 60V are acceptable regardless of the
switching frequency. In this mode, the LT3480 may enter
pulse skipping operation 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. Above 38V switching
will stop.
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
The minimum input voltage is determined by either the
LT3480’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:
according to the following equation:
⎛
⎞
⎛
⎞
VOUT + VD
fSWΔIL
VOUT + VD
L =
1–
⎜
⎟
⎜
⎟
V
⎝
⎠
⎝
⎠
IN(MAX)
VOUT + VD
V
=
– VD + VSW
IN(MIN)
where V is the voltage drop of the catch diode (~0.4V),
1– fSW OFF(MIN)
t
D
V
is the maximum input voltage, V
is the output
IN(MAX)
OUT
voltage, f is the switching frequency (set by RT), and L
whereV
istheminimuminputvoltage,andt
SW
IN(MIN)
OFF(MIN)
is in the inductor value.
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 0.1 , 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
anddecreaseswithhigherinductanceandfasterswitching
3480fb
10
LT3480
APPLICATIONS INFORMATION
Table 1. Inductor Vendors
necessary. This can be provided with a lower performance
electrolytic capacitor.
VENDOR
Murata
TDK
URL
PART SERIES
TYPE
www.murata.com
LQH55D
Open
Step-downregulatorsdrawcurrentfromtheinputsupplyin
pulses with very fast rise and fall times. The input capaci-
tor is required to reduce the resulting voltage ripple at the
LT3480 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 LT3480 and the catch diode (see the PCB Layout
section). A second precaution regarding the ceramic input
capacitorconcernsthemaximuminputvoltageratingofthe
LT3480. A ceramic input capacitor combined with trace or
cable inductance forms a high quality (under damped) tank
circuit. If the LT3480 circuit is plugged into a live supply, the
input voltage can ring to twice its nominal value, possibly
exceedingtheLT3480’svoltagerating.Thissituationiseasily
avoided (see the Hot Plugging Safety section).
www.componenttdk.com SLF7045
SLF10145
Shielded
Shielded
Toko
www.toko.com
D62CB
D63CB
D75C
Shielded
Shielded
Shielded
Open
D75F
Sumida
www.sumida.com
CR54
Open
CDRH74
CDRH6D38
CR75
Shielded
Shielded
Open
Of course, such a simple design guide will not always result
in the optimum inductor for your application. A larger value
inductor provides a slightly higher maximum load current
andwillreducetheoutputvoltageripple.Ifyourloadislower
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.Lowinductancemayresultindiscontinuousmode
operation, which is okay but further reduces maximum
load current. For details of maximum output current and
discontinuous mode operation, see Linear Technology Ap-
plication Note 44. Finally, for duty cycles greater than 50%
Forspacesensitiveapplications,a2.2μFceramiccapacitorcan
beusedforlocalbypassingoftheLT3480input. However, the
lower input capacitance will result in increased 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 LT3480 to ~3.7V.
Output Capacitor and Output Ripple
The output capacitor has two essential functions. Along
withtheinductor,itfiltersthesquarewavegeneratedbythe
LT3480toproducetheDCoutput. 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
LT3480’s control loop. Ceramic capacitors have very low
equivalent series resistance (ESR) and provide the best
ripple performance. A good starting value is:
(V /V > 0.5), there is a minimum inductance required
OUT IN
to avoid subharmonic oscillations. See AN19.
Input Capacitor
BypasstheinputoftheLT3480circuitwithaceramiccapaci-
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
bypasstheLT3480andwilleasilyhandletheripplecurrent.
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
100
COUT
=
VOUT SW
f
where f is in MHz, and C
is the recommended output
OUT
SW
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
3480fb
11
LT3480
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
www.murata.com
www.avxcorp.com
Ceramic,
Tantalum
Ceramic
TPS Series
Taiyo Yuden
(864) 963-6300
www.taiyo-yuden.com
to the regulator input voltage. Use a Schottky diode with a
reverse voltage rating greater than the input voltage. The
overvoltage protection feature in the LT3480 will keep the
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 Frequency Compensation
section to choose an appropriate compensation network.
switch off when V > 38V which allows the use of 40V
IN
rated Schottky even when V ranges up to 60V. Table 3
IN
lists several Schottky diodes and their manufacturers.
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
useinswitchingregulators.TheESRshouldbespecifiedby
the supplier, and should be 0.05 or less. Such a capacitor
willbelargerthanaceramiccapacitorandwillhavealarger
capacitance,becausethecapacitormustbelargetoachieve
low ESR. Table 2 lists several capacitor vendors.
Table 3. Diode Vendors
V
I
V AT 1A
V AT 2A
R
AVE
F
F
PART NUMBER
(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
Catch Diode
The catch diode conducts current only during switch off
time. Average forward current in normal operation can
be calculated from:
Ceramic Capacitors
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
whenusedwiththeLT3480duetotheirpiezoelectricnature.
When in Burst Mode operation, the LT3480’s switching
frequency depends on the load current, and at very light
loads the LT3480 can excite the ceramic capacitor at audio
frequencies, generating audible noise. Since the LT3480
operates at a lower current limit during Burst Mode
3480fb
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
12
LT3480
APPLICATIONS INFORMATION
operation, 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.
current proportional to the voltage at the V pin. Note that
C
the output capacitor integrates this current, and that the
capacitor on the V pin (C ) integrates the error amplifier
C
C
output current, resulting in two poles in the loop. In most
A final precaution regarding ceramic capacitors concerns
themaximuminputvoltageratingoftheLT3480.Aceramic
input capacitor combined with trace or cable inductance
forms a high quality (under damped) tank circuit. If the
LT3480 circuit is plugged into a live supply, the input volt-
agecanringtotwiceitsnominalvalue, possiblyexceeding
the LT3480’s rating. This situation is easily avoided (see
the Hot Plugging Safely section).
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 LT3480 uses current mode control to regulate the
output.Thissimplifiesloopcompensation.Inparticular,the
LT3480 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
LT3480
CURRENT MODE
POWER STAGE
SW
FB
OUTPUT
ERROR
g
m
= 3.5mho
C
R1
AMPLIFIER
PL
–
g
=
m
420μmho
ESR
+
0.8V
C1
V pin, as shown in Figure 2. Generally a capacitor (C )
C
C
+
3M
C1
and a resistor (R ) in series to ground are used. In addi-
C
tion, there may be lower value capacitor in parallel. This
POLYMER
OR
TANTALUM
CERAMIC
V
C
GND
capacitor (C ) is not part of the loop compensation but
F
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.
R
C
R2
C
F
C
C
3480 F02
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 LT3480 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
Figure 2. Model for Loop Response
V
OUT
100mV/DIV
I
L
0.5A/DIV
V
= 12V; FRONT PAGE APPLICATION
10μs/DIV
IN
3480 F03
Figure 3. Transient Load Response of the LT3480 Front Page
Application as the Load Current is Stepped from 500mA to
1500mA. VOUT = 3.3V
3480fb
13
LT3480
APPLICATIONS INFORMATION
operationataloweroutputloadcurrentthanwheninBurst
Mode. The front page application circuit will switch at full
frequency at output loads higher than about 60mA.
V
SW
5V/DIV
BOOST and BIAS Pin Considerations
I
L
0.2A/DIV
Capacitor C3 and the internal boost Schottky diode (see the
Block Diagram) are used to generate a boost voltage that is
higherthantheinputvoltage.Inmostcasesa0.22μFcapacitor
willworkwell.Figure2showsthreewaystoarrangetheboost
circuit. The BOOST pin must be more than 2.3V above the
SW pin for best efficiency. For outputs of 3V and above, the
standardcircuit(Figure5a)isbest. Foroutputsbetween2.8V
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 while using the internal boost diode. For
reliable BOOST pin 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
voltagestheboostdiodecanbetiedtotheinput(Figure5c),or
V
OUT
10mV/DIV
3480 F04
5μs/DIV
V
LOAD
= 12V; FRONT PAGE APPLICATION
IN
I
= 10mA
Figure 4. Burst Mode Operation
Low-Ripple Burst Mode and Pulse-Skip Mode
The LT3480 is capable of operating in either Low-Ripple
BurstModeorPulse-SkipModewhichareselectedusingthe
SYNC pin. See the Synchronization section for details.
To enhance efficiency at light loads, the LT3480 can be
operated in Low-Ripple Burst Mode operation which keeps
the output capacitor charged to the proper voltage while
minimizing the input quiescent current. During Burst Mode
operation,theLT3480deliverssinglecycleburstsofcurrent
to the output capacitor followed by sleep periods where the
outputpowerisdeliveredtotheloadbytheoutputcapacitor.
BecausetheLT3480deliverspowertotheoutputwithsingle,
low current pulses, the output ripple is kept below 15mV
to another supply greater than 2.8V. Tying BD to V reduces
IN
the maximum input voltage to 30V. The circuit in Figure 5a
is more efficient because the BOOST pin current and BD pin
quiescent current comes from a lower voltage source. You
must also be sure that the maximum voltage ratings of the
BOOST and BD pins are not exceeded.
The minimum operating voltage of an LT3480 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 LT3480 is turned on with its RUN/SS pin when the
output is already in regulation, then the boost capacitor
may not be fully charged. Because the boost capacitor is
chargedwiththeenergystoredintheinductor,thecircuitwill
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 start. The plots show
for a typical application. In addition, V and BD quiescent
IN
currents are reduced to typically 30μA and 80μA respec-
tively during the sleep time. As the load current decreases
towards a no load condition, the percentage of time that the
LT3480 operates in sleep mode increases and the average
input current is greatly reduced resulting in high efficiency
even at very low loads. See Figure 4. At higher output loads
(above 140mA for the front page application) the LT3480
will be running at the frequency programmed by the R
T
resistor, and will be operating in standard PWM mode. The
transition between PWM and Low-Ripple Burst Mode is
seamless, and will not disturb the output voltage.
If low quiescent current is not required the LT3480 can
operate in Pulse-Skip mode. The benefit of this mode is
that the LT3480 will enter full frequency standard PWM
3480fb
14
LT3480
APPLICATIONS INFORMATION
V
6.0
5.5
5.0
4.5
4.0
3.5
3.0
OUT
BD
TO START
(WORST CASE)
BOOST
V
V
IN
LT3480
GND
IN
C3
SW
4.7μF
TO RUN
V
A
= 3.3V
OUT
T
= 25°C
(5a) For V
> 2.8V
OUT
2.5
2.0
L = 8.2μH
f = 700kHz
V
1
10
100
1000
10000
OUT
LOAD CURRENT (A)
D2
BD
BOOST
8.0
7.0
6.0
5.0
4.0
3.0
2.0
V
V
IN
LT3480
IN
C3
TO START
(WORST CASE)
SW
GND
4.7μF
TO RUN
(5b) For 2.5V < V
< 2.8V
OUT
V
T
= 5V
OUT
A
V
OUT
= 25°C
L = 8.2μH
f = 700kHz
BD
BOOST
V
1
10
100
1000
10000
V
IN
LT3480
IN
LOAD CURRENT (A)
C3
3480 F06
SW
GND
4.7μF
Figure 6. The Minimum Input Voltage Depends on
Output Voltage, Load Current and Boost Circuit
3480 FO5
(5c) For V
< 2.5V; V
= 30V
IN(MAX)
Soft-Start
OUT
Figure 5. Three Circuits For Generating The Boost Voltage
TheRUN/SSpincanbeusedtosoft-starttheLT3480,reduc-
ingthemaximuminputcurrentduringstart-up.TheRUN/SS
pin is driven through an external RC filter to create a voltage
rampatthispin. Figure7showsthestart-upandshut-down
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 of the resistor so that it can
supply 20μA when the RUN/SS pin reaches 2.5V.
the worst-case situation where V is ramping very slowly.
IN
For lower start-up voltage, the boost diode can be tied to
V ; however, this restricts the input range to one-half of
IN
the absolute maximum rating of the BOOST pin.
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
Synchronization
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 LT3480, requiring a higher
input voltage to maintain regulation.
To select Low-Ripple Burst Mode operation, tie the SYNC
pin below 0.3V (this can be ground or a logic output).
3480fb
15
LT3480
APPLICATIONS INFORMATION
D4
MBRS140
I
V
L
V
BOOST
SW
IN
IN
RUN
15k
1A/DI
LT3480
V
RUN/SS
OUT
RUN/SS
GND
V
RUN/
2V/DI
V
C
0.22μF
GND FB
V
OUT
2V/DI
BACKUP
3480 F07
2ms/DIV
3480 F08
Figure 7. To Soft-Start the LT3480, Add a Resisitor
and Capacitor to the RUN/SS Pin
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 LT3480
Runs Only When the Input is Present
Synchronizing the LT3480 oscillator to an external fre-
quency can be done by connecting a square wave (with
20% to 80% duty cycle) to the SYNC pin. The square
wave amplitude should have valleys that are below 0.3V
and peaks that are above 0.8V (up to 6V).
or in battery backup systems where a battery or some
other supply is diode OR-ed with the LT3480’s output. If
the V pin is allowed to float and the RUN/SS pin is held
IN
The LT3480 will not enter Burst Mode at low output loads
while synchronized to an external clock, but instead will
skip pulses to maintain regulation.
high (either by a logic signal or because it is tied to V ),
IN
then the LT3480’s internal circuitry will pull its quiescent
current through its SW pin. This is fine if your system
can tolerate a few mA in this state. If you ground the
RUN/SS pin, the SW pin current will drop to essentially
The LT3480 may be synchronized over a 250kHz to 2MHz
range. The R resistor should be chosen to set the LT3480
T
zero. However, if the V pin is grounded while the output
switching frequency 20% below the lowest synchronization
IN
is held high, then parasitic diodes inside the LT3480 can
input.Forexample,ifthesynchronizationsignalwillbe250kHz
pull large currents from the output through the SW pin
and higher, the R should be chosen for 200kHz. To assure
T
and the V pin. Figure 8 shows a circuit that will run only
reliable and safe operation the LT3480 will only synchronize
whentheoutputvoltageisnearregulationasindicatedbythe
PG flag. It is therefore necessary to choose a large enough
inductor value to supply the required output current at the
IN
whentheinputvoltageispresentandthatprotectsagainst
a shorted or reversed input.
PCB Layout
frequency set by the R resistor. See Inductor Selection sec-
T
tion. It is also important to note that slope compensation
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
is set by the R value: When the sync frequency is much
T
higher than the one set by R , the slope compensation will
T
be significantly reduced which may require a larger inductor
value to prevent subharmonic oscillation.
currents flow in the LT3480’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
Iftheinductorischosensothatitwon’tsaturateexcessively,
an LT3480 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 LT3480
is absent. This may occur in battery charging applications
3480fb
16
LT3480
APPLICATIONS INFORMATION
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 LT3480 to additional ground planes within the circuit
board and on the bottom side.
L1
C2
V
OUT
C
C
R
RT
R
C
R2
Hot Plugging Safely
R1
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypasscapacitorofLT3480circuits.However,thesecapaci-
tors can cause problems if the LT3480 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
C1
D1
R
GND
PG
3480 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
VIAS TO SYNC
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
LT3480
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
LT3480
4.7μF
+
0.1μF
I
IN
10A/DIV
20μs/DIV
(10b)
V
IN
20V/DIV
LT3480
4.7μF
+
+
22μF
35V
AI.EI.
I
IN
10A/DIV
3480 F10
20μs/DIV
(10c)
Figure 10. A Well Chosen Input Network Prevents Input Voltage Overshoot and
Ensures Reliable Operation when the LT3480 is Connected to a Live Supply
3480fb
17
LT3480
APPLICATIONS INFORMATION
source, forms an under damped tank circuit, and the
the thermal resistance from die (or junction) to ambient can
be reduced to = 35°C/W or less. With 100 LFPM airflow,
voltage at the V pin of the LT3480 can ring to twice the
IN
JA
nominal input voltage, possibly exceeding the LT3480’s
rating and damaging the part. If the input supply is poorly
controlled or the user will be plugging the LT3480 into an
energized supply, the input network should be designed
topreventthisovershoot. Figure10showsthewaveforms
that result when an LT3480 circuit is connected to a 24V
supply through six feet of 24-gauge twisted pair. The
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 0.7 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.
this resistance can fall by another 25%. Further increases in
airflow will lead to lower thermal resistance. Because of the
large output current capability of the LT3480, it is possible
to dissipate enough heat to raise the junction temperature
beyond the absolute maximum of 125°C. When operating at
highambienttemperatures,themaximumloadcurrentshould
be derated as the ambient temperature approaches 125°C.
Power dissipation within the LT3480 can be estimated by
calculatingthetotalpowerlossfromanefficiencymeasure-
ment and subtracting the catch diode loss and inductor
loss. The die temperature is calculated by multiplying the
LT3480 power dissipation by the thermal resistance from
junction to ambient.
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.
High Temperature Considerations
The PCB must provide heat sinking to keep the LT3480 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 layers will
spread the heat dissipated by the LT3480. Place additional
vias can reduce thermal resistance further. With these steps,
TYPICAL APPLICATIONS
5V Step-Down Converter
V
V
5V
2A
IN
OUT
6.8V TO 36V
TRANSIENT
TO 60V*
V
BD
IN
RUN/SS
BOOST
ON OFF
L
0.47μF
D
6.8μH
V
SW
C
LT3480
GND
4.7μF
RT
16.2k
PG
536k
SYNC
40.2k
FB
470pF
22μF
100k
f = 800kHz
3480 TA02
D: DIODES INC. DFLS240L
L: TAIYO YUDEN NP06DZB6R8M
3480fb
18
LT3480
TYPICAL APPLICATIONS
3.3V Step-Down Converter
V
V
3.3V
2A
IN
OUT
4.4V TO 36V
TRANSIENT
TO 60V*
V
BD
IN
RUN/SS
BOOST
ON OFF
L
0.47μF
D
4.7μH
V
SW
C
LT3480
GND
4.7μF
RT
14k
PG
316k
SYNC
40.2k
FB
470pF
22μF
100k
f = 800kHz
3480 TA03
D: DIODES INC. DFLS240L
L: TAIYO YUDEN NP06DZB4R7M
2.5V Step-Down Converter
V
V
2.5V
2A
IN
OUT
4V TO 36V
TRANSIENT
TO 60V*
V
BD
D2
IN
RUN/SS
BOOST
ON OFF
L
1μF
D1
4.7μH
V
SW
C
4.7μF
LT3480
GND
RT
20k
PG
215k
56.2k
SYNC
FB
330pF
47μF
100k
f = 600kHz
3480 TA04
D1: DIODES INC. DFLS240L
D2: MBR0540
L: TAIYO YUDEN NP06DZB4R7M
3480fb
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.
19
LT3480
TYPICAL APPLICATIONS
5V, 2MHz Step-Down Converter
V
V
5V
2A
IN
OUT
8.6V TO 22V
TRANSIENT TO 38V
V
BD
IN
RUN/SS
BOOST
ON OFF
L
0.47μF
D
2.2μH
V
SW
C
LT3480
GND
2.2μF
RT
14k
PG
536k
SYNC
11.5k
FB
470pF
22μF
100k
f = 2MHz
3480 TA05
D: DIODES INC. DFLS240L
L: SUMIDA CDRH4D22/HP-2R2
12V Step-Down Converter
V
V
12V
2A
IN
OUT
15V TO 36V
TRANSIENT
TO 60V*
V
BD
IN
RUN/SS
BOOST
ON OFF
L
0.47μF
D
10μH
V
SW
C
LT3480
GND
10μF
RT
26.1k
PG
715k
SYNC
40.2k
FB
330pF
22μF
50k
f = 800kHz
3480 TA06
D: DIODES INC. DFLS240L
L: NEC/TOKIN PLC-0755-100
3480fb
20
LT3480
TYPICAL APPLICATIONS
1.8V Step-Down Converter
V
V
1.8V
2A
IN
OUT
3.5V TO 27V
V
IN
BD
RUN/SS
BOOST
ON OFF
L
0.47μF
D
3.3μH
V
SW
C
LT3480
GND
4.7μF
RT
18.2k
PG
127k
SYNC
68.1k
FB
330pF
47μF
100k
f = 500kHz
3480 TA08
D: DIODES INC. DFLS240L
L: TAIYO YUDEN NP06DZB3R3M
3480fb
21
LT3480
PACKAGE DESCRIPTION
DD Package
10-Lead Plastic DFN (3mm × 3mm)
(Reference LTC DWG # 05-08-1660)
R = 0.115
TYP
6
0.38 p 0.10
10
0.675 p 0.05
3.50 p 0.05
2.15 p 0.05 (2 SIDES)
1.65 p 0.05
3.00 p 0.10 1.65 p 0.10
(4 SIDES)
(2 SIDES)
PIN 1
PACKAGE
OUTLINE
TOP MARK
(SEE NOTE 6)
(DD) DFN 1103
5
1
0.25 p 0.05
0.50 BSC
0.75 p 0.05
0.200 REF
0.25 p 0.05
0.50
BSC
2.38 p 0.10
(2 SIDES)
2.38 p 0.05
(2 SIDES)
0.00 – 0.05
BOTTOM VIEW—EXPOSED PAD
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
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
3480fb
22
LT3480
PACKAGE DESCRIPTION
MSE Package
10-Lead Plastic MSOP
(Reference LTC DWG # 05-08-1660)
BOTTOM VIEW OF
EXPOSED PAD OPTION
2.06 p 0.102
2.794 p 0.102
(.110 p .004)
0.889 p 0.127
(.035 p .005)
(.081 p .004)
1
1.83 p 0.102
(.072 p .004)
5.23
(.206)
MIN
2.083 p 0.102 3.20 – 3.45
(.082 p .004) (.126 – .136)
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)
REF
10 9
8
7 6
RECOMMENDED SOLDER PAD LAYOUT
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) 0307 REV B
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
3480fb
23
LT3480
TYPICAL APPLICATION
1.2V Step-Down Converter
V
V
1.2V
2A
IN
OUT
3.6V TO 27V
V
BD
IN
RUN/SS
BOOST
ON OFF
L
0.47μF
D
3.3μH
V
SW
C
LT3480
GND
4.7μF
RT
16.2k
PG
52.3k
SYNC
68.1k
FB
330pF
100k
47μF
f = 500kHz
3480 TA09
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)
Regulator in SOT-23
LT3437
60V, 400mA (I ), MicroPower Step-Down
V : 3.3V to 80V, V
= 1.25V, I = 100μA, I <1μA, 10-Pin 3mm x 3mm
Q SD
OUT
IN
OUT(MIN)
DC/DC Converter with Burst Mode
DFN and 16-Pin TSSOP Packages
LT1936
36V, 1.4A (I ), 500kHz High Efficiency
V : 3.6V to 36V, V
= 1.2V, I = 1.9mA, I <1μA, MS8E Package
Q SD
OUT
IN
OUT(MIN)
OUT(MIN)
OUT(MIN)
Step-Down DC/DC Converter
LT3493
36V, 1.2A (I ), 750kHz High Efficiency
V : 3.6V to 40V, V
= 0.8V, I = 1.9mA, I <1μA, 6-Pin 2mm x 3mm DFN
Q SD
OUT
IN
Step-Down DC/DC Converter
Package
LT1976/LT1977
LT1767
60V, 1.2A (I ), 200kHz/500kHz, High Efficiency
V : 3.3V to 60V, V
IN
= 1.2V, I = 100μA, I <1μA, 16-Pin TSSOP Package
Q SD
OUT
Step-Down DC/DC Converter with Burst Mode
25V, 1.2A (I ), 1.1MHz, High Efficiency
V : 3V to 25V, V
= 1.2V, I = 1mA, I <6μA, MS8E Package
OUT
IN
OUT(MIN) Q SD
Step-Down DC/DC Converter
LT1940
Dual 25V, 1.4A (I ), 1.1MHz, High Efficiency
V : 3.6V to 25V, V
= 1.2V, I = 3.8mA, I <30μA, 16-Pin TSSOP
Q SD
OUT
IN
OUT(MIN)
OUT(MIN)
OUT(MIN)
OUT(MIN)
Step-Down DC/DC Converter
Package
LT1766
60V, 1.2A (I ), 200kHz, High Efficiency
V : 5.5V to 60V, V
= 1.2V, I = 2.5mA, I = 25μA, 16-Pin TSSOP
Q SD
OUT
IN
Step-Down DC/DC Converter
Package
LT3434/LT3435
LT3481
60V, 2.4A (I ), 200/500kHz, High Efficiency
V : 3.3V to 60V, V
IN
= 1.2V, I = 100μA, I <1μA, 16-Pin TSSOP Package
Q SD
OUT
Step-Down DC/DC Converter with Burst Mode
36V, 2A (I ), 2.8MHz, High Efficiency
V : 3.6V to 34V, V
= 1.26V, I = 50μA, I <1μA, 10-Pin 3mm x 3mm
Q SD
OUT
IN
Step-Down DC/DC Converter with Burst Mode
DFN and 10-Pin MSOP Packages
LT3684
36V, 2A (I ), 2.8MHz, High Efficiency
V : 3.6V to 34V, V = 1.26V, I = 1.5mA, I <1μA, 10-Pin 3mm x 3mm
OUT
IN
OUT(MIN)
Q
SD
Step-Down DC/DC Converter
DFN and 10-Pin MSOP Packages
3480fb
LT 0308 REV B • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
24
●
●
© LINEAR TECHNOLOGY CORPORATION 2008
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
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