LT3480
36V, 2A, 2.4MHz Step-Down
Switching Regulator with
70µA Quiescent Current
FEATURES
DESCRIPTION
Wide Input Range:
Operation from 3.6V to 36V
Over-Voltage Lockout Protects Circuits
through 60V Transients
n 2A Maximum Output Current
n Low Ripple Burst Mode® Operation
70µA IQ at 12VIN to 3.3VOUT
Output Ripple < 15mV
n Adjustable Switching Frequency: 200kHz to 2.4MHz
n Low Shutdown Current: I < 1µA
Q
n Integrated Boost Diode
n Synchronizable Between 250kHz to 2MHz
n Power Good Flag
n Saturating Switch Design: 0.25 On-Resistance
n 0.790V Feedback Reference Voltage
n Output Voltage: 0.79V to 20V
n Soft-Start Capability
n Small 10-Lead Thermally Enhanced MSOP and
(3mm × 3mm) DFN Packages
The LT®3480 is an adjustable frequency (200kHz to
2.4MHz) monolithic buck switching regulator that accepts 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, 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 LT3480 can
further enhance low output current efficiency by drawing bias current from the output when VOUT is above 3V.
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 VOUT reaches 86% of the programmed
output voltage. The LT3480 is available in 10-lead MSOP
and 3mm × 3mm DFN packages with exposed pads for
low thermal resistance.
APPLICATIONS
, 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.
n
n
n
n
n
Automotive Battery Regulation
Power for Portable Products
Distributed Supply Regulation
Industrial Supplies
TYPICAL APPLICATION
3.3V Step-Down Converter
BD
VIN
OFF ON
14k
4.7µF
VOUT
3.3V
2A
RUN/SS
BOOST
LT3480
4.7µH
SW
RT
470pF
PG
40.2k
SYNC
VOUT = 5V
90
0.47µF
VC
100
EFFICIENCY (%)
VIN
4.5V TO 36V
TRANSIENT
TO 60V
Efficiency
GND
22µF
100k
70
60
316k
FB
VOUT = 3.3V
80
VIN = 12V
L = 5.6µH
F = 800 kHz
50
0
3480 TA01
0.5
1.0
1.5
LOAD CURRENT (A)
2
3480 TA01b
3480fe
For more information www.linear.com/LT3480
1
LT3480
ABSOLUTE MAXIMUM RATINGS
(Note 1)
VIN, RUN/SS Voltage (Note 5)....................................60V
BOOST Pin Voltage....................................................56V
BOOST Pin Above SW Pin..........................................30V
FB, RT, VC Voltage........................................................5V
PG, BD, SYNC Voltage...............................................30V
Operating Junction Temperature Range (Note 2)
LT3480E.............................................. –40°C to 125°C
LT3480I............................................... –40°C to 125°C
LT3480H............................................. –40°C to 150°C
LT3480MP.......................................... –55°C to 150°C
Storage Temperature Range.................... –65°C to 150°C
Lead Temperature (Soldering, 10 sec)
(MSE Only)........................................................ 300°C
PIN CONFIGURATION
TOP VIEW
BD
1
10 RT
BOOST
2
9 VC
SW
3
VIN
4
7 PG
RUN/SS
5
6 SYNC
11
TOP VIEW
BD
BOOST
SW
VIN
RUN/SS
8 FB
1
2
3
4
5
11
10
9
8
7
6
RT
VC
FB
PG
SYNC
MSE PACKAGE
10-LEAD PLASTIC MSOP
DD PACKAGE
10-LEAD (3mm × 3mm) PLASTIC DFN
JA = 45°C/W, JC = 10°C/W
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
JA = 45°C/W, JC = 10°C/W
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3480EDD#PBF
LT3480EDD#TRPBF
LCTP
10-Lead (3mm × 3mm) Plastic DFN
–40°C to 125°C
LT3480IDD#PBF
LT3480IDD#TRPBF
LCTP
10-Lead (3mm × 3mm) Plastic DFN
–40°C to 125°C
LT3480EMSE#PBF
LT3480EMSE#TRPBF
LTCTM
10-Lead Plastic MSOP
–40°C to 125°C
LT3480IMSE#PBF
LT3480IMSE#TRPBF
LTCTM
10-Lead Plastic MSOP
–40°C to 125°C
LT3480HMSE#PBF
LT3480HMSE#TRPBF
LTCTM
10-Lead Plastic MSOP
–40°C to 150°C
LT3480MPMSE#PBF
LT3480MPMSE#TRPBF
LTCTM
10-Lead Plastic MSOP
–55°C to 150°C
LEAD BASED FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3480EDD
LT3480EDD#TR
LCTP
10-Lead (3mm × 3mm) Plastic DFN
–40°C to 125°C
LT3480IDD
LT3480IDD#TR
LCTP
10-Lead (3mm × 3mm) Plastic DFN
–40°C to 125°C
LT3480EMSE
LT3480EMSE#TR
LTCTM
10-Lead Plastic MSOP
–40°C to 125°C
LT3480IMSE
LT3480IMSE#TR
LTCTM
10-Lead Plastic MSOP
–40°C to 125°C
LT3480HMSE
LT3480HMSE#TR
LTCTM
10-Lead Plastic MSOP
–40°C to 150°C
LT3480MPMSE
LT3480MPMSE#TR
LTCTM
10-Lead Plastic MSOP
–55°C to 150°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/
3480fe
2
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LT3480
ELECTRICAL
CHARACTERISTICS l denotes the specifications which apply over the full operating temperature
The
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
Minimum Input Voltage
l
VIN Overvoltage Lockout
l
Quiescent Current from VIN
Quiescent Current from BD
36
TYP
MAX
3
3.6
UNITS
V
38
40
V
VRUN/SS = 0.2V
VBD = 3V, Not Switching
VBD = 0, Not Switching
l
0.01
30
105
0.5
100
160
µA
µA
µA
VRUN/SS = 0.2V
VBD = 3V, Not Switching
VBD = 0, Not Switching
l
0.01
80
1
0.5
120
5
µA
µA
µA
2.7
3
V
790
790
800
805
mV
mV
7
30
nA
0.002
0.01
%/V
Minimum Bias Voltage (BD Pin)
Feedback Voltage
l
FB Pin Bias Current (Note 3)
VFB = 0.8V, VC = 0.4V
FB Voltage Line Regulation
4V < VIN < 36V
780
775
l
Error Amp gm
400
Error Amp Gain
1000
µMho
VC Source Current
45
µA
VC Sink Current
45
µA
VC Pin to Switch Current Gain
3.5
A/V
VC Clamp Voltage
Switching Frequency
2
RT = 8.66k
RT = 29.4k
RT = 187k
2.1
0.9
160
Minimum Switch Off-Time
l
Switch Current Limit
Duty Cycle = 5%
Switch VCESAT
ISW = 2A
Boost Schottky Reverse Leakage
VBD = 0V
3
V
2.4
1
200
2.7
1.15
240
MHz
MHz
kHz
60
150
nS
3.5
4
A
500
Minimum Boost Voltage (Note 4)
l
mV
0.02
2
µA
1.5
2.1
V
BOOST Pin Current
ISW = 1A
22
35
mA
RUN/SS Pin Current
VRUN/SS = 2.5V
5
10
µA
2.5
V
RUN/SS Input Voltage High
RUN/SS Input Voltage Low
PG Threshold Offset from Feedback Voltage
0.2
VFB Rising
PG Hysteresis
PG Leakage
VPG = 5V
PG Sink Current
VPG = 0.4V
V
100
mV
12
mV
0.1
l
SYNC Low Threshold
100
600
V
0.7
VSYNC = 0V
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.
µA
µA
0.5
SYNC High Threshold
SYNC Pin Bias Current
1
0.1
V
µA
Note 2: The LT3480E is guaranteed to meet performance specifications
from 0°C to 125°C. Specifications over the –40°C to 125°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls. The LT3480I specifications are
3480fe
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3
LT3480
ELECTRICAL CHARACTERISTICS
guaranteed over the –40°C to 125°C temperature range. The LT3480H
specifications are guaranteed over the –40°C to 150°C temperature range.
The LT3480MP specifications are guaranteed over the –55°C to 150°C
temperature range.
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 5: For operation at TJ ≤ 125°C, the absolute maximum voltage at VIN
and RUN/SS pins is 40V for continuous operation and 60V for up to
1 second nonrepetitive transients. For operation at TJ > 125°C, the
absolute maximum voltage at VIN and RUN/SS pins is 36V.
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency
VIN = 12V
80
EFFICIENCY (%)
VIN = 24V
70
80
VIN = 34V
75
VIN = 24V
70
65
1
60
50
0
L: NEC PLC-0745-5R6
f: 800kHz
0.1
L: NEC PLC-0745-5R6
f: 800kHz
VOUT = 3.3V
50
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
LOAD CURRENT (A)
0
350
SUPPLY CURRENT (µA)
100
80
60
40
20
30
35
3480 G04
1.0
1.5
LOAD CURRENT (A)
300
Maximum Load Current
CATCH DIODE: DIODES, INC. PDS360
VIN = 12V
VOUT = 3.3V
3.5
INCREASED SUPPLY
CURRENT DUE TO CATCH
DIODE LEAKAGE AT
HIGH TEMPERATURE
250
200
150
TYPICAL
3.0
2.5
MINIMUM
2.0
VOUT = 3.3V
TA = 25 °C
L = 4.7µH
f = 800 kHz
100
0
–50 –25
1.5
0
0.01
2
4.0
50
10
20
15
25
INPUT VOLTAGE (V)
0.5
3480 G27
No Load Supply Current
400
VOUT = 3.3V
5
0
3480 G02
No Load Supply Current
0
30
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
LOAD CURRENT (A)
3480 G01
120
VIN = 12V
VOUT = 3.3V
L = 5.6µH
F = 800 kHz
40
55
LOAD CURRENT (A)
VOUT = 5V
50
SUPPLY CURRENT (µA)
70
60
60
0
10
POWER LOSS (W)
VIN = 34V
80
90
VIN = 7V
85
VIN = 12V
90
EFFICIENCY (%)
Efficiency
90
EFFICIENCY (%)
Efficiency
100
25 50 75 100 125 150
TEMPERATURE (°C)
3480 G05
1.0
5
10
20
15
INPUT VOLTAGE (V)
25
30
3480 G06
3480fe
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LT3480
TYPICAL PERFORMANCE CHARACTERISTICS
Maximum Load Current
Switch Current Limit
SWITCH CURRENT LIMIT(A)
LOAD CURRENT (A)
2.5
MINIMUM
2.0
VOUT = 5V
TA = 25 °C
L = 4.7µH
f = 800kHz
1.5
10
5
20
15
INPUT VOLTAGE (V)
25
4.5
4.0
3.5
SWITCH CURRENT LIMIT (A)
TYPICAL
3.0
1.0
Switch Current Limit
4.0
3.5
3.0
2.5
2.0
1.5
1.0
30
0
20
60
40
DUTY CYCLE (%)
80
1.5
1.0
0
–50 –25
100
70
300
200
0
25 50 75 100 125 150
TEMPERATURE (°C)
3480 G09
Feedback Voltage
840
FEEDBACK VOLTAGE (mV)
600
100
60
50
40
30
20
820
800
780
10
0
500
1500
1000
2000
SWITCH CURRENT (mA)
0
2500
0
500
1000
1500
2000
SWITCH CURRENT (mA)
3480 G10
Switching Frequency
1.05
1.00
0.95
0.90
0.85
0
25 50 75 100 125 150
TEMPERATURE (°C)
4380 G13
25 50 75 100 125 150
TEMPERATURE (°C)
Minimum Switch On-Time
1200
140
1000
120
MINIMUM SWITCH ON TIME (ns)
SWITCHING FREQUENCY (kHz)
1.10
0
4380 G12
Frequency Foldback
1.15
0.80
–50 –25
760
–50 –25
2500
3480 G11
1.20
FREQUENCY (MHz)
2.0
Boost Pin Current
80
BOOST PIN CURRENT (mA)
VOLTAGE DROP (mV)
Switch Voltage Drop
400
DUTY CYCLE = 90 %
3480 G08
700
0
3.0
2.5
0.5
3480 G07
500
DUTY CYCLE = 10 %
3.5
800
600
400
200
0
0
100 200 300 400 500 600 700 800 900
FB PIN VOLTAGE (mV)
3480 G14
100
80
60
40
20
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (˚C)
3480 G15
3480fe
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5
LT3480
TYPICAL PERFORMANCE CHARACTERISTICS
Soft-Start
RUN/SS Pin Current
RUN/SS PIN CURRENT (µA)
SWITCH CURRENT LIMIT (A)
3.5
3.0
2.5
2.0
1.5
1.0
1.4
10
1.2
8
6
4
2
0.5
0
0
0.5
2.5
2
1.5
RUN/SS PIN VOLTAGE (V)
1
3
0
3.5
Error Amp Output Current
0
5
20
30
15
25
10
RUN/SS PIN VOLTAGE (V)
10
0
–10
–20
0.5
1.0
1.5
BOOST DIODE CURRENT (A)
Minimum Input Voltage
6.0
4.0
3.5
3.0
–30
2.5
–40
2.0
VOUT = 3.3V
TA = 25°C
L = 4.7µH
f = 800kHz
1
200
10
5.5
5.0
VOUT = 5V
TA = 25 °C
L = 4.7µH
f = 800kHz
4.5
4.0
100
1000
LOAD CURRENT (A)
10000
1
10
100
1000
LOAD CURRENT (A)
3480 G20
3480 G19
VC Voltages
10000
3480 G21
Power Good Threshold
2.50
2.0
6.5
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
VC PIN CURRENT (µA)
20
0
3480 G18
4.5
30
Switching Waveforms; Burst Mode
95
THRESHOLD VOLTAGE (%)
2.00
CURRENT LIMIT CLAMP
1.50
SWITCHING THRESHOLD
0.50
0
–50 –25
0.4
Minimum Input Voltage
40
1.00
0.6
0
35
5.0
–100
100
0
FB PIN ERROR VOLTAGE (V)
0.8
3480 G17
50
–50
–200
1.0
0.2
3480 G16
VC VOLTAGE (V)
Boost Diode
12
BOOST DIODE Vf (V)
4.0
0
25 50 75 100 125 150
TEMPERATURE (°C)
3480 G22
VSW
5V/DIV
90
85
IL
0.2A/DIV
80
VOUT
10mV/DIV
75
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3480 G23
5µs/DIV
VIN = 12V; FRONT PAGE APPLICATION
ILOAD = 10mA
3480 G24
3480fe
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LT3480
TYPICAL PERFORMANCE CHARACTERISTICS
Switching Waveforms; Transition
from Burst Mode to Full Frequency
Switching Waveforms; Full
Frequency Continuous Operation
VSW
5V/DIV
VSW
5V/DIV
IL
0.2A/DIV
IL
0.5A/DIV
VOUT
10mV/DIV
VOUT
10mV/DIV
1µs/DIV
VIN = 12V; FRONT PAGE APPLICATION
ILOAD = 110mA
3480 G25
1µs/DIV
VIN = 12V; FRONT PAGE APPLICATION
ILOAD = 1A
3480 G26
PIN FUNCTIONS
BD (Pin 1): This pin connects to the anode of the boost
Schottky diode. BD also supplies current to the internal
regulator. BD must be locally bypassed when not tied to
VOUT with a low ESR capacitor (1µF).
BOOST (Pin 2): This pin is used to provide a drive
voltage, higher than the input voltage, to the internal bipolar
NPN power switch.
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.
VIN (Pin 4): The VIN pin supplies current to the LT3480’s
internal regulator and to the internal power switch. This
pin must be locally bypassed.
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 VIN
pin. RUN/SS also provides a soft-start function; see the
Applications Information section.
SYNC (Pin 6): This is the external clock synchronization
input. Ground this pin for low ripple Burst Mode operation at
low output loads. Tie to a clock source for synchronization.
Clock edges should have rise and fall times faster than 1µs.
See synchronizing section in Applications Information.
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
valid when VIN is above 3.6V and RUN/SS is high.
FB (Pin 8): The LT3480 regulates the FB pin to 0.790V.
Connect the feedback resistor divider tap to this pin.
VC (Pin 9): The VC pin is the output of the internal error
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.
RT (Pin 10): Oscillator Resistor Input. Connecting a resistor
to ground from this pin sets the switching frequency.
Exposed Pad (Pin 11): Ground. The exposed pad must
be soldered to PCB.
3480fe
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7
LT3480
BLOCK DIAGRAM
VIN
4
VIN
C1
–
+
INTERNAL 0.79V REF
5
10
RT
6
RUN/SS
Σ
RT
SLOPE COMP
OSCILLATOR
200kHz–2.4MHz
SWITCH
LATCH
BOOST
1
2
C3
R
S
Q
SW
DISABLE
SYNC
L1
VOUT
3
C2
D1
Burst Mode
DETECT
SOFT-START
7
BD
PG
VC CLAMP
ERROR AMP
+
–
+
–
0.7V
GND
11
VC
9
CC
RC
FB
CF
8
R2
R1
3480 BD
OPERATION
The LT3480 is a constant frequency, current mode stepdown 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
flowing between the VIN and SW pins, turning the switch
off when this current reaches a level determined by the
voltage at VC. An error amplifier measures the output
voltage through an external resistor divider tied to the FB
pin and servos the VC pin. If the error amplifier’s output
increases, more current is delivered to the output; if it
decreases, less current is delivered. An active clamp on the
VC pin provides current limit. The VC pin is also clamped to
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.
(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.
An internal regulator provides power to the control circuitry.
The bias regulator normally draws power from the VIN pin,
but if the BD pin is connected to an external voltage higher
than 3V bias power will be drawn from the external source
The oscillator reduces the LT3480’s operating frequency when
the voltage at the FB pin is low. This frequency foldback helps
to control the output current during startup and overload.
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.
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.
3480fe
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LT3480
OPERATION
The LT3480 contains a power good comparator which trips
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
enabled and VIN is above 3.6V.
The LT3480 has an overvoltage protection feature which
disables switching action when the VIN goes above 38V
typical (36V minimum). When switching is disabled, the
LT3480 can safely sustain input voltages up to 60V.
APPLICATIONS INFORMATION
FB Resistor Network
Operating Frequency Tradeoffs
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the 1% resistors according to:
Selection of the operating frequency is a tradeoff between
efficiency, component size, minimum dropout voltage, and
maximum input voltage. The advantage of high frequency
operation is that smaller inductor and capacitor values may
be used. The disadvantages are lower efficiency, lower
maximum input voltage, and higher dropout voltage. The
highest acceptable switching frequency (fSW(MAX)) for a
given application can be calculated as follows:
V
R1= R2 OUT − 1
0.79V
Reference designators refer to the Block Diagram.
Setting the Switching Frequency
The LT3480 uses a constant frequency PWM architecture
that can be programmed to switch from 200kHz to 2.4MHz
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.
SWITCHING FREQUENCY (MHz)
RT VALUE (kΩ)
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
88.7
68.1
56.2
46.4
40.2
34
29.4
23.7
19.1
16.2
13.3
11.5
9.76
8.66
Figure 1. Switching Frequency vs. RT Value
fSW(MAX) =
VD + VOUT
tON(MIN) ( VD + VIN – VSW )
where VIN is the typical input voltage, VOUT is the output
voltage, VD is the catch diode drop (~0.5V) and VSW is the
internal switch drop (~0.5V at max load). This equation
shows that slower switching frequency is necessary to
safely accommodate high VIN/VOUT ratio. Also, as shown
in the next section, lower frequency allows a lower dropout
voltage. The reason input voltage range depends on the
switching frequency is because the LT3480 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.
Typical minimum on time at 25°C is 80ns. This means that
the minimum and maximum duty cycles are:
DC MIN = fSW tON(MIN)
DC MAX = 1– fSW tOFF(MIN)
where fSW is the switching frequency, the tON(MIN) is the
minimum switch on time (~150ns), and the tOFF(MIN) is
the minimum switch off time (~150ns). These equations
show that duty cycle range increases when switching
frequency is decreased.
3480fe
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9
LT3480
APPLICATIONS INFORMATION
A good choice of switching frequency should allow adequate input voltage range (see next section) and keep
the inductor and capacitor values small.
Input Voltage Range
The maximum input voltage for LT3480 applications depends
on switching frequency, the Absolute Maximum Ratings of
the VIN and BOOST pins, and the operating mode.
The LT3480 can operate from input voltages up to 38V,
and safely withstand input voltages up 60V. Note that while
VIN>38V (typical), the LT3480 will stop switching, allowing
the output to fall out of regulation.
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 frequency must be set low enough to satisfy VIN(MAX) ≥ 40V
according to the following equation. If lower VIN(MAX) is
desired, this equation can be used directly.
VIN(MAX) =
VOUT + VD
–V +V
fSW tON(MIN) D SW
where VIN(MAX) is the maximum operating input voltage,
VOUT is the output voltage, VD is the catch diode drop
(~0.5V), VSW is the internal switch drop (~0.5V at max
load), fSW is the switching frequency (set by RT), and
tON(MIN) is the minimum switch on time (~150ns). Note that
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.
If the output is in regulation and no short-circuit, startup, 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.
The minimum input voltage is determined by either the
LT3480’s minimum operating voltage of ~3.6V or by its
10
maximum duty cycle (see equation in previous section).
The minimum input voltage due to duty cycle is:
VIN(MIN) =
VOUT + VD
–V +V
1– fSW tOFF(MIN) D SW
where VIN(MIN) is the minimum input voltage, and tOFF(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.
Inductor Selection
For a given input and output voltage, the inductor value
and switching frequency will determine the ripple current.
The ripple current ΔIL increases with higher VIN or VOUT
and decreases with higher inductance and faster switching
frequency. A reasonable starting point for selecting the
ripple current is:
ΔIL = 0.4(IOUT(MAX))
where IOUT(MAX) is the maximum output load current. To
guarantee sufficient output current, peak inductor current
must be lower than the LT3480’s switch current limit (ILIM).
The peak inductor current is:
IL(PEAK) = IOUT(MAX) + ΔIL/2
where IL(PEAK) is the peak inductor current, IOUT(MAX) is
the maximum output load current, and ΔIL is the inductor
ripple current. The LT3480’s switch current limit (ILIM) is
at least 3.5A at low duty cycles and decreases linearly to
2.5A at DC = 0.8. The maximum output current is a function of the inductor ripple current:
IOUT(MAX) = ILIM – ΔIL/2
Be sure to pick an inductor ripple current that provides
sufficient maximum output current (IOUT(MAX)).
The largest inductor ripple current occurs at the highest
VIN. To guarantee that the ripple current stays below the
specified maximum, the inductor value should be chosen
according to the following equation:
V +V V +V
L = OUT D 1– OUT D
VIN(MAX)
fSW ∆IL
For more information www.linear.com/LT3480
3480fe
LT3480
APPLICATIONS INFORMATION
where VD is the voltage drop of the catch diode (~0.4V),
VIN(MAX) is the maximum input voltage, VOUT is the output
voltage, fSW is the switching frequency (set by RT), and L
is in the inductor value.
The inductor’s RMS current rating must be greater than the
maximum load current and its saturation current should be
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.
Table 1. Inductor Vendors
VENDOR
URL
PART SERIES
TYPE
Murata
www.murata.com
LQH55D
Open
TDK
www.componenttdk.com
SLF7045
SLF10145
Shielded
Shielded
Toko
www.toko.com
D62CB
D63CB
D75C
D75F
Shielded
Shielded
Shielded
Open
Sumida
www.sumida.com
CR54
CDRH74
CDRH6D38
CR75
Open
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 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. Finally,
for duty cycles greater than 50% (VOUT/VIN > 0.5), there
is a minimum inductance required to avoid subharmonic
oscillations. See AN19.
Input Capacitor
Bypass the input of the LT3480 circuit with a ceramic capacitor 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
bypass the LT3480 and will easily handle the ripple current.
Note that larger input capacitance is required when a lower
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 lower performance
electrolytic capacitor.
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 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 capacitor concerns the maximum input
voltage rating of the 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 exceeding the LT3480’s
voltage rating. This situation is easily avoided (see the Hot
Plugging Safely section).
For space sensitive applications, a 2.2µF ceramic capacitor can be used for local bypassing of the LT3480 input.
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
with the inductor, it filters the square wave generated by the
LT3480 to produce the DC output. In this role it determines
the output ripple, and low impedance at the switching
frequency is important. The second function is to store
3480fe
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11
LT3480
APPLICATIONS INFORMATION
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:
COUT =
100
VOUT fSW
Catch Diode
The catch diode conducts current only during switch off
time. Average forward current in normal operation can
be calculated from:
ID(AVG) = IOUT (VIN – VOUT)/VIN
where fSW is in MHz, and COUT is the recommended 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 Frequency Compensation
section to choose an appropriate compensation network.
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
electrolytic capacitors can be used for the output capacitor.
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 0.05 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.
where IOUT is the output load current. The only reason to
consider a diode with a larger current rating than necessary
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 Schottky diode with a
reverse voltage rating greater than the input voltage. The
overvoltage protection feature in the LT3480 will keep the
switch off when VIN > 38V which allows the use of 40V
rated Schottky even when VIN ranges up to 60V. Table 3
lists several Schottky diodes and their manufacturers.
Table 3. Diode Vendors
PART NUMBER
VR
(V)
IAVE
(A)
VF AT 1A
(mV)
VF AT 2A
(mV)
On Semiconductor
MBRM120E
MBRM140
20
40
1
1
530
550
595
Diodes Inc.
B120
B130
B220
B230
DFLS240L
20
30
20
30
40
1
1
2
2
2
500
500
International Rectifier
10BQ030
20BQ030
30
30
1
2
420
500
500
500
470
470
Table 2. Capacitor Vendors
VENDOR
PHONE
URL
PART SERIES
COMMANDS
Panasonic
(714) 373-7366
www.panasonic.com
Ceramic,
Polymer,
Tantalum
EEF Series
Kemet
Sanyo
Murata
(864) 963-6300
(408) 749-9714
(408) 436-1300
AVX
Taiyo Yuden
(864) 963-6300
www.kemet.com
www.sanyovideo.com
Ceramic,
Tantalum
Ceramic,
Polymer,
Tantalum
www.murata.com
Ceramic
www.avxcorp.com
Ceramic,
Tantalum
www.taiyo-yuden.com
Ceramic
T494, T495
POSCAP
TPS Series
3480fe
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LT3480
APPLICATIONS INFORMATION
A final precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the 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 exceeding
the LT3480’s rating. This situation is easily avoided (see
the Hot Plugging Safely section).
Frequency Compensation
The LT3480 uses current mode control to regulate the
output. This simplifies loop compensation. In particular, 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
VC pin, as shown in Figure 2. Generally a capacitor (CC)
and a resistor (RC) in series to ground are used. In addition, there may be lower value capacitor in parallel. This
capacitor (CF) is not part of the loop compensation but
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.
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
LT3480
CURRENT MODE
POWER STAGE
gm = 3.5mho
SW
ERROR
AMPLIFIER
OUTPUT
R1
3M
VC
CF
RC
CPL
FB
gm =
420µmho
+
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT3480 due to their piezoelectric nature.
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
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.
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
current proportional to the voltage at the VC pin. Note that
the output capacitor integrates this current, and that the
capacitor on the VC pin (CC) integrates the error amplifier
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 RC in series with CC.
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 (CPL) across the feedback divider may improve
the transient response. Figure 3 shows the transient
response when the load current is stepped from 500mA
to 1500mA and back to 500mA.
–
Ceramic Capacitors
ESR
0.8V
C1
C1
+
POLYMER
OR
TANTALUM
GND
CERAMIC
R2
CC
3480 F02
Figure 2. Model for Loop Response
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13
LT3480
APPLICATIONS INFORMATION
VOUT
100mV/DIV
VSW
5V/DIV
IL
0.2A/DIV
IL
0.5A/DIV
VOUT
10mV/DIV
VIN = 12V; FRONT PAGE APPLICATION
10µs/DIV
5µs/DIV
VIN = 12V; FRONT PAGE APPLICATION
ILOAD = 10mA
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
Low-Ripple Burst Mode and Pulse-Skip Mode
The LT3480 is capable of operating in either Low-Ripple
Burst Mode or Pulse-Skip Mode which are selected using
the SYNC pin. See the Synchronization section for details.
3480 F04
Figure 4. Burst Mode Operation
that the LT3480 will enter full frequency standard PWM
operation at a lower output load current than when in
Burst Mode. The front page application circuit will switch
at full frequency at output loads higher than about 60mA.
BOOST and BIAS Pin Considerations
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, the LT3480 delivers single cycle bursts of current
to the output capacitor followed by sleep periods where
the output power is delivered to the load by the output
capacitor. Because the LT3480 delivers power to the output
with single, low current pulses, the output ripple is kept
below 15mV for a typical application. In addition, VIN and
BD quiescent currents are reduced to typically 30µA and
80µA respectively 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 RT 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.
Capacitor C3 and the internal boost Schottky diode (see
the Block Diagram) are used to generate a boost voltage 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
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 voltages the
boost diode can be tied to the input (Figure 5c), or to
another supply greater than 2.8V. Tying BD to VIN reduces
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.
If low quiescent current is not required the LT3480 can
operate in Pulse-Skip mode. The benefit of this mode is
The minimum operating voltage of an LT3480 application
is limited by the minimum input voltage (3.6V) and by the
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APPLICATIONS INFORMATION
6.0
VOUT
BD
5.5
BOOST
VIN
LT3480
GND
4.7µF
C3
INPUT VOLTAGE (V)
VIN
SW
TO START
(WORST CASE)
5.0
4.5
4.0
TO RUN
3.5
3.0
(5a) For VOUT > 2.8V
VOUT = 3.3V
TA = 25°C
L = 8.2µH
f = 700kHz
2.5
2.0
D2
BD
BOOST
VIN
LT3480
GND
4.7µF
VOUT
BD
4.7µF
LT3480
GND
6.0
5.0
TO RUN
4.0
VOUT = 5V
TA = 25°C
L = 8.2µH
f = 700kHz
3.0
2.0
BOOST
VIN
10000
TO START
(WORST CASE)
7.0
SW
(5b) For 2.5V < VOUT < 2.8V
VIN
100
1000
LOAD CURRENT (A)
8.0
C3
INPUT VOLTAGE (V)
VIN
10
1
VOUT
1
C3
10
100
1000
LOAD CURRENT (A)
10000
3480 F06
SW
Figure 6. The Minimum Input Voltage Depends on
Output Voltage, Load Current and Boost Circuit
3480 FO5
(5c) For VOUT < 2.5V; VIN(MAX) = 30V
Figure 5. Three Circuits For Generating The Boost Voltage
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
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
start. The plots show the worst-case situation where VIN
is ramping very slowly. For lower start-up voltage, the
boost diode can be tied to VIN; however, this restricts the
input range to one-half of the absolute maximum rating
of the BOOST pin.
At light loads, the inductor current becomes discontinuous and the effective duty cycle can be very high. This
reduces the minimum input voltage to approximately
300mV above VOUT. At higher load currents, the inductor
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.
3480fe
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15
LT3480
APPLICATIONS INFORMATION
Soft-Start
The RUN/SS pin can be used to soft-start the LT3480,
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 startup and shut-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.
Synchronization
To select Low-Ripple Burst Mode operation, tie the SYNC
pin below 0.3V (this can be ground or a logic output).
Synchronizing the LT3480 oscillator to an external frequency 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).
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.
The LT3480 may be synchronized over a 250kHz to 2MHz
range. The RT resistor should be chosen to set the LT3480
switching frequency 20% below the lowest synchronization
input. For example, if the synchronization signal will be
250kHz and higher, the RT should be chosen for 200kHz.
To assure reliable and safe operation the LT3480 will only
synchronize when the output voltage is near regulation
as indicated by the PG flag. It is therefore necessary to
choose a large enough inductor value to supply the required
output current at the frequency set by the RT resistor. See
Inductor Selection section. It is also important to note that
slope compensation is set by the RT value: When the sync
frequency is much higher than the one set by RT, the slope
compensation will be significantly reduced which may
require a larger inductor value to prevent subharmonic
oscillation.
Shorted and Reversed Input Protection
If the inductor is chosen so that it won’t saturate excessively,
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
or in battery backup systems where a battery or some
other supply is diode OR-ed with the LT3480’s output. If
the VIN pin is allowed to float and the RUN/SS pin is held
high (either by a logic signal or because it is tied to VIN),
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 zero.
However, if the VIN pin is grounded while the output is
held high, then parasitic diodes inside the LT3480 can
pull large currents from the output through the SW pin
and the VIN pin. Figure 8 shows a circuit that will run only
when the input voltage is present and that protects against
a shorted or reversed input.
D4
MBRS140
VIN
VIN
BOOST
LT3480
RUN/SS
IL
1A/DIV
RUN
GND FB
15k
RUN/SS
0.22µF
VOUT
SW
VC
VRUN/SS
2V/DIV
GND
BACKUP
VOUT
2V/DIV
3480 F08
2ms/DIV
3480 F07
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
3480fe
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APPLICATIONS INFORMATION
PCB Layout
Hot Plugging Safely
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
currents flow in the LT3480’s VIN and SW pins, the catch
diode (D1) and the input capacitor (C1). The loop formed
by these components should be as small as possible. These
components, along with the inductor and output capacitor,
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.
Finally, keep the FB and VC nodes small so that the ground
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.
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LT3480 circuits. However, these capacitors 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
source, forms an under damped tank circuit, and the
voltage at the VIN pin of the LT3480 can ring to twice the
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
to prevent this overshoot. Figure 10 shows the waveforms
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.
L1
C2
VOUT
RRT
CC
High Temperature Considerations
RC
R2
R1
D1
C1
GND
RPG
3480 F09
VIAS TO LOCAL GROUND PLANE
VIAS TO VOUT
VIAS TO SYNC
VIAS TO RUN/SS
VIAS TO PG
VIAS TO VIN
OUTLINE OF LOCAL
GROUND PLANE
Figure 9. A Good PCB Layout Ensures Proper, Low EMI Operation
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, the thermal resistance from die (or junction)
to ambient can be reduced to JA = 35°C/W or less. With
100 LFPM airflow, 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
3480fe
For more information www.linear.com/LT3480
17
LT3480
APPLICATIONS INFORMATION
CLOSING SWITCH
SIMULATES HOT PLUG
IIN
VIN
+
LOW
IMPEDANCE
ENERGIZED
24V SUPPLY
DANGER
RINGING VIN MAY EXCEED
ABSOLUTE MAXIMUM RATING
LT3480
4.7µF
IIN
10A/DIV
STRAY
INDUCTANCE
DUE TO 6 FEET
(2 METERS) OF
TWISTED PAIR
0.7Ω
+
VIN
20V/DIV
0.1µF
20µs/DIV
(10a)
VIN
20V/DIV
LT3480
4.7µF
IIN
10A/DIV
(10b)
+
22µF
35V
AI.EI.
+
20µs/DIV
VIN
20V/DIV
LT3480
4.7µF
IIN
10A/DIV
(10c)
20µs/DIV
3480 F10
Figure 10. A Well Chosen Input Network Prevents Input Voltage Overshoot and
Ensures Reliable Operation when the LT3480 Is Connected to a Live Supply
the LT3480, it is possible to dissipate enough heat to raise
the junction temperature beyond the absolute maximum of
125°C. When operating at high ambient temperatures, the
maximum load current should be derated as the ambient
temperature approaches 125°C.
Power dissipation within the LT3480 can be estimated by
calculating the total power loss from an efficiency measurement 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.
3480fe
18
For more information www.linear.com/LT3480
LT3480
TYPICAL APPLICATIONS
5V Step-Down Converter
VIN
6.8V TO 36V
TRANSIENT
TO 60V*
BD
VIN
RUN/SS
ON OFF
VOUT
5V
2A
BOOST
0.47µF
VC
4.7µF
SW
LT3480
D
RT
16.2k
PG
SYNC
40.2k
470pF
L
6.8µH
536k
FB
GND
22µF
100k
f = 800kHz
3480 TA02
D: DIODES INC. DFLS240L
L: TAIYO YUDEN NP06DZB6R8M
3.3V Step-Down Converter
VIN
4.4V TO 36V
TRANSIENT
TO 60V*
BD
VIN
RUN/SS
ON OFF
VOUT
3.3V
2A
BOOST
L
4.7µH
0.47µF
VC
4.7µF
SW
LT3480
D
RT
14k
PG
SYNC
40.2k
470pF
316k
FB
GND
22µF
100k
f = 800kHz
3480 TA03
D: DIODES INC. DFLS240L
L: TAIYO YUDEN NP06DZB4R7M
2.5V Step-Down Converter
VIN
4V TO 36V
TRANSIENT
TO 60V*
BD
VIN
ON OFF
RUN/SS
D2
BOOST
1µF
VC
4.7µF
LT3480
D1
PG
56.2k
330pF
SYNC
f = 600kHz
GND
L
4.7µH
SW
RT
20k
VOUT
2.5V
2A
215k
FB
47µF
100k
3480 TA04
D1: DIODES INC. DFLS240L
D2: MBR0540
L: TAIYO YUDEN NP06DZB4R7M
3480fe
For more information www.linear.com/LT3480
19
LT3480
TYPICAL APPLICATIONS
5V, 2MHz Step-Down Converter
VIN
8.6V TO 22V
TRANSIENT TO 38V
BD
VIN
RUN/SS
ON OFF
VOUT
5V
2A
BOOST
L
2.2µH
0.47µF
VC
2.2µF
SW
LT3480
D
RT
14k
PG
SYNC
11.5k
470pF
536k
FB
GND
22µF
100k
f = 2MHz
3480 TA05
D: DIODES INC. DFLS240L
L: SUMIDA CDRH4D22/HP-2R2
12V Step-Down Converter
VIN
15V TO 36V
TRANSIENT
TO 60V*
BD
VIN
ON OFF
VOUT
12V
2A
RUN/SS
BOOST
0.47µF
VC
10µF
SW
LT3480
D
RT
26.1k
PG
SYNC
40.2k
330pF
L
10µH
715k
FB
GND
22µF
50k
f = 800kHz
3480 TA06
D: DIODES INC. DFLS240L
L: NEC/TOKIN PLC-0755-100
1.8V Step-Down Converter
VOUT
1.8V
2A
VIN
3.5V TO 27V
BD
VIN
ON OFF
RUN/SS
BOOST
0.47µF
VC
4.7µF
LT3480
SW
D
RT
18.2k
PG
68.1k
330pF
SYNC
f = 500kHz
L
3.3µH
GND
127k
FB
47µF
100k
3480 TA08
D: DIODES INC. DFLS240L
L: TAIYO YUDEN NP06DZB3R3M
20
For more information www.linear.com/LT3480
3480fe
LT3480
PACKAGE DESCRIPTION
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
DD Package
10-Lead Plastic DFN (3mm × 3mm)
(Reference LTC DWG # 05-08-1699 Rev C)
0.70 ±0.05
3.55 ±0.05
1.65 ±0.05
2.15 ±0.05 (2 SIDES)
PACKAGE
OUTLINE
0.25 ± 0.05
0.50
BSC
2.38 ±0.05
(2 SIDES)
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
3.00 ±0.10
(4 SIDES)
R = 0.125
TYP
6
0.40 ± 0.10
10
1.65 ± 0.10
(2 SIDES)
PIN 1 NOTCH
R = 0.20 OR
0.35 × 45°
CHAMFER
PIN 1
TOP MARK
(SEE NOTE 6)
0.200 REF
0.75 ±0.05
0.00 – 0.05
5
1
(DD) DFN REV C 0310
0.25 ± 0.05
0.50 BSC
2.38 ±0.10
(2 SIDES)
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
3480fe
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 representaFor of
more
information
www.linear.com/LT3480
tion that the interconnection
its circuits
as described
herein will not infringe on existing patent rights.
21
LT3480
PACKAGE DESCRIPTION
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
MSE Package
10-Lead Plastic MSOP, Exposed Die Pad
(Reference LTC DWG # 05-08-1664 Rev G)
BOTTOM VIEW OF
EXPOSED PAD OPTION
1.88 ± 0.102
(.074 ± .004)
5.23
(.206)
MIN
1
0.889 ± 0.127
(.035 ± .005)
0.05 REF
10
0.305 ± 0.038
(.0120 ± .0015)
TYP
RECOMMENDED SOLDER PAD LAYOUT
3.00 ± 0.102
(.118 ± .004)
(NOTE 3)
DETAIL “B”
CORNER TAIL IS PART OF
DETAIL “B” THE LEADFRAME FEATURE.
FOR REFERENCE ONLY
NO MEASUREMENT PURPOSE
10 9 8 7 6
DETAIL “A”
0° – 6° TYP
1 2 3 4 5
GAUGE PLANE
0.53 ± 0.152
(.021 ± .006)
DETAIL “A”
0.18
(.007)
0.497 ± 0.076
(.0196 ± .003)
REF
3.00 ± 0.102
(.118 ± .004)
(NOTE 4)
4.90 ± 0.152
(.193 ± .006)
0.254
(.010)
0.29
REF
1.68
(.066)
1.68 ± 0.102 3.20 – 3.45
(.066 ± .004) (.126 – .136)
0.50
(.0197)
BSC
1.88
(.074)
SEATING
PLANE
0.86
(.034)
REF
1.10
(.043)
MAX
0.17 – 0.27
(.007 – .011)
TYP
0.50
(.0197)
BSC
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
6. EXPOSED PAD DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH ON E-PAD
SHALL NOT EXCEED 0.254mm (.010") PER SIDE.
0.1016 ± 0.0508
(.004 ± .002)
MSOP (MSE) 0910 REV G
3480fe
22
For more information www.linear.com/LT3480
LT3480
REVISION HISTORY
(Revision history begins at Rev D)
REV
DATE
DESCRIPTION
D
10/11
Added H- and MP-grades for the MSE package
E
8/13
PAGE NUMBER
2, 3
Revised BD pin description
7
Revised Figure 5 to add capacitors
15
Clarified maximum temperature range of LT3480E
2, 3
3480fe
For more information www.linear.com/LT3480
23
LT3480
TYPICAL APPLICATION
1.2V Step-Down Converter
VOUT
1.2V
2A
VIN
3.6V TO 27V
BD
VIN
RUN/SS
ON OFF
BOOST
0.47µF
VC
4.7µF
LT3480
SW
D
RT
16.2k
PG
SYNC
68.1k
330pF
L
3.3µH
GND
52.3k
FB
100k
f = 500kHz
47µF
3480 TA09
D: DIODES INC. DFLS240L
L: TAIYO YUDEN NP06DZB3R3M
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LT1933
500mA (IOUT), 500kHz Step-Down Switching
Regulator in SOT-23
VIN: 3.6V to 36V, VOUT(MIN) = 1.2V, IQ = 1.6mA, ISD
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