ANALOG DEVICES LT 3480 EMSE Datasheet

LT3480

36V, 2A, 2.4MHz Step-Down

Switching Regulator with

70µA Quiescent Current

FEATURES

n

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 3.3V

OUT

Output Ripple < 15mV

n

Adjustable Switching Frequency: 200kHz to 2.4MHz

n

Low Shutdown Current: IQ < 1µA

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

APPLICATIONS

n

Automotive Battery Regulation

n

Power for Portable Products

n

Distributed Supply Regulation

n

Industrial Supplies

DESCRIPTION

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
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 V
Shutdown reduces input supply current to less than 1µA
while a resistor and capacitor on the RUN/SS pin provide a
controlled output voltage ramp (soft-start). A power good
flag signals when V
output voltage. The LT3480 is available in 10-lead MSOP
and 3mm × 3mm DFN packages with exposed pads for
low thermal resistance.

, 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.

switch is included on the die along with

is above 3V.

OUT

reaches 86% of the programmed

OUT

TYPICAL APPLICATION

3.3V Step-Down Converter

V

IN

4.5V TO 36V
TRANSIENT

TO 60V

4.7µF

OFF ON

14k

470pF

40.2k

V

IN

RUN/SS BOOST

V

LT3480

C

RT

PG

SYNC

GND

Efficiency

V

OUT

3.3V

BD

0.47µF

SW

FB

For more information www.linear.com/LT3480

4.7µH

316k

100k

3480 TA01

2A

22µF

100

90

80

70

EFFICIENCY (%)

60

50

0

V

= 5V

OUT

V

= 3.3V

OUT

VIN = 12V
L = 5.6µH

0.5 1.0 1.5 2
LOAD CURRENT (A)

F = 800 kHz

3480 TA01b

3480fe

1

LT3480

ABSOLUTE MAXIMUM RATINGS

VIN, RUN/SS Voltage (Note 5) ...................................60V

BOOST Pin Voltage
BOOST Pin Above SW Pin
FB, RT, V

Voltage .......................................................5V

C

PG, BD, SYNC Voltage

...................................................56V

.........................................30V

..............................................30V

Operating Junction Temperature Range (Note 2)

LT3480E
LT3480I

............................................. –40°C to 125°C

.............................................. –40°C to 125°C

LT3480H ............................................ –40°C to 150°C

LT3480MP.......................................... –55°C to 150°C

PIN CONFIGURATION

TOP VIEW

10

BD

1

BOOST

2

11

3

SW

4

V

IN

5

RUN/SS

10-LEAD (3mm × 3mm) PLASTIC DFN

EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB

DD PACKAGE

= 45°C/W, JC = 10°C/W

JA

RT

9

V

C

FB

8
7

PG

6

SYNC

(Note 1)

Storage Temperature Range ................... –65°C to 150°C

Lead Temperature (Soldering, 10 sec)

(MSE Only)

....................................................... 300°C

TOP VIEW

10

1

BD

2

BOOST

SW
V

IN

RUN/SS

10-LEAD PLASTIC MSOP

EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB

11

3
4
5

MSE PACKAGE

= 45°C/W, JC = 10°C/W

JA

RT

9

V

C

FB

8

PG

7

SYNC

6

ORDER INFORMATION

LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE

LT3480EDD#PBF LT3480EDD#TRPBF LCTP

LT3480IDD#PBF LT3480IDD#TRPBF LCTP

10-Lead (3mm × 3mm) Plastic DFN

10-Lead (3mm × 3mm) Plastic DFN

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

LT3480IDD LT3480IDD#TR LCTP

10-Lead (3mm × 3mm) Plastic DFN

10-Lead (3mm × 3mm) Plastic DFN

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/

–40°C to 125°C

–40°C to 125°C

–40°C to 125°C

–40°C to 125°C

3480fe

2

For more information www.linear.com/LT3480

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, V

PARAMETER CONDITIONS MIN TYP MAX UNITS

Minimum Input Voltage

Overvoltage Lockout

V

IN

V

Quiescent Current from V

IN

Quiescent Current from BD V

Minimum Bias Voltage (BD Pin) 2.7 3 V

Feedback Voltage

FB Pin Bias Current (Note 3) V

FB Voltage Line Regulation 4V < V

Error Amp g

m

Error Amp Gain 1000

Source Current 45 µA

V

C

Sink Current 45 µA

V

C

Pin to Switch Current Gain 3.5 A/V

V

C

Clamp Voltage 2 V

V

C

Switching Frequency R

Minimum Switch Off-Time

Switch Current Limit Duty Cycle = 5% 3 3.5 4 A

Switch V

CESAT

Boost Schottky Reverse Leakage V

Minimum Boost Voltage (Note 4)

BOOST Pin Current I

RUN/SS Pin Current V

RUN/SS Input Voltage High 2.5 V

RUN/SS Input Voltage Low 0.2 V

PG Threshold Offset from Feedback Voltage V

PG Hysteresis 12 mV

PG Leakage V

PG Sink Current V

SYNC Low Threshold 0.5 V

SYNC High Threshold 0.7 V

SYNC Pin Bias Current V

= 0.2V

RUN/SS

V

= 3V, Not Switching

BD

V

= 0, Not Switching

BD

= 0.2V

RUN/SS

V

= 3V, Not Switching

BD

V

= 0, Not Switching

BD

= 0.8V, VC = 0.4V

FB

IN

= 8.66k

T

R

= 29.4k

T

R

= 187k

T

ISW = 2A 500 mV

= 0V 0.02 2 µA

BD

= 1A 22 35 mA

SW

RUN/SS

Rising 100 mV

FB

= 5V 0.1 1 µA

PG

= 0.4V

PG

= 0V 0.1 µA

SYNC

RUN/SS

= 10V, V

= 15V, VBD = 3.3V unless otherwise noted. (Note 2)

BOOST

l

l

36 38 40 V

l

3 3.6 V

0.01
30

105

l

0.01
80

1

780

l

775

l

790
790

7 30 nA

0.5
100
160

0.5
120

5

800
805

µA
µA
µA

µA
µA
µA

mV
mV

< 36V 0.002 0.01 %/V

400 µMho

2.1

0.9

160

l

l

2.4
1

200

2.7

1.15
240

MHz
MHz

kHz

60 150 nS

1.5 2.1 V

= 2.5V 5 10 µA

l

100 600 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.

For more information www.linear.com/LT3480

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

3

LT3480

EFFICIENCY (%)

EFFICIENCY (%)

POWER LOSS (W)

SUPPLY CURRENT (µA)

120

SUPPLY CURRENT (µA)

LOAD CURRENT (A)

4.0

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.

TYPICAL PERFORMANCE CHARACTERISTICS

Efficiency

100

90

80

70

60

V

50

0

VIN = 12V

VIN = 24V

= 5V

OUT

0.2 0.4 0.6 0.8 1.2 1.4 1.6 1.8 2.01.0
LOAD CURRENT (A)

No Load Supply Current

100

80

60

40

20

0

5 10 20

0

15

INPUT VOLTAGE (V)

VIN = 34V

L: NEC PLC-0745-5R6
f: 800kHz

3480 G01

V

= 3.3V

OUT

25 30 35

3480 G04

Efficiency

90

85

80

75

70

65

EFFICIENCY (%)

60

55

50

0 0.2 0.4 0.6 0.8 1.2 1.4 1.6 1.8 2.01.0

400

350

300

250

200

150

100

50

Note 4: This is the minimum voltage across the boost capacitor needed to
guarantee full saturation of the switch.

Note 5: For operation at T
and RUN/SS pins is 40V for continuous operation and 60V for up to
1 second nonrepetitive transients. For operation at T
absolute maximum voltage at V

VIN = 7V

VIN = 12V

VIN = 24V

V

= 3.3V

OUT

LOAD CURRENT (A)

L: NEC PLC-0745-5R6
f: 800kHz

No Load Supply Current

CATCH DIODE: DIODES, INC. PDS360

VIN = 12V

= 3.3V

V

OUT

INCREASED SUPPLY
CURRENT DUE TO CATCH
DIODE LEAKAGE AT
HIGH TEMPERATURE

0

–50

–25 0 50

25

TEMPERATURE (°C)

VIN = 34V

3480 G02

75 100 150125

3480 G05

≤ 125°C, the absolute maximum voltage at VIN

J

> 125°C, the

and RUN/SS pins is 36V.

IN

J

Efficiency

90

80

70

60

50

VIN = 12V

= 3.3V

V

40

30

0

0.5 1.0 1.5 2
LOAD CURRENT (A)

OUT

L = 5.6µH
F = 800 kHz

3480 G27

Maximum Load Current

3.5

3.0

2.5

2.0

1.5

1.0
5

TYPICAL

MINIMUM

10 20

15

INPUT VOLTAGE (V)

V

OUT

= 25 °C

T

A

L = 4.7µH
f = 800 kHz

25 30

10

1

0.1

0.01

= 3.3V

3480 G06

4

3480fe

For more information www.linear.com/LT3480

4.0

SWITCH CURRENT LIMIT (A)

TYPICAL PERFORMANCE CHARACTERISTICS

BOOST PIN CURRENT (mA)

80

FEEDBACK VOLTAGE (mV)

FREQUENCY (MHz)

1.20

SWITCHING FREQUENCY (kHz)

1200

140

LT3480

Maximum Load Current

3.5

TYPICAL

3.0

2.5

MINIMUM

2.0

LOAD CURRENT (A)

1.5

1.0
10 20

5

INPUT VOLTAGE (V)

Switch Voltage Drop

700

600

500

400

300

VOLTAGE DROP (mV)

200

100

0

500 1000 2000 2500

0

SWITCH CURRENT (mA)

Switching Frequency

Switch Current Limit

3.5

3.0

2.5

2.0

V

= 5V

OUT

= 25 °C

T

A

L = 4.7µH
f = 800kHz

15

25 30

3480 G07

SWITCH CURRENT LIMIT(A)

1.5

1.0
20 60

0

40

DUTY CYCLE (%)

80 100

3480 G08

Boost Pin Current

70

60

50

40

30

20

10

1500

3480 G10

0

0 1500500 1000 2000 2500

SWITCH CURRENT (mA)

3480 G11

Frequency Foldback

Switch Current Limit

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0

–50 25–25 0 50 75 100 150125

DUTY CYCLE = 10 %

DUTY CYCLE = 90 %

TEMPERATURE (°C)

Feedback Voltage

840

820

800

780

760

–50 25–25 0 50 75 100 150125

TEMPERATURE (°C)

Minimum Switch On-Time

3480 G09

4380 G12

1.15

1.10

1.05

1.00

0.95

0.90

0.85

0.80
–50 25–25 0 50 75 100 150125

TEMPERATURE (°C)

4380 G13

1000

800

600

400

200

0

0

200 400

100 300

FB PIN VOLTAGE (mV)

500

700 900

600

For more information www.linear.com/LT3480

800

3480 G14

120

100

80

60

40

MINIMUM SWITCH ON TIME (ns)

20

0

–50 25–25 0 50 75 100 150125

TEMPERATURE (˚C)

3480 G15

3480fe

5

LT3480

5.0

6.5

V

VOLTAGE (V)

2.50

95

4.0

RUN/SS PIN CURRENT (µA)

12

BOOST DIODE V

(V)

TYPICAL PERFORMANCE CHARACTERISTICS

Soft-Start

3.5

3.0

2.5

2.0

1.5

1.0

SWITCH CURRENT LIMIT (A)

0.5

0

0.5 1 2

0

1.5

RUN/SS PIN VOLTAGE (V)

Error Amp Output Current

50

40

30

20

10

0

–10

PIN CURRENT (µA)

C

–20

V

–30

–40

–50

–200

–100 100
FB PIN ERROR VOLTAGE (V)

Voltages

V

C

2.5 3 3.5

3480 G16

0 200

3480 G19

RUN/SS Pin Current

10

8

6

4

2

0

0

5 10

RUN/SS PIN VOLTAGE (V)

Minimum Input Voltage

4.5

4.0

3.5

3.0

INPUT VOLTAGE (V)

V

= 3.3V

OUT

2.5

= 25°C

T

A

L = 4.7µH
f = 800kHz

2.0
10 100 1000

1

LOAD CURRENT (A)

Power Good Threshold

20 30 35

15 25

3480 G17

10000

3480 G20

Boost Diode

1.4

1.2

1.0

f

0.8

0.6

0.4

0.2

0

0

0.5 1.0 1.5

BOOST DIODE CURRENT (A)

Minimum Input Voltage

6.0

5.5

5.0

INPUT VOLTAGE (V)

V

= 5V

OUT

4.5

= 25 °C

T

A

L = 4.7µH
f = 800kHz

4.0
1 1000010 100 1000

LOAD CURRENT (A)

Switching Waveforms; Burst Mode

2.0

3480 G18

3480 G21

2.00

1.50

1.00

C

0.50

0

–50 25–25 0 50 75 100 150125

6

CURRENT LIMIT CLAMP

SWITCHING THRESHOLD

TEMPERATURE (°C)

3480 G22

90

85

80

THRESHOLD VOLTAGE (%)

75

–50 25–25 0 50 75 100 150125

TEMPERATURE (°C)

For more information www.linear.com/LT3480

3480 G23

V

5V/DIV

0.2A/DIV

V

OUT

10mV/DIV

SW

I

L

5µs/DIV

VIN = 12V; FRONT PAGE APPLICATION

= 10mA

I

LOAD

3480 G24

3480fe

TYPICAL PERFORMANCE CHARACTERISTICS

LT3480

Switching Waveforms; Transition
from Burst Mode to Full Frequency

V

SW

5V/DIV

I

L

0.2A/DIV

V

OUT

10mV/DIV

1µs/DIV

VIN = 12V; FRONT PAGE APPLICATION
I

= 110mA

LOAD

3480 G25

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

with a low ESR capacitor (1µF).

V

OUT

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.

(Pin 4): The VIN pin supplies current to the LT3480’s

V

IN

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 V
pin. RUN/SS also provides a soft-start function; see the
Applications Information section.

IN

Switching Waveforms; Full
Frequency Continuous Operation

V

SW

5V/DIV

I

L

0.5A/DIV

V

OUT

10mV/DIV

1µs/DIV

VIN = 12V; FRONT PAGE APPLICATION
I

= 1A

LOAD

3480 G26

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 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.

(Pin 9): The VC pin is the output of the internal error

V

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.

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.

For more information www.linear.com/LT3480

3480fe

7

LT3480

BLOCK DIAGRAM

V

V

IN

R

IN

4

C1

INTERNAL 0.79V REF

RUN/SS

5

RT

10

T

SYNC

6

SOFT-START

PG

7

+

0.7V

–

GND

11 8

ERROR AMP

FB

–

BOOST

SW

BD

1

2

3

V

C

9

C3

L1

D1

C

C

C

F

R

C

V

OUT

C2

+

Σ

SLOPE COMP

OSCILLATOR

200kHz–2.4MHz

SWITCH

LATCH

R

S

DISABLE

Burst Mode

DETECT

CLAMP

V

C

Q

+
–

R2

R1

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 V
off when this current reaches a level determined by the
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
increases, more current is delivered to the output; if it
decreases, less current is delivered. An active clamp on the

pin provides current limit. The VC pin is also clamped to

V

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.

and SW pins, turning the switch

IN

pin. If the error amplifier’s output

C

3480 BD

(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.

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.

An internal regulator provides power to the control circuitry.
The bias regulator normally draws power from the V

IN

pin,
but if the BD pin is connected to an external voltage higher
than 3V bias power will be drawn from the external source

8

For more information www.linear.com/LT3480

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.

3480fe

OPERATION

VD+ V

)

DC

= fSWt

)

LT3480

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 V

is above 3.6V.

IN

APPLICATIONS INFORMATION

FB Resistor Network

The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the 1% resis­tors according to:

V



R1=R2





0.79V

OUT

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 R
frequency is in Figure 1.

SWITCHING FREQUENCY (MHz)

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

Figure 1. Switching Frequency vs. RT Value

−1






value for a desired switching

T

R

VALUE (kΩ)

T

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

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.

Operating Frequency Tradeoffs

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 (f

SW(MAX)

) for a

given application can be calculated as follows:

f

SW(MAX)

=

t

ON(MIN)VD

where VIN is the typical input voltage, V
voltage, V

is the catch diode drop (~0.5V) and VSW is the

D

(

OUT

+ VIN– V

SW

is the output

OUT

internal switch drop (~0.5V at max load). This equation
shows that slower switching frequency is necessary to
safely accommodate high V

IN/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:

MIN

DC

MAX

where fSW is the switching frequency, the t
minimum switch on time (~150ns), and the t

ON(MIN)

= 1– fSWt

OFF(MIN

ON(MIN)

OFF(MIN)

is the

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

V

+ V

V

+ V

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

and BOOST pins, and the operating mode.

the V

IN

The LT3480 can operate from input voltages up to 38V,
and safely withstand input voltages up 60V. Note that while

>38V (typical), the LT3480 will stop switching, allowing

V

IN

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 V

IN(MAX)

according to the following equation. If lower V

≥ 40V

IN(MAX)

is

desired, this equation can be used directly.

maximum duty cycle (see equation in previous section).
The minimum input voltage due to duty cycle is:

V

IN(MIN)

where V

=

IN(MIN)

OUT

1– fSWt

is the minimum input voltage, and t

D

OFF(MIN)

– VD+ V

SW

OFF(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 ΔI

increases with higher VIN or V

L

OUT

and decreases with higher inductance and faster switching
frequency. A reasonable starting point for selecting the
ripple current is:

ΔIL = 0.4(I

OUT(MAX)

)

V

IN(MAX)

where V
V

OUT

IN(MAX)

is the output voltage, VD is the catch diode drop
(~0.5V), V
load), f
t

ON(MIN)

SW

is the minimum switch on time (~150ns). Note that

OUT

=

fSWt

is the maximum operating input voltage,

is the internal switch drop (~0.5V at max

SW

is the switching frequency (set by RT), and

D

ON(MIN)

– VD+ V

SW

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

where I

OUT(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 (I

LIM

).

The peak inductor current is:

I

L(PEAK)

where I
the maximum output load current, and ΔI
ripple current. The LT3480’s switch current limit (I

= I

L(PEAK)

OUT(MAX)

+ ΔIL/2

is the peak inductor current, I

is the inductor

L

OUT(MAX)

LIM

is

) 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:

I

OUT(MAX)

= I

LIM

– ΔIL/2

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

. To guarantee that the ripple current stays below the

V

IN

specified maximum, the inductor value should be chosen
according to the following equation:



V





OUT

fSW∆I

L =

+ V





D

L

V

OUT

1–







V



IN(MAX)

+ V



D




3480fe

10

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APPLICATIONS INFORMATION

LT3480

where VD is the voltage drop of the catch diode (~0.4V),
V

IN(MAX)

voltage, f

is the maximum input voltage, V

is the switching frequency (set by RT), and L

SW

is the output

OUT

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

Toko www.toko.com D62CB

D63CB
D75C
D75F

Sumida www.sumida.com CR54

CDRH74
CDRH6D38
CR75

Shielded
Shielded

Shielded
Shielded
Shielded
Open

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% (V

OUT/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

OUTfSW

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:

C

OUT

where fSW is in MHz, and C

100

=

V

is the recommended output

OUT

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.

Catch Diode

The catch diode conducts current only during switch off
time. Average forward current in normal operation can
be calculated from:

I

where I

D(AVG)

OUT

= I

(VIN – V

OUT

OUT

)/V

IN

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 V
rated Schottky even when V

> 38V which allows the use of 40V

IN

ranges up to 60V. Table 3

IN

lists several Schottky diodes and their manufacturers.

Table 3. Diode Vendors

PART NUMBER

On Semiconductor
MBRM120E
MBRM140

Diodes Inc.
B120
B130
B220
B230
DFLS240L

International Rectifier
10BQ030
20BQ030

VR

(V)

20
40

20
30
20
30
40

30
30

I

AVE

(A)

1
1

1
1
2
2
2

1
2

VF AT 1A

(mV)

530
550

500
500

420

VF AT 2A

(mV)

595

500
500
500

470
470

Table 2. Capacitor Vendors

VENDOR PHONE URL PART SERIES COMMANDS

Panasonic [phone] www.panasonic.com Ceramic,

Kemet [phone] www.kemet.com Ceramic,

Sanyo [phone] www.sanyovideo.com Ceramic,

Murata [phone] www.murata.com Ceramic

AVX www.avxcorp.com Ceramic,

Taiyo Yuden [phone] www.taiyo-yuden.com Ceramic

12

For more information www.linear.com/LT3480

Polymer,
Tantalum

Tantalum

Polymer,
Tantalum

Tantalum

EEF Series

T494, T495

POSCAP

TPS Series

3480fe

APPLICATIONS INFORMATION

LT3480

Ceramic Capacitors

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.

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

pin, as shown in Figure 2. Generally a capacitor (CC)

V

C

and a resistor (R

) in series to ground are used. In addi-

C

tion, there may be lower value capacitor in parallel. This
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.

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 V

pin. Note that

C

the output capacitor integrates this current, and that the
capacitor on the V

pin (CC) integrates the error amplifier

C

output current, resulting in two poles in the loop. In most
cases a zero is required and comes from either the output
capacitor ESR or from a resistor R

in series with CC.

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.

LT3480

CURRENT MODE

POWER STAGE

= 3.5mho

g

m

3M

V

C

ERROR

AMPLIFIER

gm =

420µmho

SW

C

R1

FB

–

+

0.8V

POLYMER

GND

TANTALUM

PL

ESR

C1

OR

OUTPUT

+

C1

CERAMIC

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

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R

C

C

F

C

C

Figure 2. Model for Loop Response

R2

3480 F02

3480fe

13

LT3480

APPLICATIONS INFORMATION

V

OUT

100mV/DIV

I

L

0.5A/DIV

VIN = 12V; FRONT PAGE APPLICATION

10µs/DIV

Figure 3. Transient Load Response of the LT3480 Front Page
Application as the Load Current Is Stepped from 500mA to
1500mA. V

OUT

= 3.3V

3480 F03

V

SW

5V/DIV

I

L

0.2A/DIV

V

OUT

10mV/DIV

5µs/DIV

VIN = 12V; FRONT PAGE APPLICATION

= 10mA

I

LOAD

Figure 4. Burst Mode Operation

3480 F04

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.

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, V

IN

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 R

resistor, and will be operating in

T

standard PWM mode. The transition between PWM and
Low-Ripple Burst Mode is seamless, and will not disturb
the output voltage.

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

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 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.

If low quiescent current is not required the LT3480 can
operate in Pulse-Skip mode. The benefit of this mode is

14

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The minimum operating voltage of an LT3480 application
is limited by the minimum input voltage (3.6V) and by the

3480fe

APPLICATIONS INFORMATION

LT3480

V

4.7µF

V

4.7µF

V

4.7µF

V

OUT

BD

BOOST

IN

IN

IN

V

LT3480

IN

SW

GND

(5a) For V

V

LT3480

IN

BD

GND

OUT

BOOST

SW

(5b) For 2.5V < V

BD

BOOST

V

LT3480

IN

SW

GND

C3

> 2.8V

C3

< 2.8V

OUT

C3

V

OUT

D2

V

OUT

6.0

5.5

TO START
(WORST CASE)

5.0

4.5

4.0
TO RUN

3.5

INPUT VOLTAGE (V)

3.0

V

= 3.3V

OUT

= 25°C

T

A

2.5

L = 8.2µH
f = 700kHz

2.0

1

8.0

7.0

6.0

5.0

4.0

INPUT VOLTAGE (V)

3.0

2.0

1 1000010 100 1000

10 100 1000

LOAD CURRENT (A)

TO START
(WORST CASE)

TO RUN

V

= 5V

OUT

= 25°C

T

A

L = 8.2µH
f = 700kHz

LOAD CURRENT (A)

10000

3480 F06

Figure 6. The Minimum Input Voltage Depends on
Output Voltage, Load Current and Boost Circuit

3480 FO5

(5c) For V

< 2.5V; V

OUT

IN(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

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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 V

IN

is ramping very slowly. For lower start-up voltage, the
boost diode can be tied to V

; however, this restricts the

IN

input range to one-half of the absolute maximum rating
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 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.

3480fe

15

LT3480

3480 F07

1A/DIV

RUN/SS

2V/DIV

OUT

2V/DIV

2ms/DIV

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 R

resistor should be chosen to set the LT3480

T

switching frequency 20% below the lowest synchronization
input. For example, if the synchronization signal will be
250kHz and higher, the R

should be chosen for 200kHz.

T

To assure reliable and safe operation the LT3480 will only
synchronize when the output voltage is near regulation

I

L

V

V

0.22µF

RUN

15k

RUN/SS

GND

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 R

resistor. See

T

Inductor Selection section. It is also important to note that
slope compensation is set by the R
frequency is much higher than the one set by R

value: When the sync

T

, the slope

T

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 V
high (either by a logic signal or because it is tied to V

pin is allowed to float and the RUN/SS pin is held

IN

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 zero.
However, if the V

pin is grounded while the output is

IN

held high, then parasitic diodes inside the LT3480 can
pull large currents from the output through the SW pin
and the V

pin. Figure 8 shows a circuit that will run only

IN

when the input voltage is present and that protects against
a shorted or reversed input.

D4

MBRS140

V

IN

V

IN

RUN/SS

V

C

LT3480

GND FB

BOOST

SW

3480 F08

V

OUT

BACKUP

Figure 7. To Soft-Start the LT3480, Add a Resisitor
and Capacitor to the RUN/SS Pin

16

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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LT3480

PCB Layout

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 V

and SW pins, the catch

IN

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 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

V

OUT

C2

C

R

RT

C

Hot Plugging Safely

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 V

pin of the LT3480 can ring to twice the

IN

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.

High Temperature Considerations

R

C

R2

R1

C1

D1

GND

VIAS TO LOCAL GROUND PLANE

VIAS TO V

Figure 9. A Good PCB Layout Ensures Proper, Low EMI Operation

OUT

VIAS TO SYNC

R

VIAS TO RUN/SS

VIAS TO PG

PG

VIAS TO V

OUTLINE OF LOCAL
GROUND PLANE

3480 F09

IN

For more information www.linear.com/LT3480

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

= 35°C/W or less. With

JA

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

17

LT3480

APPLICATIONS INFORMATION

+

LOW
IMPEDANCE
ENERGIZED
24V SUPPLY

+

+

CLOSING SWITCH

SIMULATES HOT PLUG

STRAY
INDUCTANCE
DUE TO 6 FEET
(2 METERS) OF
TWISTED PAIR

+

22µF

35V

AI.EI.

I

IN

0.7Ω

V

IN

V

IN

LT3480

4.7µF

20V/DIV

10A/DIV

I

IN

DANGER

RINGING V

MAY EXCEED

IN

ABSOLUTE MAXIMUM RATING

20µs/DIV

(10a)

V

IN

(10b)

20V/DIV

10A/DIV

20V/DIV

10A/DIV

I

IN

20µs/DIV

V

IN

I

IN

LT3480

4.7µF0.1µF

LT3480

4.7µF

(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

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.

20µs/DIV

3480 F10

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.

18

3480fe

For more information www.linear.com/LT3480

TYPICAL APPLICATIONS

V

V

V

V

IN

6.8V TO 36V
TRANSIENT

TO 60V*

4.7µF

16.2k

470pF

D: DIODES INC. DFLS240L
L: TAIYO YUDEN NP06DZB6R8M

5V Step-Down Converter

V

IN

ON OFF

40.2k

RUN/SS BOOST

V

C

LT3480

RT

PG

SYNC

f = 800kHz

3.3V Step-Down Converter

GND

BD

SW

LT3480

OUT

5V
2A

0.47µF

D

FB

100k

L

6.8µH

536k

22µF

3480 TA02

3480 TA03

V

OUT

3.3V
2A

22µF

V

4.4V TO 36V

TRANSIENT

TO 60V*

4.7µF

IN

ON OFF

14k

40.2k

470pF

D: DIODES INC. DFLS240L
L: TAIYO YUDEN NP06DZB4R7M

V

IN

RUN/SS BOOST

V

C

LT3480

RT

PG

SYNC

GND

f = 800kHz

BD

SW

0.47µF

D

FB

100k

L

4.7µH

316k

2.5V Step-Down Converter

IN

4V TO 36V

TRANSIENT

TO 60V*

4.7µF

330pF

D1: DIODES INC. DFLS240L
D2: MBR0540
L: TAIYO YUDEN NP06DZB4R7M

ON OFF

20k

56.2k

V

IN

RUN/SS BOOST

V

C

RT

PG

SYNC

f = 600kHz

LT3480

GND

BD

SW

D2

1µF

D1

FB

100k

L

4.7µH

215k

3480 TA04

OUT

2.5V
2A

47µF

3480fe

For more information www.linear.com/LT3480

19

LT3480

TYPICAL APPLICATIONS

5V, 2MHz Step-Down Converter

V

8.6V TO 22V

TRANSIENT TO 38V

2.2µF

V

IN

15V TO 36V
TRANSIENT

TO 60V*

10µF

IN

ON OFF

14k

11.5k

470pF

D: DIODES INC. DFLS240L
L: SUMIDA CDRH4D22/HP-2R2

ON OFF

26.1k

40.2k

330pF

D: DIODES INC. DFLS240L
L: NEC/TOKIN PLC-0755-100

LT3480

GND

BD

SW

FB

V

IN

RUN/SS BOOST

V

C

RT

PG

SYNC

f = 2MHz

12V Step-Down Converter

LT3480

GND

BD

SW

FB

V

IN

RUN/SS BOOST

V

C

RT

PG

SYNC

f = 800kHz

100k

0.47µF

D

50k

0.47µF

D

715k

536k

10µH

L

2.2µH

L

3480 TA05

3480 TA06

V

OUT

12V
2A

22µF

V

OUT

5V
2A

22µF

20

V

3.5V TO 27V

4.7µF

IN

ON OFF

18.2k

68.1k

330pF

D: DIODES INC. DFLS240L
L: TAIYO YUDEN NP06DZB3R3M

For more information www.linear.com/LT3480

1.8V Step-Down Converter

LT3480

GND

BD

SW

FB

V

IN

RUN/SS BOOST

V

C

RT

PG

SYNC

f = 500kHz

0.47µF

D

100k

127k

L

3.3µH

3480 TA08

V

OUT

1.8V
2A

47µF

3480fe

PACKAGE DESCRIPTION

DD Package

PIN 1 NOTCH

Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.

10-Lead Plastic DFN (3mm × 3mm)

(Reference LTC DWG # 05-08-1699 Rev C)

0.70 ±0.05

LT3480

3.55 ±0.05

1.65 ±0.05
(2 SIDES)2.15 ±0.05

PACKAGE
OUTLINE

0.25 ± 0.05

RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS

0.50
BSC

2.38 ±0.05
(2 SIDES)

3.00 ±0.10
(4 SIDES)

PIN 1

TOP MARK

(SEE NOTE 6)

0.200 REF

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

0.75 ±0.05

1.65 ± 0.10
(2 SIDES)

0.00 – 0.05

R = 0.125

TYP

2.38 ±0.10
(2 SIDES)

BOTTOM VIEW—EXPOSED PAD

106

15

0.25 ± 0.05

0.50 BSC

0.40 ± 0.10

R = 0.20 OR

0.35 × 45°
CHAMFER

(DD) DFN REV C 0310

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 representa­tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.

For more information www.linear.com/LT3480

21

LT3480

(.0120

BOTTOM VIEW OF

NO MEASUREMENT PURPOSE

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)

EXPOSED PAD OPTION

1.88 ± 0.102
(.074 ± .004)

0.889 ± 0.127
(.035 ± .005)

1

1.88

(.074)

1.68

(.066)

0.29
REF

5.23

(.206)

MIN

0.305 ± 0.038
± .0015)

TYP

RECOMMENDED SOLDER PAD LAYOUT

0.254

(.010)

GAUGE PLANE

0.18

(.007)

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.

1.68 ± 0.102

(.066 ± .004)

0.50

(.0197)

BSC

DETAIL “A”

DETAIL “A”

0° – 6° TYP

0.53 ± 0.152
(.021 ± .006)

3.20 – 3.45

(.126 – .136)

SEATING

PLANE

10

3.00 ± 0.102
(.118 ± .004)

(NOTE 3)

4.90 ± 0.152

(.193 ± .006)

1.10

(.043)

MAX

0.17 – 0.27

(.007 – .011)

TYP

0.50

(.0197)

BSC

1 2

8910

7

6

4 5

3

DETAIL “B”

0.497 ± 0.076
(.0196 ± .003)

3.00 ± 0.102

(.118 ± .004)

(NOTE 4)

0.86

(.034)

REF

0.1016 ± 0.0508
(.004 ± .002)

MSOP (MSE) 0910 REV G

0.05 REF

DETAIL “B”

CORNER TAIL IS PART OF

THE LEADFRAME FEATURE.

FOR REFERENCE ONLY

REF

22

3480fe

For more information www.linear.com/LT3480

LT3480

REVISION HISTORY

REV DATE DESCRIPTION PAGE NUMBER

D 10/11 Added H- and MP-grades for the MSE package

Revised BD pin description
Revised Figure 5 to add capacitors

E 8/13 Clarified maximum temperature range of LT3480E 2, 3

(Revision history begins at Rev D)

2, 3

7

15

For more information www.linear.com/LT3480

3480fe

23

LT3480

TYPICAL APPLICATION

1.2V Step-Down Converter

V

IN

3.6V TO 27V

4.7µF

ON OFF

16.2k

68.1k

330pF

D: DIODES INC. DFLS240L
L: TAIYO YUDEN NP06DZB3R3M

RUN/SS BOOST

V

RT

PG

SYNC

f = 500kHz

V

IN

C

BD

LT3480

GND

RELATED PARTS

PART NUMBER DESCRIPTION COMMENTS

LT1933 500mA (I

), 500kHz Step-Down Switching

OUT

Regulator in SOT-23

LT3437 60V, 400mA (I

OUT

DC/DC Converter with Burst Mode

LT1936 36V, 1.4A (I

OUT

Step-Down DC/DC Converter

LT3493 36V, 1.2A (I

OUT

Step-Down DC/DC Converter

LT1976/LT1977 60V, 1.2A (I

OUT

Step-Down DC/DC Converter with Burst Mode

LT1767 25V, 1.2A (I

OUT

Step-Down DC/DC Converter

LT1940 Dual 25V, 1.4A (I

Step-Down DC/DC Converter

LT1766 60V, 1.2A (I

OUT

Step-Down DC/DC Converter

LT3434/LT3435 60V, 2.4A (I

OUT

Step-Down DC/DC Converter with Burst Mode

LT3481 36V, 2A (I

), 2.8MHz, High Efficiency

OUT

Step-Down DC/DC Converter with Burst Mode

LT3684 36V, 2A (I

), 2.8MHz, High Efficiency

OUT

Step-Down DC/DC Converter

), MicroPower Step-Down

), 500kHz High Efficiency

), 750kHz High Efficiency

), 200kHz/500kHz, High Efficiency

), 1.1MHz, High Efficiency

), 1.1MHz, High Efficiency

OUT

), 200kHz, High Efficiency

), 200/500kHz, High Efficiency

VIN: 3.6V to 36V, V

VIN: 3.3V to 80V, V
DFN and 16-Pin TSSOP Packages

VIN: 3.6V to 36V, V

VIN: 3.6V to 40V, V
Package

VIN: 3.3V to 60V, V

VIN: 3V to 25V, V

VIN: 3.6V to 25V, V
Package

VIN: 5.5V to 60V, V
Package

VIN: 3.3V to 60V, V

VIN: 3.6V to 34V, V
DFN and 10-Pin MSOP Packages

VIN: 3.6V to 34V, V
DFN and 10-Pin MSOP Packages

SW

FB

V

OUT

1.2V
2A

0.47µF

D

100k

= 1.2V, IQ = 1.6mA, ISD <1µA, ThinSOT Package

OUT(MIN)

= 1.25V, IQ = 100µA, ISD <1µA, 10-Pin 3mm x 3mm

OUT(MIN)

= 1.2V, IQ = 1.9mA, ISD <1µA, MS8E Package

OUT(MIN)

= 0.8V, IQ = 1.9mA, ISD <1µA, 6-Pin 2mm x 3mm DFN

OUT(MIN)

= 1.2V, IQ = 100µA, ISD <1µA, 16-Pin TSSOP Package

OUT(MIN)

= 1.2V, IQ = 1mA, ISD <6µA, MS8E Package

OUT(MIN)

= 1.2V, IQ = 3.8mA, ISD <30µA, 16-Pin TSSOP

OUT(MIN)

= 1.2V, IQ = 2.5mA, ISD = 25µA, 16-Pin TSSOP

OUT(MIN)

= 1.2V, IQ = 100µA, ISD <1µA, 16-Pin TSSOP Package

OUT(MIN)

= 1.26V, IQ = 50µA, ISD <1µA, 10-Pin 3mm x 3mm

OUT(MIN)

= 1.26V, IQ = 1.5mA, ISD <1µA, 10-Pin 3mm x 3mm

OUT(MIN)

L

3.3µH

52.3k

47µF

3480 TA09

24

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

[phone] ● FAX: [phone]

For more information www.linear.com/LT3480

●

www.linear.com/LT3480

3480fe

LT 0813 REV E • PRINTED IN USA

 LINEAR TECHNOLOGY CORPORATION 2008