Linear Technology LT3480 Manuel utilisateur

LT3480

3480fb

TYPICAL APPLICATION

FEATURES

APPLICATIONS

DESCRIPTION

36V, 2A, 2.4MHz Step-Down

Switching Regulator with

70µA Quiescent Current

The LT

3480 is an adjustable frequency (200kHz to

2.4MHz) monolithic buck switching regulator that ac-

cepts input voltages up to 36V (60V maximum). A high

efﬁciency 0.25 switch is included on the die along with

a boost Schottky diode and the necessary oscillator, con-

trol, and logic circuitry. Current mode topology is used

for fast transient response and good loop stability. Low

ripple Burst Mode operation maintains high efﬁciency at

low output currents while keeping output ripple below

15mV in a typical application. In addition, the LT3480 can

further enhance low output current efﬁciency by draw-

ing 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-Pin MSOP

and 3mm ×3mm DFN packages with exposed pads for

low thermal resistance.

nWide Input Range:

Operation from 3.6V to 36V

Over-Voltage Lockout Protects Circuits

through 60V Transients

n2A Maximum Output Current

nLow Ripple Burst Mode

Operation

70μA IQ at 12VIN to 3.3VOUT

Output Ripple < 15mV

nAdjustable Switching Frequency: 200kHz to 2.4MHz

nLow Shutdown Current: IQ < 1μA

nIntegrated Boost Diode

nSynchronizable Between 250kHz to 2MHz

nPower Good Flag

nSaturating Switch Design: 0.25 On-Resistance

n0.790V Feedback Reference Voltage

nOutput Voltage: 0.79V to 20V

nSoft-Start Capability

nSmall 10-Pin Thermally Enhanced MSOP and

(3mm ×3mm) DFN Packages

nAutomotive Battery Regulation

nPower for Portable Products

nDistributed Supply Regulation

nIndustrial Supplies

, 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.

3.3V Step-Down Converter

VIN BD

VIN

4.5V TO 36V

TRANSIENT

TO 60V

VOUT

3.3V

4.7μF

0.47μF

470pF

22μF

100k

14k

40.2k

4.7μH

316k

GND

OFF ON

LT3480

3480 TA01

RUN/SS BOOST

SYNC

Efﬁciency

LOAD CURRENT (A)

EFFICIENCY (%)

0.5 1.0 1.5 2

3480 TA01b

100

VIN = 12V

L = 5.6μH

F = 800 kHz

VOUT = 3.3V

VOUT = 5V

LT3480

3480fb

ABSOLUTE MAXIMUM RATINGS

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

BOOST Pin Voltage ...................................................56V

BOOST Pin Above SW Pin.........................................30V

FB, RT, VCVoltage .......................................................5V

PG, BD, SYNC Voltage ..............................................30V

Maximum Junction Temperature........................... 125°C

(Note 1)

TOP VIEW

DD PACKAGE

10-LEAD (3mm s3mm) PLASTIC DFN

311

1RT

SYNC

BOOST

VIN

RUN/SS

JA = 45°C/W, JC = 10°C/W

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

BOOST

VIN

RUN/SS

SYNC

TOP VIEW

MSE PACKAGE

10-LEAD PLASTIC MSOP

JA = 45°C/W, JC = 10°C/W

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

PIN CONFIGURATION

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 85°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 85°C

LT3480IMSE#PBF LT3480IMSE#TRPBF LTCTM 10-Lead Plastic MSOP –40°C to 125°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 85°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 85°C

LT3480IMSE LT3480IMSE#TR LTCTM 10-Lead Plastic MSOP –40°C to 125°C

Consult LTC Marketing for parts speciﬁed with wider operating temperature ranges. *The temperature grade is identiﬁed by a label on the shipping container.

For more information on lead free part marking, go to: http://www.linear.com/leadfree/

For more information on tape and reel speciﬁ

cations, go to: http://www.linear.com/tapeandreel/

Operating Temperature Range (Note 2)

LT3480E............................................... –40°C to 85°C

LT3480I.............................................. –40°C to 125°C

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

Lead Temperature (Soldering, 10 sec)

(MSE Only) ....................................................... 300°C

PARAMETER CONDITIONS MIN TYP MAX UNITS

Minimum Input Voltage l3 3.6 V

VIN Overvoltage Lockout l36 38 40 V

The ldenotes the speciﬁcations which apply over the full operating temperature

range, otherwise speciﬁcations are at TA= 25°C. VIN = 10V, VRUN/SS = 10V, VBOOST = 15V, VBD = 3.3V unless otherwise noted. (Note 2)

ELECTRICAL CHARACTERISTICS

LT3480

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Note 1: Stresses beyond those listed under Absolute Maximum Ratings may

cause permanent damage to the device. Exposure to any Absolute Maximum

Rating condition for extended periods may affect device reliability and lifetime.

Note 2: The LT3480E is guaranteed to meet performance speciﬁcations

from 0°C to 85°C. Speciﬁcations over the –40°C to 85°C operating

temperature range are assured by design, characterization and correlation

with statistical process controls. The LT3480I speciﬁcations are

guaranteed over the –40°C to 125°C temperature range.

The ldenotes the speciﬁcations which apply over the full operating temperature

range, otherwise speciﬁcations are at TA= 25°C. VIN = 10V, VRUN/SS = 10V, VBOOST = 15V, VBD = 3.3V unless otherwise noted. (Note 2)

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: Absolute Maximum Voltage at VIN and RUN/SS pins is 60V for

nonrepetitive 1 second transients, and 40V for continious operation.

PARAMETER CONDITIONS MIN TYP MAX UNITS

Quiescent Current from VIN VRUN/SS = 0.2V

VBD = 3V, Not Switching

VBD = 0, Not Switching

0.01

105

0.5

100

160

μA

Quiescent Current from BD VRUN/SS = 0.2V

VBD = 3V, Not Switching

VBD = 0, Not Switching

0.01

0.5

120

μA

Minimum Bias Voltage (BD Pin) 2.7 3 V

Feedback Voltage

780

775

790

800

805

FB Pin Bias Current (Note 3) VFB = 0.8V, VC= 0.4V l730 nA

FB Voltage Line Regulation 4V < VIN < 36V 0.002 0.01 %/V

Error Amp gm400 μMho

Error Amp Gain 1000

VCSource Current 45 μA

VCSink Current 45 μA

VCPin to Switch Current Gain 3.5 A/V

VCClamp Voltage 2V

Switching Frequency RT= 8.66k

RT= 29.4k

RT= 187k

2.1

0.9

160

2.4

200

2.7

1.15

240

MHz

kHz

Minimum Switch Off-Time l60 150 nS

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

Switch VCESAT ISW = 2A 500 mV

Boost Schottky Reverse Leakage VSW = 10V, VBD = 0V 0.02 2 μA

Minimum Boost Voltage (Note 4) l1.5 2.1 V

BOOST Pin Current ISW = 1A 22 35 mA

RUN/SS Pin Current VRUN/SS = 2.5V 5 10 μA

RUN/SS Input Voltage High 2.5 V

RUN/SS Input Voltage Low 0.2 V

PG Threshold Offset from Feedback Voltage VFB Rising 100 mV

PG Hysteresis 12 mV

PG Leakage VPG = 5V 0.1 1 μA

PG Sink Current VPG = 0.4V l100 600 μA

SYNC Low Threshold 0.5 V

SYNC High Threshold 0.7 V

SYNC Pin Bias Current VSYNC = 0V 0.1 μA

ELECTRICAL CHARACTERISTICS

LT3480

3480fb

TYPICAL PERFORMANCE CHARACTERISTICS

LOAD CURRENT (A)

EFFICIENCY (%)

100

0.2 0.4 0.6 0.8 1.2 1.4 1.6 1.8 2.01.0

3480 G01

VIN = 24V

VIN = 34V

VIN = 12V

L: NEC PLC-0745-5R6

f: 800kHz

VOUT = 5V

0 0.2 0.4 0.6 0.8 1.2 1.4 1.6 1.8 2.01.0

LOAD CURRENT (A)

EFFICIENCY (%)

3480 G02

VIN = 12V

VIN = 7V

L: NEC PLC-0745-5R6

f: 800kHz

VIN = 24V

VIN = 34V

VOUT = 3.3V

LOAD CURRENT (A)

EFFICIENCY (%)

POWER LOSS (W)

0.5 1.0 1.5 2

3480 G27

0.01

0.1

VIN = 12V

VOUT = 3.3V

L = 5.6μH

F = 800 kHz

INPUT VOLTAGE (V)

SUPPLY CURRENT (μA)

3480 G04

510 20

120

100

25 30 35

VOUT = 3.3V

TEMPERATURE (°C)

–50

SUPPLY CURRENT (μA)

350

3480 G05

200

100

–25 0 50

400

300

250

150

75 100 150125

VIN = 12V

VOUT = 3.3V

CATCH DIODE: DIODES, INC. PDS360

INCREASED SUPPLY

CURRENT DUE TO CATCH

DIODE LEAKAGE AT

HIGH TEMPERATURE

INPUT VOLTAGE (V)

LOAD CURRENT (A)

3480 G06

2.5

10 20

1.5

1.0

4.0

3.5

3.0

2.0

25 30

TYPICAL

MINIMUM

VOUT = 3.3V

TA= 25 °C

L = 4.7μH

f = 800 kHz

INPUT VOLTAGE (V)

LOAD CURRENT (A)

3480 G07

2.5

10 20

1.5

1.0

3.5

3.0

2.0

25 30

TYPICAL

MINIMUM

VOUT = 5V

TA= 25 °C

L = 4.7μH

f = 800kHz

DUTY CYCLE (%)

SWITCH CURRENT LIMIT(A)

3480 G08

2.5

20 60

1.5

1.0

4.0

3.5

3.0

2.0

80 100

TEMPERATURE (°C)

SWITCH CURRENT LIMIT (A)

2.0

2.5

3.5

3.0

3480 G09

1.5

1.0

0.5

4.5

4.0 DUTY CYCLE = 10 %

DUTY CYCLE = 90 %

–50 25–25 0 50 75 100 150125

Efﬁciency Efﬁciency Efﬁciency

No Load Supply Current No Load Supply Current Maximum Load Current

Maximum Load Current Switch Current Limit Switch Current Limit

LT3480

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TYPICAL PERFORMANCE CHARACTERISTICS

SWITCH CURRENT (mA)

400

500

700

1500

3480 G10

300

200

500 1000 2000 2500

100

600

VOLTAGE DROP (mV)

SWITCH CURRENT (mA)

BOOST PIN CURRENT (mA)

3480 G11

0 1500500 1000 2000 2500

TEMPERATURE (°C)

FEEDBACK VOLTAGE (mV)

800

4380 G12

760

840

780

820

–50 25–25 0 50 75 100 150125

TEMPERATURE (°C)

FREQUENCY (MHz)

1.00

1.10

4380 G13

0.90

0.80

1.20

0.95

1.05

0.85

1.15

–50 25–25 0 50 75 100 150125

FB PIN VOLTAGE (mV)

SWITCHING FREQUENCY (kHz)

800

1000

1200

600

3480 G14

600

400

200 400 800

500

100 300 700 900

200

TEMPERATURE (˚C)

MINIMUM SWITCH ON TIME (ns)

100

120

3480 G15

140

–50 25–25 0 50 75 100 150125

RUN/SS PIN VOLTAGE (V)

SWITCH CURRENT LIMIT (A)

3.5

1.5

3480 G16

2.0

1.0

0.5 1 2

0.5

4.0

3.0

2.5

1.5

2.5 3 3.5

RUN/SS PIN VOLTAGE (V)

RUN/SS PIN CURRENT (μA)

15 25

3480 G17

510 20 30 35

BOOST DIODE CURRENT (A)

BOOST DIODE Vf(V)

0.8

1.0

1.2

2.0

3480 G18

0.6

0.4

00.5 1.0 1.5

0.2

1.4

Switch Voltage Drop Boost Pin Current Feedback Voltage

Switching Frequency Frequency Foldback Minimum Switch On-Time

Soft-Start RUN/SS Pin Current Boost Diode

LT3480

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FB PIN ERROR VOLTAGE (V)

–200

–50

VCPIN CURRENT (μA)

–20

0 200

3480 G19

–40

–100 100

–10

–30

LOAD CURRENT (A)

INPUT VOLTAGE (V)

3.0

3.5

10000

3480 G20

2.5

2.0

10 100 1000

5.0

4.5

4.0

VOUT = 3.3V

TA= 25°C

L = 4.7μH

f = 800kHz

1 1000010 100 1000

LOAD CURRENT (A)

INPUT VOLTAGE (V)

5.0

5.5

3480 G21

4.5

4.0

6.5

6.0

VOUT = 5V

TA= 25 °C

L = 4.7μH

f = 800kHz

TEMPERATURE (°C)

VCVOLTAGE (V)

1.50

2.00

2.50

3480 G22

1.00

0.50

CURRENT LIMIT CLAMP

SWITCHING THRESHOLD

–50 25–25 0 50 75 100 150125

TEMPERATURE (°C)

THRESHOLD VOLTAGE (%)

3480 G23

–50 25–25 0 50 75 100 150125 3480 G24

0.2A/DIV

VSW

5V/DIV

VOUT

10mV/DIV

5μs/DIV

VIN = 12V; FRONT PAGE APPLICATION

ILOAD = 10mA

Error Amp Output Current Minimum Input Voltage Minimum Input Voltage

VCVoltages Power Good Threshold Switching Waveforms; Burst Mode

3480 G25

0.2A/DIV

VSW

5V/DIV

VOUT

10mV/DIV

VIN = 12V; FRONT PAGE APPLICATION

ILOAD = 110mA

1μs/DIV 3480 G26

0.5A/DIV

VSW

5V/DIV

VOUT

10mV/DIV

VIN = 12V; FRONT PAGE APPLICATION

ILOAD = 1A

1μs/DIV

Switching Waveforms; Transition

from Burst Mode to Full Frequency

Switching Waveforms; Full

Frequency Continuous Operation

TYPICAL PERFORMANCE CHARACTERISTICS

LT3480

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PIN FUNCTIONS

BD (Pin 1): This pin connects to the anode of the boost

Schottky diode. BD also supplies current to the internal

regulator.

BOOST (Pin 2): This pin is used to provide a drive

voltage,higherthanthe inputvoltage, tothe internalbipolar

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 thispinfor lowripple BurstModeoperation 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 VCpin 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 ResistorInput. Connectingaresistor

to ground from this pin sets the switching frequency.

Exposed Pad (Pin 11): Ground. The Exposed Pad must

be soldered to PCB.

BLOCK DIAGRAM

–

OSCILLATOR

200kHz–2.4MHz

BurstMode

DETECT

VCCLAMP

SOFT-START

SLOPE COMP

VIN

RUN/SS

BOOST

SWITCH

LATCH

VOUT

DISABLE

GND

ERROR AMP

0.7V

3480 BD

11 8

INTERNAL 0.79V REF

SYNC

LT3480

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OPERATION

The LT3480 is a constant frequency, current mode step-

down regulator. An oscillator, with frequency set by RT,

enables an RS ﬂip-ﬂop, turning on the internal power

switch. An ampliﬁer and comparator monitor the current

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 VCpin. If the error ampliﬁer’s output

increases, more current is delivered to the output; if it

decreases,lesscurrent isdelivered. An activeclamp on the

VCpin provides current limit. The VCpin 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.

Aninternal regulatorprovidespowertothe controlcircuitry.

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

(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.

TheoscillatorreducestheLT3480’soperatingfrequencywhen

the voltage at the FB pin is low. This frequency foldback helps

to control the output current during startup and overload.

TheLT3480 contains apower goodcomparator 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.

LT3480

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APPLICATIONS INFORMATION

FB Resistor Network

The output voltage is programmed with a resistor divider

between the output and the FB pin. Choose the 1% resis-

tors according to:

RR V

OUT

079 1=−

⎛

⎝

⎜⎞

⎠

⎟

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) RTVALUE (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

29.4

23.7

19.1

16.2

13.3

11.5

9.76

8.66

Figure 1. Switching Frequency vs. RTValue

Operating Frequency Tradeoffs

Selection of the operating frequency is a tradeoff between

efﬁciency, componentsize, minimumdropout voltage,and

maximum input voltage. The advantage of high frequency

operationis thatsmallerinductor andcapacitor valuesmay

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:

fVV

tVVV

SW MAX D OUT

ON MIN D IN SW

() () –

()

where VIN is the typical input voltage, VOUT is the output

voltage, VDis 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

switchingfrequency isbecause theLT3480switch hasﬁnite

minimum on and off times. The switch can turn on for a

minimumof~150ns andturn offfora minimumof ~150ns.

Typical minimum on time at 25°C is 80ns. This means that

the minimum and maximum duty cycles are:

DC f t

MIN SW ON MIN

MAX SW OFF MIN

()

–1

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.

A good choice of switching frequency should allow ad-

equate input voltage range (see next section) and keep

the inductor and capacitor values small.

Input Voltage Range

ThemaximuminputvoltageforLT3480 applicationsdepends

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 LT3480will 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 fre-

quency 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.

LT3480

3480fb

APPLICATIONS INFORMATION

VVV

ft VV

IN MAX OUT D

SW ON MIN DSW() ()

–=++

where VIN(MAX) is the maximum operating input voltage,

VOUT is the output voltage, VDis 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) isthe minimumswitch ontime (~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, start-

up, or overload events are expected, then input voltage

transients of up to 60V are acceptable regardless of the

switching frequency. In this mode, the LT3480 may enter

pulse skipping operation where some switching pulses

are skipped to maintain output regulation. In this mode

the output voltage ripple and inductor current ripple will

be higher than in normal operation. Above 38V switching

will stop.

The minimum input voltage is determined by either the

LT3480’s minimum operating voltage of ~3.6V or by its

maximum duty cycle (see equation in previous section).

The minimum input voltage due to duty cycle is:

VVV

ft VV

IN MIN OUT D

SW OFF MIN DSW() ()

––=++

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 ΔILincreases with higher VIN or VOUT

anddecreaseswith higherinductance and fasterswitching

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

mustbe lower thanthe LT3480’s switchcurrent limit(ILIM).

The peak inductor current is:

L(PEAK) = IOUT(MAX) + ΔIL/2

where IL(PEAK) is the peak inductor current, IOUT(MAX) is

the maximum output load current, and ΔILis 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 func-

tion of the inductor ripple current:

OUT(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:

LVV

OUT D

SW L

OUT D

IN MAX

⎛

⎝

⎜⎞

⎠

⎟+

⎛

⎝

⎜⎞

⎠

Δ1–

()

⎟⎟

where VDis 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.

Theinductor’s RMScurrentrating mustbe greater thanthe

maximumload current andits saturationcurrent shouldbe

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 des matières

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