LT3470ETS8#TRM_技术文档

LT3470

Micropower Buck Regulator

with Integrated Boost and

Catch Diodes

U

FEATURES

DESCRIPTIO

TheLT^®3470isamicropowerstep-downDC/DCconverter

that integrates a 300mA power switch, catch diode

and boost diode into low profile 3mm x 2mm DD and

ThinSOT^TMpackages. The LT3470 combines Burst Mode

and continuous operation to allow the use of tiny inductor

and capacitors while providing a low ripple output to

loads of up to 200mA.

■

Low Quiescent Current: 26µA at 12V_INto 3.3V_OUT

■

Integrated Boost and Catch Diodes

Input Range: 4V to 40V

■

Low Output Ripple: <10mV

<1µA in Shutdown Mode

Output Voltage: 1.25V to 16V

200mA Output Current

Hysteretic Mode Control

With its wide input range of 4V to 40V, the LT3470 can

regulate a wide variety of power sources, from 2-cell

Li-Ionbatteriestounregulatedwalltransformersandlead-

acid batteries. Quiescent current in regulation is just 26µA

in a typical application while a zero current shutdown

mode disconnects the load from the input source, simpli-

fying power management in battery-powered systems.

Fast current limiting and hysteretic control protects the

LT3470 and external components against shorted out-

puts, even at 40V input.

– Low Ripple Burst Mode^®Operation at Light Loads

– Continuous Operation at Higher Loads

Solution Size as Small as 50mm²

Low Profile (0.75mm) 3mm x 2mm Thermally

Enhanced 8-Lead DD and 1mm ThinSOT Packages

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■

APPLICATIO S

■

Automotive Battery Regulation

■

Power for Portable Products

■

Distributed Supply Regulation

, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.

Burst Mode is a registered trademark of Linear Technology Corporation. ThinSOT is a

trademark of Linear Technology Corporation. All other trademarks are the property of their

respective owners.

■

Industrial Supplies

■

Wall Transformer Regulation

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TYPICAL APPLICATIO

Efficiency and Power Loss vs Load Current

90

80

70

60

50

40

30

20

10

1000

100

10

V

= 12V

IN

7V TO 40V

0.22µF

33µH

V

BOOST

IN

LT3470

SHDN

V

OUT

5V

OFF ON

SW

200mA

BIAS

604k

1%

22pF

FB

22µF

2.2µF

GND

200k

1%

1

0.1

1

10

100

LOAD CURRENT (mA)

3470 TA02

3470fc

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LT3470

ABSOLUTE AXI U RATI GS

W W

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(Note 1)

V_IN, SHDN Voltage.................................................. 40V

BOOST Pin Voltage ................................................. 47V

BOOST Pin Above SW Pin ...................................... 25V

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

BIAS Voltage............................................................ 25V

SW Voltage ................................................................V_IN

Maximum Junction Temperature

LT3470H .......................................................... 150°C

Operating Temperature Range (Note 2)

LT3470E ............................................. –40°C to 85°C

LT3470I ............................................ –40°C to 125°C

LT3470H .......................................... –40°C to 150°C

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

Lead Temperature (Soldering, 10 sec).................. 300°C

LT3470E, LT3470I ............................................ 125°C

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PI CO FIGURATIO

TOP VIEW

FB

BIAS

1

2

3

4

8

7

6

5

SHDN

NC

TOP VIEW

SHDN 1

NC 2

8 FB

9

7 BIAS

6 BOOST

5 SW

BOOST

SW

V

IN

V

IN

3

GND

GND 4

TS8 PACKAGE

8-LEAD PLASTIC TSOT-23

DDB8 PACKAGE

8-LEAD (3mm × 2mm) PLASTIC DFN

θ

_JA= 140°C/ W

θ

_JA= 80°C/ W

EXPOSED PAD (PIN 9) IS GROUND (MUST BE SOLDERED TO PCB)

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W

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ORDER I FOR ATIO

LEAD FREE FINISH

LT3470EDDB#PBF

LT3470IDDB#PBF

LT3470HDDB#PBF

LT3470ETS8#PBF

LT3470ITS8#PBF

LT3470HTS8#PBF

LEAD BASED FINISH

LT3470EDDB

LT3470IDDB

LT3470HDDB

LT3470ETS8

LT3470ITS8

TAPE AND REEL

PART MARKING

LBPN

LBPP

LCNR

LTBDM

LTBPW

LTCNQ

PART MARKING

LBPN

LBPP

LCNR

LTBDM

LTBPW

LTCNQ

PACKAGE DESCRIPTION

TEMPERATURE RANGE

–40°C to 85°C

–40°C to 125°C

–40°C to 150°C

–40°C to 85°C

–40°C to 125°C

–40°C to 150°C

TEMPERATURE RANGE

–40°C to 85°C

–40°C to 125°C

–40°C to 150°C

–40°C to 85°C

LT3470EDDB#TRPBF

LT3470IDDB#TRPBF

LT3470HDDB#TRPBF

LT3470ETS8#TRPBF

LT3470ITS8#TRPBF

LT3470HTS8#TRPBF

TAPE AND REEL

LT3470EDDB#TR

LT3470ETS8#TR

8-Lead (3mm × 2mm) Plastic DFN

8-Lead Plastic TSOT-23

PACKAGE DESCRIPTION

8-Lead (3mm × 2mm) Plastic DFN

8-Lead Plastic TSOT-23

LT3470ITS8#TR

LT3470HTS8#TR

–40°C to 125°C

–40°C to 150°C

LT3470HTS8

Consult LTC Marketing for parts specified with wider operating temperature ranges.

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/

3470fc

2

LT3470

ELECTRICAL CHARACTERISTICS

The

IN

●

denotes specifications which apply over the full operating temperature range, otherwise specifications are T = 25°C.

A

V

= 10V, V

= 15V, V

BIAS

= 3V unless otherwise specified.

CONDITIONS

SHDN

BOOST

PARAMETER

MIN

TYP

MAX

UNITS

Minimum Input Voltage

Quiescent Current from V

●

4

V

= 0.2V

SHDN

0.1

10

35

0.5

18

50

µA

IN

= 3V, Not Switching

= 0V, Not Switching

BIAS

Quiescent Current from Bias

V

= 0.2V

= 3V, Not Switching

= 0V, Not Switching

0.1

25

0.1

0.5

60

1.5

µA

SHDN

●

BIAS

FB Comparator Trip Voltage

FB Pin Bias Current (Note 3)

V

Falling

1.228

1.250

1.265

V

FB

= 1V, E and I-Grade

35

80

150

225

nA

●

H-Grade

4V < V < 40V

FB Voltage Line Regulation

0.0006

500

0.01

%/V

ns

IN

Minimum Switch Off-Time (Note 5)

Switch Leakage Current

0.7

1.5

µA

Switch V

I

= 100mA (TS8 Package)

= 100mA (DD8 Package)

215

300

mV

CESAT

SW

Switch Top Current Limit

Switch Bottom Current Limit

Catch Schottky Drop

V

= 0V

250

325

225

435

775

mA

FB

I

= 100mA (TS8 Package)

= 100mA (DD8 Package)

630

mV

SH

Catch Schottky Reverse Leakage

Boost Schottky Drop

V

= 10V

0.2

650

0.2

1.7

7

2

775

2

µA

mV

µA

V

SW

I

= 30mA

SH

Boost Schottky Reverse Leakage

Minimum Boost Voltage (Note 4)

BOOST Pin Current

V

= 10V, V

= 0V

BIAS

SW

●

2.2

12

5

I

= 100mA

mA

µA

V

SW

SHDN Pin Current

V

= 2.5V

1

SHDN

SHDN Input Voltage High

SHDN Input Voltage Low

2.5

0.2

V

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.

with statistical process controls. The LT3470I specifications are

guaranteed over the –40°C to 125°C temperature range. LT3470H

specifications are guaranteed over –40°C to 150°C temperature range.

Note 3: Bias current flows out of the FB pin.

Note 2: The LT3470E 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

Note 4: This is the minimum voltage across the boost capacitor needed to

guarantee full saturation of the switch.

Note 5: This parameter is assured by design and correlation with statistical

process controls.

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LT3470

TYPICAL PERFOR A CE CHARACTERISTICS

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Efficiency, V

= 3.3V

Efficiency, V

= 5V

V

FB

vs Temperature

OUT

90

1.260

1.255

1.250

1.245

L = TOKO D52LC 47µH

T

= 25°C

T

= 25°C

V

= 7V

A

IN

V

= 12V

IN

80

70

80

70

V

= 12V

= 24V

IN

V

= 36V

IN

V

IN

V

= 36V

V

= 24V

IN

60

50

60

50

40

30

40

30

1.240

–50 –25

0

25

50

75 100 125

0.1

1

10

100

0.1

1

10

100

TEMPERATURE (°C)

LOAD CURRENT (mA)

3470 G01

3470 G02

3470 G03

V

Quiescent Current

Top and Bottom Switch Current

Limits (V = 0V) vs Temperature

BIAS Quiescent Current

(Bias > 3V) vs Temperature

IN

vs Temperature

FB

30

25

20

15

50

400

350

300

40

30

20

10

0

BIAS < 3V

250

200

150

100

50

10

5

BIAS > 3V

0

50

100 125 150

–50 –25

0

25

75

–25

0

50 75 100 125 150

25

TEMPERATURE (°C)

–50

–50 –25

0

25 50 75 100 125 150

TEMPERATURE (°C)

3470 G06

3470 G04

3470 G05

SHDN Bias Current

vs Temperature

FB Bias Current (V = 1V)

FB

vs Temperature

60

50

40

30

20

10

0

9

8

7

6

5

4

3

2

1

V

= 36V

SHDN

V

= 2.5V

SHDN

25

0

–50 –25

0

25 50 75 100 125 150

TEMPERATURE (°C)

–50 –25

0

125 150

50 75 100

TEMPERATURE (°C)

3470 G08

3470 G07

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LT3470

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TYPICAL PERFOR A CE CHARACTERISTICS

FB Bias Current (V = 0V)

Switch V

(I = 100mA)

Boost Diode V (I = 50mA)

FB

CESAT SW

F

vs Temperature

300

250

200

150

0.8

120

100

80

0.7

0.6

0.5

0.4

0.3

0.2

0.1

60

100

50

0

40

20

0

50

100 125 150

–50 –25

0

25

75

–25

0

50 75 100 125 150

25

TEMPERATURE (°C)

50

100 125 150

–50

–50 –25

0

25

75

TEMPERATURE (°C)

3470 G10

3470 G11

3470 G09

Diode Leakage (V = 36V)

Catch Diode V (I = 100mA)

R

F

Switch V

vs Temperature

CESAT

60

55

50

45

40

35

30

25

20

15

10

5

700

600

500

0.7

0.6

CATCH

BOOST

0.5

0.4

0.3

0.2

0.1

400

300

200

100

0

50

100 125 150

–50 –25

0

25

75

100

200

400

0

300

–50 –25

0

25 50 75 100 125 150

TEMPERATURE (°C)

SWITCH CURRENT (mA)

3470 G12

3470 G14

3470 G13

BOOST Pin Current

Catch Diode Forward Voltage

1.0

0.8

0.6

0.4

0.2

0

14

12

10

8

6

4

2

0

100

200

300

400

100

200

400

0

300

CATCH DIODE CURRENT (mA)

SWITCH CURRENT (mA)

3470 G16

3470 G15

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LT3470

TYPICAL PERFOR A CE CHARACTERISTICS

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Boost Diode Forward Voltage

Minimum Input Voltage, V

= 3.3V

Minimum Input Voltage, V

= 5V

OUT

6.0

5.5

8

7

6

5

4

900

800

700

600

500

400

300

200

100

0

T

A

= 25°C

T

= 25°C

A

V

IN

TO START

V

TO START

IN

5.0

4.5

V

TO RUN

IN

4.0

3.5

3.0

V

IN

TO RUN

100

0

50

150

200

100

0

100

150

200

0

50

150

200

50

LOAD CURRENT (mA)

BOOST DIODE CURRENT (mA)

LOAD CURRENT (mA)

3470 G18

3470 G17

3470 G19

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PI FU CTIO S

(ThinSOT/DD)

SHDN (Pin 1/Pin 8): The SHDN pin is used to put the

LT3470inshutdownmode.Tietogroundtoshutdownthe

LT3470. Apply 2V or more for normal operation. If the

shutdown feature is not used, tie this pin to the V_INpin.

BOOST (Pin 6/Pin 3): The BOOST pin is used to provide a

drive voltage, which is higher than the input voltage, to the

internal bipolar NPN power switch.

BIAS (Pin 7/Pin 2): The BIAS pin connects to the internal

NC (Pin 2/Pin 7): This pin can be left floating or connected

to V_IN.

boost Schottky diode and to the internal regulator. Tie to

V_OUTwhen V_OUT> 2V or to V_INotherwise. When V_BIAS>

3V the BIAS pin will supply current to the internal

regulator.

V_IN(Pin 3/Pin 6): The V_INpin supplies current to the

LT3470’s internal regulator and to the internal power

switch. This pin must be locally bypassed.

FB (Pin 8/Pin 1): The LT3470 regulates its feedback pin

to 1.25V. Connect the feedback resistor divider tap to this

pin. Set the output voltage according to V_OUT= 1.25V

(1 + R1/R2) or R1 = R2 (V_OUT/1.25 – 1).

GND (Pin 4/Pin 5): Tie the GND pin to a local ground plane

below the LT3470 and the circuit components. Return the

feedback divider to this pin.

Exposed Pad (DD, Pin 9): Ground. Must be soldered

to PCB.

SW (Pin 5/Pin 4): The SW pin is the output of the internal

powerswitch.Connectthispintotheinductor,catchdiode

and boost capacitor.

3470fc

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LT3470

W

BLOCK DIAGRA

V

IN

BIAS

V

IN

C1

+

–

BOOST

500ns

ONE SHOT

R

S

Q′

C3

Q

L1

C2

–

+

SW

V

OUT

ENABLE

BURST MODE

DETECT

NC

SHDN

V

REF

1.25V

g

m

GND

FB

R2

R1

3470 BD

3470fc

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LT3470

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OPERATIO

The LT3470 uses a hysteretic control scheme in conjunc-

tion with Burst Mode operation to provide low output

ripple and low quiescent current while using a tiny induc-

tor and capacitors.

comparatortripsandresetsthelatchcausingtheswitchto

turnoff. Whiletheswitchisoff, theinductorcurrentramps

down through the catch diode. When both the bottom

current comparator trips and the minimum off-time one-

shot expires, the latch turns the switch back on thus

completingafullcycle.Thehystereticactionofthiscontrol

scheme results in a switching frequency that depends on

inductor value, input and output voltage. Since the switch

only turns on when the catch diode current falls below

threshold,thepartwillautomaticallyswitchslowertokeep

inductor current under control during start-up or short-

circuit conditions.

Operation can best be understood by studying the Block

Diagram. An error amplifier measures the output voltage

through an external resistor divider tied to the FB pin. If

the FB voltage is higher than V_REF, the error amplifier will

shut off all the high power circuitry, leaving the LT3470 in

its micropower state. As the FB voltage falls, the error

amplifier will enable the power section, causing the chip

to begin switching, thus delivering charge to the output

capacitor. Iftheloadislightthepartwillalternatebetween

micropower and switching states to keep the output in

regulation (See Figure 1a). At higher loads the part will

switch continuously while the error amp servos the top

and bottom current limits to regulate the FB pin voltage to

1.25V (See Figure 1b).

The switch driver operates from either the input or from

the BOOST pin. An external capacitor and internal diode is

used to generate a voltage at the BOOST pin that is higher

than the input supply. This allows the driver to fully

saturatetheinternalbipolarNPNpowerswitchforefficient

operation.

The switching action is controlled by an RS latch and two

current comparators as follows: The switch turns on, and

the current through it ramps up until the top current

If the SHDN pin is grounded, all internal circuits are turned

off and V_INcurrent reduces to the device leakage current,

typically a few nA.

NO LOAD

200mA LOAD

V

OUT

20mV/DIV

OUT

20mV/DIV

I

L

100mA/DIV

I

L

100mA/DIV

1ms/DIV

1µs/DIV

10mA LOAD

150mA LOAD

V

OUT

20mV/DIV

OUT

20mV/DIV

I

L

100mA/DIV

3470 F01a

3470 F1b

5µs/DIV

1µs/DIV

(1a) Burst Mode Operation

(1b) Continuous Operation

Figure 1. Operating Waveforms of the LT3470 Converting 12V to 5V Using a 33µH Inductor and 10µF Output Capacitor

3470fc

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LT3470

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APPLICATIO S I FOR ATIO

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Input Voltage Range

where V_IN(MAX)is the maximum input voltage for the

application, t_ON-TIME(MIN)is ~150ns and I_MAXis the maxi-

mum allowable increase in switch current during a mini-

mumswitchon-time(150mA).Whilethisequationprovides

a safe inductor value, the resulting application circuit may

switch at too high a frequency to yield good efficiency. It

is advised that switching frequency be below 1.2MHz

during normal operation:

Theminimuminputvoltagerequiredtogenerateaparticu-

lar output voltage in an LT3470 application is limited by

either its 4V undervoltage lockout or by its maximum duty

cycle. Thedutycycleisthefractionoftimethattheinternal

switch is on and is determined by the input and output

voltages:

V_OUT+ V_D

DC =

1– DC V + V

(

)(

)

D

OUT

V – V_SW+ V_D

IN

f =

L • ∆I_L

where V_Dis the forward voltage drop of the catch diode

(~0.6V) and V_SWis the voltage drop of the internal switch

at maximum load (~0.4V). Given DC_MAX= 0.90, this leads

to a minimum input voltage of:

wherefistheswitchingfrequency,∆I_Listheripplecurrent

in the inductor (~150mA), V_Dis the forward voltage drop

of the catch diode, and V_OUTis the desired output voltage.

If the application circuit is intended to operate at high duty

cycles (V_INclose to V_OUT), it is important to look at the

calculated value of the switch off-time:

⎛

⎜

⎝

⎞

⎟

⎠

V_OUT+ V_D

DC_MAX

V

=

+ V_SW– V_D

IN(MIN)

Thisanalysisassumestheparthasstartedupsuchthatthe

capacitor tied between the BOOST and SW pins is charged

to more than 2V. For proper start-up, the minimum input

voltage is limited by the boost circuit as detailed in the

section BOOST Pin Considerations.

1–DC

t_OFF-TIME

=

f

The calculated t_OFF-TIMEshould be more than LT3470’s

minimum t_OFF-TIME(See Electrical Characteristics), so the

application circuit is capable of delivering full rated output

current. If the full output current of 200mA is not required,

the calculated t_OFF-TIMEcan be made less than minimum

t_OFF-TIMEpossibly allowing the use of a smaller inductor.

See Table 1 for an inductor value selection guide.

The maximum input voltage is limited by the absolute

maximum V_INrating of 40V, provided an inductor of

sufficient value is used.

Inductor Selection

Table 1. Recommended Inductors for Loads up to 200mA

The switching action of the LT3470 during continuous

operation produces a square wave at the SW pin that

results in a triangle wave of current in the inductor. The

hysteretic mode control regulates the top and bottom

current limits (see Electrical Characteristics) such that the

average inductor current equals the load current. For safe

operation,itmustbenotedthattheLT3470cannotturnthe

switch on for less than ~150ns. If the inductor is small and

the input voltage is high, the current through the switch

may exceed safe operating limit before the LT3470 is able

to turn off. To prevent this from happening, the following

equation provides a minimum inductor value:

V

Up to 16V

10µH

V

IN

Up to 40V

33µH

OUT

IN

2.5V

3.3V

5V

10µH

33µH

15µH

33µH

12V

33µH

47µH

Choose an inductor that is intended for power applica-

tions. Table 2 lists several manufacturers and inductor

series.

For robust output short-circuit protection at high V_IN(up

to 40V) use at least a 33µH inductor with a minimum

450mA saturation current. If short-circuit performance is

not required, inductors with I_SATof 300mA or more may

V_IN(MAX)• t_ON-TIME(MIN)

L_MIN

=

I_MAX

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LT3470

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APPLICATIO S I FOR ATIO

Table 2. Inductor Vendors

VENDOR

URL

PART SERIES

INDUCTANCE RANGE (µH) SIZE (mm)

Coilcraft

www.coilcraft.com

DO1605

ME3220

DO3314

10 to 47

1.8 × 5.4 × 4.2

2.0 × 3.2 × 2.5

1.4 × 3.3 × 3.3

Sumida

www.sumida.com

CR32

10 to 47

10 to 33

10 to 47

10 to 15

3.0 × 3.8 × 4.1

1.8 × 4.0 × 4.0

3.0 × 4.0 × 4.0

2.0 × 3.2 × 3.2

CDRH3D16/HP

CDRH3D28

CDRH2D18/HP

Toko

www.tokoam.com

DB320C

D52LC

10 to 27

10 to 47

2.0 × 3.8 × 3.8

2.0 × 5.0 × 5.0

Würth Elektronik

www.we-online.com

WE-PD2 Typ S

WE-TPC Typ S

10 to 47

10 to 22

3.2 × 4.0 × 4.5

1.6 × 3.8 × 3.8

Coiltronics

Murata

www.cooperet.com

www.murata.com

SD10

10 to 47

1.0 × 5.0 × 5.0

LQH43C

LQH32C

10 to 47

10 to 15

2.6 × 3.2 × 4.5

1.6 × 2.5 × 3.2

be used. It is important to note that inductor saturation

current is reduced at high temperatures—see inductor

vendors for more information.

LT3470’s switching frequency. The capacitor’s equivalent

seriesresistance(ESR)determinesthisimpedance.Choose

onewithlowESRintendedforuseinswitchingregulators.

The contribution to ripple voltage due to the ESR is

approximatelyI_LIM•ESR.ESRshouldbelessthan~150mΩ.

The value of the output capacitor must be large enough to

accept the energy stored in the inductor without a large

changeinoutputvoltage. Settingthisvoltagestepequalto

1% of the output voltage, the output capacitor must be:

Input 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 V_INpin of the LT3470 and to force this switching

current into a tight local loop, minimizing EMI. The input

capacitor must have low impedance at the switching

frequency to do this effectively. A 1µF to 2.2µF ceramic

capacitor satisfies these requirements.

2

⎛ I_LIM

⎝ V_OUT

⎞

C_OUT> 50 •L •

⎜

⎟

⎠

Where I_LIMis the top current limit with V_FB= 0V (see

Electrical Characteristics). For example, an LT3470 pro-

ducing 3.3V with L = 33µH requires 22µF. The calculated

value can be relaxed if small circuit size is more important

than low output ripple.

If the input source impedance is high, a larger value

capacitor may be required to keep input ripple low. In this

case, an electrolytic of 10µF or more in parallel with a 1µF

ceramic is a good combination. Be aware that the input

capacitor is subject to large surge currents if the LT3470

circuit is connected to a low impedance supply, and that

some electrolytic capacitors (in particular tantalum) must

be specified for such use.

Sanyo’s POSCAP series in B-case and provides very good

performance in a small package for the LT3470. Similar

performance in traditional tantalum capacitors requires a

larger package (C-case). With a high quality capacitor

filtering the ripple current from the inductor, the output

voltage ripple is determined by the delay in the LT3470’s

feedback comparator. This ripple can be reduced further

by adding a small (typically 22pF) phase lead capacitor

between the output and the feedback pin.

Output Capacitor and Output Ripple

The output capacitor filters the inductor’s ripple current

and stores energy to satisfy the load current when the

LT3470isquiescent. Inordertokeepoutputvoltageripple

low, the impedance of the capacitor must be low at the

3470fc

10

LT3470

W U U

APPLICATIO S I FOR ATIO

U

BOOST and BIAS Pin Considerations

Ceramic Capacitors

Capacitor C3 and the internal boost Schottky diode (see

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 two ways to

arrange the boost circuit. The BOOST pin must be more

than2.5VabovetheSWpinforbestefficiency. Foroutputs

of 3.3V and above, the standard circuit (Figure 2a) is best.

For outputs between 2.5V and 3V, use a 0.47µF. For lower

output voltages the boost diode can be tied to the input

Ceramic capacitors are small, robust and have very low

ESR. However, ceramic capacitors can cause problems

whenusedwiththeLT3470. Notallceramiccapacitorsare

suitable. X5R and X7R types are stable over temperature

and applied voltage and give dependable service. Other

types, including Y5V and Z5U have very large temperature

and voltage coefficients of capacitance. In an application

circuit they may have only a small fraction of their nominal

capacitanceresultinginmuchhigheroutputvoltageripple

than expected.

V

IN

Ceramiccapacitorsarepiezoelectric.TheLT3470’sswitch-

ing frequency depends on the load current, and at light

loadstheLT3470canexcitetheceramiccapacitorataudio

frequencies, generating audible noise. Since the LT3470

operates at a lower current limit during Burst Mode

operation, the noise is typically very quiet to a casual ear.

If this audible noise is unacceptable, use a high perfor-

mance electrolytic capacitor at the output. The input

capacitor can be a parallel combination of a 2.2µF ceramic

capacitor and a low cost electrolytic capacitor.

C3

0.22µF

V

BOOST

LT3470

IN

V

SW

OUT

BIAS

GND

V

– V

≅ V

BOOST

SW OUT

MAX V

≅ V + V

IN OUT

BOOST

(2a)

V

IN

C3

0.22µF

V

IN

BOOST

SW

LT3470

A final precaution regarding ceramic capacitors concerns

the maximum input voltage rating of the LT3470. A ce-

ramic input capacitor combined with trace or cable induc-

tance forms a high quality (under damped) tank circuit. If

the LT3470 circuit is plugged into a live supply, the input

voltage can ring to twice its nominal value, possibly

exceeding the LT3470’s rating. This situation is easily

avoided; see the Hot-Plugging Safely section.

V

BIAS

OUT

GND

3470 F02

V

– V

≅ V

BOOST

SW IN

MAX V

≅ 2V

IN

BOOST

(2b)

Figure 2. Two Circuits for Generating the Boost Voltage

Table 3. Capacitor Vendors

Vendor

Phone

URL

Part Series

Comments

Panasonic

(714) 373-7366

www.panasonic.com

Ceramic,

Polymer,

Tantalum

EEF Series

Kemet

Sanyo

(864) 963-6300

(408) 749-9714

www.kemet.com

Ceramic,

Tantalum

T494, T495

POSCAP

www.sanyovideo.com Ceramic,

Polymer,

Tantalum

Murata

AVX

(404) 436-1300

www.murata.com

www.avxcorp.com

Ceramic

Ceramic,

Tantalum

TPS Series

Taiyo Yuden (864) 963-6300

www.taiyo-yuden.com Ceramic

3470fc

11

LT3470

W U U

U

APPLICATIO S I FOR ATIO

(Figure 2b). The circuit in Figure 2a is more efficient

because the BOOST pin current and BIAS pin quiescent

currentcomesfromalowervoltagesource. Youmustalso

be sure that the maximum voltage ratings of the BOOST

and BIAS pins are not exceeded.

minimum V_INto start and to run. 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_OUT. At

higher load currents, the inductor current is continuous

and the duty cycle is limited by the maximum duty cycle of

the LT3470, requiring a higher input voltage to maintain

regulation.

The minimum operating voltage of an LT3470 application

is limited by the undervoltage lockout (4V) and by the

maximum duty cycle as outlined in a previous section. For

proper start-up, the minimum input voltage is also limited

by the boost circuit. If the input voltage is ramped slowly,

or the LT3470 is turned on with its SHDN pin when the

output is already in regulation, then the boost capacitor

may not be fully charged. The plots in Figure 3 show

Shorted Input Protection

If the inductor is chosen so that it won’t saturate exces-

sively at the top switch current limit maximum of 450mA,

an LT3470 buck regulator will tolerate a shorted output

even if V_IN= 40V. There is another situation to consider in

systems where the output will be held high when the input

to the LT3470 is absent. This may occur in battery charg-

ing applications or in battery backup systems where a

battery or some other supply is diode OR-ed with the

LT3470’s output. If the V_INpin is allowed to float and the

SHDN pin is held high (either by a logic signal or because

it is tied to V_IN), then the LT3470’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 SHDN pin, the SW pin current will drop to

essentially zero. However, if the V_INpin is grounded while

the output is held high, then parasitic diodes inside the

LT3470 can pull large currents from the output through

the SW pin and the V_INpin. Figure 4 shows a circuit that

will run only when the input voltage is present and that

protects against a shorted or reversed input.

Minimum Input Voltage, V

= 3.3V

OUT

6.0

T

= 25°C

A

V

TO START

IN

5.5

5.0

4.5

4.0

3.5

3.0

V

TO RUN

100

IN

0

50

150

200

LOAD CURRENT (mA)

3470 G18

Minimum Input Voltage, V

= 5V

OUT

8

T

= 25°C

A

V

TO START

IN

D1

7

6

5

4

V

IN

V

BOOST

IN

LT3470 SOT-23

100k

1M

V

SHDN

SW

OUT

V

TO RUN

IN

BIAS

FB

GND

BACKUP

3470 F04

100

150

0

200

50

LOAD CURRENT (mA)

3470 G19

Figure 4. Diode D1 Prevents a Shorted Input from Discharging a

Backup Battery Tied to the Output; It Also Protects the Circuit

from a Reversed Input. The LT3470 Runs Only When the Input is

Present Hot-Plugging Safely

Figure 3. The Minimum Input Voltage Depends on Output

Voltage, Load Current and Boost Circuit

3470fc

12

LT3470

W U U

APPLICATIO S I FOR ATIO

U

PCB Layout

location, ideally at the ground terminal of the output

capacitor C2. Additionally, the SW and BOOST nodes

should be kept as small as possible. Unshielded inductors

can induce noise in the feedback path resulting in instabil-

ity and increased output ripple. To avoid this problem, use

vias to route the V_OUTtrace under the ground plane to the

feedback divider (as shown in Figure 5). Finally, keep the

FB node as small as possible so that the ground pin and

groundtraceswillshielditfromtheSWandBOOSTnodes.

Figure 5 shows component placement with trace, ground

plane and via locations. Include vias near the GND pin, or

pad, of the LT3470 to help remove heat from the LT3470

to the ground plane.

For proper operation and minimum EMI, care must be

taken during printed circuit board layout. Note that large,

switched currents flow in the power switch, the internal

catch diode and the input capacitor. The loop formed by

these components should be as small as possible. Fur-

thermore, the system ground should be tied to the regu-

lator ground in only one place; this prevents the switched

current from injecting noise into the system ground.

These components, along with the inductor and output

capacitor, should be placed on the same side of the circuit

board,andtheirconnectionsshouldbemadeonthatlayer.

Place a local, unbroken ground plane below these compo-

nents, and tie this ground plane to system ground at one

SHDN

V

IN

V

IN

C1

GND

C2

V

OUT

V

OUT

3470 F05

VIAS TO FEEDBACK DIVIDER

VIAS TO LOCAL GROUND PLANE

OUTLINE OF LOCAL GROUND PLANE

(A)

(B)

Figure 5. A Good PCB Layout Ensures Proper, Low EMI Operation

3470fc

13

LT3470

W U U

U

APPLICATIO S I FOR ATIO

Hot-Plugging Safely

the voltage overshoot (it also reduces the peak input

current).A0.1µFcapacitorimproveshighfrequencyfilter-

ing. This solution is smaller and less expensive than the

electrolyticcapacitor.Forhighinputvoltagesitsimpacton

efficiency is minor, reducing efficiency less than one half

percent for a 5V output at full load operating from 24V.

The small size, robustness and low impedance of ceramic

capacitors make them an attractive option for the input

bypass capacitor of LT3470. However, these capacitors

can cause problems if the LT3470 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_INpin of the LT3470 can ring to twice the

nominal input voltage, possibly exceeding the LT3470’s

rating and damaging the part. If the input supply is poorly

controlled or the user will be plugging the LT3470 into an

energizedsupply, theinputnetworkshouldbedesignedto

prevent this overshoot. Figure 6 shows the waveforms

that result when an LT3470 circuit is connected to a 24V

supply through six feet of 24-gauge twisted pair. The first

plot is the response with a 2.2µF ceramic capacitor at the

input. The input voltage rings as high as 35V and the input

current peaks at 20A. One method of damping the tank

circuit is to add another capacitor with a series resistor to

the circuit. In Figure 6b an aluminum electrolytic capacitor

has been added. This capacitor’s high equivalent series

resistance damps the circuit and eliminates the voltage

overshoot. The extra capacitor improves low frequency

ripple filtering and can slightly improve the efficiency of

the circuit, though it is likely to be the largest component

in the circuit. An alternative solution is shown in Figure 6c.

A 1Ω resistor is added in series with the input to eliminate

High Temperature Considerations

ThediejunctiontemperatureoftheLT3470mustbelower

thanthemaximumratingof125°C(150°CfortheHgrade).

This is generally not a concern unless the ambient tem-

perature is above 85°C. For higher temperatures, care

should be taken in the layout of the circuit to ensure good

heat sinking of the LT3470. The maximum load current

shouldbederatedastheambienttemperatureapproaches

the maximum junction rating. The die temperature is

calculated by multiplying the LT3470 power dissipation

by the thermal resistance from junction to ambient.

Power dissipation within the LT3470 can be estimated by

calculating the total power loss from an efficiency mea-

surement. Thermal resistance depends on the layout of

the circuit board and choice of package. The DD package

with the exposed pad has a thermal resistance of approxi-

mately 80°C/W while the ThinSOT is approximately

150°C/W. Finally, be aware that at high ambient tempera-

tures the internal Schottky diode will have significant

leakage current (see Typical Performance Characteris-

tics) increasing the quiescent current of the LT3470

converter.

3470fc

14

LT3470

W U U

APPLICATIO S I FOR ATIO

U

CLOSING SWITCH

SIMULATES HOT PLUG

I

IN

V

IN

LT3470

2.2µF

V

IN

10V/DIV

+

I

IN

10A/DIV

LOW

STRAY

IMPEDANCE

ENERGIZED

24V SUPPLY

INDUCTANCE

DUE TO 6 FEET

(2 METERS) OF

TWISTED PAIR

10µs/DIV

(6a)

LT3470

2.2µF

+

10µF

35V

AI.EI.

(6b)

1Ω

LT3470

2.2µF

0.1µF

3470 F06

(6c)

Figure 6: A Well Chosen Input Network Prevents Input Voltage Overshoot and

Ensures Reliable Operation When the LT3470 is Connected to a Live Supply

3470fc

15

LT3470

TYPICAL APPLICATIO S

U

3.3V Step-Down Converter

V

IN

5.5V TO 40V

C3

0.22µF, 6.3V

L1

33µH

V

BOOST

IN

LT3470

SHDN

V

OUT

3.3V

OFF ON

SW

200mA

BIAS

R1

22pF

324k

C1

1µF

C2

22µF

FB

GND

R2

200k

3470 TA03

C1: TDK C3216JB1H105M

C2: CE JMK316 BJ226ML-T

L1: TOKO A993AS-270M=P3

5V Step-Down Converter

V

IN

7V TO 40V

C3

0.22µF, 6.3V

L1

33µH

V

BOOST

IN

LT3470

SHDN

V

OUT

5V

200mA

OFF ON

SW

BIAS

R1

22pF

604k

C1

1µF

C2

22µF

FB

GND

R2

200k

3470 TA04

C1: TDK C3216JB1H105M

C2: CE JMK316 BJ226ML-T

L1: TOKO A914BYW-330M=P3

2.5V Step-Down Converter

V

IN

4.7V TO 40V

C3

0.47µF, 6.3V

L1

33µH

V

BOOST

IN

LT3470

SHDN

V

OUT

2.5V

OFF ON

SW

200mA

BIAS

R1

22pF

200k

C1

1µF

C2

22µF

FB

GND

R2

200k

3470 TA07

C1: TDK C3216JB1H105M

C2: TDK C2012JB0J226M

L1: SUMIDA CDRH3D28

3470fc

16

LT3470

U

TYPICAL APPLICATIO S

1.8V Step-Down Converter

V

IN

C3

4V TO 23V

0.22µF, 25V

L1

22µH

V

BOOST

SW

IN

LT3470

V

OUT

1.8V

OFF ON

SHDN

200mA

R1

BIAS

22pF

147k

C1

1µF

C2

22µF

FB

GND

R2

332k

3470 TA05

C1: TDK C3216JB1H105M

C2: TDK C2012JB0J226M

L1: MURATA LQH32CN150K53

12V Step-Down Converter

V

IN

C3

15V TO 34V

0.22µF, 16V

L1

33µH

V

BOOST

IN

LT3470

SHDN

V

OUT

12V

OFF ON

SW

200mA

BIAS

R1

22pF

866k

C1

1µF

C2

10µF

FB

GND

R2

100k

3470 TA06

C1: TDK C3216JB1H105M

C2: TDK C3216JB1C106M

L1: MURATA LQH32CN150K53

3470fc

17

LT3470

U

PACKAGE DESCRIPTIO

TS8 Package

8-Lead Plastic TSOT-23

(Reference LTC DWG # 05-08-1637)

2.90 BSC

(NOTE 4)

0.52

MAX

0.65

REF

1.22 REF

1.50 – 1.75

(NOTE 4)

2.80 BSC

1.4 MIN

3.85 MAX 2.62 REF

PIN ONE ID

RECOMMENDED SOLDER PAD LAYOUT

PER IPC CALCULATOR

0.22 – 0.36

8 PLCS (NOTE 3)

0.65 BSC

0.80 – 0.90

0.20 BSC

DATUM ‘A’

0.01 – 0.10

1.00 MAX

0.30 – 0.50 REF

1.95 BSC

0.09 – 0.20

(NOTE 3)

TS8 TSOT-23 0802

NOTE:

1. DIMENSIONS ARE IN MILLIMETERS

2. DRAWING NOT TO SCALE

3. DIMENSIONS ARE INCLUSIVE OF PLATING

4. DIMENSIONS ARE EXCLUSIVE OF MOLD FLASH AND METAL BURR

5. MOLD FLASH SHALL NOT EXCEED 0.254mm

6. JEDEC PACKAGE REFERENCE IS MO-193

3470fc

18

LT3470

U

PACKAGE DESCRIPTIO

DDB Package

8-Lead Plastic DFN (3mm × 2mm)

(Reference LTC DWG # 05-08-1702 Rev B)

0.61 ±0.05

(2 SIDES)

R = 0.115

0.40 ± 0.10

3.00 ±0.10

(2 SIDES)

TYP

5

R = 0.05

TYP

8

0.70 ±0.05

2.55 ±0.05

1.15 ±0.05

2.00 ±0.10

PIN 1 BAR

TOP MARK

PIN 1

(2 SIDES)

R = 0.20 OR

0.25 × 45°

(SEE NOTE 6)

PACKAGE

OUTLINE

0.56 ± 0.05

(2 SIDES)

CHAMFER

4

1

(DDB8) DFN 0905 REV B

0.25 ± 0.05

0.75 ±0.05

0.200 REF

0.50 BSC

2.20 ±0.05

(2 SIDES)

0.50 BSC

2.15 ±0.05

(2 SIDES)

0 – 0.05

RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS

BOTTOM VIEW—EXPOSED PAD

NOTE:

1. DRAWING CONFORMS TO VERSION (WECD-1) IN JEDEC PACKAGE OUTLINE M0-229

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

3470fc

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 represen-

tationthattheinterconnectionofitscircuitsasdescribedhereinwillnotinfringeonexistingpatentrights.

19

LT3470

RELATED PARTS

PART NUMBER

DESCRIPTION

COMMENTS

= 3.6V to 25V, V

LT1616

25V, 500mA (I ), 1.4MHz, High Efficiency

V

= 1.25V, I = 1.9mA, I = <1µA,

Q SD

OUT

IN

OUT

Step-Down DC/DC Converter

ThinSOT Package

LT1676

60V, 440mA (I ), 100kHz, High Efficiency

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V

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Q SD

OUT

IN

OUT

S8 Package

LT1765

25V, 2.75A (I ), 1.25MHz, High Efficiency

Step-Down DC/DC Converter

V

= 3V to 25V, V

= 1.2V, I = 1mA, I = 15µA,

OUT Q SD

OUT

IN

S8, TSSOP16E Packages

LT1766

60V, 1.2A (I ), 200kHz, High Efficiency

Step-Down DC/DC Converter

V

IN

= 5.5V to 60V, V = 1.2V, I = 2.5mA, I = 25µA,

OUT

Q

SD

TSSOP16/E Package

= 3V to 25V; V = 1.2V, I = 1mA, I = 6µA,

OUT Q SD

LT1767

25V, 1.2A (I ), 1.25MHz, High Efficiency

V

IN

OUT

Step-Down DC/DC Converter

MS8/E Packages

LT1776

40V, 550mA (I ), 200kHz, High Efficiency

Step-Down DC/DC Converter

V

= 7.4V to 40V; V

= 1.24V, I = 3.2mA, I = 30µA,

OUT Q SD

OUT

IN

N8, S8 Packages

LTC^®1877

LTC1879

LT1933

600mA (I ), 550kHz, Synchronous

Step-Down DC/DC Converter

V

= 2.7V to 10V; V

= 0.8V, I = 10µA, I = <1µA,

Q SD

OUT

IN

OUT

MS8 Package

1.2A (I ), 550kHz, Synchronous

V

IN

= 2.7V to 10V; V

= 0.8V, I = 15µA, I = <1µA,

Q SD

OUT

Step-Down DC/DC Converter

TSSOP16 Package

36V, 600mA, 500kHz, High Efficiency

Step-Down DC/DC Converter

V

IN

= 3.6V to 36V; V

= 1.25V, I = 2.5µA, I = <1µA,

Q SD

??? Package

LT1934

34V, 250mA (I ), Micropower, Step-Down

V

IN

= 3.2V to 34V; V

= 1.25V, I = 12µA, I = <1µA,

Q SD

OUT

DC/DC Converter

??? Package

LT1956

60V, 1.2A (I ), 500kHz, High Efficiency

V

IN

= 5.5V to 60V, V

= 1.2V, I = 2.5mA, I = 25µA,

Q SD

OUT

Step-Down DC/DC Converter

TSSOP16/E Package

LTC3405/LTC3405A

LTC3406/LTC3406B

LTC3411

LTC3412

LTC3430

300mA (I ), 1.5MHz, Synchronous

Step-Down DC/DC Converter

V

= 2.7V to 6V, V

= 0.8V, I = 20µA, I = <1µA,

OUT Q SD

OUT

IN

ThinSOT Package

600mA (I ), 1.5MHz, Synchronous

V

IN

= 2.5V to 5.5V, V

= 0.6V, I = 20µA, I = <1µA,

OUT Q SD

OUT

Step-Down DC/DC Converter

ThinSOT Package

1.25A (I ), 4MHz, Synchronous

V

IN

= 2.5V to 5.5V, V

= 0.8V, I = 60µA, I = <1µA,

Q SD

OUT

Step-Down DC/DC Converter

MS Package

2.5A (I ), 4MHz, Synchronous

V

IN

= 2.5V to 5.5V, V

= 0.8V, I = 60µA, I = <1µA,

Q SD

OUT

Step-Down DC/DC Converter

TSSOP16E Package

60V, 2.75A (I ), 200kHz, High Efficiency

V

IN

= 5.5V to 60V, V

= 1.2V, I = 2.5mA, I = 30µA,

Q SD

OUT

Step-Down DC/DC Converter

TSSOP16E Package

3470fc

LT 0907 REV C • PRINTED IN THE USA

LinearTechnology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

20

●

(408) 432-1900 FAX: (408) 434-0507 www.linear.com


型号：	LT3470ETS8#TRM
厂家：	Linear
描述：	LT3470 - Micropower Buck Regulator with Integrated Boost and Catch Diodes; Package: SOT; Pins: 8; Temperature Range: -40°C to 85°C 开关光电二极管
文件：	总20页 (文件大小：253K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

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