LT3470ETS8_技术文档

LT3470

Micropower Buck Regulator

with Integrated Boost and

Catch Diodes

FeaTures

DescripTion

The LT^®3470 is a micropower step-down DC/DC con-

verter that integrates a 300mA power switch, catch diode

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

ThinSOT™ packages. The LT3470 combines Burst Mode

and continuous operation to allow the use of tiny induc-

tor and capacitors while providing a low ripple output to

loads of up to 200mA.

n

Low Quiescent Current: 26µA at 12V to 3.3V

IN

OUT

n

Integrated Boost and Catch Diodes

n

Input Range: 4V to 40V

n

Low Output Ripple: <10mV

n

<1µA in Shutdown Mode

n

Output Voltage: 1.25V to 16V

n

200mA Output Current

n

Hysteretic Mode Control

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

regulateawidevarietyofpowersources,from2-cellLi-Ion

batteries to unregulated wall transformers and lead-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, simplifying

power management in battery-powered systems. Fast

currentlimitingandhystereticcontrolprotectstheLT3470

and external components against shorted outputs, even

at 40V input.

– Low Ripple Burst Mode^®Operation at Light Loads

– Continuous Operation at Higher Loads

2

n

Solution Size as Small as 50mm

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

Enhanced 8-Lead DD and 1mm ThinSOT Packages

applicaTions

n

Automotive Battery Regulation

n

Power for Portable Products

L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks

and ThinSOT is a trademark of Linear Technology Corporation. All other trademarks are the

property of their respective owners.

n

Distributed Supply Regulation

n

Industrial Supplies

n

Wall Transformer Regulation

Typical applicaTion

Efficiency and Power Loss vs Load Current

90

80

70

60

50

40

30

20

10

1000

100

10

V

IN

V

= 12V

IN

7V TO 40V

0.22µF

33µH

V

BOOST

IN

LT3470

SHDN

V

OUT

5V

OFF ON

2.2µF

SW

200mA

BIAS

604k

1%

22pF

FB

22µF

GND

200k

1%

1

3470 TA01a

0.1

1

10

100

LOAD CURRENT (mA)

3470 TA02

3470fd

1

LT3470

absoluTe MaxiMuM raTings ^{(Note 1)}

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

Operating Temperature Range (Note 2)

IN

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

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

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

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

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

SW Voltage ................................................................V

Maximum Junction Temperature

IN

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

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

pin conFiguraTion

TOP VIEW

FB

BIAS

1

2

3

4

8

7

6

5

SHDN

SHDN 1

NC 2

8 FB

NC

7 BIAS

6 BOOST

5 SW

9

BOOST

SW

V

IN

V

IN

3

GND

GND 4

TS8 PACKAGE

8-LEAD PLASTIC TSOT-23

= 140°C/W

DDB8 PACKAGE

8-LEAD (3mm × 2mm) PLASTIC DFN

= 180°C/W

θ

JA

θ

JA

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

orDer inForMaTion

LEAD FREE FINISH

LT3470EDDB#PBF

LT3470IDDB#PBF

LT3470HDDB#PBF

TAPE AND REEL

PART MARKING

LBPN

PACKAGE DESCRIPTION

TEMPERATURE RANGE

–40°C to 85°C

LT3470EDDB#TRPBF

LT3470IDDB#TRPBF

LT3470HDDB#TRPBF

8-Lead (3mm × 2mm) Plastic DFN

8-Lead Plastic TSOT-23

LBPP

–40°C to 125°C

–40°C to 150°C

–40°C to 85°C

LCNR

LT3470ETS8#PBF

LT3470ITS8#PBF

LT3470HTS8#PBF

LEAD BASED FINISH

LT3470EDDB

LT3470ETS8#TRPBF

LT3470ITS8#TRPBF

LT3470HTS8#TRPBF

TAPE AND REEL

LTBDM

LTBPW

8-Lead Plastic TSOT-23

–40°C to 125°C

–40°C to 150°C

TEMPERATURE RANGE

–40°C to 85°C

LTCNQ

8-Lead Plastic TSOT-23

PART MARKING

LBPN

PACKAGE DESCRIPTION

LT3470EDDB#TR

8-Lead (3mm × 2mm) Plastic DFN

8-Lead Plastic TSOT-23

LT3470IDDB

LT3470HDDB

LT3470ETS8

LT3470ITS8

LT3470HTS8

LT3470IDDB#TR

LT3470HDDB#TR

LT3470ETS8#TR

LT3470ITS8#TR

LT3470HTS8#TR

LBPP

–40°C to 125°C

–40°C to 150°C

–40°C to 85°C

LCNR

LTBDM

LTBPW

LTCNQ

8-Lead Plastic TSOT-23

–40°C to 125°C

–40°C to 150°C

8-Lead Plastic TSOT-23

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/

3470fd

2

LT3470

The l denotes the specifications which apply over the full operating

elecTrical characTerisTics

temperature range, otherwise specifications are at T_A= 25°C. V_IN= 10V, V_SHDN= 10V, V_BOOST= 15V, V_BIAS= 3V unless otherwise specified.

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

Minimum Input Voltage

Quiescent Current from V

●

4

V

= 0.2V

0.1

10

35

0.5

18

50

µA

IN

SHDN

= 3V, Not Switching

= 0V, Not Switching

BIAS

Quiescent Current from Bias

V

= 0.2V

0.1

25

0.5

60

µA

SHDN

= 3V, Not Switching

= 0V, Not Switching

●

BIAS

0.1

1.5

FB Comparator Trip Voltage

FB Pin Bias Current (Note 3)

V

FB

V

FB

Falling

1.228

1.250

1.265

V

= 1V, E- and I-Grade

35

80

150

nA

●

H-Grade

4V < V < 40V

35

0.0006

500

225

nA

%/V

ns

FB Voltage Line Regulation

Minimum Switch Off-Time (Note 5)

Switch Leakage Current

0.01

IN

0.7

1.5

µA

Switch V

I

SW

I

SW

= 100mA (TS8 Package)

= 100mA (DD8 Package)

215

300

mV

CESAT

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

SH

= 30mA

Boost Schottky Reverse Leakage

Minimum Boost Voltage (Note 4)

BOOST Pin Current

V

= 10V, V

= 0V

BIAS

SW

●

2.2

12

5

I

SW

= 100mA

mA

µA

V

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.

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 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 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.

3470fd

3

LT3470

Typical perForMance characTerisTics

Efficiency, V_OUT= 3.3V

Efficiency, V_OUT= 5V

V_FBvs Temperature

90

1.260

1.255

1.250

1.245

L = TOKO D52LC 47µH

T

= 25°C

T = 25°C

A

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

0.1

1

10

100

0.1

1

10

100

–50 –25

0

25

50

75 100 125

LOAD CURRENT (mA)

TEMPERATURE (°C)

3470 G01

3470 G02

3470 G03

Top and Bottom Switch Current

Limits (V_FB= 0V) vs Temperature

V_INQuiescent Current

vs Temperature

BIAS Quiescent Current

(Bias > 3V) vs Temperature

30

25

20

15

50

40

30

20

10

0

400

350

300

BIAS < 3V

BIAS > 3V

250

200

150

100

50

10

5

0

50

100 125 150

–50 –25

0

25

75

–25

0

50 75 100 125 150

25

TEMPERATURE (°C)

–50

50

125 150

–50 –25

0

25

75 100

TEMPERATURE (°C)

3470 G06

3470 G04

3470 G05

SHDN Bias Current

vs Temperature

FB Bias Current (V_FB= 1V)

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

0

–50 –25

0

25 50 75 100 125 150

TEMPERATURE (°C)

–50 –25

0

25

125 150

50 75 100

TEMPERATURE (°C)

3470 G08

3470 G07

3470fd

4

LT3470

Typical perForMance characTerisTics

Boost Diode V_F(I_F= 50mA)

vs Temperature

FB Bias Current (V_FB= 0V)

vs Temperature

Switch V_CESAT(I_SW= 100mA)

vs Temperature

120

100

80

300

250

200

150

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

60

40

20

0

100

50

0

50

100 125 150

50

100 125 150

–25

0

50 75 100 125 150

25

TEMPERATURE (°C)

–50 –25

0

25

75

–50 –25

0

25

75

–50

TEMPERATURE (°C)

3470 G09

3470 G10

3470 G11

Catch Diode V_F(I_F= 100mA)

vs Temperature

Diode Leakage (V_R= 36V)

vs Temperature

Switch V_CESAT

60

55

50

45

40

35

30

25

20

15

10

5

0.7

0.6

700

600

500

CATCH

BOOST

0.5

0.4

0.3

0.2

0.1

400

300

200

100

0

50

100 125 150

100

200

400

–50 –25

0

25

75

50

TEMPERATURE (°C)

0

300

–50 –25

0

25

75 100

125 150

TEMPERATURE (°C)

SWITCH CURRENT (mA)

3470 G12

3470 G14

3470 G13

BOOST Pin Current

Catch Diode Forward Voltage

14

12

10

1.0

0.8

0.6

0.4

0.2

0

8

6

4

2

0

100

200

400

0

300

0

100

200

300

400

SWITCH CURRENT (mA)

CATCH DIODE CURRENT (mA)

3470 G15

3470 G16

3470fd

5

LT3470

Typical perForMance characTerisTics

Boost Diode Forward Voltage

Minimum Input Voltage, V_OUT= 3.3V

Minimum Input Voltage, V_OUT= 5V

8

7

6

5

4

6.0

5.5

900

800

700

600

500

400

300

200

100

0

T

= 25°C

T

A

= 25°C

A

V

TO START

IN

V

TO START

IN

5.0

4.5

V

TO RUN

IN

4.0

3.5

3.0

V

TO RUN

100

IN

100

150

0

200

100

BOOST DIODE CURRENT (mA)

0

50

150

200

50

0

50

150

200

LOAD CURRENT (mA)

3470 G19

3470 G18

3470 G17

(ThinSOT/DD)

pin FuncTions

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

LT3470 in shutdown mode. Tie to ground to shut down

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

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.

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

IN

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

boost Schottky diode and to the internal regulator. Tie to

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

to V .

V

when V

> 2V or to V otherwise. When V

>

IN

OUT

IN

BIAS

3VtheBIASpinwillsupplycurrenttotheinternalregulator.

V

(Pin 3/Pin 6): The V pin supplies current to the

IN

LT3470’s internal regulator and to the internal power

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).

switch. This pin must be locally bypassed.

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.

3470fd

6

LT3470

block DiagraM

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

3470fd

7

LT3470

operaTion

TheLT3470usesahystereticcontrolschemeinconjunction

with Burst Mode operation to provide low output ripple

and low quiescent current while using a tiny inductor and

capacitors.

comparator trips and resets the latch causing the switch

to turn off. While the switch is off, the inductor current

ramps down through the catch diode. When both the bot-

tom 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, the part will automatically switch slower

to keep 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. If the load is light the part will alternate between

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 saturate the internal bipolar NPN power switch for

efficient 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 current reduces to the device leakage current,

IN

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

3470fd

8

LT3470

applicaTions inForMaTion

Input Voltage Range

where V

is the maximum input voltage for the ap-

IN(MAX)

plication,t

is~150nsandI

isthemaximum

ON-TIME(MIN)

MAX

The minimum input voltage required to generate a par-

ticular output voltage in an LT3470 application is limited

by either its 4V undervoltage lockout or by its maximum

duty cycle. The duty cycle is the fraction of time that the

internal switch is on and is determined by the input and

output voltages:

allowable increase in switch current during a minimum

switch on-time (150mA). While this equation provides 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:

V_OUT+ V_D

DC=

1–DC V + V

(

)

(

)

D

OUT

V – V_SW+ V_D

f =

IN

L • ∆I_L

where V is the forward voltage drop of the catch diode

D

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

wherefistheswitchingfrequency, ∆I istheripplecurrent

SW

L

at maximum load (~0.4V). Given DC

to a minimum input voltage of:

= 0.90, this leads

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

MAX

D

of the catch diode, and V

is the desired output voltage.

OUT





If the application circuit is intended to operate at high duty

V_OUT+ V_D

DC_MAX

V

=

+ V_SW– V_D

IN(MIN)





cycles (V close to V ), it is important to look at the

IN

OUT





calculated value of the switch off-time:

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

should be more than LT3470’s

OFF-TIME

minimum t

(See Electrical Characteristics), so

OFF-TIME

the application circuit is capable of delivering full rated

The maximum input voltage is limited by the absolute

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

maximum V rating of 40V, provided an inductor of suf-

IN

required, the calculated t

can be made less than

OFF-TIME

ficient value is used.

minimum t

possibly allowing the use of a smaller

OFF-TIME

inductor. See Table 1 for an inductor value selection guide.

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, it must be noted that the LT3470 cannot

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

smallandtheinputvoltageishigh, thecurrentthroughthe

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

Chooseaninductorthatisintendedforpowerapplications.

Table 2 lists several manufacturers and inductor series.

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

IN

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

450mA saturation current. If short-circuit performance is

not required, inductors with I of 300mA or more may

SAT

V_IN(MAX)• t_ON-TIME(MIN)

L_MIN

=

I_MAX

3470fd

9

LT3470

applicaTions inForMaTion

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 ap-

Input Capacitor

proximately I • ESR. ESR should be less than ~150mΩ.

LIM

The value of the output capacitor must be large enough to

accept the energy stored in the inductor without a large

change in output voltage. Setting this voltage step equal

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

Step-down regulators draw current from the input sup-

ply in pulses with very fast rise and fall times. The input

capacitor is required to reduce the resulting voltage ripple

at the V pin of the LT3470 and to force this switching

IN

 ²

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.



I_LIM

C_OUT> 50 •L •





V





OUT

where I is the top current limit with V = 0V (see Elec-

LIM

FB

trical Characteristics). For example, an LT3470 producing

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 ca-

pacitor 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

LT3470 is quiescent. In order to keep output voltage ripple

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

3470fd

10

LT3470

applicaTions inForMaTion

Ceramic Capacitors

BOOST and BIAS Pin Considerations

Ceramic capacitors are small, robust and have very low

ESR. However, ceramic capacitors can cause problems

when used with the LT3470. Not all ceramic capacitors are

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.

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 ar-

range the boost circuit. The BOOST pin must be more than

2.5V above the SW pin for best efficiency. For outputs 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

V

IN

C3

Ceramiccapacitorsarepiezoelectric.TheLT3470’sswitch-

ing frequency depends on the load current, and at light

loads the LT3470 can excite the ceramic capacitor at audio

frequencies, generating audible noise. Since the LT3470

operates at a lower current limit during Burst Mode opera-

tion, the noise is typically very quiet to a casual ear. If this

audible noise is unacceptable, use a high performance

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.

0.22µF

V

BOOST

LT3470

IN

V

SW

OUT

BIAS

GND

V

– V

BOOST

≅ V

SW OUT

BOOST

MAX V

≅ V + V

IN OUT

(2a)

V

IN

C3

0.22µF

V

BOOST

SW

IN

LT3470

A final precaution regarding ceramic capacitors concerns

themaximuminputvoltageratingoftheLT3470.Aceramic

input capacitor combined with trace or cable inductance

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

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

agecanringtotwiceitsnominalvalue, possiblyexceeding

the LT3470’s rating. This situation is easily avoided; see

the Hot-Plugging Safely section.

V

BIAS

OUT

GND

3470 F02

V

– V

BOOST

≅ V

SW IN

BOOST

MAX V

≅ 2V

IN

(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

(864) 963-6300

www.murata.com

www.avxcorp.com

Ceramic

Ceramic,

Tantalum

TPS Series

Taiyo Yuden

www.taiyo-yuden.com Ceramic

3470fd

11

LT3470

applicaTions inForMaTion

(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.

V to start and to run. At light loads, the inductor current

IN

becomes discontinuous and the effective duty cycle can

be very high. This reduces the minimum input voltage to

approximately300mVaboveV .Athigherloadcurrents,

OUT

the inductor current is continuous and the duty cycle is

limitedbythemaximumdutycycleoftheLT3470,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

outputisalreadyinregulation,thentheboostcapacitormay

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

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 = 40V. There is another situation to consider

IN

in systems where the output will be held high when the

input to the LT3470 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

Minimum Input Voltage, V_OUT= 3.3V

6.0

T

= 25°C

A

V

TO START

IN

5.5

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

IN

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

5.0

4.5

it is tied to V ), then the LT3470’s internal circuitry will

IN

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 es-

4.0

3.5

3.0

V

TO RUN

IN

sentially zero. However, if the V pin is grounded while

IN

the output is held high, then parasitic diodes inside the

LT3470 can pull large currents from the output through

0

50

100

150

200

LOAD CURRENT (mA)

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

IN

3470 G18

will run only when the input voltage is present and that

protects against a shorted or reversed input.

Minimum Input Voltage, V_OUT= 5V

8

7

6

5

4

T

A

= 25°C

D1

V

TO START

IN

V

IN

V

BOOST

IN

LT3470 SOT-23

100k

1M

V

SHDN

SW

OUT

BIAS

V

TO RUN

IN

FB

GND

BACKUP

3470 F04

100

150

0

200

50

LOAD CURRENT (mA)

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

3470 G19

Figure 3. The Minimum Input Voltage Depends on Output

Voltage, Load Current and Boost Circuit

3470fd

12

LT3470

applicaTions inForMaTion

PCB Layout

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 instability and increased

output ripple. To avoid this problem, use vias to route the

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

thesecomponentsshouldbeassmallaspossible.Further-

more, the system ground should be tied to the regulator

ground in only one place; this prevents the switched cur-

rent from injecting noise into the system ground. These

components,alongwiththeinductorandoutputcapacitor,

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

and their connections should be made on that layer. Place

a local, unbroken ground plane below these components,

andtiethisgroundplanetosystemgroundatonelocation,

V

trace under the ground plane to the feedback divider

OUT

(as shown in Figure 5). Finally, keep the FB node as small

as possible so that the ground pin and ground traces

will shield it from the SW and BOOST nodes. 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.

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

(5a)

(5b)

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

3470fd

13

LT3470

applicaTions inForMaTion

Hot-Plugging Safely

(it also reduces the peak input current). A 0.1µF capacitor

improves high frequency filtering. This solution is smaller

and less expensive than the electrolytic capacitor. For high

input voltages its impact on 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

bypasscapacitorofLT3470.However,thesecapacitorscan

cause problems if the LT3470 is plugged into a live supply

(seeLinearTechnologyApplicationNote88foracomplete

discussion).Thelowlossceramiccapacitorcombinedwith

stray inductance in series with the power source forms an

High Temperature Considerations

under damped tank circuit, and the voltage at the V pin

The die junction temperature of the LT3470 must be

lower than the maximum rating of 125°C (150°C for the

H-grade). Thisisgenerallynotaconcernunlesstheambi-

ent temperature 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 should be derated as the ambient temperature

approaches the maximum junction rating. The die tem-

perature 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 measurement. 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 approximately 80°C/W while the ThinSOT

is approximately 150°C/W. Finally, be aware that at high

ambient temperatures the internal Schottky diode will

havesignificantleakagecurrent(seeTypicalPerformance

Characteristics) increasing the quiescent current of the

LT3470 converter.

IN

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 energized supply, the input

network should be designed to 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

likelytobethelargestcomponentinthecircuit. Analterna-

tive solution is shown in Figure 6c. A 1Ω resistor is added

in series with the input to eliminate the voltage overshoot

3470fd

14

LT3470

applicaTions inForMaTion

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

10µs/DIV

DUE TO 6 FEET

(2 METERS) OF

TWISTED PAIR

(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

3470fd

15

LT3470

Typical applicaTions

3.3V Step-Down Converter

5V Step-Down Converter

V

IN

C3

5.5V TO 40V

7V TO 40V

0.22µF, 6.3V

V

BOOST

V

BOOST

IN

L1

33µH

L1

33µH

LT3470

SHDN

LT3470

SHDN

V

OUT

3.3V

200mA

5V

OFF ON

SW

OFF ON

SW

200mA

BIAS

R1

22pF

324k

604k

C1

1µF

C2

22µF

C1

1µF

C2

FB

22µF

GND

R2

200k

R2

200k

3470 TA03

3470 TA04

C1: TDK C3216JB1H105M

C2: CE JMK316 BJ226ML-T

L1: TOKO A914BYW-330M=P3

C1: TDK C3216JB1H105M

C2: CE JMK316 BJ226ML-T

L1: TOKO A993AS-270M=P3

2.5V Step-Down Converter

1.8V Step-Down Converter

V

IN

C3

4V TO 23V

4.7V TO 40V

0.22µF, 25V

0.47µF, 6.3V

V

BOOST

SW

V

BOOST

LT3470

SHDN

IN

L1

22µH

L1

33µH

LT3470

V

OUT

1.8V

2.5V

OFF ON

SHDN

OFF ON

SW

200mA

BIAS

R1

BIAS

R1

22pF

147k

200k

C1

1µF

C2

C1

1µF

C2

22µF

FB

22µF

GND

R2

332k

R2

200k

3470 TA07

3470 TA05

C1: TDK C3216JB1H105M

C2: TDK C2012JB0J226M

L1: MURATA LQH32CN150K53

C1: TDK C3216JB1H105M

C2: TDK C2012JB0J226M

L1: SUMIDA CDRH3D28

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

3470fd

16

LT3470

package DescripTion

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

TS8 Package

8-Lead Plastic TSOT-23

(Reference LTC DWG # 05-08-1637 Rev A)

2.90 BSC

(NOTE 4)

0.40

MAX

0.65

REF

1.22 REF

1.4 MIN

1.50 – 1.75

(NOTE 4)

2.80 BSC

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

TS8 TSOT-23 0710 REV A

0.09 – 0.20

(NOTE 3)

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

3470fd

17

LT3470

package DescripTion

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

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

TYP

5

R = 0.05

(2 SIDES)

TYP

8

0.70 0.05

2.55 0.05

2.00 ±0.10

(2 SIDES)

PIN 1 BAR

TOP MARK

(SEE NOTE 6)

1.15 0.05

PIN 1

R = 0.20 OR

0.25 × 45°

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

3470fd

18

LT3470

revision hisTory ^{(Revision history begins at Rev D)}

REV

DATE

DESCRIPTION

PAGE NUMBER

D

09/11 Corrected lead-based tape and reel part numbers in the Order Information section.

2

3470fd

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-

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

19

LT3470

relaTeD parTs

PART NUMBER

DESCRIPTION

COMMENTS

= 3.6V to 25V, V

LT1616

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

Step-Down DC/DC Converter

V

= 1.25V, I = 1.9mA, I < 1µA,

Q SD

OUT

IN

OUT

ThinSOT Package

LT1676

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

V

= 7.4V to 60V, V

= 1.24V, I = 3.2mA, I = 2.5µA,

Q SD

OUT

IN

Step-Down DC/DC Converter

S8 Package

LT1765

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

V

= 3V to 25V, V

= 1.2V, I = 1mA, I = 15µA,

OUT Q SD

OUT

IN

Step-Down DC/DC Converter

S8, TSSOP16E Packages

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

IN

LT1766

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

OUT

Q

SD

Step-Down DC/DC Converter

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

OUT

IN

Step-Down DC/DC Converter

MS8/E Packages

LT1776

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

V

= 7.4V to 40V; V

= 1.24V, I = 3.2mA, I = 30µA,

OUT Q SD

OUT

IN

Step-Down DC/DC Converter

N8, S8 Packages

LTC^®1877

LTC1879

LT1933

600mA (I ), 550kHz, Synchronous

V

= 2.7V to 10V; V

= 0.8V, I = 10µA, I ≤ 1µA,

Q SD

OUT

IN

OUT

Step-Down DC/DC Converter

MS8 Package

1.2A (I ), 550kHz, Synchronous

V

= 2.7V to 10V; V

= 0.8V, I = 15µA, I ≤ 1µA,

Q SD

OUT

IN

Step-Down DC/DC Converter

TSSOP16 Package

36V, 600mA, 500kHz, High Efficiency

Step-Down DC/DC Converter

V

= 3.6V to 36V; V

= 1.25V, I = 2.5µA, I ≤ 1µA,

Q SD

IN

??? Package

LT1934

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

V

= 3.2V to 34V; V

= 1.25V, I = 12µA, I ≤ 1µA,

Q SD

OUT

IN

DC/DC Converter

??? Package

LT1956

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

V

= 5.5V to 60V, V

= 1.2V, I = 2.5mA, I = 25µA,

Q SD

OUT

IN

Step-Down DC/DC Converter

TSSOP16/E Package

= 2.7V to 6V, V = 0.8V, I = 20µA, I ≤ 1µA,

OUT Q SD

LTC3405/LTC3405A

LTC3406/LTC3406B

LTC3411

LTC3412

LTC3430

300mA (I ), 1.5MHz, Synchronous

V

OUT

IN

Step-Down DC/DC Converter

ThinSOT Package

600mA (I ), 1.5MHz, Synchronous

V

= 2.5V to 5.5V, V

= 0.6V, I = 20µA, I ≤ 1µA,

OUT Q SD

OUT

IN

Step-Down DC/DC Converter

ThinSOT Package

1.25A (I ), 4MHz, Synchronous

V

= 2.5V to 5.5V, V

= 0.8V, I = 60µA, I ≤ 1µA,

Q SD

OUT

IN

OUT

Step-Down DC/DC Converter

MS Package

2.5A (I ), 4MHz, Synchronous

V

= 2.5V to 5.5V, V

= 0.8V, I = 60µA, I ≤ 1µA,

Q SD

OUT

IN

Step-Down DC/DC Converter

TSSOP16E Package

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

V

= 5.5V to 60V, V

= 1.2V, I = 2.5mA, I = 30µA,

Q SD

OUT

IN

Step-Down DC/DC Converter

TSSOP16E Package

3470fd

LT 0911 REV D • PRINTED IN USA

LinearTechnology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

20

●

 LINEAR TECHNOLOGY CORPORATION 2004

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


型号：	LT3470ETS8
厂家：	Linear
描述：	Micropower Buck Regulator with Integrated Boost and Catch Diodes 微功率降压型调节器，集成的升压和钳位二极管稳压器开关式稳压器或控制器调节器电源电路二极管开关式控制器光电二极管
文件：	总20页 (文件大小：240K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

LT3470ETS8 [Linear]

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