LT3470A Datasheet by Analog Devices Inc.

Datasheet Feedback/Errors

L7L|nt I “2 LT347OA TECHNOLOGY M— _ L7 LJUW

LT3470A

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TYPICAL APPLICATION

FEATURES

APPLICATIONS

DESCRIPTION

Micropower Buck Regulator

with Integrated Boost and

Catch Diodes

The LT

3470A is a micropower step-down DC/DC con-

verter that integrates a 440mA power switch, catch diode

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

package. The LT3470A 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 250mA.

With its wide input range of 4V to 40V, the LT3470A can

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

batteries to unregulated wall transformers and lead-acid

batteries. Quiescent current in regulation is just 35µA 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 cur-

rent limiting and hysteretic control protects the LT3470A

and external components against shorted outputs, even

at 40V input. The LT3470A has higher output current and

improved start-up and dropout performance compared

to the LT3470.

Efficiency and Power Loss vs Load Current

n Low Quiescent Current: 35μA at 12VIN to 3.3VOUT

Integrated Boost and Catch Diodes

Input Range: 4V to 40V

3.3V at 250mA from 4V to 40V Input

5V at 250mA from 5.7V to 40V Input

n Low Output Ripple: <10mV

n <1µA in Shutdown Mode

Output Voltage: 1.25V to 16V

Hysteretic Mode Control

– Low Ripple Burst Mode

Operation at Light Loads

– Continuous Operation at Higher Loads

Solution Size as Small as 50mm2

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

Enhanced 8-Lead DFN Package

n Automotive Battery Regulation

Power for Portable Products

Distributed Supply Regulation

Industrial Supplies

n Wall Transformer Regulation

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

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

owners.

VIN BOOST

LT3470A

SWSHDN

0.22µF

22pF

22µF

2.2µF

VIN

5.7V TO 40V

VOUT

250mA

604k

200k

33µH

BIAS

GND

OFF ON

3470a TA01a

LOAD CURRENT (mA)

EFFICIENCY (%)

POWER LOSS (mW)

0.1 10 100 300

3470a TA01b

1000

100

0.1

VIN = 12V

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LT3470A

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PIN CONFIGURATION ABSOLUTE MAXIMUM RATINGS

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

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

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

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

BIAS Voltage .............................................................15V

SW Voltage ................................................................VIN

Maximum Junction Temperature

LT3470AE, LT3470AI ........................................ 125°C

LT3470AH ......................................................... 150°C

Operating Temperature Range (Note 2)

LT3470AE ............................................–40°C to 85°C

LT3470AI ........................................... –40°C to 125°C

LT3470AH .......................................... –40°C to 150°C

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

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

(Note 1)

TOP VIEW

DDB8 PACKAGE

8-LEAD (3mm × 2mm) PLASTIC DFN

1FB

BIAS

BOOST

SHDN

VIN

GND

θJA = 80°C/W

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

ORDER INFORMATION

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

LT3470AEDDB#PBF LT3470AEDDB#TRPBF LDPR 8-Lead (3mm × 2mm) Plastic DFN –40°C to 85°C

LT3470AIDDB#PBF LT3470AIDDB#TRPBF LDPR 8-Lead (3mm × 2mm) Plastic DFN –40°C to 125°C

LT3470AHDDB#PBF LT3470AHDDB#TRPBF LDPR 8-Lead (3mm × 2mm) Plastic DFN –40°C to 150°C

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

Consult LTC Marketing for information on non-standard lead based finish parts.

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/

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LT3470A

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ELECTRICAL CHARACTERISTICS

The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C.

VIN = 10V, VSHDN = 10V, VBOOST = 15V, VBIAS = 3V unless otherwise specified.

PARAMETER CONDITIONS MIN TYP MAX UNITS

Minimum Input Voltage ●4V

Quiescent Current from VIN VSHDN = 0.2V

VBIAS = 3V, Not Switching

VBIAS = 0V, Not Switching

●

0.1

0.5

µA

Quiescent Current from Bias VSHDN = 0.2V

VBIAS = 3V, Not Switching

VBIAS = 0V, Not Switching

●

0.1

0.5

1.5

µA

FB Comparator Trip Voltage VFB Falling ●1.228 1.250 1.265 V

FB Pin Bias Current (Note 3) VFB = 1V

H-Grade

●

150

225

FB Voltage Line Regulation 4V < VIN < 40V 0.0006 0.02 %/V

Minimum Switch Off-Time (Note 5) 500 ns

Maximum Duty Cycle ●90 95 %

Switch Leakage Current 0.7 1.5 µA

Switch VCESAT ISW = 100mA 150 mV

Switch VCESAT Without Boost VBOOST = VSW 0.9 1.2 V

Switch Top Current Limit VFB = 0V 320 440 560 mA

Switch Bottom Current Limit VFB = 0V 280 mA

Catch Schottky Drop ISW = 100mA 600 mV

Catch Schottky Reverse Leakage VSW = 10V 0.2 2 µA

Boost Schottky Drop IBIAS = 50mA 690 775 mV

Boost Schottky Reverse Leakage VSW = 10V, VBIAS = 0V 0.2 2 µA

Minimum Boost Voltage (Note 4) ●1.7 2.2 V

BOOST Pin Current ISW = 100mA 2.3 5 mA

Bias Pin Preload VBOOST = 10V 50 mA

SHDN Pin Current VSHDN = 2.5V 1 5 µA

SHDN Input Voltage High 2V

SHDN Input Voltage Low 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 LT3470AE is guaranteed to meet performance speciﬁ cations

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

temperature range are assured by design, characterization and correlation

with statistical process controls. The LT3470AI speciﬁ cations are

guaranteed over the –40°C to 125°C temperature range. LT3470AH

speciﬁ cations are guaranteed over the –40°C to 150°C temperature range.

Note 3: Bias current ﬂ ows 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.

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LT3470A

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TYPICAL PERFORMANCE CHARACTERISTICS

Efficiency, VOUT = 3.3V Efficiency, VOUT = 5V VFB vs Temperature

Top and Bottom Switch Current

Limits (VFB = 0V) vs Temperature

VIN Quiescent Current

vs Temperature

BIAS Quiescent Current

(Bias > 3V) vs Temperature

SHDN Bias Current

vs Temperature

LOAD CURRENT (mA)

EFFICIENCY (%)

0.1 10 100

3470a G01

L = TOKO D52LC 47µH

TA = 25°C VIN = 7V

VIN = 36V

VIN = 24V

VIN = 12V

TA = 25°C unless otherwise noted.

TEMPERATURE (°C)

–50

CURRENT LIMIT (mA)

550

3470a G04

400

300

–25 0 50

250

200

600

500

450

350

75 100 150125

TEMPERATURE (°C)

–50 –25

VIN CURRENT (µA)

050 75

3470a G05

25 100 150125

BIAS < 3V

BIAS > 3V

TEMPERATURE (°C)

–50

BIAS CURRENT (µA)

25 75

3470a G06

–25 0 50 100 150125

TEMPERATURE (°C)

–50

SHDN CURRENT (µA)

3470a G07

–25 75 100

25 150125

8VSHDN = 36V

VSHDN = 2.5V

TEMPERATURE (°C)

–50

1.240

VFB (V)

1.245

1.250

1.255

1.260

–25 0 25 50

3470a G03

75 100 150125

LOAD CURRENT (mA)

EFFICIENCY (%)

0.1 10 100

3470a G02

L = TOKO D52LC 47µH

TA = 25°C VIN = 12V

VIN = 36V

VIN = 24V

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LT3470A

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FB Bias Current (VFB = 1V)

vs Temperature

TYPICAL PERFORMANCE CHARACTERISTICS

FB Bias Current (VFB = 0V)

vs Temperature

Switch VCESAT (ISW = 100mA)

vs Temperature

Boost Diode VF (IF = 50mA)

vs Temperature

Catch Diode VF (IF = 100mA)

vs Temperature

Diode Leakage (VR = 36V)

vs Temperature

TA = 25°C unless otherwise noted.

TEMPERATURE (°C)

–50 –25

FB CURRENT (nA)

050 75

3470a G08

25 100 150125

TEMPERATURE (°C)

–50

FB CURRENT (µA)

100

120

25 75

3470a G09

–25 0 50 100 150125

TEMPERATURE (°C)

–50

SWITCH VCESAT (mV)

200

250

300

25 75

3470a G10

150

100

–25 0 50 100 150125

TEMPERATURE (°C)

–50

SCHOTTKY VF (V)

0.7

3470a G11

0.4

0.2

–25 0 50

0.1

0.8

0.6

0.5

0.3

75 100 150125

TEMPERATURE (°C)

–50

0.4

0.5

0.7

25 75

3470a G12

0.3

0.2

–25 0 50 100 150125

0.1

0.6

SCHOTTKY VF (V)

TEMPERATURE (°C)

–50 –25

SCHOTTKY DIODE LEAKAGE (mA)

050 75

3470a G13

25 100 150

CATCH

BOOST

125

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LT3470A

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Boost Diode Forward Voltage

Minimum Input Voltage, VOUT = 3.3V Minimum Input Voltage, VOUT = 5V

TYPICAL PERFORMANCE CHARACTERISTICS

BOOST DIODE CURRENT (mA)

SCHOTTKY VF (V)

500

600

700

200

3470a G17

400

300

050 100 150

100

200

900

800

Switch VCESAT BOOST Pin Current

Catch Diode Forward Voltage

SWITCH CURRENT (mA)

400

500

700

300 400

3470a G14

300

200

100 200 500

100

600

SWITCH VCESAT (mV)

SWITCH CURRENT (mA)

300 400

3470a G15

100 200 500

BOOST PIN CURRENT (mA)

CATCH DIODE CURRENT (mA)

SCHOTTKY VF (V)

0.4

0.6

400

3470a G16

0.2

0100 200 300

1.0

0.8

LOAD CURRENT (mA)

3.0

INPUT VOLTAGE (V)

3.5

4.0

4.5

5.0

5.5

6.0

50 100 150 200 250

3470a G18

TA = 25°C

VIN TO RUN/START

LOAD CURRENT (mA)

INPUT VOLTAGE (V)

250

3470a G19

450 100 150 200

8TA = 25°C

VIN TO RUN/START

TA = 25°C unless otherwise noted.

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LT3470A

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SHDN (Pin 8): The SHDN pin is used to put the LT3470A in

shutdown mode. Tie to ground to shut down the LT3470A.

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

feature is not used, tie this pin to the VIN pin.

NC (Pin 7): This pin can be left floating, connected to VIN,

or tied to GND.

VIN (Pin 6): The VIN pin supplies current to the LT3470A’s

internal regulator and to the internal power switch. This

pin must be locally bypassed.

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

below the LT3470A and the circuit components. Return

the feedback divider to this pin.

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

switch. Connect this pin to the inductor, catch diode and

boost capacitor.

BOOST (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 2):

The BIAS pin connects to the internal boost

Schottky diode and to the internal regulator. Tie to VOUT

when VOUT > 2.5V or to VIN otherwise. When VBIAS > 3V

the BIAS pin will supply current to the internal regulator.

FB (

Pin 1):

The LT3470A regulates its feedback pin to

1.25V. Connect the feedback resistor divider tap to this

pin. Set the output voltage according to VOUT = 1.25V (1

+ R1/R2) or R1 = R2 (VOUT/1.25 – 1).

Exposed Pad (

Pin 9):

Ground. Must be soldered to PCB.

PIN FUNCTIONS

BLOCK DIAGRAM

–

RQʹ

500ns

ONE SHOT

VREF

1.25V

BURST MODE

DETECT

GND

3470a BD

R2 R1

SHDN

ENABLE

VIN

BIAS

BOOST

VOUT

LT347OA /\/\/\/\ /\/\/V/\v/\/\/ L7Hﬂ§ﬂ§

LT3470A

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Figure 1. Operating Waveforms of the LT3470A Converting 12V to 5V Using a 33μH Inductor and 10μF Output Capacitor

OPERATION

The LT3470A 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 inductor and

capacitors.

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 VREF, the error amplifier will

shut off all the high power circuitry, leaving the LT3470A

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 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

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

completing a full cycle. The hysteretic action of this control

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.

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.

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

off and VIN current reduces to the device leakage current,

typically 100nA.

(1a) Burst Mode Operation (1b) Continuous Operation

VOUT

20mV/DIV

100mA/DIV

1ms/DIV

VOUT

20mV/DIV

100mA/DIV

5µs/DIV 3470a F01a

NO LOAD

10mA LOAD

VOUT

20mV/DIV

100mA/DIV

1µs/DIV

VOUT

20mV/DIV

100mA/DIV

1µs/DIV 3470a F1b

200mA LOAD

150mA LOAD

VD UT VD sz VOUT VD DCMAX L7 LJUW LT347OA

LT3470A

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Input Voltage Range

The minimum input voltage required to generate a par-

ticular output voltage in an LT3470A 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:

DC =VOUT +VD

IN –V

SW +VD

where VD is the forward voltage drop of the catch diode

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

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

to a minimum input voltage of:

IN(MIN) =VOUT +VD

DCMAX

⎛

⎝

⎜⎞

⎠

⎟+VSW –V

This analysis assumes the part has started up such that the

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.

The maximum input voltage is limited by the absolute

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

ficient value is used.

Inductor Selection

The switching action of the LT3470A 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 induc-

tor current equals the load current. For safe operation, it

must be noted that the LT3470A cannot turn the 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 LT3470A is able to

turn off. To prevent this from happening, the following

equation provides a minimum inductor value:

LMIN =V

IN(MAX) •t

ON-TIME(MIN)

IMAX

APPLICATIONS INFORMATION

where VIN(MAX) is the maximum input voltage for the ap-

plication, tON-TIME(MIN)

is ~150ns and IMAX is the maximum

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:

f=1–DC

()

D+VOUT

()

L•ΔIL

where f is the switching frequency, ΔIL is the ripple current

in the inductor (~200mA), VD is the forward voltage drop

of the catch diode, and VOUT is the desired output voltage.

If the application circuit is intended to operate at high duty

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

calculated value of the switch off-time:

tOFF-TIME =1–DC

The calculated tOFF-TIME should be more than LT3470A’s

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

application circuit is capable of delivering full rated output

current. If the full output current of 250mA is not required,

the calculated tOFF-TIME can be made less than minimum

tOFF-TIME possibly allowing the use of a smaller inductor.

See Table 1 for an inductor value selection guide.

Table 1. Recommended Inductors for Loads up to 250mA

VOUT VIN Up to 16V VIN Up to 40V

2.5V 10µH 33µH

3.3V 10µH 33µH

5V 15µH 33µH

12V 33µH 47µH

Choose an inductor that is intended for power applications.

Table 2 lists several manufacturers and inductor series.

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

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

450mA saturation current. If short-circuit performance is

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

be used. It is important to note that inductor saturation

current is reduced at high temperatures—see inductor

vendors for more information.

LT347OA ILIM \Vom) L7LJCUEN2

LT3470A

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APPLICATIONS INFORMATION

Input Capacitor

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 VIN pin of the LT3470A 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.

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 LT3470A

circuit is connected to a low impedance supply, and that

some electrolytic capacitors (in particular tantalum) must

be specified for such use.

Output Capacitor and Output Ripple

The output capacitor filters the inductor’s ripple current

and stores energy to satisfy the load current when the

LT3470A is quiescent. In order to keep output voltage

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

the LT3470A’s switching frequency. The capacitor’s equiva-

lent series resistance (ESR) determines this impedance.

Choose one with low ESR intended for use in switching

regulators. The contribution to ripple voltage due to the

ESR is approximately ILIM • ESR. ESR should be less than

~150m. 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:

COUT >50 • L • ILIM

VOUT

⎛

⎝

⎜⎞

⎠

⎟

Where ILIM is the top current limit with VFB

= 0V (see Electri-

cal Characteristics). For example, an LT3470A 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.

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

performance in a small package for the LT3470A. 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 LT3470A’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.

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

CDRH3D16/HP

CDRH3D28

CDRH2D18/HP

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

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 www.cooperet.com SD10 10 to 47 1.0 × 5.0 × 5.0

Murata www.murata.com LQH43C

LQH32C

10 to 47

10 to 15

2.6 × 3.2 × 4.5

1.6 × 2.5 × 3.2

LT347OA _—| 1—4— :: l E I _=_ T ——I l——|— __ l”: " T L7 LJUW

LT3470A

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APPLICATIONS INFORMATION

Ceramic Capacitors

Ceramic capacitors are small, robust and have very low

ESR. However, ceramic capacitors can cause problems

when used with the LT3470A. 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

capacitance resulting in much higher output voltage ripple

than expected.

Ceramic capacitors are piezoelectric. The LT3470A’s

switching frequency depends on the load current, and at

light loads the LT3470A can excite the ceramic capacitor

at audio frequencies, generating audible noise. Since the

LT3470A operates at a lower current limit during Burst

Mode operation, the noise is typically very quiet to a ca-

sual 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.

A final precaution regarding ceramic capacitors concerns

the maximum input voltage rating of the LT3470A. A ceramic

input capacitor combined with trace or cable inductance

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

LT3470A circuit is plugged into a live supply, the input volt-

age can ring to twice its nominal value, possibly exceeding

the LT3470A’s rating. This situation is easily avoided; see

the Hot-Plugging Safely section.

BOOST and BIAS Pin Considerations

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

Figure 2. Two Circuits for Generating the Boost Voltage

Table 2. Capacitor Vendors

Vendor Phone URL Part Series Comments

Panasonic (714) 373-7366 www.panasonic.com Ceramic,

Polymer,

Tantalum

EEF Series

Kemet (864) 963-6300 www.kemet.com Ceramic,

Tantalum

T494, T495

Sanyo (408) 749-9714 www.sanyovideo.com Ceramic,

Polymer,

Tantalum

POSCAP

Murata (404) 436-1300 www.murata.com Ceramic

AVX www.avxcorp.com Ceramic,

Tantalum

TPS Series

Taiyo Yuden (864) 963-6300 www.taiyo-yuden.com Ceramic

VIN BOOST

LT3470A

(2a)

(2b)

0.22µF

VIN

VOUT

VBOOST – VSW ≅ VOUT

MAX VBOOST ≅ VIN + VOUT

BIAS

GND

VIN BOOST

LT3470A

SWBIAS

0.22µF

VIN

VOUT

3470a F02

VBOOST – VSW ≅ VIN

MAX VBOOST ≅ 2•VIN

GND

LT347OA ‘ _+f'—I |—_|_ m EMF START // § :: —§_|_I _ ? L7LJCUEN2

LT3470A

3470afb

APPLICATIONS INFORMATION

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

because the BOOST pin current and BIAS pin quiescent

current comes from a lower voltage source. You must also

be sure that the maximum voltage ratings of the BOOST

and BIAS pins are not exceeded.

The LT3470A monitors the boost capacitor for sufficient

voltage such that the switch is allowed to fully saturate.

When boost voltage falls below adequate levels (1.8V

typical) the switch will operate with about 1V of drop, and

an internal current source will begin to pull 50mA (typi-

cal) from the BIAS pin which is typically connected to the

output. This current forces the LT3470A to switch more

often and with more inductor current, which recharges

the boost capacitor. When the boost capacitor voltage is

above 1.8V (typical) the current source turns off, and the

part may enter BurstMode. This cycle will repeat anytime

there is an undervoltage condition on the boost capaci-

tor. See Figure 3 for minimum input voltage for outputs

of 3.3V and 5V.

Shorted Input Protection

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

sively at the top switch current limit maximum of 525mA,

an LT3470A buck regulator will tolerate a shorted output

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

in systems where the output will be held high when the

input to the LT3470A 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

LT3470A’s output. If the VIN pin is allowed to float and the

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

it is tied to VIN), then the LT3470A’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 es-

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

the output is held high, then parasitic diodes inside the

LT3470A can pull large currents from the output through

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

will run only when the input voltage is present and that

protects against a shorted or reversed input.

Figure 3. The Minimum Input Voltage Depends on Output

Voltage, Load Current and Boost Circuit

Minimum Input Voltage, VOUT = 3.3V

Minimum Input Voltage, VOUT = 5V

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 LT3470A Runs Only When the Input is

Present Hot-Plugging Safely

VIN BOOST

LT3470A

SWSHDN

3470a F04

VIN

100k

VOUT

BACKUP

BIAS

GND

LOAD CURRENT (mA)

3.0

INPUT VOLTAGE (V)

3.5

4.0

4.5

5.0

5.5

6.0

50 100 150 200 250

3470a F03a

TA = 25°C

VIN TO RUN/START

LOAD CURRENT (mA)

INPUT VOLTAGE (V)

250

3470a F03b

450 100 150 200

8TA = 25°C

VIN TO RUN/START

LT347OA LT n ® " CCEC m .W .EEI_LL Amw 1111 E ............................. T Lywmg

LT3470A

3470afb

APPLICATIONS INFORMATION

PCB Layout

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. 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, along with the inductor and output capacitor,

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

and their connections should be made on that layer. Place

a local, unbroken ground plane below these components,

and tie this ground plane to system ground at one 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 instability and increased

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

VOUT trace 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 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

LT3470A to help remove heat from the LT3470A to the

ground plane.

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

VOUT

3470a F05

SHDN

VIN

GND

LT347OA L7LJCUEN2

LT3470A

3470afb

APPLICATIONS INFORMATION

Hot-Plugging Safely

The small size, robustness and low impedance of ceramic

capacitors make them an attractive option for the input

bypass capacitor of LT3470A. However, these capacitors

can cause problems if the LT3470A 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 volt-

age at the VIN pin of the LT3470A can ring to twice the

nominal input voltage, possibly exceeding the LT3470A’s

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

controlled or the user will be plugging the LT3470A into an

energized supply, the input network should be designed

to prevent this overshoot. Figure 6 shows the waveforms

that result when an LT3470A 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 1

resistor is added in series with the input to eliminate the

voltage overshoot (it also reduces the peak input current).

A 0.1µF capacitor improves high frequency filtering. 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.

High Temperature Considerations

The die junction temperature of the LT3470A must be

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

H-grade). This is generally not a concern unless the ambi-

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 LT3470A. 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 LT3470A power

dissipation by the thermal resistance from junction to

ambient. Power dissipation within the LT3470A 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 DFN package with the exposed pad has a thermal

resistance of approximately 80°C/W. Finally, be aware

that at high ambient temperatures the internal Schottky

diode will have significant leakage current (see Typical

Performance Characteristics) increasing the quiescent

current of the LT3470A converter.

L7 LJUW LT347OA

LT3470A

3470afb

APPLICATIONS INFORMATION

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

Ensures Reliable Operation When the LT3470A is Connected to a Live Supply

LT3470A

2.2µF

VIN

10V/DIV

IIN

10A/DIV

10µs/DIV

VIN

10V/DIV

IIN

10A/DIV

10µs/DIV

VIN

10V/DIV

IIN

10A/DIV

10µs/DIV

VIN

CLOSING SWITCH

SIMULATES HOT PLUG

IIN

(6a)

(6b)

(6c)

LOW

IMPEDANCE

ENERGIZED

24V SUPPLY

STRAY

INDUCTANCE

DUE TO 6 FEET

(2 METERS) OF

TWISTED PAIR

LT3470A

2.2µF

10µF

35V

AI.EI.

LT3470A

2.2µF0.1µF

1Ω

3470a F06

LT347OA ﬂ _—l 11 m Tm _: Lil I: _—I m him L7LJCUEN2

LT3470A

3470afb

APPLICATIONS INFORMATION

3.3V Step-Down Converter

5V Step-Down Converter

2.5V Step-Down Converter

VIN BOOST

LT3470A

SWSHDN

0.22µF, 6.3V

22pF C2

22µF

3470a TA02

1µF

VIN

4V TO 40V

VOUT

3.3V

250mA

324k

200k

C1: TDK C3216JB1H105M

C2: CE JMK316 BJ226ML-T

L1: TOKO A993AS-270M=P3

33µH

BIAS

GND

OFF ON

VIN BOOST

LT3470A

SWSHDN

0.22µF, 6.3V

22pF C2

22µF

3470a TA03

1µF

VIN

5.7V TO 40V

VOUT

250mA

604k

200k

33µH

BIAS

GND

OFF ON

C1: TDK C3216JB1H105M

C2: CE JMK316 BJ226ML-T

L1: TOKO A914BYW-330M=P3

VIN BOOST

LT3470A

SWSHDN

0.47µF, 6.3V

22pF C2

22µF

3470a TA04

1µF

VIN

4V TO 40V

VOUT

2.5V

250mA

200k

C1: TDK C3216JB1H105M

C2: TDK C2012JB0J226M

L1: SUMIDA CDRH3D28

33µH

BIAS

GND

OFF ON

M— M— 11H L7 LJUW LT347OA

LT3470A

3470afb

TYPICAL APPLICATIONS

1.8V Step-Down Converter

12V Step-Down Converter

VIN BOOST

LT3470A

SWSHDN

BIAS

0.22µF, 25V

22pF

22µF

3470a TA05

1µF

VIN

4V TO 23V

VOUT

1.8V

250mA

147k

332k

22µH

GND

OFF ON

C1: TDK C3216JB1H105M

C2: TDK C2012JB0J226M

L1: MURATA LQH32CN150K53

VIN BOOST

LT3470A

SWSHDN

0.22µF, 16V

22pF C2

10µF

3470a TA06

1µF

VIN

15V TO 34V

VOUT

12V

250mA

866k

100k

C1: TDK C3216JB1H105M

C2: TDK C3216JB1C106M

L1: MURATA LQH32CN150K53

33µH

BIAS

GND

OFF ON

LT3470A i ‘ 4 7‘ L 1 fgﬁﬁﬁﬁ% 4 T LAU 1U U,:T J ‘ 13‘ {My}? ‘—\ i film W] _ ,wq : “ 9‘ L ii L” “A FWD—77 L7LJCUEN2

LT3470A

3470afb

PACKAGE DESCRIPTION

DDB Package

8-Lead Plastic DFN (3mm × 2mm)

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

2.00 ±0.10

(2 SIDES)

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

0.40 ± 0.10

BOTTOM VIEW—EXPOSED PAD

0.56 ± 0.05

(2 SIDES)

0.75 ±0.05

R = 0.115

TYP

R = 0.05

TYP

2.15 ±0.05

(2 SIDES)

3.00 ±0.10

(2 SIDES)

PIN 1 BAR

TOP MARK

(SEE NOTE 6)

0.200 REF

0 – 0.05

(DDB8) DFN 0905 REV B

0.25 ± 0.05

2.20 ±0.05

(2 SIDES)

RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS

0.61 ±0.05

(2 SIDES)

1.15 ±0.05

0.70 ±0.05

2.55 ±0.05

PACKAGE

OUTLINE

0.25 ± 0.05

0.50 BSC

PIN 1

R = 0.20 OR

0.25 × 45°

CHAMFER

0.50 BSC

LT347OA L7 LJUW

LT3470A

3470afb

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.

REVISION HISTORY

REV DATE DESCRIPTION PAGE NUMBER

B 01/11 Added H-grade part information. 2 to 5, 15

(Revision history begins at Rev B)

LT347OA L7LJCUEN2

LT3470A

3470afb

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

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

LT 0111 REV B • PRINTED IN USA

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LT3470A Datasheet by Analog Devices Inc.

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