LTC3538EDCB-TR_技术文档

LTC3538

800mA Synchronous

Buck-Boost

DC/DC Converter

FEATURES

DESCRIPTION

The LTC^®3538 is a highly efﬁcient, low noise, buck-boost

DC/DC converter that operates from input voltages above,

below, and equal to the output voltage. The topology

incorporated in the IC provides a continuous transfer

function through all operating modes, making the product

ideal for single Lithium Ion or multicell Alkaline or NiMH

applications where the output voltage is within the battery

voltage range.

■

Regulated Output with Input Voltages Above,

Below, or Equal to the Output

■

800mA Continuous Output Current from a Single

Lithium-Ion/Polymer Cell

■

Single Inductor

1.8V to 5.25V V

■

Range

OUT

IN

■

2.4V to 5.5V V Range

1MHz Fixed Frequency Operation

Output Disconnect in Shutdown

35μA Quiesecent Current in Burst Mode Operation

<5μA Shutdown Current

The LTC3538 is suited for use in Micro Hard Disk Drive

(μHDD)applicationswithits800mAcurrentcapability.Burst

Mode^®operation provides high efﬁciency at light loads.

Internal Soft-Start

Small, Thermally Enhanced 8-Lead (2mm x 3mm)

DFN package

The LTC3538 includes two 0.17Ω N-channel and two

0.2Ω P-channel MOSFET switches. Operating frequency

is internally set to 1MHz to minimize solution footprint

while maximizing efﬁciency.

APPLICATIONS

Other features include <5μA shutdown current, internal

soft-start, short circuit protection and thermal shutdown.

The LTC3538 is available in a low proﬁle (0.75mm), ther-

mally enhanced 8-lead (2mm × 3mm) DFN package.

, LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technology

Corporation. All other trademarks are the property of their respective owners. Protected by

U.S. Patents including 5481178, 6304066, 6580258, 6166527, 6404251.

■

Miniature Hard Disk Drives

■

MP3 Players

■

Digital Cameras

■

Cellular Handsets

■

PDAs, Handheld PC

■

GPS Receivers

TYPICAL APPLICATION

Li-Ion/Polymer to 3.3V at 800mA

Efﬁciency vs V_IN

100

L1

3.3μH

V

LOAD

= 3.3V

= 200mA

OUT

I

V

OUT

3.3V

800mA

95

90

85

80

LTC3538

R1

464k

10k

SW1

SW2

V

IN

2.9V TO 4.2V

V

IN

OUT

FB

33pF

C

IN

C

OUT

10μF

22μF

PWM

BURST

GND

V

C

15k

330pF

SD

*

R2

200k

ON OFF

2.4

2.9

3.4

3.9

(V)

4.4

4.9

5.4

V

IN

3538 TA01

3538 TA01b

*μP OPEN DRAIN I/O

3538fb

1

LTC3538

ABSOLUTE MAXIMUM RATINGS

PIN CONFIGURATION

(Note 1)

TOP VIEW

V ,V

Voltage.......................................... –0.3V to 6V

IN OUT

SW1,SW2 Voltage

8

7

6

5

FB

VC

1

2

3

4

V

IN

DC............................................................ –0.3V to 6V

Pulsed < 100ns........................................ –0.3V to 7V

BURST, FB, VC Voltage................................. –0.3V to 6V

Operating Temperature (Note 2)............... –40°C to 85°C

Maximum Junction Temperature (Note 3)............. 125°C

Storage Temperature Range................... –65°C to 125°C

SW1

SW2

9

GND

BURST

V

OUT

DCB PACKAGE

8-LEAD (2mm × 3mm) PLASTIC DFN

T

= 125°C

JMAX

θ

= 75°C/W 4-LAYER BOARD, θ = 13.5°C/W

JA

JC

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

ORDER INFORMATION

LEAD FREE FINISH

LTC3538EDCB#PBF

LEAD BASED FINISH

LTC3538EDCB

TAPE AND REEL

PART MARKING

LCRB

PACKAGE DESCRIPTION

TEMPERATURE RANGE

LTC3538EDCB#TRPBF

TAPE AND REEL

8-Lead (2mm × 3mm) Plastic DFN

PACKAGE DESCRIPTION

–40°C to 85°C

PART MARKING

LCRB

TEMPERATURE RANGE

–40°C to 85°C

LTC3538EDCB#TR

8-Lead (2mm × 3mm) Plastic DFN

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/

ELECTRICAL CHARACTERISTICS ^The● denotes the speciﬁcations which apply over the full operating

temperature range, otherwise speciﬁcations are at T_A= 25°C. V_IN= V_OUT= 3.6V, BURST = 0V, unless otherwise noted.

PARAMETER

CONDITIONS

MIN

2.4

TYP

MAX

5.5

1.020

50

UNITS

V

●

Input Voltage

Feedback Voltage

Feedback Input Current

(Note 4)

0.980

1.00

1

V

nA

μA

mA

μA

Ω

V

Quiescent Current – Shutdown

Quiescent Current – Active

Quiescent Current – Sleep

V = 0V, Not Including Switch Leakage

C

1.5

1

5

IN

FB = 0.8V

1.8

60

FB = 1.2V, BURST = V

Switches B and C

Switches A and D

Switches B and C

Switches A and D

35

IN

NMOS Switch Leakage

0.1

0.17

0.2

2

7

PMOS Switch Leakage

10

NMOS Switch On-Resistance

PMOS Switch On-Resistance

Input Current Limit

Ω

●

1.4

A

Reverse Current Limit

0.5

0.9

88

A

Burst Mode Operational Peak Current

Maximum Duty Cycle

A

●

Boost (%Switch C On)

Buck (% Switch A On)

Buck (% Switch D On)

70

100

%

3538fb

2

LTC3538

ELECTRICAL CHARACTERISTICS ^The● denotes the speciﬁcations which apply over the full operating

temperature range, otherwise speciﬁcations are at T_A= 25°C. V_IN= V_OUT= 3.6V, unless otherwise noted.

PARAMETER

CONDITIONS

MIN

TYP

MAX

0

UNITS

%

●

Minimum Duty Cycle

Frequency Accuracy

Internal Soft-Start Time

FB = 1.2V

0.8

1

1.2

MHz

ms

dB

μA

V

1.5

80

Error Amp A

VOL

Error Amp Source Current

Error Amp Sink Current

V = 1.5V, FB = OV

C

–13

130

V = 1.5V, FB = 1.2V

C

●

V Shutdown Threshold (Off)

C

IC is Disabled

0.25

–3

V Output Current in Shutdown

C

V = GND

C

–1

μA

V

●

BURST Threshold (High)

BURST Threshold (Low)

BURST Input Current

1.4

0.4

1

V

= 3.6V

0.1

μA

BURST

Note 3: This IC includes over-temperature protection that is intended

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 LTC3538 is guaranteed to meet performance speciﬁcations

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

temperature range are assured by design, characterization and correlation

with statistical process controls.

to protect the device during momentary overload conditions. Junction

temperature will exceed 125°C when over-temperature protection is active.

Continuous operation above the speciﬁed maximum operating junction

temperature may result in device degradation or failure.

Note 4: The IC is tested in a feedback loop to make the measurement.

TYPICAL PERFORMANCE CHARACTERISTICS ^T_A^{= 25°C unless otherwise noted}

Efﬁciency and Power Loss vs

Load Current

Switch Pins Before Entering

Boost Mode

Li-Ion to 3.3V Efﬁciency

100

90

80

70

60

50

40

30

20

10

100

90

80

70

60

50

40

30

20

10

0

1000

100

10

FIXED FREQUENCY

SW1

2V/DIV

Burst Mode

OPERATION

SW2

2V/DIV

Burst Mode

OPERATION

3538 G03

POWER LOSS

FIXED FREQUENCY

50ns/DIV

= 3.3V AT 500mA

V

= 2.9V

IN

OUT

1

V

= 2.7V

= 3.6V

= 4.2V

IN

POWER LOSS BURST

0.1

1000

0.1

1

10

100

1000

0.1

1

10

100

LOAD CURRENT (mA)

3538 G01

3538 G02

3538fb

3

LTC3538

TYPICAL PERFORMANCE CHARACTERISTICS ^T_A^{= 25°C unless otherwise noted}

Switch Pins Before Entering

Buck Mode

V_OUTRipple in Buck, Buck-Boost

and Boost Modes at 500mA Load

Burst Mode Sleep Current vs

Temperature

45

40

35

30

V

= 2.5V

= 3.3V

= 4.2V

IN

SW1

2V/DIV

SW2

2V/DIV

3538 G05

3538 G04

1μs/DIV

50ns/DIV

V

= 3.3V, AC-COUPLED

V

= 3.9V

OUT

IN

OUT

20mV/DIV

= 3.3V AT 500mA

25

20

C

= 22μF

= 500mA

OUT

I

LOAD

–50

–25

0

25

50

75

100

TEMPERATURE (°C)

3538 G16

Error Ampliﬁer Source Current vs

Temperature

Oscillator Frequency vs

Temperature

Feedback Voltage vs Temperature

–12.5

–13.0

–13.5

–14.0

1025

1000

975

1.010

1.005

1.000

0.995

0.990

V

IN

= V

= 3.6V

OUT

–14.5

–15.0

–50

–25

0

25

50

75

100

–50

–25

0

25

50

75

100

–50

–25

0

25

50

75

100

TEMPERATURE (°C)

3538 G06

3538 G07

3538 G08

Maximum Output Current

Capability vs V_IN

Feedback Voltage Line

Regulation

Minimum Start-Up Voltage

0.4

0.3

0.2

0.1

0

2.3045

2.3040

2.3035

2.3030

2.3025

2.3020

2.3015

2.3010

2.3005

1800

1600

1400

1200

1000

800

600

400

200

0

V

OUT

= 3.3V

V

= 3.3V

OUT

–0.1

–0.2

2.4

3.4

4.4

5.4

–50

–25

0

25

50

75

100

2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5

(V)

V

(V)

TEMPERATURE (°C)

V

IN

3538 G09

3538 G10

3538 G17

3538fb

4

LTC3538

TYPICAL PERFORMANCE CHARACTERISTICS ^T_A^{= 25°C unless otherwise noted}

V_COn/Off Threshold vs

Temperature

Load Transient in Fixed

Frequency Mode

Current Limit vs Temperature

0.80

0.75

0.70

0.65

0.60

0.55

0.50

0.45

0.40

4.0

3.5

3.0

2.5

2.0

1.5

V

= V

= 3.6V

OUT

IN

V

OUT

100mV/DIV

PEAK CURRENT LIMIT

V

ON THRESHOLD

C

I

LOAD

200mA/DIV

3538 G13

100μs/DIV

V

OFF THRESHOLD

C

V

I

= 3.3V

IN

OUT

= 3.3V

= 0mA TO 500mA

= 22μF X5R CERAMIC

LINEAR CURRENT LIMIT

LOAD

OUT

C

–50

0

50

100

–50

–25

0

25

50

75

100

TEMPERATURE (°C)

3538 G10

3538 G12

Transition From Burst Mode

Operation to Fixed Frequency

Burst Mode Operation

BURST

2V/DIV

V

OUT

50mV/DIV

V

OUT

100mV/DIV

I

L

500mA/DIV

3538 G14

3538 G15

10μs/DIV

50μs/DIV

V

I

= 3.3V

V

I

= 3.3V

IN

OUT

IN

OUT

= 3.3V

= 10mA

= 30mA

LOAD

C

= 22μF X5R CERAMIC

C

= 22μF X5R CERAMIC

OUT

3538fb

5

LTC3538

PIN FUNCTIONS

FB (Pin 1): Feedback Input to Error Amplifer. Connect

SW2 (Pin 6): Switch Pin where the Internal Switches

resistive divider tap from V

to this pin to set the output

C and D are Connected. An optional Schottky diode can

OUT

voltage. The output voltage can be adjusted from 1.8V to

5.25V. Referring to the Block Diagram the output voltage

is given by:

be connected from SW2 to V

for a moderate efﬁciency

OUT

improvement. Keep the trace length as short as possible

to minimize EMI.

V

= 1V • (1 + R1/R2)

SW1(Pin7):SwitchPinwheretheInternalSwitchesAand

B are Connected. Connect an inductor from SW1 to SW2.

AnoptionalSchottkydiodecanbeconnectedfromSW1to

ground for a moderate efﬁciency improvement. Keep the

trace length as short as possible to minimize EMI.

OUT

V (Pin2):ErrorAmpliﬁerOutput.Afrequencycompensa-

C

tion network should be connected between this pin and FB

to compensate the loop. See Closing the Feedback Loop

section of the datasheet for further information. Pulling

V below 0.25V disables the LTC3538.

V

IN

(Pin 8): Input Supply. This input provides power to

C

the IC and also supplies current to switch A. A ceramic

bypass capacitor (4.7μF or larger) is recommended as

GND (Pin 3): Ground.

BURST (Pin 4): Burst Mode Select Input.

BURST = Low for ﬁxed frequency PWM operation

BURST = High for Burst Mode operation

close to V and GND as possible.

IN

Exposed Pad (Pin 9): GND. The exposed pad must be

electrically connected to the board ground for proper

electrical and thermal performance.

V

(Pin 5): Power Supply Output. This pin should be

OUT

connected to a low ESR output capacitor. The capacitor

should be placed as close to the IC as possible and should

have a short return to GND.

3538fb

6

LTC3538

BLOCK DIAGRAM

L1

SW1

SW2

7

6

ANTI-RING

V

OUT

V

IN

V

OUT

A

D

8

5

+

V

IN

2.4V TO 5.5V

GATE DRIVERS

AND

C

IN

0.5A

ANTICROSS

CONDUCTION

C

B

+

–

R1

REVERSE

CURRENT

LIMIT

C

Z1

C

OUT

AVERAGE

CURRENT LIMIT

+

–

2A

3.5A

2.3V

PEAK

–

+

–

CURRENT LIMIT

+

–

PWM LOGIC

AND

OUTPUT PHASING

1V

PWM

COMPARATORS

FB

–

+

1

2

+

–

UVLO

C

P1

C

P2

R

SOFT-START

R2

Z

V

C

THERMAL

SHUTDOWN

OSC

1MHz

OFF ON

BURST

INTERNAL

SOFT-START

TSD

UVLO

SLEEP

BURST

5μs

DELAY

4

MODE

SS DONE

CONTROL

1 = BurstMode OPERATION

0 = FIXED FREQUENCY

FB

GND

3

3538 BD

3538fb

7

LTC3538

OPERATION

Internal Current Limit

The LTC3538 provides high efﬁciency, low noise power

for a wide variety of handheld electronic devices. The LTC

proprietary topology allows input voltages above, below

and equal to the output voltage through proper phasing

of the four on-chip MOSFET switches. The error ampliﬁer

TherearetwocurrentlimitcircuitsintheLTC3538.Theﬁrst

is a high speed peak current limit ampliﬁer that will shut

off switch A once the input current exceeds ~ 3.5A typical.

The delay to output of this ampliﬁer is typically 50ns.

outputvoltageonV determinestheoutputdutycycleofthe

C

The second current limit sources current out of the FB pin

to drop the output voltage once the input average current

exceeds 2A typical. This method provides a closed loop

means of clamping the input current. During conditions

switches. Since V is a ﬁltered signal, it provides rejection

C

of frequencies from well below the switching frequency.

The low R

, low gate charge synchronous switches

DS(ON)

provide high frequency pulse width modulation control at

high efﬁciency. High efﬁciency is achieved at light loads

when Burst Mode operation is selected.

when V

is near ground, such as during a short circuit

OUT

or during start-up, this threshold is cut to 1A typical,

providingafoldbackfeaturetolimitpowerdissipation. For

this current limit feature to be most effective, the Thevenin

resistance (typically the parallel combination of R1 and

R2) from FB to ground should be greater than 100k.

LOW NOISE FIXED FREQUENCY OPERATION

Operating Frequency

The operating frequency is internally ﬁxed to 1MHz to

maximize overall converter efﬁciency while minimizing

external component size.

Reverse Current Limit

Duringﬁxedfrequencyoperation,theLTC3538operatesin

forced continuous conduction mode. The reverse current

limit comparator monitors the inductor current from the

output through switch D. Should this negative inductor

current exceed 500mA typical, the LTC3538 shuts off

switch D.

Error Ampliﬁer

The error ampliﬁer controls the duty cycle of the internal

switches. The loop compensation components are con-

ﬁgured around the ampliﬁer to provide converter loop

stability. Pulling down the output of the error ampliﬁer

Four-Switch Control

(V ) below 0.25V will disable the LTC3538. In shutdown

C

V

IN

OUT

5

the LTC3538 will draw only 1.5μA typical from the input

8

supply. During normal operation the V pin should be

C

allowed to ﬂoat.

PMOS A

PMOS D

SW1

7

SW2

6

L1

Soft-Start

The converter has an internal voltage mode soft-start

circuit with a nominal duration of 1.5ms. The converter

remains in regulation during soft-start and will therefore

respond to output load transients that occur during this

time. In addition, the output voltage risetime has minimal

dependency on the size of the output capacitor or load.

During soft-start, the converter is forced into PWM

operation regardless of the state of the BURST pin.

NMOS B

NMOS C

3538 FO1

Figure 1. Simpliﬁed Diagram of Output Switches

Figure1showsasimpliﬁeddiagramofhowthefourinternal

switchesareconnectedtotheinductor,V ,V andGND.

IN OUT

Figure 2 shows the regions of operation for the LTC3538

as a function of the internal control voltage.

3538fb

8

LTC3538

OPERATION

Depending on the V voltage, the LTC3538 will operate in

Buck-Boost or Four Switch (V ~ V

)

OUT

C

IN

either buck, buck-boost or boost mode. The four power

switches are properly phased so the transfer between

operatingmodesiscontinuous,smoothandtransparentto

When the control voltage, V , is above voltage V2, switch

C

pairADremainsonfordutycycleD

,andtheswitch

MAX_BUCK

pair AC begins to phase in. As switch pair AC phases in,

theuser.WhenV approachesV thebuck-boostregion

IN

OUT

switch pair BD phases out accordingly. When V reaches

C

is entered, where the conduction time of the four-switch

region is typically 150ns. Referring to Figures 1 and 2, the

various regions of operation will now be described.

the edge of the buck-boost range, at voltage V3, the AC

switchpaircompletelyphaseouttheBDpair,andtheboost

phase begins at duty cycle D4 . The input voltage, V ,

SW

IN

where the four switch region begins is given by:

88%

D

MAX

V4 (~2.2V)

V = V (1 – D4 ) ≈ 0.85 • V

OUT

BOOST

IN

OUT

SW

A ON, B OFF

PWM C, D SWITCHES

BOOST REGION

The point at which the four-switch region ends is given

by:

D

MIN

V3 (~1.8V)

V2 (~1.7V)

BOOST

BUCK-BOOST

REGION

FOUR-SWITCH PWM

D

MAX

V_OUT

1−D4_SW

BUCK

V =

V ≈1.18• V_OUT

IN

D ON, C OFF

PWM A, B SWITCHES

BUCK REGION

0%

V1 (~1.2V)

Boost Region (V < V

)

OUT

IN

DUTY

CYCLE

CONTROL

VOLTAGE, V

3538 F02

C

Switch A is always on and switch B is always off during

this mode. When the control voltage, V , is above volt-

Figure 2. Switch Control vs Control Voltage, V_C

C

age V3, switch pair CD will alternately switch to provide

a boosted output voltage. This operation is typical to a

synchronous boost regulator. The maximum duty cycle

of the converter is limited to 88% typical and is reached

Buck Region (V > V

)

OUT

IN

Switch D is always on and switch C is always off during

this mode. When the control voltage, V , is above volt-

C

when V is above V4.

C

age V1, output A begins to switch. During the off time of

switchA,synchronousswitchBturnsonfortheremainder

of the period. Switches A and B will alternate similar to a

typical synchronous buck regulator. As the control volt-

age increases, the duty cycle of switch A increases until

the maximum duty cycle of the converter in buck mode

Burst Mode OPERATION

BurstModeoperationreducesquiescentcurrentconsump-

tion of the LTC3538 at light loads and improves overall

conversionefﬁciency, increasingbatterylife. DuringBurst

Mode operation the LTC3538 delivers energy to the out-

put until it is regulated and then goes into a sleep mode

where the outputs are off and the quiescent current drops

to 35μA. In this mode the output ripple has a variable

frequency component that depends upon load current,

and will typically be about 2% peak-to-peak. Burst Mode

operation ripple can be reduced slightly by using more

output capacitance. Another method of reducing Burst

Mode operation ripple is to place a small feed-forward

reaches D , given by:

MAX_BUCK

D

= 100 – D4

%

MAX_BUCK

SW

where D4 = duty cycle % of the four switch range.

SW

D4 = (150ns • f) • 100 %

SW

where f = operating frequency, Hz.

Beyond this point the four switch, or buck-boost region

is reached.

capacitor across the upper resistor in the V

feedback

OUT

divider network (as in Type III compensation).

3538fb

9

LTC3538

OPERATION

During the period when the LTC3538 is delivering energy

to the output, the peak inductor current will be equal to

800mA typical and the inductor current will terminate

each cycle at zero current. In Burst Mode operation the

maximum average output current that can be delivered

while maintaining output regulation is given by:

Inadditiontoaffectingoutputcurrentripple, thesizeofthe

inductor can also affect the stability of the feedback loop.

In boost mode, the converter transfer function has a right

halfplanezeroatafrequencythatisinverselyproportional

to the value of the inductor. As a result, a large inductor

can move this zero to a frequency low enough to degrade

the phase margin of the feedback loop. It is recommended

that the inductor value be chosen less than 10μH.

V

IN

I

_{OUT _BURST(BOOST)}= 0.25•

A; V_OUT> V

IN

V_OUT

_{OUT _BURST(BUCK)}= 0.27A; V_OUT< V

For high efﬁciency, choose a ferrite inductor with a high

frequency core material to reduce core loses. The induc-

tor should have low ESR (equivalent series resistance) to

I

IN

The maximum average Burst Mode output current that

can be delivered in the four-switch buck-boost region is

limited to the boost equation speciﬁed above.

2

reduce the I R losses, and must be able to handle the peak

inductorcurrentwithoutsaturating.Moldedchokesorchip

inductors usually do not have enough core to support the

peak inductor currents in the 1A to 2A region. To minimize

radiated noise, use a shielded inductor. See Table 1 for a

suggested list of inductor suppliers.

INDUCTOR SELECTION

To achieve high efﬁciency, a low ESR inductor should be

utilized for the converter. The inductor must have a satura-

tion rating greater than the worst case average inductor

current plus half the ripple current. The peak-to-peak cur-

rent ripple will be larger in buck and boost mode than in

the buck-boost region. The peak-to-peak inductor current

ripple for each mode can be calculated from the following

formulas, where f is the frequency (1MHz typical) and L

is the inductance in μH.

Output Capacitor Selection

The bulk value of the output ﬁlter capacitor is selected to

reduce the ripple due to charge into the capacitor each

cycle. The steady state ripple due to charge is given by:

ΔV

I

• (V

– V )/(C

• V

• f)V

OUT

P-P, BOOST = LOAD

OUT

IN

OUT

2

= (V – V ) • V /(8 • L • V • C • f )V

OUT

P-P,BUCK

IN

OUT

IN

where C

= output ﬁlter capacitor, F

V_OUT• V – V

/ V

IN

OUT

(

)

IN

OUT

ΔI_L,P-P,BUCK

=

A

I

= Output load current, A

f •L

LOAD

V_OUT• V_OUT– V / V

(

)

IN

OUT

ΔI_L,P-P,BOOST

=

A

f •L

where f = frequency (1MHz typical), Hz

L = inductor, H

Table 1. Inductor Vendor Information

SUPPLIER

Coilcraft

PHONE

FAX

WEB SITE

(847) 639-6400

(800) 227-7040

(847) 639-1469

(650) 361-2508

(814) 238-0490

www.coilcraft.com

CoEv Magnetics

Murata

www.tycoelectronics.com

www.murata.com

(814) 237-1431

(800) 831-9172

Sumida

USA: (847) 956-0666

Japan: 81 (3) 3607-5111

USA: (847) 956-0702

Japan: 81(3) 3607-5144

www.sumida.com

TDK

(847) 803-6100

(847) 803-6296

(847) 699-7864

www.component.tdk.com

www.tokoam.com

TOKO

(847) 297-0070

3538fb

10

LTC3538

OPERATION

Since the output current is discontinuous in boost mode,

the ripple in this mode will generally be much larger than

the magnitude of the ripple in buck mode.

importantly, leakage and parasitic capacitance need to

be minimized. During start-up, 1.5μA is typically sourced

from V . The leakage of an external pull-down device and

C

compensation components tied to V , must therefore be

C

Minimizing solution size is usually a priority. Please be

aware that ceramic capacitors can exhibit a signiﬁcant

reduction in effective capacitance when a bias is applied.

The capacitors exhibiting the highest reduction are those

packaged in the smallest case size.

minimized to ensure proper start-up. Capacitance from

the pull-down device should also be minimized as it can

affect converter stability. An N-channel MOSFET such as

the FDV301N or similar is recommended if an external

discrete N-channel MOSFET is needed.

Input Capacitor Selection

PCB Layout Considerations

SinceV isthesupplyvoltagefortheICitisrecommended

The LTC3538 switches large currents at high frequencies.

Special care should be given to the PCB layout to ensure

stable, noise-free operation. Figure 3 depicts the recom-

mended PCB layout to be utilized for the LTC3538. A few

key guidelines follow:

IN

to place at least a 4.7μF, low ESR ceramic bypass capaci-

tor close to V and GND. It is also important to minimize

IN

any stray resistance from the converter to the battery or

other power source.

Optional Schottky Diodes

1. All circulating current paths should be kept as short as

possible. This can be accomplished by keeping the routes

to all components (except the FB divider network) in

Figure3asshortandaswideaspossible.Capacitorground

connections should via down to the ground plane in the

Schottky diodes across the synchronous switches B and

D are not required, but do provide a lower drop during the

break-before-make time (typically 15ns), thus improving

efﬁciency. Use a surface mount Schottky diode such as an

MBRM120T3 or equivalent. Do not use ordinary rectiﬁer

diodes since their slow recovery times will compromise

efﬁciency.

shortestroutepossible.ThebypasscapacitoronV should

IN

be placed as close to the IC as possible and should have

the shortest possible paths to ground.

2. Thesmallsignalgroundpad(GND)shouldhaveasingle

point connection to the power ground. A convenient way

to achieve this is to short this pin directly to the Exposed

Pad as shown in Figure 3.

Table 2. Capacitor Vendor Information

SUPPLIER PHONE

FAX

WEB SITE

AVX

(803) 448-9411 (803) 448-1943 www.avxcorp.com

(619) 661-6322 (619) 661-1055 www.sanyovideo.com

(408) 573-4150 (408) 573-4159 www.t-yuden.com

Sanyo

3. The components in bold and their connections should

all be placed over a complete ground plane.

Taiyo

Yuden

TDK

(847) 803-6100 (847) 803-6296 www.component.tdk.com

4. To prevent large circulating currents from disrupting

the output voltage sensing, the ground for the resistor

divider should be returned directly to the small signal

ground (GND) as shown.

Shutdown MOSFET Selection

A discrete external N-channel MOSFET, open-drain pull-

down device or other suitable means can be used to put

5. Use of vias in the attach pad will enhance the thermal

environment of the converter especially if the vias extend

to a ground plane region on the exposed bottom surface

of the PCB.

the part in shutdown by pulling V below 0.25V. Since

C

the error ampliﬁer sources 13μA typically when active

and 1.5μA in shutdown, a relatively high resistance pull-

down device can be used to pull V below 0.25V. More

C

3538fb

11

LTC3538

OPERATION

V

IN

ƒ_{FILTER _POLE}

=

Hz

2• V_OUT•π• L•C_OUT

(in boost mode)

where L is in Henries and C

1

FB

8

IN

V

is in Farads.

OUT

The output ﬁlter zero is given by:

2

7

V

C

SW1

1

ƒ_{FILTER _ ZERO}

=

Hz

2•π•R_ESR•C_OUT

3

GND

6

SW2

where R

is the equivalent series resistance of the

ESR

V

4

5

OUT

output capacitor.

BURST

V

Atroublesomefeatureinboostmodeistheright-halfplane

zero (RHP), given by:

2

3538 F03

V

IN

ƒ_RHPZ

=

Hz

VIA TO GND PLANE

2•π•I_OUT•L• V_OUT

Figure 3. LTC3538 Recommended PCB Layout

The loop gain is typically rolled off before the RHP zero

frequency.

Closing the Feedback Loop

The LTC3538 incorporates voltage mode PWM control.

The control to output gain varies with operation region

(buck, boost, buck-boost), but is usually no greater than

15. The output ﬁlter exhibits a double pole response, as

given by:

AsimpleTypeIcompensationnetworkcanbeincorporated

tostabilizetheloop,butatacostofreducedbandwidthand

slowertransientresponse.Toensureproperphasemargin

using Type I compensation, the loop must be crossed

over a decade before the LC double pole. Referring to

Figure 4, the unity-gain frequency of the error ampliﬁer

with the Type I compensation is given by:

1

ƒ_{FILTER _POLE}

=

Hz

2•π• L•C_OUT

1

ƒ_UG

=

Hz

(in buck mode)

2•π•R1•C_P1

V

OUT

1V

+

–

R1

FB

1

R2

C

P1

V

C

2

3538 F04

Figure 4. Error Ampliﬁer with Type I Compensation

3538fb

12

LTC3538

OPERATION

Mostapplicationsdemandanimprovedtransientresponse

toallowasmalleroutputﬁltercapacitor.Toachieveahigher

bandwidth, Type III compensation is required, providing

two zeros to compensate for the double-pole response of

the output ﬁlter. Referring to Figure 5, the location of the

poles and zeros are given by:

1

ƒ_ZERO1

ƒ_ZERO2

ƒ_POLE2

=

Hz

2•π•R_Z•C_P1

1

2•π•R1•C_Z1

1

2•π•R_Z•C_P2

1

ƒ_POLE1

≅

Hz

2•π•32e³•R1•C_P1

where resistance is in Ohms and capacitance is in

Farads.

(which is extremly close to DC)

V

OUT

R1

C

Z1

1V

FB

+

–

1

R2

C

P2

C

P1

V

C

RZ

2

3538 F05

Figure 5. Error Ampliﬁer with Type III Compensation

3538fb

13

LTC3538

TYPICAL APPLICATION

High Efﬁciency 5V/500mA from USB Input

L1

3.3μH

V

OUT

5V, 500mA

LTC3538

R1

806k

10k

33pF

SW1

SW2

USB

4.35V TO 5.25V

V

IN

V

OUT

C

IN

C

OUT

10μF

FB

22μF

PWM

BURST

V

C

15k

GND

330pF

R2

200k

ON OFF

M1

1Ω

3538 TA03

C

: TAIYO YUDEN JMK212BJ106MG

IN

: TAIYO YUDEN JMK325BJ226MM

OUT

L1: SUMIDA CDRH2D18/HP-3R3NC

M1: μP OPEN DRAIN I/O OR FAIRCHILD FDV301N

3538fb

14

LTC3538

PACKAGE DESCRIPTION

DCB Package

8-Lead Plastic DFN (2mm × 3mm)

(Reference LTC DWG # 05-08-1718 Rev A)

0.70 0.05

1.35 0.05

1.65 0.05

3.50 0.05

2.10 0.05

PACKAGE

OUTLINE

0.25 0.05

0.45 BSC

1.35 REF

RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS

APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED

R = 0.115

2.00 0.10

(2 SIDES)

0.40 0.10

TYP

5

R = 0.05

TYP

8

1.35 0.10

1.65 0.10

3.00 0.10

(2 SIDES)

PIN 1 NOTCH

R = 0.20 OR 0.25

× 45° CHAMFER

PIN 1 BAR

TOP MARK

(SEE NOTE 6)

(DCB8) DFN 0106 REV A

4

1

0.23 0.05

0.45 BSC

0.75 0.05

0.200 REF

1.35 REF

BOTTOM VIEW—EXPOSED PAD

0.00 – 0.05

NOTE:

1. DRAWING IS NOT A JEDEC PACKAGE OUTLINE

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

3538fb

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.

15

LTC3538

RELATED PARTS

PART NUMBER

DESCRIPTION

COMMENTS

V : 2.5V to 5.5V, V = 0.8V

OUT(MIN)

LTC3407

600mA (I ), 1.5MHz Dual Synchronous Step-Up DC/DC Converter

OUT

IN

I = 40μA, I ≤1μA, SC70 Package

Q

SD

LTC3410

300mA (I ), 2.25MHz Synchronous Step-Down DC/DC Converter in SC70 V : 2.5V to 5.5V, V

= 0.8V

SW

IN

OUT(MIN)

I = 26μA, I ≤1μA, MS Package

Q

SD

LTC3411

1.25A (I ), 4MHz Synchronous Step-Down DC/DC Converter

V : 2.625V to 5.5V, V

= 0.8V

OUT

IN

OUT(MIN)

I = 62μA, I ≤1μA, MS Package

Q

SD

LTC3412

2.5A (I ), 4MHz Synchronous Step-Down DC/DC Converter

V : 2.625V to 5.5V, V

= 0.8V

OUT

IN

OUT(MIN)

I = 62μA, I ≤1μA, TSSOP16E Package

Q

SD

LTC3421

3A (I ), 3MHz Synchronous Step-Up DC/DC Converter

V : 0.5V to 4.5V, V

= 5.25V

SW

IN

OUT(MAX)

I = 12μA, I <1μA, QFN Package

Q

SD

LTC3422

1.5A (I ), 3MHz Synchronous Step-Up DC/DC Converter with

V : 0.5V to 4.5V, V

= 5.25V

SW

IN

OUT(MAX)

Output Disconnect

I = 25μA, I <1μA, DFN Package

Q SD

LTC3425

5A (I ), 8MHz Multiphase Synchronous Step-Up DC/DC Converter

V : 0.5V to 4.5V, V

= 5.25V

SW

IN

OUT(MAX)

I = 12μA, I <1μA, QFN Package

Q

SD

LTC3427

500mA (I ), 1.25MHz Step-Up DC/DC Converter with

V : 1.8V to 5V, V

= 5.25V,

SW

IN

OUT(MAX)

Output Disconnect in 2mm × 2mm DFN

I = 350μA, I <1μA, DFN Package

Q SD

LTC3429

600mA (I ), 500KHz Synchronous Step-Up DC/DC Converter

V : 0.5V to 4.4V, V

= 5V

SW

IN

OUT(MAX)

I = 20μA, I <1μA, ThinSOT™ Package

Q

SD

LTC3440

600mA (I ), 2MHz Synchronous Buck-Boost DC/DC Converter

V : 2.5V to 5.5V, V : 2.5V to 5.5V

IN OUT

OUT

I = 25μA, I <1μA, MS, DFN Package

Q

SD

LTC3441/LTC3443

LTC3442

1.2A (I ), Synchronous Buck-Boost DC/DC Converters, LTC3441(1MHz), V : 2.5V to 5.5V, V : 2.4V to 5.25V

OUT IN OUT

LTC3443 (600kHz)

I = 25μA, I <1μA, DFN Package

Q SD

1.2A (I ), 2MHz Synchronous Buck-Boost DC/DC Converter

V : 2.4V to 5.5V, V : 2.4V to 5.25V

IN OUT

I = 28μA, I <1μA, MS Package

OUT

Q

SD

LTC3522

400mA, Synchronous Buck-Boost and 200mA Buck Converters

V : 2.4V to 5.5V, V

Buck-Boost: 2.2V to 5.25V,

IN

OUT

I = 25μA, I <1μA, DFN Package

Q

SD

LTC3525

400mA (I ), Synchronous Step-Up DC/DC Converter with

V : 0.5V to 4.5V, V

= 3, 3.3, 5V

SW

IN

OUT

Output Disconnect

I = 7μA, I <1μA, SC70 Package

Q SD

LTC3526/LTC3526B

LTC3530

500mA (I ), 1MHz Synchronous Step-Up DC/DC Converter with

V : 0.5V to 4.5V, V : 1.6V to 5.25V

IN OUT

SW

Output Disconnect in 2mm × 2mm DFN

I = 9μA, I <1μA, DFN Package

Q SD

600mA (I ), 2MHz Synchronous Buck-Boost DC/DC Converter

V : 1.8V to 5.5V, V : 1.6V to 5.25V

IN OUT

OUT

I = 40μA, I <1μA, DFN, MS Packages

Q

SD

LTC3531

200mA (I ) Synchronous Buck-Boost DC/DC Converter

V : 1.8V to 5.5V, V : 2V to 5V

IN OUT

OUT

I = 16μA, I <1μA, DFN, ThinSOT Packages

Q

SD

LTC3532

500mA (I ), 2MHz Synchronous Buck-Boost DC/DC Converter

V : 2.4V to 5.5V, V : 2.2V to 5.25V

IN OUT

OUT

I = 35μA, I <1μA, DFN, MS Packages

Q

SD

LTC3533

2A (I ), 2MHz Synchronous Buck-Boost DC/DC Converter

V : 1.8V to 5.5V, V : 1.6V to 5.25V

IN OUT

OUT

I = 40μA, I <1μA, DFN Package

Q

SD

ThinSOT is a trademark of Linear Technology Corporation.

3538fb

LT 1007 REV B • PRINTED IN USA

LinearTechnology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

16

●

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


型号：	LTC3538EDCB-TR
厂家：	Linear
描述：	800mA Synchronous Buck-Boost DC/DC Converter 800毫安同步降压 - 升压型DC / DC转换器转换器
文件：	总16页 (文件大小：207K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

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