LTC3252EDE_技术文档

LTC3252

Dual, Low Noise,

Inductorless Step-Down

DC/DC Converter

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FEATURES

DESCRIPTIO

The LTC^®3252 is a switched capacitor step-down DC/DC

converter that produces two adjustable regulated outputs

from a single 2.7V to 5.5V input. The part uses switched

capacitor fractional conversion to achieve a typical effi-

ciency increase of 50% over that of a linear regulator. No

inductors are required.

■

2.7V to 5.5V Input Voltage Range

■

No Inductors

■

Typical Efficiency 50% Higher than LDOs

■

Spread Spectrum Operation

Low Input and Output Noise

■

Shutdown Disconnects Load from V_IN

■

Dual Adjustable Independent Outputs

A unique constant frequency, spread spectrum architec-

ture provides a very low noise regulated output as well as

low noise at the input. The part also has Burst Mode^®

operationtoprovidehighefficiencyatlowoutputcurrents,

as well as ultralow current shutdown.

(Range: 0.9V to 1.6V)

■

Output Current: 250mA Each Output

■

Low Operating Current: I_IN= 60µA Typ

(35µA with One Output Enabled)

■

Low Shutdown Current: I_IN= 0.01µA Typ

■

Low operating currents (60µA with both outputs enabled,

35µA with one output enabled) and low external parts

count make the LTC3252 ideally suited for space-con-

strained battery-powered applications. The part is short-

circuit and overtemperature protected and is available in a

tiny 4mm × 3mm 12-pin DFN package.

Soft-Start Limits Inrush Current at Turn On

Short Circuit and Over Temperature Protected

Available in 4mm × 3mm 12-Pin DFN Package

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APPLICATIO S

■

Handheld Electronic Devices

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

Burst Mode is a registered trademark of Linear Technology Corporation.

■

Cellular Phones

■

Low Voltage Logic Supplies

■

DSP Power Supplies

■

3.3V to 1.5V Conversion

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TYPICAL APPLICATIO

1.5V and 1.2V Output Voltages with Shutdown

1.5V/1.2V Efficiency vs Input Voltage

100

I

(1.5V) = 100mA

(1.2V) = 100mA

2

11

3

5

1

OUT

V

= 1.5V

OUT

90

80

70

60

50

40

30

20

10

0

EN1

EN2

OUT1

FB1

OFF ON

I

≤ 250mA

470k

510k

LTC3252-1.5V

1-CELL

Li-ION

OR 3-CELL

NiMH

V

4.7µF

IN

4

6

4.7µF

+

–

+

–

LTC3252-1.2V

C1

C2

LTC3252

1µF

LDO-1.5V

LDO-1.2V

10

8

9

V

= 1.2V

≤ 250mA

OUT

OUT2

FB2

I

OUT

261k

510k

7

12

4.7µF

GND

2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5

(V)

3252 TA01

V

IN

3252 TA01a

3252f

1

LTC3252

W W U W

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W

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ABSOLUTE AXI U RATI GS

PACKAGE/ORDER I FOR ATIO

(Notes 1, 6)

ORDER PART

TOP VIEW

V_INto GND................................................–0.3V to 6.0V

EN1, EN2, FB1, FB2 to GND .......... –0.3V to (V_IN+ 0.3V)

I_OUT1, I_OUT2(Note 3)........................................... 400mA

Operating Ambient Temperature Range

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

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

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

NUMBER

FB1

EN1

1

2

3

4

5

6

12 FB2

11 EN2

LTC3252EDE

+

V

10 C2

IN

+

C1

9

8

7

OUT2

–

OUT1

C2

GND

–

DE PART

MARKING

C1

DE PACKAGE

12-LEAD (4mm × 3mm) PLASTIC DFN

3252

EXPOSED PAD IS GROUND

(MUST BE SOLDERED TO PCB)

T_JMAX= 125°C, θ_JA= 40°C/W, θ_JC= 4.3°C/W

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

ELECTRICAL CHARACTERISTICS

The ● denotes the specifications which apply over the full operating

temperature range, otherwise specifications are at T_A= 25°C. V_IN= 3.6V, V_OUT1= V_OUT2= 1.5V, C1 = C2 = 1µF, Cin = C_OUT1= C_OUT2

4.7µF (all capacitors ceramic) unless otherwise noted.

=

SYMBOL

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

V

Min Operating Voltage

Max Operating Voltage

(Note 4)

●

2.7

V

IN

5.5

I

Operating Current,

Both Outputs Enabled

I

= 0mA, V

= V , V

= V ,

60

35

100

µA

VIN

OUT

EN1

IN EN2

IN

2.7V ≤ V ≤ 5.5V

IN

Operating Current,

I

= 0mA, V

= 0, V

= V or V

= V ,

●

60

µA

OUT

EN1

EN2

IN

EN1

IN

One Output Enabled

V

= 0, 2.7V ≤ V ≤ 5.5V

EN2

IN

Shutdown Current

V

= 0V, V = 0V, 2.7V ≤ V ≤ 5.5V

●

0.01

0.8

1

µA

V

M0

M1

IN

V

, V

Feedback Voltage

I

= 0mA, 2.7V ≤ V ≤ 5.5V

0.78

250

–50

0.82

FB1 FB2

OUT

IN

I

Output Current

V

= V

mA

nA

OUT1

OUT2

FB

EN1

EN2

FB1

IN

Output Current

FB1, FB2 Input Current

Output Ripple (OUT1 or OUT2)

Spread Spectrum Frequency Range

= V = 0.85V

50

FB2

V

I

= 250mA

10

mV

P-P

RIPPLE

OUT

f

Switching Frequency

●

0.8

1.2

1.0

1.6

MHz

MIN

2.0

MAX

V

EN1, EN2 Input High Voltage

EN1, EN2 Input Low Voltage

EN1, EN2 Input High Current

EN1, EN2 Input Low Current

Turn On Time

2.7V ≤ V ≤ 5.5V

●

0.8

V

IH

IL

IN

2.7V ≤ V ≤ 5.5V

0.4

1

IN

I

t

EN1 = V , EN2 = V

IN

–1

µA

IH

IN

EN1 = 0V, EN2 = 0V

= 3Ω

1

µA

IL

R

0.8

0.08

0.2

1

ms

ON

OUT

OUT1, OUT2 Load Regulation (Referred to FB pin)

Line Regulation

mV/mA

%/V

Ω

0 ≤ I

≤ 250mA or 0 ≤ I

≤ 250mA

OUT2

OUT1

R

Open Loop Output Impedance,

(OUT1 or OUT2)

V

= 3.0V, I

= 200mA, V = 0.74V (Note 5)

●

1.4

OL

IN

OUT

FB

3252f

2

LTC3252

ELECTRICAL CHARACTERISTICS

Note 1: Absolute Maximum Ratings are those values beyond which the life

Note 4: Minimum operating voltage required for regulation is:

> 2 • (V + R • I

of a device may be impaired.

V

)

OL OUT

IN

OUT(MIN)

Note 2: The LTC3252EDE is guaranteed to meet specified performance

from 0°C to 70°C. Specifications over the –40°C and 85°C operating

temperature range are assured by design characterization and correlation

with statistical process control.

Note 5: Output not in regulation; R = (V /2 – V )/I .

OUT OUT

OL

IN

Note 6: This IC includes overtemperature protection that is intended to

protect the device during momentary overload conditions. Junction

temperature will exceed 125°C when overtemperature protection is active.

Continuous operation above the specified maximum operating junction

temperature may impair device reliability.

Note 3: Based on long-term current density limitations.

U W

TYPICAL PERFOR A CE CHARACTERISTICS

No Load Supply Current vs Supply

Voltage (One Output Enabled)

No Load Supply Current vs Supply

Voltage (Both Outputs Enabled)

FB Voltage vs Load Current

60

50

40

30

20

10

0

100

90

80

70

60

50

40

30

20

10

0

0.82

0.81

0.80

0.79

0.78

0.77

0.76

V

IN

= 3.6V

25°C

85°C

25°C

85°C

25°C

–40°C

–45°C

85°C

2.7

3.7

4.2

(V)

4.7

5.2

2.7

3.7

4.2

(V)

4.7

5.2

0

50

100

I

OUT

150

(mA)

200

250

3.2

V

IN

3252 G01

3252 G02

3252 G03

EN1/EN2 Input Threshold Voltage

vs Supply Voltage

1.5V Output Voltage vs Supply

Voltage

1.5V Output Efficiency vs Output

Current

1.2

1.1

1.0

0.9

0.8

0.7

0.6

0.5

0.4

1.60

1.58

1.56

1.54

1.52

1.50

1.48

1.46

1.44

1.42

1.40

100

80

60

40

20

0

T

= 25°C

A

T

= 25°C

A

25°C

I

= 50mA

OUT

–40°C

I

= 0mA

OUT

V

= 3.1V

= 3.3V

= 3.6V

= 4V

IN

I

= 250mA

85°C

OUT

= 5V

2.7

3.7

4.2

(V)

4.7

5.2

3

3.5

4

4.5

5

5.5

0.1

1

10

(mA)

100

1000

3.2

V

IN

(V)

I

OUT

IN

3252 G04

3252 G05

3252 G06

3252f

3

LTC3252

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TYPICAL PERFOR A CE CHARACTERISTICS

Oscillator Max/Min Frequency vs

Supply Voltage

1.2V Output Voltage vs Supply

Voltage

1.2V Efficiency vs Load Current

2.0

1.9

1.8

1.7

1.6

1.5

1.4

1.3

1.2

1.1

1.0

0.9

0.8

1.30

1.28

1.26

1.24

1.22

1.20

1.18

1.16

1.14

1.12

1.10

100

80

60

40

20

0

T

= 25°C

A

T

= 25°C

A

25°C MAX

–40°C MAX

I

= 0mA

OUT

85°C MAX

= 50mA

= 250mA

V

= 2.8V

= 3.1V

= 3.5V

= 4.5V

IN

25°C MIN

–40°C MIN

85°C MIN

2.7

3.7

4.2

(V)

4.7

5.2

2.7

3.7

4.2

(V)

4.7

5.2

0.1

1

10

(mA)

100

1000

3.2

V

IN

I

OUT

IN

3252 G07

3252 G08

3252 G09

Output Current Transient

Response

Line Transient Response

4.5V

3.5V

250mA

20mA

V

IN

I

OUT

V

OUT

20mV/DIV

AC

V

OUT

10mV/DIV

AC

3252 G12

3252 G11

V

I

= 1.5V

= 150mA

V

= 3.6V

OUT

IN

OUT

= 1.5V

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PI FU CTIO S

FB1 (Pin 1): Feedback Input Pin 1. An output divider C1⁺(Pin 4): Flying Capacitor 1 Positive Terminal (C1).

should be connected from OUT1 to FB1 to program the

output voltage.

OUT1 (Pin 5): Regulated Output Voltage 1. OUT1 is

disconnected from V_INwhen in shutdown. Bypass OUT1

EN1 (Pin 2): Input Enable Pin 1. When EN1 is high, OUT1 with a low ESR ceramic capacitor to GND (C_O1). See

is enabled. When EN1 is low OUT1 is shut down.

Output Capacitor Selection section for size requirements.

V_IN(Pin 3): Input Supply Voltage. Operating V_INmay be C1^–(Pin 6): Flying Capacitor 1 Negative Terminal (C1).

between 2.7V and 5.5V. Bypass V_INwith a ≥ 4.7µF (1µF

min) low ESR ceramic capacitor to GND (C_IN).

performance.

GND (Pin 7): Ground. Connect to a ground plane for best

3252f

4

LTC3252

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PI FU CTIO S

C2^–(Pin 8): Flying Capacitor 2 Negative Terminal (C2).

EN2 (Pin 11): Input Enable Pin 2. When EN2 is high, OUT2

is enabled. When EN2 is low OUT2 is shut down.

OUT2 (Pin 9): Regulated Output Voltage 2. OUT2 is

disconnected from V_INwhen in shutdown. Bypass OUT2

with a low ESR ceramic capacitor to GND (C_O2). See

Output Capacitor Selection section for size requirements.

FB2 (Pin 12): Feedback Input Pin 2. An output divider

should be connected from OUT2 to FB2 to program the

output voltage.

C2⁺(Pin 10): Flying Capacitor 2 Positive Terminal (C2).

W

SI PLIFIED BLOCK DIGRAM

EN1

2

EN2

11

SWITCH

CONTROL

SPREAD SPECTRUM

OSCILLATOR

OVERTEMPERATURE

CHARGE

PUMP 1

V

IN

3

+

4

5

6

C1

OUT1

–

C1

1

FB1

–

+

BURST

DETECT

CIRCUIT

CHARGE

PUMP 2

+

10 C2

9

8

OUT2

–

C2

12 FB2

–

+

7

GND

3252 SBD

3252f

5

LTC3252

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OPERATIO

(Refer to Simplified Block Diagram)

TheLTC3252hastwoswitchedcapacitorchargepumpsto

step down V_INto two regulated output voltages. The two

charge pumps operate 180° out of phase to reduce input

ripple. Regulation is achieved by sensing each output

voltage through an external resistor divider and modulat-

ing the charge pump output current based on the error

signal. A 2-phase nonoverlapping clock activates the two

chargepumpsrunningthemoutofphasefromeachother.

On the first phase of the clock current is transferred from

V_IN,throughtheexternalflyingcapacitor1,toOUT1viathe

switches of charge pump 1. Not only is current being

delivered to OUT1 on the first phase, but the flying capaci-

tor is also being charged up. On the second phase of the

clock, flying capacitor 1 is connected from OUT1 to

ground, transferring the charge stored during the first

phase of the clock to OUT1 via the switches of charge

pump 1. Charge pump 2 operates in the same manner to

supply current to OUT2, but with the phases of the clock

reversed relative to charge pump 1. Using this method of

switching, only half of the output current for each output

is delivered from V_IN, thus achieving a 50% increase in

efficiency over a conventional LDO. A spread spectrum

oscillator, which utilizes random switching frequencies

between 1MHz and 1.6MHz, sets the rate of charging and

discharging of the flying capacitors. This architecture

achievesextremelylowoutputnoise.Inputnoiseissignifi-

cantly reduced compared to conventional charge pumps.

Theoutputsalsohavealowcurrentburstmodetoimprove

efficiency even at light loads.

Short-Circuit/Thermal Protection

The LTC3252 has built-in short-circuit current limiting as

well as over temperature protection. During short-circuit

conditions, internal circuitry automatically limits each

output to approximately 500mA of current. If fault condi-

tions (such as shorted outputs) cause excessive self

heating on chip such that the junction temperature ex-

ceeds approximately 160°C, the thermal shutdown cir-

cuitry will disable the charge pumps. The IC resumes

operation once the junction temperature drops back to

approximately155°C. TheLTC3252willcycleinandoutof

thermal shutdown without latchup or damage until the

overstress condition is removed. Long term overstress

(I_OUT1or I_OUT2> 400mA, and/or T_J> 125°C) should be

avoided as it can degrade the performance or shorten the

life of the part.

Soft-Start

To prevent excessive current flow at V_INduring start-up,

the LTC3252 has built-in soft-start circuitry on each out-

put. When an output is enabled, the soft-start circuitry

increases the amount of current available from the output

linearly over a period of approximately 500µs. The soft-

start circuitry is disabled shortly after the output achieves

regulation.

Spread Spectrum Operation

Switchingregulatorscanbeparticularlytroublesomewhere

electromagnetic interference (EMI) is concerned. Switch-

ingregulatorsoperateonacycle-by-cyclebasistotransfer

power to an output. In most cases, the frequency of

operation is either fixed or is a constant based on the

output load. This method of conversion creates large

componentsofnoiseatthefrequencyofoperation(funda-

mental) and multiples of the operating frequency (har-

monics).

In shutdown mode all circuitry is turned off and the

LTC3252 draws only leakage current from the V_INsupply.

Furthermore, OUT1 and OUT2 are disconnected from V_IN.

The EN1 and EN2 pins are CMOS inputs with threshold

voltages of approximately 0.8V to allow regulator control

with low voltage logic levels. The LTC3252 is in shutdown

when a logic low is applied to both enable pins. Since the

mode pins are high impedance CMOS inputs, they should

neverbeallowedtofloat. Alwaysdrivetheenablepinswith

valid logic levels.

3252f

6

LTC3252

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OPERATIO

(Refer to Simplified Block Diagram)

Unlikeconventionalbuckconverters,theLTC3252’sinter-

nal oscillator is designed to produce a clock pulse whose

period is random on a cycle-by-cycle basis but fixed

between1MHzand1.6MHz.Thishasthebenefitofspread-

ing the switching noise over a range of frequencies, thus

significantly reducing the peak noise. Figures 1 and 2

show how the spread spectrum feature of the LTC3252

significantly reduces the peak harmonic noise and virtu-

ally elliminates harmonics compared to a conventional

buck converter.

threshold (30mA typ). When this occurs, the part shuts

down the internal oscillator and goes into a low current

operating state. The LTC3252 will remain in the low

current operating state until either output has dropped

enough to require another burst of current. The LTC3252

resumes continuous operation when the load on one or

both outputs exceeds the internally set threshold. Unlike

traditional charge pumps where the burst current is highly

dependant on many factors (i.e., supply, switch strength,

capacitor selection, etc.), the LTC3252’s burst current is

set by the burst threshold and hysteresis. This means that

the output ripple voltage in Burst Mode operation is

relatively consistent and is typically about 12mV with a

4.7µFoutputcapacitorona1.5Voutput.Theripplevoltage

amplitude is a direct function of the output capacitor size.

BurstModeoperationripplevoltagedoesincreaseslightly

at lower output voltages due to the increase in loop gain.

Userscancounteractoutputvoltagerippleincreasethrough

the use of a slightly larger output capacitor. See Recom-

mended Output Capacitance guidelines of Figure 3.

Spread spectrum operation is always enabled but is most

effective when the LTC3252’s outputs are out of Burst

Mode operation and the oscillator is running continuously

(see the Low Current Burst Mode Operation section).

Low Current Burst Mode Operation

Toimproveefficiencyatlowoutputcurrents,aBurstMode

operation function is included in the LTC3252. An output

current sense is used to detect when the required output

current of both outputs drop below an internally set

Figure 1. Conventional Buck Input Noise

Figure 2. LTC3252 Input Noise

3252f

7

LTC3252

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OPERATIO

(Refer to Simplified Block Diagram)

Output Capacitor Selection

tance required for good transient response (see the Ce-

ramic Capacitor Selection Guidelines section).

The style and value of capacitors used with the LTC3252

determineseveralimportantparameterssuchasregulator

control loop stability, output ripple and charge pump

strength.

Likewise excessive ESR on the output capacitor will tend

to degrade the loop stability of the LTC3252. The closed

loop output impedance of the LTC3252 is approximately:

The switching nature of the LTC3252 minimizes output

noise significantly but not completely. What small ripple

that exists at an output is controlled by the value of output

capacitordirectly. Increasingthesizeoftheoutputcapaci-

tor will proportionately reduce the output ripple. The ESR

(equivalentseriesresistance)oftheoutputcapacitorplays

the dominant role in output noise. When the LTC3252

switches between clock phases there is a period where all

switches are turned off. This “blanking period” shows up

asaspikeattheoutputandisadirectfunctionoftheoutput

current times the ESR value. To reduce output noise and

ripple, it is suggested that a low ESR (<0.08Ω) ceramic

capacitor be used for the output capacitor. Tantalum and

aluminum capacitors are not recommended because of

their high ESR.

V_OUT

R_O= 0.08Ω •

0.8V

For example, with the output programmed to 1.5V, the R_O

is 0.15Ω, which produces a 38mV output change for a

250mA load current step. For stability and good load

transient response it is important for the output capacitor

to have 0.1Ω or less of ESR. Ceramic capacitors typically

have exceptional ESR and combined with a tight board

layout should yield excellent stability and load transient

performance.

Furtheroutputnoisereductioncanbeachievedbyfiltering

the LTC3252 outputs through a very small series inductor

as shown in Figure 4. A 10nH inductor will reject the fast

output transients caused by the blanking period, thereby

presenting a nearly constant output voltage. For economy

the 10nH inductor can be fabricated on the PC board with

about 1cm (0.4") of PC board trace.

Both the style and value of the output capacitors can

significantly affect the stability of the LTC3252. As shown

in the Simplified Block Diagram, the LTC3252 uses a

control loop to adjust the strength of each charge pump to

match the current required at the output. The error signal

of each loop is stored directly on each output capacitor.

Thus the output capacitors also serve to form the domi-

nant pole in each control loop. Figure 3 is a graph of the

recommended output capacitance, and minimum capaci-

10nH

V

OUT

LTC3252

GND

OUT

4.7µF

0.47µF

3252 F04

Figure 4. 10nH Inductor Used for

Additional Output Noise Reduction

8

V_INCapacitor Selection

RECOMMENDED

CAPACITANCE

7

6

5

4

3

2

Thelownoise,dualphasearchitectureusedbytheLTC3252

makes input noise filtering much less demanding than

conventional charge pump regulators. The LTC3252 input

current will transition between I_OUT1/2 and I_OUT2/2 for

each half cycle of the oscillator. The blanking period

described in the V_OUTsection also effects the input. For

this reason it is recommended that a low ESR 4.7µF (1µF

min) or greater ceramic capacitor be used for C_IN(see the

Ceramic Capacitor Selection Guidelines section). Alumi-

num and tantalum capacitors can be used but are not

MINIMUM

CAPACITANCE

0.9

1

1.1 1.2 1.3 1.4 1.5 1.6

V

(V)

OUT

3252 F03

Figure 3. Output Capacitance vs Output Voltage

recommended because of their high ESR.

3252f

8

LTC3252

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OPERATIO

(Refer to Simplified Block Diagram)

Further input noise reduction can be achieved by filtering

the input through a very small series inductor as shown in

Figure 5. A 10nH inductor will reject the fast input tran-

sients caused by the blanking period, thereby presenting

anearlyconstantloadtotheinputsupply.Foreconomythe

10nH inductor can be fabricated on the PC board with

about 1cm (0.4") of PC board trace.

discussing the specified capacitance value. For example,

over rated voltage and temperature conditions, a 4.7µF,

10V, Y5V ceramic capacitor in a 0805 case may not

provide any more capacitance than a 1µF, 10V, X7R

available in the same 0805 case. In fact, over bias and

temperature range, the 1µF, 10V, X7R will provide more

capacitance than the 4.7µF, 10V, Y5V. The capacitor

manufacturer’s data sheet should be consulted to deter-

mine what value of capacitor is needed to ensure mini-

mum capacitance values are met over operating

temperature and bias voltage.

10nH

V

IN

V

IN

SUPPLY

4.7µF

LTC3252

GND

Below is a list of ceramic capacitor manufacturers and

how to contact them:

3252 F05

Figure 5. 10nH Inductor Used for

Additional Input Noise Reduction

AVX

Kemet

www.avxcorp.com

www.kemet.com

www.murata.com

www.t-yuden.com

www.vishay.com

Flying Capacitor Selection

Murata

Taiyo Yuden

Vishay

Warning: A polarized capacitor such as tantalum or alumi-

num should never be used for the flying capacitors since

their voltages can reverse upon start-up of the LTC3252.

Ceramic capacitors should always be used for the flying

capacitors.

Layout Considerations

Duetothehighswitchingfrequencyandtransientcurrents

produced by the LTC3252 careful board layout is neces-

sary for optimal performance. A true ground plane and

short connections to all capacitors will improve perfor-

mance and ensure proper regulation under all conditions.

Figure 7 shows the suggested layout configuration. Note

the exposed paddle of the package is ground (GND) and

must be soldered to the PCB ground.

The flying capacitors control the strength of the charge

pump. In order to achieve the rated output current it is

necessary for the flying capacitor to have at least 0.4µF of

capacitance over operating temperature with a 2V bias

(seetheCeramicCapacitorSelectionGuidelines).If100mA

or less of current is required from an output then its asso-

ciatedflyingcapacitorminimumcanbereducedto0.15µF.

Ceramic Capacitor Selection Guidelines

The flying capacitor pins C1⁺, C1^–, C2⁺and C2^–will have

very high edge rate wave forms. The large dv/dt on these

pins can couple energy capacitively to adjacent printed

circuit board runs. Magnetic fields can also be generated

if the flying capacitors are not close to the LTC3252 (i.e.,

the loop area is large). To decouple capacitive energy

transfer, a Faraday shield may be used. This is a grounded

PC trace between the sensitive node and the LTC3252

pins. For a high quality AC ground, it should be returned to

a solid ground plane that extends all the way to the

LTC3252. Keep the FB traces away from or shielded from

the flying capacitor traces or degraded performance could

result.

Capacitors of different materials lose their capacitance

withhighertemperatureandvoltageatdifferentrates. For

example, a ceramic capacitor made of X7R material will

retainmostofitscapacitancefrom–40°Cto85°Cwhereas

a Z5U or Y5V style capacitor will lose considerable

capacitance over that range (60% to 80% loss typical).

Z5U and Y5V capacitors may also have a very strong

voltage coefficient causing them to lose an additional

60% or more of their capacitance when the rated voltage

is applied. Therefore, when comparing different capaci-

tors it is often more appropriate to compare the amount

of achievable capacitance for a given case size rather than

3252f

9

LTC3252

U

OPERATIO

(Refer to Simplified Block Diagram)

Thermal Management

I_OUT1= 150mA and OUT1 regulating at 1.5V the measured

efficiency is 80.6% which is in close agreement with the

theoretical 83.3% calculation.

To reduce the maximum junction temperature, a good

thermal connection to the PC board is recommended.

Soldering the exposed paddle of the IC to the PCB and

maintaining a solid ground plane under the device on one

or more layers of the PC board, the thermal resistance of

the package can be as small as 40°C/W. By applying the

suggested thermal management techniques the IC junc-

tion temperature should never exceed 125°C even under

worst case operating conditions.

Programming the LTC3252 Output Voltages (FB1 and

FB2 Pin)

EachoutputoftheLTC3252isprogrammedtoanarbitrary

voltageviaanexternalresistivedivider.Figure7showsthe

required voltage divider connection. The voltage divider

ratio is given by the expression:

R_AOUT

R_B0.8V

Power Efficiency

=

−1

The power efficiency (η) of the LTC3252 is approximately

50% higher than a conventional linear regulator. This

occurs because the input current for a 2-to-1 step-down

charge pump is approximately half the output current. For

an ideal 2-to-1 step-down charge pump the power effi-

ciency is given by:

Typical values for total voltage divider resistance can

range from several kΩs up to 1MΩ.

The user may want to consider load regulation when

setting the desired output voltage. The closed loop output

impedance of the LTC3252 is approximately:

P_OUT

P

IN

V_OUT•I_OUT2V_OUT

OUT

R_O= 0.08Ω •

0.8V

η ≡

=

1

V

IN

V • I_OUT

IN

2

For a 1.5V output, R_Ois 0.15Ω, which produces a 38mV

output change for a 250mA load current step. Thus, the

user may want to target an unloaded output voltage

slightly higher than desired to compensate for the output

load conditions. The output may be programmed for

regulation voltages of 0.9V to 1.6V.

TheswitchinglossesandquiescentcurrentoftheLTC3252

are designed to minimize efficiency loss over the entire

output current range, causing only a couple % error from

the theoretical efficiency. For example with V_IN= 3.6V,

R

R '

B

R '

A

B

Since the LTC3252 employs a 2-to-1 charge pump archi-

tecture, it is not possible to achieve output voltages

greater than half the available input voltage. The minimum

V_INsupply required for regulation can be determined by

the following equation:

1

EN2

EN1

V

IN

LTC3252

C2

C

IN

V_IN(MIN) ≤ 2 • (V_OUT(MIN) + I_OUT• R_OL)

C1

OUT1

OUT2

FB2

OUT2

R

OUT1

0.8V

R ' OUT2

A

B

R

R '

A

LTC3252

=

– 1

=

– 1

OUT1

OUT2

R

R '

B

0.8V

C

O2

O1

C

O1

FB1

C

O2

R '

B

GND

(CONNECT DIRECTLY TO GROUND PLANE)

3252 F06

3252 F07

Figure 6. Suggested Layout for the LTC3252

Figure 7. Programming the LTC3252

3252f

10

LTC3252

U

TYPICAL APPLICATIO

Li-Ion to 1.5V/1.2V Outputs

3

2

5

11

9

V

EN2

OFF ON

IN

OUT2

1.5V

EN1

OUT2

OFF ON

1µF

OUT1

1.2V

250mA

10

+

OUT1

C2

470k

510k

1µF

LTC3252

250mA

4

6

1

8

+

–

261k

C1

C2

4.7µF

12

7

–

C1

FB2

4.7µF

FB1

GND

Li-ION

510k

4.7µF

3252 TA02

U

PACKAGE DESCRIPTIO

DE/UE Package

12-Lead Plastic DFN (4mm × 3mm)

(Reference LTC DWG # 05-08-1695)

0.58 ±0.05

3.40 ±0.05

2.24 ±0.05 (2 SIDES)

1.70 ±0.05

PACKAGE OUTLINE

0.25 ± 0.05

0.50

BSC

3.30 ±0.05

(2 SIDES)

RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS

0.38 ± 0.10

4.00 ±0.10

(2 SIDES)

R = 0.115

TYP

7

12

R = 0.20

TYP

3.00 ±0.10 1.70 ± 0.10

(2 SIDES)

PIN 1

TOP MARK

PIN 1

NOTCH

(UE12/DE12) DFN 0802

6

0.25 ± 0.05

1

0.75 ±0.05

0.200 REF

0.50

BSC

3.30 ±0.10

(2 SIDES)

0.00 – 0.05

BOTTOM VIEW—EXPOSED PAD

NOTE:

1. DRAWING PROPOSED TO BE A VARIATION OF VERSION

(WGED) IN JEDEC PACKAGE OUTLINE M0-229

2. ALL DIMENSIONS ARE IN MILLIMETERS

3. 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

4. EXPOSED PAD SHALL BE SOLDER PLATED

3252f

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

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

tationthattheinterconnectionofitscircuitsasdescribedhereinwillnotinfringeonexistingpatentrights.

11

LTC3252

U

TYPICAL APPLICATIO S

Fixed 3.3V_INto 1.5V_OUTat 300mA

3-Cell NiMH with Digitally Selectable 1.2V/1.5V Output

OFF ON

3

2

5

11

9

3

2

5

11

9

V

EN2

IN

V

EN2

3.3V

IN

I

OUT

250mA

EN1

OUT2

4.7µF

1µF

1.5V

EN1

OUT2

10

+

300mA

10

OUT1

C2

261k

+

OUT1

C2

470k

510k

1µF

LTC3252

4

6

1

8

4.7µF

+

–

1µF

LTC3252

4

6

1

8

C1

C2

+

–

C1

C2

1µF

4.7µF

3-CELL

NiMH

12

7

–

10µF

C1

FB2

12

7

–

C1

FB2

FB1

GND

EN1 EN2 OUT

FB1

GND

412k

100k

OFF

ON

OFF

ON

OFF 1.5V

0V

1.2V

3252 TA03

ON

1.5V

3252 TA04

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DESCRIPTION

COMMENTS

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V

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250mA, 1.5MHz, High Efficiency

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85% Efficiency, V = 3.1V to 5.5V, V

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Up to 85% Efficiency, V = 2.7V to 5.5V, V

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I = 8µA, I

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600mA (I ), 1.4MHz, Synchronous

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95% Efficiency, V = 2.7V to 6V, V

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OUT

I = 10µA, I

= <1µA, MS8

Q

SHDN

LTC3405/LTC3405A 300mA (I ), 1.5MHz, Synchronous

95% Efficiency, V = 2.7V to 6V, V

IN

OUT

Step-Down DC/DC Converter

I = 20µA, I

= <1µA, ThinSOT

Q

SHDN

LTC3406/LTC3406B 600mA (I ), 1.5MHz, Synchronous

95% Efficiency, V = 2.5V to 5.5V, V

Min = 0.6V,

Min = 0.8V,

Min = 2.5V,

OUT

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OUT

Step-Down DC/DC Converter

I = 20µA, I

= <1µA, ThinSOT

Q

SHDN

LTC3411

LTC3412

LTC3440

1.25A (I ), 4MHz, Synchronous Step-Down 95% Efficiency, V = 2.5V to 5.5V, V

DC/DC Converter

OUT

IN

I = 60µA, I

= <1µA, MS10

Q

SHDN

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

95% Efficiency, V = 2.5V to 5.5V, V

IN

OUT

DC/DC Converter

I = 60µA, I

= <1µA, TSSOP-16E

Q

SHDN

600mA (I ), 2MHz, Synchronous

95% Efficiency, V = 2.5V to 5.5V, V

IN

OUT

Buck-Boost DC/DC Converter

I = <25µA, I

= 1µA, MS10

Q

SHDN

3252f

LT/TP 0503 1K • PRINTED IN USA

12 ^{LinearTechnology Corporation}

1630 McCarthy Blvd., Milpitas, CA 95035-7417

●

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

LINEAR TECHNOLOGY CORPORATION 2003


型号：	LTC3252EDE
厂家：	Linear
描述：	Dual, Low Noise, Inductorless Step-Down DC/DC Converter 双通道，低噪声，无电感器降压型DC / DC转换器转换器稳压器开关式稳压器或控制器电源电路开关式控制器电感器光电二极管
文件：	总12页 (文件大小：288K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

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