LTC1265CS-5#PBF_技术文档

LTC1265/LTC1265-3.3/LTC1265-5

1.2A, High Efficiency

Step-Down DC/DC Converter

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FEATURES

DESCRIPTIO

The LTC^®1265 is a monolithic step-down current mode

DC/DC converter featuring Burst Mode ^TMoperation at low

output current. The LTC1265 incorporates a 0.3Ω switch

(V_IN=10V) allowing up to 1.2A of output current.

■

High Efficiency: Up to 95%

■

Current Mode Operation for Excellent Line and Load

Transient Response

■

Internal 0.3

Ω Power Switch (V_IN= 10V)

■

Short-Circuit Protection

Under no load condition, the converter draws only 160µA.

In shutdown it typically draws a mere 5µA making this

converter ideal for current sensitive applications. In drop-

out the internal P-channel MOSFET switch is turned on

continuouslymaximizingthelifeofthebatterysource.The

LTC1265incorporatesautomaticpowersavingBurstMode

operation to reduce gate charge losses when the load

currents drop below the level required for continuous

operation.

Low Dropout Operation: 100% Duty Cycle

Low-Battery Detector

Low 160µA Standby Current at Light Loads

Active-High Micropower Shutdown: I_Q< 15µA

Peak Inductor Current Independent of Inductor Value

^{Available in 14-p}U^{in SO Package}

APPLICATIO S

■

5V to 3.3V Conversion

Theinductorcurrentisuser-programmableviaanexternal

current sense resistor. Operation up to 700kHz permits

the use of small surface mount inductors and capacitors.

■

Distributed Power Systems

■

Step-Down Converters

■

Inverting Converters

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

Burst Mode is a trademark of Linear Technology Corporation.

■

Memory Backup Supply

■

Portable Instruments

■

Battery-Powered Equipment

■

Cellular Telephones

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TYPICAL APPLICATIO

V

IN

5.4V TO

12V

LTC1265-5 Efficiency

†††

+

C

IN

100

68µF

20V

0.1µF

L1*

R

**

SENSE

V

PWR V

SHDN

IN

33µH

V

5V

1A

0.1Ω

OUT

V

= 6V

= 9V

IN

SW

95

90

85

80

75

70

V

D1^†

IN

LTC1265-5

1k

††

+

C

OUT

I

TH

220µF

PGND

V

= 12V

IN

10V

3900pF

+

C

T

SENSE

130pF

1000pF

L = 33µH

V = 5V

OUT

–

SENSE

R

T

= 0.1Ω

* COILTRONICS CTX33-4

SENSE

SGND

C

= 130pF

** IRC LRC2010-01-R100-J

†

MBRS130LT3

^††AVX TPSE227K010

^†††AVX TPSE686K020

0.01

0.10

LOAD CURRENT (A)

1.00

LTC1265-FO1

LTC1265 TA01

Figure 1. High Efficiency Step-Down Converter

1

LTC1265/LTC1265-3.3/LTC1265-5

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ABSOLUTE MAXIMUM RATINGS

PACKAGE/ORDER INFORMATION

(Voltages Refer to GND Pin) (Note 1)

ORDER

Input Supply Voltage (Pins 1, 2, 13)..........–0.3V to 13V

DC Switch Current (Pin 14) .................................... 1.2A

Peak Switch Current (Pin 14) ................................. 1.6A

Switch Voltage (Pin 14) ..................................V_IN– 13.0

Operating Temperature Range

LTC1265C ............................................... 0° to 70°C

LTC1265I ........................................ –40°C to 85°C

Junction Temperature (Note 2)............................. 125°C

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

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

TOP VIEW

PART NUMBER

PWR V

1

2

3

4

5

6

7

14

13

12

11

10

9

SW

IN

V

PWR V

PGND

SGND

SHDN

IN

LTC1265CS

LB

OUT

LTC1265CS-5

LTC1265CS-3.3

LTC1265IS

LB

IN

C

T

I

N/C (V *)

FB

TH

–

+

SENSE

8

SENSE

S PACKAGE

14-LEAD PLASTIC SO

*ADJUSTABLE OUTPUT VERSION

= 125°C, θ = 110°C/W

T

JMAX

JA

Consult factory for Military grade parts.

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

temperature range, otherwise specifications are at T_A= 25°C. V_IN= 10V, V_SHDN= 0V, unless otherwise specified.

SYMBOL

PARAMETER

CONDITIONS

LTC1265

MIN

TYP

MAX

UNITS

I

Feedback Current into Pin 9

Feedback Voltage

0.2

1

µA

FB

V

LTC1265C

●

1.22

1.20

1.25

1.28

1.30

V

FB

V

= 9V, LTC1265I

IN

V

Regulator Output Voltage

LTC1265-3.3: I

= 800mA

LOAD

●

3.22

4.9

3.3

5

3.40

5.2

V

OUT

LTC1265-5: I

= 800mA

LOAD

∆V

Output Voltage Line Regulation

Output Voltage Load Regulation

V

= 6.5V to 10V, I = 800mA

LOAD

–40

0

40

mV

OUT

IN

LTC1265-3.3: 10mA < I

LTC1265-5: 10mA < I

< 800mA

40

60

65

100

mV

LOAD

< 800mA

LOAD

Burst Mode Operation Output Ripple

Input DC Supply Current (Note 3)

I

= 0mA

50

mV

P-P

LOAD

I

Active Mode: 3.5V < V < 10V

Sleep Mode: 3.5V < V < 10V

Sleep Mode: 5V < V < 10V (LTC1265-5)

Shutdown: V

1.8

160

5

2.4

230

15

mA

µA

Q

IN

= V , 3.5V < V < 10V

SHDN

IN

V

Low-Battery Trip Point

Current into Pin 4

1.15

0.5

1.25

1.35

0.5

V

LBTRIP

I

µA

LBIN

Current Sunk by Pin 3

V

= 0.4V, V = 0V

LBIN

1.0

1.5

1.0

mA

µA

LBOUT

= 5V, V

= 10V

LBIN

–

V – V

Current Sense Threshold Voltage

LTC1265: V

V

LTC1265-3.3: V

= 5V, V = V /4 + 25mV (Forced)

25

150

25

150

25

mV

8

7

SENSE

9

OUT

–

= 5V, V = V /4 – 25mV (Forced)

130

180

SENSE

9

OUT

–

= V

OUT

+ 100mV (Forced)

– 100mV (Forced)

+ 100mV (Forced)

– 100mV (Forced)

SENSE

OUT

–

V

SENSE

–

OUT

LTC1265-5: V

V

= V

SENSE

–

= V

150

OUT

R

ON Resistance of Switch

LTC1265C

LTC1265I

●

0.3

0.60

0.70

Ω

ON

–

I

t

C Pin Discharge Current

V

in Regulation, V

= 0V

= V

OUT

40

60

2

100

10

µA

5

T

OUT

SENSE

Switch Off Time (Note 4)

C = 390pF, I

= 800mA (LTC1265C)

= 800mA (LTC1265I)

●

4

3.5

5

6

7

µs

OFF

T

LOAD

T

2

LTC1265/LTC1265-3.3/LTC1265-5

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

temperature range, otherwise specifications are at T_A= 25°C. V_IN= 10V, V_SHDN= 0V, unless otherwise specified.

SYMBOL

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

V

Shutdown Pin High

Shutdown Pin Low

Shutdown Pin Input Current

Min Voltage at Pin 10 for Device to be in Shutdown

Max Voltage at Pin 10 for Device to be Active

1.2

V

IH

IL

0.6

0.5

I

V

= 8V

SHDN

µA

10

Note 3: Dynamic supply current is higher due to the gate charge being

delivered at the switching frequency.

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

of the device may be impaired.

Note 4: In applications where R

off time increases by approximately 40%.

is placed at ground potential, the

Note 2: T is calculated from the ambient temperature T and power

SENSE

J

A

dissipation P according to the following formulas:

D

LTC1265CS, LTC1265CS-3.3, LTC1265CS-5:

T = T + (P • 110°C/W)

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A

D

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TYPICAL PERFORMANCE CHARACTERISTICS

Efficiency vs Input Voltage

(V_OUT= 5V)

Efficiency vs Input Voltage

(V_OUT= 3.3V)

Efficiency vs Load Current

100

98

96

94

92

90

88

86

84

82

80

100

98

96

94

92

90

88

86

84

82

80

100

95

90

85

80

75

70

LTC1265-3.3

R

= 0.1Ω

SENSE

T

COIL = CTX33-4

C

= 130pF

V

= 5V

IN

I

= 250mA

LOAD

V

= 9V

IN

I

= 250mA

LOAD

I

= 800mA

LOAD

V

= 12V

IN

LTC1265-3.3

= 3.3V

I

= 800mA

LOAD

V

R

OUT

LTC1265-5

R = 0.1Ω

SENSE

= 0.1Ω

SENSE

T

C

= 130pF

C = 130pF

T

COIL = CTX33-4

0.01

0.10

1.00

11

4

5

6

7

8

9

10

12 13

4

5

6

7

8

9

10

12 13

LOAD CURRENT (A)

INPUT VOLTAGE (V)

1265 G01

1265 G02

LTC1265 G03

Operating Frequency

vs (V_IN– V_OUT

Switch Leakage Current

Switch Resistance

)

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

1.2

1.0

0.8

0.6

0.4

0.2

0

300

270

V

= 12V

IN

0°C

25°C

240

210

70°C

T = 125°C

J

180

150

T = 70°C

J

120

90

60

30

0

T = 25°C

J

T = 0°C

J

0

1

2

3

4

5

6

7

8

9

10

3

10 11 12 13

INPUT VOLTAGE (V)

4

5

6

7

8

9

0

20

40

60

80

100

(V

–

V

) VOLTAGE (V)

OUT

IN

TEMPERATURE (°C)

1265 G04

1265 G05

1265 G06

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LTC1265/LTC1265-3.3/LTC1265-5

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TYPICAL PERFORMANCE CHARACTERISTICS

DC Supply Current

Supply Current in Shutdown

Gate Charge Losses

2.1

1.8

1.5

1.2

0.9

0.6

0.3

0

5.5

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0

8

7

6

5

4

3

2

1

0

DOES NOT INCLUDE

GATE CHARGE

SHUTDOWN = 3V

T

= 25C

A

V

IN

= 12V

ACTIVE MODE

V

= 9V

= 6V

IN

V

IN

SLEEP MODE

0

2

4

6

8

10

12

14

0

200

400

600

800

1000

3

4

5

6

7

8

9

10 11 12 13

FREQUENCY (kHz)

INPUT VOLTAGE (V)

1265 G07

1265 G09

1265 G08

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PIN FUNCTIONS

PWR V_IN(Pins 1, 13): Supply for the Power MOSFET and

itsDriver.Mustdecouplethispinproperlytoground.Must

always tie Pins 1 and 13 together.

SENSE⁺(Pin 8): The (+) Pin to the Current Comparator. A

built-in offset between Pins 7 and 8 in conjunction with

R_SENSEsets the current trip threshold.

V_IN(Pin 2): Main Supply for All the Control Circuitry in the

N/C,V_FB(Pin 9): For the LTC1265 adjustable version, this

pin serves as the feedback pin from an external resistive

dividerusedtosettheoutputvoltage. OntheLTC1265-3.3

and LTC1265-5 versions, this pin is not used.

LTC1265.

LB_OUT(Pin 3): Open-Drain Output of the Low-Battery

Comparator. This pin will sink current when Pin 4 (LB_IN)

goes below 1.25V. During shutdown, this pin is high

impedance.

SHDN (Pin 10): Pulling this pin HIGH keeps the internal

switch off and puts the LTC1265 in micropower shut-

down. Do not float this pin.

LB_IN(Pin4):The(–)InputoftheLow-BatteryComparator.

The (+) input is connected to a reference voltage of 1.25V.

SGND (Pin 11): Small-Signal Ground. Must be routed

separately from other grounds to the (–) terminal of C_OUT

.

C_T(Pin 5): External capacitor C_Tfrom Pin 5 to ground sets

the switch off time. The operating frequency is dependent

on the input voltage and C_T.

PGND (Pin 12): Switch Driver Ground. Connects to the

(–) terminal of C_IN. Anode of the Schottky diode must be

connected close to this pin.

I_TH(Pin 6): Feedback Amplifier Decoupling Point. The

current comparator threshold is proportional to Pin 6

voltage.

SENSE^–(Pin 7): Connect to the (–) input of the current

comparator. For LTC1265-3.3 and LTC1265-5, it also

connects to an internal resistive divider which sets the

output voltage.

SW (Pin 14): Drain of the P-Channel MOSFET Switch.

Cathode of the Schottky diode must be connected close to

this pin.

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LTC1265/LTC1265-3.3/LTC1265-5

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(Pin 9 connection shown for LTC1265-3.3 and LTC1265-5; change create LTC1265)

FUNCTIONAL DIAGRA

PWR V

1, 13

IN

+

–

SENSE

8

SENSE

7

14 SW

12 PGND

V

FB

–

+

9

ADJUSTABLE

VERSION

V

–

+

SLEEP

R

S

25mV TO 150mV

C

+

Q

5pF

V

OS

S

–

+

–

V

TH2

13k

6

G

I

TH

100k

V

–

TH1

T

V

IN

2

3

LB

0UT

+

–

REFERENCE

10

SHDN

5

C

T

OFF-TIME

CONTROL

A3

–

SENSE

V

FB

4

LB

IN

1265 FD

11 SGND

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OPERATION

(Refer to Functional Diagram)

The LTC1265 uses a constant off-time architecture to

switch its internal P-channel power MOSFET. The off time

is set by an external timing capacitor at C_T(Pin 5). The

operatingfrequencyisthendeterminedbytheofftimeand

the voltage across the shunt reaches the comparator’s

threshold value, its output signal will change state, setting

theflipflopandturningtheinternalP-channelMOSFEToff.

The timing capacitor connected to Pin 5 is now allowed to

discharge at a rate determined by the off-time controller.

the difference between V_INand V_OUT

.

The output voltage is set by an internal resistive divider

(LTC1265-3.3 and LTC1265-5) connected to SENSE^–

(Pin 7) or an external divider returned to V_FB(Pin 9 for

LTC1265). A voltage comparator V, and a gain block G,

compare the divided output voltage with a reference

voltage of 1.25V.

When the voltage on the timing capacitor has discharged

past V_TH1, comparator T trips, sets the flip flop and causes

the switch to turn on. Also, the timing capacitor is re-

charged. The inductor current will again ramp up until the

current comparator C trips. The cycle then repeats.

When the load current increases, the output voltage de-

creases slightly. This causes the output of the gain stage

(Pin 6) to increase the current comparator threshold, thus

tracking the load current.

Tooptimizeefficiency,theLTC1265automaticallyswitches

between continuous and Burst Mode operation. The volt-

age comparator is the primary control element when the

device is in Burst Mode operation, while the gain block

controls the output voltage in continuous mode.

When the load is relatively light, the LTC1265 automati-

cally goes into Burst Mode operation. The current loop is

interrupted when the output voltage exceeds the desired

regulated value. The hysteretic voltage comparator V trips

when V_OUTis above the desired output voltage, shutting

off the switch and causing the capacitor to discharge. This

When the load is heavy, the LTC1265 is in continuous

operation. During the switch ON time, current comparator

C monitors the voltage between Pins 7 and 8 connected

across an external shunt in series with the inductor. When

5

LTC1265/LTC1265-3.3/LTC1265-5

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OPERATION ^{(Refer to Functional Diagram)}

capacitor discharges past V_TH1until its voltage drops

below V_TH2. Comparator S then trips and a sleep signal is

generated. Thecircuitnowentersintosleepmodewiththe

power MOSFET turned off. In sleep mode, the LTC1265 is

in standby and the load current is supplied by the output

capacitor. All unused circuitry is shut off, reducing quies-

cent current from 2mA to 160µA. When the output capaci-

tor discharges by the amount of the hysteresis of the

comparatorV,theP-channelswitchturnsonagainandthe

process repeats itself. During Burst Mode operation the

To avoid the operation of the current loop interfering with

BurstModeoperation, abuilt-inoffsetV_OSisincorporated

in the gain stage. This prevents the current from increas-

inguntiltheoutputvoltagehasdroppedbelowaminimum

threshold.

Using constant off-time architecture, the operating fre-

quency is a function of the voltage. To minimize the

frequencyvariationasdropoutisapproached,theoff-time

controller increases the discharge current as V_INdrops

below V_OUT+ 2V. In dropout the P-channel MOSFET is

turned on continuously (100% duty cycle) providing low

dropout operation with V_OUTV_IN.

peak inductor current is set at 25mV/R_SENSE

.

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APPLICATIONS INFORMATION

The basic LTC1265 application circuit is shown in

Figure 1. External component selection is driven by the

150mV

25mV

2 • R

I

=

(Amps)

OUT(MAX)

–

R

SENSE

loadrequirement, andbeginswiththeselectionofR_SENSE

.

137.5mV

=

Once R_SENSEis known, C_Tand L can be chosen. Next, the

(Amps)

R

SENSE

Schottky diode D1 is selected followed by C_INand C_OUT

.

Solving for R_SENSEand allowing a margin of variations in

the LTC1265 and extended component values yields:

R_SENSESelection for Output Current

R_SENSEis chosen based on the required output current.

Withthecurrentcomparatormonitoringthevoltagedevel-

oped across R_SENSE, the threshold of the comparator

determines the peak inductor current. Depending on the

load current condition, the threshold of the comparator

liesbetween25mV/R_SENSEand150mV/R_SENSE. Themaxi-

mum output current of the LTC1265 is:

100mV

OUT(MAX)

R

=

(Ω)

SENSE

I

TheLTC1265isratedwithacapabilitytosupplyamaximum

of1.2Aofoutputcurrent.Therefore, theminimumvalueof

R_SENSEthat can be used is 0.083Ω. A graph for selecting

R_SENSEversus maximum output is given in Figure 2.

150mV

I

RIPPLE

I

=

(Amps)

OUT(MAX)

–

0.5

0.4

0.3

0.2

0.1

0

R

2

SENSE

where I_RIPPLEis the peak-to-peak inductor ripple current.

At a relatively light load, the LTC1265 is in Burst Mode

operation. In this mode the peak inductor current is set at

25mV/R_SENSE.TofullybenefitfromBurstModeoperation,

the inductor current should be continuous during burst

periods. Hence, the peak-to-peak inductor ripple current

must not exceed 25mV/R_SENSE

.

Toaccountforlightandheavyloadconditions,theI_OUT(MAX)

is then given by:

0

0.2

0.4

0.6

0.8

1

MAXIMUM OUTPUT CURRENT (A)

1265 G10

Figure 2. Selecting R_SENSE

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LTC1265/LTC1265-3.3/LTC1265-5

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APPLICATIONS INFORMATION

Under short-circuit condition, the peak inductor current is

determined by:

2V, the LTC1265 reduces t_OFFby increasing the discharge

current in C_T. This prevents audible operation prior to

dropout. (See shelving effect shown in the Operating

Frequency curve under Typical Performance Character-

istics.)

150mV

I

=

(Amps)

SC(PK)

R

SENSE

In this condition, the LTC1265 automatically extends the

off time of the P-channel MOSFET to allow the inductor

current to decay far enough to prevent any current build-

up. The resulting ripple current causes the average short-

To maintain continuous inductor current at light load, the

inductor must be chosen to provide no more than 25mV/

R_SENSEof peak-to-peak ripple current. This results in the

following expression for L:

circuit current to be approximately I_OUT(MAX)

.

L ≥ 5.2(10⁵)R_SENSE(C_T)V_REG

C_Tand L Selection for Operating Frequency

Using an inductance smaller than the above value will

result in the inductor current being discontinuous. A

consequenceofthisisthattheLTC1265willdelayentering

Burst Mode operation and efficiency will be degraded at

low currents.

The LTC1265 uses a constant off-time architecture with

t_OFFdetermined by an external capacitor C_T. Each time the

P-channel MOSFET turns on, the voltage on C_Tis reset to

approximately 3.3V. During the off time, C_Tis discharged

by a current that is proportional to V_OUT. The voltage on C_T

is analogous to the current in inductor L, which likewise,

decays at a rate proportional to V_OUT. Thus the inductor

value must track the timing capacitor value.

Inductor Core Selection

With the value of L selected, the type of inductor must be

chosen. Basically, there are two kinds of losses in an

inductor; core and copper losses.

The value of C_Tis calculated from the desired continuous

mode operating frequency:

Core losses are dependent on the peak-to-peak ripple

current and core material. However it is independent of

the physical size of the core. By increasing the induc-

tance, the peak-to-peak inductor ripple current will de-

crease, therefore reducing core loss. Utilizing low core

loss material, such as molypermalloy or Kool Mµ^®will

allow user to concentrate on reducing copper loss and

preventing saturation.

V – V

V + V

IN

1

IN

OUT

D

C =

T

(Farads)

)

4

1.3(10 )f

where V_Dis the drop across the Schottky diode.

As the operating frequency is increased, the gate charge

losses will reduce efficiency. The complete expression for

operating frequency is given by:

Although higher inductance reduces core loss, it in-

creases copper loss as it requires more windings. When

space is not at a premium, larger wire can be used to

reduce the wire resistance. This also prevents excessive

heat dissipation.

V – V

V + V

IN

1

OFF

IN

OUT

D

(Hz)

f ≈

)

t

where:

V

REG

OUT

CATCH DIODE SELECTION

4

(sec)

t

= 1.3(10 )C

OFF

T

)

Losses in the catch diode depend on forward drop and

switching times. Therefore Schottky diodes are a good

choice for low drop and fast switching times.

V_REGis the desired output voltage (i.e. 5V, 3.3V). V_OUTis

the measured output voltage. Thus V_REG/V_OUT= 1

in regulation.

The catch diode carries load current during the off time.

The average diode current is therefore dependent on the

Note that as V_INdecreases, the frequency decreases.

When the input-to-output voltage differential drops below

Kool Mµ is a registered trademark of Magnetics, Inc.

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LTC1265/LTC1265-3.3/LTC1265-5

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APPLICATIONS INFORMATION

P-channel switch duty cycle. At high input voltages, the

worst-case RMS ripple current in the output capacitor is

given by:

diode conducts most of the time. As V_INapproaches V_OUT

,

the diode conducts only a small fraction of the time. The

most stressful condition for the diode is when the output

is short circuited. Under this condition, the diode must

safely handle I_SC(PK)at close to 100% duty cycle. Most

LTC1265 circuits will be well served by either a 1N5818 or

a MBRS130LT3 Schottky diode. An MBRS0520 is a good

choice for I_OUT(MAX)≤ 500mA.

150mV

I

≈

(A

)

RMS

2(R

)

SENSE

Generally, once the ESR requirement for C_OUThas been

met, the RMS current rating far exceeds the I_RIPPLE(P-P)

requirement.

ESR is a direct function of the volume of the capacitor.

ManufacturerssuchasNichicon,AVXandSpragueshould

be considered for high performance capacitors. The

OS-CON semiconductor dielectric capacitor available

from Sanyo has the lowest ESR for its size at a somewhat

higher price.

C_IN

In continuous mode, the input current of the converter is

a square wave of duty cycle V_OUT/V_IN. To prevent large

voltage transients, a low ESR input capacitor must be

used. In addition, the capacitor must handle a high RMS

current. The C_INRMS current is given by:

In surface mount applications, multiple capacitors may

have to be paralleled to meet the capacitance, ESR or RMS

current handling requirement of the application. Alumi-

num electrolyte and dry tantalum capacitors are both

available in surface mount configurations. In the case of

tantalum, it is critical that the capacitors are both available

in surface mount configuration and are surge tested for

useinswitchingpowersupplies. Anexcellentchoiceisthe

AVX TPS series of surface mount tantalums, available in

case heights ranging from 2mm to 4mm. Consult the

manufacturer for other specific recommendations.

1

/

2

I

[V

(V – V )]

OUT OUT IN OUT

(A

)

I

≈

RMS

V

IN

This formula has a maximum at V_IN= 2V_OUT, where I_RMS

= I_OUT/2. This simple worst case is commonly used for

design because even significant deviations do not offer

much relief. Note that capacitor manufacturer’s ripple

current ratings are often based on only 2000 hours life-

time. This makes it advisable to further derate the capaci-

tor, or to choose a capacitor rated at a higher temperature

than required. Do not underspecify this component. An

additional 0.1µF ceramic capacitor is also required on

PWR V_INfor high frequency decoupling.

When the capacitance of C_OUTis made too small, the

outputrippleatlowfrequencieswillbelargeenoughtotrip

the voltage comparator. This causes Burst Mode opera-

tion to be activated when the LTC1265 would normally be

in continuous operation. The effect will be most pro-

nounced with low value of R_SENSEand can be improved at

higher frequencies with lower values of L.

C_OUT

The selection of C_OUTis based upon the effective series

resistance(ESR)forproperoperationoftheLTC1265.The

required ESR of C_OUTis:

Low-Battery Detection

ESR_COUT< 50mV/I_RIPPLE

The low-battery comparator senses the input voltage

through an external resistive divider. This divided voltage

connects to the (–) input of a voltage comparator (Pin 4)

which is compared with a 1.25V reference voltage. Ne-

glecting Pin 4 bias current, the following expression is

used for setting the trip limit:

where I_RIPPLEis the ripple current of the inductor. For the

case where the I_RIPPLEis 25mV/R_SENSE, the required ESR

of C_OUTis:

ESR_COUT< 2(R_SENSE

)

To avoid overheating, the output capacitor must be sized

to handle the ripple current generated by the inductor. The

R4

R3

1 +

= 1.25

V

LB_TRIP

)

8

LTC1265/LTC1265-3.3/LTC1265-5

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APPLICATIONS INFORMATION

Theoutput,Pin3,isanN-channelopendrainthat goeslow

when the battery voltage is below the threshold set by R3

and R4. In shutdown, the comparator is disabled and Pin

3 is in a high impedance state.

THERMAL CONSIDERATIONS

In a majority of applications, the LTC1265 does not

dissipate much heat due to its high efficiency. However, in

applications where the switching regulator is running at

high duty cycles or the part is in dropout with the switch

turnedoncontinuously(DC),theuserwillneedtodosome

thermal analysis. The goal of the thermal analysis is to

determine whether the power dissipated by the regulator

exceeds the maximum junction temperature of the part.

The temperature rise is given by:

V

IN

R4

4

LTC1265

3

–

+

R3

T_R= P(θ_JA)

1.25V REFERENCE

where P is the power dissipated by the regulator and θ_JA

is the thermal resistance from the junction of the die to the

ambient temperature.

LTC1265 F03

The junction temperature is simply given by:

T_J= T_R+ T_A

Figure 3. Low-Battery Comparator

As an example, consider the LTC1265 is in dropout at an

input voltage of 4V with a load current of 0.5A. From the

Typical Performance Characteristics graph of Switch Re-

sistance, the ON resistance of the P-channel is 0.55Ω.

Therefore power dissipated by the part is:

LTC1265 ADJUSTABLE APPLICATIONS

The LTC1265 develops a 1.25V reference voltage between

the feedback (Pin 9) terminal and signal ground (see

Figure 4). By selecting resistor R1, a constant current is

caused to flow through R1 and R2 to set overall output

voltage. The regulated output voltage is determined by:

P = I²(R_DSON) = 0.1375W

For the SO package, the θ_JAis 110°C/W.

R2

R1

1 +

= 1.25

V

OUT

Therefore the junction temperature of the regulator when

it is operating in ambient temperature of 25°C is:

)

For most applications a 30k resistor is suggested for R1.

To prevent stray pickup, a 100pF capacitor is suggested

across R1 located close to the LTC1265.

T_J= 0.1375(110) + 25 = 40.1°C

Remembering that the above junction temperature is

obtained from a R_DSONat 25°C, we need to recalculate the

junction temperature based on a higher R_DSONsince it

increases with temperature. However, we can safely as-

sume that the actual junction temperature will not exceed

the absolute maximum junction temperature of 125°C.

V

OUT

R2

9

LTC1265

V

FB

Now consider the case of a 1A regulator with V_IN= 4V and

T_A= 65°C. Starting with the same 0.55Ω assumption for

R_DSON, the T_Jcalculation will yield 125°C. But from the

graph, this will increase the R_DSONto 0.76Ω, which when

used in the above calculation yields an actual T_J> 148°C.

ThereforetheLTC1265wouldbeunsuitablefora4Vinput,

1A output regulator operating at T_A= 65°C.

SGND

11

100pF

R1

LTC1265 F04

Figure 4. LTC1265 Adjustable Configuration

9

LTC1265/LTC1265-3.3/LTC1265-5

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APPLICATIONS INFORMATION

Board Layout Checklist

When laying out the printed circuit board, the following

checklist should be used to ensure proper operation of the

LTC1265. These items are also illustrated graphically in

the layout diagram of Figure 5. Check the following in your

layout:

4. Is the Schottky diode closely connected between the

power ground (Pin 12) and switch (Pin 14)?

5. Does the LTC1265 SENSE^–(Pin 7) connect to a point

close to R_SENSEand the (+) plate of C_OUT? In adjustable

applications, the resistive divider, R1 and R2, must be

connected between the (+) plate of C_OUTand signal

ground.

6. AretheSENSE^–andSENSE⁺leadsroutedtogetherwith

minimum PC trace spacing? The 1000pF capacitor

between Pins 7 and 8 should be as close as possible to

the LTC1265.

1. Are the signal and power grounds segregated? The

LTC1265 signal ground (Pin 11) must return to the (–)

plate of C_OUT. The power ground (Pin 12) returns to the

anode of the Schottky diode, and the (–) plate of C_IN,

whose leads should be as short as possible.

2. Does the (+) plate of the C_INconnect to the power V_IN

(Pins 1,13) as close as possible? This capacitor pro-

vides the AC current to the internal P-channel MOSFET

and its driver.

7. Is SHDN (Pin 10) actively pulled to ground during

normal operation? The SHDN pin is high impedance

and must not be allowed to float.

3. Is the input decoupling capacitor (0.1µF) connected

closely between power V_IN(Pins 1,13) and power

ground (Pin 12)? This capacitor carries the high fre-

quency peak currents.

PWR V

1

2

IN

V

IN

14

13

12

11

10

9

V

IN

SW

D1

PWR V

IN

+

LTC1265

C

IN

0.1µF

3

4

L

PGND

SGND

SHDN

LB

OUT

1k

1000pF

IN

3900pF

5

6

R1

SHDN

C

T

C

OUT

+

I

TH

N/C (V

SENSE

)

FB

R

SENSE

7

8

R2

+

–

SENSE

V

OUT

1000pF

OUTPUT DIVIDER REQUIRED

WITH ADJUSTABLE VERSION ONLY

BOLD LINES INDICATE

HIGH PATH CURRENTS

LTC1265 F05

Figure 5. LTC1265 Layout Diagram (See Board Layout Checklist)

10

LTC1265/LTC1265-3.3/LTC1265-5

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APPLICATIONS INFORMATION

Troubleshooting Hints

When the load currents are low (I_LOAD< I_BURST) Burst

Mode operation occurs. The voltage on C_Tpin now falls to

ground for periods of time as shown in Figure 6b. During

this time the LTC1265 is in sleep mode with quiescent

current reduced to 160µA.

Since efficiency is critical to LTC1265 applications, it is

very important to verify that the circuit is functioning

correctlyinbothcontinuousandBurstModeoperation.As

the LTC1265 is highly tolerant of poor layout, the output

voltage will still be regulated. Therefore, monitoring the

output voltage will not tell you whether you have a good or

bad layout. The waveform to monitor is the voltage on the

timing capacitor Pin 5.

The inductor current should also be monitored. If the

circuit is poorly decoupled, the peak inductor current will

be haphazard as in Figure 7a. A well decoupled LTC1265

has a clean inductor current as in Figure 7b.

IncontinuousmodethevoltageontheC_Tpinisasawtooth

with approximately 0.9V_P-Pswing. This voltage should

never dip below 2V as shown in Figure 6a.

SLEEP MODE

3.3V

2.4V

3.3V

2.4V

0V

TIME

(a) CONTINUOUS MODE OPERATION

(b) Burst Mode OPERATION

LTC1265 F06a

LTC1265 F06b

Figure 6. C_TWaveforms

(b) WELL DECOUPLED LTC1265

(a) POORLY DECOUPLED LTC1265

Figure 7. Inductor Waveforms

11

LTC1265/LTC1265-3.3/LTC1265-5

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APPLICATIONS INFORMATION

Design Example

V

IN

As a design example, assume V_IN= 5V, V_OUT= 3.3V, I_MAX

= 0.8A and f = 250kHz. With this information we can easily

calculate all the important components.

5V

+

C

IN

V

PWR V

SHDN

IN

0.1µF

From (1),

22µH

V

3.3V

0.8A

0.125Ω

OUT

SW

R_SENSE= 100mV/0.8 = 0.125Ω

From (2) and assuming V_D= 0.4V,

C_T100pF

LTC1265-3.3

D1

1k

+

I

TH

C

OUT

PGND

3900pF

100pF

+

SENSE

C

T

1000pF

Using (3), the value of the inductor is:

L ≥ 5.2(10⁵)(0.125)(100pF)3.3V = 22µH

–

LTC1265 F08

SENSE

SGND

For the catch diode, a MBRS130LT3 or 1N5818 will be

sufficient in this application.

Figure 8. Design Example Circuit

C_INwill require an RMS current rating of at least 0.4A at

temperature, and C_OUTwill require an ESR of (from 5):

100

95

90

85

80

75

70

L = DALE LPT4545-220 (22µH)

ESR_COUT< 0.25Ω

V

C

= 3.3V

OUT

T

= 100pF

The inductor ripple current is given by:

V

)

+ V

D

L

OUT

t

= 0.22A

I

=

OFF

RIPPLE

)

At light loads the peak inductor current is at:

I_PEAK= 25mV/0.125 = 0.2A

Therefore, at load current less than 0.1A the LTC1265 will

be in Burst Mode operation. Figure 8 shows the complete

circuit and Figure 9 shows the efficiency curve with the

above calculated component values.

0.01

0.1

LOAD CURRENT (mA)

1.0

1265 G11

Figure 9. Design Example Efficiency Curve

12

LTC1265/LTC1265-3.3/LTC1265-5

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TYPICAL APPLICATIONS

High Efficiency 5V to 3.3V Converter

V

IN

5V

+

C

*

2

1, 13

IN

L1^†

47µH

††

0.1µF

100µF

R

SENSE

V

PWR V

IN

10V

V

3.3V

1A

0.1Ω

OUT

4

14

LB

SW

IN

+

C

**

OUT

MBRS130LT1

LTC1265-3.3

PGND

220µF

3

5

12

11

10

9

LB

10V

OUT

270pF

C

SGND

SHDN

NC

T

SHDN

3900pF

1k

6

7

I

THR

8

–

+

SENSE

*AVX TPSD107K010

**AVX TPSE227K010

1000pF

^†COILCRAFT D03316-473

^††DALE WSL2010-0.1-1%

LTC1265 TA02

Positive-to-Negative (–5V) Converter

*AVX TPSD226K025

V

IN

3.5V TO 7.5V

**AVX TPSD107K010

C

*

IN

2

1, 13

SHDN

D1

+

^†L1 SELECTION

22µF

25V

× 2

L1^†

50µH

0.1µF

TP0610L

14

V

PWR V

IN

PART NO.

MANUFACTURER

4

LB

IN

SW

DO3316-473

CTX50-4

LPT4545-500LA

CD74-470

COILCRAFT

COILTRONICS

DALE

LTC1265-5

3

5

12

11

V

OUT

LB

PGND

OUT

SUMIDA

–5V

220pF

^††IRC LRC2010-01-R100-J

C

SGND

T

D1= MBRS130LT3

2200pF

V

(V)

I

(mA)

1k

100k

IN

OUT(MAX)

6

7

10

8

C

**

OUT

I

SHDN

THR

3.5

4.0

5.0

6.0

7.0

7.5

360

430

540

630

720

740

100µF

+

10V

–

+

SENSE

††

1000pF

R

SENSE

0.1Ω

LTC1265 TA03

13

LTC1265/LTC1265-3.3/LTC1265-5

U

TYPICAL APPLICATIONS

5V Buck-Boost Converter

V

IN

V

(V)

I

(mA)

OUT(MAX)

IN

3.5V TO 7.5V

C

*

2

1, 13

+

IN

33µF

3.5

4.0

5.0

6.0

7.0

7.5

240

275

365

490

610

665

L1A^††

0.1µF

100µF

10V*

V

PWR V

IN

16V

33µH

4

14

V

OUT

LB

SW

IN

5V

2

1

LTC1265

1N5818

3

5

12

11

10

9

LB

PGND

SGND

SHDN

OUT

75pF

4

•

L1B^††

33µH

75k

25k

L1A

C

L1B

T

2

3

+

3

SHDN

C

*

OUT

TOP VIEW

4

3300pF

100µF

1k

•

6

7

1

10V

V

I

FB

THR

8

–

+

SENSE

SANYO OS-CON CAPACITOR

IRC LRC2010-01-R162-J

L1A, L2A SELECTION

*

100pF

0.01µF

R

**

SENSE

†

0.162Ω

PART NO.

MANUFACTURER

CTX33-4

LPT4545-330LA

COILTRONICS

DALE

LTC1265 F09

9V to 12V and – 12V Outputs

MBRS130LT3

V

OUT

V

(V)

I

(mA)

OUT(MAX)

IN

–12V

V

IN

C

*

4.0

5.0

6.0

7.0

8.0

9.0

10.0

11.0

12.0

40

4V TO 12V

OUT

+

C *

IN

68µF

60

2

1, 13

+

0.1µF

68µF

33µF**

L1A^††

50µH

20V

80

20V

25V

V

PWR V

IN

100

115

130

150

165

180

1N914

4

14

V

OUT

LB

IN

SW

12V

2

1

LTC1265

MBRS130LT3

3

5

12

11

10

9

LB

PGND

SGND

SHDN

OUT

75pF

4

SI19430DY

•

301k

34k

C

I

L1B^††

50µH

T

L1A

L1B

2

3

+

SHDN

C

*

OUT

3

3300pF

TOP VIEW

68µF

1k

6

7

•

20V

4

1

V

FB

THR

8

–

+

SENSE

*AVX TPSE686K020

**AVX TPSE336K025

100pF

0.01µF

R

*

^†IRC LRC2010-01-R162-J

^††L1A,L2A SELECTION

SENSE

0.162Ω

PART NO.

MANUFACTURER

LTC1265 TA05

CTX50-4

LPT4545-500LA

COILTRONICS

DALE

14

LTC1265/LTC1265-3.3/LTC1265-5

U

TYPICAL APPLICATIONS

2.5mm Max Height 5V-to-3.3V (500mA)

*AVX TAJB156K010

**AVX TAJB226K06

V

IN

3.5V TO 12.5V

2

1, 13

C

*

+

^†IRC LRC2010-01-R200-J

^††SUMIDA CLS62-180

IN

0.1µF

15µF

V

PWR V

IN

10V × 2

4

14

LB

IN

SW

MBRS0520LT1

LTC1265-3.3

PGND

3

5

12

11

10

9

LB

OUT

51pF

C

T

SGND

SHDN

N/C

L1^††

18µH

SHDN

3300pF

1k

C

**

6

7

OUT

I

22µF

THR

+

6.3V × 2

8

–

+

SENSE

†

1000pF

R

SENSE

0.20Ω

V

OUT

3.3V

LTC1265 TA06

500mA

Logic Selectable 0V/3.3V/5V 700mA Regulator

*DALE 593D68X0020E2W

**DALE 593D107X0010D2W

^†IRC LRC2010-01-R15-J

^††L1 SELECTION

PART NO.

MANUFACTURER

DO3316-333

CTX33-4

LPT4545-330LA

CD74-330

COILCRAFT

COILTRONICS

DALE

V

IN

3.5V TO 12.5V

C

*

2

1, 13

+

IN

SUMIDA

0.1µF

68µF

^†††V

= 0V: V

= 5V: V

= 3.3V/5V

= 0V

V

PWR V

IN

20V

SHDN

OUT

4

14

0V: V

5V: V

= 5V

= 3.3V

OUT

LB

SW

IN

MBRS130LT3

100pF

LTC1265

3

5

12

11

10

9

LB

PGND

SGND

SHDN

OUT

75pF

C

T

L1^††

33µH

45.3k

†††

V

SHDN

3300pF

1k

C

**

6

7

OUT

100µF

V

I

FB

THR

+

10V

8

–

+

SENSE

56.2k

75k

†

1000pF

R

SENSE

0.15Ω

V

OUT

0V/3.3V/5V

700mA

LTC1265 TA07

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.

15

LTC1265/LTC1265-3.3/LTC1265-5

U

TYPICAL APPLICATIONS

4-NiCad Battery Charger

*DALE 593D226X0025D2W

**DALE 593D107X0016E2W

^†DALE WSL2010-0.10-1%

^††L1 SELECTION

V

IN

PART NO.

MANUFACTURER

8V TO 12.5V

2

1, 13

+

C

*

IN

22µF, 25V

0.1µF

DO3316-104

CTX100-4P

CD105-101

COILCRAFT

COILTRONICS

SUMIDA

V

PWR V

IN

4

14

LB

IN

SW

MBRS130LT3

LTC1265

51Ω

3

5

12

11

10

9

LB

PGND

SGND

SHDN

OUT

C

T

L1^††

100µH

FAST CHARGE: = 0V

TRICKLE CHARGE: > 2V

30k

CHARGER

ON/OFF

100pF

270pF

C

**

OUT

6

7

V

I

100µF

FB

THR

+

10V

8

1k

–

+

VN2222L

SENSE

138k

†

1000pF

3300pF

R

SENSE

0.10Ω

MBRS130LT3

V

OUT

4 NICAD

1A FAST CHARGE

0.1A TRICKLE CHARGE

LTC1265 TA08

U

PACKAGE DESCRIPTION ^{Dimension in inches (millimeters) unless otherwise noted.}

0.337 – 0.344*

(8.560 – 8.738)

0.010 – 0.020

(0.254 – 0.508)

14

13

12

11 10

9

8

× 45°

0.053 – 0.069

(1.346 – 1.752)

0.004 – 0.010

(0.101 – 0.254)

0.008 – 0.010

(0.203 – 0.254)

0° – 8° TYP

0.228 – 0.244

(5.791 – 6.197)

0.150 – 0.157**

(3.810 – 3.988)

0.050

(1.270)

BSC

0.014 – 0.019

(0.355 – 0.483)

TYP

0.016 – 0.050

(0.406 – 1.270)

S14 1298

DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE

*

7

DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE

**

1

2

3

4

5

6

RELATED PARTS

PART NUMBER

DESCRIPTION

COMMENTS

Dual Version of LTC1147

Nonsynchronous, 8-Pin, V ≤ 16V

LTC1143

Dual Step-Down Switching Regulator Controller

Step-Down Switching Regulator Controller

Step-Down Switching Regulator with Internal 0.5A Switch

LTC1147

IN

LTC1148HV

LTC1174

Synchronous, V ≤ 20V

IN

V

≤ 18.5V, Comparator/Low Battery Detector

IN

LTC1474/LTC1475 Low Quiescent Current Step-Down Regulators

Monolithic, I = 40µA, 400mA, MS8

Q

LTC1574

Step-Down Switching Regulator with Internal 0.5A Switch

and Schottky Diode

V ≤ 18.5V, Comparator

IN

LTC1622

LTC1627

LTC1772

Low Input Voltage Step-Down DC/DC Controller

Constant Frequency, 2V to 10V V , MS8

IN

Monolithic Synchronous Step-Down Switching Regulator Constant Frequency, I

to 500mA, 2.65V to 8.5V V

IN

OUT

Constant Frequency Step-Down DC/DC Controller

SOT-23, 2.2V to 9.8V V

IN

126535fa LT/TP 1299 2K REV A • PRINTED IN USA

LINEAR TECHNOLOGY CORPORATION 1995

LinearTechnology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

16

●

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


型号：	LTC1265CS-5#PBF
厂家：	Linear
描述：	LTC1265 - 1.2A, High Efficiency Step-Down DC/DC Converter; Package: SO; Pins: 14; Temperature Range: 0°C to 70°C 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光电二极管
文件：	总16页 (文件大小：230K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

LTC1265CS-5#PBF [Linear]

相关型号：

LTC1265CS-5#TR

LTC1265CS-5#TRPBF

LTC1265I

LTC1265IS

LTC1265IS#PBF

LTC1265IS#TR

LTC1265IS#TRPBF

LTC1265_15

LTC1266

LTC1266-3.3

LTC1266-3.3_15

LTC1266-33