LT1302-5_技术文档

LT1302/LT1302-5

Micropower

High Output Current

Step-Up Adjustable and

Fixed 5V DC/DC Converters

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DESCRIPTIO

EATURE

S

F

The LT^®1302/LT1302-5 are micropower step-up DC/DC

converters that maintain high efficiency over a wide

range of output current. They operate from a supply

voltage as low as 2V and feature automatic shifting

between Burst Mode operation at light load, and current

mode operation at heavy load.

■

5V at 600mA or 12V at 120mA from 2-Cell Supply

200µA Quiescent Current

Logic Controlled Shutdown to 15µA

Low V_CESATSwitch: 310mV at 2A Typical

Burst Mode^TMOperation at Light Load

Current Mode Operation for Excellent

Line and Load Transient Response

Available in 8-Lead SO or PDIP

The internal low loss NPN power switch can handle

current in excess of 2A and switch at frequencies up to

400kHz. Quiescent current is just 200µA and can be

further reduced to 15µA in shutdown.

■

Operates with Supply Voltage as Low as 2V

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PPLICATI

S

A

Availablein8-pinPDIPor8-pinSOpackaging,theLT1302/

LT1302-5 have the highest switch current rating of any

similarly packaged switching regulators presently on the

market.

■

Notebook and Palmtop Computers

Portable Instruments

Personal Digital Assistants

Cellular Telephones

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

Burst Mode is a trademark of Linear Technology Corporation.

Flash Memory

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TYPICAL APPLICATI

2-Cell to 5V Converter Efficiency

NC

5

L1

6

90

10µH

V

I

T

IN

3

4

7

88

86

84

SW

SHDN

LT1302-5

SHUTDOWN

C3

0.1µF

V

= 3V

IN

8

+

C1

100µF

PGND

GND

1

SENSE

V

C

2 CELLS

D1

V

= 2.5V

IN

82

80

2

V

= 2V

IN

R

C

20k

C

0.01µF

+

78

76

74

72

70

C2

C

100µF

OUTPUT

5V

LT1302 • F01

600mA

1

10

100

1000

C1 = C2 = SANYO OS-CON

L1 = COILTRONICS CTX10-3

COILCRAFT DO3316-103

D1 = MOTOROLA MBRS130LT3

LOAD CURRENT (mA)

LT1302 • TA02

Figure 1. 2-Cell to 5V/600mA DC/DC Converter

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LT1302/LT1302-5

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ABSOLUTE AXI U RATI GS

PACKAGE RDER I FOR ATIO

TOP VIEW

V_INVoltage ............................................................. 10V

SW Voltage ............................................................. 25V

FB Voltage .............................................................. 10V

SHDN Voltage ......................................................... 10V

V_CVoltage ................................................................ 4V

I_TVoltage.................................................................. 4V

Maximum Power Dissipation ............................ 700mW

Operating Temperature Range .................... 0°C to 70°C

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

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

ORDER PART

NUMBER

GND

1

2

3

4

PGND

SW

8

7

6

5

V

C

LT1302CN8

LT1302CS8

LT1302CN8-5

LT1302CS8-5

SHDN

V

IN

(SENSE*)FB

I

T

N8 PACKAGE

8-LEAD PDIP

S8 PACKAGE

8-LEAD PLASTIC SO

*FIXED VERSION

PINS 1 AND 8 ARE INTERNALLY

CONNECTED IN SOIC PACKAGE

S8 PART MARKING

1302

13025

T_JMAX= 125°C, θ_JA= 100°C/W (N8)

_JMAX= 125°C, θ_JA= 80°C/W (S8)

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Consult factory for Industrial and Military grade parts.

DC ELECTRICAL CHARACTERISTICS

T_A= 25°C, V_IN= 2.5V, unless otherwise noted.

SYMBOL

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

I

Quiescent Current

V

SHDN

V

SHDN

= 0.5V, V = 1.3V

●

200

15

300

25

µA

Q

FB

= 1.8V

V

Input Voltage Range

2.0

2.2

1.22

V

nA

IN

●

8

1.26

Feedback Voltage (LT1302)

Feedback Pin Bias Current (LT1302)

Output Sense Voltage (LT1302-5)

Output Ripple Voltage (LT1302-5)

Sense Pin Resistance to Ground (LT1302-5)

Offset Voltage

V = 0.4V

1.24

100

5.05

50

420

15

FB

C

V

FB

= 1V

V = 0.4V

●

4.85

5.25

V

C

V = 0.4V

C

mV

kΩ

mV

V

OS

See Block Diagram

(Note 1)

Comparator Hysteresis

5

Oscillator Frequency

Current Limit Not Asserted (Note 2)

175

160

75

220

265

310

95

kHz

kHZ

%

µs

%/V

mV

µA

A

V/V

V

●

DC

Maximum Duty Cycle

Switch On Time

Switch Off Time

Output Line Regulation

Switch Saturation Voltage

86

3.9

0.7

0.06

310

t

Current Limit Not Asserted

ON

OFF

2 < V < 8V

0.15

400

475

IN

V

CESAT

I

= 2A

SW

●

Switch Leakage Current

Switch Current Limit

V

SW

= 5V, Switch Off

0.1

1

2.8

10

V = 0.4V (Burst Mode Operation)

C

V = 1.25V (Full Power) (Note 3)

●

2.0

50

1.8

3.9

C

Error Amplifier Voltage Gain

Shutdown Pin High

Shutdown Pin Low

0.9V ≤ V ≤ 1.2V, ∆V /∆V

75

C

FB

V

●

SHDNH

SHDNL

SHDN

0.5

20

V

I

Shutdown Pin Bias Current

V

SHDN

V

SHDN

V

SHDN

= 5V

= 2V

= 0V

8

3

0.1

µA

●

1

I Pin Resistance to Ground

T

3.9

kΩ

The

● denotes specifications which apply over the 0°C to 70°C

Note 2: The LT1302 operates in a variable frequency mode. Switching

frequency depends on load inductance and operating conditions and may

be above specified limits.

temperature range.

Note 1: Hysteresis is specified at DC. Output ripple depends on capacitor

size and ESR.

Note 3: Minimum switch current 100% tested. Maximum switch current

guaranteed by design.

2

LT1302/LT1302-5

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TYPICAL PERFORMANCE CHARACTERISTICS

No-Load Quiescent Current

Circuit of Figure 1

Switch Saturation Voltage

600

500

400

300

200

100

0

400

350

300

250

200

150

100

500

450

400

350

300

250

200

150

100

50

T

A

= 25°C

T

A

= 25°C

I

= 2A

SW

0

1

2

3

4

–50

0

25

50

75

100

–25

2.0

2.5

3.0

3.5

4.0

4.5

5.0

SWITCH CURRENT (A)

TEMPERATURE (°C)

SUPPLY VOLTAGE (V)

1302 G02

1302 G03

1302 G01

LT1302 Feedback Voltage

LT1302-5 Sense Pin Resistance

Quiescent Current

300

250

200

150

100

50

1.250

1.245

1.240

1.235

1.230

1.225

1.220

1.215

1.210

1.205

1.200

600

500

400

300

200

100

0

V

= 2.5V

IN

SWITCH OFF

0

–50

0

25

50

75

100

–50

0

25

50

75

100

–25

–50

0

25

50

75

100

–25

TEMPERATURE (°C)

1302 G06

1302 G04

1302 G05

Error Amplifier Offset Voltage

LT1302-5 Output Voltage

Maximum On-Time

5.0

4.5

4.0

3.5

3.0

2.5

2.0

5.100

5.075

5.050

5.025

5.000

4.975

4.950

4.925

4.900

30

25

20

15

10

5

0

–50

0

25

50

75

100

–50

0

25

50

75

100

–25

–50

0

25

50

75

100

–25

TEMPERATURE (°C)

1302 G09

1302 G08

1302 G07

3

LT1302/LT1302-5

TYPICAL PERFORMANCE CHARACTERISTICS

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Shutdown Pin Bias Current

Oscillator Frequency

Maximum Duty Cycle

100

90

80

70

60

50

300

275

250

225

200

175

150

20

18

16

14

12

10

8

T

A

= 25°C

6

4

2

0

–50

0

25

50

75

100

–50

0

25

50

75

100

–25

0

4

6

7

1

2

3

5

8

TEMPERATURE (°C)

SHUTDOWN VOLTAGE (V)

1302 G10

1302 G11

1302 G12

LT1302-5 Output Voltage vs

Load Current

Maximum Output Power*

Boost Mode

5.20

5.15

5.10

5.05

5.00

4.95

4.90

4.85

4.80

20

16

12

8

V

= 4V

IN

V

IN

= 2.2V

V

IN

= 3V

4

0

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

LOAD CURRENT (A)

0

2

4

6

8

10

0.1

INPUT VOLTAGE (V)

1302 G13

1302 G14

* APPROXIMATE

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I_T(Pin5):Normallyleftfloating. Additionofa3.3kresistor

toGNDforcestheLT1302intocurrentmodeatlightloads.

Efficiency drops at light load but increases at medium

loads. See Applications Information section.

GND (Pin 1): Signal Ground. Feedback resistor and 0.1µF

ceramic bypass capacitor from V_INshould be connected

directly to this pin.

V_C(Pin 2): Frequency Compensation Pin. Connect series

RC to GND. Keep trace short.

V_IN(Pin6):SupplyPin.Mustbebypassedwith:(1)a0.1µF

ceramictoGND, and(2)alargevalueelectrolytictoPGND.

When V_INis greater than 5V, a low value resistor (2Ω to

10Ω) is recommended to isolate the V_INpin from input

supply noise.

SHDN (Pin 3): Shutdown. Pull high to effect shutdown; tie

to ground for normal operation.

FB/Sense (Pin 4): Feedback/Sense. On the LT1302 this

pin connects to CMP1 input. On the LT1302-5 this pin

connects to the output resistor string.

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LT1302/LT1302-5

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SW (Pin 7): Switch Pin. Connect inductor and diode here.

Keep layout short and direct.

and 8 are thermally connected to the die. One square inch

of PCB copper provides an adequate heat sink for the

device.

PGND (Pin 8): Power Ground. Pins 8 and 1 should be

connected under the package. In the SO package, pins 1

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BLOCK DIAGRA SM

D1

L1

V

IN

V

OUT

+

C2

0.1µF

C1

C3

6

7

V

SW

IN

36mV

R4

+

–

1.75Ω

R5

730Ω

A2

CMP1

1.24V

REFERENCE

OFF

+

–

ENABLE

Q3

220kHz

OSCILLATOR

Q4

160X

A3

R1

R2

HYSTERETIC

DRIVER

FB

COMPARATOR

2

µA

4

3

V

OS

C5

100pF

V

IN

15mV

Q5

V

IN

–

Q1

Q2

A1

BIAS

SHDN

SHUTDOWN

+

ERROR

AMPLIFIER

300Ω

3.6k

GND

V

I

PGND

C

T

1

2

5

8

R3

22k

1302 F02

C4

0.01µF

Figure 2. LT1302 Block Diagram

5

LT1302/LT1302-5

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BLOCK DIAGRA SM

SENSE

4

V

IN

6

SW

7

36mV

R4

1.75Ω

+

–

R5

A2

730Ω

R1

315k

CMP1

1.24V

OFF

220kHz

OSCILLATOR

+

–

REFERENCE

ENABLE

Q3

Q4

A3

160X

HYSTERETIC

COMPARATOR

DRIVER

2

µA

V

OS

V

IN

15mV

Q5

R2

105k

V

IN

–

Q1

Q2

A1

BIAS

SHDN

3

SHUTDOWN

+

ERROR

AMPLIFIER

300Ω

3.6k

1

2

5

I

8

1302 F03

GND

V

C

PGND

T

Figure 3. LT1302-5 Block Diagram

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OPERATIO

CMP1’s hysteresis (about 5mV) CMP1 turns the oscilla-

tor off. In this mode, peak switch current is limited to

approximately 1A by A2, Q2, and Q3. Q2’s current, set at

34µA, flows through R5, causing A2’s negative input to

be 25mV lower than V_IN. This node must fall more than

36mV below V_INfor A2 to trip and turn off the oscillator.

The remaining 11mV is generated by Q3’s current flow-

ing through R4. Emitter-area scaling sets Q3’s collector

current to 0.625% of switch Q4’s current. When Q4’s

current is 1A, Q3’s current is 6.25mA, creating an 11mV

drop across R4 which, added to R5’s 25mV drop, is

enough to trip A2.

The LT1302’s operation can best be understood by

examining the block diagram in Figure 2. The LT1302

operates in one of two modes, depending on load. With

light loads, comparator CMP1 controls the output; with

heavy loads, control is passed to error amplifier A1.

Burst Mode operation consists of monitoring the FB pin

voltage with hysteretic comparator CMP1. When the FB

voltage, related to the output voltage by external attenu-

ator R1 and R2, falls below the 1.24V reference voltage,

the oscillator is enabled. Switch Q4 alternately turns on,

causing current buildup in inductor L1, then turns off,

allowing the built-up current to flow into output capaci-

tor C3 via D1. As the output voltage increases, so does

the FB voltage; when it exceeds the reference plus

When the output load is increased to the point where the

1A peak current cannot support the output voltage,

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LT1302/LT1302-5

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OPERATIO

CMP1 stays on and the peak switch current is regulated

by the voltage on the V_Cpin (A1’s output). V_Cdrives the

base of Q1. As the V_Cvoltage rises, Q2 conducts less

current, resulting in less drop across R5. Q4’s peak

current must then increase in order for A2 to trip. This

currentmodecontrolresultsingoodstabilityandimmu-

nity to input voltage variations. Because this is a linear,

closed-loopsystem,frequencycompensationisrequired.

A series RC from V_Cto ground provides the necessary

pole-zero combination.

The LT1302-5 incorporates feedback resistors R1 and

R2 into the device. Output voltage is set at 5.05V in Burst

Mode, dropping to 4.97V in current mode.

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APPLICATIONS INFORMATION

Inductor Selection

t

×

ON

V − V

(

)

IN

SW

L ≤

Inductors used with the LT1302 must fulfill two require-

ments. First, the inductor must be able to handle current

of 2.5A to 3A without runaway saturation. Rod or drum

coreunitsusuallysaturategraduallyanditisacceptableto

exceed manufacturers’ published saturation currents by

20% or so. Second, it should have low DCR, under 0.05Ω

so that copper loss is kept low. Inductance value is not

critical. Generally, for low voltage inputs down to 2V, a

10µHinductorisrecommended(suchasCoilcraftDO3316-

103). For inputs above 4V to 5V use a 22µH unit (such as

CoilcraftDO3316-223). Switchingfrequencycanreachup

to 400kHz so the core material should be able to handle

high frequency without loss. Ferrite or molypermalloy

cores are a better choice than powdered iron. If EMI is a

concernatoroidalinductorissuggested,suchasCoiltronics

CTX20-4.

2A

With the 2V input a value of 3.3µH is acceptable. Since the

inductance is so low, usually a smaller core size can be

used. Efficiency will not be as high as for the continuous

case since peak currents will necessarily be higher.

Table1listsinductorsuppliersalongwithappropriatepart

numbers.

Table 1. Recommended Inductors

VENDOR

Coilcraft

PART NO.

VALUE(

µH)

PHONE NO.

(708) 639-6400

DO3316-103

DO3316-153

DO3316-223

10

15

22

10

20

10

20

10

15

22

Coiltronics

Dale

CTX10-2

CTX20-4

(407) 241-7876

(605) 665-9301

(708) 956-0666

LPT4545-100LA

LPT4545-200LA

CD105-100

CD105-150

CDR125-220

For a boost converter, duty cycle can be calculated by the

following formula:

Sumida

V

IN

DC = 1–

V_OUT

Capacitor Selection

AspecialsituationexistswheretheV_OUT/V_INdifferentialis

high, such as a 2V-to-12V converter. The required duty

cycle is higher than the LT1302 can provide, so the

converter must be designed for discontinuous operation.

This means that inductor current goes to zero during the

switch off-time. In the 2V-to-12V case, inductance must

be low enough so that current in the inductor can reach

2A in a single cycle. Inductor value can be defined by:

The output capacitor should have low ESR for proper

performance. A high ESR capacitor can result in “mode-

hopping” between current mode and Burst Mode at high

load currents because the output voltage will increase by

I

_SW× ESR when the inductor current is flowing into the

diode. Figure 4 shows output voltage of an LT1302-5

boostconverterwithtwo220µFAVXTPScapacitorsatthe

output. Ripple voltage at a 510mA load is about 30mV_P-P

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LT1302/LT1302-5

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APPLICATIONS INFORMATION

Input Capacitor

and there is no low frequency component. The total ESR

is under 0.03Ω. If a single 100µF aluminum electrolytic

capacitor is used instead, the converter mode-hops be-

tween current mode and Burst Mode due to high ESR,

causing the voltage comparator to trip as shown in Figure

5. The ripple voltage is now over 500mV_P-Pand contains

a low frequency component. Maximum allowable output

capacitor ESR can be calculated by the following formula:

The input supply should be decoupled with a good quality

electrolytic capacitor close to the LT1302 to provide a

stable input supply. Long leads or traces from power

source to the switcher can have considerable impedance

at the LT1302’s switching frequency. The input capacitor

provides a low impedance at high frequency. A 0.1µF

ceramiccapacitorisrequiredrightattheV_INpin. Whenthe

input voltage can be above 5V, a 10Ω/1µF decoupling

network for V_INis recommended as detailed in Figure 6.

This network is also recommended when driving a trans-

former.

V_OS× V_OUT

V_REF×1A

ESR_MAX

=

where,

V_OS= 15mV

V

> 5V

IN

V

_REF= 1.24V

10Ω

V

IN

SW

LT1302

+

47µF

TO

100µF

+

1µF

• • •

V_OUT

50mV/DIV

PGND

GND

AC COUPLED

1302 F06

510mA

I_LOAD

10mA

Figure 6. A 10Ω/1µF Decoupling Network at V_INIs

Recommended When Input Voltage Is Above 5V

500µs/DIV

1302 F04

Figure 4. Low ESR Output Capacitor Results in Stable

Operation. Ripple Voltage is Under 30mV_P-P

Table 2 lists capacitor vendors along with device types.

Table 2. Recommended Capacitors

VENDOR

AVX

Sanyo

SERIES

TPS

OS-CON

595D

TYPE

PHONE NO.

Surface Mount

Through Hole

Surface Mount

(803) 448-9411

(619) 661-6835

(603) 224-1961

V_OUT

200mV/DIV

AC COUPLED

Sprague

510mA

Diode Selection

I_LOAD

10mA

A 2A Schottky diode such as Motorola MBRS130LT3 has

been found to be the best available. Other choices include

1N5821 or MBRS130T3. Do not use “general purpose”

diodes such as 1N4001. They are much too slow for use

in switching regulator applications.

500µs/DIV

1302 F05

Figure 5. Inexpensive Electrolytic Capacitor Has High

ESR, Resulting in Mode-Hop, Ripple Voltage Amplitude Is

Over 500mV_P-Pand Includes Low Frequency Component

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LT1302/LT1302-5

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APPLICATIONS INFORMATION

Frequency Compensation

behavior in the 4th graticule is the result of the LT1302’s

BurstModecomparatorturningoffallswitching asoutput

voltage rises above its threshold.

Obtaining proper RC values for the frequency compensa-

tion network is largely an empirical procedure, since

variations in input and output voltage, topology, capacitor

ESR and inductance make a simple formula elusive. As an

example, considerthecaseofa2.5Vto5Vboostconverter

supplying 500mA. To determine optimum compensation,

the circuit is built and a transient load is applied to the

circuit. Figure 7 shows the setup.

In Figure 7c, the 0.1µF capacitor has been replaced by a

0.01µF unit. Undershoot is less but the response is still

underdamped. Figure 7d shows the results of the 0.1µF

capacitor and a 10k resistor in series. Now some amount

of damping is observed, and behavior is more controlled.

Figure 7e details response with a 0.01µF/10k series net-

work. Undershoot is down to around 100mV, or 2%. A

slight underdamping is still noticeable.

In Figure 7a, the V_Cpin is simply left floating. Although

output voltage is maintained and transient response is

good, switch current rises instantaneously to the internal

current limit upon application of load. This is an undesir-

able situation as it places maximum stress on the switch

and the other power components. Additionally, efficiency

iswelldownfromitsoptimalvalue.Next,a0.1µFcapacitor

is connected with no resistor. Figure 7b details response.

Although the circuit eventually stabilizes, the loop is quite

underdamped. Initial output “sag” exceeds 5%. Aberrant

Finally, a 0.01µF/24k series network results in the re-

sponse shown in Figure 7f. This has optimal damping,

undershoot less than 100mV and settles in less than 1ms.

The V_Cpin is sensitive to high frequency noise. Some

layouts may inject enough noise to modulate peak switch

current at 1/2 the switching frequency. A small capacitor

connected from V_Cto ground will eliminate this. Do not

exceed 1/10 of the compensation capacitor value.

V

IN

2.5V

NC

L1

10µH

V

I

T

SHDN

IN

+

C1

330µF

SW

LT1302-5

D1

0.1µF

PGND

GND

SENSE

V

C

10Ω

2W

500Ω

+

C2

220µF

C3

220µF

R

PULSE

GENERATOR

C

MTP3055EL

50Ω

C1, C2, C3 = AVX TPS SERIES

D1 = MOTOROLA MBRS130LT3

L1 = COILCRAFT DO3316-103K

1302 F07

Figure 7. Boost Converter with Simulated Load

V_OUT

100mV/DIV

AC COUPLED

V_OUT

100mV/DIV

AC COUPLED

510mA

I_LOAD

510mA

I_LOAD

10mA

2ms/DIV

1302 F07b

2ms/DIV

1302 F07a

Figure 7b. 0.1µF from V_Cto Ground.

Better, but More Improvement Needed

Figure 7a. V_CPin Left Unconnected. Output Shows

Low Frequency Components Under Load

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LT1302/LT1302-5

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APPLICATIONS INFORMATION

I_TPin

V_OUT

100mV/DIV

AC COUPLED

The I_Tpin is used to disable Burst Mode, forcing the

LT1302 to operate in current mode even at light load. To

disable Burst Mode, 3.3k resistor R1 is connected from I_T

to gound. More conservative frequency compensation

must be used when in this mode. A 0.1µF capacitor and

4.7k resistor from V_Cto ground has been found to be

adequate. Low frequency Burst Mode ripple can be

reduced or eliminated using this technique in many appli-

cations.

510mA

I_LOAD

10mA

2ms/DIV

1302 F07c

Figure 7c. 0.01µF from V_Cto Ground.

Underdamped Response Requires Series R

To illustrate, the transient load response of Figure 8’s

circuit is pictured without and with R1. Figure 8a shows

output voltage and inductor current without the resistor.

Note the 6kHz burst rate when the converter is delivering

25mA. By adding the 3.3k resistor, the low frequency

bursting is eliminated, as shown in Figure 8b. This feature

is useful in systems that contain audio circuitry. At very

light or zero load, switching frequency drops and eventu-

V_OUT

100mV/DIV

AC COUPLED

510mA

I_LOAD

10mA

2ms/DIV

1302 F07d

Figure 7d. 0.1µF with 10k Series RC.

Classic Overdamped Response

V

IN

2.5V

10µH

V

SENSE

IN

SW

+

C1

330µF

V_OUT

100mV/DIV

AC COUPLED

LT1302-5

MBRS130LT3

0.1µF

PGND

GND

V

C

I

T

+

220µF

10V

220µF

10V

4.7k

0.1µF

R1

3.3k

510mA

I_LOAD

10mA

1302 F08

V

2ms/DIV

1302 F07e

OUT

5V

600mA

Figure 7e. 0.01µF, 10k Series RC Shows Good

Transient Response. Slight Underdamping

Still Noticeable

Figure 8. Addition of R1 Eliminates Low Frequency

Output Ripple in This 2.5V to 5V Boost Converter

V_OUT

100mV/DIV

AC COUPLED

INDUCTOR

CURRENT

1A/DIV

510mA

I_LOAD

10mA

525mA

I_LOAD

25mA

1ms/DIV

1302 F08a

2ms/DIV

1302 F07f

Figure 7f. 0.01µF, 24k Series RC

Results in Optimum Response

Figure 8a. I_TPin Floating. Note 6kHz Burst Rate at

I_LOAD= 25mA. 0.1µF/4.7k Compensation Network

Causes 220mV Undershoot

10

LT1302/LT1302-5

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APPLICATIONS INFORMATION

ally reaches audio frequencies, but at a much lighter load

than without the I_Tfeature. At some input voltage/load

current combinations, some residual bursting may occur

at frequencies out of the audio band.

The I_Tpin cannot be used as a soft-start. Large capacitors

connected to the pin will cause erratic operation. If oper-

ating the device in Burst Mode, let the pin float. Keep high

dV/dt signals away from the pin.

Figure8cdetailsefficiencywithandwithouttheadditionof

R1. Burst Mode operation keeps efficiency high at light

load with I_Tfloating. Efficiency falls off at light load with

R1addedbecausetheLT1302cannottransitionintoBurst

Mode.

V_OUT

100mV/DIV

AC COUPLED

INDUCTOR

CURRENT

1A/DIV

Layout

525mA

I_LOAD

25mA

1ms/DIV

1302 F08b

The high speed, high current switching associated with

the LT1302 mandates careful attention to layout. Follow

thesuggestedcomponentplacementinFigure9forproper

operation. High current functions are separated by the

package from sensitive control functions. Feedback resis-

tors R1 and R2 should be close to the feedback pin (pin4).

Noisecaneasilybecoupledintothispinifcareisnottaken.

A small capacitor (100pF to 200pF) from FB to ground

provides a high frequency bypass. If the LT1302 is oper-

ated off a three-cell or higher input, R3 (2Ω to 10Ω) in

series with V_INis recommended. This isolates the device

from noise spikes on the input supply. Do not put in R3 if

the device must operate from a 2V input, as input current

will cause the voltage at the LT1302’s V_INpin to go below

2V. The 0.1µF ceramic bypass capacitor C3 (use X7R, not

Z5U) should be mounted as close as possible to the

package. When R3 is used, C3 should be a 1µF tantalum

unit. Grounding should be segregated as illustrated. C3’s

ground trace should not carry switch current. Run a

Figure 8b. 3.3k Resistor from I_TPin to Ground Forces

LT1302 into Current Mode Regardless of Load. Audio

Frequency Component Eliminated

90

I

T

FLOATING

80

70

60

50

40

30

3.3k I TO GND

T

1

10

100

1000

OUTPUT CURRENT (mA)

1302 F08c

Figure 8c. 3.3k Resistor for I_Tto Ground Increases

Efficiency at Moderate Load, Decreases at Light Load

V

IN

R2

5

6

7

8

4

C3

R3

2Ω

L1

3

2

1

R1

200pF

R

SHUTDOWN

LT1302

C

+

C1

C

D1

C2

V

OUT

GND (BATTERY AND LOAD RETURN)

1302 F09

Figure 9. Suggested Component Placement for LT1302

11

LT1302/LT1302-5

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APPLICATIONS INFORMATION

separate ground trace up under the package as shown.

The battery and load return should go to the power side of

the ground copper.

Table 3. S8 Package, 8-Lead Plastic SO

COPPER AREA

THERMAL RESISTANCE

TOPSIDE*

BACKSIDE BOARD AREA (JUNCTION-TO-AMBIENT)

2500 sq. mm 2500 sq. mm 2500 sq. mm

1000 sq. mm 2500 sq. mm 2500 sq. mm

60°C/W

62°C/W

65°C/W

69°C/W

73°C/W

80°C/W

83°C/W

Thermal Considerations

The LT1302 contains a thermal shutdown feature which

protects against excessive internal (junction) tempera-

ture. If the junction temperature of the device exceeds the

protection threshold, the device will begin cycling be-

tween normal operation and an off state. The cycling is not

harmful to the part. The thermal cycling occurs at a slow

rate, typically10mstoseveralseconds, whichdependson

the power dissipation and the thermal time constants of

the package and heat sinking. Raising the ambient tem-

perature until the device begins thermal shutdown gives a

good indication of how much margin there is in the

thermal design.

225 sq. mm

100 sq. mm

2500 sq. mm 2500 sq. mm

1000 sq. mm 2500 sq. mm

225 sq. mm 2500 sq. mm

100 sq. mm 2500 sq. mm

* Pins 1 and 8 attached to topside copper

N8 Package, 8-Lead DIP:

Thermal Resistance (Junction-to-Ambient) = 100°C/W

Calculating Temperature Rise

Power dissipation internal to the LT1302 in a boost

regulator configuration is approximately equal to:

For surface mount devices heat sinking is accomplished

by using the heat spreading capabilities of the PC board

and its copper traces. Experiments have shown that the

heat spreading copper layer does not need to be electri-

cally connected to the tab of the device. The PCB material

can be very effective at transmitting heat between the pad

area attached to pins 1 and 8 of the device, and a ground

or power plane layer either inside or on the opposite side

of the board. Although the actual thermal resistance of the

PCB material is high, the length/area ratio of the thermal

resistance between the layer is small. Copper board stiff-

eners and plated through holes can also be used to spread

the heat generated by the device.

2

V_OUT+ V_D

I_{OUT OUT}

V_OUT+ V_D

I_{OUT OUT}R

P = I²_OUT

R

−

D

V

R

V

V_IN−

V_IN

I_OUT

V

+ V_D− V_IN

(

)

OUT

+

27

The first term in this equation is due to switch “on-

resistance.” The second term is from the switch driver. R

is switch resistance, typically 0.15Ω. V_Dis the diode

forward drop.

Table 3 lists thermal resistance for the SO package.

Measured values of thermal resistance for several differ-

ent board sizes and copper areas are listed for each

surface mount package. All measurements were taken in

The temperature rise can be calculated from:

∆T = P_D× θ_JA

still air on 3/32" FR-4 board with 1oz copper. This data can

where:

be used as a rough guideline in estimating thermal resis-

tance. The thermal resistance for each application will be

affectedbythermalinteractionswithothercomponentsas

well as board size and shape.

∆T = Temperature Rise

P_D= Device Power Dissipation

θ_JA= Thermal Resistance (Junction-to-Ambient)

12

LT1302/LT1302-5

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APPLICATIONS INFORMATION

As an example, consider a boost converter with the

following specifications:

V_IN= 3V

V_OUT= 6V

I_OUT= 700mA

Total power loss in the LT1302, assuming R = 0.15Ω and

V_D= 0.45V, is:

2

0.7 6 + 0.45 − 3

2

) (

(

)(

)

6 + 0.45

0.7× 6 × 0.15

6 + 0.45

0.7× 6 × 0.15

3

P = 700mA 0.15Ω

−

+

(

)

D

27

3 −

3

= 223mW + 89mW = 312mW

Using the CS8 package with 100 sq. mm topside and

backside heat sinking:

∆T = (312mW)(84°C/W) = 25.9°C rise

With the N8 package:

∆T = 31.2°C

At a 70°C ambient, die temperature would be 101.2°C.

13

LT1302/LT1302-5

U

TYPICAL APPLICATIONS

Single Cell to 5V/150mA Converter

5V/150mA

OUTPUT

L1

3.3µH

D1

R1

301k

220Ω

10Ω

100k

1%

2N3906

1.5V

CELL

(169k FOR 3.3V)

100k

I

V

IN

SW

SHDN

FB

L

IN

SET

SW1

LT1073

LT1302

56.2k

1%

A

O

I

T

FB

GND

V

C

GND

SW2

PGND

100pF

20k

4.99k

1%

+

C1

47µF

C2

220µF

0.1µF

0.01µF

36.5k

1%

L1 = COILCRAFT DO3316-332

D1 = MOTOROLA MBRS130LT3

C1 = AVX TPSD476M016R0150

C2 = AVX TPSE227M010R0100

COILCRAFT (708) 639-2361

1302 TA03

2V to 12V/120mA Converter

NC

5

L1

3.3µH

6

V

I

T

IN

3

4

7

8

SW

SHDN

SHUTDOWN

C3

0.1µF

LT1302

+

C1

100µF

PGND

GND

1

FB

V

C

2 CELLS

D1

2

R1

100k

1%

R2

866k

1%

R

C

100pF

+

20k

C2

33µF

C

0.02µF

OUTPUT

12V

LT1302 • TA04

120mA

C1 = AVX TPSD107M010R0100

C2 = AVX TPSD336M025R0200

D1 = MOTOROLA MBRS130LT3

L1 = COILCRAFT DO3316-332

14

LT1302/LT1302-5

U

TYPICAL APPLICATIONS

3 Cell to 3.3V Buck-Boost Converter with Auxiliary 12V Regulated Output

V

IN

2.5V-8V

7

6

10Ω

C3

+

SHUTDOWN

47µF

T1D

T1E

5

16V

4

SHDN

FB

V

IN

SW

D2

13V

LT1302

100k

1%

0.1µF

I

T

2

V

C

C1

+

GND

PGND

100µF

T1B

16V

IN

12V

OUT

ADJ

9

+

120mA

22µF

25V

D1

330k

1%

169k

1%

24k

LT1121

200pF

SHDN

+

3

1

T1A

10

GND

3.3µF

C2

+

4700pF

330µF

T1C

150k

1%

6.3V

8

1302 TA05

3.3V OUTPUT

400mA

T1 = DALE LPE-6562-A069, 1:3:1:1:1 TURNS RATIO, 10µH PRIMARY. DALE (605) 665-9301

D1, D2 = MOTOROLA MBRS130LT3

C1 = AVX TPSE107016R0100

C2 = AVX TPSE337006R0100

C3 = AVX TPSD476016R0150

2 Li-Ion Cell to 5.8V/600mA DC/DC Converter

C2

220µF

L1

10V

22µH

+

V

IN

4V TO 9V

10Ω

L2

MBRS130LT3

22µH

365k

1%

SW

FB

V

T

V

5.8V

600mA

IN

OUT

I

+

C1

100µF

16V

1µF

LT1302

+

C3

220µF

10V

SHDN

GND

V

C

100k

1%

PGND

20k

10nF

SHUTDOWN

V

OUT

1302 TA07

DUTY CYCLE =

L1, L2

=

COILCRAFT DO3316-223

AVX TPSE107016R0100

AVX TPSE227010R0100

V

+ V

OUT

IN

PEAK SWITCH VOLTAGE = V + V

C1

C2, C3

IN

OUT

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

LT1302/LT1302-5

U

PACKAGE DESCRIPTION

Dimensions in inches (millimeters) unless otherwise noted.

N8 Package

8-Lead Plastic DIP

0.400*

(10.160)

MAX

8

7

6

3

5

4

0.255 ± 0.015*

(6.477 ± 0.381)

1

2

0.130 ± 0.005

0.300 – 0.325

0.045 – 0.065

(3.302 ± 0.127)

(1.143 – 1.651)

(7.620 – 8.255)

0.065

(1.651)

TYP

0.009 – 0.015

(0.229 – 0.381)

0.125

0.015

(0.380)

MIN

(3.175)

MIN

+0.025

0.045 ± 0.015

(1.143 ± 0.381)

0.325

–0.015

+0.635

8.255

(

)

–0.381

0.100 ± 0.010

(2.540 ± 0.254)

0.018 ± 0.003

(0.457 ± 0.076)

N8 0694

*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.

MOLD FLASH OR PROTURSIONS SHALL NOT EXCEED 0.010 INCH (0.254mm).

S8 Package

8-Lead Plastic SOIC

0.189 – 0.197*

(4.801 – 5.004)

7

5

8

6

0.150 – 0.157*

(3.810 – 3.988)

0.228 – 0.244

(5.791 – 6.197)

1

3

4

2

0.010 – 0.020

(0.254 – 0.508)

× 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

0.016 – 0.050

0.406 – 1.270

0.050

(1.270)

BSC

0.014 – 0.019

(0.355 – 0.483)

SO8 0294

*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.

MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.006 INCH (0.15mm).

LT/GP 0295 10K • PRINTED IN USA

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7487

16

●

LINEAR TECHNOLOGY CORPORATION 1995

(408) 432-1900 FAX: (408) 434-0507 TELEX: 499-3977


型号：	LT1302-5
厂家：	Linear
描述：	Micropower High Output Current Step-Up Adjustable and Fixed 5V DC/DC Converters 微功耗高输出电流的升压调节和固定5V DC / DC转换器转换器
文件：	总16页 (文件大小：360K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

LT1302-5 [Linear]

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