LT1111CS8-5应用文章市场行情现货热卖使用介绍供应商报价-中国IC网-芯三七

LT1111

Micropower

DC/DC Converter

Adjustable and Fixed 5V, 12V

U

DESCRIPTIO

EATURE

Operates at Supply Voltages from 2V to 30V

72kHz Oscillator

Works with Surface Mount Inductors

Only Three External Components Required

Step-Up or Step-Down Mode

Low-Battery Detector Comparator On-Chip

User Adjustable Current Limit

Internal 1A Power Switch

Fixed or Adjustable Output Voltage Versions

Space Saving 8-Pin MiniDIP or SO-8 Package

S

F

■

The LT1111 is a versatile micropower DC/DC converter.

The device requires only three external components to

deliver a fixed output of 5V or 12V. Supply voltage ranges

from 2V to 12V in step-up mode and to 30V in step-down

mode. The LT1111 functions equally well in step-up, step-

down, or inverting applications.

The LT1111 oscillator is set at 72kHz, optimizing the

devicetoworkwithoff-the-shelfsurfacemountinductors.

The device can deliver 5V at 100mA from a 3V input in

step-up mode or 5V at 200mA from a 12V input in step-

down mode.

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PPLICATI

A

S

Switch current limit can be programmed with a single

resistor. An auxiliary open-collector gain block can be

configuredasalow-batterydetector,linearpostregulator,

undervoltage lock-out circuit, or error amplifier.

■

3V to 5V, 5V to 12V Converters

9V to 5V, 12V to 5V Converters

Remote Controls

Peripherals and Add-On Cards

Battery Backup Supplies

Uninterruptible Supplies

Laptop and Palmtop Computers

Cellular Telephones

Portable Instruments

Flash Memory VPP Generators

For input sources of less than 2V use the LT1110.

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TYPICAL APPLICATI

Typical Load Regulation

All Surface Mount 3V to 5V Step-Up Converter

6

SUMIDA

5

CD54-220M

MBRS120T3

22µH

5V

V

= 2V 2.2 2.4

2.6

2.8 3V

IN

3V INPUT

4

3

2

1

0

100mA

I

V

IN

LIM

SW1

LT1111CS8-5

SENSE

SW2

+

µ

10 F*

33µF

GND

0

25

50 75 100 125 150 175 200

LOAD CURRENT (mA)

*OPTIONAL

LT1111 • TA01

LT1111 • TA02

1

LT1111

W W W

U

ABSOLUTE AXI U RATI GS

Supply Voltage (V_IN) ............................................... 36V

SW1 Pin Voltage (V_SW1) ......................................... 50V

SW2 Pin Voltage (V_SW2) ............................ –0.5V to V_IN

Feedback Pin Voltage (LT1111) ............................. 5.5V

Switch Current....................................................... 1.5A

Maximum Power Dissipation ............................ 500mW

Operating Temperature Range

LT1111C............................................... 0°C to 70°C

LT1111I ......................................... –40°C to 105°C

LT1111M ....................................... –55°C to 125°C

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

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

W

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PACKAGE RDER I FOR ATIO

TOP VIEW

ORDER PART

NUMBER

I

1

2

3

4

8

7

6

5

FB (SENSE)*

I

1

2

3

4

FB (SENSE)*

8

7

6

5

LIM

LT1111CN8

LT1111CS8

LT1111CS8-5

LT1111CS8-12

V

IN

SET

A0

V

SET

A0

IN

LT1111CN8-5

LT1111CN8-12

LT1111MJ8

SW1

SW2

SW1

SW2

GND

S8 PART MARKING

S8 PACKAGE

8-LEAD PLASTIC SO

J8 PACKAGE

N8 PACKAGE

LT1111MJ8-5

LT1111MJ8-12

8-LEAD CERAMIC DIP 8-LEAD PLASTIC DIP

1111

11115

11111

*FIXED VERSION

*FIXED VERSIONS

T_JMAX= 90°C, θ_JA= 150°C/W

T_JMAX= 150°C, θ_JA= 120°C/W (J)

_JMAX= 90°C, θ_JA= 130°C/W (N)

T

Consult factory for Industrial grade parts

ELECTRICAL CHARACTERISTICS ^V_IN= 3V, Military or Commercial Version

SYMBOL

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

I

Quiescent Current

Input Voltage

Switch OFF

300

400

µA

Q

V

Step-Up Mode

Step-Down Mode

●

2.0

12.6

30.0

V

IN

Comparator Trip Point Voltage

Output Sense Voltage

LT1111 (Note 1)

●

1.20

1.25

1.30

V

OUT

LT1111-5 (Note 2)

LT1111-12 (Note 2)

●

4.75

11.40

5.00

12.00

5.25

12.60

V

Comparator Hysteresis

Output Hysteresis

LT1111

●

8

12.5

mV

LT1111-5

LT1111-12

●

32

75

50

120

mV

f

Oscillator Frequency

54

72

88

kHz

OSC

DC

Duty Cycle: Step-Up Mode

Step-Down Mode

Full Load

43

24

50

34

59

50

%

t

Switch ON Time: Step-Up Mode

Step-Down Mode

I

Tied to V

IN

5

3.3

7

5

9

7.8

µs

ON

LIM

V

, = 5V, V = 12V

OUT

IN

V

SAT

SW Saturation Voltage, Step-Up Mode

V

IN

V

IN

= 3.0V, I = 650mA

0.5

0.8

0.65

1.0

V

SW

= 5.0V, I = 1A

SW

SW Saturation Voltage, Step-Down Mode

Feedback Pin Bias Current

V

= 12V, I = 650mA

1.1

70

1.5

V

IN

SW

I

LT1111, V = 0V

●

120

nA

FB

SET

FB

Set Pin Bias Current

V

= V

●

70

300

0.4

nA

V

SET

REF

V

Gain Block Output Low

I

= 300µA, V = 1.00V

SET

0.15

OL

SINK

2

LT1111

ELECTRICAL CHARACTERISTICS ^V_IN= 3V, Military or Commercial Version

SYMBOL

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

Reference Line Regulation

5V ≤ V ≤ 30V

●

0.02

0.20

0.075

0.400

%/V

IN

2V ≤ V ≤ 5V

IN

A

V

Gain Block Gain

R = 100k (Note 3)

L

1000

6000

400

–0.3

1

V/V

mA

I

Current Limit

220Ω from I to V

LIM IN

LIM

Current Limit Temperature Coefficient

Switch OFF Leakage Current

Maximum Excursion Below GND

●

%/°C

µA

Measured at SW1 Pin, V

= 12V

10

SW1

I

≤ 10µA, Switch OFF

–400

–350

mV

SW1

V_IN= 3V, –55°C ≤ T_A≤ 125°C unless otherwise noted.

LT1111M

TYP

SYMBOL

PARAMETER

CONDITIONS

MIN

MAX

500

100

UNITS

µA

I

f

Quiescent Current

Oscillator Frequency

Switch OFF

●

300

72

Q

45

kHz

OSC

DC

Duty Cycle: Step-Up Mode

Step-Down Mode

Full Load

●

40

20

50

62

55

%

t

Switch ON Time: Step-Up Mode

Step-Down Mode

I

V

Tied to V

IN

●

5

3

7

11

9

µs

ON

LIM

= 5V, V = 12V

OUT

IN

Reference Line Regulation

2V ≤ V ≤ 5V, 25°C ≤ T ≤ 125°C

0.2

0.5

0.4

0.8

%/V

IN

A

2.4V ≤ V ≤ 5V, T = –55°C

IN

A

V

SAT

SW Saturation Voltage, Step-Up Mode

SW Saturation Voltage, Step-Down Mode

0°C ≤ T ≤ 125°C, I = 500mA,

0.65

V

A

SW

T = –55°C, I = 400mA

A

SW

V

IN

= 12V,

0°C ≤ T ≤ 125°C

1.5

2.0

V

A

I

= 500mA

T = –55°C

A

SW

V_IN= 3V, 0°C ≤ T_A≤ 70°C unless otherwise noted.

LT1111C

TYP

SYMBOL

PARAMETER

CONDITIONS

MIN

300

54

MAX

450

95

UNITS

µA

I

f

Quiescent Current

Oscillator Frequency

Switch OFF

●

Q

72

kH

OSC

DC

Duty Cycle: Step-Up Mode

Step-Down Mode

Full Load

●

43

24

50

34

59

50

%

t

Switch ON Time: Step-Up Mode

Step-Down Mode

I

V

Tied to V

IN

●

5.0

3.3

7

5

9.0

7.8

µs

ON

LIM

= 5V, V = 12V

OUT

IN

Reference Line Regulation

2V ≤ V ≤ 5V

●

0.2

0.7

%/V

IN

V

SAT

SW Saturation Voltage, Step-Up Mode

SW Saturation Voltage, Step-Down Mode

V

IN

V

IN

= 3V, I = 650mA

●

0.5

1.1

0.65

1.50

V

SW

= 12V, I = 650mA

SW

Note 2: The output voltage waveform will exhibit a sawtooth shape due to

the comparator hysteresis. The output voltage on the fixed output versions

will always be within the specified range.

The

●

denotes specifications which apply over the full operating

temperature range.

Note 1: This specification guarantees that both the high and low trip points

Note 3: 100k resistor connected between a 5V source and the A0 pin.

of the comparator fall within the 1.20V to 1.30V range.

3

LT1111

TYPICAL PERFOR A CE CHARACTERISTICS

U W

Oscillator Frequency

Switch ON Time

10

100

90

75

74

9.5

9.0

8.5

8.0

7.5

7.0

6.5

6.0

5.5

5.0

73

72

71

70

69

68

67

80

70

60

50

40

25

TEMPERATURE (°C)

100

–50 –25

0

50

75

125

3

6

15 18 21

27

30

–50 –25

0

25

50

75 100 125

0

9

12

24

INPUT VOLTAGE (V)

TEMPERATURE (°C)

LT1111 • TPC01

LT1111 • TPC02

LT111 • TPC03

Saturation Voltage

Step-Up Mode

Saturation Voltage

Step-Up Mode

Duty Cycle

1.4

60

58

56

54

52

50

48

46

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

V

= 3V

= 5V

V

SW

= 3V

IN

I

=

650mA

1.2

1.0

V

= 2V

IN

0.8

0.6

0.4

0.2

0

V

IN

44

42

40

–50 –25

0

25

50

75 100 125

–50 – 25

0

25

50

75 100 125

0

0.2

1.0

0.4 0.6 0.8

SWITCH CURRENT (A)

1.6

1.2 1.4

TEMPERATURE (°C)

LT1111 • TPC04

LT1111 • TPC05

LT1111 • TPC06

Switch ON Voltage

Step-Down Mode

Switch ON Voltage

Step-Down Mode

Minimum/Maximum Frequency

vs ON Time

100

90

80

70

60

50

40

2.00

1.75

1.50

1.25

1.00

0.75

0.50

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0

V

SW

= 12V

V

= 12V

IN

I

=

650mA

0°C ≤ T ≤ 70°C

A

–55°C ≤ T ≤ 125°C

A

–50 –25

0

25

50

75 100 125

0

0.2

0.4

0.6

0.8

1.0

5

6

7

8

9

10 11 12

4

TEMPERATURE (°C)

SWITCH CURRENT (A)

SWITCH ON TIME (µs)

LT1111 • TPC07

LT1111 • TPC08

LT1111 • TPC09

4

LT1111

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TYPICAL PERFOR A CE CHARACTERISTICS

Maximum Switch Current

vs R_LIM

Quiescent Current

400

500

1.5

1.4

1.3

1.2

1.1

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

380

360

340

320

300

280

260

240

220

200

450

400

350

300

250

200

150

100

STEP-UP

2V ≤ V ≤ 5V

IN

STEP-DOWN

= 12V

V

IN

0

3

6

9

12 15 18 21 24 27 30

–50 –25

0

25

50

75 100 125

10

100

1000

INPUT VOLTAGE (V)

TEMPERATURE (°C)

R

LIM

(Ω)

LT1111 • TPC10

LT1111 • TPC11

LT1111 • TPC12

Set Pin Bias Current

Feedback Bias Current

100

90

80

70

60

50

40

30

20

10

0

100

90

80

70

60

50

40

30

20

10

0

–50 –25

0

25

50

75 100 125

–50 –25

0

25

50

75 100 125

TEMPERATURE (°C)

LT1111 • TPC13

LT1111 • TPC14

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PI

FU CTI

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I_LIM(Pin 1): Connect this pin to V_INfor normal use. Where

lower current limit is desired, connect a resistor between

I_LIMand V_IN. A 220Ω resistor will limit the switch current

to approximately 400mA.

GND (Pin 5): Ground.

A0(Pin6):AuxiliaryGainBlock(GB)Output.Opencollector,

can sink 300µA.

SET (Pin 7): GB Input. GB is an op amp with positive input

connected to SET pin and negative input connected to

1.25V reference.

FB/SENSE (Pin 8): On the LT1111 (adjustable) this pin

goes to the comparator input. On the LT1111-5 and

LT1111-12,thispingoestotheinternalapplicationresistor

that sets output voltage.

V_IN(Pin 2): Input Supply Voltage.

SW1 (Pin 3): Collector of Power Transistor. For step-up

mode connect to inductor/diode. For step-down mode

connect to V_IN.

SW2 (Pin 4): Emitter of Power Transistor. For step-up

mode connect to ground. For step-down mode connect to

inductor/diode. Thispinmustneverbeallowedtogomore

than a Schottky diode drop below ground.

5

LT1111

W

BLOCK DIAGRA S

LT1111

LT1111-5/LT1111-12

+

–

SET

+

–

A2

A0

A2

A0

V

IN

GAIN BLOCK/

ERROR AMP

GAIN BLOCK/

ERROR AMP

I

LIM

SW1

SW2

I

LIM

SW1

1.25V

+

–

1.25V

+

–

REFERENCE

OSCILLATOR

A1

OSCILLATOR

A1

DRIVER

COMPARATOR

SENSE

COMPARATOR

R2

220k

SW2

LT1111 • BD01

R1

LT1111-5: R1 = 73.5k

LT1111-12: R1 = 25.5k

GND

FB

GND

LT1111 • BD02

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LT11

11 OPERATI

The LT1111 is a gated oscillator switcher. This type

architecture has very low supply current because the

switch is cycled when the feedback pin voltage drops

below the reference voltage. Circuit operation can best be

understood by referring to the LT1111 block diagram.

Comparator A1 compares the feedback (FB) pin voltage

with the 1.25V reference signal. When FB drops below

1.25V, A1 switches on the 72kHz oscillator. The driver

amplifier boosts the signal level to drive the output NPN

power switch. The switch cycling action raises the output

voltage and FB pin voltage. When the FB voltage is suffi-

cient to trip A1, the oscillator is gated off. A small amount

of hysteresis built into A1 ensures loop stability without

external frequency compensation. When the comparator

output is low, the oscillator and all high current circuitry is

turnedoff,loweringdevicequiescentcurrenttojust300µA.

Gain block A2 can serve as a low-battery detector. The

negative input of A2 is the 1.25V reference. A resistor

divider from V_INto GND, with the mid-point connected to

the SET pin provides the trip voltage in a low-battery

detector application. AO can sink 300µA (use a 22k

resistor pull-up to 5V).

A resistor connected between the I_LIMpin and V_INsets

maximum switch current. When the switch current ex-

ceeds the set value, the switch cycle is prematurely

terminated. If current limit is not used, I_LIMshould be tied

directly to V_IN. Propagation delay through the current limit

circuitry is approximately 1µs.

In step-up mode the switch emitter (SW2) is connected to

ground and the switch collector (SW1) drives the induc-

tor; in step-down mode the collector is connected to V_IN

and the emitter drives the inductor.

Theoscillatorissetinternallyfor7µsONtimeand7µsOFF

time, optimizingthedeviceforcircuitswhereV_OUTandV_IN

differ by roughly a factor of 2. Examples include a 3V to 5V

step-up converter or a 9V to 5V step-down converter.

The LT1111-5 and LT1111-12 are functionally identical to

the LT1111. The -5 and -12 versions have on-chip voltage

setting resistors for fixed 5V or 12V outputs. Pin 8 on the

fixed versions should be connected to the output. No

external resistors are needed.

6

LT1111

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PPLICATI

A

S I FOR ATIO

Inductor Selection — General

P /

(02)

f

L

OSC

A DC/DC converter operates by storing energy as mag-

netic flux in an inductor core, and then switching this

energy into the load. Since it is flux, not charge, that is

stored, the output voltage can be higher, lower, or oppo-

site in polarity to the input voltage by choosing an

appropriate switching topology. To operate as an efficient

energy transfer element, the inductor must fulfill three

requirements. First, the inductance must be low enough

for the inductor to store adequate energy under the worst

case condition of minimum input voltage and switch-on

time. The inductance must also be high enough so maxi-

mum current ratings of the LT1111 and inductor are not

exceeded at the other worst case condition of maximum

input voltage and ON time. Additionally, the inductor core

must be able to store the required flux; i.e., it must not

saturate. At power levels generally encountered with

LT1111 based designs, small surface mount ferrite core

units with saturation current ratings in the 300mA to 1A

range and DCR less than 0.4Ω (depending on application)

are adequate. Lastly, the inductor must have sufficiently

low DC resistance so excessive power is not lost as heat

in the windings. An additional consideration is Electro-

Magnetic Interference (EMI). Toroid and pot core type

inductors are recommended in applications where EMI

must be kept to a minimum; for example, where there are

sensitive analog circuitry or transducers nearby. Rod core

types are a less expensive choice where EMI is not a

problem. Minimum and maximum input voltage, output

voltage and output current must be established before an

inductor can be selected.

in order for the converter to regulate the output.

When the switch is closed, current in the inductor builds

according to:

–R′t

L

V

R′

IN

I (t) =

1–e

(03)

L

where R′ is the sum of the switch equivalent resistance

(0.8Ω typical at 25°C) and the inductor DC resistance.

WhenthedropacrosstheswitchissmallcomparedtoV_IN,

the simple lossless equation:

V

L

IN

I t =

t

(04)

( )

L

can be used. These equations assume that at t = 0,

inductor current is zero. This situation is called “discon-

tinuous mode operation” in switching regulator parlance.

Setting “t” to the switch-on time from the LT1111 speci-

fication table (typically 7µs) will yield I_PEAKfor a specific

“L” and V_IN. Once I_PEAKis known, energy in the inductor

at the end of the switch-on time can be calculated as:

1

2

E = LI

(05)

L

PEAK

E_LmustbegreaterthanP_L/f_OSCfortheconvertertodeliver

the required power. For best efficiency I_PEAKshould be

kept to 1A or less. Higher switch currents will cause

excessive drop across the switch resulting in reduced

efficiency. In general, switch current should be held to as

low a value as possible in order to keep switch, diode and

inductor losses at a minimum.

Inductor Selection — Step-Up Converter

In a step-up, or boost converter (Figure 4), power gener-

ated by the inductor makes up the difference between

input and output. Power required from the inductor is

determined by:

As an example, suppose 12V at 60mA is to be generated

from a 4.5V to 8V input. Recalling equation (01),

P = 12V + 0.5V – 4.5V 60mA = 480mW (06)

(

)(

)

L

P = V

+ V – V

I

) (

OUT

(01)

(

)

MIN

L

OUT

D

IN

Energy required from the inductor is

where V_Dis the diode drop (0.5V for a 1N5818 Schottky).

Energy required by the inductor per cycle must be equal or

greater than:

P

480mW

72kHz

L

=

= 6.7µJ

(07)

f

OSC

7

LT1111

PPLICATI

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A

S I FOR ATIO

Picking an inductor value of 47µH with 0.2Ω DCR results

I_OUT= output current

V_OUT= output voltage

V_IN= minimum input voltage

in a peak switch current of:

–1.0Ω ×7µs

4.5V

1.0Ω

I

=

1– e

= 623mA.

(08)

V_SWisactuallyafunctionofswitchcurrentwhichisinturn

a function of V_IN, L, time, and V_OUT. To simplify, 1.5V can

be used for V_SWas a very conservative value.

47µH

PEAK

Substituting I_PEAKinto Equation 04 results in:

Once I_PEAKis known, inductor value can be derived from:

1

2

E = 47µH 0.623A = 9.1µJ

) (

(09)

(

)

L

V

− V

OUT

IN MIN

SW

L =

× t

(11)

ON

I

PEAK

Since 9.1µJ > 6.7µJ, the 47µH inductor will work. This

trial-and-error approach can be used to select the opti-

mum inductor. Keep in mind the switch current maximum

rating of 1.5A. If the calculated peak current exceeds this,

consider using the LT1110. The 70% duty cycle of the

LT1110 allows more energy per cycle to be stored in the

inductor, resulting in more output power.

where t_ON= switch-on time (7µs).

Next, the current limit resistor R_LIMis selected to give

I_PEAKfrom the R_LIMStep-Down Mode curve. The addition

ofthisresistorkeepsmaximumswitchcurrentconstantas

the input voltage is increased.

As an example, suppose 5V at 300mA is to be generated

from a 12V to 24V input. Recalling Equation (10),

A resistor can be added in series with the I_LIMpin to invoke

switch current limit. The resistor should be picked so the

calculated I_PEAKat minimum V_INis equal to the Maximum

Switch Current (from Typical Performance Characteristic

curves). Then, as V_INincreases, switch current is held

constant, resulting in increasing efficiency.

2 300mA

(

)

5 + 0.5

I

=

= 600mA (12)

PEAK

0.50

12 – 1.5 + 0.5

Next, inductor value is calculated using Equation (11):

Inductor Selection — Step-Down Converter

12 – 1.5 – 5

The step-down case (Figure 5) differs from the step-up in

that the inductor current flows through the load during

both the charge and discharge periods of the inductor.

Current through the switch should be limited to ~650mA

in this mode. Higher current can be obtained by using an

external switch (see Figure 6). The I_LIMpin is the key to

successful operation over varying inputs.

L =

7µs = 64µH.

(13)

600mA

Use the next lowest standard value (56µH).

Then pick R_LIMfrom the curve. For I_PEAK= 600mA, R_LIM

= 56Ω.

Inductor Selection — Positive-to-Negative Converter

After establishing output voltage, output current and input

voltagerange,peakswitchcurrentcanbecalculatedbythe

formula:

Figure 7 shows hookup for positive-to-negative conver-

sion. Alloftheoutputpowermustcomefromtheinductor.

In this case,

2I

DC

V

+ V

OUT D

OUT

P_L= ( V_OUT+ V_D)(I_OUT)

In this mode the switch is arranged in common collector

or step-down mode. The switch drop can be modeled as

a 0.75V source in series with a 0.65Ω resistor. When the

(14)

I

=

(10)

PEAK

V

– V

+ V

IN

SW D

where DC = duty cycle (0.50)

V_SW= switch drop in step-down mode

V_D= diode drop (0.5V for a 1N5818)

8

LT1111

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PPLICATI

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S I FOR ATIO

capacitors provide still better performance at more ex-

pense. We recommend OS-CON capacitors from Sanyo

Corporation (San Diego, CA). These units are physically

quite small and have extremely low ESR. To illustrate,

Figures 1, 2, and 3 show the output voltage of an LT1111

based converter with three 100µF capacitors. The peak

switch current is 500mA in all cases. Figure 1 shows a

Sprague 501D, 25V aluminum capacitor. V_OUTjumps by

over 120mV when the switch turns off, followed by a drop

in voltage as the inductor dumps into the capacitor. This

works out to be an ESR of over 0.24Ω. Figure 2 shows the

same circuit, but with a Sprague 150D, 20V tantalum

capacitor replacing the aluminum unit. Output jump is

now about 35mV, corresponding to an ESR of 0.07Ω.

Figure 3 shows the circuit with a 16V OS-CON unit. ESR

is now only 0.02Ω.

switch closes, current in the inductor builds according to

–R′t

L

V

R′

L

I

t =

( )

1– e

(15)

L

where R′ = 0.65Ω + DCR_L

V_L= V_IN– 0.75V

As an example, suppose –5V at 50mA is to be generated

from a 4.5V to 5.5V input. Recalling Equation (14),

P_L= ( -5V +0.5V)(50mA) = 275mW

Energy required from the inductor is:

(16)

P

275mW

72kHz

L

=

= 3.8µJ.

(17)

f

OSC

Picking an inductor value of 56µH with 0.2Ω DCR results

in a peak switch current of:

–0.85Ω × 7µs

56µH

4.5V – 0.75V

(

)

1– e

)

I

=

= 445mA.

(18)

PEAK

0.65Ω + 0.2Ω

LT1111 • F01

Substituting I_PEAKinto Equation (04) results in:

5µs/DIV

Figure 1. Aluminum

1

2

E = 56µH 0.445A = 5.54µJ.

) (

(19)

(

)

L

Since 5.54µJ > 3.82µJ, the 56µH inductor will work.

With this relatively small input range, R_LIMis not usually

necessary and the I_LIMpin can be tied directly to V_IN. As in

the step-down case, peak switch current should be limited

to ~650mA.

LT1111 • F02

5µs/DIV

Capacitor Selection

Figure 2. Tantalum

Selecting the right output capacitor is almost as important

as selecting the right inductor. A poor choice for a filter

capacitor can result in poor efficiency and/or high output

ripple. Ordinaryaluminumelectrolytics, whileinexpensive

andreadilyavailable, mayhaveunacceptablypoorEquiva-

lent Series Resistance (ESR) and ESL (inductance). There

are low ESR aluminum capacitors on the market specifi-

cally designed for switch mode DC/DC converters which

work much better than general-purpose units. Tantalum

LT1111 • F01

5µs/DIV

Figure 3. OS-CON

9

LT1111

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PPLICATI

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S I FOR ATIO

Diode Selection

At the end of the switch ON time the current in L1 is¹:

Speed, forward drop, and leakage current are the three

main considerations in selecting a catch diode for LT1111

converters.Generalpurposerectifierssuchasthe1N4001

are unsuitable for use in any switching regulator applica-

tion. Although they are rated at 1A, the switching time of

a 1N4001 is in the 10µs to 50µs range. At best, efficiency

will be severely compromised when these diodes are

used; at worst, the circuit may not work at all. Most

LT1111 circuits will be well served by a 1N5818 Schottky

diode, or its surface mount equivalent, the MBRS130T3.

The combination of 500mV forward drop at 1A current,

fast turn ON and turn OFF time, and 4µA to 10µA leakage

current fit nicely with LT1111 requirements. At peak

switch currents of 100mA or less, a 1N4148 signal diode

may be used. This diode has leakage current in the 1nA to

5nArangeat25°Candlowercostthana1N5818. (Youcan

also use them to get your circuit up and running, but

beware of destroying the diode at 1A switch currents.)

V

IN

I

=

t

(20)

PEAK

ON

L

Immediately after switch turn-off, the SW1 voltage pin

starts to rise because current cannot instantaneously stop

flowing in L1. When the voltage reaches V_OUT+ V_D, the

inductor current flows through D1 into C1, increasing

_OUT. This action is repeated as needed by the LT1111 to

keep V_FBat the internal reference voltage of 1.25V. R1 and

R2 set the output voltage according to the formula

V

R2

V

= 1+

1.25V

(

(21)

)

OUT

R1

Step-Down (Buck Mode) Operation

A step-down DC/DC converter converts a higher voltage

to a lower voltage. The usual hookup for an LT1111 based

step-down converter is shown in Figure 5.

Step-Up (Boost Mode) Operation

V

A step-up DC/DC converter delivers an output voltage

higher than the input voltage. Step-up converters are not

short-circuit protected since there is a DC path from input

to output.

IN

R3

100Ω

+

I

V

IN

SW1

FB

LIM

C2

The usual step-up configuration for the LT1111 is shown

in Figure 4. The LT1111 first pulls SW1 low causing V_IN–

V_CESATto appearacrossL1. Acurrentthenbuildsup inL1.

LT1111

GND

L1

V

SW2

OUT

R2

R1

D1

1N5818

+

C1

D1

L1

V

IN

OUT

R3*

LT1111 • F05

R2

R1

I

V

IN

SW1

LIM

Figure 5. Step-Down Mode Hookup

+

C1

LT1111 FB

When the switch turns on, SW2 pulls up to V_IN– V_SW. This

puts a voltage across L1 equal to V_IN– V_SW– V_OUT

causing a current to build up in L1. At the end of the switch

ON time, the current in L1 is equal to:

,

GND

SW2

*OPTIONAL

LT1111 • F04

V

− V − V

IN

SW

OUT

I

=

t

(22)

PEAK

ON

L

Figure 4. Step-Up Mode Hookup.

Refer to Table 1 for Component Values.

Note 1: This simple expression neglects the effect of switch and coil

resistance. This is taken into account in the “Inductor Selection” section.

10

LT1111

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PPLICATI

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S I FOR ATIO

Q1

When the switch turns off, the SW2 pin falls rapidly and

actually goes below ground. D1 turns on when SW2

reaches 0.4V below ground. D1 MUST BE A SCHOTTKY

DIODE. The voltage at SW2 must never be allowed to go

below –0.5V. Asilicondiodesuchasthe1N4933willallow

SW2togoto –0.8V, causingpotentiallydestructivepower

dissipation inside the LT1111. Output voltage is deter-

mined by:

R1

MJE210 OR

ZETEX ZTX749

0.3Ω

L1

V

30V

MAX

IN

V

OUT

R2

220

D1

1N5821

R3

330

V

I

L

SW1

IN

+

C2

C1

LT1111

R4

FB

SW2

GND

R2

R1

R5

R4

V

= 1.25V

(

1 +

)

OUT

V

= 1+

1.25V

(23)

R5

(

)

OUT

LT1111 • TA08

R3programsswitchcurrentlimit.Thisisespeciallyimpor-

tant in applications where the input varies over a wide

range.WithoutR3,theswitchstaysonforafixedtimeeach

cycle. Under certain conditions the current in L1 can build

up to excessive levels, exceeding the switch rating and/or

saturating the inductor. The 100Ω resistor programs the

switch to turn off when the current reaches approximately

700mA. When using the LT1111 in step-down mode,

output voltage should be limited to 6.2V or less. Higher

output voltages can be accommodated by inserting a

1N5818 diode in series with the SW2 pin (anode con-

nected to SW2).

Figure 6. Q1 Permits Higher Current Switching.

LT1111 Functions as Controller.

Inverting Configurations

The LT1111 can be configured as a positive-to-negative

converter (Figure 7), or a negative-to-positive converter

(Figure 8). In Figure 7, the arrangement is very similar to

a step-down, except that the high side of the feedback is

referredtoground.Thislevelshiftstheoutputnegative.As

in the step-down mode, D1 must be a Schottky diode,

and V_OUTshould be less than 6.2V. More negative out-

put voltages can be accommodated as in the prior section.

Higher Current Step-Down Operation

V

IN

R3

Output current can be increased by using a discrete PNP

pass transistor as shown in Figure 6. R1 serves as a

current limit sense. When the voltage drop across R1

equals a V_BE, the switch turns off. For temperature com-

pensation a Schottky diode can be inserted in series with

theI_LIMpin.ThisalsolowersthemaximumdropacrossR1

to V_BE– V_D, increasing efficiency. As shown, switch

current is limited to 2A. Inductor value can be calculated

based on formulas in the “Inductor Selection — Step-

Down Converter” section with the following conservative

I

V

IN

SW1

FB

LIM

+

C2

LT1111

GND

L1

SW2

R1

R2

D1

1N5818

+

C1

–V

OUT

LT1111 • F07

Figure 7. Positive-to-Negative Converter

expression for V_SW

= V + V ≈ 1.0V

Q1SAT

:

In Figure 8, the input is negative while the output is

positive. In this configuration, the magnitude of the input

voltage can be higher or lower than the output voltage. A

levelshift, providedbythePNPtransistor, suppliesproper

polarity feedback information to the regulator.

V

(24)

SW

R1

R2providesacurrentpathtoturnoffQ1. R3providesbase

drivetoQ1. R4andR5setoutputvoltage. APMOSFETcan

be used in place of Q1 when V_INis between 10V and 20V.

11

LT1111

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PPLICATI

A

I FOR ATIO

D1

L1

V

OUT

+

R1

I

L

C1

I

V

2N3906

LIM

IN

SW1

+

LT1111

C2

ON

A0

GND

SWITCH

FB

SW2

LT1111 • F08

OFF

LT1111 • F09

R2

R1

V

=

1.25V + 0.6V

(_R2)

OUT

–V

Figure 9. No Current Limit Causes Large Inductor

Current Build-Up

IN

Figure 8. Negative-to-Positive Converter

PROGRAMMED CURRENT LIMIT

Using the I_LIMPin

I

L

The LT1111 switch can be programmed to turn off at a set

switch current, a feature not found on competing devices.

This enables the input to vary over a wide range without

exceeding the maximum switch rating or saturating the

inductor. Consider the case wh ere analysis shows the

LT1111 must operate at an 800mA peak switch current

with a 2V input. If V_INrises to 4V, the peak switch current

will rise to 1.6A, exceeding the maximum switch current

rating. With the proper resistor selected (see the “Maxi-

mum Switch Current vs I_LIM” characteristic), the switch

current will be limited to 800mA, even if the input voltage

increases.

ON

SWITCH

OFF

LT1111 • F10

Figure 10. Current Limit Keeps Inductor Current Under Control

Figure 11 details current limit circuitry. Sense transistor

Q1, whose base and emitter are paralleled with power

switch Q2, is ratioed such that approximately 0.5% of

Q2’s collector current flows in Q1’s collector. This current

is passed through internal 80Ω resistor R1 and out

through the I_LIMpin. The value of the external resistor

connected between I_LIMand V_INsets the current limit.

When sufficient switch current flows to develop a V_BE

across R1 + R_LIM, Q3 turns on and injects current into the

oscillator, turning off the switch. Delay through this cir-

cuitry is approximately 1µs. The current trip point be-

comes less accurate for switch ON times less than 3µs.

Resistor values programming switch ON time for 1µs or

less will cause spurious response in the switch circuitry

although the device will still maintain output regulation.

Another situation where the I_LIMfeature is useful occurs

when the device goes into continuous mode operation.

This occurs in step-up mode when:

V

+

V

1

OUT

DIODE

<

(25)

V − V

1− DC

IN

SW

When the input and output voltages satisfy this relation-

ship, inductor current does not go to zero during the

switch OFF time. When the switch turns on again, the

current ramp starts from the non-zero current level in the

inductor just prior to switch turn-on. As shown in Figure

9, theinductorcurrentincreasestoahighlevelbeforethe

comparator turns off the oscillator. This high current can

causeexcessiveoutputrippleandrequiresoversizingthe

output capacitor and inductor. With the I_LIMfeature,

however, the switch current turns off at a programmed

level as shown in Figure 10, keeping output ripple to a

minimum.

R

I

LIM

(EXTERNAL)

LIM

V

IN

R1

80Ω

(INTERNAL)

Q3

SW1

Q2

DRIVER

Q1

OSCILLATOR

SW2

LT1111 • F11

Figure 11. LT1111 Current Limit Circuitry

12

LT1111

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S I FOR ATIO

A

Using the Gain Block

when the trip point is reached. Values in the 1M to 10M

range are optimal. However, the addition of R3 will

The gain block (GB) on the LT1111 can be used as an error

amplifier, low-battery detector or linear post regulator.

ThegainblockitselfisaverysimplePNPinputopampwith

an open collector NPN output. The negative input of the

gain block is tied internally to the 1.25V reference. The

positive input comes out on the SET pin.

change the trip point.

5V

LT1111

V

IN

47k

R1

R2

1.25V

REF

–

+

A0

TO

PROCESSOR

SET

V

BAT

Arrangement of the gain block as a low-battery detector

is straightforward. Figure 12 shows hookup. R1 and R2

need only be low enough in value so that the bias current

of the SET input does not cause large errors. 33k for R2

isadequate.R3canbeaddedtointroduceasmallamount

of hysteresis. This will cause the gain block to “snap”

V

– 1.25V

LB

35.1µA

R1 =

GND

V

= BATTERY TRIP POINT

R2 = 33k

LB

R3

R3 = 1.6M

LT1111 • F12

Figure 12. Setting Low-Battery Detector Trip Point

Table 1. Component Selection for Common Converters

INPUT

VOLTAGE

OUTPUT

VOLTAGE

OUTPUT

CURRENT (MIN)

CIRCUIT

FIGURE

INDUCTOR

VALUE

INDUCTOR

PART NUMBER

CAPACITOR

VALUE

NOTES

2 to 3.1

5

90mA

10mA

30mA

10mA

90mA

30mA

50mA

300mA

75mA

250mA

4

5

6

15µH

47µH

15µH

47µH

33µH

47µH

15µH

56µH

120µH

56µH

120µH

S CD75-750K

S CD54-470K, C CTX50-1

S CD75-150K

S CD54-470K, C CTX50-1

S CD75-330K

S CD75-470K, C CTX50-1

S CD54-150K

S CD105-560K, C CTX50-4

S CD105-121K, C CTX100-4

S CD75-560K, C CTX50-4

S CD105-121K, C CTX100-4

33µF

10µF

22µF

10µF

22µF

15µF

47µF

100µF

*

12

5

–5

5

6.5 to 11

12 to 20

20 to 30

5

**

12

**

S = Sumida

C = Coiltronics

* Add 47Ω from I

** Add 220Ω from I to V

to V

IN

LIM

IN

Table 3. Capacitor Manufacturers

MANUFACTURER

Table 2. Inductor Manufacturers

MANUFACTURER

PART NUMBERS

Coiltronics Incorporated

6000 Park of Commerce Blvd.

Boca Raton, FL 33487

407-241-7876

CTX100-4 Series

Surface Mount

Sanyo Video Components

1201 Sanyo Avenue

San Diego, CA 92073

619-661-6322

OS-CON Series

Nichicon America Corporation

927 East State Parkway

Schaumberg, IL 60173

708-843-7500

PL Series

Toko America Incorporated

1250 Feehanville Drive

Mount Prospect, IL 60056

312-297-0070

Type 8RBS

Sprague Electric Company

Lower Main Street

Sanford, ME 04073

207-324-4140

150D Solid Tantalums

550D Tantalex

Sumida Electric Co. USA

708-956-0666

CD54

CDR74

CDR105

Surface Mount

Matsuo

714-969-2491

267 Series

Surface Mount

13

LT1111

U

O

TYPICAL APPLICATI S

3V to –22V LCD Bias Generator

L1*

27µH

1N4148

R1

100Ω

732k

1%

I

V

LIM

IN

SW1

2 × 1.5V

CELLS

LT1111

3V

0.1µF

FB

GND

SW2

+

4.7µF

39.2k

1%

MBRS130T3

+

22µF

220k

* L1 = SUMIDA CD54-270K

–22V OUTPUT

FOR 5V INPUT CHANGE R1 TO 47Ω.

7mA AT 2V INPUT

CONVERTER WILL DELIVER –22V AT 40mA.

LT1111 • TA03

20V to 5V Step-Down Converter

9V to 5V Step-Down Converter

V

IN

12V TO 28V

100Ω

V

I

IN

SW1

LIM

I

V

LIM

IN

SW1

9V

BATTERY

LT1111-5

SENSE

SW2

SENSE

SW2

GND

L1*

GND

L1*

68µH

15µH

5V OUTPUT

150mA AT 9V INPUT

50mA AT 6.5V INPUT

5V OUTPUT

300mA

+

MBRS130T3

22µF

+

47µF

MBRS130T3

* L1 = SUMIDA CD54-150K

LT1111 • TA04

* L1 = SUMIDA CD74-680M

LT1111 • TA06

14

LT1111

U

O

TYPICAL APPLICATI S

5V to –5V Converter

V

IN

5V INPUT

100Ω

I

V

LIM

IN

SW1

+

22µF

LT1111-5

SENSE

SW2

GND

L1*

33µH

MBRS130T3

33µF

+

–5V OUTPUT

75mA

* L1= SUMIDA CD54-330K

LT1111 • TA05

Voltage Controlled Positive-to-Negative Converter

L1*

20µH, 3A

ZETEX^†

ZTX788A

0.22Ω

V

IN

5V TO 12V

+

47µF

–V

BAT54

MBRD320

220Ω

= –5.13 × V

OUT

C

2W MAXIMUM OUTPUT

V

I

IN

LIM

SW1

220Ω

V

200k

IN

39k

–

+

LT1111

V

(0V TO 5V)

C

LT1006

FB

SW2

GND

LT1111 • TA07

* L1 = COILTRONICS CTX20-4

^†ZETEX INC. 516-543-7100

High Power, Low Quiescent Current Step-Down Converter

L1*

10µH, 3A

0.22Ω

MTM20P08

5V

500mA

V

IN

8V TO 18V

+

BAT54

2k

51Ω

MBRD320

220µF

2N3904

V

I

LIM

IN

SW1

1N4148

LT1111

121k

FB

SW2

GND

40.2k

* L1 = SUMIDA CDR105-100M

OPERATE STANDBY

LT1111 • TA20

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

LT1111

U

Dimensions in inches (millimeters) unless otherwise noted.

PACKAGE DESCRIPTIO

J8 Package

8-Lead Ceramic DIP

0.405

(10.287)

MAX

CORNER LEADS OPTION

(4 PLCS)

0.005

(0.127)

MIN

0.200

(5.080)

MAX

0.290 – 0.320

(7.366 – 8.128)

6

5

4

8

7

0.023 – 0.045

(0.584 – 1.143)

HALF LEAD

OPTION

0.015 – 0.060

(0.381 – 1.524)

0.025

(0.635)

RAD TYP

0.220 – 0.310

(5.588 – 7.874)

0.045 – 0.068

(1.143 – 1.727)

FULL LEAD

OPTION

0.008 – 0.018

(0.203 – 0.457)

0° – 15°

1

2

3

0.045 – 0.068

(1.143 – 1.727)

0.385 ± 0.025

(9.779 ± 0.635)

0.125

3.175

MIN

0.100 ± 0.010

0.014 – 0.026

(2.540 ± 0.254)

(0.360 – 0.660)

NOTE: LEAD DIMENSIONS APPLY TO SOLDER DIP OR TIN PLATE LEADS.

N8 Package

8-Lead Plastic DIP

0.400

(10.160)

MAX

0.130 ± 0.005

0.045 – 0.065

0.300 – 0.320

(3.302 ± 0.127)

(1.143 – 1.651)

(7.620 – 8.128)

8

1

7

6

5

0.065

(1.651)

TYP

0.250 ± 0.010

(6.350 ± 0.254)

0.009 – 0.015

(0.229 – 0.381)

0.125

0.020

(0.508)

MIN

(3.175)

MIN

+0.025

–0.015

0.045 ± 0.015

(1.143 ± 0.381)

0.100 ± 0.010

(2.540 ± 0.254)

2

4

3

0.325

+0.635

8.255

(

)

–0.381

0.018 ± 0.003

(0.457 ± 0.076)

S8 Package

8-Lead Plastic SOIC

0.189 – 0.197

(4.801 – 5.004)

0.010 – 0.020

(0.254 – 0.508)

7

5

8

6

× 45°

0.004 – 0.010

(0.101 – 0.254)

0.053 – 0.069

(1.346 – 1.752)

0.008 – 0.010

(0.203 – 0.254)

0°– 8° TYP

0.150 – 0.157

(3.810 – 3.988)

0.228 – 0.244

(5.791 – 6.197)

0.016 – 0.050

0.406 – 1.270

0.050

(1.270)

BSC

0.014 – 0.019

(0.355 – 0.483)

*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.

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

1

3

4

2

SO8 0294

LT/GP 0594 5K REV C • PRINTED IN USA

LINEAR TECHNOLOGY CORPORATION 1994

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7487

16

●

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

型号:	LT1111CS8-5
Brand Name：	Linear Technology
是否Rohs认证：	不符合
生命周期：	Transferred
零件包装代码：	SOIC
包装说明：	SOP, SOP8,.25
针数：	8
制造商包装代码：	S8
Reach Compliance Code：	not_compliant
ECCN代码：	EAR99
HTS代码：	8542.39.00.01
风险等级：	5.07
模拟集成电路 - 其他类型：	SWITCHING REGULATOR
控制模式：	VOLTAGE-MODE
控制技术：	PULSE FREQUENCY MODULATION
最大输入电压：	30 V
最小输入电压：	2 V
标称输入电压：	3 V
JESD-30 代码：	R-PDSO-G8
JESD-609代码：	e0
长度：	4.9 mm
湿度敏感等级：	1
功能数量：	1
端子数量：	8
最高工作温度：	70 °C
最低工作温度：
最大输出电流：	1.5 A
标称输出电压：	5 V
封装主体材料：	PLASTIC/EPOXY
封装代码：	SOP
封装等效代码：	SOP8,.25
封装形状：	RECTANGULAR
封装形式：	SMALL OUTLINE
峰值回流温度（摄氏度）：	235
认证状态：	Not Qualified
座面最大高度：	1.75 mm
子类别：	Switching Regulator or Controllers
表面贴装：	YES
切换器配置：	SINGLE
最大切换频率：	88 kHz
技术：	BIPOLAR
温度等级：	COMMERCIAL
端子面层：	Tin/Lead (Sn/Pb)
端子形式：	GULL WING
端子节距：	1.27 mm
端子位置：	DUAL
处于峰值回流温度下的最长时间：	20
宽度：	3.9 mm
Base Number Matches：	1

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