LT1173CS8-5#TR_技术文档

LT1173

Micropower

DC/DC Converter

Adjustable and Fixed 5V, 12V

U

EATURE

S

DESCRIPTIO

F

■

Operates at Supply Voltages From 2.0V to 30V

Consumes Only 110µA Supply Current

Works in Step-Up or Step-Down Mode

Only Three External Components Required

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 SO8 Package

The LT1173 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

from2.0Vto12Vin step-upmodeandto30Vinstep-down

mode. The LT1173 functions equally well in step-up, step-

down or inverting applications.

The LT1173 consumes just 110µA supply current at

standby, making it ideal for applications where low quies-

cent current is important. The device can deliver 5V at

80mA from a 3V input in step-up mode or 5V at 200mA

from a 12V input in step-down mode.

O U

PPLICATI

S

A

■

Flash Memory Vpp Generators

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

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

LCD Bias Generators

Peripherals and Add-On Cards

Battery Backup Supplies

Laptop and Palmtop Computers

Cellular Telephones

Portable Instruments

Switch current limit can be programmed with a single

resistor. An auxiliary gain block can be configured as a low

battery detector, linear post regulator, under voltage lock-

out circuit or error amplifier.

For input sources of less than 2V, use the LT1073.

and LTC are registered trademarks and LT is a trademark of Linear Technology Corporation.

U

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TYPICAL APPLICATI S

Logic Controlled Flash Memory VPP Generator

VPP Output

L1*

100µH

1N5818

12V

100mA

5V

IN

47Ω

V_OUT

5V/DIV

I

V

IN

SW1

LIM

1.07M^†

0V

SANYO

+

LT1173

OS-CON

10µF

µ

100

F

FB

SW2

PROGRAM

5V/DIV

GND

124k^†

5ms/DIV

1173 TA02

1N4148

PROGRAM

LT1173 • TA01

*L1 = GOWANDA GA20-103K

COILTRONICS CTX100-4

EFFICIENCY = 81%

†

= 1% METAL FILM

NO OVERSHOOT

1

LT1173

W W W

U

ABSOLUTE AXI U RATI GS

/O

PACKAGE RDER I FOR ATIO

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

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

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

Feedback Pin Voltage (LT1173) ................................. 5V

Sense Pin Voltage (LT1173, -5, -12) ....................... 36V

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

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

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

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

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

TOP VIEW

ORDER PART

I

1

2

3

4

FB (SENSE)*

8

7

6

5

LIM

NUMBER

V

IN

SET

AO

LT1173CN8

LT1173CN8-5

LT1173CN8-12

SW1

SW2

GND

N8 PACKAGE

8-LEAD PLASTIC DIP

*FIXED VERSIONS

T_JMAX= 90°C, θ_JA= 130°C/W

TOP VIEW

LT1173CS8

LT1173CS8-5

LT1173CS8-12

I

1

2

3

4

8

7

6

5

FB (SENSE)*

LIM

V

IN

SET

AO

Consult factory for Industrial and Military grade parts

SW1

SW2

S8 PART MARKING

GND

S8 PACKAGE

8-LEAD PLASTIC SOIC

*FIXED VERSIONS

1173

11735

117312

T_JMAX= 90°C, θ_JA= 150°C/W

ELECTRICAL CHARACTERISTICS ^T_A^{= 25°C, V}_IN= 3V, unless otherwise noted.

SYMBOL

PARAMETER

CONDITIONS

Switch Off

No Load

MIN

TYP

110

135

250

MAX

UNITS

I

Quiescent Current

●

150

µA

V

Q

Quiescent Current, Boost

Mode Configuration

LT1173-5

LT1173-12

V

Input Voltage

Step-Up Mode

Step-Down Mode

LT1173 (Note 1)

LT1173-5 (Note 2)

LT1173-12 (Note 2)

LT1173

●

2.0

12.6

30

IN

V

Comparator Trip Point Voltage

Output Sense Voltage

1.20

4.75

11.4

1.245

5.00

12.0

5

1.30

5.25

12.6

10

V

OUT

V

Comparator Hysteresis

Output Hysteresis

mV

kHz

%

LT1173-5

20

40

LT1173-12

50

100

30

f

t

Oscillator Frequency

Duty Cycle

18

43

17

23

OSC

ON

Full Load

51

59

Switch ON Time

I

tied to V

22

32

µs

nA

V

LIM

IN

Feedback Pin Bias Current

Set Pin Bias Current

Gain Block Output Low

Reference Line Regulation

LT1173, V = 0V

10

50

FB

V

= V

20

100

0.4

0.075

0.65

1.0

1.4

SET

REF

V

I

= 100µA, V = 1.00V

0.15

0.2

0.02

0.5

0.8

OL

SINK

SET

2.0V ≤ V ≤ 5V

%/V

V

IN

5V ≤ V ≤ 30V

IN

SW

Voltage, Step-Up Mode

V

= 3.0V, I = 650mA

SW

SAT

IN

= 5.0V, I = 1A

V

SW

●

V

2

LT1173

ELECTRICAL CHARACTERISTICS _{T = 25°C, V}

_IN= 3V, unless otherwise noted.

A

SYMBOL

PARAMETER

CONDITIONS

MIN

TYP

MAX

1.5

UNITS

V

SAT

SW Voltage, Step-Down Mode

V

= 12V, I = 650mA

1.1

SAT

IN

SW

●

1.7

V

A

Gain Block Gain

R = 100kΩ (Note 3)

400

1000

400

V/V

mA

%/°C

µA

V

L

Current Limit

220Ω to I to V

LIM IN

Current Limit Temperature Coeff.

Switch OFF Leakage Current

Maximum Excursion Below GND

●

–0.3

Measured at SW1 Pin

≤ 10µA, Switch Off

1

10

V

SW2

I

–400

–350

mV

SW1

The

●

denotes the specifications which apply over the full operating

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.

temperature range.

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

Note 3: 100kΩ resistor connected between a 5V source and the AO pin.

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

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TYPICAL PERFOR A CE CHARACTERISTICS

Switch ON Voltage

Saturation Voltage Step-Up Mode

(SW2 Pin Grounded)

Step-Down Mode

Maximum Switch Current vs

R_LIMStep-Up Mode

(SW1 Pin Connected to V_IN)

1.2

1.0

0.8

0.6

0.4

0.2

0

1200

1100

1000

900

800

700

600

500

400

300

200

100

1.4

1.3

1.2

1.1

1.0

0.9

0.8

0.7

2V ≤ V ≤ 5V

IN

V

= 3.0V

IN

V

= 5.0V

V

= 2.0V

IN

0

0.2

0.4

0.6

0.8

1.0

1.2

10

100

(Ω)

1000

0

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

I

(A)

R

I

(A)

SWITCH

LIM

SWITCH

LT1173 • TPC01

LT1173 • TPC03

LT1173 • TPC02

Maximum Switch Current vs

R_LIMStep-Down Mode

Set Pin Bias Current vs

Temperature

Feedback Pin Bias Current vs

Temperature

1000

900

800

700

600

500

400

300

200

100

0

20

15

10

5

18

16

14

V

= 5V

OUT

V

= 3V

IN

V

= 3V

IN

V

= 24V

IN

L = 500µH

V

= 12V

IN

L = 250µH

12

10

8

100

1000

–50 –25

0

25

TEMPERATURE (°C)

50

75

–50 –25

0

25

TEMPERATURE (°C)

50

75 100

125

100 125

R

(Ω)

LIM

LT1173 • TPC09

LT1173 •TPC04

LT1173 •TPC05

3

LT1173

TYPICAL PERFOR A CE CHARACTERISTICS

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Quiescent Current vs Temperature

Supply Current vs Switch Current

Oscillator Frequency

26.0

25.5

120

110

100

90

50

40

30

V

= 3V

IN

25.0

24.5

24.0

23.5

23.0

22.5

22.0

V

= 5V

IN

20

10

0

V

= 2V

IN

–50 –25

0

25

50

75 100 125

0

200

400

600

800

1000

0

5

10

15

(V)

20

25

30

TEMPERATURE (°C)

SWITCH CURRENT (mA)

V

IN

LT1173 •TPC06

LT1173 •TPC07

LT1173 • TPC08

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PI

FU CTI

S

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.

AO (Pin 6): Auxiliary Gain Block (GB) output. Open collec-

tor, can sink 100µA.

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

connected to SET pin and negative input connected to

1.245V reference.

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.

FB/SENSE (Pin 8): On the LT1173 (adjustable) this pin

goes to the comparator input. On the LT1173-5 and

LT1173-12, this pin goes to the internal application resis-

tor that sets output voltage.

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.

W

BLOCK DIAGRA S

LT1173

LT1173-5, -12

SET

A2

AO

SET

V

IN

A2

AO

V

GAIN BLOCK/

ERROR AMP

IN

I

LIM

SW1

GAIN BLOCK/

ERROR AMP

I

LIM

SW1

1.245V

REFERENCE

1.245V

REFERENCE

OSCILLATOR

A1

OSCILLATOR

DRIVER

COMPARATOR

SENSE

DRIVER

SW2

R2

753kΩ

COMPARATOR

R1

LT1173-5: R1 = 250kΩ

LT1173-12: R1 = 87.4kΩ

GND

SW2

GND

FB

LT1173 • BD01

LT1173 • BD02

4

LT1173

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LT1173 OPERAT

The LT1173 is a gated oscillator switcher. This type archi-

tecture has very low supply current because the switch is

cycledonlywhenthefeedbackpinvoltagedropsbelowthe

reference voltage. Circuit operation can best be under-

stoodbyreferringtotheLT1173blockdiagram.Compara-

tor A1 compares the feedback pin voltage with the 1.245V

referencevoltage.Whenfeedbackdropsbelow1.245V, A1

switches on the 24kHz oscillator. The driver amplifier

boosts the signal level to drive the output NPN power

switch. An adaptive base drive circuit senses switch

current and provides just enough base drive to ensure

switchsaturationwithoutoverdrivingtheswitch,resulting

in higher efficiency. The switch cycling action raises the

output voltage and feedback pin voltage. When the feed-

back voltage is sufficient to trip A1, the oscillator is gated

off.AsmallamountofhysteresisbuiltintoA1ensuresloop

stability without external frequency compensation. When

the comparator is low the oscillator and all high current

circuitryisturned off, lowering device quiescent current

to just 110µA, for the reference, A1 and A2.

A2 is a versatile gain block that can serve as a low battery

detector, a linear post regulator, or drive an under voltage

lockout circuit. The negative input of A2 is internally

connected to the 1.245V 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 applica-

tion.Thegainblockoutput(AO)cansink100µA(usea47k

resistor pull-up to +5V). This line can signal a microcon-

troller that the battery voltage has dropped below the

preset level.

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 2µs.

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.

The oscillator is set internally for 23µs ON time and 19µs

OFF time, optimizing the device for circuits where V_OUT

and V_INdiffer by roughly a factor of 2. Examples include a

3V to 5V step-up converter or a 9V to 5V step-down

converter.

The LT1173-5 and LT1173-12 are functionally identical to

the LT1173. 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.

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Measuring Input Current at Zero or Light Load

approach is required to measure the 100µA off-state and

Obtaining meaningful numbers for quiescent current and

efficiency at low output current involves understanding

howtheLT1173operates. Atveryloworzeroloadcurrent,

the device is idling for seconds at a time. When the output

voltage falls enough to trip the comparator, the power

switch comes on for a few cycles until the output voltage

rises sufficiently to overcome the comparator hysteresis.

When the power switch is on, inductor current builds up

to hundreds of milliamperes. Ordinary digital multimeters

are not capable of measuring average current because of

bandwidth and dynamic range limitations. A different

500mA on-state currents of the circuit.

Quiescent current can be accurately measured using the

circuit in Figure 1. V_SETis set to the input voltage of the

LT1173. The circuit must be “booted” by shorting V2 to

V_SET. After the LT1173 output voltage has settled, discon-

nect the short. Input voltage is V2, and average input

current can be calculated by this formula:

V2 − V1

100Ω

I

=

01

( )

IN

5

LT1173

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S

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PPLICATI

A

1MΩ

theinductiveeventsaddtotheinputvoltagetoproducethe

output voltage. Power required from the inductor is deter-

mined by

+12V

–

1µF*

100Ω

LT1173

LTC1050

+

P_L= (V_OUT+ V_D– V_IN) (I_OUT

)

(02)

CIRCUIT

V1

V2

+

1000µF

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

V

SET

*NON-POLARIZED

LT1173 • TA06

Figure 1. Test Circuit Measures No Load Quiescent Current of

LT1073 Converter

P

L

03

( )

F

OSC

Inductor Selection

in order for the converter to regulate the output.

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 that

maximum current ratings of the LT1173 and inductor are

not exceeded at the other worst case condition of maxi-

mum input voltage and ON time. Additionally, the inductor

coremustbeabletostoretherequiredflux;i.e., itmustnot

saturate. At power levels generally encountered with

LT1173 based designs, small axial leaded units with

saturation current ratings in the 300mA to 1A range

(depending on application) are adequate. Lastly, the in-

ductor must have sufficiently low DC resistance so that

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 recom-

mended 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.

When the switch is closed, current in the inductor builds

according to

–R't

L

V

R'

IN

I t =

1– e

04

( )

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

05

( )

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 LT1173 speci-

fication table (typically 23µs) 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

06

( )

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.

Specifying a proper inductor for an application requires

first establishing minimum and maximum input voltage,

outputvoltage,andoutputcurrent.Ina step-upconverter,

6

LT1173

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PPLICATI

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As an example, suppose 9V at 50mA is to be generated

from a 3V input. Recalling Equation 02,

In the negative-to-positive case, the switch saturates and

the 0.8Ω switch ON resistance value given for Equation 04

can be used. In both cases inductor design proceeds from

Equation 03.

P_L= (9V + 0.5V – 3V) (50mA) = 325mW.

Energy required from the inductor is

(07)

The step-down case is different than the preceeding three

in that the inductor current flows through the load in a

step-downtopology(Figure6).Currentthroughtheswitch

shouldbelimitedto~650mAinstep-downmode. Thiscan

beaccomplishedbyusingtheI_LIMpin. Withinputvoltages

in the range of 12V to 25V, a 5V output at 300mA can be

generated with a 220µH inductor and 100Ω resistor in

series with the I_LIMpin. With a 20V to 30V input range, a

470µH inductor should be used along with the 100Ω

resistor.

P

325mW

24kHz

L

=

= 13.5µJ.

08

( )

F

OSC

Pickingan inductor valueof100µH with 0.2ΩDCR results

in a peak switch current of

–1Ω •23µs

100µH

3V

i

=

1– e

= 616mA.

09

( )

PEAK

1Ω

Substituting i_PEAKinto Equation 04 results in

Capacitor Selection

2

1

2

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

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 2, 3, and 4 show the output voltage of an LT1173

based converter with three 100µF capacitors. The peak

switch current is 500mA in all cases. Figure 2 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

worksouttobeanESRofover240mΩ.Figure3showsthe

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 70mΩ.

Figure 4 shows the circuit with a 16V OS-CON unit. ESR is

now only 20mΩ.

E = 100µH 0.616A = 19.0µJ.

)(

10

(

)

( )

L

Since 19µJ > 13.5µJ the 100µH 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 LT1073. The 70% duty cycle of the

LT1073 allows more energy per cycle to be stored in the

inductor, resulting in more output power.

An inductor’s energy storage capability is proportional to

its physical size. If the size of the inductor is too large for

a particular application, considerable size reduction is

possible by using the LT1111. This device is pin compat-

ible with the LT1173 but has a 72kHz oscillator, thereby

reducing inductor and capacitor size requirements by a

factor of three.

For both positive-to-negative (Figure 7) and negative-to-

positive configurations (Figure 8), all the output power

must be generated by the inductor. In these cases

P_L= ( V_OUT+ V_D) (I_OUT).

(11)

In the positive-to-negative case, switch drop can be mod-

eled as a 0.75V voltage source in series with a 0.65Ω

resistor so that

V_L= V_IN– 0.75V – I_L(0.65Ω).

(12)

7

LT1173

PPLICATI

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5µs/DIV

LT1173 • TA07

LT1173 • TA09

LT1173 • TA08

Figure 2. Aluminum

Figure 3. Tantalum

Figure 4. OS-CON

In very low power applications where every microampere

is important, leakage current of the capacitor must be

considered. The OS-CON units do have leakage current in

the 5µA to 10µA range. If the load is also in the microam-

pere range, a leaky capacitor will noticeably decrease

efficiency. In this type application tantalum capacitors are

the best choice, with typical leakage currents in the 1µA to

5µA range.

Step-Up (Boost Mode) Operation

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.

The usual step-up configuration for the LT1173 is shown

in Figure 5. The LT1173 first pulls SW1 low causing V_IN–

V_CESATtoappearacrossL1. Acurrentthenbuilds up in L1.

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

Diode Selection

Speed, forward drop, and leakage current are the three

main considerations in selecting a catch diode for LT1173

converters.Generalpurposerectifierssuchasthe1N4001

are unsuitable for use in any switching regulator applica-

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

a1N4001isinthe10µs-50µsrange.Atbest,efficiencywill

be severely compromised when these diodes are used; at

worst,thecircuitmaynotworkatall.MostLT1173circuits

will be well served by a 1N5818 Schottky diode. 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 LT1173 requirements. At peak

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

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

5nArangeat25°Candlowercostthana1N5818. (Youcan

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

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

situations where the load is intermittent and the LT1173 is

idling most of the time, battery life can sometimes be

extended by using a silicon diode such as the 1N4933,

which can handle 1A but has leakage current of less than

1µA. Efficiency will decrease somewhat compared to a

1N5818 while delivering power, but the lower idle current

may be more important.

V

IN

i

=

t

13

( )

PEAK

ON

L

D1

L1

V

IN

OUT

R3*

I

V

LIM

IN

SW1

R2

R1

+

C1

LT1173 FB

GND

SW2

* = OPTIONAL

LT1173 • TA10

Figure 5. Step-Up Mode Hookup.

Refer to Table 1 for Component Values

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

V_OUT. This action is repeated as needed by the LT1173 to

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

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

8

LT1173

O U

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U

PPLICATI

A

S I FOR ATIO

keep V_FBat the internal reference voltage of 1.245V. R1

and R2 set the output voltage according to the formula

R3 programs switch current limit. This is especially im-

portant in applications where the input varies over a wide

range. Without R3, the switch stays on for a fixed time

each 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 pro-

grams the switch to turn off when the current reaches

approximately 800mA. When using the LT1173 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 connected to SW2).

R2

R1

V

= 1+

1.245V .

14

( )

(

)

OUT

Step-Down (Buck Mode) Operation

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

to a lower voltage. The usual hookup for an LT1173 based

step-down converter is shown in Figure 6.

V

IN

R3

100Ω

Inverting Configurations

+

I

V

IN

SW1

FB

LIM

C2

The LT1173 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

referred to ground. This level shifts the output negative.

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

diode, and V_OUTshould be less than 6.2V. More nega-

tive output voltages can be accomodated as in the prior

section.

LT1173

GND

L1

V

SW2

OUT

R2

R1

D1

1N5818

+

C1

LT1173 • TA11

Figure 6. Step-Down Mode Hookup

+V

IN

Whentheswitchturnson, SW2 pullsuptoV_IN–V_SW. This

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

R3

,

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

ON time, the current in L1 is equal to

+

I

V

IN

SW1

FB

LIM

C2

LT1173

GND

V

− V − V

IN

L1

SW

OUT

i

=

t

.

ON

15

SW2

( )

PEAK

L

R1

R2

D1

1N5818

+

C1

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 LT1173. Output voltage is deter-

mined by

–V

OUT

LT1173 • F07

Figure 7. Positive-to-Negative Converter

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.

R2

R1

V

= 1+

1.245V .

16

( )

(

)

OUT

9

LT1173

O U

S

W U

I FOR ATIO

PPLICATI

A

D1

L1

+V

OUT

+

R1

C1

I

L

I

V

2N3906

LIM

IN

SW1

+

LT1173

C2

ON

AO

GND

FB

SW2

SWITCH

OFF

LT1173 • TA14

R2

R1

V

=

1.245V + 0.6V

( _R2)

OUT

Figure 9. No Current Limit Causes Large Inductor

Current Build-Up

–V

LT1173 • TA13

IN

Figure 8. Negative-to-Positive Converter

PROGRAMMED CURRENT LIMIT

Using the I_LIMPin

I

L

The LT1173 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 where analysis shows the

LT1173 must operate at an 800mA peak switch current

witha2.0Vinput.IfV_INrisesto4V,thepeakswitchcurrent

will rise to 1.6A, exceeding the maximum switch current

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

mum Switch Current vs R_LIM” characteristic), the switch

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

increases.

ON

SWITCH

OFF

LT1173 • TA15

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

switchQ2,isratioedsuchthatapproximately0.5%ofQ2’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 suffi-

cient switch current flows to develop a V_BEacross R1 +

R_LIM, Q3 turns on and injects current into the oscillator,

turning off the switch. Delay through this circuitry is

approximately 2µs. The current trip point becomes less

accurate for switch ON times less than 4µs. Resistor

values programming switch ON time for 2µs 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

<

.

17

( )

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, the inductor current increases to a high level before the

comparator turns off the oscillator. This high current can

cause excessive output ripple and requires oversizing the

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

LT1173 • TA28

Figure 11. LT1173 Current Limit Circuitry

10

LT1173

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A

S I FOR ATIO

+5V

Using the Gain Block

V

IN

The gain block (GB) on the LT1173 can be used as an error

amplifier, lowbatterydetectororlinearpostregulator. The

gain block itself is a very simple PNP input op amp with an

open collector NPN output. The negative input of the gain

block is tied internally to the 1.245V reference. The posi-

tive input comes out on the SET pin.

LT1173

100k

R1

R2

1.245V

REF

–

+

V

AO

TO

BAT

PROCESSOR

SET

GND

Arrangement of the gain block as a low battery detector is

straightforward.Figure12showshookup.R1andR2need

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

SET input does not cause large errors. 100kΩ for R2 is

adequate. R3 can be added to introduce a small amount of

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

the trip point is reached. Values in the 1M-10M range are

optimal. The addition of R3 will change the trip point,

however.

R3

V

– 1.245V

11.7µA

= BATTERY TRIP POINT

R2 = 100kΩ

R3 = 4.7MΩ

LB

R1 =

V

LB

LT1173 • TA16

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

2.0-3.1

5

90mA

10mA

50mA

10mA

90mA

30mA

50mA

25mA

50mA

300mA

75mA

250mA

150mA

75mA

5

6

7

8

47µH

220µH

47µH

G GA10-472K, C CTX50-1

G GA10-223K, C CTX

G GA10-472K, C CTX50-1

G GA10-153K

100µF

22µF

*

12

15

30

5

47µF

*

150µH

120µH

150µH

120µH

100µH

47µH

22µF

G GA10-123K

100µF

47µF

5

G GA10-153K

**

5

G GA10-123K C CTX100-4

G GA10-103K, C CTX100-4

G GA10-472K, C CTX50-1

G GA20-223K

47µF

5

10µF, 50V

100µF

220µF

470µF

100µF

220µF

47µF

6.5-9.5

12-20

20-30

5

**

5

220µH

470µH

100µH

470µH

100µH

5

G GA20-473K

–5

5

G GA10-103K, C CTX100-4

G GA40-473K

12

–5

G GA10-103K, C CTX100-4

–5

12

G = Gowanda

C = Coiltronics

* Add 68Ω from I to V

LIM

IN

** Add 100Ω from I

to V

IN

LIM

11

LT1173

PPLICATI

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Table 2. Inductor Manufacturers

MANUFACTURER

Table 3. Capacitor Manufacturers

MANUFACTURER

PART NUMBERS

Gowanda Electronics Corporation

1 Industrial Place

Gowanda, NY 14070

GA10 Series

GA40 Series

Sanyo Video Components

2001 Sanyo Avenue

San Diego, CA 92173

619-661-6835

OS-CON Series

716-532-2234

Caddell-Burns

7300 Series

6860 Series

Nichicon America Corporation

927 East State Parkway

Schaumberg, IL 60173

708-843-7500

PL Series

258 East Second Street

Mineola, NY 11501

516-746-2310

Coiltronics International

984 S.W. 13th Court

Pompano Beach, FL 33069

305-781-8900

Custom Toroids

Surface Mount

Sprague Electric Company

Lower Main Street

Sanford, ME 04073

207-324-4140

150D Solid Tantalums

550D Tantalex

Renco Electronics Incorporated

60 Jefryn Boulevard, East

Deer Park, NY 11729

800-645-5828

RL1283

RL1284

U

O

TYPICAL APPLICATI S

3V to –22V LCD Bias Generator

L1*

100µH

1N4148

R1

100Ω

2.21M

1%

I

V

LIM

IN

SW1

2 X 1.5V

CELLS

LT1173

3V

FB

GND

SW2

+

4.7µF

0.1µF

118k

1%

1N5818

+

22µF

220k

* L1 = GOWANDA GA10-103K

COILTRONICS CTX100-4

–22V OUTPUT

7mA AT 2.0V INPUT

FOR 5V INPUT CHANGE R1 TO 47Ω.

70% EFFICIENCY

CONVERTER WILL DELIVER –22V AT 40mA.

LT1173 • TA19

12

LT1173

U

O

TYPICAL APPLICATI S

3V to 5V Step-Up Converter

9V to 5V Step-Down Converter

L1*

100µH

100Ω

V

IN

SW1

I

LIM

I

V

LIM

IN

SW1

9V

BATTERY

LT1173-5

2 X 1.5V

CELLS

1N5818

LT1173-5

SENSE

SW2

5V OUTPUT

150mA AT 3V INPUT

60mA AT 2V INPUT

GND

L1*

47µH

SENSE

SW2

5V OUTPUT

GND

+

150mA AT 9V INPUT

50mA AT 6.5V INPUT

100µF

+

1N5818

100µF

* L1 = GOWANDA GA10-472K

COILTRONICS CTX50-1

FOR HIGHER OUTPUT CURRENTS SEE LT1073 DATASHEET

* L1 = GOWANDA GA10-103K

LT1173 • TA18

COILTRONICS CTX100-1 (SURFACE MOUNT)

LT1173 • TA17

+5V to –5V Converter

+20V to 5V Step-Down Converter

+V

IN

5V INPUT

+V

IN

12V-28V

100Ω

I

V

I

V

LIM

IN

LIM

IN

SW1

+

22µF

LT1173-5

SENSE

SW2

SENSE

SW2

GND

L1*

100µH

L1*

220µH

5V OUTPUT

300mA

+

µ

1N5818

100µF

1N5818

100

F

–5V OUTPUT

75mA

* L1= GOWANDA GA10-103K

COILTRONICS CTX100-1

* L1 = GOWANDA GA20-223K

LT1173 • TA21

LT1173 • TA20

Telecom Supply

L1*

500µH

MUR110

+5V

100mA

44mH

~

+

–

+

47µF

+

48V DC

390kΩ

220µF

10V

100V

3.6MΩ

~

44mH

10k

VN2222

12V

2N5400

10nF

*L1 = CTX110077

IRF530

I

= 120µA

Q

100Ω

1N4148

15V

I

V

IN

LIM

SW1

+

10µF

16V

1N965B

LT1173

FB

GND

SW2

110kΩ

LT1173 • TA22

13

LT1173

U

O

TYPICAL APPLICATI S

“5 to 5” Step-Up or Step-Down Converter

L1*

100µH

1N5818

SI9405DY

+5V

OUTPUT

56Ω

1

2

470k

75k

I

V

LIM

IN

SW1

3

6

8

4 X NICAD

+

7

OR

SET LT1173 AO

470µF

ALKALINE

CELLS

FB

+

GND

5

SW2

4

240Ω

470µF

24k

*L1 = COILTRONICS CTX100-4

GOWANDA GA20-103K

V

OUT

= 2.6V TO 7.2V

= 5V AT 100mA

IN

LT1173 • TA23

V

2V to 5V at 300mA Step-Up Converter with Under Voltage Lockout

L1*

20µH, 5A

1N5820

47k

100k

2.2M

220

I

V

IN

LIM

100k

100

2N3906

SW1

2N4403

AO

LT1173

2 X NICAD

+5V OUTPUT

300mA

†

301k

SET

GND

FB

LOCKOUT AT

1.85V INPUT

SW2

5Ω

+

100µF

OS-CON

MJE200

†

100k

47Ω

*L1 = COILTRONICS CTX-20-5-52

1% METAL FILM

LT1173 • TA24

†

14

LT1173

U

O

TYPICAL APPLICATI S

Voltage Controlled Positive-to-Negative Converter

L1*

50µH, 2.5A

0.22

MJE210

V

IN

5V-12V

+

100µF

–V

1N5818

1N5820

220

= –5.13 • V

OUT

C

2W MAXIMUM OUTPUT

V

I

LIM

IN

150

V

200k

SW1

IN

39k

–

+

LT1173

V

(0V TO 5V)

C

LT1006

FB

SW2

GND

LT1173 • TA25

* L1 = GOWANDA GT10-101

High Power, Low Quiescent Current Step-Down Converter

L1*

25µH, 2A

0.22Ω

MTM20P08

V

5V

500mA

IN

7V-24V

18V

1W

+

1N5818

2k

51Ω

1N5820

470µF

2N3904

100Ω

1/2W

V

I

LIM

IN

SW1

1N4148

LT1173

121k

FB

SW2

GND

40.2k

* L1 = GOWANDA GT10-100

EFFICIENCY ≥ 80% FOR 10mA ≤ I

≤ 500mA

LOAD

STANDBY I ≤ 150µA

Q

OPERATE STANDBY

LT1173 • TA26

2 Cell Powered Neon Light Flasher

0.02µF

L1*

470µH

1N4148

95V REGULATED

I

V

LIM

IN

SW1

0.02µF

3V

LT1173

100M

FB

NE-2

BLINKS AT

0.5Hz

GND

SW2

0.68µF

200V

1.3M

3.3M

LT1173 • TA27

*TOKO 262LYF-0100K

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

LT1173

U

PACKAGE DESCRIPTIO Dimensions in inches (milimeters) unless otherwise noted.

N8 Package

8-Lead Plastic DIP

0.400*

(10.160)

MAX

8

7

6

5

4

0.255 ± 0.015*

(6.477 ± 0.381)

1

2

3

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 0894 2K REV B • PRINTED IN USA

LINEAR TECHNOLOGY CORPORATION 1994

Linear Technology Corporation

16

1630 McCarthy Blvd., Milpitas, CA 95035-7487

●

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


型号：	LT1173CS8-5#TR
厂家：	Linear
描述：	LT1173 - Micropower DC/DC Converter Adjustable and Fixed 5V, 12V; Package: SO; Pins: 8; Temperature Range: 0°C to 70°C 开关光电二极管
文件：	总16页 (文件大小：272K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

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