Datasheet LT1173 Datasheet (Linear Technology)

LT1173

Micropower

DC/DC Converter

Adjustable and Fixed 5V, 12V

EATU

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

PPLICATI

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

RE

S

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DUESCRIPTIO

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
from 2.0V to 12V in step-up mode and to 30V in step-down
mode. The LT1173 functions equally well in step-up, step­down or inverting applications.

The LT1173 consumes just 110µA supply current at
standby, making it ideal for applications where low quies­cent 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.

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 lock­out 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.

O

PPLICATITYPICAL

Logic Controlled Flash Memory VPP Generator

5V

IN

+

10 F

µ

*L1 = GOWANDA GA20-103K
COILTRONICS CTX100-4

NO OVERSHOOT

47Ω

V

I

LIM

GND SW2

PROGRAM

IN

LT1173

SW1

100µH

FB

1N4148

L1*

1N5818

EFFICIENCY = 81%

†

= 1% METAL FILM

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SA



1.07M

124k

VPP Output

12V
100mA

V

OUT

5V/DIV

†

SANYO

+

OS-CON

µ

100 F

†

LT1173 • TA01

0V

PROGRAM

5V/DIV

5ms/DIV

1173 TA02

1

LT1173

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PACKAGE

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O

RDER I FOR ATIO

A

W

O

LUTEXI T

S

A

WUW

ARB

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Supply Voltage (VIN)................................................ 36V

SW1 Pin Voltage (V
SW2 Pin Voltage (V

) .......................................... 50V

SW1

) .............................–0.5V to V

SW2

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

Consult factory for Industrial and Military grade parts



I

1

LIM



V

2

IN

SW1

3

SW2

4

N8 PACKAGE

8-LEAD PLASTIC DIP

*FIXED VERSIONS

T

= 90°C, θJA = 130°C/W

JMAX

I



1

LIM



V

2

IN

SW1

3

SW2

4

S8 PACKAGE

8-LEAD PLASTIC SOIC

*FIXED VERSIONS

T

= 90°C, θJA = 150°C/W

JMAX

TOP VIEW

TOP VIEW

FB (SENSE)*

8

SET

7

AO

6

GND

5



FB (SENSE)*

8

SET

7

AO

6

GND

5

ORDER PART

NUMBER

LT1173CN8
LT1173CN8-5
LT1173CN8-12

LT1173CS8
LT1173CS8-5
LT1173CS8-12

S8 PART MARKING

1173
11735
117312

LECTRICAL C CHARA TERIST

E

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

I
I

V

V

f

t

V

V

Q

Q

IN

OUT

OSC

ON

OL

SAT

Quiescent Current Switch Off ● 110 150 µA
Quiescent Current, Boost No Load LT1173-5 135 µA

Mode Configuration
Input Voltage Step-Up Mode ● 2.0 12.6 V

Comparator Trip Point Voltage LT1173 (Note 1) ● 1.20 1.245 1.30 V
Output Sense Voltage LT1173-5 (Note 2) ● 4.75 5.00 5.25 V

Comparator Hysteresis LT1173 ● 510 mV
Output Hysteresis LT1173-5 ● 20 40 mV

Oscillator Frequency ● 18 23 30 kHz
Duty Cycle Full Load ● 43 51 59 %
Switch ON Time I
Feedback Pin Bias Current LT1173, VFB = 0V ● 10 50 nA
Set Pin Bias Current V
Gain Block Output Low I
Reference Line Regulation 2.0V ≤ VIN ≤ 5V ● 0.2 0.4 %/V

SW

Voltage, Step-Up Mode VIN = 3.0V, ISW = 650mA ● 0.5 0.65 V

SAT

ICS

TA = 25°C, VIN = 3V, unless otherwise noted.

LT1173-12 250 µA

Step-Down Mode ● 30 V

LT1173-12 (Note 2) ● 11.4 12.0 12.6 V

LT1173-12 ● 50 100 mV

tied to V

LIM

SET

SINK

5V ≤ VIN ≤ 30V ● 0.02 0.075 %/V

VIN = 5.0V, ISW = 1A 0.8 1.0 V

IN

= V

REF

= 100µA, V

= 1.00V ● 0.15 0.4 V

SET

● 17 22 32 µs

● 20 100 nA

● 1.4 V

2

LT1173

TEMPERATURE (°C)

–50

FEEDBACK PIN BIAS CURRENT ( A)

14

16

18

–25 0 25 50

LT1173 •TPC05

75

100

125

V = 3V

IN

µ

12

10

8

R ( )

10

100

SWITCH CURRENT (mA)

400

800

1200

100

LT1173 • TPC03

Ω

1000

900

700
600

500

300
200

LIM

1000

1100

2V ≤ VIN ≤ 5V

LECTRICAL C CHARA TERIST

E

ICS

TA = 25°C, V

= 3V, unless otherwise noted.

IN

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

V

SAT

A

V

SW

Voltage, Step-Down Mode VIN = 12V, ISW = 650mA 1.1 1.5 V

SAT

● 1.7 V

Gain Block Gain RL = 100kΩ (Note 3) ● 400 1000 V/V
Current Limit 220Ω to I

LIM

to V

IN

400 mA

Current Limit Temperature Coeff. ● –0.3 %/°C
Switch OFF Leakage Current Measured at SW1 Pin 1 10 µA

V

SW2

The ● denotes the specifications which apply over the full operating
temperature range.

Note 1: This specification guarantees that both the high and low trip points
of the comparator fall within the 1.20V to 1.30V range.

Maximum Excursion Below GND I

≤ 10µA, Switch Off –400 –350 mV

SW1

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.

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

UW

Y

PICA

1.2

LPER

F

O

R

AT

CCHARA TERIST

E

C

ICS

Switch ON Voltage
Saturation Voltage Step-Up Mode Step-Down Mode Maximum Switch Current vs
(SW2 Pin Grounded) (SW1 Pin Connected to VIN)R

1.4

Step-Up Mode

LIM

1.0

0.8

0.6

CESAT

V (V)

0.4

0.2

0

0

0.2 0.4 0.6 0.8

Maximum Switch Current vs Set Pin Bias Current vs Feedback Pin Bias Current vs
R

Step-Down Mode Temperature Temperature

LIM

1000

900
800
700
600
500
400
300

SWITCH CURRENT (mA)

200
100

0

100

= 12V

V

IN

L = 250µH

V = 3.0V

V = 2.0V

IN

I (A)

SWITCH

VIN = 24V
L = 500µH

R ( )

LIM

1.3

IN

V = 5.0V

IN

1.0 1.2

LT1173 • TPC01

V

= 5V

OUT

1000

Ω

LT1173 • TPC09

1.2

1.1

1.0

0.9

SWITCH ON VOLTAGE (V)

0.8

0.7

0.1 0.2 0.3 0.4

0

I (A)

SWITCH

20

15

10

SET PIN BIAS CURRENT (nA)

5

–25 0 25 50

–50

TEMPERATURE (°C)

0.5 0.6

V = 3V

IN

75

0.7 0.8

LT1173 • TPC02

100

LT1173 •TPC04

125

3

LT1173

VIN(V)

0

22.0

F

OSC

(kHz)

22.5

23.0

23.5

24.5

25.0

5101520

LT1173 • TPC08

24.0

25 30

25.5

26.0

UW

Y

PICA

120

LPER

F

O

R

AT

CCHARA TERIST

E

C

ICS

Quiescent Current vs Temperature Supply Current vs Switch Current Oscillator Frequency

50

V = 3V

IN

40

110

µ

IN

I ( A)

100

90

–25 0 25 50

–50

TEMPERATURE (°C)

U

PI

I

FUUC

(Pin 1): Connect this pin to VIN for normal use. Where

LIM

TI

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U
S

75

100

LT1173 •TPC06

125

30

20

SUPPLY CURRENT (mA)

10

0

0

V = 5V

200

400 600 800

SWITCH CURRENT (mA)

lower current limit is desired, connect a resistor between
I

and VIN. A 220Ω resistor will limit the switch current

LIM

to approximately 400mA.

V

(Pin 2): Input supply voltage.

IN

SW1 (Pin 3): Collector of power transistor. For step-up

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

SW2 (Pin 4): Emitter of power transistor. For step-up

mode connect to ground. For step-down mode connect to
inductor/diode. This pin must never be allowed to go more
than a Schottky diode drop below ground.

IN

V = 2V

IN

1000

LT1173 •TPC07

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.
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 resis­tor that sets output voltage.

BLOCK

V

IN

1.245V

REFERENCE

GND

4

SET

IDAGRA

A2

GAIN BLOCK/
ERROR AMP

COMPARATOR

FB

LT1173

A1

W

S

AO

OSCILLATOR

I

LIM

DRIVER

SW1

SW2

LT1173 • BD01

REFERENCE

GND

V

IN

1.245V

R1

SET

R2

753k

A2

GAIN BLOCK/

ERROR AMP

Ω

LT1173-5, -12

A1

COMPARATOR

SENSE

LT1173-12:

AO

OSCILLATOR

LT1173-5:

I

DRIVER

R1 = 250k
R1 = 87.4k

LIM

Ω

Ω

SW1

SW2

LT1173 • BD02

LT1173 OPER

I

VV

IN

=

−

()

21

10001Ω

AT

LT1173

U

O

I

The LT1173 is a gated oscillator switcher. This type archi­tecture has very low supply current because the switch is
cycled only when the feedback pin voltage drops below the
reference voltage. Circuit operation can best be under­stood by referring to the LT1173 block diagram. Compara­tor A1 compares the feedback pin voltage with the 1.245V
reference voltage. When feedback drops below 1.245V, 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
switch saturation without overdriving the switch, resulting
in higher efficiency. The switch cycling action raises the
output voltage and feedback pin voltage. When the feed­back voltage is sufficient 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 is low the oscillator and all high current
circuitry is turned off, lowering device quiescent current
to just 110µA, for the reference, A1 and A2.

The oscillator is set internally for 23µs ON time and 19µs
OFF time, optimizing the device for circuits where V
and VIN differ by roughly a factor of 2. Examples include a
3V to 5V step-up converter or a 9V to 5V step-down
converter.

OUT

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
VIN to GND, with the mid-point connected to the SET pin
provides the trip voltage in a low battery detector applica­tion. The gain block output (AO) can sink 100µA (use a 47k
resistor pull-up to +5V). This line can signal a microcon­troller that the battery voltage has dropped below the
preset level.

A resistor connected between the I
maximum switch current. When the switch current ex­ceeds the set value, the switch cycle is prematurely
terminated. If current limit is not used, I
directly to VIN. 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 induc­tor; in step-down mode the collector is connected to V
and the emitter drives the inductor.

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.

pin and VIN sets

LIM

should be tied

LIM

IN

U

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PPLICATI

A

Measuring Input Current at Zero or Light Load

Obtaining meaningful numbers for quiescent current and
efficiency at low output current involves understanding
how the LT1173 operates. At very low or zero load current,
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

S

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approach is required to measure the 100µA off-state and
500mA on-state currents of the circuit.

Quiescent current can be accurately measured using the
circuit in Figure 1. V
LT1173. The circuit must be “booted” by shorting V2 to
V

. After the LT1173 output voltage has settled, discon-

SET

nect the short. Input voltage is V2, and average input
current can be calculated by this formula:

is set to the input voltage of the

SET

5

LT1173

P

F

L

OSC

03

()

It

V

R

e

L

IN

Rt
L

()

=











()

'

–

–'

104

It

V

L

t

L

IN

()

=

()

05

ELi

L

PEAK

=

()

1
2

06

2

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PPLICATI

A

+12V

–

LTC1050

+

V

SET

Figure 1. Test Circuit Measures No Load Quiescent Current of
LT1073 Converter

S

I FOR ATIO

µ1 F*

100

V1 V2

1000 F

*NON-POLARIZED

Ω1M

Ω

+

µ

WU

LT1173

CIRCUIT

LT1173 • TA06

U

Inductor Selection

A DC-DC converter operates by storing energy as mag­netic 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 oppo­site 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 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
LT1173 based designs, small axial leaded units with
saturation current ratings in the 300mA to 1A range
(depending on application) are adequate. Lastly, the in­ductor 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 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.

Specifying a proper inductor for an application requires
first establishing minimum and maximum input voltage,
output voltage, and output current. In a step-up converter,

the inductive events add to the input voltage to produce the
output voltage. Power required from the inductor is deter­mined by

PL = (V

+ VD – VIN) (I

OUT

) (02)

OUT

where VD is the diode drop (0.5V for a 1N5818 Schottky).
Energy required by the inductor per cycle must be equal or
greater than

in order for the converter to regulate the output.
When the switch is closed, current in the inductor builds

according to

where R' is the sum of the switch equivalent resistance
(0.8Ω typical at 25°C) and the inductor DC resistance.
When the drop across the switch is small compared to VIN,
the simple lossless equation

can be used. These equations assume that at t = 0,
inductor current is zero. This situation is called “discon­tinuous mode operation” in switching regulator parlance.
Setting “t” to the switch ON time from the LT1173 speci­fication table (typically 23µs) will yield i
“L” and VIN. Once i

is known, energy in the inductor at

PEAK

for a specific

PEAK

the end of the switch ON time can be calculated as

EL must be greater than PL/F
the required power. For best efficiency i

for the converter to deliver

OSC

should be

PEAK

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.

6

LT1173

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PPLICATI

A

As an example, suppose 9V at 50mA is to be generated
from a 3V input. Recalling Equation 02,

PL = (9V + 0.5V – 3V) (50mA) = 325mW. (07)

Energy required from the inductor is

P

F

OSC

Picking an inductor value of 100µH with 0.2Ω DCR results
in a peak switch current of

i

PEAK

Substituting i

EHAJ

Since 19µJ > 13.5µJ the 100µH inductor will work. This
trial-and-error approach can be used to select the opti­mum 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 compat­ible 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-to­positive configurations (Figure 8), all the output power
must be generated by the inductor. In these cases

PL = ( V

In the positive-to-negative case, switch drop can be mod­eled as a 0.75V voltage source in series with a 0.65Ω
resistor so that

325

L

==

kHz

24

V

3

=

Ω

1

PEAK

1

=

100 0 616 19 0 10

()( )

L

2

+ VD) (I

OUT

S

I FOR ATIO

mW

–µ



emA

–.

1 616 09




into Equation 04 results in

µµ...

J

13 5 08..µ

s

•

123

Ω

100

OUT



H

µ




2

=

). (11)

WU

=

U

()

()

()

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.

The step-down case is different than the preceeding three
in that the inductor current flows through the load in a
step-down topology (Figure 6). Current through the switch
should be limited to ~650mA in step-down mode. This can
be accomplished by using the I
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
470µH inductor should be used along with the 100Ω
resistor.

Capacitor Selection

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. Ordinary aluminum electrolytics, while inexpensive
and readily available, may have unacceptably poor equiva­lent series resistance (ESR) and ESL (inductance). There
are low-ESR aluminum capacitors on the market specifi­cally designed for switch mode DC-DC converters which
work much better than general-purpose units. Tantalum
capacitors provide still better performance at more ex­pense. 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
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 240mΩ. Figure 3 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 70mΩ.
Figure 4 shows the circuit with a 16V OS-CON unit. ESR is
now only 20mΩ.

pin. With a 20V to 30V input range, a

LIM

pin. With input voltages

LIM

jumps by

OUT

VL = VIN – 0.75V – IL (0.65Ω). (12)

7

LT1173

i

V

L

t

PEA K

IN

ON

=

()

13

PPLICATI

A

50mV/DIV

Figure 2. Aluminum Figure 3. Tantalum Figure 4. OS-CON

5 s/DIV

µ

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LT1173 • TA07

WU

50mV/DIV

U

5 s/DIV

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 microam­pere 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.

Diode Selection

50mV/DIV

5 s/DIV

µ

LT1173 • TA08

µ

LT1173 • TA09

Step-Up (Boost Mode) Operation

A step-up DC-DC converter delivers an output voltage
higher than the input voltage. Step-up converters are
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 VIN –
V

to appear across L1. A current then builds up in L1.

CESAT

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

not

Speed, forward drop, and leakage current are the three
main considerations in selecting a catch diode for LT1173
converters. General purpose rectifiers such as the 1N4001
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-50µs range. At best, efficiency will
be severely compromised when these diodes are used; at
worst, the circuit may not work at all. Most LT1173 circuits
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 1nA­5nA range at 25°C and lower cost than a 1N5818. (You can
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.

D1

V

OUT

R2

+

C1

R1

LT1173 • TA10

V

IN

* = OPTIONAL

L1

R3*

V

I

LIM

GND SW2

Figure 5. Step-Up Mode Hookup.
Refer to Table 1 for Component Values

LT1173

IN

SW1

FB

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

+ VD, the

OUT

inductor current flows through D1 into C1, increasing
V

. This action is repeated as needed by the LT1173 to

OUT

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

PPLICATI

A

U

O

S

I FOR ATIO

WU

U

keep VFB at the internal reference voltage of 1.245V. R1
and R2 set the output voltage according to the formula

V

OUT



=+

1







R

2

() ()



R

1



V

1 245 14..

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
100

Ω

+

C2

V

I

LIM

Figure 6. Step-Down Mode Hookup

IN

LT1173

GND

SW1

SW2

FB

L1

D1

+

1N5818

C1

V

R2

R1

LT1173 • TA11

OUT

R3 programs switch current limit. This is especially im­portant 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 pro­grams the switch to turn off when the current reaches
approximately 800mA. When using the LT1173 in step­down 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).

Inverting Configurations

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

should be less than 6.2V. More nega-

OUT

tive output voltages can be accomodated as in the prior
section.

When the switch turns on, SW2 pulls up to V
puts a voltage across L1 equal to VIN – VSW – V

– VSW. This

IN

OUT

,
causing a current to build up in L1. At the end of the switch
ON time, the current in L1 is equal to

V

VV

−−

IN

i

PEAK

=

SW OUT

L

t

.15

ON

()

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.

DIODE

. The voltage at SW2 must never be allowed to go

D1 MUST BE A SCHOTTKY

below –0.5V. A silicon diode such as the 1N4933 will allow
SW2 to go to – 0.8V, causing potentially destructive power
dissipation inside the LT1173. Output voltage is deter­mined by

V

OUT



=+

1







R

2

() ()



R

1



V

1 245 16..

+V

IN

R3

R1

R2

LT1173 • F07

+

C2

–V

OUT

V

I

LIM

Figure 7. Positive-to-Negative Converter

IN

LT1173

GND

SW1

SW2

FB

L1

D1
1N5818

+

C1

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
level shift, provided by the PNP transistor, supplies proper
polarity feedback information to the regulator.

9

LT1173

PPLICATI

A

+

C2

–V

IN

Using the I

U

O

S

I FOR ATIO

L1

V

I

LIM

AO

GND SW2

Figure 8. Negative-to-Positive Converter

Pin

LIM

IN

SW1

LT1173

FB

WU

D1

+

C1

R2

R1

V = 1.245V + 0.6V

( )

OUT

R2

U

+V

OUT

R1

2N3906

LT1173 • TA13

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
with a 2.0V input. If VIN rises to 4V, the peak switch current
will rise to 1.6A, exceeding the maximum switch current
rating. With the proper resistor selected (see the “Maxi­mum Switch Current vs R

” characteristic), the switch

LIM

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

Another situation where the I

feature is useful occurs

LIM

when the device goes into continuous mode operation.
This occurs in step-up mode when

V

+

OUT DIODE

V

VV DC

−

IN SW

1

<

−

1

17.

()

When the input and output voltages satisfy this relation­ship, 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

feature,

LIM

however, the switch current turns off at a programmed
level as shown in Figure 10, keeping output ripple to a
minimum.

I

L

ON

SWITCH

OFF

Figure 9. No Current Limit Causes Large Inductor
Current Build-Up

PROGRAMMED CURRENT LIMIT

I

L

ON

SWITCH

OFF

Figure 10. Current Limit Keeps Inductor Current Under Control

LT1173 • TA14

LT1173 • TA15

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
between I

pin. The value of the external resistor connected

LIM

and VIN sets the current limit. When suffi-

LIM

cient switch current flows to develop a VBE across R1 +
R

, Q3 turns on and injects current into the oscillator,

LIM

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.



R

LIM

V

IN

Q3

OSCILLATOR

Figure 11. LT1173 Current Limit Circuitry

(EXTERNAL)

DRIVER

I

LIM

R1
80Ω
(INTERNAL)

Q1

SW1

Q2

SW2

LT1173 • TA28

10

LT1173

PPLICATI

A

U

O

S

I FOR ATIO

WU

U

Using the Gain Block

The gain block (GB) on the LT1173 can be used as an error
amplifier, low battery detector or linear post regulator. 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 posi­tive input comes out on the SET pin.

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. 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.

V

IN

–

+

LT1173

GND

R3

V

– 1.245V

LB

R1 =

11.7µA

= BATTERY TRIP POINT

V

LB

R2 = 100kΩ
R3 = 4.7MΩ

AO

R1

1.245V

V

BAT

R2

REF

SET

Figure 12. Setting Low Battery Detector Trip Point

+5V

100k

TO 
PROCESSOR

LT1173 • TA16

Table 1. Component Selection for Common Converters

INPUT OUTPUT OUTPUT CIRCUIT INDUCTOR INDUCTOR CAPACITOR

VOLTAGE VOLTAGE CURRENT (MIN) FIGURE VALUE PART NUMBER VALUE NOTES

2.0-3.1 5 90mA 5 47µH G GA10-472K, C CTX50-1 100µF*

2.0-3.1 5 10mA 5 220µH G GA10-223K, C CTX 22µF

2.0-3.1 12 50mA 5 47µH G GA10-472K, C CTX50-1 47µF*

2.0-3.1 12 10mA 5 150µH G GA10-153K 22µF
5 12 90mA 5 120µH G GA10-123K 100µF
5 12 30mA 5 150µH G GA10-153K 47µF**
5 15 50mA 5 120µH G GA10-123K C CTX100-4 47µF
5 30 25mA 5 100µH G GA10-103K, C CTX100-4 10µF, 50V

6.5-9.5 5 50mA 6 47µH G GA10-472K, C CTX50-1 100µF**

12-20 5 300mA 6 220µH G GA20-223K 220µF**
20-30 5 300mA 6 470µH G GA20-473K 470µF**

5 –5 75mA 7 100µH G GA10-103K, C CTX100-4 100µF**

12 –5 250mA 7 470µH G GA40-473K 220µF**
–5 5 150mA 8 100µH G GA10-103K, C CTX100-4 220µF
–5 12 75mA 8 100µH G GA10-103K, C CTX100-4 47µF

G = Gowanda
C = Coiltronics
* Add 68Ω from I
** Add 100Ω from I

LIM

to V

LIM

IN

to V

IN

11

LT1173

PPLICATI

A

U

O

S

I FOR ATIO

WU

U

Table 2. Inductor Manufacturers

MANUFACTURER PART NUMBERS

Gowanda Electronics Corporation GA10 Series
1 Industrial Place GA40 Series
Gowanda, NY 14070
716-532-2234

Caddell-Burns 7300 Series
258 East Second Street 6860 Series
Mineola, NY 11501
516-746-2310

Coiltronics International Custom Toroids
984 S.W. 13th Court Surface Mount
Pompano Beach, FL 33069
305-781-8900

Renco Electronics Incorporated RL1283
60 Jefryn Boulevard, East RL1284
Deer Park, NY 11729
800-645-5828

Table 3. Capacitor Manufacturers

MANUFACTURER PART NUMBERS

Sanyo Video Components OS-CON Series
2001 Sanyo Avenue
San Diego, CA 92173
619-661-6835

Nichicon America Corporation PL Series
927 East State Parkway
Schaumberg, IL 60173
708-843-7500

Sprague Electric Company 150D Solid Tantalums
Lower Main Street 550D Tantalex
Sanford, ME 04073
207-324-4140

U

O

PPLICATITYPICAL

SA

3V to –22V LCD Bias Generator

R1
100Ω

LT1173

V

SW2

IN

SW1

FB

I

LIM

2 X 1.5V

CELLS

* L1 = GOWANDA GA10-103K
COILTRONICS CTX100-4
FOR 5V INPUT CHANGE R1 TO 47Ω.
CONVERTER WILL DELIVER –22V AT 40mA.

3V

GND

L1*

100µH

1N5818

1N4148

2.21M
1%

+

4.7µF

1N5818

118k
1%

22µF+220k

0.1µF

–22V OUTPUT
7mA AT 2.0V INPUT
70% EFFICIENCY

LT1173 • TA19

12

PPLICATITYPICAL

3V to 5V Step-Up Converter

L1*

µ

100 H

I

2 X 1.5V

CELLS

GND

* L1 = GOWANDA GA10-103K
COILTRONICS CTX100-1 (SURFACE MOUNT)

+VIN

5V INPUT

V

LIM

IN

SW1

LT1173-5

SENSE
SW2

+5V to –5V Converter +20V to 5V Step-Down Converter

100

Ω

O

+

U
SA

1N5818

100 Fµ

5V OUTPUT
150mA AT 3V INPUT
60mA AT 2V INPUT

LT1173 • TA17

9V to 5V Step-Down Converter

100

Ω

V

LT1173-5

100

IN

SW1

SENSE
SW2

Ω

47µH

1N5818

L1*

+

I

LIM

9V

BATTERY

GND

* L1 = GOWANDA GA10-472K
COILTRONICS CTX50-1
FOR HIGHER OUTPUT CURRENTS SEE LT1073 DATASHEET

+VIN

12V-28V

100 Fµ

LT1173

5V OUTPUT
150mA AT 9V INPUT
50mA AT 6.5V INPUT

LT1173 • TA18

+

22µF

* L1 = GOWANDA GA10-103K
COILTRONICS CTX100-1

44mH

48V DC

44mH

*L1 = CTX110077

I

= 120µA

Q

I

LIM

LT1173-5

GND

1N5818

~

~

V

IN

SW1

SENSE
SW2

+

–

100µH

+

L1*

100 F

47µF
100V

µ

10nF

1N965B

+

–5V OUTPUT
75mA

LT1173 • TA20

3.6MΩ

10k

15V

Telecom Supply

VN2222

12V

I

LIM

+

10µF
16V

GND

I

LIM

GND

* L1 = GOWANDA GA20-223K

L1*

500µH

100Ω

V

IN

SW1

LT1173

FB

SW2

LT1173-5

1N4148

V

IN

SW1

SENSE
SW2

IRF530

220µH

1N5818

MUR110

220µF

10V

L1*

5V OUTPUT
300mA

+

100 Fµ

LT1173 • TA21

+5V

+

100mA

390kΩ

2N5400

110kΩ

LT1173 • TA22

13

LT1173

PPLICATITYPICAL

4 X NICAD

OR

ALKALINE

CELLS

*L1 = COILTRONICS CTX100-4
GOWANDA GA20-103K

470µF

U

O

SA

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

L1*

100µH

56Ω

I

LIM

+

7

SET AO

GND

5

V

IN

V

OUT

21

V

IN

SW1

LT1173

FB

SW2

4

= 2.6V TO 7.2V
= 5V AT 100mA

3

6

8

1N5818

SI9405DY

470k 75k

+5V
OUTPUT

+

470µF

+

470µF

240Ω

24k

LT1173 • TA23

47k

2N3906

2 X NICAD

100k 100k

*L1 = COILTRONICS CTX-20-5-52

†

1% METAL FILM

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

100k

100k

2.2M

AO

SET

I

LIM

GND

L1*

20µH, 5A

V

IN

100

SW1 2N4403

LT1173

FB

SW2

301k

†

220

†

5Ω

47Ω

1N5820

MJE200

+

+5V OUTPUT
300mA
LOCKOUT AT

1.85V INPUT

100µF
OS-CON

LT1173 • TA24

14

LT1173

U

O

PPLICATITYPICAL

V



IN

5V-12V

SA

Voltage Controlled Positive-to-Negative Converter

V

GND

IN

LT1173

0.22

I

LIM

SW2

1N5818

SW1

FB

150

MJE210

220

V

IN

LT1006

200k

–

+

L1*

50µH, 2.5A

1N5820

39k

+

100µF

–V
2W MAXIMUM OUTPUT

V

C

= –5.13 • VC

OUT

(0V TO 5V)

* L1 = GOWANDA GT10-101

0.22Ω



V

IN

7V-24V

V

IN

LT1173

GND

OPERATE STANDBY

High Power, Low Quiescent Current Step-Down Converter

L1*

1N5820

121k

≤ 150µA

Q

25µH, 2A

+

I

LIM

SW2

1N5818

SW1

FB

100Ω

1/2W

18V
1W

2k

MTM20P08

51Ω

2N3904

1N4148

40.2k
* L1 = GOWANDA GT10-100

EFFICIENCY ≥ 80% FOR 10mA ≤ I
STANDBY I

2 Cell Powered Neon Light Flasher

0.02µF

470µF

LOAD

LT1173 • TA25

5V
500mA

≤ 500mA

LT1173 • TA26

3V

*TOKO 262LYF-0100K

L1*

470µH

I

LIM

GND

V

IN

SW1

LT1173

FB

SW2

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 represen­tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

1N4148

0.02µF

1.3M

1N4148

100M

1N4148

95V REGULATED

0.02µF

0.68µF

3.3M

200V

NE-2
BLINKS AT

0.5Hz

LT1173 • TA27

15

LT1173

PACKAGEDESCRIPTI

U

O

Dimensions in inches (milimeters) unless otherwise noted.

N8 Package

8-Lead Plastic DIP

0.400*

(10.160)

MAX

876

5

0.255 ± 0.015*
(6.477 ± 0.381)

0.300 – 0.325

(7.620 – 8.255)

0.065

(1.651)

0.009 – 0.015

(0.229 – 0.381)

+0.025

0.325
–0.015

+0.635

8.255

()

–0.381

*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTURSIONS SHALL NOT EXCEED 0.010 INCH (0.254mm).

TYP

0.045 ± 0.015

(1.143 ± 0.381)





1234

0.045 – 0.065

(1.143 – 1.651)

0.100 ± 0.010

(2.540 ± 0.254)

S8 Package

8-Lead Plastic SOIC

0.189 – 0.197*
(4.801 – 5.004)

8

7

0.018 ± 0.003

(0.457 ± 0.076)

5

6

0.130 ± 0.005

(3.302 ± 0.127)

0.125

(3.175)

MIN

(0.380)



0.015

MIN

N8 0694

16

0.010 – 0.020

(0.254 – 0.508)

0.008 – 0.010

(0.203 – 0.254)

*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.006 INCH (0.15mm).

× 45°

0°– 8° TYP

0.016 – 0.050

0.406 – 1.270

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7487

[phone]

●

FAX

: [phone]

●

TELEX

: 499-3977

0.228 – 0.244

(5.791 – 6.197)

0.053 – 0.069

(1.346 – 1.752)

0.014 – 0.019

(0.355 – 0.483)

0.150 – 0.157*
(3.810 – 3.988)

1

3

2

4

0.004 – 0.010

(0.101 – 0.254)

0.050

(1.270)

BSC

SO8 0294

LT/GP 0894 2K REV B • PRINTED IN USA

 LINEAR TECHNOLOGY CORPORATION 1994