Datasheet LT1073 Datasheet (LINEAR TECHNOLOGY)

FEATURES

LT1073

Micropower

DC/DC Converter

Adjustable and Fixed 5V, 12V

U

DESCRIPTIO

■

No Design Required

■

Operates at Supply Voltages from 1V to 30V

■

Consumes Only 95µA Supply Current

■

Works in Step-Up or Step-Down Mode

■

Only Three External Off-the-Shelf 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 PDIP or SO-8 Package

U

APPLICATIO S

■

Pagers

■

Cameras

■

Single-Cell to 5V Converters

■

Battery Backup Supplies

■

Laptop and Palmtop Computers

■

Cellular Telephones

■

Portable Instruments

■

4mA to 20mA Loop Powered Instruments

■

Hand-Held Inventory Computers

■

Battery-Powered α, β, and γ Particle Detectors

The LT®1073 is a versatile micropower DC/DC converter.
The device requires only three external components to
deliver a fixed output of 5V or 12V. The very low minimum
supply voltage of 1V allows the use of the LT1073 in
applications where the primary power source is a single
cell. An on-chip auxiliary gain block can function as a low­battery detector or linear post-regulator.

Average current drain of the LT1073-5 used as shown in
the Typical Application circuit below is just 135µA un-
loaded, making it ideal for applications where long battery
life is important. The circuit shown can deliver 5V at 40mA
from an input as low as 1.25V and 5V at 10mA from a 1V
input.

The device can easily be configured as a step-up or step­down converter, although for most step-down applica­tions or input sources greater than 3V, the LT1173 is
recommended. Switch current limiting is user-adjustable
by adding a single external resistor. Unique reverse­battery protection circuitry limits reverse current to safe,
nondestructive levels at reverse supply voltages up to

1.6V.

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

TYPICAL APPLICATION

Single-Cell to 5V Converter

CADDELL-BURNS

7300-12

82µH

2

1

I

V

LIM

1.5V
AA CELL*

OPERATES WITH CELL VOLTAGE ≥1V

*

ADD 10µF DECOUPLING CAPACITOR IF
BATTERY IS MORE THAN 2" AWAY FROM LT1073

LT1073-5

SENSE

5

SW1

SW2GND

IN

3

8

4

U

1N5818

Single Alkaline “AA” Cell Operating

Hours vs DC Load Current

1000

5V
40mA

100

L = 180µH

10

+

100µF
SANYO
0S-CON

1073 TA01

CONTINUOUS OPERATION (HOURS)

1

1

LOAD CURRENT (mA)

L = 82µH

10 100

LT1073 TA02

1

LT1073

WW

W

ABSOLUTE AXI U RATI GS

U

UUW

PACKAGE/ORDER I FOR ATIO

(Note 1)

Supply Voltage, Step-Up Mode................................ 15V

Supply Voltage, Step-Down Mode ........................... 36V

SW1 Pin Voltage...................................................... 50V

SW2 Pin Voltage........................................... –0.4 to V

IN

Feedback Pin Voltage (LT1073) ................................. 5V

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

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

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

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

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

I

1

LIM

V

2

IN

SW1

3

SW2

4

N8 PACKAGE
8-LEAD PDIP

*FIXED VERSIONS

T

JMAX

T

JMAX

TOP VIEW

FB (SENSE)*

8

SET

7

A0

6

GND

5

S8 PACKAGE

8-LEAD PLASTIC SO

= 125°C, θJA = 100°C/W (N8)
= 125°C, θJA = 120°C/W (S8)

ORDER PART

NUMBER

LT1073CN8
LT1073CN8-5
LT1073CN8-12
LT1073CS8
LT1073CS8-5
LT1073CS8-12

S8 PART MARKING

1073
10735
107312

Consult factory for parts specified with wider operating temperature ranges.

ELECTRICAL CHARACTERISTICS

temperature range, otherwise specifications are at TA = 25°C. VIN = 1.5V unless otherwise specified.

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

I

Q

I

Q

V

IN

V

OUT

f

OSC

DC Duty Cycle Full Load (VFB = V
t

ON

I

FB

I

SET

V

AO

V

CESAT

A

V

Quiescent Current Switch Off ● 95 130 µA
Quiescent Current, Step-Up No Load LT1073-5 135 µA

Mode Configuration LT1073-12 250 µA
Input Voltage Step-Up Mode ● 1.15 12.6 V

Comparator Trip Point Voltage LT1073 (Note 2) ● 202 212 222 mV
Output Sense Voltage LT1073-5 (Note 3) ● 4.75 5 5.25 V

Comparator Hysteresis LT1073 ● 510 mV
Output Hysteresis LT1073-5 ● 125 250 mV

Oscillator Frequency ● 15 19 23 kHz

Switch ON Time ● 30 38 50 µs
Feedback Pin Bias Current LT1073, VFB = 0V ● 10 50 nA
Set Pin Bias Current V
AO Output Low IAO = –100µA ● 0.15 0.4 V
Reference Line Regulation 1V ≤ VIN ≤ 1.5V ● 0.35 1.0 %V

Switch Saturation Voltage VIN = 1.5V, ISW = 400mA 300 400 mV
Set-Up Mode

A2 Error Amp Gain RL = 100kΩ (Note 4) ● 400 1000 V/V

The ● denotes the specifications which apply over the full operating

1.0 12.6 V

Step-Down Mode ● 30 V

LT1073-12 (Note 3)

LT1073-12

) ● 65 72 80 %

REF

= V

SET

REF

1.5V ≤ V

V

VIN = 5V, ISW = 1A 700 1000 mV

≤ 12V ● 0.05 0.1 %V

IN

= 1.5V, ISW = 500mA 400 550 mV

IN

● 11.4 12 12.6 V

● 300 600 mV

● 60 120 nA

● 600 mV

● 750 mV

● 1500 mV

2

LT1073

ELECTRICAL CHARACTERISTICS

The ● denotes the specifications which apply over the full operating

temperature range, otherwise specifications are at TA = 25°C. VIN = 1.5V unless otherwise specified.

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

I
I

REV

LIM

Reverse Battery Current (Note 5) 750 mA
Current Limit 220Ω Between I

LIM

and V

IN

400 mA

Current Limit Temperature Coefficient –0.3 %/°C

I

LEAK

V

SW2

Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.

Note 2: This specification guarantees that both the high and low trip point
of the comparator fall within the 202mV to 222mV range.

Switch OFF Leakage Current Measured at SW1 Pin 1 10 µA
Maximum Excursion Below GND I

≤ 10µA, Switch Off –400 –350 mV

SW1

Note 4: 100k resistor connected between a 5V source and the AO pin.
Note 5: The LT1073 is guaranteed to withstand continuous application of

, I

1.6V applied to the GND and SW2 pins while V

and SW1 pins are

IN

LIM

grounded.

Note 3: This specification guarantees that the output voltage of the fixed
versions will always fall within the specified range. The waveform at the
SENSE pin will exhibit a sawtooth shape due to the comparator hysteresis.

UW

TYPICAL PERFOR A CE CHARACTERISTICS

Saturation Voltage Step-Up Mode
(SW2 Pin Grounded)

1.2

1.0

0.8

(V)

0.6

CESAT

V

0.4

0.2

0

0

VIN = 1V

0.2

VIN = 1.5V

VIN = 1.25V

VIN = 2V

0.4 0.6 0.8
I

(A)

SWITCH

VIN = 3V

VIN = 5V

1.0 1.2

1073 G01

Switch ON Voltage Step-Down Mode
(SW1 Pin Connected to VIN)

1.4

1.3

1.2

1.1

1.0

0.9

SWITCH ON VOLTAGE (V)

0.8

0.7

0.1 0.3 0.5 0.7

0.2 0.4 0.8

0

I

SWITCH

0.6

(A)

1073 G02

Maximum Switch Current vs R

1200
1100
1000

900
800
700
600
500
400

SWITCH CURRENT (mA)

300
200
100

10

VIN = 3V

100 1000

R

(Ω)

LIM

LIM

L = 82µH

VIN = 1.5V

1073 G03

FB Pin Bias Current vs
Temperature

20

18

16

14

12

10

FB BIAS CURRENT (nA)

8

6

4

–50

050

–25 25 75 125

TEMPERATURE (°C)

100

1073 G04

SET Pin Bias Current vs
Temperature

200

175

150

125

100

75

50

SET PIN BIAS CURRENT (nA)

25

0

–25 25 75 125

–50

050

TEMPERATURE (°C)

100

1073 G05

“Gain Block” Gain

1800

1600

1400

1200

1000

800

GB GAIN (V/V)

600

400

200

0

–25 25 75 125

–50

VIN = 1.5V
R

L

050

TEMPERATURE (°C)

= 100k

100

1073 G06

3

LT1073

UW

TYPICAL PERFOR A CE CHARACTERISTICS

Recommended Minimum

Supply Current vs Temperature

150

VIN = 1.5V

140
130
120
110
100

90
80

SUPPLY CURRENT (µA)

70
60
50

–25 25 75 125

–50

U

050

TEMPERATURE (°C)

UU

100

1073 G07

Inductance Value

300

R

= 0V

LIM

250

200

150

100

MINIMUM INDUCTANCE (µH)

50

0

1.0

1.5 2.5

2.0
INPUT VOLTAGE (V)

PI FU CTIO S

I

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

LIM

lower current limit is desired, connect a resistor between
I

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

LIM

to approximately 400mA.

VIN (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 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.

Guaranteed Minimum Output

3.0

3.5

4.0

4.5

1073 G08

5.0

Current at 5V vs V

1000

100

OUTPUT CURRENT (mA)

10

1.0

1.5 2.0 2.5 3.0 3.5

IN

FOR VIN > 1.6V A
68Ω RESISTOR
MUST BE CONNECTED
BETWEEN I

VIN (V)

LIM

AND V

IN

1073 G09

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
212mV reference.

FB/SENSE (Pin 8): On the LT1073 (adjustable) this pin
goes to the comparator input. On the LT1073-5 and
LT1073-12, this pin goes to the internal application resis­tor that sets output voltage.

W

BLOCK DIAGRA S

LT1073

SET

+

V

IN

212mV

REFERENCE

GND

A2

–

GAIN BLOCK/ERROR AMP

A1

FB

COMPARATOR

A0

OSCILLATOR

4

I

LIM

DRIVER

SW1

SW2

1073 BD01

LT1073-5 and LT1073-12

SET

V

IN

212mV

REFERENCE

Q1

R1R2940k

GND

+

A2

–

GAIN BLOCK/ERROR AMP

A1

COMPARATOR

SENSE

A0

OSCILLATOR

LT1073-5: R1 = 40k

LT1073-12: R2 = 16.3k

I

LIM

DRIVER

SW1

Q1

SW2

1073 BD02

OPERATIO

VmV

R

R

OUT

=

()

+











212

2
1

1

LT1073

LT1073

U

The LT1073 is 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 LT1073 Block Diagram. Com­parator A1 compares the FB pin voltage with the 212mV
reference signal. When FB drops below 212mV, A1 switches
on the 19kHz oscillator. The driver amplifier boosts the
signal level to drive the output NPN power switch Q1. 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 FB pin voltage. When the FB 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 95µA for the
reference, A1 and A2.

The oscillator is set internally for 38µs ON time and 15µs
OFF time, optimizing the device for step-up circuits where
V

≈ 3VIN, e.g., 1.5V to 5V. Other step-up ratios as well

OUT

as step-down (buck) converters are possible at slight
losses in maximum achievable power output.

A2 is a versatile gain block that can serve as a low-battery
detector, a linear post-regulator, or drive an undervoltage
lockout circuit. The negative input of A2 is internally
connected to the 212mV reference. An external resistor
divider from VIN to GND provides the trip point for A2. The
AO output can sink 100µA (use a 56k 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 is turned off. This feature
is especially useful when small inductance values are used
with high input voltages. If the internal current limit of 1.5A
is desired, I
delay through the current-limit circuitry is about 2µs.

In step-up mode, SW2 is connected to ground and SW1
drives the inductor. In step-down mode, SW1 is con­nected to VIN and SW2 drives the inductor. Output voltage
is set by the following equation in either step-up or step­down modes where R1 is connected from FB to GND and
R2 is connected from V

LT1073-5 and LT1073-12

The LT1073-5 and LT1073-12 fixed output voltage ver­sions have the gain-setting resistor on-chip. Only three
external components are required to construct a fixed­output converter. 5µA flows through R1 and R2 in the
LT1073-5, and 12.3µA flows in the LT1073-12. This
current represents a load and the converter must cycle
from time to time to maintain the proper output voltage.
Output ripple, inherently present in gated-oscillator de­signs, will typically run around 150mV for the LT1073-5
and 350mV for the LT1073-12 with the proper inductor/
capacitor selection. This output ripple can be reduced
considerably by using the gain block amp as a preamplifier
in front of the FB pin. See the Applications Information
section for details.

should be tied directly to VIN. Propagation

LIM

to FB.

OUT

pin and VIN adjusts

LIM

5

LT1073

I

VV

IN

=

Ω

21

100

–

WUUU

APPLICATIO S I FOR ATIO

Table 1. Component Selection for Step-Up Converters

INPUT BATTERY OUTPUT OUTPUT INDUCTOR INDUCTOR CAPACITOR

VOLTAGE (V) TYPE VOLTAGE (V) CURRENT (MIN) VALUE (µH) PART NUMBER VALUE (µF) NOTES

1.55-1.25 Single Alkaline 3 60mA 82 G GA10-822K, CB 7300-12 150

1.30-1.05 Single Ni-Cad 3 20mA 180 G GA10-183K, CB 7300-16 47

1.55-1.25 Single Alkaline 5 30mA 82 G GA10-822K, CB 7300-12 100

1.30-1.05 Single Ni-Cad 5 10mA 180 G GA10-183K, CB 7300-16 22

3.1-2.1 Two Alkaline 5 80mA 120 G GA10-123K, CB 7300-14 470 *

3.1-2.1 Two Alkaline 5 25mA 470 G GA10-473K, CB 7300-21 150 *

3.3-2.5 Lithium 5 100mA 150 G GA40-153K, CB 6860-15 470 *

3.1-2.1 Two Alkaline 12 25mA 120 G GA10-123K, CB 7300-14 220

3.1-2.1 Two Alkaline 12 5mA 470 G GA10-473K, CB 7300-21 100

3.3-2.5 Lithium 12 30mA 150 G GA10-153K, CB 7300-15 220

4.5-5.5 TTL Supply 12 90mA 220 G GA40-223K, CB 6860-17 470 *

4.5-5.5 TTL Supply 12 22mA 1000 G GA10-104K, CB 7300-25 100 *

4.5-5.5 TTL Supply 24 35mA 220 G GA40-223K, CB 6860-17 150 *

G = GOWANDA CB = CADDELL-BURNS *Add 68Ω from I

LIM

to V

IN

Measuring Input Current at Zero or Light Load

Obtaining meaningful numbers for quiescent current and
efficiency at low output current involves understanding
how the LT1073 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
approach is required to measure the 100µA off-state and
500mA on-state currents of the circuit.

1MΩ

12V

–

LTC1050

+

V

SET

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

*NONPOLARIZED

1µF*

100Ω

V1 V2

1000µF

LT1073

CIRCUIT

+

1073 F01

Quiescent current can be accurately measured using the
circuit in Figure 1. V

is set to the input voltage of the

SET

LT1073. The circuit must be “booted” by shorting V2 to
V

. After the LT1073 output voltage has settled, discon-

SET

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

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 LT1073 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
LT1073-based designs, small axial-lead units with

6

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APPLICATIO S I FOR ATIO

LT1073

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

OUT

)
where VD is the diode drop (0.5V for a 1N5818 Schottky).
Maximum power in the inductor is

PEf

==•

L L OSC

1

Li f

2

2

•

PEAK OSC

where

i

PEAK



V

IN ON

=







R







1–

e



Rt

–



L







R = Switch equivalent resistance (1Ω maximum)
added to the DC resistance of the inductor and tON = ON
time of the switch.

At maximum VIN and ON time, i

should not be allowed

PEAK

to exceed the maximum switch current shown in Figure 2.
Some input/output voltage combinations will cause con­tinuous1 mode operation. In these cases a resistor is
needed between I
current under control. See the “Using the I

(Pin 1) and VIN (Pin 2) to keep switch

LIM

Pin” section

LIM

for details.

NOTE 1: i.e., inductor current does not go to zero when the switch is off.

1200

1000

800

(mA)

600

SWITCH

I

400

200

0

Figure 2. Maximum Switch Current vs Input Voltage

1234

0

VIN (V)

5

1073 F02

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 3, 4, and 5 show the output voltage of an LT1073
based converter with three 100µF capacitors. The peak
switch current is 500mA in all cases. Figure 3 shows a
Sprague 501D aluminum capacitor. V

jumps by over

OUT

150mV 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 300mΩ. Figure 4 shows the
same circuit, but with a Sprague 150D tantalum capacitor
replacing the aluminum unit. Output jump is now about
30mV, corresponding to an ESR of 60mΩ. Figure 5 shows
the circuit with an OS-CON unit. ESR is now only 30mΩ.

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

7

LT1073

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APPLICATIO S I FOR ATIO

50mV/DIV

20µs/DIV

Figure 3. Aluminum Figure 4. Tantalum Figure 5. OS-CON

50mV/DIV

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

Speed, forward drop and leakage current are the three
main considerations in selecting a catch diode for LT1073
converters. “General-purpose” rectifiers such as the
1N4001 are

unsuitable

for use in

any

switching regulator

application. 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 and at worst, the circuit may not work at
all. Most LT1073 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 LT1073 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 5nA 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 LT1073
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.

Step-Up (Boost Mode) Operation

50mV/DIV

20µs/DIV

20µs/DIV

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

The usual step-up configuration for the LT1073 is shown
in Figure 6. The LT1073 first pulls SW1 low causing VIN­V

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

CESAT

At the end of the switch ON time the current in L1 is2:

V

i

PEAK

NOTE 2: This simple expression neglects the effect of switch and coil resistance. These are taken
into account in the “Inductor Selection” section.

IN

t

=

V

IN

Figure 6. Step-Up Mode Hookup.
(Refer to Table 1 for Component Values)

L

I

LIM

ON

R3*

LT1073

V

SW1

SW2GND

IN

FB

L1

*= OPTIONAL

D1

R2

+

R1

1073 F06

V

OUT

C1

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 LT1073 to

OUT

keep VFB at the internal reference voltage of 212mV. R1
and R2 set the output voltage according to the formula:

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

not

8

WUUU

APPLICATIO S I FOR ATIO

LT1073

V

OUT



=+







R

2

1

R

1

mV

212

()





Step-Down (Buck Mode) Operation

A step-down DC/DC converter converts a higher voltage to
a lower voltage. It is short-circuit protected because the

V

IN

R3

220Ω

I

VINSW1

LIM

FB

+

C2

LT1073

SW2

GND

D1

1N5818

Figure 7. Step-Down Mode Hookup

L1

+

C1

R2

R1

1073 FO7

V

OUT

switch is in series with the output. Step-down converters
are characterized by low output voltage ripple but high
input current ripple. The usual hookup for an LT1073­based step-down converter is shown in Figure 7.

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

VV V

––

i

PEAK

IN SW OUT

=

L

t

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 LT1073. Output voltage is deter­mined by

V

OUT



=+







R

2

1

R

1

mV

212

()





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 220Ω resistor pro­grams the switch to turn off when the current reaches
approximately 400mA. When using the LT1073 in step­down mode, output voltage should be limited to 6.2V or
less.

Inverting Configurations

The LT1073 can be configured as a positive-to-negative
converter (Figure 8), or a negative-to-positive converter
(Figure 9). In Figure 8, 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.

OUT

+V

IN

+

C2

R3

SW1

I

LIMVIN

FB

LT1073

SW2

GND

D1

1N5818

Figure 8. Positive-to-Negative Converter

L1

I

V

LIM

+

C2

–V

IN

Figure 9. Negative-to-Positive Converter

IN

SW1

LT1073

FBAO

SW2GND

L1

+

C1

D1

+

C1

R2

R1

V

= ( )212mV + 0.6V

OUT

R2

R1

R2

+V

R1

2N3906

1073 F09

–V

1073 FO8

OUT

OUT

9

LT1073

VV

VV DC

OUT DIODE

IN SW

+

<

––

1

1

PROGRAMMED CURRENT LIMIT

1073 F11

ON

OFF

SWITCH

I

L

WUUU

APPLICATIO S I FOR ATIO

In Figure 9, 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.

Using the I

The LT1073 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
LT1073 must operate at an 800mA peak switch current
with a 2V 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 (see the “Maximum Switch
Current vs R
rent will be limited to 800mA, even if the input voltage
increases. The LT1073 does this by sampling a small
fraction of the switch current and passing this current
through the external resistor. When the voltage on the I
pin drops a VBE below VIN, the oscillator terminates the
cycle. Propagation delay through this loop is about 2µs.

Pin

LIM

” characteristic) selected, the switch cur-

LIM

LIM

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 nonzero current level in the
inductor just prior to switch turn-on. As shown in
Figure 10, 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

LIM

feature, however, the switch current turns off at a pro­grammed level as shown in Figure 11, keeping output
ripple to a minimum.

Using the Gain Block

The gain block (GB) on the LT1073 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 (–) input of the gain
block is tied internally to the 212mV reference. The (+)
input comes out on the SET pin.

Another situation where the I

feature is useful is when

LIM

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

I

L

ON

SWITCH

OFF

Figure 10. No Current Limit Causes
Large Inductor Current Build-Up

1073 F10

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.

Output ripple of the LT1073, normally 150mV at 5V

OUT

,
can be reduced significantly by placing the gain block in
front of the FB input as shown in Figure 13. This effectively
reduces the comparator hysteresis by the gain of the gain
block. Output ripple can be reduced to just a few millivolts
using this technique. Ripple reduction works with step­down or inverting modes as well.

Figure 11. Current Limit Keeps Inductor Current Under Control

10

WUUU

+

R3

680k

L1

D1

V

OUT

R1

C1

R2

1073 F13

LT1073

I

LIM

V

IN

SW1

SETFB

AO

SW2GND

V

BAT

V

OUT

=

+ 1 212mV

(

)

R2
R1

(

)

APPLICATIO S I FOR ATIO

LT1073

V

IN

LT1073

212mV

SET

REF

–

+

GND

R1

V

BAT

R2

5V

100k

A0

R1 = R2
VLB = BATTERY TRIP POINT

V

LB

(

–1

212mV

TO
PROCESSOR

)

1073 F12

Figure 12. Settling Low Battery Detector Trip Point Figure 13. Output Ripple Reduction Using Gain Block

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

Toko America Incorporated Type 8RBS
1250 Feehanville Drive
Mount Prospect, IL 60056
312-297-0070

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
1201 Sanyo Avenue
San Diego, CA 92073
619-661-6322

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
Stanford, ME 04073
207-324-4140

U

TYPICAL APPLICATIO S

1.5V to 3V Step-Up Converter 1.5V to 9V Step-Up Converter

†

L1

1N5818

536k*

40.2k*

+

3V OUTPUT
20mA AT
V

BATTERY

100µF

1073 TA03

= 1V

I

LIM

1.5V
LT1073

CELL

* 1% METAL FILM

†

L1 = GOWANDA GA10-123k

OR CADDELL-BURNS 7300-14

120µH

220Ω

I

V

LIM

1.5V
CELL

* 1% METAL FILM

†

L1 = GOWANDA GA10-123k

OR CADDELL-BURNS 7300-14

LT1073

IN

SW1

FB

SW2GND

V

SW1

SW2GND

IN

FB

L1

120µH

†

1N5818

1M*

9V OUTPUT
7mA AT V
16mA AT V

BATTERY

BATTERY

= 1V

= 1.5V

+

47µF

24.3k*

1073 TA04

11

LT1073

+

L1

†

68µH

15V OUTPUT
27mA AT
V

BATTERY

= 2V

47µF

LT1073

I

LIM

V

IN

SW1

FB

SW2GND

TWO

1.5V
CELLS

1N5818

* 1% METAL FILM

†

L1 = GOWANDA GA10-682k

OR CADDELL-BURNS 7300-11

100Ω

1073 TA08

1M*

14.3k*

+

L1

†

150µH

15V OUTPUT
100mA AT 4.5V

IN

100µF

+

100µF

LT1073

I

LIM

V

IN

SW1

FB

SW2GND

1N5818

* 1% METAL FILM
† L1 = GOWANDA GA20-153k

OR CADDELL-BURNS 7200-15

50Ω

1073 TA10

1M*

14.3k*

5V

IN

TYPICAL APPLICATIO S

1.5V to 12V Step-Up Converter 3V to 5V Step-Up Converter

†

L1

1N5818

+

1.5V
CELL

I

LIM

LT1073-12

V

SW1

SENSE

SW2GND

120µH

IN

U

12V OUTPUT
5mA AT V
16mA AT V

47µF

BATTERY

BATTERY

= 1V

= 1.5V

TWO

1.5V
CELLS

100Ω

I

LIM

LT1073-5

V

SW1

SENSE

SW2GND

†

L1

1N5818

68µH

IN

5V OUTPUT
100mA AT
V

BATTERY

= 2V

+

100µF

†

L1 = GOWANDA GA10-123k

OR CADDELL-BURNS 7300-14

3V to 12V Step-Up Converter

100Ω

I

LIM

TWO

1.5V

LT1073-12

CELLS

†

L1 = GOWANDA GA10-682k

OR CADDELL-BURNS 7300-11

V

SW1

SENSE

SW2GND

1073 TA05

†

L1

1N5818

68µH

IN

12V OUTPUT
35mA AT

= 2V

V

BATTERY

+

47µF

1073 TA07

†

L1 = GOWANDA GA10-682k

OR CADDELL-BURNS 7300-11

3V to 15V Step-Up Converter

1073 TA06

5V to 12V Step-Up Converter 5V to 15V Step-Up Converter

†

L1

1N5818

50Ω

V

SW1

SENSE

SW2GND

150µH

IN

5V

IN

I

LIM

+

100µF

LT1073-12

†

L1 = GOWANDA GA20-153k

OR CADDELL-BURNS 7200-15

+

12V OUTPUT
130mA AT 4.5V

100µF

1073 TA09

IN

12

TYPICAL APPLICATIO S

+

L1

†

100µH

5V OUTPUT

100µF

LT1073-5

I

LIM

V

IN

SW1

SENSE

SW2GND

1N5818

1073 TA14

9V
BATTERY

220Ω

†

L1 = GOWANDA GA10-103k

OR CADDELL-BURNS 7300-13

LT1073

U

1.5V to 5V Step-Up Converter with Logic Shutdown

†

L1

1N5818

I

LIM

1.5V

CELL

SHUTDOWN

* 1% METAL FILM

†

L1 = GOWANDA GA10-822k

OR CADDELL-BURNS 7300-12

LT1073

OPERATE

V

SW1

SW2GND

82µH

IN

FB

1N4148

74C04

909k*

40.2k*

5V OUTPUT

+

100µF

1073 TA11

9V to 3V Step-Down Converter

3V OUTPUT

220Ω

I

V

LIM

9V

BATTERY

LT1073

SW1

SW2GND

IN

FB

L1

100µH

1N5818

†

+

100µF

536k*

40.2k*

1.5V to 5V Step-Up Converter with Low-Battery Detector

†

L1

1N5818

82µH

442k*

1.5V
CELL

100k*

* 1% METAL FILM

†

L1 = GOWANDA GA10-822k

OR CADDELL-BURNS 7300-12

I

LIM

SET

LT1073-5

AO

V

SW1

SENSE

SW2GND

IN

+

100µF

1073 TA12

9V to 5V Step-Down Converter

5V OUTPUT

100k

LO BAT
GOES LOW
AT V
= 1.15V

BATTERY

†

MINIMUM START-UP VOLTAGE = 1.1V

* 1% METAL FILM

†

L1 = GOWANDA GA10-103k

OR CADDELL-BURNS 7300-13

1.5V to 5V Bootstrapped Step-Up Converter

†

L1

1N5818

47µH

2N3906

56Ω

2.2k

1.5V
CELL

L1 = GOWANDA GA10-123k

I

LIM

LT1073-5

SENSE

OR CADDELL-BURNS 7300-14

V

IN

SW1

SW2GND

+

1073 TA13

5V OUTPUT
50mA

100µF

1073 TA15

Memory Backup Supply

I

V

LIM

1.5V
CELL

* 1% METAL FILM
**OPTIONAL

†

L1 = GOWANDA GA10-822k

OR CADDELL-BURNS 7300-12

IN

SW1

LT1073

FB

SW2GND

L1

82µH

5V MAIN
SUPPLY

†

5V TO MEMORY,

4.5V WHEN MAIN
SUPPLY OPEN

1N5818

806k*

+

100µF**

40.2k*

1073 TA16

13

LT1073

+

680k

5V OUTPUT
20mV

P-P

RIPPLE

40.2k*

909k*

100µF
OS-CON

1073 TA18

LT1073

I

LIM

V

IN

SW1

SETAO

FB

SW2GND

1.5V

L1

†

82µH

* 1% METAL FILM

†

L1 = GOWANDA GA10-822k

OR CADDELL-BURNS 7300-12

1N5818

TYPICAL APPLICATIO S

U

3V to 5V Step-Up Converter with Undervoltage Lockout

†

L1

1N5818

68µH

I

LIM

AO

SET

100Ω100k

LT1073

V

SW1

SW2GND

IN

FB

1M*

40.3k*

909k*

2N3906

3V

100k*

* 1% METAL FILM

†

L1 = GOWANDA GA10-682k

OR CADDELL-BURNS 7300-11

100k

2.2M

1.5V to 5V Very Low Noise Step-Up Converter 9V to 5V Reduced Noise Step-Down Converter

†

L1

1N5818

909k*

40.2k*

+

5V OUTPUT
5mA AT V
10mV

P-P

100µF
OS-CON

RIPPLE

680k

1.5V

I

LIM

FB

LT1073

V

SW1

SETAO

SW2GND

470µH

IN

+

BATTERY

5V OUTPUT
100mA
LOCKOUT
AT 1.8V

100µF

1073 TA17

= 1V

+V

6.5V TO 12V

1.5V to 5V Low Noise Step-Up Converter

†

L1

IN

680k 220Ω

I

LIMVIN

FB

LT1073

SW1

SETAO

SW2GND

47µH

1N5818

100µF

OS-CON

5V

OUT

90mA AT 6.5VIN

909k*

+

40.2k*

* 1% METAL FILM

†

L1 = GOWANDA GA10-473k

OR CADDELL-BURNS 7300-21

EFFICIENCY = 83% AT 5mA LOAD

3V to 6V at 1A Step-Up Converter 1.5V Powered 350ps Risetime Pulse Generator

INPUT

3V TO 6V

(2 LITHIUM CELLS)

560k

549k*

+

1000µF

* 1% METAL FILM

†

L1 = COILTRONICS CTX25-5-52

LOW I

Q

(<250µA)

20k*

I

LIM

FB

LT1073

V

IN

SW1

SETAO

SW2GND

1N5818

5.1k

2N3906

L1

25µH

51Ω

†

1073 TA19

6V OUTPUT

1A AT

= 3V

V

IN

1N5820

+

MTP3055EL

2200µF

1073 TA21

* 1% METAL FILM

†

L1 = GOWANDA GA10-472k

OR CADDELL-BURNS 7300-09

†

V

SW1

SW2GND

IN

FB

L1

150µH

0.1µF

1.5V

220Ω

I

LIM

LT1073

†

L1 = TOKO 262LYF-0095K

SELECT Q1 AND C1 FOR OPTIMUM RISE AND FALL

EFFICIENCY ≈ 80%
I

Q

OUTPUT NOISE ≈ 100mV

MUR120

MUR120

MUR120

0.1µF

≈ 130µA

0.1µF

24k 10k

90V BIAS

10M

P-P

1M

1073 TA20

C1

2pF TO 4pF

Q1
2N2369

OUTPUT
5V INTO
50Ω PULSE
WIDTH ≈ 1ns

50Ω

1073 TA22

14

PACKAGE DESCRIPTIO

0.300 – 0.325

(7.620 – 8.255)

U

Dimensions in inches (millimeters) unless otherwise noted.

N8 Package

8-Lead PDIP (Narrow 0.300)

(LTC DWG # 05-08-1510)

0.045 – 0.065

(1.143 – 1.651)

0.130 ± 0.005

(3.302 ± 0.127)

LT1073

0.400*

(10.160)

MAX

876

5

0.065

(1.651)

0.009 – 0.015

(0.229 – 0.381)

+0.035

0.325
–0.015

+0.889

8.255

()

–0.381

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

TYP

0.100
(2.54)

BSC

8-Lead Plastic Small Outline (Narrow 0.150)

0.010 – 0.020

(0.254 – 0.508)

0.008 – 0.010

(0.203 – 0.254)

*

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

**

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

× 45°

0.016 – 0.050

(0.406 – 1.270)

0.053 – 0.069

(1.346 – 1.752)

0°– 8° TYP

0.014 – 0.019

(0.355 – 0.483)

TYP

0.125

(3.175)

MIN

0.018 ± 0.003

(0.457 ± 0.076)

S8 Package

(LTC DWG # 05-08-1610)

0.004 – 0.010

(0.101 – 0.254)

0.050

(1.270)

BSC

0.020

(0.508)

MIN

0.255 ± 0.015*
(6.477 ± 0.381)

0.228 – 0.244

(5.791 – 6.197)

0.189 – 0.197*
(4.801 – 5.004)

7

8

1

2

12

6

3

3

5

0.150 – 0.157**
(3.810 – 3.988)

4

4

N8 1098

SO8 1298

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.

15

LT1073

TYPICAL APPLICATIO S

1.5V Powered Temperature Compensated Crystal Oscillator

30.1k* 27.4k*

6.81K*

LM134-3

1.5V

* 1% METAL FILM

†

L1 = J.W. MILLER #100267

= AT CUT –35° 20' ANGLE

2N3906

X1

2N3904

1.5V

T1 = COILTRONICS CTX10052-1
X1 = PROJECTS UNLIMITED AT­ 11k OR 8Ω SPEAKER
D1, D2, D3 = MUR1100
R1 = VICTOREEN MOX-300
U1 = LND-712 CORP., OCEANSIDE, NY

U

150k*

73.2k*

10k

+

150k

LT1017

–

10M*

0.02µF

1.5V

100k

2N3906

47µF47µF

OUTPUT

0.05ppm/°C

100Ω 100k

2N3904

1MHz

2k

1.5V Powered α, β, γ Particle Detector

1M1M3M 330Ω

I

LIMVIN

FB

0.01µF

1N976

1N4148

SW1

LT1073

SETAO

SW2GND

210k

1.5V

I

LIMVIN

FB

LT1073

A0

SW2

510pF

510pF

T1

3

4

NC

5

NC

1

2

NC

1N5818

R1

500M

SW1

SET

10

7

820µH

150k*

560k

68pF
600V

†

L1

2N3906

39.2k*

1MHz

MV209

0.01µF

D1 D2 D3

0.01µF

0.01µF

10M

1073 TA24

U1

1N4148

100k

1k

1.5V

++

1073 TA23

500V
REGULATED

RELATED PARTS

PART NUMBER DESCRIPTION COMMENTS

LT1307 Single Cell Micropower 600kHz PWM DC/DC Converter 3.3V at 75mA from One Cell, MSOP Package
LT1316 Burst ModeTM Operation DC/DC with Programmable Current Limit 1.5V Minimum, Precise Control of Peak Current Limit
LT1317 2-Cell Micropower DC/DC with Low-Battery Detector 3.3V at 200mA from Two Cells, 600kHz Fixed Frequency
LT1610 Single Cell Micropower DC/DC Converter 3V at 30mA from 1V, 1.7MHz Fixed Frequency
LT1611 Inverting 1.4MHz Switching Regulator in 5-Lead SOT-23 –5V at 150mA from 5V Input, Tiny SOT-23 Package
LT1613 1.4MHz Switching Regulator in 5-Lead SOT-23 5V at 200mA from 3.3V Input, Tiny SOT-23 Package
LT1615 Micropower Constant Off-Time DC/DC Converter in 5-Lead SOT-23 20V at 12mA from 2.5V, Tiny SOT-23 Package
LT1617 Micropower Inverting DC/DC Converter in 5-Lead SOT-23 –15V at 12mA from 2.5V, Tiny SOT-23 Package
LT1930/LT1930A 1.2MHz/2.2MHz, 1A Switching Regulator in 5-Lead SOT-23 5V at 450mA from 3.3V Input, Tiny SOT-23 Package
LT1931/LT1931A 1.2MHz/2.2MHz, 1A Inverting Switching Regulator in 5-Lead SOT-23 –5V at 350mA from 5V Input, Tiny SOT-23 Package

Burst Mode operation is a trademark of Linear Technology Corporation.

1073fa LT/TP 0301 2K REV A • PRINTED IN USA

 LINEAR TECHNOLOGY CORPORATION 2000

16

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

[phone] ● FAX: [phone]

●

www.linear-tech.com