LINEAR TECHNOLOGY LT1073 User Manual

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

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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