LT1073
Micropower
DC/DC Converter
Adjustable and Fixed 5V, 12V
FEATURES
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DESCRIPTION
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 low-battery detector
or linear post-regulator.
Average current drain of the LT1073-5 used as shown in
the Typical Application circuit below is just 135µA unloaded,
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 step-
down converter, although for most step-down applications
or input sources greater than 3V, the LT1173 is recom-
mended. Switch current limiting is user-adjustable by
adding a single external resistor. Unique reverse-battery
protection circuitry limits reverse current to safe, non-
destructive levels at reverse supply voltages up to 1.6V.
L,
LTC, and LT are registered trademarks of Linear Technology Corporation.
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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
APPLICATIONS
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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
TYPICAL APPLICATION
Single-Cell to 5V Converter
CADDELL-BURNS
7300-12
1N5818
82µH
5V
40mA
CONTINUOUS OPERATION (HOURS)
Single Alkaline “AA” Cell Operating
Hours vs DC Load Current
1000
1
I
LIM
1.5V
AA CELL*
2
V
IN
SW1
3
100
L = 180µH
10
LT1073-5
SENSE
GND
5
SW2
4
8
+
100µF
SANYO
0S-CON
L = 82µH
1
1
10
LOAD CURRENT (mA)
100
LT1073 TA02
OPERATES WITH CELL VOLTAGE ≥1V
*ADD 10µF DECOUPLING CAPACITOR IF
BATTERY IS MORE THAN 2" AWAY FROM LT1073
1073 TA01
1
LT1073
ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
ORDER PART
NUMBER
TOP VIEW
I
LIM
1
V
IN
2
SW1 3
SW2 4
N8 PACKAGE
8-LEAD PDIP
8
7
6
5
FB (SENSE)*
SET
A0
GND
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
S8 PACKAGE
8-LEAD PLASTIC SO
LT1073CN8
LT1073CN8-5
LT1073CN8-12
LT1073CS8
LT1073CS8-5
LT1073CS8-12
S8 PART MARKING
1073
10735
107312
*FIXED VERSIONS
T
JMAX
= 125°C,
θ
JA
= 100°C/W (N8)
T
JMAX
= 125°C,
θ
JA
= 120°C/W (S8)
Consult factory for parts specified with wider operating temperature ranges.
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 1.5V unless otherwise specified.
SYMBOL
I
Q
I
Q
V
IN
PARAMETER
Quiescent Current
Quiescent Current, Step-Up
Mode Configuration
Input Voltage
CONDITIONS
Switch Off
No Load
Step-Up Mode
Step-Down Mode
Comparator Trip Point Voltage
V
OUT
Output Sense Voltage
Comparator Hysteresis
Output Hysteresis
f
OSC
DC
t
ON
I
FB
I
SET
V
AO
Oscillator Frequency
Duty Cycle
Switch ON Time
Feedback Pin Bias Current
Set Pin Bias Current
AO Output Low
Reference Line Regulation
V
CESAT
Switch Saturation Voltage
Set-Up Mode
LT1073, V
FB
= 0V
V
SET
= V
REF
I
AO
= –100µA
1V ≤ V
IN
≤ 1.5V
1.5V ≤ V
IN
≤ 12V
V
IN
= 1.5V, I
SW
= 400mA
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ELECTRICAL CHARACTERISTICS
MIN
LT1073-5
LT1073-12
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TYP
95
135
250
MAX
130
UNITS
µA
µA
µA
1.15
1.0
202
4.75
11.4
212
5
12
5
125
300
15
65
30
19
72
38
10
60
0.15
0.35
0.05
300
400
12.6
12.6
30
222
5.25
12.6
10
250
600
23
80
50
50
120
0.4
1.0
0.1
400
600
550
750
1000
1500
V
V
V
mV
V
V
mV
mV
mV
kHz
%
µs
nA
nA
V
%V
%V
mV
mV
mV
mV
mV
mV
V/V
LT1073 (Note 2)
LT1073-5 (Note 3)
LT1073-12 (Note 3)
LT1073
LT1073-5
LT1073-12
Full Load (V
FB
= V
REF
)
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V
IN
= 1.5V, I
SW
= 500mA
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V
IN
= 5V, I
SW
= 1A
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700
l
A
V
A2 Error Amp Gain
R
L
= 100kΩ (Note 4)
400
1000
2
LT1073
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 1.5V unless otherwise specified.
SYMBOL
I
REV
I
LIM
I
LEAK
V
SW2
PARAMETER
Reverse Battery Current
Current Limit
Current Limit Temperature Coefficient
Switch OFF Leakage Current
Maximum Excursion Below GND
Measured at SW1 Pin
I
SW1
≤ 10µA, Switch Off
CONDITIONS
(Note 5)
220Ω Between I
LIM
and V
IN
MIN
TYP
750
400
–0.3
1
–400
10
–350
MAX
UNITS
mA
mA
%/°C
µA
mV
ELECTRICAL CHARACTERISTICS
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.
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.
Note 4:
100k resistor connected between a 5V source and the AO pin.
Note 5:
The LT1073 is guaranteed to withstand continuous application of
1.6V applied to the GND and SW2 pins while V
IN
, I
LIM
and SW1 pins are
grounded.
TYPICAL PERFORMANCE CHARACTERISTICS
Saturation Voltage Step-Up Mode
(SW2 Pin Grounded)
1.2
1.0
SWITCH ON VOLTAGE (V)
V
IN
= 1.25V
0.8
V
CESAT
(V)
V
IN
= 1V
0.6
0.4
0.2
0
V
IN
= 5V
V
IN
= 3V
V
IN
= 2V
V
IN
= 1.5V
1.4
1.3
SWITCH CURRENT (mA)
1.2
1.1
1.0
0.9
0.8
0
0.2
0.4
0.6
I
SWITCH
(A)
1073 G01
Switch ON Voltage Step-Down Mode
(SW1 Pin Connected to V
IN
)
1200
1100
1000
900
800
700
600
500
400
300
200
Maximum Switch Current vs R
LIM
L = 82µH
V
IN
= 3V
V
IN
= 1.5V
0.8
1.0
1.2
0.7
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
100
10
I
SWITCH
(A)
1073 G02
100
R
LIM
( )
1000
1073 G03
FB Pin Bias Current vs
Temperature
20
18
SET PIN BIAS CURRENT (nA)
FB BIAS CURRENT (nA)
16
14
12
10
8
6
4
–50 –25
0
25
50
75
100
125
200
175
150
SET Pin Bias Current vs
Temperature
1800
1600
1400
GB GAIN (V/V)
1200
1000
800
600
400
200
0
25
50
75
100
125
“Gain Block” Gain
V
IN
= 1.5V
R
L
= 100k
125
100
75
50
25
0
–50 –25
0
–50 –25
0
25
50
75
100
125
TEMPERATURE (°C)
1073 G04
TEMPERATURE (°C)
1073 G05
TEMPERATURE (°C)
1073 G06
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LT1073
TYPICAL PERFORMANCE CHARACTERISTICS
Supply Current vs Temperature
150
140
130
SUPPLY CURRENT (µA)
120
110
100
90
80
70
60
50
–50 –25
0
25
50
75
100
125
V
IN
= 1.5V
MINIMUM INDUCTANCE (µH)
300
250
200
150
100
50
0
1.0 1.5
10
1.0
OUTPUT CURRENT (mA)
Recommended Minimum
Inductance Value
R
LIM
= 0V
1000
Guaranteed Minimum Output
Current at 5V vs V
IN
100
FOR V
IN
> 1.6V A
68 RESISTOR
MUST BE CONNECTED
BETWEEN I
LIM
AND V
IN
1.5
2.0
2.5
V
IN
(V)
3.0
3.5
1073 G09
2.0
TEMPERATURE (°C)
1073 G07
2.5 3.0 3.5 4.0
INPUT VOLTAGE (V)
4.5
5.0
1073 G08
PIN FUNCTIONS
I
LIM
(Pin 1):
Connect this pin to V
IN
for normal use. Where
lower current limit is desired, connect a resistor between
I
LIM
and V
IN
. A 220Ω resistor will limit the switch current
to approximately 400mA.
V
IN
(Pin 2):
Input Supply Voltage
SW1 (Pin 3):
Collector of Power Transistor. For step-up
mode connect to inductor/diode. For step-down mode
connect to V
IN
.
SW2 (Pin 4):
Emitter of Power Transistor. For step-up
mode connect to ground. For step-down mode connect
to inductor/diode. This pin must never be allowed to go
more than a Schottky diode drop below ground.
GND (Pin 5):
Ground.
AO (Pin 6):
Auxiliary Gain Block (GB) Output. Open col-
lector, 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.
BLOCK DIAGRAMS
LT1073
SET
SET
LT1073-5 and LT1073-12
+
A2
A0
+
A2
A0
–
V
IN
212mV
REFERENCE
GAIN BLOCK/ERROR AMP
I
LIM
A1
COMPARATOR
R1
GND
R2
940k
OSCILLATOR
DRIVER
SW1
Q1
SW2
–
V
IN
212mV
REFERENCE
GND
FB
COMPARATOR
GAIN BLOCK/ERROR AMP
I
LIM
A1
OSCILLATOR
DRIVER
SW1
Q1
SW2
1073 BD01
SENSE
LT1073-5: R1 = 40k
LT1073-12: R2 = 16.3k
1073 BD02
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LT1073
OPERATION
LT1073
The LT1073 is gated oscillator switcher. This type archi-
tecture 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 adap-
tive 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
OUT
≈ 3V
IN
, e.g., 1.5V to 5V. Other step-up ratios as well
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 undervolt-
age lockout circuit. The negative input of A2 is internally
connected to the 212mV reference. An external resistor
divider from V
IN
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
LIM
pin and V
IN
adjusts
maximum switch current. When the switch current exceeds
the set value, the switch is turned off. This feature is espe-
cially useful when small inductance values are used with
high input voltages. If the internal current limit of 1.5A is
desired, I
LIM
should be tied directly to V
IN
. Propagation
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 V
IN
and SW2 drives the inductor. Output voltage is set
by the following equation in either step-up or step-down
modes where R1 is connected from FB to GND and R2 is
connected from V
OUT
to FB.
R2
V
OUT
=
(
212mV
)
+1
R1
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 fixed-output con-
verter. 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, inher-
ently 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.
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