LT1173
Micropower
DC/DC Converter
Adjustable and Fixed 5V, 12V
FEATURES
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DESCRIPTIO
Operates at Supply Voltages From 2.0V to 30V
Consumes Only 110µA Supply Current
Works in Step-Up or Step-Down Mode
Only Three External Components Required
Low Battery Detector Comparator On-Chip
User-Adjustable Current Limit
Internal 1A Power Switch
Fixed or Adjustable Output Voltage Versions
Space Saving 8-Pin MiniDIP or SO8 Package
The LT1173 is a versatile micropower DC-DC converter.
The device requires only three external components to
deliver a fixed output of 5V or 12V. Supply voltage ranges
from 2.0V to 12V in step-up mode and to 30V in step-down
mode. The LT1173 functions equally well in step-up, step-
down or inverting applications.
The LT1173 consumes just 110µA supply current at
standby, making it ideal for applications where low quies-
cent current is important. The device can deliver 5V at
80mA from a 3V input in step-up mode or 5V at 200mA
from a 12V input in step-down mode.
Switch current limit can be programmed with a single
resistor. An auxiliary gain block can be configured as a low
battery detector, linear post regulator, under voltage lock-
out circuit or error amplifier.
For input sources of less than 2V, use the LT1073.
APPLICATI
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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
and LTC are registered trademarks and LT is a trademark of Linear Technology Corporation.
TYPICAL APPLICATI
L1*
100µH
5V
IN
47Ω
I
LIM
10
µ
F
V
IN
SW1
S
VPP Output
12V
100mA
Logic Controlled Flash Memory VPP Generator
1N5818
V
OUT
5V/DIV
1.07M
†
+
+
LT1173
FB
SW2
SANYO
OS-CON
100
µ
F
0V
GND
124k
†
PROGRAM
5V/DIV
5ms/DIV
1173 TA02
1N4148
PROGRAM
LT1173 • TA01
*L1 = GOWANDA GA20-103K
COILTRONICS CTX100-4
NO OVERSHOOT
EFFICIENCY = 81%
†
= 1% METAL FILM
U
UO
UO
1
LT1
173
ABSOLUTE
AXI U
RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
I
LIM
1
V
IN
2
SW1 3
SW2 4
8
7
6
5
FB (SENSE)*
SET
AO
GND
Supply Voltage (V
IN
) ................................................ 36V
SW1 Pin Voltage (V
SW1
) .......................................... 50V
SW2 Pin Voltage (V
SW2
) ............................. – 0.5V to V
IN
Feedback Pin Voltage (LT1173) ................................. 5V
Sense Pin Voltage (LT1173, -5, -12) ....................... 36V
Maximum Power Dissipation ............................. 500mW
Maximum Switch Current ....................................... 1.5A
Operating Temperature Range ..................... 0°C to 70°C
Storage Temperature Range .................. –65°C to 150°C
Lead Temperature, (Soldering, 10 sec.)................ 300°C
Consult factory for Industrial and Military grade parts
ORDER PART
NUMBER
LT1173CN8
LT1173CN8-5
LT1173CN8-12
N8 PACKAGE
8-LEAD PLASTIC DIP
*FIXED VERSIONS
T
JMAX
= 90°C,
θ
JA
= 130°C/W
TOP VIEW
I
LIM
1
V
IN
2
SW1 3
SW2 4
8 FB (SENSE)*
7 SET
6 AO
5 GND
LT1173CS8
LT1173CS8-5
LT1173CS8-12
S8 PART MARKING
1173
11735
117312
S8 PACKAGE
8-LEAD PLASTIC SOIC
*FIXED VERSIONS
T
JMAX
= 90°C,
θ
JA
= 150°C/W
ELECTRICAL CHARACTERISTICS
T
A
= 25°C, V
IN
= 3V, unless otherwise noted.
SYMBOL
I
Q
I
Q
V
IN
PARAMETER
Quiescent Current
Quiescent Current, Boost
Mode Configuration
Input Voltage
Comparator Trip Point Voltage
V
OUT
Output Sense Voltage
Comparator Hysteresis
Output Hysteresis
f
OSC
t
ON
Oscillator Frequency
Duty Cycle
Switch ON Time
Feedback Pin Bias Current
Set Pin Bias Current
V
OL
Gain Block Output Low
Reference Line Regulation
V
SAT
SW
SAT
Voltage, Step-Up Mode
Full Load
I
LIM
tied to V
IN
LT1173, V
FB
= 0V
V
SET
= V
REF
I
SINK
= 100µA, V
SET
= 1.00V
2.0V
≤
V
IN
≤
5V
5V
≤
V
IN
≤
30V
V
IN
= 3.0V, I
SW
= 650mA
V
IN
= 5.0V, I
SW
= 1A
q
CONDITIONS
Switch Off
No Load
Step-Up Mode
Step-Down Mode
LT1173 (Note 1)
LT1173-5 (Note 2)
LT1173-12 (Note 2)
LT1173
LT1173-5
LT1173-12
LT1173-5
LT1173-12
q
q
q
q
q
q
q
q
q
q
q
q
q
q
q
q
q
q
MIN
TYP
110
135
250
MAX
150
UNITS
µA
µA
µA
2.0
1.20
4.75
11.4
1.245
5.00
12.0
5
20
50
18
43
17
23
51
22
10
20
0.15
0.2
0.02
0.5
0.8
12.6
30
1.30
5.25
12.6
10
40
100
30
59
32
50
100
0.4
0.4
0.075
0.65
1.0
1.4
2
U
V
V
V
V
V
mV
mV
mV
kHz
%
µs
nA
nA
V
%/V
%/V
V
V
V
W
U
U
W W
W
LT1173
ELECTRICAL CHARACTERISTICS
T
A
= 25°C, V
IN
= 3V, unless otherwise noted.
SYMBOL
V
SAT
A
V
PARAMETER
SW
SAT
Voltage, Step-Down Mode
Gain Block Gain
Current Limit
Current Limit Temperature Coeff.
Switch OFF Leakage Current
V
SW2
Maximum Excursion Below GND
Measured at SW1 Pin
I
SW1
≤
10µA, Switch Off
CONDITIONS
V
IN
= 12V, I
SW
= 650mA
q
MIN
TYP
1.1
MAX
1.5
1.7
UNITS
V
V
V/V
mA
%/°C
R
L
= 100kΩ (Note 3)
220Ω to I
LIM
to V
IN
q
400
1000
400
– 0.3
1
– 400
10
– 350
q
µA
mV
The
q
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.
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.
TYPICAL PERFOR A CE CHARACTERISTICS
Saturation Voltage Step-Up Mode
(SW2 Pin Grounded)
1.2
1.0
SWITCH ON VOLTAGE (V)
1.4
1.3
SWITCH CURRENT (mA)
V
IN
= 3.0V
0.8
V
CESAT
(V)
0.6
0.4
0.2
0
0
0.2
0.4
V
IN
= 2.0V
0.6
I
SWITCH
(A)
0.8
1000
900
800
Maximum Switch Current vs
R
LIM
Step-Down Mode
V
OUT
= 5V
SET PIN BIAS CURRENT (nA)
FEEDBACK PIN BIAS CURRENT (
µ
A)
SWITCH CURRENT (mA)
V
IN
= 24V
L = 500µH
700
600
500
400
300
200
100
0
100
R
LIM
(
Ω
)
LT1173 • TPC09
V
IN
= 12V
L = 250µH
U W
V
IN
= 5.0V
1.0
LT1173 • TPC01
Switch ON Voltage
Step-Down Mode
(SW1 Pin Connected to V
IN
)
1200
1100
1000
900
800
700
600
500
400
300
200
100
Maximum Switch Current vs
R
LIM
Step-Up Mode
2V
≤
V
IN
≤
5V
1.2
1.1
1.0
0.9
0.8
0.7
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
I
SWITCH
(A)
LT1173 • TPC02
1.2
10
100
R
LIM
(
Ω
)
1000
LT1173 • TPC03
Set Pin Bias Current vs
Temperature
20
V
IN
= 3V
15
18
Feedback Pin Bias Current vs
Temperature
V
IN
= 3V
16
14
12
10
10
1000
5
–50
8
–25
0
25
50
75
100
125
–50
–25
0
25
50
75
100
TEMPERATURE (°C)
LT1173 •TPC04
125
TEMPERATURE (°C)
LT1173 •TPC05
3
LT1
173
TYPICAL PERFOR A CE CHARACTERISTICS
Quiescent Current vs Temperature
120
V
IN
= 3V
110
SUPPLY CURRENT (mA)
F
OSC
(kHz)
I
IN
(
µ
A)
100
90
–50
–25
0
25
50
75
TEMPERATURE (°C)
LT1173 •TPC06
PI FU CTI
S
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 resis-
tor that sets output voltage.
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.
BLOCK DIAGRA S
LT1173
SET
SET
A2
V
IN
GAIN BLOCK/
ERROR AMP
1.245V
REFERENCE
A1
OSCILLATOR
DRIVER
GND
FB
COMPARATOR
SW2
LT1173 • BD01
4
U W
100
Supply Current vs Switch Current
50
26.0
25.5
Oscillator Frequency
40
25.0
30
V
IN
= 5V
24.5
24.0
23.5
23.0
22.5
20
V
IN
= 2V
10
0
125
0
200
400
600
800
1000
SWITCH CURRENT (mA)
LT1173 •TPC07
22.0
0
5
10
15
V
IN
(V)
LT1173 • TPC08
20
25
30
W
UO
U
U
LT1173-5, -12
A2
AO
AO
V
IN
GAIN BLOCK/
ERROR AMP
I
LIM
SW1
I
LIM
SW1
1.245V
REFERENCE
A1
OSCILLATOR
DRIVER
COMPARATOR
R1
GND
R2
753k
Ω
SW2
SENSE
LT1173-5: R1 = 250k
Ω
LT1173-12: R1 = 87.4k
Ω
LT1173 • BD02
LT1173
LT1173 OPERATI
The LT1173 is a 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 under-
stood by referring to the LT1173 block diagram. Compara-
tor 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 feed-
back 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
OUT
and V
IN
differ by roughly a factor of 2. Examples include a
3V to 5V step-up converter or a 9V to 5V step-down
converter.
APPLICATI
S I FOR ATIO
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
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
SET
is set to the input voltage of the
LT1173. The circuit must be “booted” by shorting V2 to
V
SET
. After the LT1173 output voltage has settled, discon-
nect the short. Input voltage is V2, and average input
current can be calculated by this formula:
I
IN
=
V2
−
V1
100
Ω
U
W
UO
A2 is a versatile gain block that can serve as a low battery
detector, a linear post regulator, or drive an under voltage
lockout circuit. The negative input of A2 is internally
connected to the 1.245V reference. A resistor divider from
V
IN
to GND, with the mid-point connected to the SET pin
provides the trip voltage in a low battery detector applica-
tion. The gain block output (AO) can sink 100µA (use a 47k
resistor pull-up to + 5V). This line can signal a microcon-
troller that the battery voltage has dropped below the
preset level.
A resistor connected between the I
LIM
pin and V
IN
sets
maximum switch current. When the switch current ex-
ceeds the set value, the switch cycle is prematurely
terminated. If current limit is not used, I
LIM
should be tied
directly to V
IN
. Propagation delay through the current limit
circuitry is approximately 2µs.
In step-up mode the switch emitter (SW2) is connected to
ground and the switch collector (SW1) drives the induc-
tor; in step-down mode the collector is connected to V
IN
and the emitter drives the inductor.
The 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.
U
UO
(
01
)
5
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