LT1173
1
Micropower
DC/DC Converter
Adjustable and Fixed 5V, 12V
V
OUT
5V/DIV
0V
PROGRAM
5V/DIV
5ms/DIV
1173 TA02
L1*
100
H
LT1173 TA01
+
GND
SW2
FB
SW1
LIM
I
IN
V
LT1173
10 F
SANYO
OS-CON
100 F
12V
100mA
1N5818
+
5V
IN
1N4148
PROGRAM
47
*L1 = GOWANDA GA20-103K
COILTRONICS CTX100-4
1.07M
124k
NO OVERSHOOT
EFFICIENCY = 81%
= 1% METAL FILM
Logic Controlled Flash Memory VPP Generator
VPP Output
D
U
ESCRIPTIO
S
FEATURE
U
S
A
O
PPLICATI
and LTC are registered trademarks and LT is a trademark of Linear Technology Corporation.
s
Operates at Supply Voltages From 2.0V to 30V
s
Consumes Only 110
A Supply Current
s
Works in Step-Up or Step-Down Mode
s
Only Three External Components Required
s
Low Battery Detector Comparator On-Chip
s
User-Adjustable Current Limit
s
Internal 1A Power Switch
s
Fixed or Adjustable Output Voltage Versions
s
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 110
A 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.
s
Flash Memory Vpp Generators
s
3V to 5V, 5V to 12V Converters
s
9V to 5V, 12V to 5V Converters
s
LCD Bias Generators
s
Peripherals and Add-On Cards
s
Battery Backup Supplies
s
Laptop and Palmtop Computers
s
Cellular Telephones
s
Portable Instruments
U
S
A
O
PPLICATI
TYPICAL
LT1173
2
W
U
U
PACKAGE/ORDER I FOR ATIO
LT1173CN8
LT1173CN8-5
LT1173CN8-12
A
U
G
W
A
W
U
W
A
R
BSOLUTE
XI
TI
S
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
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
I
Q
Quiescent Current
Switch Off
q
110
150
A
I
Q
Quiescent Current, Boost
No Load LT1173-5
135
A
Mode Configuration
LT1173-12
250
A
V
IN
Input Voltage
Step-Up Mode
q
2.0
12.6
V
Step-Down Mode
q
30
V
Comparator Trip Point Voltage
LT1173 (Note 1)
q
1.20
1.245
1.30
V
V
OUT
Output Sense Voltage
LT1173-5 (Note 2)
q
4.75
5.00
5.25
V
LT1173-12 (Note 2)
q
11.4
12.0
12.6
V
Comparator Hysteresis
LT1173
q
5
10
mV
Output Hysteresis
LT1173-5
q
20
40
mV
LT1173-12
q
50
100
mV
f
OSC
Oscillator Frequency
q
18
23
30
kHz
Duty Cycle
Full Load
q
43
51
59
%
t
ON
Switch ON Time
I
LIM
tied to V
IN
q
17
22
32
s
Feedback Pin Bias Current
LT1173, V
FB
= 0V
q
10
50
nA
Set Pin Bias Current
V
SET
= V
REF
q
20
100
nA
V
OL
Gain Block Output Low
I
SINK
= 100
A, V
SET
= 1.00V
q
0.15
0.4
V
Reference Line Regulation
2.0V
V
IN
5V
q
0.2
0.4
%/V
5V
V
IN
30V
q
0.02
0.075
%/V
V
SAT
SW
SAT
Voltage, Step-Up Mode
V
IN
= 3.0V, I
SW
= 650mA
q
0.5
0.65
V
V
IN
= 5.0V, I
SW
= 1A
0.8
1.0
V
q
1.4
V
T
A
= 25
C, V
IN
= 3V, unless otherwise noted.
ELECTRICAL C
C
HARA TERISTICS
1
2
3
4
8
7
6
5
TOP VIEW
I
LIM
V
IN
SW1
SW2
FB (SENSE)*
SET
AO
GND
N8 PACKAGE
8-LEAD PLASTIC DIP
*FIXED VERSIONS
T
JMAX
= 90
C,
JA
= 130
C/W
TOP VIEW
S8 PACKAGE
8-LEAD PLASTIC SOIC
*FIXED VERSIONS
1
2
3
4
8
7
6
5
I
LIM
V
IN
SW1
SW2
FB (SENSE)*
SET
AO
GND
T
JMAX
= 90
C,
JA
= 150
C/W
LT1173CS8
LT1173CS8-5
LT1173CS8-12
ORDER PART
NUMBER
S8 PART MARKING
1173
11735
117312
LT1173
3
V
SAT
SW
SAT
Voltage, Step-Down Mode
V
IN
= 12V, I
SW
= 650mA
1.1
1.5
V
q
1.7
V
A
V
Gain Block Gain
R
L
= 100k
(Note 3)
q
400
1000
V/V
Current Limit
220
to I
LIM
to V
IN
400
mA
Current Limit Temperature Coeff.
q
0.3
%/
C
Switch OFF Leakage Current
Measured at SW1 Pin
1
10
A
V
SW2
Maximum Excursion Below GND
I
SW1
10
A, Switch Off
400
350
mV
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
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: 100k
resistor connected between a 5V source and the AO pin.
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.
ELECTRICAL C
C
HARA TERISTICS
T
A
= 25
C, V
IN
= 3V, unless otherwise noted.
Maximum Switch Current vs
Set Pin Bias Current vs
Feedback Pin Bias Current vs
R
LIM
Step-Down Mode
Temperature
Temperature
C
C
HARA TERISTICS
U
W
A
TYPICAL PERFOR
CE
I (A)
0
0
V (V)
0.2
0.4
0.6
1.0
1.2
0.2
0.4
0.6
0.8
LT1173 TPC01
0.8
1.0
1.2
SWITCH
CESAT
V = 5.0V
IN
V = 3.0V
IN
V = 2.0V
IN
I (A)
0
0.7
SWITCH ON VOLTAGE (V)
0.8
0.9
1.1
1.3
1.4
0.1
0.2
0.3
0.4
LT1173 TPC02
1.2
0.5
0.6
SWITCH
0.7
0.8
1.0
Switch ON Voltage
Saturation Voltage Step-Up Mode
Step-Down Mode
Maximum Switch Current vs
(SW2 Pin Grounded)
(SW1 Pin Connected to V
IN
)
R
LIM
Step-Up Mode
TEMPERATURE (C)
50
SET PIN BIAS CURRENT (nA)
10
15
20
25
0
25
50
LT1173 TPC04
75
100
125
V = 3V
IN
5
TEMPERATURE (C)
50
FEEDBACK PIN BIAS CURRENT ( A)
14
16
18
25
0
25
50
LT1173 TPC05
75
100
125
V = 3V
IN
12
10
8
R ( )
100
0
SWITCH CURRENT (mA)
400
800
1000
LT1173 TPC09
900
700
600
500
300
200
LIM
1000
V
OUT
= 5V
100
V
IN
= 24V
L = 500
H
V
IN
= 12V
L = 250
H
R ( )
10
100
SWITCH CURRENT (mA) 400
800
1200
100
LT1173 TPC03
1000
900
700
600
500
300
200
LIM
1000
1100
2V
V
IN
5V
LT1173
4
C
C
HARA TERISTICS
U
W
A
TYPICAL PERFOR
CE
LT1173 BD02
IN
V
GND
SET
AO
A2
1.245V
REFERENCE
A1
OSCILLATOR
DRIVER
R1
SW1
SW2
LIM
I
R2
753k
SENSE
LT1173-5:
LT1173-12:
R1 = 250k
R1 = 87.4k
GAIN BLOCK/
ERROR AMP
COMPARATOR
LT1173 BD01
IN
V
GND
SET
AO
1.245V
REFERENCE
A1
A2
DRIVER
FB
SW1
SW2
LIM
I
OSCILLATOR
GAIN BLOCK/
ERROR AMP
COMPARATOR
LT1173
LT1173-5, -12
W
I
D AGRA
BLOCK
S
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 220
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 collec-
tor, can sink 100
A.
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.
PI
U
FU
U
C
U
S
O
TI
Quiescent Current vs Temperature
Supply Current vs Switch Current
Oscillator Frequency
SWITCH CURRENT (mA)
0
SUPPLY CURRENT (mA)
30
40
50
400
600
800
LT1173 TPC07
1000
V = 5V
IN
20
10
0
V = 2V
IN
200
TEMPERATURE (C)
50
I ( A)
100
110
120
25
0
25
50
LT1173 TPC06
75
100
125
V = 3V
IN
IN
90
V
IN
(V)
0
22.0
F
OSC
(kHz)
22.5
23.0
23.5
24.5
25.0
5
10
15
20
LT1173 TPC08
24.0
25
30
25.5
26.0
LT1173
5
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 110
A, for the reference, A1 and A2.
The oscillator is set internally for 23
s ON time and 19
s
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.
U
LT1173 OPER
O
ATI
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 100
A (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 2
s.
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
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PPLICATI
W
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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 100
A 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
V
V
I N
=
-
( )
2
1
100
01
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