LINER LT1173 Micropower dc/dc converter adjustable and fixed 5v, 12v Datasheet

LT1173
Micropower
DC/DC Converter
Adjustable and Fixed 5V, 12V
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DESCRIPTIO
FEATURES
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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, stepdown or inverting applications.
The LT1173 consumes just 110µA supply current at
standby, making it ideal for applications where low quiescent 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.
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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
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 lockout circuit or error amplifier.
For input sources of less than 2V, use the LT1073.
and LTC are registered trademarks and LT is a trademark of Linear Technology Corporation.
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TYPICAL APPLICATI
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Logic Controlled Flash Memory VPP Generator
L1*
100µH
VPP Output
1N5818
12V
100mA
5VIN
47Ω
I LIM
10 µ F
+
VOUT
5V/DIV
V IN
SW1
1.07M†
+
LT1173
GND
FB
SW2
SANYO
OS-CON
100 µ F
0V
PROGRAM
5V/DIV
124k†
5ms/DIV
1173 TA02
1N4148
PROGRAM
LT1173 • TA01
*L1 = GOWANDA GA20-103K
COILTRONICS CTX100-4
EFFICIENCY = 81%
† = 1% METAL FILM
NO OVERSHOOT
1
LT1173
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ABSOLUTE
PACKAGE/ORDER I FOR ATIO
Supply Voltage (VIN) ................................................ 36V
SW1 Pin Voltage (VSW1) .......................................... 50V
SW2 Pin Voltage (VSW2) ............................. – 0.5V to VIN
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
TOP VIEW
ILIM 1
8
FB (SENSE)*
VIN 2
7
SET
SW1 3
6
AO
SW2 4
5
GND
LT1173CN8
LT1173CN8-5
LT1173CN8-12
N8 PACKAGE
8-LEAD PLASTIC DIP
*FIXED VERSIONS
TJMAX = 90°C, θJA = 130°C/W
TOP VIEW
Consult factory for Industrial and Military grade parts
ORDER PART
NUMBER
ILIM 1
8 FB (SENSE)*
VIN 2
7 SET
SW1 3
6 AO
SW2 4
5 GND
LT1173CS8
LT1173CS8-5
LT1173CS8-12
S8 PART MARKING
1173
11735
117312
S8 PACKAGE
8-LEAD PLASTIC SOIC
*FIXED VERSIONS
TJMAX = 90°C, θJA = 150°C/W
ELECTRICAL CHARACTERISTICS TA = 25°C, VIN = 3V, unless otherwise noted.
SYMBOL
PARAMETER
IQ
Quiescent Current
Switch Off
IQ
Quiescent Current, Boost
Mode Configuration
No Load
Input Voltage
Step-Up Mode
●
Step-Down Mode
●
Comparator Trip Point Voltage
LT1173 (Note 1)
●
1.20
Output Sense Voltage
LT1173-5 (Note 2)
●
4.75
LT1173-12 (Note 2)
●
11.4
Comparator Hysteresis
LT1173
●
5
10
mV
Output Hysteresis
LT1173-5
●
20
40
mV
LT1173-12
●
VIN
VOUT
fOSC
tON
VOL
VSAT
CONDITIONS
MIN
●
MAX
110
150
UNITS
µA
LT1173-5
135
µA
LT1173-12
250
µA
Oscillator Frequency
2.0
12.6
V
30
V
1.245
1.30
V
5.00
5.25
V
12.0
12.6
V
50
100
mV
●
18
23
30
kHz
%
Duty Cycle
Full Load
●
43
51
59
Switch ON Time
ILIM tied to VIN
●
17
22
32
µs
Feedback Pin Bias Current
LT1173, VFB = 0V
●
10
50
nA
Set Pin Bias Current
VSET = VREF
●
20
100
nA
Gain Block Output Low
ISINK = 100µA, VSET = 1.00V
●
0.15
0.4
V
Reference Line Regulation
2.0V ≤ VIN ≤ 5V
●
0.2
0.4
%/V
5V ≤ VIN ≤ 30V
●
0.02
0.075
%/V
VIN = 3.0V, ISW = 650mA
●
0.5
0.65
SWSAT Voltage, Step-Up Mode
VIN = 5.0V, ISW = 1A
0.8
●
2
TYP
V
1.0
V
1.4
V
LT1173
ELECTRICAL CHARACTERISTICS TA = 25°C, VIN = 3V, unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
VSAT
SWSAT Voltage, Step-Down Mode
VIN = 12V, ISW = 650mA
TYP
MAX
1.1
●
AV
Gain Block Gain
RL = 100kΩ (Note 3)
Current Limit
220Ω to ILIM to VIN
Current Limit Temperature Coeff.
VSW2
400
●
●
Switch OFF Leakage Current
Measured at SW1 Pin
Maximum Excursion Below GND
ISW1 ≤ 10µA, Switch Off
The ● denotes the specifications which apply over the full operating
temperature range.
UNITS
1.5
V
1.7
V
1000
V/V
400
mA
– 0.3
%/°C
1
10
µA
– 400
– 350
mV
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.
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.
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TYPICAL PERFOR A CE CHARACTERISTICS
Switch ON Voltage
Step-Down Mode
(SW1 Pin Connected to VIN)
Saturation Voltage Step-Up Mode
(SW2 Pin Grounded)
1.2
1.4
1.0
1.3
Maximum Switch Current vs
RLIM Step-Up Mode
1200
2V ≤ VIN ≤ 5V
VIN = 5.0V
VIN= 2.0V
0.6
0.4
0.2
1.2
1.1
1.0
0.9
0
0
0.2
0.4
0.6
0.8
1.0
0.1
0.2
0.3
0.6
0.7
VIN = 24V
L = 500µH
600
VIN = 12V
L = 250µH
400
300
200
1000
R LIM (Ω)
LT1173 • TPC03
Feedback Pin Bias Current vs
Temperature
18
VIN = 3V
15
10
5
–50
VIN = 3V
16
14
12
10
8
–25
0
25
50
75
100
125
TEMPERATURE (°C)
LT1173 • TPC09
1000
100
R LIM (Ω)
100
0
100
10
0.8
20
VOUT = 5V
SET PIN BIAS CURRENT (nA)
SWITCH CURRENT (mA)
0.5
Set Pin Bias Current vs
Temperature
700
500
0.4
LT1173 • TPC02
Maximum Switch Current vs
RLIM Step-Down Mode
800
500
400
ISWITCH (A)
LT1173 • TPC01
900
600
100
0
ISWITCH (A)
1000
700
200
0.7
1.2
900
800
300
0.8
FEEDBACK PIN BIAS CURRENT (µA)
VCESAT (V)
0.8
1000
SWITCH CURRENT (mA)
VIN= 3.0V
SWITCH ON VOLTAGE (V)
1100
–50
–25
0
25
50
75
100
125
TEMPERATURE (°C)
LT1173 •TPC04
LT1173 •TPC05
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LT1173
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TYPICAL PERFOR A CE CHARACTERISTICS
Quiescent Current vs Temperature
Supply Current vs Switch Current
120
Oscillator Frequency
26.0
50
25.5
VIN = 3V
IIN (µA)
110
100
25.0
VIN = 5V
30
FOSC (kHz)
SUPPLY CURRENT (mA)
40
20
VIN = 2V
24.5
24.0
23.5
23.0
10
22.5
90
–50
0
–25
0
25
75
50
125
100
22.0
0
TEMPERATURE (°C)
200
400
600
800
1000
0
5
10
SWITCH CURRENT (mA)
LT1173 •TPC06
15
20
25
30
VIN(V)
LT1173 •TPC07
LT1173 • TPC08
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ILIM (Pin 1): Connect this pin to VIN for normal use. Where
lower current limit is desired, connect a resistor between
ILIM and VIN. A 220Ω resistor will limit the switch current
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.
GND (Pin 5): Ground.
AO (Pin 6): Auxiliary Gain Block (GB) output. Open collector, 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 resistor that sets output voltage.
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BLOCK DIAGRA S
LT1173
LT1173-5, -12
SET
A2
SET
A2
AO
V IN
GAIN BLOCK/
ERROR AMP
GAIN BLOCK/
ERROR AMP
I LIM
SW1
1.245V
REFERENCE
OSCILLATOR
OSCILLATOR
DRIVER
COMPARATOR
FB
SW1
COMPARATOR
DRIVER
GND
I LIM
1.245V
REFERENCE
A1
A1
4
AO
V IN
SW2
LT1173 • BD01
R1
GND
R2
753k Ω
SW2
SENSE
LT1173-5: R1 = 250k Ω
LT1173-12: R1 = 87.4k Ω
LT1173 • BD02
LT1173
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LT1173 OPERATI
The LT1173 is a 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 LT1173 block diagram. Comparator 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 feedback 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 VOUT
and VIN differ by roughly a factor of 2. Examples include a
3V to 5V step-up converter or a 9V to 5V step-down
converter.
A resistor connected between the ILIM pin and VIN sets
maximum switch current. When the switch current exceeds the set value, the switch cycle is prematurely
terminated. If current limit is not used, ILIM should be tied
directly to VIN. 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 inductor; in step-down mode the collector is connected to VIN
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.
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APPLICATI
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
VIN to GND, with the mid-point connected to the SET pin
provides the trip voltage in a low battery detector application. The gain block output (AO) can sink 100µA (use a 47k
resistor pull-up to + 5V). This line can signal a microcontroller that the battery voltage has dropped below the
preset level.
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. VSET is set to the input voltage of the
LT1173. The circuit must be “booted” by shorting V2 to
VSET. After the LT1173 output voltage has settled, disconnect the short. Input voltage is V2, and average input
current can be calculated by this formula:
IIN =
V2 − V1
100Ω
(01)
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LT1173
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1MΩ
the inductive events add to the input voltage to produce the
output voltage. Power required from the inductor is determined by
+12V
1µF*
–
100 Ω
LTC1050
V1
V2
+
1000µF
V SET
*NON-POLARIZED
LT1173
CIRCUIT
PL = (VOUT + VD – VIN) (IOUT)
(02)
+
LT1173 • TA06
Figure 1. Test Circuit Measures No Load Quiescent Current of
LT1073 Converter
where VD is the diode drop (0.5V for a 1N5818 Schottky).
Energy required by the inductor per cycle must be equal or
greater than
(03)
PL
FOSC
Inductor Selection
in order for the converter to regulate the output.
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 LT1173 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
LT1173 based designs, small axial leaded units with
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.
When the switch is closed, current in the inductor builds
according to
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,
6
–R't 
V 
IL t = IN  1 – e L 
R' 

()
(04)
where R' is the sum of the switch equivalent resistance
(0.8Ω typical at 25°C) and the inductor DC resistance.
When the drop across the switch is small compared to VIN,
the simple lossless equation
()
V
IL t = IN t
L
(05)
can be used. These equations assume that at t = 0,
inductor current is zero. This situation is called “discontinuous mode operation” in switching regulator parlance.
Setting “t” to the switch ON time from the LT1173 specification table (typically 23µs) will yield iPEAK for a specific
“L” and VIN. Once iPEAK is known, energy in the inductor at
the end of the switch ON time can be calculated as
EL =
1 2
Li
2 PEAK
(06)
EL must be greater than PL/FOSC for the converter to deliver
the required power. For best efficiency iPEAK should be
kept to 1A or less. Higher switch currents will cause
excessive drop across the switch resulting in reduced
efficiency. In general, switch current should be held to as
low a value as possible in order to keep switch, diode and
inductor losses at a minimum.
LT1173
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As an example, suppose 9V at 50mA is to be generated
from a 3V input. Recalling Equation 02,
PL = (9V + 0.5V – 3V) (50mA) = 325mW.
(07)
Energy required from the inductor is
PL
FOSC
=
325mW
= 13.5µJ.
24kHz
(08)
Picking an inductor value of 100µH with 0.2Ω DCR results
in a peak switch current of
iPEAK =
3V
1Ω
–1Ω •23µ s 

–
e
1
100µ H  = 616m A.



(09)
Substituting iPEAK into Equation 04 results in
EL =
(
)(
)
2
1
100µH 0.616 A = 19.0µJ.
2
(10)
An inductor’s energy storage capability is proportional to
its physical size. If the size of the inductor is too large for
a particular application, considerable size reduction is
possible by using the LT1111. This device is pin compatible with the LT1173 but has a 72kHz oscillator, thereby
reducing inductor and capacitor size requirements by a
factor of three.
For both positive-to-negative (Figure 7) and negative-topositive configurations (Figure 8), all the output power
must be generated by the inductor. In these cases
(11)
In the positive-to-negative case, switch drop can be modeled as a 0.75V voltage source in series with a 0.65Ω
resistor so that
VL = VIN – 0.75V – IL (0.65Ω).
The step-down case is different than the preceeding three
in that the inductor current flows through the load in a
step-down topology (Figure 6). Current through the switch
should be limited to ~650mA in step-down mode. This can
be accomplished by using the ILIM pin. With input voltages
in the range of 12V to 25V, a 5V output at 300mA can be
generated with a 220µH inductor and 100Ω resistor in
series with the ILIM pin. With a 20V to 30V input range, a
470µH inductor should be used along with the 100Ω
resistor.
Capacitor Selection
Since 19µJ > 13.5µJ the 100µH inductor will work. This
trial-and-error approach can be used to select the optimum inductor. Keep in mind the switch current maximum
rating of 1.5A. If the calculated peak current exceeds this,
consider using the LT1073. The 70% duty cycle of the
LT1073 allows more energy per cycle to be stored in the
inductor, resulting in more output power.
PL = ( VOUT + VD) (IOUT).
In the negative-to-positive case, the switch saturates and
the 0.8Ω switch ON resistance value given for Equation 04
can be used. In both cases inductor design proceeds from
Equation 03.
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 2, 3, and 4 show the output voltage of an LT1173
based converter with three 100µF capacitors. The peak
switch current is 500mA in all cases. Figure 2 shows a
Sprague 501D, 25V aluminum capacitor. VOUT jumps by
over 120mV 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 240mΩ. Figure 3 shows the
same circuit, but with a Sprague 150D, 20V tantalum
capacitor replacing the aluminum unit. Output jump is
now about 35mV, corresponding to an ESR of 70mΩ.
Figure 4 shows the circuit with a 16V OS-CON unit. ESR is
now only 20mΩ.
(12)
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LT1173
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50mV/DIV
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50mV/DIV
APPLICATI
5µs/DIV
Figure 2. Aluminum
5µs/DIV
LT1173 • TA07
Figure 3. Tantalum
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 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 LT1173
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-50µs range. At best, efficiency will
be severely compromised when these diodes are used; at
worst, the circuit may not work at all. Most LT1173 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 LT1173 requirements. At peak
switch currents of 100mA or less, a 1N4148 signal diode
may be used. This diode has leakage current in the 1nA5nA 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 LT1173 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.
8
5µs/DIV
LT1173 • TA08
LT1173 • TA09
Figure 4. OS-CON
Step-Up (Boost Mode) Operation
A step-up DC-DC converter delivers an output voltage
higher than the input voltage. Step-up converters are not
short circuit protected since there is a DC path from input
to output.
The usual step-up configuration for the LT1173 is shown
in Figure 5. The LT1173 first pulls SW1 low causing VIN –
VCESAT to appear across L1. A current then builds up in L1.
At the end of the switch ON time the current in L1 is1:
i PEAK =
VIN
L
(13)
t ON
L1
D1
V IN
V OUT
R3*
I LIM
V IN
SW1
LT1173
GND
R2
+
C1
FB
SW2
R1
* = OPTIONAL
LT1173 • TA10
Figure 5. Step-Up Mode Hookup.
Refer to Table 1 for Component Values
Immediately after switch turn off, the SW1 voltage pin
starts to rise because current cannot instantaneously stop
flowing in L1. When the voltage reaches VOUT + VD, the
inductor current flows through D1 into C1, increasing
VOUT. This action is repeated as needed by the LT1173 to
Note 1: This simple expression neglects the effect of switch and coil
resistance. This is taken into account in the “Inductor Selection” section.
LT1173
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S I FOR ATIO
keep VFB at the internal reference voltage of 1.245V. R1
and R2 set the output voltage according to the formula
 R2 
VOUT =  1 +  1.245V .
R1

(
)
(14)
Step-Down (Buck Mode) Operation
A step-down DC-DC converter converts a higher voltage
to a lower voltage. The usual hookup for an LT1173 based
step-down converter is shown in Figure 6.
VIN
R3
100 Ω
+
C2
I LIM
Inverting Configurations
V IN
SW1
FB
LT1173
L1
VOUT
SW2
GND
R2
D1
1N5818
+
C1
R1
LT1173 • TA11
Figure 6. Step-Down Mode Hookup
When the switch turns on, SW2 pulls up to VIN – VSW. This
puts a voltage across L1 equal to VIN – VSW – VOUT,
causing a current to build up in L1. At the end of the switch
ON time, the current in L1 is equal to
i PEAK =
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 100Ω resistor programs the switch to turn off when the current reaches
approximately 800mA. When using the LT1173 in stepdown mode, output voltage should be limited to 6.2V or
less. Higher output voltages can be accommodated by
inserting a 1N5818 diode in series with the SW2 pin
(anode connected to SW2).
VIN − VSW − VOUT
L
t ON.
(15)
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. D1 MUST BE A SCHOTTKY
DIODE. The voltage at SW2 must never be allowed to go
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 LT1173. Output voltage is determined by
 R2 
VOUT =  1 +  1.245 V .
R1

(
)
(16)
The LT1173 can be configured as a positive-to-negative
converter (Figure 7), or a negative-to-positive converter
(Figure 8). In Figure 7, 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 VOUTshould be less than 6.2V. More negative output voltages can be accomodated as in the prior
section.
+VIN
R3
I LIM
V IN
+
SW1
C2
FB
LT1173
L1
SW2
GND
R1
D1
1N5818
+
C1
R2
–VOUT
LT1173 • F07
Figure 7. Positive-to-Negative Converter
In Figure 8, 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.
9
LT1173
U
W
L1
D1
U
UO
APPLICATI
S I FOR ATIO
+VOUT
+
C1
I LIM
+
C2
V IN
SW1
R1
IL
2N3906
LT1173
AO
GND
FB
SW2
ON
SWITCH
OFF
R2
VOUT =
LT1173 • TA14
( )
R1 1.245V + 0.6V
R2
–VIN
Figure 9. No Current Limit Causes Large Inductor
Current Build-Up
LT1173 • TA13
Figure 8. Negative-to-Positive Converter
PROGRAMMED CURRENT LIMIT
Using the ILIM Pin
The LT1173 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
LT1173 must operate at an 800mA peak switch current
with a 2.0V 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 selected (see the “Maximum Switch Current vs RLIM” characteristic), the switch
current will be limited to 800mA, even if the input voltage
increases.
Another situation where the ILIM feature is useful occurs
when the device goes into continuous mode operation.
This occurs in step-up mode when
VOUT + VDIODE
1
<
.
VIN − VSW
1 − DC
(17)
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 non-zero current level in the
inductor just prior to switch turn on. As shown in Figure
9, 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 ILIM feature,
however, the switch current turns off at a programmed
level as shown in Figure 10, keeping output ripple to a
minimum.
10
IL
SWITCH
ON
OFF
LT1173 • TA15
Figure 10. Current Limit Keeps Inductor Current Under Control
Figure 11 details current limit circuitry. Sense transistor
Q1, whose base and emitter are paralleled with power
switch Q2, is ratioed such that approximately 0.5% of Q2’s
collector current flows in Q1’s collector. This current is
passed through internal 80Ω resistor R1 and out through
the ILIM pin. The value of the external resistor connected
between ILIM and VIN sets the current limit. When sufficient switch current flows to develop a VBE across R1 +
RLIM, Q3 turns on and injects current into the oscillator,
turning off the switch. Delay through this circuitry is
approximately 2µs. The current trip point becomes less
accurate for switch ON times less than 4µs. Resistor
values programming switch ON time for 2µs or less will
cause spurious response in the switch circuitry although
the device will still maintain output regulation.
RLIM
(EXTERNAL)
VIN
ILIM
R1
80Ω
(INTERNAL)
Q3
SW1
DRIVER
OSCILLATOR
Q1
Q2
SW2
LT1173 • TA28
Figure 11. LT1173 Current Limit Circuitry
LT1173
W
U
U
UO
APPLICATI
S I FOR ATIO
Using the Gain Block
+5V
V IN
The gain block (GB) on the LT1173 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 negative input of the gain
block is tied internally to the 1.245V reference. The positive input comes out on the SET pin.
LT1173
100k
R1
VBAT
1.245V
REF
–
SET
+
AO
R2
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. R3 can be added to introduce a small amount of
hysteresis. This will cause the gain block to “snap” when
the trip point is reached. Values in the 1M-10M range are
optimal. The addition of R3 will change the trip point,
however.
TO
PROCESSOR
GND
R3
V – 1.245V
R1 = LB
11.7µA
VLB = BATTERY TRIP POINT
R2 = 100kΩ
R3 = 4.7MΩ
LT1173 • TA16
Figure 12. Setting Low Battery Detector Trip Point
Table 1. Component Selection for Common Converters
INPUT
VOLTAGE
OUTPUT
VOLTAGE
OUTPUT
CURRENT (MIN)
CIRCUIT
FIGURE
INDUCTOR
VALUE
INDUCTOR
PART NUMBER
CAPACITOR
VALUE
NOTES
2.0-3.1
5
90mA
5
47µH
G GA10-472K, C CTX50-1
100µF
*
2.0-3.1
5
10mA
5
220µH
G GA10-223K, C CTX
22µF
2.0-3.1
12
50mA
5
47µH
G GA10-472K, C CTX50-1
47µF
2.0-3.1
12
10mA
5
150µH
G GA10-153K
22µF
5
12
90mA
5
120µH
G GA10-123K
100µF
5
12
30mA
5
150µH
G GA10-153K
47µF
5
15
50mA
5
120µH
G GA10-123K C CTX100-4
47µF
*
**
5
30
25mA
5
100µH
G GA10-103K, C CTX100-4
10µF, 50V
6.5-9.5
5
50mA
6
47µH
G GA10-472K, C CTX50-1
100µF
**
12-20
5
300mA
6
220µH
G GA20-223K
220µF
**
20-30
5
300mA
6
470µH
G GA20-473K
470µF
**
5
–5
75mA
7
100µH
G GA10-103K, C CTX100-4
100µF
**
12
–5
250mA
7
470µH
G GA40-473K
220µF
**
–5
5
150mA
8
100µH
G GA10-103K, C CTX100-4
220µF
–5
12
75mA
8
100µH
G GA10-103K, C CTX100-4
47µF
G = Gowanda
C = Coiltronics
* Add 68Ω from ILIM to VIN
** Add 100Ω from ILIM to VIN
11
LT1173
W
U
U
UO
APPLICATI
S I FOR ATIO
Table 2. Inductor Manufacturers
Table 3. Capacitor Manufacturers
MANUFACTURER
PART NUMBERS
MANUFACTURER
PART NUMBERS
Gowanda Electronics Corporation
1 Industrial Place
Gowanda, NY 14070
716-532-2234
GA10 Series
GA40 Series
Sanyo Video Components
2001 Sanyo Avenue
San Diego, CA 92173
619-661-6835
OS-CON Series
Caddell-Burns
258 East Second Street
Mineola, NY 11501
516-746-2310
7300 Series
6860 Series
Nichicon America Corporation
927 East State Parkway
Schaumberg, IL 60173
708-843-7500
PL Series
Coiltronics International
984 S.W. 13th Court
Pompano Beach, FL 33069
305-781-8900
Custom Toroids
Surface Mount
Sprague Electric Company
Lower Main Street
Sanford, ME 04073
207-324-4140
150D Solid Tantalums
550D Tantalex
Renco Electronics Incorporated
60 Jefryn Boulevard, East
Deer Park, NY 11729
800-645-5828
RL1283
RL1284
UO
TYPICAL APPLICATI
S
3V to –22V LCD Bias Generator
L1*
100µH
1N4148
R1
100Ω
2.21M
1%
ILIM
V IN
SW1
2 X 1.5V
CELLS
3V
LT1173
FB
GND
SW2
+
4.7µF
0.1µF
118k
1%
1N5818
1N5818
+
22µF
* L1 = GOWANDA GA10-103K
COILTRONICS CTX100-4
FOR 5V INPUT CHANGE R1 TO 47Ω.
CONVERTER WILL DELIVER –22V AT 40mA.
220k
–22V OUTPUT
7mA AT 2.0V INPUT
70% EFFICIENCY
LT1173 • TA19
12
LT1173
UO
TYPICAL APPLICATI
S
3V to 5V Step-Up Converter
9V to 5V Step-Down Converter
L1*
100 µ H
100 Ω
V IN
ILIM
ILIM
V IN
9V
BATTERY
SW1
2 X 1.5V
CELLS
LT1173-5
1N5818
LT1173-5
SW2
SENSE
5V OUTPUT
150mA AT 3V INPUT
60mA AT 2V INPUT
SENSE
GND
SW1
+
GND
SW2
L1*
47µH
100 µ F
+
1N5818
* L1 = GOWANDA GA10-103K
COILTRONICS CTX100-1 (SURFACE MOUNT)
5V OUTPUT
150mA AT 9V INPUT
50mA AT 6.5V INPUT
100 µ F
* L1 = GOWANDA GA10-472K
COILTRONICS CTX50-1
FOR HIGHER OUTPUT CURRENTS SEE LT1073 DATASHEET
LT1173 • TA17
+5V to –5V Converter
LT1173 • TA18
+20V to 5V Step-Down Converter
+VIN
5V INPUT
+VIN
12V-28V
100 Ω
100 Ω
ILIM
V IN
ILIM
V IN
SW1
+
22µF
SW1
LT1173-5
LT1173-5
SENSE
GND
SENSE
SW2
GND
L1*
100µH
100 µ F
1N5818
SW2
L1*
220µH
+
+
1N5818
5V OUTPUT
300mA
100 µ F
–5V OUTPUT
75mA
* L1 = GOWANDA GA10-103K
COILTRONICS CTX100-1
* L1 = GOWANDA GA20-223K
LT1173 • TA20
LT1173 • TA21
Telecom Supply
L1*
500µH
44mH
~
+
48V DC
44mH
~
+
47µF
100V
–
MUR110
220µF
10V
3.6MΩ
+
+5V
100mA
390kΩ
10k
VN2222
12V
10nF
*L1 = CTX110077
IQ = 120µA
2N5400
IRF530
100Ω
1N4148
15V
ILIM
+
1N965B
V IN
SW1
10µF
16V
LT1173
FB
GND
SW2
110kΩ
LT1173 • TA22
13
LT1173
UO
TYPICAL APPLICATI
S
“5 to 5” Step-Up or Step-Down Converter
L1*
100µH
1N5818
SI9405DY
+5V
OUTPUT
56Ω
1
2
V IN
ILIM
4 X NICAD
OR
ALKALINE
CELLS
+
470µF
470k
SW1
7
SET LT1173
AO
FB
GND
5
75k
3
+
6
470µF
8
+
SW2
4
240Ω
470µF
24k
*L1 = COILTRONICS CTX100-4
GOWANDA GA20-103K
VIN = 2.6V TO 7.2V
VOUT = 5V AT 100mA
LT1173 • TA23
2V to 5V at 300mA Step-Up Converter with Under Voltage Lockout
L1*
20µH, 5A
47k
1N5820
100k
220
ILIM
100k
2N3906
100
SW1
2N4403
LT1173
2.2M
2 X NICAD
V IN
AO
+5V OUTPUT
300mA
LOCKOUT AT
1.85V INPUT
301k†
SET
GND
FB
SW2
5Ω
+
MJE200
100k
*L1 = COILTRONICS CTX-20-5-52
†1% METAL FILM
14
100k†
100µF
OS-CON
47Ω
LT1173 • TA24
LT1173
UO
TYPICAL APPLICATI
S
Voltage Controlled Positive-to-Negative Converter
0.22
VIN
5V-12V
L1*
50µH, 2.5A
MJE210
+
1N5818
V IN
220
–VOUT = –5.13 • VC
2W MAXIMUM OUTPUT
ILIM
150
V IN
SW1
200k
39k
–
LT1173
VC (0V TO 5V)
LT1006
FB
GND
100µF
1N5820
SW2
+
* L1 = GOWANDA GT10-101
LT1173 • TA25
High Power, Low Quiescent Current Step-Down Converter
0.22Ω
VIN
7V-24V
L1*
25µH, 2A
MTM20P08
18V
1W
1N5818
2k
51Ω
1N5820
5V
500mA
+
470µF
2N3904
V IN
100Ω
1/2W
ILIM
SW1
1N4148
LT1173
121k
FB
GND
SW2
40.2k
* L1 = GOWANDA GT10-100
EFFICIENCY ≥ 80% FOR 10mA ≤ ILOAD ≤ 500mA
STANDBY IQ ≤ 150µA
OPERATE STANDBY
LT1173 • TA26
2 Cell Powered Neon Light Flasher
0.02µF
L1*
470µH
ILIM
1N4148
1N4148
95V REGULATED
V IN
SW1
3V
1N4148
0.02µF
0.02µF
LT1173
100M
FB
GND
SW2
1.3M
3.3M
0.68µF
200V
*TOKO 262LYF-0100K
NE-2
BLINKS AT
0.5Hz
LT1173 • TA27
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
LT1173
U
PACKAGE DESCRIPTIO
Dimensions in inches (milimeters) unless otherwise noted.
N8 Package
8-Lead Plastic DIP
0.400*
(10.160)
MAX
8
7
6
5
1
2
3
4
0.255 ± 0.015*
(6.477 ± 0.381)
0.300 – 0.325
(7.620 – 8.255)
0.065
(1.651)
TYP
0.009 – 0.015
(0.229 – 0.381)
(
+0.025
0.325 –0.015
8.255
+0.635
–0.381
0.130 ± 0.005
(3.302 ± 0.127)
0.045 – 0.065
(1.143 – 1.651)
0.125
(3.175)
MIN
0.045 ± 0.015
(1.143 ± 0.381)
)
0.018 ± 0.003
(0.457 ± 0.076)
0.100 ± 0.010
(2.540 ± 0.254)
0.015
(0.380)
MIN
N8 0694
*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTURSIONS SHALL NOT EXCEED 0.010 INCH (0.254mm).
S8 Package
8-Lead Plastic SOIC
0.189 – 0.197*
(4.801 – 5.004)
8
7
6
5
0.150 – 0.157*
(3.810 – 3.988)
0.228 – 0.244
(5.791 – 6.197)
1
0.010 – 0.020
× 45°
(0.254 – 0.508)
0.008 – 0.010
(0.203 – 0.254)
0.053 – 0.069
(1.346 – 1.752)
0°– 8° TYP
0.016 – 0.050
0.406 – 1.270
0.014 – 0.019
(0.355 – 0.483)
2
3
4
0.004 – 0.010
(0.101 – 0.254)
0.050
(1.270)
BSC
SO8 0294
*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.006 INCH (0.15mm).
16
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7487
(408) 432-1900 ● FAX: (408) 434-0507 ● TELEX: 499-3977
LT/GP 0894 2K REV B • PRINTED IN USA
 LINEAR TECHNOLOGY CORPORATION 1994