LT1103/LT1105
Offline Switching Regulator
THE LT1103 IS OBSOLETE:
FOR INFORMATION PURPOSES ONLY
Contact Analog Devices for Potential Replacement
THE LT1105 IS AVAILABLE, BUT NOT
RECOMMENDED FOR NEW DESIGNS
FEATURES
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DESCRIPTION
The
LT
®
1105
Offline Switching Regulator is designed
for high input voltage applications using an external FET
switch. The LT1105 is available and its totem pole out-
put drives the gate of an external FET. Unique design of
the LT1105 eliminates the need for an optocoupler while
still providing ±1% load and line regulation in a magnetic
flux-sensed converter. This significantly simplifies the
design of offline power supplies and reduces the number
of components which must cross the isolation barrier to
one, the transformer.
The LT1105 current mode switching techniques are well
suited to transformer isolated flyback and forward topolo-
gies while providing ease of frequency compensation with
a minimum of external components.
All registered trademarks and trademarks are the property of their respective owners.
±1% Line and Load Regulation with No Optocoupler
Switch Frequency Up to 200kHz
Internal 2A Switch and Current Sense (LT1103)
Internal 1A Totem-Pole Driver (LT1105)
Start-Up Mode Draws Only 200µA
Fully Protected Against Overloads
Overvoltage Lockout of Main Supply
Protected Against Underdrive or Overdrive to FET
Operates in Continuous or Discontinuous Mode
Ideal for Flyback and Forward Topologies
Isolated Flyback Mode Has Fully Floating Outputs
APPLICATIONS
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Up to 250W Isolated Mains Converter
Up to 50W Isolated Telecom Converter
Fully Isolated Multiple Outputs
Distributed Power Conversion Networks
TYPICAL APPLICATION
Fully Isolated Flyback 100kHz 50W Converter with Load Regulation Compensation
OPTIONAL OUTPUT FILTER
85V
AC
TO 270V
AC
MBR2045
10µH
5V
10A
50V
470µF
+
+
220µF
385V
220k
1W
499
1000pF 100
–
BRIDGE
RECTIFIER
+
LINE
FILTER
R!!
NGE AGE!!
DA LT
VO
IGH
H
1N4148
1.5KE300A
5W
MUR150
+
+
*50V
3600µF
*OUTPUT CAPACITOR IS THREE 1200µF,
50V CAPACITORS IN PARALLEL TO
ACHIEVE REQUIRED RIPPLE CURRENT
RATING AND LOW ESR.
WINDINGS FOR
OPTIONAL
±12V
DC
OUTPUTS
5.25
5.20
5.15
10
15V
TRANSFORMER DATA:
COILTRONICS CTX110228-3
L
(PRI)
= 1.6mH
N
PRI
:N
SEC
= 1:0.05
N
BIAS
:N
SEC
= 1:0.27
5.10
V
OUT
(V)
5.05
5.00
4.95
4.90
4.85
4.80
LT1103 TA13
Load Regulation
BAV21
BAV21
13k
1%
1N4148
39µF
35V
18.7k
FB
GND
4.75k
1%
0.047µF
330
0.047µF
0.1µF
V
C
V
IN
V
SW
LT1103
BUK426-800A
+
220V
AC
OSC
390pF
+
1µF
25V
270V
AC
110V
AC
85V
AC
4.75
0
1
2
3
4
5
I
OUT
(A)
6
7
8
9
10
Danger!! Lethal Voltages Present – See Text
LT1103 TA02
Rev. E
Document Feedback
For more information
www.analog.com
1
LT1103/LT1105
DESCRIPTION
200kHz maximum switching frequency to achieve high
power density. Performance at switching frequencies
above 100kHz may be degraded due to internal timing
constraints associated with fully isolated flyback mode.
Included are the oscillator, control, and protection cir-
cuitry such as current limit and overvoltage lockout.
Switch frequency and maximum duty cycle are adjustable.
Bootstrap circuitry draws 200µA for start-up of isolated
topologies. A 5V reference as well as a 15V gate bias
are available to power external primary-side circuitry. No
external current sense resistor is necessary with LT1103
because it is integrated with the high current switch. The
LT1105 brings out the input to the current limit amplifier
and requires the use of an external sense resistor.
The LT1103/LT1105 have unique features not found on
other offline switching regulators. Adaptive antisat switch
drive allows wide ranging load currents while maintain-
ing high efficiency. The external FET is protected from
insufficient or excessive gate drive voltage with a drive
detection circuit. An externally activated shutdown mode
reduces total supply current to less than 200µA, typical
for standby operation. Fully isolated and regulated outputs
can be generated in the optional isolated flyback mode
without the need for optocouplers or other isolated feed-
back paths.
WARNING
DANGEROUS AND LETHAL POTENTIALS ARE
PRESENT IN OFFLINE CIRCUITS!
BEFORE PROCEEDING ANY FURTHER, THE
READER IS WARNED THAT CAUTION MUST
BE USED IN THE CONSTRUCTION, TESTING
AND USE OF OFFLINE CIRCUITS. HIGH
VOLTAGE, AC LINE-CONNECTED POTENTIALS
ARE PRESENT IN THESE CIRCUITS. EXTREME
CAUTION MUST BE USED IN WORKING WITH
AND MAKING CONNECTIONS TO THESE
CIRCUITS. REPEAT: OFFLINE CIRCUITS
CONTAIN DANGEROUS, AC LINE-CONNECTED
HIGH VOLTAGE POTENTIALS. USE CAUTION.
ALL TESTING PERFORMED ON AN OFFLINE
CIRCUIT MUST BE DONE WITH AN ISOLATION
TRANSFORMER CONNECTED BETWEEN THE
OFFLINE CIRCUIT’S INPUT AND THE AC LINE.
USERS AND CONSTRUCTORS OF OFFLINE
CIRCUITS MUST OBSERVE THIS PRECAUTION
WHEN CONNECTING TEST EQUIPMENT TO
THE CIRCUIT TO AVOID ELECTRIC SHOCK.
REPEAT: AN ISOLATION TRANSFORMER MUST
BE CONNECTED BETWEEN THE CIRCUIT INPUT
AND THE AC LINE IF ANY TEST EQUIPMENT IS
TO BE CONNECTED.
ABSOLUTE MAXIMUM RATINGS
(Note 1)
V
IN
............................................................................30V
V
SW
Output Voltage (LT1103)....................................50V
V
SW
Output Current (200ns)(LT1105) .................... ±1.5A
V
C
, FB, OSC, SS ..........................................................6V
I
LIM
(LT1105)...............................................................3V
OVLO Input Current..................................................1mA
Lead Temperature (Soldering, 10 sec.).................. 300°C
Maximum Operating Ambient Temperature Range
LT1103C
(OBSOLETE)
.............................. 0°C to 70°C
LT1105C ................................................... 0°C to 70°C
Maximum Operating Temperature Range
LT1103C
(OBSOLETE)
............................ 0°C to 100°C
LT1105C ................................................. 0°C to 100°C
LT1105I .............................................. –40°C to 125°C
Storage Temperature Range .................. –65°C to 150°C
Rev. E
2
For more information
www.analog.com
LT1103/LT1105
PIN CONFIGURATION
LT1105
PWRGND
OVLO
FB
V
C
5V
SS
GND
1
2
3
4
5
6
7
TOP VIEW
14 V
SW
13 NC
12 NC
11 15V
10 V
IN
9
8
OSC
I
LIM
GND
I
LIM
FB
V
C
1
2
3
4
LT1105
LT1103
FRONT VIEW
TOP VIEW
8
7
6
5
V
SW
15V
V
IN
OSC
7
6
5
4
3
2
1
15V
V
IN
OSC
GND
V
C
FB
V
SW
N PACKAGE
14-LEAD PDIP
PINS 1 AND 7 MUST BE TIED TOGETHER
T
JMAX
= 100°C,
JA
= 100°C/W
N8 PACKAGE
8-LEAD PDIP
T
JMAX
= 100°C,
JA
= 130°C/W
T7 PACKAGE
7-LEAD TO-220
CASE IS CONNECTED TO GROUND. LEADS ARE FORMED
T
JMAX
= 100°C,
JA
= 50°C/W
OBSOLETE PACKAGE
ORDER INFORMATION
LEAD FREE FINISH
LT1105CN#PBF
LT1105IN#PBF
LT1105CN8#PBF
LT1105IN8#PBF
LT1103CT7#PBF
TAPE AND REEL
LT1105CN#TRPBF
LT1105IN#TRPBF
LT1105CN8#TRPBF
LT1105IN8#TRPBF
LT1103CT7#TRPBF
PART MARKING
LT1105CN
LT1105IN
LT1105CN8
LT1105IN8
LT1103CT7
PACKAGE DESCRIPTION
14-Lead PDIP
14-Lead PDIP
8-Lead PDIP
8-Lead PDIP
7-Lead TO-220
TEMPERATURE RANGE
0°C to 100°C
–40°C to 125°C
0°C to 100°C
–40°C to 125°C
0°C to 100°C
OBSOLETE PACKAGE
Contact the factory for parts specified with wider operating temperature ranges.
Tape and reel specifications.
Some packages are available in 500 unit reels through designated sales channels with #TRMPBF suffix.
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 20V, V
C
= 0.85V, OVLO = 0V, V
SW
Open, unless otherwise noted.
SYMBOL PARAMETER
I
Q
I
START
Supply Current
Start-Up Current
V
IN
Start Threshold
V
IN
Shutdown Threshold
V
REF
5V Reference Voltage
V
REF
Line Regulation
V
REF
Load Regulation
V
REF
Short-Circuit Current
15V Short-Circuit Current
10V < V
IN
< 30V
0mA < I
L
< 20mA
Commercial Grade
Industrial Grade
Commercial Grade
Industrial Grade
Note: Switching Stops When V
SW
< 10V (LT1103)
Note: Switching Stops When V
GATE
< 10V (LT1105)
CONDITIONS
8V < V
IN
< 30V, After Device Has Started
V
IN
< V
IN
Start Threshold
Industrial Grade
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ELECTRICAL CHARACTERISTICS
MIN
10
TYP
20
200
MAX
30
400
450
17.5
8.0
5.20
0.1
0.05
110
120
130
140
UNITS
mA
µA
µA
V
V
V
%V
%mA
mA
mA
mA
mA
14.5
5.0
4.80
16.0
7.0
4.95
0.025
0.025
25
20
30
25
60
Rev. E
For more information
www.analog.com
3
LT1103/LT1105
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 20V, V
C
= 0.85V, OVLO = 0V, V
SW
Open, unless otherwise noted.
SYMBOL PARAMETER
V
GATE
15V Gate Bias Reference
15V Dropout Voltage
15V Short-Circuit Current
SF
Oscillator Scaling Factor
Oscillator Valley Voltage
Oscillator Peak Voltage
DC
Preset Max Switch Duty Cycle
(LT1103)
Preset Max Switch Duty Cycle
(LT1105)
OVLO Threshold
OVLO Input Bias Current
V
FB
FB Threshold Voltage
FB Input Bias Current
Change in FB Input
Bias Current with Change in V
C
FB Threshold Line Regulation
g
m
A
V
Error Amp Transconductance
FB = 4V, V
C
= Open, f
OSC
= 40kHz, I
SW
= 25mA,
Note: Maximum Duty Cycle Can Be Altered at OSC Pin
FB = 4V, V
C
= Open, f
OSC
= 40kHz, I
SW
= 25mA,
Note: Maximum Duty Cycle Can Be Altered at OSC Pin
Industrial Grade
Overvoltage Lockout Threshold at Which Switching is Inhibited
Industrial Grade
OVLO = 2V, Measured Out of Pin (Note 2)
I(V
C
) = 0mA
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ELECTRICAL CHARACTERISTICS
CONDITIONS
MIN
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TYP
15.0
2.0
70
40
40
2.0
4.5
MAX
16.2
2.5
130
44
48
UNITS
V
V
mA
Hz • µF
Hz • µF
V
V
17 < V
IN
< 30V, 0mA < I
L
< 30mA
V
IN
= 15V, I
L
= 30mA
FB = 4V, V
C
= Open, Measured at V
SW
, I
SW
= 25mA,
OVLO = 5V, f
OSC
= SF/C
OSC
, 40kHz < f
OSC
< 200kHz
13.8
30
36
32
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56
55
2.3
2.2
4.425
4.400
5
4
8
7
6
9000
6000
5000
500
450
0.85
50
50
150
1.5
0.7
0.3
4.2
25
20
1
65
63
72
70
75
%
%
%
V
V
µA
V
V
µA
µA
µA/V
µA/V
µA/V
%/V
µmho
µmho
µmho
V/V
V/V
2.5
1.0
4.50
4.50
10
11
11
0.025
12000
12000
1250
1.25
150
275
3
0.7
4.4
40
2
2.7
2.8
3.0
4.575
4.600
20
22
13
14
15
0.10
17500
20000
24000
FB = V
FB
(Note 3)
Industrial Grade
FB = V
FB
, V
C
= 1V to 4V (Note 3)
Industrial Grade
10V < V
IN
< 30V
∆I(V
C
) = ±50µA
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Error Amp Voltage Gain
V
C
Switching Threshold
Shutdown Threshold Voltage
Error Amp Source Current
Error Amp Sink Current
Error Amp Clamp Voltage
Soft-Start Charging Current
Soft-Start Reset Current
Output Switch Leakage
(LT1103)
1V < V
C
< 3V
Industrial Grade
Switch Duty Cycle = 0%
Industrial Grade
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1.4
250
300
4.5
4.5
0.9
4.6
60
75
500
200
mA
mV
mV
µA
mA
mA
V
V
µA
µA
mA
µA
µA
V
Industrial Grade
FB = 4.75V
FB = 4.0V
SS = 0V
Industrial Grade
V
IN
= 6V, SS = 0.3V
V
SW
= 45V
V
SW
= 15V
I
SW
= 5mA
Duty Cycle = 25% (Note 4)
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BV
Switch Breakdown Voltage
(LT1103)
V
SW
Current Limit (LT1103)
Output Switch On Resistance
(LT1103)
50
2.0
70
2.5
0.4
3.0
0.75
A
Rev. E
4
For more information
www.analog.com
LT1103/LT1105
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 20V, V
C
= 0.85V, OVLO = 0V, V
SW
Open, unless otherwise noted.
SYMBOL PARAMETER
∆I
IN
∆I
SW
I
Q
Increase During Switch On Time
(LT1103)
Switch Output High Level
(LT1105)
Switch Output High Level
Industrial Grade
Switch Output Low Level
(LT1105)
Rise Time (LT1105)
Fall Time (LT1105)
I
LIM
Threshold Voltage (LT1105)
Low Switch Drive Lockout
Threshold
High Switch Drive Lockout
Threshold
CONDITIONS
I
SW
= 0.5A to 1.5A
I
SW
= 200mA, V
GATE
= 15V
I
SW
= 750mA, V
GATE
= 15V
I
SW
= 200mA, V
GATE
= 15V
I
SW
= 750mA, V
GATE
= 15V
I
SW
= 200mA
I
SW
= 750mA
C
L
= 1000pF
C
L
= 1000pF
Duty Cycle = 25% (Note 5)
Measured at V
SW
(LT1103)
Measured at 15V Gate Bias Reference (LT1105)
Measured at V
SW
(LT1103)
Measured at 15V Gate Bias Reference (LT1105)
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ELECTRICAL CHARACTERISTICS
MIN
TYP
30
MAX
50
UNITS
mA/A
V
V
V
V
13.00
12.50
12.75
12.25
13.5
13.2
0.25
0.75
50
20
300
9.0
17.0
375
9.5
18.5
0.50
1.50
V
V
ns
ns
450
10.5
20.0
mV
V
V
Note 1:
Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2:
The OVLO pin is clamped with a 5.5V Zener and can sink a
maximum input current of 1mA.
Note 3:
FB input bias current changes as a function of the V
C
pin voltage.
Rate of change of FB input bias current is 11µA/V of change on V
C
. By
including a resistor in series with the FB pin, load regulation can be set to
zero.
Note 4:
Current limit on V
SW
is constant for DC < 35% and decreases for
DC > 35% due to internal slope compensation circuity. The LT1103 switch
current limit is given by I
LIM
= 1.76 (1.536 – DC) above 35% duty cycle.
Note 5:
The current limit threshold voltage is constant for DC < 35% and
decreases for DC > 35% due to internal slope compensation circuitry. The
LT1105 switch current limit threshold voltage is given by V
LIM
= 0.225
(1.7 – DC) above 35% duty cycle.
TYPICAL PERFORMANCE CHARACTERISTICS
Supply Current vs Input Voltage
25
25°C
START-UP SUPPLY CURRENT (µA)
500
QUIESCENT SUPPLY CURRENT (mA)
450
400
350
300
250
200
150
100
50
0
0
3
9
6
INPUT VOLTAGE (V)
12
15
LT1103 G02
Start-Up Supply Current vs
Input Voltage
22
21
20
19
18
17
16
15
14
13
12
Quiescent Supply Current vs
Input Voltage
SUPPLY CURRENT (mA)
20
125°C
25°C
15
125°C
25°C
10
–55°C
–55°C
5
I
SHUT
0
0
5
I
START
10
15
20
INPUT VOLTAGE (V)
25
30
0
5
10
15 20 25 30
INPUT VOLTAGE (V)
35
40
LT1103 G01
LT1103 G03
Rev. E
For more information
www.analog.com
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