LT1103/LT1105
Offline Switching Regulator
FEATURES
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DESCRIPTION
The LT
®
1103 Offline Switching Regulator is designed for
high input voltage applications using an external FET
switch whose source is driven by the open collector output
of the LT1103. The LT1103 is optimized for 15W to 100W
applications. For higher power applications or additional
switch current flexibility, the LT1105 is available and its
totem pole output drives the gate of an external FET.
Unique design of the LT1103/LT1105 eliminates the need
for an optocoupler while still providing
±1%
load and line
regulation in a magnetic flux-sensed converter. This sig-
nificantly simplifies the design of offline power supplies
and reduces the number of components which must cross
the isolation barrier to one, the transformer.
The LT1103/LT1105 current mode switching techniques
are well suited to transformer isolated flyback and forward
topologies while providing ease of frequency compensa-
tion with a minimum of external components. Low exter-
nal part count for a typical application combines with a
, LTC and LT are registered trademarks of Linear Technology Corporation.
±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
+
+
–
1000pF 100Ω
BRIDGE
RECTIFIER
+
LINE
FILTER
220µF
385V
499Ω
220k
1W
R!!
NGE AGE!!
DA LT
VO
HIGH
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
BAV21
BAV21
1N4148
39µF
35V
18.7k
FB
GND
4.75k
1%
0.047µF
330Ω
0.047µF
0.1µF
LT1103 TA13
V
IN
V
SW
LT1103
BUK426-800A
13k
1%
+
10Ω
15V
V
C
OSC
390pF
+
1µF
25V
TRANSFORMER DATA:
COILTRONICS CTX110228-3
L
(PRI)
= 1.6mH
N
PRI
:N
SEC
= 1:0.05
N
BIAS
:N
SEC
= 1:0.27
V
OUT
(V)
Danger!! Lethal Voltages Present – See Text
U
U
U
+
50V
470µF
Load Regulation
5.10
5.05
5.00
4.95
4.90
4.85
4.80
4.75
0
1
2
3
4
5
I
OUT
(A)
LT1103 TA02
220V
AC
270V
AC
110V
AC
85V
AC
6
7
8
9
10
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 maintaining
high efficiency. The external FET is protected from insuf-
ficient or excessive gate drive voltage with a drive detec-
tion 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.
WAR I G!
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
0VLO Input Current ............................................... 1mA
Lead Temperature (Soldering, 10 sec.)................ 300°C
Maximum Operating Ambient Temperature Range
LT1103C .............................................. 0°C to 70°C
LT1105C .............................................. 0°C to 70°C
Maximum Operating Junction Temperature Range
LT1103C ............................................. 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
2
UU
U
W W
U
W
LT1103/LT1105
PACKAGE/ORDER INFORMATION
ORDER PART
NUMBER
TOP VIEW
PWRGND
OVLO
FB
V
C
5V
SS
GND
1
2
3
4
5
6
7
14 V
SW
13 NC
12 NC
11 15V
10 V
IN
9
8
OSC
I
LIM
FRONT VIEW
7
6
5
4
3
2
1
15V
V
IN
OSC
GND
V
C
FB
V
SW
TOP VIEW
GND
I
LIM
1
2
3
4
N8 PACKAGE
8-LEAD PDIP
8
7
6
5
V
SW
15V
V
IN
OSC
LT1105CN
LT1105IN
N PACKAGE
14-LEAD PDIP
PINS 1 AND 7 MUST BE TIED TOGETHER
T
JMAX
= 100°C,
θ
JA
= 100°C/W
Consult factory for Military grade parts.
ELECTRICAL CHARACTERISTICS
SYMBOL
I
Q
I
START
PARAMETER
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
V
GATE
15V Gate Bias Reference
15V Dropout Voltage
15V Short-Circuit Current
SF
Oscillator Scaling Factor
Oscillator Valley Voltage
Oscillator Peak Voltage
CONDITIONS
V
IN
= 20V, V
C
= 0.85V, OVLO = 0V, V
SW
Open, T
A
=
25°C, unless otherwise noted.
MIN
q
q
q
q
8V < V
IN
< 30V, After Device Has Started
V
IN
< V
IN
Start Threshold
Industrial Grade
Note: Switching Stops When V
SW
< 10V (LT1103)
Note: Switching Stops When V
GATE
< 10V (LT1105)
10V < V
IN
< 30V
0mA < I
L
< 20mA
Commercial Grade
Industrial Grade
Commercial Grade
Industrial Grade
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
U
W
U
ORDER PART
NUMBER
LT1105CN8
LT1105IN8
FB
V
C
T
JMAX
= 100°C,
θ
JA
= 130°C/W
ORDER PART
NUMBER
LT1103CT7
T7 PACKAGE
7-LEAD TO-220
CASE IS CONNECTED TO GROUND. LEADS ARE FORMED
T
JMAX
= 100°C,
θ
JA
= 50°C/W
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
V
V
mA
Hz •
µF
Hz •
µF
V
V
10
14.5
5.0
4.80
16.0
7.0
4.95
0.025
0.025
q
q
q
q
q
q
q
q
q
q
q
q
25
20
30
25
13.8
30
36
32
60
15.0
2.0
70
40
40
2.0
4.5
16.2
2.5
130
44
48
3
LT1103/LT1105
V
IN
= 20V, V
C
= 0.85V, OVLO = 0V, V
SW
Open, T
A
=
25°C, unless otherwise noted.
SYMBOL
DC
PARAMETER
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
gm
Error Amp Transconductance
CONDITONS
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
q
q
q
q
q
q
q
ELECTRICAL CHARACTERISTICS
MIN
58
56
55
2.3
2.2
4.425
4.400
5
4
8
7
6
TYP
65
63
MAX
72
70
75
UNIT
%
%
%
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
2.7
2.8
3.0
4.575
4.600
20
22
13
14
15
0.10
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
Industrial Grade
q
q
q
q
q
q
q
q
q
q
q
q
q
9000 12000 17500
6000 12000 20000
5000
24000
500
450
0.85
50
50
150
1.5
0.7
0.3
4.2
25
20
1
1250
1.25
150
275
3
0.7
4.4
40
2
500
200
50
2.0
70
2.5
0.4
30
13.00
12.50
12.75
12.25
13.5
13.2
3.0
0.75
50
4.5
4.5
0.9
4.6
60
75
1.4
250
300
A
V
Error Amp Voltage Gain
V
C
Switching Threshold
Shutdown Threshold Voltage
1V < V
C
< 3V
Industrial Grade
Switch Duty Cycle = 0%
Industrial Grade
V
mV
mV
µA
mA
mA
V
V
µA
µA
mA
µA
µA
V
A
Ω
mA/A
V
V
V
V
Error Amp Source Current
Error Amp Sink Current
Industrial Grade
Error Amp Clamp Voltage
Soft-Start Charging Current
Soft-Start Reset Current
Output Switch Leakage
(LT1103)
BV
Switch Breakdown Voltage
(LT1103)
V
SW
Current Limit (LT1103)
Output Switch On Resistance
(LT1103)
∆I
IN
∆I
SW
I
Q
Increase During Switch On Time
(LT1103)
Switch Output High Level
(LT1105)
Switch Output High Level
Industrial Grade
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
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)
q
q
q
q
q
q
q
q
q
q
q
q
q
q
q
q
q
4
LT1103/LT1105
ELECTRICAL CHARACTERISTICS
V
IN
= 20V, V
C
= 0.85V, OVLO = 0V, V
SW
Open, T
A
=
25°C, unless otherwise noted.
SYMBOL
PARAMETER
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
CONDITONS
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)
q
q
q
q
q
MIN
TYP
0.25
0.75
50
20
MAX
0.50
1.50
UNIT
V
V
ns
ns
300
9.0
17.0
375
9.5
18.5
450
10.5
20.0
mV
V
V
The
q
denotes specifications which apply over the full operating
temperature range.
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
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
500
QUIESCENT SUPPLY CURRENT (mA)
START-UP SUPPLY CURRENT (µA)
20
SUPPLY CURRENT (mA)
15
10
5
I
SHUT
0
I
START
0
5
10
15
20
INPUT VOLTAGE (V)
25
30
U W
LT1103 G01
Start-Up Supply Current vs
Input Voltage
22
21
20
19
18
17
16
15
14
13
12
0
3
9
6
INPUT VOLTAGE (V)
12
15
LT1103 G02
Quiescent Supply Current vs
Input Voltage
450
400
350
300
250
200
150
100
50
0
–55°C
125°C
25°C
125°C
25°C
–55°C
0
5
10
15 20 25 30
INPUT VOLTAGE (V)
35
40
LT1103 G03
5
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