LT1725
General Purpose
Isolated Flyback Controller
FEATURES
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DESCRIPTIO
Drives External Power MOSFET with External
I
SENSE
Resistor
Application Input Voltage Limited Only by
External Power Components
Senses Output Voltage Directly from Primary Side
Winding—No Optoisolator Required
Accurate Regulation Without User Trims
Regulation Maintained Well into Discontinuous Mode
Switching Frequency from 50kHz to 250kHz with
External Capacitor
Optional Load Compensation
Optional Undervoltage Lockout
Available in 16-Pin SO and SSOP Packages
The LT
®
1725 is a monolithic switching regulator control-
ler specifically designed for the isolated flyback topology.
It drives the gate of an external MOSFET and is generally
powered from a third transformer winding. These features
allow for an application input voltage limited only by
external power path components. The third transformer
winding also provides output voltage feedback informa-
tion, such that an optoisolator is not required. Its gate
drive capability coupled with a suitable external MOSFET
can deliver load power up to tens of watts.
The LT1725 has a number of features not found on most
other switching regulator ICs. By utilizing current mode
switching techniques, it provides excellent AC and DC line
regulation. Its unique control circuitry can maintain regu-
lation well into discontinuous mode in most applications.
Optional load compensation circuitry allows for improved
load regulation. An optional undervoltage lockout pin
halts operation when the application input voltage is too
low. An optional external capacitor implements a soft-
start function. A 3V output is available at up to several mA
for powering primary side application circuitry.
APPLICATIO S
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Telecom Isolated Converters
Offline Isolated Power Supplies
Instrumentation Power Supplies
, LTC and LT are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
TYPICAL APPLICATIO
48V to Isolated 5V Converter
CTX02-14989
6
LT1725
35.7k
1%
3V
OUT
FB
3.01k
1%
1nF
47pF
SFST
VC
OSCAP
820k
51k
51k
MENAB
51k
2.7k
R
OCMP
R
CMPC
0.1µF
SGND
GATE
I
SENSE
PGND
0.18Ω
1725 TA01a
BAS16
22Ω
V
IN
36V TO 72V
1
18Ω
470pF
47k
V
CC
15µF
1µF
12CWQ06
V
OUT
(V)
+
68Ω
150pF
1.5µF
2
9
V
OUT
= 5V
I
OUT
= 0 to 2A
51Ω
1W
11
10
t
ON
ENDLY
UVLO
100pF
33k
+
150µF
4.75
0
0.5
1.0
I
LOAD
(A)
1.5
2.0
1725 F10b
4
12
IRF620
U
Output Load Regulation
5.25
V
IN
= 48V
V
IN
= 36V
V
IN
= 72V
5.00
U
U
1725fa
1
LT1725
ABSOLUTE
(Note 1)
AXI U
RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
PGND 1
I
SENSE
2
SFST 3
R
OCMP
4
R
CMPC
5
OSCAP 6
V
C
7
FB 8
16 GATE
15 V
CC
14 t
ON
13 ENDLY
12 MINENAB
11 SGND
10 UVLO
9
3V
OUT
V
CC
Supply Voltage ................................................. 22V
UVLO Pin Voltage .................................................... V
CC
I
SENSE
Pin Voltage .................................................... 2V
FB Pin Current .....................................................
±2mA
Operating Junction
Temperature Range
LT1725C .............................................. 0°C to 100°C
LT1725I ........................................... –40°C TO 125°C
Storage Temperature Range ................ – 65°C to 150°C
Lead Temperature (Soldering, 10 sec)................ 300°C
GN PACKAGE
S PACKAGE
16-LEAD PLASTIC SSOP 16-LEAD PLASTIC SO
T
JMAX
= 125°C,
θ
JA
= 110°C/W (GN)
T
JMAX
= 125°C,
θ
JA
= 100°C/W (SO)
ORDER PART NUMBER
LT1725CGN
LT1725IGN
LT1725CS
LT1725IS
GN PART MARKING
1725
1725I
Order Options
Tape and Reel: Add #TR
Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF
Lead Free Part Marking:
http://www.linear.com/leadfree/
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
SYMBOL
Power Supply
V
CC
V
CC
Turn-On Voltage
V
CC
Turn-Off Voltage
V
CC
Hysteresis (Note 3)
Supply Current
Start-Up Current
Feedback Voltage
PARAMETER
The
●
denotes specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
CC
= 14V, GATE open, V
C
= 1.4V unless otherwise noted.
CONDITIONS
●
●
●
●
●
MIN
14.0
8
4.0
6
TYP
15.1
9.7
5.4
10
120
1.245
500
MAX
16.0
11
6.5
15
280
1.260
1.270
1800
80
0.05
50
UNITS
V
V
V
mA
µA
V
V
nA
µmho
µA
V
%/V
V/V
µA
mA
1725fa
(V
TURN-ON
– V
TURN-OFF
)
V
C
= Open
I
CC
Feedback Amplifier
V
FB
I
FB
g
m
I
SRC
, I
SNK
V
CL
●
1.230
1.220
400
30
Feedback Pin Input Current
Feedback Amplifier Transconductance
Feedback Amplifier Source or Sink Current
Feedback Amplifier Clamp Voltage
Reference Voltage/Current Line Regulation
Voltage Gain
Soft-Start Charging Current
Soft-Start Discharge Current
12V
≤
V
IN
≤
18V
V
C
= 1V to 2V
V
SFST
= 0V
V
SFST
= 1.5V, V
UVLO
= 0V
25
0.8
●
∆l
C
=
±10µA
●
●
1000
50
2.5
0.01
2000
40
1.5
2
U
W
U
U
W W
W
LT1725
ELECTRICAL CHARACTERISTICS
SYMBOL
Gate Output
V
GATE
Output High Level
Output Low Level
I
GATE
t
r
t
f
V
C
V
ISENSE
Output Sink Current in Shutdown, V
UVLO
= 0V
Rise Time
Fall Time
Control Pin Threshold
Switch Current Limit
I
GATE
= 100mA
I
GATE
= 500mA
I
GATE
= 100mA
I
GATE
= 500mA
V
GATE
= 2V
C
L
= 1000pF
C
L
= 1000pF
Duty Cycle = Min
●
●
●
●
●
●
The
●
denotes specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
CC
= 14V, GATE open, V
C
= 1.4V unless otherwise noted.
PARAMETER
CONDITIONS
MIN
11.5
11.0
TYP
12.1
11.8
0.3
0.6
1.2
2.5
30
30
0.90
0.80
220
200
1.12
250
220
0.30
C
OSCAP
= 100pF
●
MAX
UNITS
V
V
0.45
1.0
V
V
mA
ns
ns
Current Amplifier
1.25
1.35
270
280
V
V
mV
mV
mV
mV
115
125
200
200
200
200
24
●
Duty Cycle
≤
30%
Duty Cycle
≤
30%
Duty Cycle = 80%
●
∆V
ISENSE
/∆V
C
Timing
f
C
OSCAP
t
ON
t
ED
t
EN
R
t
Switching Frequency
Oscillator Capacitor Value
Minimum Switch On Time
Flyback Enable Delay Time
Minimum Flyback Enable Time
Timing Resistor Value
Maximum Switch Duty Cycle
Load Compensation
Sense Offset Voltage
Current Gain Factor
UVLO Function
V
UVLO
I
UVLO
UVLO Pin Threshold
UVLO Pin Bias Current
V
UVLO
= 1.2V
V
UVLO
= 1.3V
I
LOAD
= 1mA
●
●
●
90
80
33
100
kHz
kHz
pF
ns
ns
ns
(Note 2)
R
tON
= 50k
R
ENDLY
= 50k
R
MENAB
= 50k
(Note 2)
200
90
2
5
1.05
1.29
+ 0.25
– 2.50
3.2
kΩ
%
mV
mV
V
µA
µA
V
Ω
mA
85
0.80
1.21
– 0.25
– 4.50
2.8
8
0.95
1.25
+ 0.1
– 3.5
3.0
10
15
3V Output Function
V
REF
Reference Output Voltage
Output Impedance
Current Limit
Note 1:
Absolute Maximum Ratings are those values beyond which the life of
a device may be impaired.
Note 2:
Component value range guaranteed by design.
Note 3:
The V
CC
turn-on/turn-off voltages and hysteresis voltage are
proportional in magnitude to each other-guaranteed by design.
1725fa
3
LT1725
TYPICAL PERFOR A CE CHARACTERISTICS
V
CC
Turn-On Voltage vs
Temperature
16.00
V
CC
HYSTERESIS VOLTAGE (V)
15.75
V
CC
TURN-ON VOLTAGE (V)
15.50
15.25
15.00
14.75
14.50
14.25
–50 –25
6.00
5.75
5.50
5.25
5.00
4.75
–50 –25
START-UP CURRENT (µA)
50
25
75
0
TEMPERATURE (°C)
Supply Current vs Temperature
13
UVLO PIN INPUT CURRENT (µA)
1
12
SUPPLY CURRENT (mA)
–1
–2
–3
–4
–5
–6
–50 –25
V
UVLO
= 1.3V
OSCILLATOR FREQUENCY (kHz)
11
10
9
8
–50 –25
50
25
0
75
TEMPERATURE (°C)
V
C
CLAMP VOLTAGE, SWITCHING THRESHOLD (V)
V
GATE
vs I
SINK
1.0
0.8
T
A
= 125°C
V
GATE
(V)
0.6
T
A
= 25°C
0.4
VCC-V
GATE
(V)
0.2
T
A
= –55°C
0
1
10
I
SINK
(mA)
1725 G07
100
4
U W
100
125
1725 G01
V
CC
Hysteresis Voltage vs
Temperature
6.50
6.25
Start-Up Current vs Temperature
250
200
150
100
50
50
25
75
0
TEMPERATURE (°C)
100
125
0
–50
–25
50
25
0
75
TEMPERATURE (°C)
100
125
1725
G02
1725 G03
UVLO Pin Input Current vs
Temperature
115
V
UVLO
= 1.2V
Oscillator Frequency vs
Temperature
0
110
105
100
95
90
85
–50 –25
100
125
50
25
75
0
TEMPERATURE (°C)
100
125
50
25
75
0
TEMPERATURE (°C)
100
125
1725 G04
1725
G05
1725 G06
V
CC
-V
GATE
vs I
SOURCE
0
–0.5
–1.0
T
A
= 125°C
T
A
= 25°C
–1.5
–2.0
–2.5
–3.0
1000
V
C
Clamp Voltage, Switching
Threshold vs Temperature
3.0
2.5
2.0
1.5
1.0
0.5
0
–50
CLAMP VOLTAGE
T
A
= –55°C
SWITCHING THRESHOLD
1
10
100
I
SOURCE
(mA)
1000
1725 G08
–25
50
25
75
0
TEMPERATURE (°C)
100
125
1725 G09
1725fa
LT1725
TYPICAL PERFOR A CE CHARACTERISTICS
Minimum Switch-On Time vs
Temperature
275
MINIMUM SWITCH-ON TIME (ns)
R
TON
= 50k
MINIMUM ENABLE TIME (ns)
250
225
200
175
150
125
–50
ENABLE DELAY TIME (ns)
–25
50
25
75
0
TEMPERATURE (°C)
FEEDBACK AMPLIFIER TRANSCONDUCTANCE (µmho)
Feedback Amplifier Output Current
vs FB Pin Voltage
FEEDBACK AMPLIFIER OUTPUT CURRENT (µA)
80
60
40
20
0
–20
T
A
= 125°C
–40
–60
–80
1.05 1.10
1.15 1.20 1.25 1.30 1.35 1.40
FB PIN VOLTAGE (V)
1725 G13
T
A
= 25°C
Soft-Start Charging Current vs
Temperature
60
SOFT-START CHARGING CURRENT (µA)
50
40
30
20
10
0
–50
SOFT-START SINK CURRENT (mA)
–25
U W
100
125
1725 G10
Minimum Enable Time vs
Temperature
275
250
225
200
175
150
125
–50
R
MINENAB
= 50k
Enable Delay Time vs
Temperature
275
250
225
200
175
150
125
–50
–25
50
25
75
0
TEMPERATURE (°C)
100
125
–25
50
25
75
0
TEMPERATURE (°C)
100
125
1725 G11
1725 G12
Feedback Amplifier
Transconductance vs Temperature
1600
1400
1200
1000
800
600
400
200
–50 –25
T
A
= –55°C
50
25
75
0
TEMPERATURE (°C)
100
125
1725
G14
Soft-Start Sink Current vs
Temperature
2.5
V(SFST) = 1.5V
2.0
V(SFST) = 0V
1.5
1.0
0.5
50
25
75
0
TEMPERATURE (°C)
100
125
0
–50
–25
50
25
0
75
TEMPERATURE (°C)
100
125
1725 G15
1725 G16
1725fa
5
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