LT1737
High Power
Isolated Flyback Controller
FEATURES
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DESCRIPTIO
Drives External Power MOSFET
Supply Voltage Range: 4.5V to 20V
Flyback Voltage Limited Only by
External Components
Senses Output Voltage Directly from Primary Side
Winding—No Optoisolator Required
Switching Frequency from 50kHz to 250kHz
with External Capacitor
Moderate Accuracy Regulation Without User Trims
Regulation Maintained Well into Discontinuous Mode
External I
SENSE
Resistor
Optional Load Compensation
Optional Undervoltage Lockout
Shutdown Feature Reduces I
Q
to 50µA Typ
Available in 16-Pin GN and SO Packages
The LT
®
1737 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 DC supply voltage. Output voltage feed-
back information may be supplied by a variety of methods
including a third transformer winding, the primary wind-
ing or even direct DC feedback (see Applications Informa-
tion). Its gate drive capability, coupled with a suitable
external MOSFET and other power path components, can
deliver load power up to tens of Watts.
The LT1737 has a number of features not found on other
isolated flyback controller 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.
, LTC and LT are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
APPLICATIO S
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Isolated Flyback Switching Regulators
Medical Instruments
Instrumentation Power Supplies
TYPICAL APPLICATIO
R2
35.7k
1%
D2
BAS16
12V-18V to Isolated 15V Converter
V
IN
C1
22µF
+
Q1
2N3906
T1
COILTRONICS
CTX150-4
240k
33k
C3
0.1µF
•
150µH
150µH
•
UVLO
FB
V
C
R3
3.01k
1%
1nF
OSCAP
47pF
LT1737
V
CC
GATE
I
SENSE
M1
IRFL014
R1
0.27Ω
t
ON
ENDLY MINENAB R
OCMP
75k
150k
100k
4.7k
R
CMPC
SGND
PGND
0.1µF
U
D1
MBRS1100
U
U
+
C2
33µF
R4
7.5k
V
OUT
= 15V
I
OUT
= 300mA
NOTE: SEE APPLICATIONS
INFORMATION FOR ADDITIONAL
COMPONENT SPECIFICATIONS
1737 TA01
1737fa
1
LT1737
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
LT1737C ............................................... 0°C to 100°C
LT1737I ............................................ –40°C to 125°C
Storage Temperature Range ................ – 65°C to 150°C
Lead Temperature (Soldering, 10 sec)................ 300°C
ORDER PART
NUMBER
LT1737CGN
LT1737CS
LT1737IGN
LT1737IS
GN PART MARKING
1737
1737I
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
= 110°C/W (SO)
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
SYMBOL
Power Supply
V
CC(MIN)
I
CC
Minimum Input Voltage
Supply Current
Shutdown 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
TYP
4.1
10
50
MAX
4.5
15
150
1.260
1.270
1800
80
0.05
50
UNITS
V
mA
µA
V
V
nA
µmho
µA
V
%/V
V/V
µA
mA
V
V
V
C
= Open
V
UVLO
= 0V, V
C
= Open
●
●
Feedback Amplifier
V
FB
I
FB
g
m
I
SRC
, I
SNK
V
CL
●
1.230
1.220
400
30
1.245
500
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
4.75V
≤
V
IN
≤
18V
V
C
= 1V to 2V
V
SFST
= 0V
V
SFST
= 1.5V, V
UVLO
= 0V
I
GATE
= 100mA
I
GATE
= 500mA
I
GATE
= 100mA
I
GATE
= 500mA
V
GATE
= 2V
C
L
= 1000pF
C
L
= 1000pF
●
●
●
●
●
●
∆l
C
=
±10µA
●
●
1000
50
2.5
0.01
2000
25
0.8
11.5
11.0
40
1.5
12.1
11.8
0.3
0.6
Gate Output
V
GATE
Output High Level
Output Low Level
I
GATE
t
r
t
f
Output Sink Current in Shutdown, V
UVLO
= 0V
Rise Time
Fall Time
0.45
1.0
1.2
2.5
30
30
2
U
V
V
mA
ns
ns
1737fa
W
U
U
W W
W
LT1737
ELECTRICAL CHARACTERISTICS
SYMBOL
V
C
V
ISENSE
PARAMETER
Control Pin Threshold
Switch Current Limit
Current Amplifier
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.
CONDITIONS
MIN
0.90
0.80
220
200
TYP
1.12
250
220
0.30
C
OSCAP
= 100pF
●
MAX
1.25
1.35
270
280
UNITS
V
V
mV
mV
mV
mV
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
UVLO Pin Lockout Threshold
UVLO Pin Shutdown Threshold
●
●
90
80
33
100
115
125
200
kHz
kHz
pF
ns
ns
ns
(Note 2)
R
tON
= 50k
R
ENDLY
= 50k
R
MENAB
= 50k
(Note 2)
●
200
200
200
24
85
90
2
0.80
1.21
0.4
– 0.25
– 4.50
●
●
240
kΩ
%
5
1.05
1.29
0.95
mV
mV
V
V
V
µA
µA
V
Ω
mA
0.95
1.25
0.75
I
UVLO
UVLO Pin Bias Current
V
UVLO
= 1.2V
V
UVLO
= 1.3V
I
LOAD
= 1mA
+ 0.1
– 3.5
3.0
10
15
+ 0.25
– 2.50
3.2
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.
8
2.8
1737fa
3
LT1737
TYPICAL PERFOR A CE CHARACTERISTICS
Minimum Input Voltage vs
Temperature
4.3
125
SHUTDOWN VOLTAGE V
UVLO
(V)
50
25
0
75
TEMPERATURE (°C)
100
125
MINIMUM INPUT VOLTAGE (V)
4.2
SHUTDOWN I
CC
(µA)
4.1
4.0
3.9
3.8
–50
–25
50
25
0
75
TEMPERATURE (°C)
Supply Current vs Temperature
13
UVLO PIN INPUT CURRENT (µA)
1
0
–1
–2
–3
–4
–5
12
SUPPLY CURRENT (mA)
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)
1737 G07
100
4
U W
100
1737 G01
Shutdown I
CC
vs Temperature
1.0
0.9
0.8
0.7
0.6
0.5
Shutdown Voltage (V
UVLO
) vs
Temperature
100
75
50
25
125
0
–50 –25
0.4
–50
–25
50
25
75
0
TEMPERATURE (°C)
100
125
1737 G02
1737 G03
UVLO Pin Input Current vs
Temperature
115
V
UVLO
= 1.2V
Oscillator Frequency vs
Temperature
110
105
100
95
90
85
–50 –25
V
UVLO
= 1.3V
100
125
–6
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
125
50
25
75
0
TEMPERATURE (°C)
100
125
1737 G04
1737
G05
1737 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
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
1000
1
10
100
I
SOURCE
(mA)
1000
1737 G08
–25
50
25
75
0
TEMPERATURE (°C)
100
125
1737 G09
1737fa
LT1737
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
225
200
175
150
125
–50
ENABLE DELAY TIME (ns)
–25
50
25
75
0
TEMPERATURE (°C)
100
125
–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)
1737 G13
T
A
= 25°C
Soft-Start Charging Current vs
Temperature
60
2.5
SOFT-START CHARGING CURRENT (µA)
50
40
30
20
10
0
–50
SOFT-START SINK CURRENT (mA)
–25
U W
100
Minimum Enable Time vs
Temperature
275
250
R
MINENAB
= 50k
275
250
225
200
175
150
Enable Delay Time vs
Temperature
125
125
–50
–25
50
25
75
0
TEMPERATURE (°C)
100
125
1737 G10
1737 G11
1737 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
1737
G14
Soft-Start Sink Current vs
Temperature
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
1737 G15
1737 G16
1737fa
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