LT1425
Isolated Flyback
Switching Regulator
FEATURES
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DESCRIPTION
The LT
®
1425 is a monolithic high power switching regu-
lator specifically designed for the isolated flyback topol-
ogy. No “third winding” or optoisolator is required; the
integrated circuit senses the isolated output voltage
directly from the primary side flyback waveform. A high
current, high efficiency switch is included on the die along
with all oscillator, control and protection circuitry.
The LT1425 operates with input supply voltages from 3V
to 20V and draws only 7mA quiescent current. It can
deliver output power up to 6W with no external power
devices. By utilizing current mode switching techniques, it
provides excellent AC and DC line regulation.
The LT1425 has a number of features not found on other
switching regulator ICs. Its unique control circuitry can
maintain regulation well into discontinuous mode in most
applications. Optional load compensation circuitry allows
for improved load regulation. An externally activated shut-
down mode reduces total supply current to 15µA for
standby operation.
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No Transformer “Third Winding” or Optoisolator
Required
±5%
Accurate Output Voltage Without User Trims
(See Circuit Below)
Resistor Programmable Output Voltage
Regulation Maintained Well Into Discontinuous
Mode (Light Load)
Optional Load Compensation
Operating Frequency: 285kHz
Easily Synchronized to External Clock
Available in 16-Pin Narrow SO Package
APPLICATIONS
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Isolated Flyback Switching Regulators
Ethernet Isolated 5V to – 9V Converters
Medical Instruments
Isolated Telecom Supplies
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATION
5V to Isolated – 9V
OUT
500V
ISOLATION BARRIER
5V
T1*
9.5
9.4
+
V
C2
47µF
16V
–
V
IN
V
SW
11
R1
22.6k
3 1%
•
•
LT1425
15
6
4
C3
1000pF
SHDN
SYNC
V
C
R
REF
R
OCOMP
R
CCOMP
SGND PGND
7
10
R
FB
*DALE LPE 4841-330MB
R2
3.01k
1%
5
14
13
C4
0.1µF
R3
15k
+
12
OUTPUT VOLTAGE (V)
C1
100µF
10V
D1
1N5819
ISOLATED
–9V
±
5% AT
20mA TO 200mA
F
9.3
9.2
9.1
9.0
8.9
8.8
8.7
8.6
8.5
0
50
100
150
OUTPUT CURRENT (mA)
200
1425 TA02
1425 TA01
U
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Load Regulation
1
LT1425
ABSOLUTE
MAXIMUM
RATINGS
(Note 1)
PACKAGE/ORDER INFORMATION
TOP VIEW
GND 1
NC 2
R
FB
3
V
C
4
R
REF
5
SYNC 6
SGND 7
GND 8
16 GND
15 SHDN
14 R
OCOMP
13 R
CCOMP
12 V
IN
11 V
SW
10 PGND
9
GND
Supply Voltage ........................................................ 20V
Switch Voltage ......................................................... 35V
SHDN, SYNC Pin Voltage ........................................... 7V
R
FB
Pin Current....................................................... 2mA
Operating Junction Temperature Range
Commercial .......................................... 0°C to 100°C
Industrial ......................................... – 40°C to 100°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
ORDER PART
NUMBER
LT1425CS
LT1425IS
S PACKAGE
16-LEAD PLASTIC SO
T
JMAX
= 145°C,
θ
JA
= 75°C/ W
Consult factory for Military grade parts.
ELECTRICAL CHARACTERISTICS
SYMBOL
I
REF
I
IN
g
m
V
CL
PARAMETER
Reference Current
R
REF
Pin Input Current
Feedback Amplifier Transconductance
Feedback Amplifier Clamp Voltage
Reference Voltage/Current Line Regulation
Voltage Gain
V
IN
Sense Error
Output Switch
BV
V(V
SW
)
I
LIM
Output Switch Breakdown Voltage
Output Switch ON Voltage
Switch Current Limit
Feedback Amplifier
V
IN
= 5V, T
J
= 25°C, V
SW
open, V
C
= 1.4V, unless otherwise specified.
MIN
402
396
400
30
TYP
408
500
MAX
414
420
1600
80
0.04
25
UNITS
µA
µA
nA
µmho
µA
V
%/V
V/V
mV
V
0.85
1.9
1.9
V
A
A
A
V
V
A/V
300
320
260
kHz
kHz
ns
ns
ns
95
%
CONDITIONS
Measured at R
FB
Pin with R
REF
= 3.000k
q
∆I
C
=
±10µA
(Note 2)
q
q
1000
50
1.9
0.01
500
10
I
SOURCE
, I
SINK
Feedback Amplifier Source or Sink Current
5V
≤
V
IN
≤
18V
(Note 3)
q
q
I
C
= 5mA
I
SW
= 1A
Duty Cycle = 50%, 0°C
≤
T
J
≤
100°C
Duty Cycle = 50%, – 40°C
≤
T
J
≤
100°C
Duty Cycle = 80%
Duty Cycle = Minimum
q
q
q
q
35
1.35
1.25
50
0.55
1.60
1.60
1.30
1.2
2
Current Amplifier
Control Pin Threshold
Control Voltage to Switch Transconductance
Timing
f
t
ON
t
ED
t
EN
Switching Frequency
q
q
0.95
0.85
1.3
1.4
260
240
170
285
210
150
180
Minimum Switch ON Time
Flyback Enable Delay Time
Minimum Flyback Enable Time
Maximum Switch Duty Cycle
q
85
90
2
U
W
U
U
W W
W
LT1425
ELECTRICAL CHARACTERISTICS
SYMBOL
PARAMETER
∆V
RCCOMP
/∆I
SW
SYNC Function
Minimum SYNC Amplitude
Synchronization Range
SYNC Pin Input Resistance
Power Supply
V
IN(MIN)
I
CC
Minimum Input Voltage
Supply Current
Shutdown Mode Supply Current
Shutdown Mode Threshold
Load Compensation
V
IN
= 5V, T
J
= 25°C, V
SW
Open, V
C
= 1.4V, unless otherwise specified.
MIN
TYP
0.45
q
CONDITIONS
MAX
UNITS
Ω
1.5
320
40
2.2
450
V
kHz
kΩ
q
q
q
q
2.8
7.0
15
0.4
0.9
3.1
9.5
40
1.3
V
mA
µA
V
The
q
denotes the 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:
Feedback amplifier transconductance is R
REF
referred.
Note 3:
Voltage gain is R
REF
referred.
TYPICAL PERFORMANCE CHARACTERISTICS
Switch Saturation Voltage vs
Switch Current
1.2
SWITCH SATURATION VOLTAGE (V)
1.0
125°C
0.8
25°C
0.6
0.4
0.2
0
0
–55°C
2.0
T
A
= 25°C
3.0
SWITCH CURRENT LIMIT (A)
INPUT VOLTAGE (V)
0
0.2
1.0
0.4 0.6 0.8
SWITCH CURRENT (A)
U W
1.2
1425 G01
Switch Current Limit vs
Duty Cycle
3.1
Minimum Input Voltage vs
Temperature
1.5
2.9
2.8
2.7
2.6
2.5
2.4
–50 –25
1.0
0.5
1.4
0
10 20 30 40 50 60 70 80 90 100
DUTY CYCLE (%)
1425 G02
50
25
75
0
TEMPERATURE (°C)
100
125
1425 G03
3
LT1425
TYPICAL PERFORMANCE CHARACTERISTICS
Feedback Amplifier Output
Current vs R
REF
Pin Voltage
FEEDBACK AMPLIFIER OUTPUT CURRENT (µA)
60
40
20
0
–20
–40
–60
–80
1.05 1.10
25°C
125°C
–55°C
1400
1200
V
C
PIN VOLTAGE (V)
1000
800
600
400
200
0
–50 –25
TRANSCONDUCTANCE (µmho)
1.15 1.20 1.25 1.30 1.35
R
REF
NODE VOLTAGE (V)
Switching Frequency vs
Temperature
MINIMUM SYNCHRONIZATION VOLTAGE (V
P-P
)
300
295
290
285
280
275
270
265
–50 –25
2.50
2.25
2.00
1.75
1.50
1.25
1.00
SHDN PIN INPUT CURRENT (µA)
SWITCHING FREQUENCY (kHz)
50
25
75
0
TEMPERATURE (°C)
Minimum Switch ON Time vs
Temperature
300
275
ENABLE DELAY TIME (ns)
SWITCH ON TIME (ns)
250
225
200
175
150
125
–50 –25
250
225
175
150
125
100
75
–50 –25
ENABLE TIME (ns)
50
25
75
0
TEMPERATURE (°C)
4
U W
1425 G04
Error Amplifier Transconductance
vs Temperature (R
REF
Referred)
2.50
2.25
2.00
1.75
1.50
1.25
1.00
V
C
Pin Threshold and High Clamp
Voltage vs Temperature
V
C
HIGH CLAMP
V
C
THRESHOLD
1.40
50
25
75
0
TEMPERATURE (°C)
100
125
0.75
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
125
1425 G05
1425 G06
Minimum Synchronization
Voltage vs Temperature
1
SHDN Pin Input Current vs
Voltage
T
A
= 25°C
0
–1
–2
–3
100
125
0.75
–50 –25
–4
50
25
75
0
TEMPERATURE (°C)
100
125
0
1
3
4
2
SHDN PIN VOLTAGE (V)
5
1425 G09
1425 G07
1425 G08
Flyback Enable Delay Time vs
Temperature
275
250
225
200
175
150
125
Minimum Flyback Enable Time vs
Temperature
200
100
125
50
25
75
0
TEMPERATURE (°C)
100
125
100
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
125
1425 G10
1425 G11
1425 G12
LT1425
PIN FUNCTIONS
GND (Pins 1, 8, 9, 16):
Ground. These pins connect to the
substrate of the die and are separate from the power
ground and signal ground. They should connect directly to
a good quality ground plane.
R
FB
(Pin 3):
Input Pin for External “Feedback” Resistor
Connected to Transformer Primary (V
SW
). The ratio of this
resistor to the R
REF
resistor, times the internal bandgap
(V
BG
) reference, is the primary determinant of the output
voltage (plus the effect of any nonunity transformer turns
ratio). The average current through this resistor during the
flyback period should be approximately 400µA. See Appli-
cations Information for more details.
V
C
(Pin 4):
Control Voltage. This pin is the output of the
feedback amplifier and the input of the current compara-
tor. Frequency compensation of the overall loop is effected
by placing a capacitor between this node and ground.
R
REF
(Pin 5):
Input Pin for External Ground-Referred
“Reference” Resistor. This resistor should be in the range
of 3k, but for convenience, need not be this value precisely.
See Applications Information for more details.
SYNC (Pin 6):
Pin to Synchronize Internal Oscillator to
External Frequency Reference. It is directly logic compat-
ible and can be driven with any signal between 10% and
90% duty cycle. If unused, this pin can be left floating;
however, for best noise immunity the pin should be
grounded.
SGND (Pin 7):
Signal Ground. This pin is a clean ground.
The internal reference and feedback amplifier are referred
to it. Keep the ground path connection to R
REF
and the V
C
compensation capacitor free of large ground currents.
PGND (Pin 10):
Power Ground. This pin is the emitter of
the power switch device and has large currents flowing
through it. It should be connected directly to a good quality
ground plane.
V
SW
(Pin 11):
This is the collector node of the output
switch and has large currents flowing through it. Keep the
traces to the switching components as short as possible
to minimize electromagnetic radiation and voltage spikes.
V
IN
(Pin 12):
Supply Voltage. Bypass input supply pin with
10µF or more. The part goes into undervoltage lockout
when V
IN
drops below 2.8V. Undervoltage lockout stops
switching and pulls the V
C
pin low.
R
CCOMP
(Pin 13):
Pin for the External Filter Capacitor for
Load Compensation Function. A common 0.1µF
ceramic capacitor will suffice for most applications. See
Applications Information for further details.
R
OCOMP
(Pin 14):
Input Pin for Optional External Load
Compensation Resistor. Use of this pin allows nominal
compensation for nonzero output impedance in the power
transformer secondary circuit, including secondary wind-
ing impedance, output Schottky diode impedance and
output capacitor ESR. In less demanding applications this
resistor is not needed. See Applications Information for
more details.
SHDN (Pin 15):
Shutdown. This pin is used to turn off the
regulator and reduce V
IN
input current to a few tens of
microamperes. The SHDN pin can be left floating when
unused.
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