LT1424-5
Isolated Flyback
Switching Regulator
with 5V Output
FEATURES
s
s
s
s
DESCRIPTIO
s
s
s
No Transformer “Third Winding” or Optoisolator
Required
Designed for Use with 1:1 Ratio Transformers
Fixed, Application Specific 5V Output Voltage
Regulation Maintained Well into Discontinuous
Mode (Light Load)
Load Compensation Provides Excellent
Load Regulation
Available in 8-Pin PDIP and SO Packages
Operating Frequency: 285kHz
The LT
®
1424-5 is a monolithic high power switching
regulator specifically designed for the isolated flyback
topology. 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 LT1424-5 operates with input supply voltages from
3V to 20V and draws only 7mA quiescent current. It can
deliver up to 400mA at 5V with no external power devices.
By utilizing current mode switching techniques, it pro-
vides excellent AC and DC line regulation.
The LT1424-5 has a number of features not found on other
switching regulator ICs. Its unique control circuitry can
maintain regulation well into discontinuous mode. Load
compensation circuitry allows for improved load regula-
tion. An externally activated shutdown mode reduces total
supply current to 20µA typical for standby operation.
, LTC and LT are registered trademarks of Linear Technology Corporation.
APPLICATIO S
s
s
s
Isolated Communication Supplies
Industrial Automation
Instrumentation Systems
TYPICAL APPLICATIO
5V Output Isolated Power Supply
ISOLATION
BARRIER
5V
T1
47Ω
MBRS130LT3
330pF
5.25
+
C1
100µF
10V
5V
400mA
4
0.1µF
1
2
3
4
8
SHDN R
CCOMP
7
V
C
V
IN
LT1424-5
6
SYNC
V
SW
5
SGND
PGND
1N5248
47Ω
330pF
6
•
3
+
OUTPUT VOLTAGE (V)
MBR0540T4
7
•
1
•
2
C2
100µF
10V
1.8k
1000pF
INPUT
COM
1424-5 TA01
OUT
COM
4.75
0
100
200
300
OUTPUT CURRENT (mA)
400
1424-5 TA02
0.1µF
C1, C2: AVX TPS D107M010R0080
T1: DALE LPE-4841-A307
U
Load Regulation
5.00
U
U
sn14245 14245fs
1
LT1424-5
ABSOLUTE
MAXIMUM
RATINGS
(Note 1)
PACKAGE/ORDER INFORMATION
TOP VIEW
SHDN 1
V
C
2
SYNC 3
SGND 4
N8 PACKAGE
8-LEAD PDIP
8 R
CCOMP
7 V
IN
6 V
SW
5 PGND
S8 PACKAGE
8-LEAD PLASTIC SO
Supply Voltage (V
IN
) ................................................ 20V
Switch Voltage (V
SW
) .............................................. 35V
SHDN, SYNC Pin Voltage ........................................... 7V
Operating Junction Temperature Range
Commercial .......................................... 0°C to 125°C
Industrial ......................................... – 40°C to 125°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
ORDER PART
NUMBER
LT1424CN8-5
LT1424CS8-5
LT1424IN8-5
LT1424IS8-5
S8 PART MARKING
14245
1424I5
T
JMAX
= 145°C,
θ
JA
= 130°C/ W (N)
T
JMAX
= 145°C,
θ
JA
= 110°C/ W (S)
Consult factory for Military grade parts.
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 5V, V
SW
Open, V
C
= 1.4V, unless otherwise specified.
SYMBOL
Power Supply
V
IN(MIN)
I
CC
Minimum Operating Voltage
Supply Current
Shutdown Mode Supply Current
SHDN Pin Threshold
Feedback Amplifier
V
REF
g
m
V
CL
Reference Voltage
Feedback Amplifier Transconductance
Feedback Amplifier Clamp Voltage
Reference Voltage/Current Line Regulation
Voltage Gain
Output Switch
BV
V(V
SW
)
I
LIM
Output Switch Breakdown Voltage
Output Switch ON Voltage
Switch Current Limit
I
C
= 5mA
I
SW
= 1A
Duty Cycle = 50%, 0°C
≤
T
J
≤
125°C
Duty Cycle = 50%, – 40°C
≤
T
J
≤
125°C
Duty Cycle = 80%
Duty Cycle = Minimum
q
q
q
q
q
q
q
q
q
ELECTRICAL CHARACTERISTICS
PARAMETER
CONDITIONS
MIN
TYP
2.8
7.0
15
MAX
3.1
9.5
40
1.3
5.37
5.42
1600
80
0.04
UNITS
V
mA
µA
V
V
V
µmho
µA
V
%/V
V/V
V
0.3
5.23
5.18
400
30
0.9
5.30
5.30
1000
50
1.9
0.01
500
Measured at V
SW
Pin (Note 2)
q
∆I
C
=
±10µA
(Note 3)
q
q
I
SOURCE
, I
SINK
Feedback Amplifier Source or Sink Current
5V
≤
V
IN
≤
18V
(Note 4)
q
35
1.35
1.20
50
0.55
1.6
1.6
1.3
1.2
1.2
2
0.85
1.95
1.95
Current Amplifier
Control Pin Threshold
Control Voltage to Switch Transconductance
0.95
0.85
1.3
1.4
V
V
A/V
sn14245 14245fs
2
U
W
U
U
W W
W
V
A
A
A
LT1424-5
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 5V, V
SW
Open, V
C
= 1.4V, unless otherwise specified.
SYMBOL
Timing
f
t
ON
t
ED
t
EN
Switching Frequency
q
ELECTRICAL CHARACTERISTICS
PARAMETER
CONDITIONS
MIN
260
240
170
TYP
285
285
200
150
180
MAX
300
320
260
UNITS
kHz
kHz
ns
ns
ns
%
Ω
Minimum Switch ON Time
Flyback Enable Delay Time
Minimum Flyback Enable Time
Maximum Switch Duty Cycle
q
85
90
0.9
Load Compensation
∆V
REF
/∆I
SW
SYNC Function
Minimum SYNC Amplitude
Synchronization Range
SYNC Pin Input Resistance
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be imparied.
Note 2:
V
REF
is a parameter which is measured at the V
SW
pin. It differs
from the output voltage because it accounts for output diode drop,
transformer leakage inductance, etc. Nominal output voltage is 5V in the
intended application circuit.
q
q
1.5
330
40
2.2
450
V
kHz
kΩ
Note 3:
Feedback amplifier transconductance is R
REF
referred.
Note 4:
Voltage gain is R
REF
referred.
sn14245 14245fs
3
LT1424-5
TYPICAL PERFOR A CE CHARACTERISTICS
Switch Saturation Voltage
vs Switch Current
1.2
SWITCH SATURATION VOLTAGE (V)
SWITCH CURRENT LIMIT (A)
1.0
125°C
0.8
25°C
0.6
0.4
0.2
0
–55°C
INPUT VOLTAGE (V)
0
0.2
0.4 0.6 0.8 1.0
SWITCH CURRENT (A)
Reference Voltage
vs Temperature
FEEDBACK AMPLIFIER OUTPUT CURRENT (µA)
5.36
5.34
60
40
20
0
–20
–40
–60
REFERENCE VOLTAGE (V)
5.32
5.30
4.28
4.26
4.24
– 50 – 25
V
C
PIN VOLTAGE (V)
50
25
75
0
TEMPERATURE (°C)
Switching Frequency
vs Temperature
MINIMUM SYNCHRONIZATION VOLTAGE (V
P-P
)
300
295
290
285
280
275
270
265
– 50 – 25
SHDN PIN INPUT CURRENT (µA)
SWITCHING FREQUENCY (kHz)
50
25
75
0
TEMPERATURE (°C)
4
U W
1.2
1424-5 G01
Switch Current Limit
vs Duty Cycle
2.0
T
A
= 25°C
3.0
1.5
2.9
2.8
2.7
2.6
2.5
0
3.1
Minimum Input Voltage
vs Temperature
1.0
0.5
1.4
0
10 20 30 40 50 60 70 80 90 100
DUTY CYCLE (%)
1424-5 G02
2.4
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
125
1424-5 G03
Feedback Amplifier Output
Current vs Flyback Voltage
2.50
25°C
125°C
–55°C
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
100
125
–80
4.50 4.75
5.00 5.25 5.50 5.75
FLYBACK VOLTAGE (V)
6.00
6.25
0.75
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
125
1424-5 G04
1424-5 G05
1424-5 G06
Minimum Synchronization
Voltage vs Temperature
2.50
2.25
2.00
1.75
1.50
1.25
1.00
0.75
–50 –25
1
SHDN Pin Input Current
vs Voltage
T
A
= 25°C
0
–1
–2
–3
100
125
50
25
75
0
TEMPERATURE (°C)
100
125
–4
0
1
3
4
2
SHDN PIN VOLTAGE (V)
5
1424-5 G09
1424-5 G07
1424-5 G08
sn14245 14245fs
LT1424-5
TYPICAL PERFOR A CE CHARACTERISTICS
Minimum Switch On Time
vs Temperature
275
250
250
225
ENABLE DELAY TIME (ns)
SWITCH ON TIME (ns)
200
175
150
125
100
– 50 – 25
175
150
125
100
75
– 50 – 25
ENABLE TIME (ns)
225
50
25
75
0
TEMPERATURE (°C)
PIN FUNCTIONS
SHDN (Pin 1):
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.
V
C
(Pin 2):
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.
SYNC (Pin 3):
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 should be tied to
ground.
SGND (Pin 4):
Signal Ground. This pin is a clean ground.
The internal reference and feedback amplifier are referred
to it. Keep the ground path connection to the V
C
compen-
sation capacitor free of large ground currents.
PGND (Pin 5):
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 6):
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 7):
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 8):
Pin for the External Filter Capacitor for
Load Compensation Function. A common 0.1µF
ceramic capacitor will suffice.
U W
100
1424-5 G10
Flyback Enable Delay Time
vs Temperature
275
250
225
200
175
150
125
Minimum Flyback Enable Time
vs Temperature
200
125
50
25
75
0
TEMPERATURE (°C)
100
125
100
– 50 – 25
50
25
75
0
TEMPERATURE (°C)
100
125
1424-5 G11
1424-5 G12
U
U
U
sn14245 14245fs
5
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