LT1681
Dual Transistor
Synchronous Forward Controller
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FEATURES
DESCRIPTIO
The LT®1681 controller simplifies the design of high power
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High Voltage: Operation Up to 72V
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Synchronizable Operating Frequency and Output
Switch Phase for Multiple Controller Systems
Fixed Frequency Operation to 350kHz
Adaptive and Adjustable Blanking
Synchronous Rectifier Driver
synchronous dual transistor forward DC/DC converters. The
part employs fixed frequency current mode control and
supports both isolated and nonisolated topologies. The IC
drivesexternalN-channelpowerMOSFETsandoperateswith
input voltages up to 72V.
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Local 1% Voltage Reference
The LT1681’s operating frequency is programmable and can
be synchronized up to 350kHz. Switch phase is also con-
trolled during synchronized operation to accommodate mul-
tipleconvertersystems. Internallogicguarantees50%maxi-
mum duty cycle operation to prevent transformer saturation.
Undervoltage Lockout Protection with Hysteresis
Input Overvoltage Protection
Programmable Start Inhibit
Transformer Primary Saturation Protection
Optocoupler Feedback Support
Soft-Start Control
The LT1681 incorporates a soft-start feature that provides a
controlled increase in supplied current during start-up and
after an undervoltage lockout or overvoltage/overcurrent
event.
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APPLICATIO S
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Isolated Telecommunication Systems
The part is available in a 20-lead wide SO package to support
high voltage pin-to-pin clearance.
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Personal Computers and Peripherals
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Lead Acid Battery Backup Systems
Automotive and Heavy Equipment
, LTC and LT are registered trademarks of Linear Technology Corporation.
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TYPICAL APPLICATIO
36V-72V DC to 5V/7A Synchronous Forward Converter (Half-Brick Footprint)
V
= 5V
= 7A
OUT
OUT
L1
4.7µH
L2
4.1µH
I
+
+
6
7
5
8
4
V
V
IN
OUT
Q1
MURS120T3
1
2
3
10Ω
0.25W
+
C5
MBR-
+
C2
22µF
100V
C4
1.5µF
100V
330µF
0540T1
1nF
100V
10Ω
0.25W
MURS120T3
Q3
10V
10
11
12
1nF
100V
Q6
C3
1.5µF
100V
0.025Ω
1/2W
9
4.7Ω
Q5
T1
–
V
IN
–
V
OUT
C2:SANYO 100MV22AX
ZVN3310F
C3, C4: VITRAMON VJ1825Y155MXB
C5: 4X KEMET T510X337KO10AS
L1: COILCRAFT DO1608C-472
L2: PANASONIC ETQP6F4R1LF4
Q1,Q3:100V SILICONIX SUD40N10-25
Q5,Q6: SILICONIX Si4450
BAT54
1OV
10k
2k
0.22µF
100Ω
BIAS
1OV
BIAS
0.1µF
FZT690
330pF
50V
MMBD914LT1
BAS21
100V
T1:COILTRONICS VP5-1200
Q10: ON SEMI MMBT3906LTI
CMPZ5242B
12V
73.2k
1%
4.7µF
16V
270k
5V
OUT
0.25W
20k
19
18
16 11
15
20
17
12
14
LTC1693-2
CC1
3.3Ω
V
V
BLKSENS TG BSTREF BG SENSE T
LT1681
PGND
6
8
3
1
4
CC
BST
MAX
13
CMPZ-
SG
V
V
CC2
2
1
CMPZ5248B
5248B
0.047µF
OVLO
SHDN
5
7
2
18V
15V
9
IN2
OUT2
OUT1
GND1
V
THERM SYNC SGND SS V
C
5V
F
SET
FB
REF
5
1681 TA01
IN1
51Ω
3.01k
1%
6
3
7
4
8
10
100Ω
0.1µF
10k
56k
52.3k
GND2
Q10
+
1.24k
1%
68µF
1nF 24k
20V
4.7nF
1k
1%
1µF
150pF
0.01µF
1681f
1