LT3781 Datasheet, PDF (1/20 Pages) Linear Technology – Bootstrap Start Dual Transistor Synchronous Forward Controller

Final Electrical Specifications
LT3781
“Bootstrap” Start Dual
Transistor Synchronous Forward Controller
FEATURES
DESCRIPTIO
May 2002
s _{High Voltage Operation up to 72V}
s _{Synchronizable Operating Frequency and Output}
Switch Phase for Multiple Controller Systems
s _{Synchronous Switch Output}
s _{Undervoltage Lockout Protection with 6V Hysteresis}
for Self-Biased Power
s _{Fixed Frequency Operation to 350kHz}
s _{Local 1% Voltage Reference}
s _{Input Overvoltage Protection}
s _{Low Start-Up Current}
s _{Programmable Start Inhibit for Power Supply}
Sequencing and Protection
s _{Optocoupler Support}
s _{Soft-Start ControUl}
APPLICATIO S
s _{Isolated Telecommunication Systems}
s _{Personal Computers and Peripherals}
s _{Distributed Power Step-Down Converters}
s _{Lead Acid Battery Backup Systems}
s _{Automotive and Heavy Equipment}
The LT^®3781 controller simplifies the design of high
power synchronous dual transistor forward DC/DC con-
verters. The part employs fixed frequency current mode
control and supports both isolated and nonisolated to-
pologies. The IC drives external N-channel power MOSFETs
and operates with input voltages up to 72V.
The LT3781 is ideal for output derived power schemes,
through the use of a large undervoltage lockout hysteresis
range. The part is also equipped with an 18V power supply
pin regulator which prevents exceeding absolute maxi-
mum ratings while in trickle start applications.
The LT3781’s operating frequency is programmable and
can be synchronized up to 350kHz. Switch phase is also
controlled during synchronized operation to accommo-
date multiple-converter systems. Internal logic guaran-
tees 50% maximum duty cycle operation to prevent trans-
former saturation.
The LT3781 is available in a 20-lead SSOP package.
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATIO
36V-72V to 5V/7A DC/DC
Synchronous Forward Converter (Quarter-Brick Footprint)
L1
4.7µH
V_IN+
T1
6
Q1
1
+ C2
22µF
100V
C4
1.5µF
100V
C3
1.5µF
100V
MURS120T3
Q3
MURS120T3
R9
0.025Ω
1/2W
7
2
10Ω
3
5
0.25W
1nF
8
4
10
1nF ^100V
11 100V ^10Ω
Q6
12
0.25W
9
Q5
V_IN–
ZVN3310F _BAT54
L2 VOUT = 5V
4.1µH IOUT = 7A
V_OUT+
MBR- ⁺
0540T1
C5
330µF
10V
4.7Ω
V_OUT–
0.1µF
68µF ⁺
20V
270k
0.25W
73.2k
1%
20k
1nF
10k
1OV
BIAS
MMBD914LT1
0.1µF
100V
10k
330pF
BAS21
20
13
V_CC V_BST
2 _OVLO
1 _{SHDN 5VREF}
19 18 15 11
14
TG BSTREF BG SENSE
LT3781
PGND
SG
12
FSET THERM SYNC SGND SS V_C VFB ⁹
1.24k
1%
5
6
52.3k
3
7 4 8 10
100Ω
5V
OUT
3.3Ω
0.047µF
3.01k 51Ω
1%
1µF
150pF
4.7nF
1k
0.01µF
1%
1OV
100Ω ^2k
BIAS
FZT690
0.22µF
50V
CMPZ5242B
4.7µF
12V
16V
8 ^LTC1693-2 6
V_CC1 V_CC2
3
5
IN2 OUT2
1
7
IN1 OUT1
4
2
GND2 GND1
C2:SANYO 100MV22AX
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
T1:COILTRONICS VP5-1200
3781i
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
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