LT3781_09 Datasheet, PDF (1/20 Pages) Linear Technology – “Bootstrap” Start Dual Transistor Synchronous Forward Controller

LT3781
“Bootstrap” Start Dual
Transistor Synchronous Forward Controller
FEATURES
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}
DESCRIPTIO
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 V_CC shunt
regulator, which prevents exceeding absolute maximum
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
T1
V_IN+
6
Q1
1
+ C2
C4
MURS120T3
7
2
3
5
10Ω
0.25W
22µF
100V
1.5µF
100V
C3
1.5µF
100V
MURS120T3
Q3
R9
0.025Ω
1/2W
1nF
8
4
10
1nF ^100V
11 100V ^10Ω
Q6
12
0.25W
9
Q5
V_IN–
ZVN3310F _BAT54
L2
4.7µH
V_OUT = 5V
I_OUT = 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
PGND
SG
12
LT3781
FSET THERM SYNC SGND SS V_C VFB ⁹
1.24k
1%
5
6
52.3k
3
7 4 8 10
100Ω
5V
OUT
3.3k
0.047µF
3.01k 51Ω
1%
6.8k
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
3781f
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