LTC3706
Secondary-Side Synchronous
Forward Controller with
PolyPhase Capability
DESCRIPTION
The LTC®3706 is a PolyPhase capable secondary-side
controllerforsynchronousforwardconverters.Whenused
in conjunction with the LTC3705 gate driver and primary-
side controller, the part creates a complete isolated power
supply that combines the power of PolyPhase operation
with the speed of secondary-side control.
FEATURES
n
Secondary-Side Control for Fast Transient Response
n
Self-Starting Architecture Eliminates Need for
Separate Bias Regulator
n
Proprietary Gate Drive Encoding Scheme Reduces
System Complexity
PolyPhase® Operation Reduces C Requirements
n
IN
n
Current Mode Control Ensures Current Sharing
The LTC3706 has been designed to simplify the design
of highly efficient, secondary-side forward converters.
Working in concert with the LTC3705, the LTC3706 forms
a robust, self-starting converter that eliminates the need
for the separate bias regulator that is commonly used in
secondary-side control applications. In addition, a pro-
prietary scheme is used to multiplex gate drive signals
and DC bias power across the isolation barrier through a
single, tiny pulse transformer.
n
PLL Fixed Frequency: 100kHz to 500kHz
n
1% Output Voltage Accuracy
n
True Remote Sense Differential Amplifier
n
Power Good Output Voltage Monitor
n
High Voltage Linear Regulator Controller
n
Wide Supply Range: 5V to 30V
n
Available in a Narrow 24-Lead SSOP Package
APPLICATIONS
The LTC3706 provides remote sensing, accurate power
goodandovervoltagemonitoringcircuitstosupportpreci-
sion,highcurrentapplications.Alinearregulatorcontroller
with thermal protection is also provided to simplify the
generation of secondary-side bias voltage.
n
Isolated 48V Telecommunication Systems
n
Internet Servers and Routers
n
Distributed Power Step-Down Converters
Automotive and Heavy Equipment
n
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
PolyPhase is a registered trademark of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
The LTC3706 is available in a 24-lead SSOP package.
Protected by U.S. Patents including 6144194, other patents pending.
TYPICAL APPLICATION
36V-72V to 3.3V/20A Isolated Forward Converter
+
+
V
V
OUT
IN
T1
L1
1.2μH
•
•
Si7852DP
1.2Ω
MURS120
Si7852DP
Si7336ADP
s2
1μF
100V
s3
330μF
6.3V
s3
Si7336ADP
MURS120
CMPSH1-4
2mΩ
2W
30mΩ
1W
10μF
25V
–
–
V
V
OUT
IN
CZT3019
100k
BAS21
2.2μF
16V
102k
1%
FQT7N10
L1: COILCRAFT SER2010-122
T1: PULSE PA0807
0.22μF
T2: PULSE PA0297
FG
SW SG
V
NDRV
V
CC
IN
–
+
I
I
S
S
365k
1%
FS/SYNC
1μF
NDRV
UVLO
BOOST TG TS BG IS
T2
+
+
FB
FB/IN
PT
•
•
LTC3706
ITH
680pF
LTC3705
V
CC
2.2μF
25V
–
–
FS/IN
PT
SS/FLT
22.6k
1%
RUN/SS GND PGND PHASE SLP MODE REGSD
GND PGND VSLMT
15k
1%
20k
162k
33nF
33nF
3706 TA01
3706fb
1