LTC1698 LINER [Linear Technology], LTC1698 Datasheet - Page 8

• Manufacturer Part Number: LTC1698
• Description: Isolated Secondary Synchronous Rectifier Controller
• Manufacturer: LINER [Linear Technology]

BLOCK DIAGRA
OPERATIO
LTC1698
The LTC1698 is a secondary-side synchronous rectifier
controller designed to work with the LT3781 primary-side
synchronous controller chip to form an isolated synchro-
nous forward converter. This chip set uses a dual transis-
tor forward topology that is predominantly used in distrib-
uted power supply systems where isolated low voltages
are needed to power complex electronic equipment. The
primary stage is a current mode, fixed frequency forward
converter and provides the typical PWM operation. A
power transformer is used to provide the functions of
input/output isolation and voltage step-down to achieve
the required low output voltage. Instead of using typical
8
U
(Refer to Block Diagram)
14
15
10
7
5
W
V
SYNC
MARGIN
OPTODRV
PWRGD
AUX
V
–
FB
M
PWRGD
PWRGD
0.94V
100k
BANDGAP
R
AUX GEN
SYNC IN
+
MARGIN
REF
OPTO
V
REF
+
–
M
ILIM
I-TO-V CONVERTER
1
V
DD
R
3k
R
ILIM
20k
3k
OVP
V
CC
V
Schottky diodes, synchronous rectification on the sec-
ondary offers isolation with high efficiency. It supplies
high power without the need of bulky heat sinks, which is
often a problem in any space constrained application.
The LTC1698 not only provides synchronous drivers for
the external MOSFETs, it comes with other housekeeping
functions performed on the secondary side of the power
supply, all within a single integrated controller. Figure 1
shows the typical chip-set application. Upon power up, the
LTC1698’s V
both at the ground potential. The secondary forward and
CC
GEN
ERR
OVP
I
LIM
+
+
+
–
–
–
5% V
DD
REF
25mV
+
input is low, the gate drivers TG and BG are
V
REF
I
SNSGND
V
OVPIN
I
COMP
COMP
I
V
SNS
CG
FG
FB
16
11
12
13
2
8
6
9
1698 BD
1698f