Datasheet

8
LTC1698
1698f
BLOCK DIAGRA
W
I-TO-V CONVERTER
R
MARGIN
20k
R
OVP
3k
100k
V
FB
0.94V
REF
M
PWRGD
R
ILIM
3k
M
ILIM
MARGIN
7
SYNC
15
OPTODRV
5
±5% V
REF
V
REF
BANDGAP
SYNC IN
V
CC
V
DD
V
AUX
14
–
+
–
+
–
+
ERR
–
+
I
LIM 25mV
–
+
OVP
V
REF
1698 BD
+
OPTO
PWRGD
10
OVPIN
9
I
COMP
13
I
SNS
12
I
SNSGND
11
V
COMP
6
V
FB
8
CG
2
FG
16
PWRGD
AUX GEN V
CC
GEN
1
OPERATIO
U
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
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
DD
input is low, the gate drivers TG and BG are
both at the ground potential. The secondary forward and
(Refer to Block Diagram)