ltc3900 Linear Technology Corporation, ltc3900 Datasheet - Page 6

• Manufacturer Part Number: ltc3900
• Description: Synchronous Rectifier Driver For Forward Converters
• Manufacturer: Linear Technology Corporation

BLOCK DIAGRA
LTC3900
APPLICATIO S I FOR ATIO
Overview
In a typical forward converter topology, a power trans-
former is used to provide the functions of input/output
isolation and voltage step-down to achieve the required
low output voltage. Schottky diodes are often used on
the secondary-side to provide rectification. Schottky di-
odes, though easy to use, result in a loss of efficiency due
to relatively high voltage drops. To improve efficiency,
synchronous output rectifiers utilizing N-channel MOSFETs
can be used instead of Schottky diodes. The LTC3900
provides all of the necessary functions required to drive
the synchronous rectifier MOSFETs.
Figure 1 shows a simplified forward converter application.
T1 is the power transformer; Q1 is the primary-side power
transistor driven by the primary controller, LT1952 output
(OUT). The pulse transformer T2 provides synchroniza-
tion and is driven by LT1952 synchronization signal, S
or SG from the primary controller. Q3 and Q4 are second-
ary-side synchronous switches driven by the LTC3900’s
FG and CG output. Inductor L
the output filter to provide a steady DC output voltage for
the load. Also shown in Figure 1 is the feedback path from
V
6
OUT
through the optocoupler driver LT4430 and an
U
TIMER
SYNC
CS
CS
W
U
+
–
8
1
2
7
O
M
TMR
and capacitor C
10.5mV
–
W
+
+1.4V
–1.4V
Z
11V
Z
0.5 • V
TMR
CS
TMR
S
S –
IS
+
U
OUT
CC
SYNC
SYNC
form
OUT
+
–
R1
180k
R2
45k
optocoupler, back to the primary controller to regulate
V
Each full cycle of the forward converter operation consists
of two periods. In the first period, Q1 turns on and the
primary-side delivers power to the load through T1. SG
goes high and T2 generates a negative pulse at the
LTC3900 SYNC input. The LTC3900 forces FG to turn on
and CG to turn off, Q3 conducts. Current flows to the load
through Q3, T1 and L
goes low and T2 generates a positive pulse at the LTC3900
SYNC input. The LTC3900 forces FG to turn off and CG to
turn on, Q4 conducts. Current continues to flow to the load
through Q4 and L
nization waveforms.
External MOSFET Protection
A programmable timer and a differential input current
sense comparator are included in the LTC3900 for protec-
tion of the external MOSFET during power down and Burst
Mode
V
DISABLE
DRIVER
OUT
CC
UVLO
< 4.1V.
.
®
operation. The chip also shuts off the MOSFETs if
3900 BD
DRIVER
LOGIC
SYNC
AND
TIMER
RESET
O
. Figure 2 shows the LTC3900 synchro-
O
. In the next period, Q1 turns off, SG
4
5
3
6
V
FG
CG
GND
CC
3900f