LTC3901 Linear Technology, LTC3901 Datasheet - Page 6

• Manufacturer Part Number: LTC3901
• Description: Secondary Side Synchronous Driver for Push-Pull and Full-Bridge Converters
• Manufacturer: Linear Technology

BLOCK DIAGRA
LTC3901
APPLICATIO S I FOR ATIO
Overview
Push-pull and full bridge converters use power transform-
ers to provide input-to-output isolation and voltage step-
up/down. Diodes are used as a simple solution for second-
ary side rectification. Unfortunately, as output currents
increase, the loss associated with diode forward voltage
drop results in low overall efficiency. The LTC3901 over-
comes this problem by providing control and drive for two
external N-channel synchronous MOSFETs. Synchroniza-
tion to the primary side controller is maintained through a
small signal transformer.
Figure 1 shows a simplified push-pull converter applica-
tion. T1 is the power transformer; MA and MB are the
primary side power transistors driven by the LTC3723
controller’s DRVA and DRVB outputs. The gate drive
transformer T2 is driven by the LTC3723’s SDRA and
SDRB outputs and provides the synchronization signal to
the LTC3901 on the secondary side. When both SDRA and
SDRB are high, there is no voltage across the transformer’s
primary and the LTC3901 SYNC input is approximately 0V.
According to the polarity of the transformer: if SDRA goes
low while SDRB is high, SYNC is positive; if SDRB goes
low while SDRA is high, SYNC is negative. ME and MF are
6
U
TIMER
SYNC
CSE
CSE
CSF
CSF
+
–
+
–
W
11
12
U
9
6
5
7
10.5mV
10.5mV
M
–
–
TMR
+
+
W
+1.4V
–1.4V
Z
11V
CSF
Z
11V
Z
0.5 • V
TMR
CSE
ISE
TMR
ISF
S
S –
+
CC
U
SYNC
SYNC
+
–
R1
180k
R2
45k
the secondary side synchronous switches driven by the
LTC3901’s ME and MF output. Inductor L1 and capacitor
C
the load. The feedback path from V
coupler driver and optocoupler back to the primary side
controller is also shown in Figure 1.
Each full cycle of the push-pull converter consists of four
distinct periods. Figure 2 shows the push-pull converter
waveforms.
DISABLE
DRIVER
OUT
UVLO
DRVA
DRVB
SDRA
SDRB
SYNC
form the output filter, providing DC output voltage to
Figure 2. Push-Pull Converter Switching Waveforms
ME
MF
DRIVER
LOGIC
SYNC
AND
TIMER
RESET
GND GND
8
16
14
13
10
1 PV
3
4
3901 BD
V
ME
PGND
MF
PGND
CC
CC
OUT
through the opto-
3901 F02
0V
3901f