Datasheet
LTC3901
6
3901f
BLOCK DIAGRA
W
APPLICATIO S I FOR ATIO
WUU
U
Figure 2. Push-Pull Converter Switching Waveforms
SYNC
AND
DRIVER
LOGIC
ISE
S
–
S
+
SYNC
+
SYNC
–
TMR
–1.4V
+1.4V
DISABLE
DRIVER
UVLO
Z
TMR
0.5 • V
CC
Z
CSE
11V
7
5
6
9
3
16
SYNC
CSE
+
CSE
–
ISF
Z
CSF
11V
12
11
CSF
+
CSF
–
TIMER
M
TMR
R1
180k
R2
45k
TIMER
RESET
ME
PGND
GND GND
MF
PGND
V
CC
1
PV
CC
14
4
13
108
3901 BD
+
–
+
–
10.5mV
10.5mV
DRVA
DRVB
SDRA
SDRB
SYNC
ME
3901 F02
MF
0V
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
the secondary side synchronous switches driven by the
LTC3901’s ME and MF output. Inductor L1 and capacitor
C
OUT
form the output filter, providing DC output voltage to
the load. The feedback path from V
OUT
through the opto-
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.