LTC3901_15 Datasheet, PDF (6/16 Pages) Linear Technology – Secondary Side Synchronous Driver for Push-Pull and Full-Bridge Converters

LTC3901
BLOCK DIAGRA
SYNC 9
S⁺
+1.4V
–1.4V
S–
CSE⁺ 6
10.5mV
CSE^– 5 _– +
CSF⁺ 11
10.5mV
CSF^– 12 _{– +}
TIMER 7
ISE
Z_CSE
11V
ISF
Z_CSF
11V
TMR
SYNC⁺
SYNC^–
R1
180k
M_TMR
Z_TMR
0.5 • V_CC
R2
45k
DISABLE
DRIVER
UVLO
SYNC
AND
DRIVER
LOGIC
TIMER
RESET
16 V_CC
1 PV_CC
3 ME
4 PGND
14 MF
13 PGND
8 10
3901 BD
GND GND
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
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.
DRVA
DRVB
SDRA
SDRB
SYNC
0V
ME
MF
3901 F02
Figure 2. Push-Pull Converter Switching Waveforms
3901f