LTC3901 Linear Technology, LTC3901 Datasheet - Page 12

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

APPLICATIO S I FOR ATIO
MOSFET Selection
The required MOSFET R
based on allowable power dissipation and maximum re-
quired output current.
The MOSFETs body diodes conduct during the power-up
phase, when the LTC3901 V
ME and MF signals stay low and the inductor current flows
through the body diodes. The body diodes must be able to
handle the load current during start-up until V
4.1V.
The LTC3901 drivers dissipate power while the MOSFETs
are switching. The power dissipation increases with switch-
ing frequency, PV
late the driver dissipation, the total gate charge Q
This parameter is found on the MOSFET manufacturers’
data sheets.
The power dissipated in each LTC3901 MOSFET driver is:
where f
LTC3901
12
P
DRIVER
SW
is the switching frequency of the converter.
= Q
G
• PV
CC
U
, and size of the MOSFETs. To calcu-
CC
• f
U
DS(ON)
SW
CC
supply is ramping up. The
should be determined
W
CC
U
G
reaches
is used.
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC3901:
1. Connect the 1µF C
possible to the V
C
and PGND pins.
2. Connect the two MOSFET drain terminals directly to the
transformer. The two MOSFET sources should be as close
together as possible.
3. Keep the timer, SYNC and V
from the high current path of ME, MF and T1.
4. Place the timer capacitor, C
the LTC3901.
5. Keep the PCB trace from the resistors R
R
sible. Connect the other ends of the resistors directly to the
drain and source of the MOSFET.
6. Make the connection between GND and PGND right at
the LTC3901 pins.
PVCC
CSX3
bypass capacitor as close as possible to the PV
to the LTC3901 CSX
CC
and GND pins. Connect the 4.7µF
VCC
bypass capacitor as close as
+
/CSX
TMR
CC
–
as close as possible to
regulator circuit away
pins as short as pos-
CSX1
, R
CSX2
and
3901f
CC