LTC3901_15 Datasheet, PDF (12/16 Pages) Linear Technology – Secondary Side Synchronous Driver for Push-Pull and Full-Bridge Converters

LTC3901
APPLICATIO S I FOR ATIO
MOSFET Selection
The required MOSFET R_DS(ON) should be determined
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_CC supply is ramping up. The
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_CC reaches
4.1V.
The LTC3901 drivers dissipate power while the MOSFETs
are switching. The power dissipation increases with switch-
ing frequency, PV_CC, and size of the MOSFETs. To calcu-
late the driver dissipation, the total gate charge Q_G is used.
This parameter is found on the MOSFET manufacturers’
data sheets.
The power dissipated in each LTC3901 MOSFET driver is:
P_DRIVER = Q_G • PV_CC • f_SW
where f_SW is the switching frequency of the converter.
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_VCC bypass capacitor as close as
possible to the V_CC and GND pins. Connect the 4.7µF
C_PVCC bypass capacitor as close as possible to the PV_CC
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_CC regulator circuit away
from the high current path of ME, MF and T1.
4. Place the timer capacitor, C_TMR as close as possible to
the LTC3901.
5. Keep the PCB trace from the resistors R_CSX1, R_CSX2 and
R_CSX3 to the LTC3901 CSX⁺/CSX^– pins as short as pos-
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.
3901f
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