Datasheet

LTC3901
12
3901f
APPLICATIO S I FOR ATIO
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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.