QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 542
DUAL PHASE, DUAL OUTPUT SYNCHRONOUS BUCK CONVERTER
3
OPTIONAL BIAS SUPPLY
The DC542 also contains a footprint for a 5.5V
boost converter bias supply whose output is
connected to EXTVCC. The input of this bias
supply can be connected to one of the following:
1) VIN, if VIN<5V
2) VOUT1 for VOUT1 <5V and VIN>5V.
The latter connection will provide higher effi-
ciency. If this bias supply is stuffed, please keep
the following points in mind:
1) If the input voltage of the optional bias supply
is connected to VIN, then the board will need
to be slightly modified to ensure that the
voltage on the EXTVCC pin of the
LTC3728EG does not exceed the voltage on
the VIN pin. These are the modifications:
a) Remove resistor R8.
b) Tie the (+) end of C3 to the (+) end of
C21.
2) If the input voltage of the optional bias supply
is connected to output #1, precautions need
to be taken to make sure that the voltage on
the EXTVCC pin of the LTC3728EG does not
exceed the voltage on the VIN pin during
turn-on of the main converter. Follow the
steps below:
a) Put jumper JP6 in the “0” position to tie
the RUN1 signal to ground.
b) Turn-on the main input voltage and set to
desired level.
c) Put jumper JP6 in the “1” position to turn-
on output #1.
3) If a different power MOSFET configuration is
used or the switching frequency is increased,
then be sure to measure the current flowing
into the EXTVCC pin and make sure the cur-
rent will not exceed the maximum output cur-
rent of the bias supply. At 3.3V input, the
maximum output current of the bias supply is
200mA.
For further assistance, contact the factory.
