LTC3725 Datasheet, PDF (14/20 Pages) Linear Technology – Single-Switch Forward Controller and Gate Driver

LTC3725
APPLICATIO S I FOR ATIO
Auxiliary Supply
When used with the LTC3706, the LTC3725 does not
require an auxiliary supply to provide primary-side gate-
drive bias current. After start-up, primary-side gate drive
current is provided by the LTC3706 through a small pulse
transformer and the LTC3725’s on-chip rectifier.
However, when used as a standalone primary-side con-
troller, the LTC3725 may require a conventional gate-drive
bias supply as shown in Figure 3. The bias supply must be
designed to keep the voltage on the V_CC pin between the
absolute maximum of 15V and the gate-drive undervoltage
lockout of 7V.
The auxiliary supply is connected in parallel with V_CC. The
linear regulator maintains V_CC at 8V. If the auxiliary supply
produces more than 8V, it turns off the external NMOS
before the LTC3725 can time out the linear regulator. If the
auxiliary supply produces less than 8V, the linear regulator
times out and then the voltage on the V_CC pin declines to
the voltage produced by the auxiliary supply.
Slave Mode Operation
When the LTC3725 is paired with the LTC3706, multiple
pairs can be used to form a PolyPhase converter. In
PolyPhase operation, one LTC3725 becomes the “master”
while the remainder become “slaves.” The master con-
trols start-up in the same manner as for the single-phase
converter, while the slaves do not begin switching until
receiving PWM information through their own pulse trans-
former from their corresponding LTC3706. To synchro-
nize operation, the SSFLT and V_CC pins of the master are
connected to the corresponding pins of all the slaves. The
master is designated by connection of the frequency set
resistor to the FS/IN^– pin while this resistor is omitted from
the slaves. For the slaves the NDRV pin is connected to the
V_CC pin. See the following section on PolyPhase Applica-
tions for more detail.
PolyPhase Applications
Figure 4 shows the basic connections for using the LTC3725
and LTC3706 in PolyPhase applications. One of the phases
is always identified as the “master,” while all other phases
are “slaves.” For the LTC3725 (primary side), the master
performs the open-loop start-up and supplies the initial
V_CC voltage for the master and all slaves. The LTC3725
slaves are put into that mode by omitting the resistor on
FS/IN–. The LTC3725 slaves simply stand by and wait for
PWM signals from their respective pulse transformers.
Since the SSFLT pins of master and slave LTC3725s are
interconnected, a FAULT (overcurrent, etc.) on any one of
the phases will perform a shutdown/restart on all phases
together.
For the LTC3706, the master performs soft-start and
voltage-loop regulation by driving all slaves to the same
current as the master using the I_TH pins. Faults and
shutdowns are communicated via the interconnection of
the RUN/SS pins. The LTC3706 is put into slave mode by
tying the FB pin to V_CC.
V_IN
NDRV
LTC3725
V_CC
GND
POWER
TRANSFORMER
PRIMARY
WINDING N_P
1mH
BAS21
SECONDARY
WINDING N_S
2.2µF _BAS21
AUXILIARY
WINDING N_A
3725 F03
Figure. 3. Auxiliary Supply for Primary-Side Control
3725f
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