LTC3705_15 Datasheet, PDF (13/20 Pages) Linear Technology – 2-Switch Forward Controller and Gate Driver

LTC3705
APPLICATIO S I FOR ATIO
Pulse Transformer
The pulse transformer that connects the LTC3706 to the
LTC3705 performs the dual functions of gate drive duty
cycle encoding and gate drive bias supply for the LTC3705
by way of the on-chip full-wave rectifier. The designs of the
LTC3705 and LTC3706 have been coordinated so that the
transformer turn ratio is:
N_LTC3705 = 2N_LTC3706
where N_LTC3705 is the number of turns in the winding
connected to the FB/IN⁺ and FS/IN^– pins of the LTC3705
and N_LTC3706 is the number of turns in the winding
connected to the PT⁺ and PT^– pins of the LTC3706. The
winding connected to the LTC3706 must be able to with-
stand volt-seconds equal to:
(V
–
s)_MAX
=
V_CC
2f
where V_CC is the maximum supply voltage for the LTC3706
and f is the operating frequency of the LTC3706.
Auxiliary Supply
When used with the LTC3706, the LTC3705 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 LTC3705’s on-chip rectifier.
However, when used as a standalone primary-side con-
troller, the LTC3705 may require a conventional gate-drive
bias supply as shown in Figure 3. The bias supply must be
V_IN
NDRV
LTC3705
V_CC
GND
POWER
TRANSFORMER
PRIMARY
WINDING N_P
1mH
BAS21
SECONDARY
WINDING N_S
2.2µF _BAS21
AUXILIARY
WINDING N_A
3705 F03
Figure. 3. Auxiliary Supply for Primary-Side Control
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 LTC3705 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 LTC3705 is paired with the LTC3706, multiple
pairs can be used to form a PolyPhase converter. In
PolyPhase operation, one LTC3705 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 LTC3705
and LTC3706 in PolyPhase applications. One of the phases
is always identified as the “master,” while all other phases
are “slaves.” For the LTC3705 (primary side), the master
performs the open-loop start-up and supplies the initial
V_CC voltage for the master and all slaves. The LTC3705
slaves are put into that mode by omitting the resistor on
FS/IN–. The LTC3705 slaves simply stand by and wait for
PWM signals from their respective pulse transformers.
Since the SSFLT pins of master and slave LTC3705s are
interconnected, a FAULT (overcurrent, etc.) on any one of
the phases will perform a shutdown/restart on all phases
together.
3705fb
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