LTC3705

13

3705fb

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:

winding connected to the LTC3706 must be able to with-

stand volt-seconds equal to:

(– )

Vs

V

f

MAX

CC

=

2

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

absolute maximum of 15V and the gate-drive undervoltage

lockout of 7V.

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

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-

connected to the corresponding pins of all the slaves. The

master is designated by connection of the frequency set

the slaves. For the slaves the NDRV pin is connected to the

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

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.

POWER

TRANSFORMER

2.2

µF

PRIMARY

SECONDARY

BAS21

BAS21

1mH

LTC3705

3705 F03

NDRV

GND

AUXILIARY

Figure. 3. Auxiliary Supply for Primary-Side Control