LTC3706_15 Datasheet, PDF (13/22 Pages) Linear Technology – Secondary-Side Synchronous Forward Controller with PolyPhase Capability

LTC3706
OPERATION
Slave Mode Operation
When two or more LTC3706 devices are used in PolyPhase
systems, one device becomes the “master” controller, while
the others are used as “slaves.” Slave mode is activated
by connecting the FB pin to V_CC. In this mode, the ITH pin
becomes a high impedance input, allowing it to be driven by
the master controller. In this way, equal inductor currents
are established in each of the individual phases. Also, in
slave mode the soft-start charge/discharge currents are
disabled, allowing the master device to control the charging
and discharging of the soft-start capacitor.
APPLICATIONS INFORMATION
Start-Up Considerations
In self-starting applications, the LTC3705 will initially begin
the soft-start of the converter in an open-loop fashion.
After bias is obtained on the secondary side, the LTC3706
assumes control and completes the soft-start interval. In
order to ensure that control is properly transferred from the
LTC3705 (primary-side) to the LTC3706 (secondary-side),
it is necessary to limit the rate of rise on the primary-side
soft-start ramp so that the LTC3706 has adequate time to
wake up and assume control before the output voltage gets
too high. This condition is satisfied for many applications
if the following relationship is maintained:
C_SS,SEC ≤ C_{SS PRI}
However, care should be taken to ensure that soft-start
transfer from primary-side to secondary-side is completed
well before the output voltage reaches its target value. A
good design goal is to have the transfer completed when
the output voltage is less than one-half of its target value.
Note that the fastest output voltage rise time during pri-
mary-side soft-start mode occurs with maximum input
voltage and minimum load current.
The open-loop start-up frequency on the LTC3705 is set
by placing a resistor from the FB/IN⁺ pin to GND. Although
the exact start-up frequency on the primary side is not
critical, it is generally good practice to set this approxi-
mately equal to the operating frequency on the secondary
side. The FS/IN^– start-up resistor for the LTC3705 may be
selected using the following:
f_PRI(Hz)
=
3.2 • 10¹⁰
R_FS/IN– + 10k
In the event that the secondary-side circuitry fails to
properly start up and assume control of switching, there
are several fail-safe mechanisms to help avoid overvoltage
conditions. First, the LTC3705 contains a volt-second
clamp that will keep the primary-side duty cycle at a level
that cannot produce an overvoltage condition. Second,
the LTC3705 contains a time-out feature that will detect
a FAULT if the LTC3706 fails to start up and deliver PWM
signals to the primary side. Finally, the LTC3706 has an
independent overvoltage detection circuit that will crowbar
the output of the DC/DC converter using the synchronous
MOSFET switch.
In the event that a short circuit is applied to the output of
the DC/DC converter prior to start-up, the LTC3706 will
generally not receive enough bias voltage to operate. In
this case, the LTC3705 will detect a FAULT for one of two
reasons: 1) the start-up time-out feature will be activated
since the LTC3706 never sends signals to the primary side
or 2) the primary-side overcurrent circuit will be tripped
because of current buildup in the output inductor. In either
case, the LTC3705 will initiate a shutdown followed by a
soft-start retry. See the LTC3705 data sheet for further
details.
3706fd
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