LTC3725_15 Datasheet, PDF (10/20 Pages) Linear Technology – Single-Switch Forward Controller and Gate Driver

LTC3725
U
OPERATIO
exceeds 1.26V the gate drive is immediately turned “off.”
Note that this is not considered a fault condition and the
LTC3725 can run indefinitely with the switch duty cycle
being determined by the volt-second limit circuit. The duty
cycle is always limited to 75% to ensure that the power
transformer flux always has time to reset before the start
of the next cycle.
In an alternate application, the volt-second limit can be
used for open-loop regulation of the output against changes
in V_IN.
Current Limit
Current limit for the LTC3725 is principally a safety feature
to protect the converter and is not part of a control
function. The current that flows in series through the
transformer primary and the switch is sensed by a resistor
connected between the source of the switch and GND. If
the voltage across this resistor exceeds 300mV, the
LTC3725 initiates a fault.
Voltage Feedforward
The LTC3725 uses voltage feedforward to properly modu-
late the duty cycle as a function of the input voltage. For
secondary-side control with the LTC3706, voltage
feedforward is used during start-up only. The duty cycle
during start up is determined by comparison of the voltage
on the SSFLT pin to a 75% duty cycle triangle wave with
an amplitude of 2V. To implement voltage feedforward, the
charging current for the soft-start capacitor is reduced in
proportion to the input voltage. As a result, the initial rate
of rise of the converter output voltage is held approxi-
mately constant regardless of the input voltage. At some
point during start-up, the LTC3706 begins to switch the
pulse transformer and take over the soft-start.
For operation with standalone primary-side control and
optoisolator feedback, voltage feedforward is used during
both start-up and normal operation. The duty cycle is
determined by using a 75% duty cycle triangle wave with
an amplitude equal to 66% of the voltage on the UVLO pin
which is, in turn, proportional to V_IN. The charging current
for the soft-start capacitor is a constant 5.2µA. During
soft-start, the duty cycle is determined by comparing the
voltage on the SSFLT pin to the triangle wave. Soft-start is
concluded when the voltage on the SSFLT pin exceeds the
voltage on the FB/IN⁺ pin. After the conclusion of soft-
start, the duty cycle is determined by comparison of the
voltage on the FB/IN⁺ pin to the triangle wave.
Optoisolator Bias
When the LTC3725 is used in standalone primary-side
mode, feedback is provided by an optoisolator connected
to the FB/IN⁺ pin. The LTC3725 has a built optoisolator
bias circuit which eliminates the need for external
components.
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