LTC3705_15 Datasheet, PDF (10/20 Pages) Linear Technology – 2-Switch Forward Controller and Gate Driver

LTC3705
U
OPERATIO
the volt-second limit circuit. The duty cycle is always
limited to 50% 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 LTC3705 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 top
switch, the transformer primary, and the bottom switch is
sensed by a resistor connected between the source of the
bottom switch and GND. If the voltage across this resistor
exceeds 300mV, the LTC3705 initiates a fault.
Bootstrap Refresh
The LTC3705 incorporates a unique bootstrap refresh
circuit to ensure that the bootstrap supply (BOOST) for the
top switch has adequate voltage for operation at low duty
cycles. Therefore, the LTC3705 does not require a
undervoltage lockout for the bootstrap supply and a po-
tential source of unexpected shutdowns is eliminated.
Voltage Feedforward
The LTC3705 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 50% 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 takes 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 50% 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 LTC3705 is used in standalone primary-side
mode, feedback is provided by an optoisolator connected
to the FB/IN⁺ pin. The LTC3705 has a built optoisolator
bias circuit which eliminates the need for external
components.
APPLICATIO S I FOR ATIO
UVLO
The UVLO pin is connected to a resistive voltage divider
connected to V_IN as shown in Figure 2. The voltage
threshold on the UVLO pin for V_IN rising is 1.242V. To
introduce hysteresis, the LTC3705 draws 4.9µA from the
UVLO pin when V_IN is rising. The hysteresis is therefore
user adjustable and depends on the value of R1. The UVLO
threshold for V_IN rising is:
V_{IN(UVLO, RISING)}
=
(1.242V)R1^{+ R2}
R2
+ R1(4.9µA)
10
The LTC3705 also has 16mV of voltage hysteresis on the
UVLO pin so that the UVLO threshold for V_IN falling is:
V_{IN(UVLO, FALLING)}
=
(1.226V)R1^{+ R2}
R2
To implement external Run/Stop control, connect a small
NMOS to the UVLO pin as shown in Figure 2. Turning the
NMOS on grounds the UVLO pin and prevents the LTC3705
from running.
3705fb