LTC3705 LINER [Linear Technology], LTC3705 Datasheet - Page 10

• Manufacturer Part Number: LTC3705
• Description: 2-Switch Forward Controller and Gate Driver
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC3705
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
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
UVLO
The UVLO pin is connected to a resistive voltage divider
connected to V
threshold on the UVLO pin for V
introduce hysteresis, the LTC3705 draws 4.9µA from the
UVLO pin when V
user adjustable and depends on the value of R1. The UVLO
threshold for V
10
V
IN
IN UVLO RISING
.
(
,
IN
IN
rising is:
U
IN
)
U
as shown in Figure 2. The voltage
=
is rising. The hysteresis is therefore
( .
1 242
U
V
)
R
1
R
IN
+
W
2
R
rising is 1.242V. To
2
+
R
1 4 9
( .
U
µ
A
)
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
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
start, the duty cycle is determined by comparison of the
voltage on the FB/IN
Optoisolator Bias
When the LTC3705 is used in standalone primary-side
mode, feedback is provided by an optoisolator connected
to the FB/IN
bias circuit which eliminates the need for external
components.
The LTC3705 also has 16mV of voltage hysteresis on the
UVLO pin so that the UVLO threshold for V
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.
V
IN UVLO FALLING
(
,
+
pin. The LTC3705 has a built optoisolator
)
+
+
=
pin to the triangle wave.
pin. After the conclusion of soft-
( .
1 226
V
)
R
IN
1
R
. The charging current
+
2
R
2
IN
falling is:
3705fb