LTC3765_15 Datasheet, PDF (15/24 Pages) Linear Technology – Active Clamp Forward Controller and Gate Driver

LTC3765
APPLICATIONS INFORMATION
Self-Starting Start-Up
When starting up, the LTC3765 begins switching in an
open-loop fashion to supply power to the secondary side
LTC3766. When the LTC3766 has adequate bias voltage
and has met other conditions for start-up, it begins sending
both duty cycle information and power through the pulse
transformer connected to the IN⁺/IN^– pins.
The LTC3765’s start-up switching frequency is set by a
resistor from FSUV to ground. Since the internal oscilla-
tor is only used in start-up, the frequency accuracy is not
critical; however, avoid setting the frequency excessively
low, as this will cause high currents in the transformer and
inductor. To minimize the impact on the transition due to
the duty cycle handoff from the LTC3765 to the LTC3766,
this frequency should be set to the approximately the same
frequency as the LTC3766. The frequency set resistor
(R_FS) value can be selected using the following equation:
R_FS
=
6.2 • 10⁹
f_SW
– 4.5k
Table 2 shows standard 5% resistor values of R_FS for
common switching frequencies.
Table 2. Standard R_FS Resistor Values for Common Frequencies
FREQUENCY
R_FS VALUE
150kHz
36k
200kHz
27k
250kHz
20k
275kHz
18k
350kHz
13k
The internal oscillator generates a ramp that is compared
with the voltage on the SSFLT pin to generate a duty cycle.
The internal oscillator has an offset that prevents switch-
ing until SSFLT reaches approximately 1V. When V_CC is
undervoltage or the RUN pin is below its threshold, the
SSFLT pin is internally grounded and the drivers therefore
do not switch.
When start-up conditions have been met, the SSFLT pin is
released and a current is sourced out of the pin to charge
an external capacitor connected from SSFLT to ground.
Initially, a 60µA current is sourced out of the pin; how-
ever, this current is reduced to approximately 4µA when
PG begins switching. The 60µA initial current reduces
delay due to charging the external capacitor to 1V, where
switching begins.
During the open-loop start-up, the output voltage rises
much more quickly at high line than at low line for a given
duty cycle ramp rate. This has the potential to overvolt-
age the output before the LTC3766 has begun switching,
particularly at no load. To avoid this situation, the 4µA
soft-start current is modulated by the RUN pin voltage,
which monitors V_IN through a resistive divider. When the
RUN pin voltage increases from 1.3V to 3.75V, the soft-
start current decreases from 4µA to 1.6µA.
As the external soft-start capacitor gradually charges
from 1V to 3V, the duty cycle increases linearly from 0%
to 70%. For SSFLT voltages above 3V, the duty cycle is
clamped at approximately 70% to allow for adequate active
clamp reset time. When the SSFLT voltage reaches 3.5V,
if duty cycle information has not been received at the IN⁺
and IN^– pins, the voltage is held and the linear regulator
is turned off. The PG and AG gate drivers will continue to
switch at 70% duty cycle and the V_CC supply will decrease
until it reaches its falling undervoltage lockout threshold.
At that point, the LTC3765 will fault, turn on the linear
regulator, and gradually reset the SSFLT capacitor for a
restart attempt.
For more information www.linear.com/LTC3765
3765fb
15