LTC3725
9
3725fa
Self-Starting Architecture
The LTC3725 is combined with the LTC3706 to form a
complete self-starting DC isolated power supply. When
power is first applied, and when V
CC
for the LTC3725 is
above the appropriate threshold, the LTC3725 begins
open-loop operation using its own internal oscillator.
Power is supplied to the secondary by switching the gate
driver with a gradually increasing duty cycle as controlled
by the rate of rise of the voltage on the SSFLT pin. A peak
detector power supply for the LTC3706 allows it to begin
operation even for small duty cycles. Once adequate
voltage is available for the LTC3706, it provides duty cycle
information and gate drive bias power using the pulse
transformer as shown in Figure 1. The LTC3725 detects
the appearance of this signal and transfers control of the
gate drivers to the LTC3706. Simultaneously, the LTC3725
also enables the on-chip rectifier and turns off the linear
regulator.
Alternately, when the LTC3725 is used as a standalone
primary-side controller, the gradually increasing duty cycle
powers up a secondary-side reference and optoisolator and
feedback is accomplished when the output of the
optoisolator begins pulling down in the FB/IN
+
pin.
Soft-Start and Fault
These two functions are implemented using the SSFLT
pin. (This pin is also used for linear regulator timeout as
described in the following section.)
Initiating soft-start requires that: 1) the gate drive
undervoltage (UVGD) goes low meaning that adequate
voltage is available on the V
CC
pin (7.4V for the linear
regulator or 13.4V for the trickle charger) and 2) the input
undervoltage (UVV
IN
) goes low meaning that the voltage
on the UVLO pin has reached the 1.242V rising threshold.
During soft-start, the LTC3725 gradually charges the soft-
start capacitor to ramp up the converter duty cycle. Soft-
start is over when the voltage on the SSFLT pin reaches 2.8V.
In normal operation, at some point before this, the LTC3725
makes a transition to controlling duty cycle using closed-
loop regulation of the converter output voltage.
The SSFLT pin is also used to indicate a fault. The LTC3725
recognizes faults from four origins: 1) an overcurrent fault
caused by the current sense voltage on the IS pin exceed-
ing the 300mV overcurrent threshold, 2) an input
undervoltage fault caused by the UVLO pin falling below
the 1.226V falling threshold, 3) a gate drive undervoltage
fault caused by the voltage on the V
CC
pin falling below the
7V threshold, or 4) loss of the gate drive encoding signal
from the LTC3706.
Upon sensing a fault, the LTC3725 immediately turns off
the gate drive and indicates a fault by quickly pulling the
voltage on the SSFLT pin to within 1.3V of the voltage on
the V
CC
pin. After indicating the fault, the LTC3725 quickly
ramps down the voltage on the SSFLT pin to approxi-
mately 2.8V. Then, to allow complete discharge of the
secondary-side circuit, the LTC3725 slowly ramps down
the voltage on the SSFLT pin to about 200mV. The LTC3725
then attempts a restart.
Linear Regulator Timeout
The thermal rating of the linear regulator’s external NMOS
often cannot allow it to indefinitely supply bias current to
the primary-side gate drives. The LTC3725 has a linear
regulator timeout mechanism that also uses the SSFLT
capacitor.
As described in the prior section, soft-start is over once the
voltage on the SSFLT pin reaches 2.8V. However, the
SSFLT capacitor continues to charge and the linear regu-
lator is turned off when the voltage on the SSFLT pin
reaches 3.9V. The “Applications Information” section de-
scribes linear regulator timeout in more detail.
Volt-Second Limit
The volt-second limit ensures that the power transformer
core does not saturate for any combination of duty cycle
and input voltage. The input of an R-C integrator is
connected to V
IN
and its output is connected to the V
SLMT
pin. While the gate drive is “off,” the LTC3725 grounds the
V
SLMT
pin. When the gate drive is turned “on” the V
SLMT
pin is released and the capacitor is allowed to charge in
proportion to V
IN
. If the capacitor voltage on the V
SLMT
pin
OPERATIO
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