The power sequencing features of the UCC3570 are highly
optimized for off-line operation beginning with the fact that the
BiCMOS process yields very low operating currents for this
device - one milliamp to run and less than 100 microamps
prior to starting. In addition, there are two pins
which sense the line voltage and provide turnon/
turn-off functions. VCC provides chip power
and has a UVLO circuit which sets turn-on at 13
volts with a 4-volt hysteresis. VFWD is primarily intended
to provide voltage feed-forward by adjusting
the ramp slope, but this pin is also monitored
by over and under-voltage sensing comparators
which terminate output pulses if the line voltage
is outside its defined operating range. Because
VCC will normally require an energy storage capacitor,
it will respond more slowly than VFWD
and since both must be above a minimum for operation,
turn-on will typically be initiated by VCC
while a collapsing voltage on VFWD will provide a
faster turn-off. Turn-on is further controlled by a
Soft-Start circuit which minimizes both starting
currents and output voltage overshoot.