RAMP GENERATION CIRCUITRY
An important circuit feature incorporated into the design
of the UCC3570 is a unique PWM ramp generator
which combines voltage feed-forward, duty-cycle
clamping, and fixed frequency operation. While all of
these characteristics have individually been used before,
combining them into a control which will automatically
and linearly provide an open-loop PWM correction
for large input voltage variations, and allow a maximum
duty-cycle clamp to be user-set over a 20%-to-80%
range, while maintaining constant switching frequency,
was a challenge met with the circuitry shown separately
in Figure 2.