SCY99194 Datasheet, PDF (29/40 Pages) ON Semiconductor – Fixed Frequency Current Mode Controller

SCY99194
Figure 56. Soft−Start Feature
Under some conditions, like a winding short−circuit for
instance, not all the energy stored during the on time is
transferred to the output during the off time, even if the on
time duration is at its minimum (imposed by the propagation
delay of the detector added to the LEB duration). As a result,
the current sense voltage keeps on increasing above V_ILIM,
because the controller is blind during the LEB blanking
time. Dangerously high current can grow in the system if
nothing is done to stop the controller. That’s what the
additional comparator, that senses when the current sense
voltage on CS pin reaches V_CS(stop) ( = 1.5 x V_ILIM ), does:
as soon as this comparator toggles, the controller
immediately enters the protection mode.
In order to allow the SCY99194 to operate in CCM with
a duty cycle above 50%, the fixed slope compensation is
internally applied to the current−mode control. The slope
appearing on the internal voltage setpoint for the PWM
comparator is −32.5 mV/ms typical for the 65 kHz version.
The slope compensation can be observable as a value of the
peak current at CS pin.
The internal slope compensation circuitry uses a sawtooth
signal synchronized with the internal oscillator is subtracted
from the FB voltage divided by K_FB.
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