NCP1237 ONSEMI [ON Semiconductor], NCP1237 Datasheet - Page 17

• Manufacturer Part Number: NCP1237
• Description: Fixed Frequency Current Mode Controller for Flyback Converters
• Manufacturer: ONSEMI [ON Semiconductor]

off and starts, or restarts, when V
soft−start is applied: the current sense setpoint is linearly
increased from 0 (the minimum level can be higher than 0
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
because the controller is blind during the LEB blanking
Each time the controller is starting (i.e. the controller was
Under some conditions, like a winding short−circuit for
Soft-start ramp
V
CS Setpoint
V
V
FB(fault)
ILIMI
ILIM
V
FB
CC
reaches V
over soft-start
V
CC(on)
FB
Figure 12. Soft−Start
http://onsemi.com
takes
ILIM
), a
,
17
because of the LEB and propagation delay) until it reaches
V
imposes a setpoint lower than the one imposed by the
soft−start (the 2 comparators outputs are OR’ed).
time. Dangerously high current can grow in the system if
nothing is done to stop the controller. In order to protect
against this, an additional comparator is included, that
senses when V
as this comparator toggles, the controller immediately enters
the protection mode (latched or autorecovery according to
the chosen option).
ILIM
(after a duration of t
t
SSTART
CS
reaches V
CS(stop)
SSTART
Time
Time
Time
( = 1.5 x V
), or until the FB loop
ILIM
). As soon