NCP1288
http://onsemi.com
24
Each time the controller is starting, i.e. the controller was
off and starts, or restarts, when V
CC
reaches V
CC(on)
, a
soft−start is applied: the current sense setpoint is linearly
increased from 0 (the minimum level can be higher than 0
because of the LEB and propagation delay) until it reaches
V
ILIM
(after a duration of t
SSTART
), or until the FB loop
imposes a setpoint lower than the one imposed by the
soft−start (the 2 comparators outputs are OR’ed).
Figure 49. Soft−Start
Time
V
FB
V
FB(fault)
Time
Soft-start ramp
V
ILIM
t
SSTART
Time
CS Setpoint
V
ILIMI
VFB takes
over soft-start
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. In order to protect
against this, an additional comparator is included, that
senses when V
CS
reaches V
CS(stop)
( = 1.5 x V
ILIM
). As soon
as this comparator toggles, the controller immediately enters
the protection mode (latched or autorecovery according to
the chosen option).
