NCP1256
www.onsemi.com
19
The double hiccup is operating regardless of the
brown−out level. However, when the internal comparator
toggles indicating that the controller recovers from a
brown−out situation (the input line was ok, then too low and
back again to normal), the double hiccup is interrupted and
the controller re−starts to the next available V
cc
peak.
Figure 42 displays the resulting waveform: the controller is
protecting the converter against an overload. The mains
suddenly went down, and then back again at a normal level.
Right at this moment, the double hiccup logic receives a
reset signal and ignores the next hiccup to immediately
initiate a re−start signal.
8.8 V
18 V
()
cc
Vt
()
DRV
Vt
Re−start
Brown−out
recovery
1
2
1
2
1
BONOK
BOK BOK
Figure 42. The hiccup latch is reset when a brown−out transition is detected to shorten the re−start time
Latched Short Circuit Protection with Pre Short
In some applications, the controller must be fully latched
in case of an output short circuit presence. In that case, you
would select options A in the controller list. When the error
flag is asserted, meaning the controller is asked to deliver its
full peak current, upon timer completion, the controller
latches off: all pulses are immediately stopped and V
cc
hiccups between the two levels, VCC
ON
and VCC
(min)
.
However, in presence of a small V
cc
capacitor, it can very
well be the case where the stored energy does not give
enough time to let the timer elapse before V
cc
touches
UVLO. When this happens, the latch is not acknowledged
since the timer countdown has been prematurely aborted. To
avoid this problem, NCP1256 (with latched−OCP option)
combines the error flag assertion together with the UVLO
flag to confirm a pre−short situation: upon start up, as
maximum power is asked to increase V
out
, the error flag is
temporarily raised until regulation is met. If during the time
the flag is raised an UVLO event is detected, the part latches
off immediately. When latched, V
cc
hiccups between the
two levels, VCC
ON
and VCC
(min)
until a reset occurs
(Brown−out event or V
cc
cycled down below VCC
reset
). In
normal operation, if a UVLO event is detected for any
reason while the error flag is not asserted, the controller will
naturally resume operations in a double hiccup mode.
Details of this behavior are given in Figure 43.
