Datasheet
AN4165 Functional check
Doc ID 023661 Rev 1 21/37
10 Functional check
10.1 Soft-start
At startup the current limitation value reaches IDLIM after an internally set time, t
SS
, whose
typical value is 8.5 msec. This time is divided into 16 time intervals, each corresponding to a
current limitation step progressively increasing. In this way the drain current is limited during
the output voltage increase, therefore reducing the stress on the secondary diode. The soft-
start phase is shown in
Figure 24
and
25
.
10.2 Overload protection
In case of overload or short-circuit (see
Figure 26
), the drain current reaches the IDLIM
value (or the one set by the user through the RLIM resistor). Every cycle that this condition
is met, a counter is incremented. If the fault is maintained continuously for the time t
OVL
(50 msec typical, set internally), the overload protection is tripped, the power section is
turned off and the converter is disabled for a t
RESTART
time (1 s typical). After this time has
elapsed, the IC resumes switching and, if the short is still present, the protection occurs
indefinitely in the same way (
Figure 27
). This ensures restart attempts of the converter with
low repetition rate, so that it works safely with extremely low power throughput and avoids
overheating of the IC in case of repeated overload events.
Moreover, every time the protection is tripped, the internal soft-start function (
Figure 25
) is
implemented, in order to reduce the stress on the secondary diode.
After the short removal, the IC resumes working normally. If the short is removed during t
SS
or t
OVL
, i.e. before the protection is tripped, the counter is decremented on a cycle-by-cycle
basis down to zero and the protection is not tripped.
If the short-circuit is removed during t
RESTART
, the IC waits for the t
RESTART
period to elapse
before resuming switching (
Figure 29
).
Figure 24. Soft-start at startup Figure 25. Soft-start at startup (zoom)
AM13379v1
AM13380v1