Datasheet
Functional check AN4164
20/38 Doc ID 023660 Rev 1
6 Functional check
6.1 Soft-start
At startup, the current limitation value reaches IDLIM after an internally fixed 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 28
and
29
.
6.2 Overload protection
In the case of overload or short-circuit (see
Figure 30
), the drain current reaches the IDLIM
value (or the one set by the user through the RLIM resistor). In every cycle where this
condition is met, a counter is incremented; if it is maintained continuously for the time t
OVL
(50 msec typical, internally fixed), the overload protection is tripped, the power section is
turned off and the converter is disabled for a t
RESTART
time (1 second typ.). 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 31
). This ensures restart attempts of the converter with
low repetition rate, so that it works safely with extremely low power throughput and avoids
the IC overheating in the case of repeated overload events.
Furthermore, every time the protection is tripped, the internal soft-startup function is invoked
(
Figure 32
), in order to reduce the stress on the secondary diode.
After the short removal, the IC resumes normal working. If the short is removed during t
SS
or
t
OVL
, i.e. before the protection tripping, 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 must wait for the t
RESTART
period to
elapse before switching is resumed (
Figure 33
).
Figure 28. Soft-start @ startup Figure 29. Soft-start @ startup (zoom)
AM13094v1
AM13095v1