Datasheet

Table Of Contents
Operation description VIPER37
22/35 Doc ID 022218 Rev 1
Figure 26. Timing diagram: soft-start
8.4 Power down operation
At converter power down, the system loses regulation as soon as the input voltage is so low
that the peak current limitation is reached. The V
DD
voltage drops and when it falls below
the V
DDoff
threshold (see
Tabl e 7
) the Power MOSFET is switched OFF, the energy transfers
to the IC interrupted and consequently the V
DD
voltages decrease,
Figure 25
. Later, if the
V
IN
is lower than V
DRAIN_START
(see
Tabl e 7
), the startup sequence is inhibited and the
power down completed. This feature is useful to prevent the converter’s restart attempts and
ensures monotonic output voltage decay during the system power down.
8.5 Auto-restart operation
If, after a converter power down, the V
IN
is higher than V
DRAIN_START,
the startup sequence
is not inhibited and is activated only when the V
DD
voltage drops below the V
DD(RESTART)
threshold (see
Tabl e 7
). This means that the HV startup current generator restarts the V
DD
capacitor charging only when the V
DD
voltage drops below V
DD(RESTART)
. The scenario
described above is, for instance, a power down because of a fault condition. After a fault
condition, the charging current I
DDch
is 0.6 mA (typ.) instead of the 3 mA (typ.) of a normal
startup converter phase. This feature, together with the low V
DD(RESTART)
threshold,
ensures that, after a fault, the restart attempts of the IC have a very long repetition rate and
the converter works safely with extremely low power throughput.
Figure 27
shows the IC
behavior after a short-circuit event.
FBlin
V
V
FBolp
I
DRAIN
V
FB
t
ss
t
t
I
Dlim