Datasheet

VIPER27 Operation descriptions
Doc ID 15133 Rev 5 17/31
7.4 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 Table 7 on page 6 ) the power MOSFET is switched OFF, the
energy transfers to the IC interrupted and consequently the V
DD
voltages decreases,
Figure 23 on page 16 . Later, if the V
IN
is lower than V
DRAIN_START
(see Table 7 on page 6 ),
the start up sequence is inhibited and the power down completed. This feature is useful to
prevent converter’s restart attempts and ensures monotonic output voltage decay during the
system power down.
7.5 Auto restart operation
If after a converter power down, the V
IN
is higher than V
DRAIN_START,
the start up sequence
is not inhibited and will be activated only when the V
DD
voltage drops down the
V
DD(RESTART)
threshold (see Table 7 on page 6 ). This means that the HV start up current
generator restarts the V
DD
capacitor charging only when the V
DD
voltage drops below
V
DD(RESTART)
. The scenario above described 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 start up converter phase. This feature together with the low
V
DD(RESTART)
threshold ensures that, after a fault, the restart attempts of the IC has a very
long repetition rate and the converter works safely with extremely low power throughput.
The
Figure 25 shows the IC behavioral after a short circuit event.
Figure 25. Timing diagram: behavior after short circuit
7.6 Oscillator
The switching frequency is internally fixed to 60 kHz or 115 kHz. In both case the switching
frequency is modulated by approximately ±4 kHz (60 kHz version) or ±8 kHz
(115 kHz version) at 250 Hz (typical) rate, so that the resulting spread-spectrum action
distributes the energy of each harmonic of the switching frequency over a number of side-
band harmonics having the same energy on the whole but smaller amplitudes.
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