VIPer12ADIP / VIPer12AS
Figure 11 :
Start Up Sequence
V
DD
V
DDon
Figure 12 :
Overvoltage Sequence
V
DD
V
DDovp
V
DDoff
tss
V
DDon
V
DDoff
t
I
FB
t
V
DS
t
V
OUT
t
t
the regulation point where the secondary loop
begins to send a current in the optocoupler. At this
point, the converter enters a regulated operation
where the FB pin receives the amount of current
needed to deliver the right power on secondary
side.
This sequence is shown in figure 11. Note that
during the real starting phase t
ss
, the device
consumes some energy from the V
DD
capacitor,
waiting for the auxiliary winding to provide a
continuous supply. If the value of this capacitor is
too low, the start up phase is terminated before
receiving any energy from the auxiliary winding
and the converter never starts up. This is illustrated
also in the same figure in dashed lines.
OVERVOLTAGE THRESHOLD
An overvoltage detector on the V
DD
pin allows the
VIPer12A to reset itself when V
DD
exceeds
V
DDovp
. This is illustrated in figure 12, which shows
the whole sequence of an overvoltage event. Note
that this event is only latched for the time needed
by V
DD
to reach V
DDoff
, and then the device
resumes normal operation automatically.
10/15
STMICROELECTRONICS [ STMICROELECTRONICS ]
