VIPER22AS datasheet(10/15 Pages) STMICROELECTRONICS

10 / 15 page

VIPer22ADIP / VIPer22AS

10/15

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 tss, the device

consumes some energy from the VDD 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 VDD pin allows the

VIPer22A to reset itself when VDD exceeds

VDDovp. 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 VDD to reach VDDoff, and then the device

resumes normal operation automatically.

Figure 11 : Start Up Sequence

IFB

VDDon

VOUT

VDD

VDDoff

tss

Figure 12 : Overvoltage Sequence

VDS

VDDon

VDD

VDDoff

VDDovp

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