VIPER25HD datasheet(23/40 Pages) STMICROELECTRONICS

23 / 40 page

VIPER25

Operation description

Doc ID 15585 Rev 4

23/40

7.8

Double blanking time

The blanking time, TBLANK, can have two different values: the lower one is 2,5 μs (typical

value) and the higher one is 6,3

μs (typical value). The value is linked to the voltage V

ZCD,

sampled during the time TSTROBE defined as for the over voltage protection (see the

relevant Section 7.11 on page 26). The time TBLANK has the lower value if is detected VZCD

< 1V or it has the higher value if is detected VZCD > 1V, refer to Table 8 on page 8 and

Figure 30 on page 23

The higher value of the blanking time is normally activated during the start-up phase or in

case of output short circuit; when the output voltage of the converter is quite lower than the

regulated value. In this condition can happens that during the demagnetization of the

transformer, the VZCD is very close to the arming and triggering thresholds (VZCDAth and

VZCDTth) and the ZCD circuit can be erroneously trigged, leading the system to work at

higher frequency and in continuous mode. This false trigger is inhibited by the selection of

the higher value of TBLANK when VZCD is lower than 1 V.

During the normal operation, in steady state condition, the voltage VZCD during the

demagnetization is higher than 1V and the selected TBLANK value is the lower one. The

Figure 30

shows the typical waveforms during the power up and the linked TBLANK selection.

Figure 30.

Double TBLANK timing diagram

Vaux

ZCD

(pin 3)

1. 5

0. 5

BL ANK

ST RO B E

6.3μs

2.5μs

Delay

Mosfet switched on by the starter

Quasi Resonant Operation

lim

OSC

0.8V

0. 6V

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