VIPER25HD datasheet(26/40 Pages) STMICROELECTRONICS

26 / 40 page

Operation description

VIPER25

26/40

Doc ID 15585 Rev 4

Figure 32.

ZCD pin typical external configuration

7.11

Overvoltage protection (OVP)

The VIPER25 has integrated the logic for the monitor of the output voltage using as input

signal the voltage VZCD during the OFF time of the power MOSFET. This is the time when

the voltage from the auxiliary winding tracks the output voltage, through the turn ratio NAUX /

NSEC.

The ZCD pin has to be connected to the auxiliary winding through the diode DOVP and the

resistors ROVP and RLIM as shows the Figure 32 on page 26. When, during the OFF time,

the voltage VZCD exceeds, four consecutive times, the reference voltage VOVP (reported on

Table 8 on page 8

) the over voltage protection will stop the power MOSFET and the

converter enters the auto-restart mode.

In order to bypass the noise after the turn off of the power MOSFET, the voltage VZCD is

sampled inside a short window after the time TSTROBE, see the Table 8 on page 8 and

Figure 33 on page 27

. The sampled signal, if higher than VOVP, trigger the internal OVP

digital signal and increments the internal counter. The same counter is reset every time the

signal OVP is not triggered in one oscillator cycle.

Referring to the Figure 32, the resistors divider ratio kOVP will be given by:

Equation 5

OVP

AUX

SEC

--------------

OUTOVP

DSEC

() V

DAUX

–

⋅

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