VIPER35 datasheet(27/44 Pages) STMICROELECTRONICS

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DocID026980 Rev 4

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VIPer35

Operation description

Figure 35. Timing diagram: double blanking time

8.9

Starter

If the amplitude of the voltage on ZCD pin at the end of one oscillator cycle is smaller than

VZCDAth arming threshold, (in this case MOSFET turn-on could not be triggered), the system

stops.

This is what normally happens during the converter power-up or under overload/short-circuit

conditions.

During the converter start-up phase, the voltage on ZCD pin is not so high to arm the

triggering circuit. Thus, the converter operates at a fixed frequency, FSTARTER, (see

Table 7). As the voltage developed across the auxiliary winding arms the ZCD circuit,

MOSFET turn-on is locked to transformer demagnetization, hence quasi-resonant operation

is set.

8.10

Current limit set-point and feed-forward option

The VIPER35 is a current mode converter and the drain current is limited cycle-by-cycle

according to FB pin voltage value, which is related to the feedback loop response and the

load. When the drain current, sensed by the integrated senseFET, reaches the current

limitation, after the internal propagation delay, the MOSFET switches off. The current

limitation cannot exceed a certain value, IDlim, which can vary according to the current sunk

by ZCD pin during MOSFET on-time.

Usually a resistor, RLIM, connected from ZCD pin to ground fixes this sunk current and then

the peak drain current set-point: the lower the resistor, the lower IDlim.

VIPER35 Datasheet(PDF) 27 Page - STMicroelectronics