AN4007 datasheet(22/33 Pages) STMICROELECTRONICS

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Protection features

AN4007

22/33

Doc ID 022455 Rev 2

2.2

Overvoltage protection

An output overvoltage protection is implemented monitoring the voltage across the auxiliary

winding during the MOSFET turn-off time, through the diode D4 and the resistor dividers R4

and R12 connected on the CONT pin of the VIPER37LE. If this voltage exceeds the VOVP

threshold (3 V typ.), an overvoltage event is assumed and the device is no longer allowed to

switch.

To re-enable operation, the VDD voltage must be recycled. In order to provide high noise

immunity and avoid that spikes erroneously trip the protection, a digital filter was

implemented so the CONT pin must sense a voltage higher than VOVP for four consecutive

cycles before stopping operation.

The protection can be tested by opening the resistor R9. In this way the converter operates

in open loop and the excess of power with respect to the load charges the output

capacitance, increasing the output voltage as the OVP is tripped and the converter stops

switching.

Figure 23 and 24 it is possible to see that output voltage increases and as it reaches the

value of 6.5 V the converter stops switching. In the same figure the CONT pin voltage is

reported. The crest value of the CONT pin voltage tracks the output voltage.

Figure 23.

Overvoltage event: OVP triggering

AM11362v1

Ch2 (Max): 6.48 V

M: 400.0 µs/div

VOUT

I DRAIN

VCONT

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