Figure 25. Timing diagram of OLP protection

5.11

Output overvoltage protection

The voltage on the ZCD pin is monitored at the end of the transformer’s demagnetization, where the auxiliary

winding accurately tracks the converter output voltage and compares with an internal reference.

If the sampled voltage exceeds the OVP threshold VOVP (2.5 V typ.), an overvoltage condition is assumed.

With reference to the external configuration of Figure 1, once ROPP is fixed by feed-forward considerations (see

relevant section), it is possible to calculate the value of the ROVP resistor to activate the OVP protection for a

certain output voltage level, VOUT-OVP.

Equation 9

(9)

RFB= VOVP

NAUXNSEC∙VOUT−OVP−VOVP∙ROPP

To reduce sensitivity to external noise and prevent the protection from being erroneously activated, the OVP

comparator must be triggered for four consecutive oscillator cycles before the device is stopped.

A counter, which is reset every time the OVP comparator is not triggered in one oscillator cycle, is provided for

this purpose.

Best accuracy of OVP protection is achieved when demagnetization time is longer than 2 μs.

Once the protection is tripped, the switching is inhibited for TRESTART and VDD is recycled between VDD_RESTART

and VDD_ON by the periodical activation of the HV current source. At the end of TRESTART, switching is enabled

with soft-start at the first VDD recharge to VDD-ON. If overvoltage is still present, the protection is invoked again in

the same way, resulting in a low-frequency intermittent operation, which increases the end-product’s safety and

reliability; otherwise, the IC resumes normal operation.

The OVP protection timing diagrams are shown in Figure 26.

VIPERGAN65

Output overvoltage protection

DS14005 - Rev 2

page 21/35