General description

VIPER26K

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DS12978 Rev 1

Figure 24. Timing diagram: OLP sequence

4.11

Open loop failure protection

In case the power supply is built in flyback topology and the VIPer26K is supplied by an

auxiliary winding, the converter is protected against feedback loop failure or accidental

disconnections of the winding.

If RH is opened or RL is shorted, the VIPer26K works at its drain current limitation. The

output voltage, VOUT, increases and also the auxiliary voltage, VAUX, which is coupled with

the output through the secondary-to-auxiliary turns ratio.

As the auxiliary voltage increases up to the internal VDD active clamp, VDDclamp and the

clamp current injected on the VDD pin exceeds the latch threshold, IDDol, a fault signal is

internally generated.

In order to distinguish an actual malfunction from a bad auxiliary winding design, both the

above conditions (drain current equal to the drain current limitation and current higher than

IDDol through VDD clamp) have to be verified to reveal the fault.

If RL is opened or RH is shorted, the output voltage, VOUT, is clamped to the reference

voltage VREF_FB (in case of non-isolated flyback) or to the external TL voltage reference (in

case of isolated flyback).

4.12

Thermal shutdown

When the controller temperature exceeds the shutdown threshold, TSD, the device is shut

down to prevent any dangerous overheating for the system and the VDD pin is continuously

recycled between VDDon and VDDoff to keep the controller alive.

Once the tRESTART time is elapsed, when temperature falls THYST below the OTP threshold,

the IC starts once it has reached again the VDDon.

The OTP timing diagram is shown in Figure 25.