Device description

L6731B

14/24

5.8

Monitoring and protections

The output voltage is monitored by means of pin FB. If it is not within ±10% (typ.) of the

programmed value, the Power-Good (PGOOD) output is forced low. The device provides over-

voltage-protection: when the voltage sensed on FB pin reaches a value 20% (typ.) greater than

the reference, the low-side driver is turned on as long as the over voltage is detected (see

Figure 7.).

It must be taken into account that there is an electrical network between the output terminal and

the FB pin and therefore the voltage at the pin is not a perfect replica of the output voltage.

However due to the fact that the converter can sink current, in the most of cases the low-side

will turn-on before the output voltage exceeds the over-voltage threshold, because the error

amplifier will throw off balance in advance. Even if the device doesn't report an over-voltage, the

behavior is the same, because the low-side is turned-on immediately. The following figure

shows the device behavior during an over-voltage event. The output voltage rises with a slope

of 100mV/µs, emulating in this way the breaking of the high-side MOSFET as an over-voltage

cause.

Figure 7.

OVP

LGate

FB