L6730CQ datasheet(21/50 Pages) STMICROELECTRONICS

21 / 50 page

L6730C - L6730D

Device description

21/50

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 this pin is not a perfect replica of the output voltage. If

the converter can sink current, in the most of cases the low-side will be turned-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 behaviour is the

same, because the low-side is turned-on immediately. Instead, if the sink-mode is disabled, the

low-side will be turned-on only when the over-voltage-protection (OVP) operates and not

before, because the current can’t be reversed. In this case a delay between the output voltage

rising and the FB voltage rising can appear and the OVP can operate on late. The following two

figures show an over-voltage event in case of sink enabled and disabled. 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 13. OVP latched

Figure 14. OVP with sink enabled: the low-side MOSFET is turned-on in advance.

LGate

OSC

VOUT

109%

VFB

LGate

L6730CQ 数据表(PDF) 21 Page - STMicroelectronics