A6986F datasheet(35/72 Pages) STMICROELECTRONICS

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A6986F

Functional description

A small resistor value (few ohms) in series with the VBIAS can help to limit the pin negative

voltage (see Figure 27) during heavy short-circuit events if it is connected to the regulated

output voltage.

Figure 27. VBIAS in heavy short-circuit event

5.8

Overvoltage protection

The overvoltage protection monitors the FB pin and enables the low-side MOSFET to

discharge the output capacitor if the output voltage is 20% over the nominal value.

This is a second level protection and should never be triggered in normal operating

conditions if the system is properly dimensioned. In other words, the selection of the

external power components and the dynamic performance determined by the compensation

network should guarantee an output voltage regulation within the overvoltage threshold

even during the worst case scenario in term of load transitions.

The protection is reliable and also able to operate even during normal load transitions for

a system whose dynamic performance is not in line with the load dynamic request. As

a consequence the output voltage regulation would be affected.

Figure 28 shows the overvoltage operation during a negative steep load transient for

a system designed with huge inductor value and small output capacitor. The inductor value

limits the switch current slew rate and the extra charge flowing into the small capacitor value

generates an overvoltage event. This can be considered as an example for a system with

dynamic performance not in line with the load request.

The A6986F device implements a 1 A typ. negative current limitation to limit the maximum

reversed switch current during the overvoltage operation.

VBIAS pin voltage

regulated output voltage

inductor current

switching node

(purple)

(cyan)

regulated output voltage

VBIAS pin

short-circuit current

switching node

A6986F Fiches technique(PDF) 35 Page - STMicroelectronics