L6986H datasheet(33/68 Pages) STMICROELECTRONICS

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The VBIAS pin absolute maximum ratings (see Table 1) must be satisfied over the different dynamic conditions.

If the VBIAS is connected to GND there are no issues (see Figure 31. Output voltage oscillations during heavy

short-circuit and Figure 32. Zoomed waveforms).

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

Figure 33. VBIAS in heavy short-circuit event) during heavy short-circuit events if it is connected to the regulated

output voltage.

Figure 33. VBIAS in heavy short-circuit event

4.12

Overvoltage protection

The overvoltage protection monitors the VOUT 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 34. Overvoltage operation shows the overvoltage operation during a negative steep load transient for a

system configured in low consumption mode and designed with a not optimized compensation network. This can

be considered as an example for a system with dynamic performance not in line with the load request.

The L6986H device implements a 1 A typ. negative current limitation to limit the maximum reversed switch current

during the overvoltage operation.

Moreover, the overvoltage protection also activates the internal pull-down on RST pin. Once OVP is deactivated,

the L6986H releases the RST pin after the delay programmed by DELAY capacitor (6 ms in

Figure 34. Overvoltage operation).

L6986H

Overvoltage protection

DS12905 - Rev 2

page 33/68

L6986H 数据表(PDF) 33 Page - STMicroelectronics