L6986ITR datasheet(30/60 Pages) STMICROELECTRONICS

30 / 60 page

6.3

Voltage supervisor

The embedded voltage supervisor (composed of the RST and the DELAY pins) monitors the regulated output

voltage and keeps the RST open collector output in low impedance as long as the VOUT is out of regulation (i.e.

lower than 93% of the target output voltage). In order to ensure a proper reset of digital devices with a valid power

supply, the device can delay the RST assertion with a programmable time.

Figure 34. Voltage supervisor operation

When the RST comparator detects the output voltage is in regulation, a 2 mA internal current source starts to

charge an external capacitor to implement a voltage ramp on the DELAY pin. The RST open collector is then

released as soon as VDELAY = 1.234 V (see Figure 35). The CDELAY is dimensioned as follows.

CDELAY=ISSCH∙TDELAY

VDELAY =2μA∙TDELAY

1.234V

(28)

The maximum suggested capacitor value is 270 nF.

The L6986I also activates internal pull-down on RST pin in case overvoltage protection is triggered. As soon as

the output voltage goes below OVP threshold (20% typ.), the 2 µA internal current source starts to charge an

external capacitor to implement a voltage ramp on the DELAY pin. The RST open collector is then released as

soon as VDELAY = 1.234 V (see figure below).

Figure 35. Voltage supervisor operation during OVP

L6986I

Voltage supervisor

DS13647 - Rev 1

page 30/60

L6986ITR 数据表(PDF) 30 Page - STMicroelectronics