L99VR02XP datasheet(27/50 Pages) STMICROELECTRONICS

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Every wi-positive edge switches the current source from discharging to charging. The same happens when the

lower threshold is reached. When the voltage reaches the upper threshold, Vwhth_LDO1,2, the current switches

from charging to discharging. The result is a sawtooth voltage at the watchdog timer capacitor Ctwx. If a

microcontroller operates in low power mode it will not be able to generate any pulse to refresh the voltage

regulator watchdog, so triggering the microcontroller reset. In such a case, to avoid generating the microcontroller

reset, the watchdog functionality is automatically deactivated any time the microcontroller current consumption

falls under the Iw_off_LDO1,2 threshold. On the other hand, when the current consumption rises above the

Iw_on_LDO1,2 threshold the watchdog functionality is once again activated. Once the regulator is enabled for the

first time, if IO_LDO1,2 < Iw_off_LDO1,2 the watchdog will not be activated, while if IO_LDO1,2 > Iw_off_LDO1,2 the

watchdog is activated.

Figure 45. Watchdog timing diagram

Since the RST_LDO1,2 output pins are shared between the watchdog circuit and the output voltage monitoring

circuit, for applications where the watchdog is not needed, to prevent the watchdog from generating RST pulses

without anyway losing the reset functionality of the VO_LDO1,2 monitoring, the Vcw_LDO1,2 pins have to be

connected to VO_LDO1,2. Vcw_LDO1,2 pins must be always tied to the ground by an external capacitor (Ctwx) when

watchdog function is used.

Note:

•

When the watchdog timer is used to and the regulator recovers from a thermal shut-down event or

recovers from an output undervoltage event (including the recovery from output undervoltage event at the

regulator output turning on), the reset pin might be pulled back high with a delay longer than Trd, in the

range between Trd and Trd +Twol due to the watchdog that might affect the RST pin release. The

watchdog will not affect the RST pin release in the case where recovering from a thermal shut-down

event or recovering from an output undervoltage event (including the recovery from output undervoltage

event at the regulator output turning on) the Io drops below Iw_off before Trd. The output current Io

consists in the load current, the current drawn from the RST pin and the TW pin through the pull-down

resistors connected to VO when the RST and the TW pins are asserted low and the current needed to

charge/discharge the output capacitor is CO.

•

When the watchdog timer is used, in case of an output undervoltage event not making VO drop to zero

volt, since the watchdog will still be running during the output undervoltage condition, the first watchdog

timeout after the reset pin is released coming out of the output undervoltage event might occur before

expected (0 ≤ Timeout ≤ Twop - Twol).

Table 17. Watchdog timer

Usage of watchdog timer

Connection of Vcw_LDO1,2 pin

Watchdog timer is not used

Connect to the VO_LDO1,2 pin

Watchdog timer is used

Connect to an external capacitor Ctw

L99VR02XP

Application information

DS14686 - Rev 1

page 27/50

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