4.6

Autonomous watchdog

The watchdog input Wi monitor a supplied microcontroller. If pulses are missing, the RST output pin is set low.

The watchdog timeout can be set within a wide range with the external capacitor, Ctw. The watchdog circuit

discharges the capacitor Ctw, with the constant current ICWd. If the lower threshold Vwlth is reached, a watchdog

reset is generated. To avoid it, the microcontroller must generate a positive edge during the discharge of the

capacitor before the voltage reaches the threshold Vwlth. To calculate the sawtooth period “Twop”, taking care that

the microcontroller triggers the positive edge during the discharge phase of Ctw (Td). The following equations can

be used:

Vwℎtℎ−Vwltℎ ×Ctw=ICWd×Td

Vwℎtℎ−Vwltℎ ×Ctw=ICWc×Twol

Twop=Td+Twol

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, the current switches from

charging to discharging. The result is a saw-tooth voltage at the watchdog timer capacitor Ctw. If a microcontroller

operates in low power mode, it is not able to generate any pulse to refresh the voltage regulator watchdog,

triggering so the microcontroller reset. In such a case, the watchdog functionality is automatically deactivated

anytime the microcontroller current consumption falls under the Iw_off threshold. This to avoid generating the

microcontroller reset. On the other hand, when the current consumption rises above the Iw_on threshold the

watchdog functionality is once again activated. Once the regulator is enabled for the first time, if IO < Iw_off, the

watchdog is not activated. While if IO > Iw_off, the watchdog is activated.

Figure 38. Watchdog timing diagram

Wi

VCW

IO

RST

Vwhth

Vwlth

Iw_off

Iw_on

Twol

Twop

Since the RST output pin is shared by 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

anyhow losing the reset functionality of the VO monitoring, the Vcw pin has to be connected to VO. Vcw pin must

be always tied to ground by an external capacitor (Ctw) when watchdog function is used.

Note:

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

an output under-voltage event (including the recovery from output under-voltage 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

under-voltage event (including the recovery from output under-voltage 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 CO.

Note:

When the watchdog timer is used, in the case of an output under-voltage event not making the VO drop to

zero volt, since the watchdog will still be running during the output under-voltage condition, the first watchdog

timeout after the reset pin is released coming out of the output under-voltage event might occur before expected

(0 ≤ timeout ≤ Twop - Twol).

L99VR02J

Autonomous watchdog

DS14076 - Rev 1

page 22/38