L4995 STMICROELECTRONICS [STMicroelectronics], L4995 Datasheet - Page 18

• Manufacturer Part Number: L4995
• Description: 5V, 500mA low drop voltage regulator
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

Application information
3.3
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Watchdog
A connected microcontroller is monitored by the watchdog input Wi. If pulses are missing,
the Reset output pin is set to low. The pulse sequence time 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 prevent this the microcontroller must generate a positive edge during the
discharge of the capacitor before the voltage has reached the threshold Vwlth. In order to
calculate the minimum time t, during which the micro-controller must output the positive
edge, the following equation can be used:
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.
Figure 29. Watchdog timing diagram
(Vwhth-Vwlth) x Ctw = Icwd x t
Wi
Vcw
Res
twol
Twop
Vwhth
Vwlth
L4995