l4993 STMicroelectronics, l4993 Datasheet - Page 11

• Manufacturer Part Number: l4993
• Description: Low Drop Voltage Regulator
• Manufacturer: STMicroelectronics

L4993
3.3
Vrhth = 2.35 V (typ) and Vrlth =0.65 V (typ) are two voltage thresholds
Ctr is an external capacitance.
Figure 4.
Watchdog
The watchdog input Wi monitors a connected microcontroller. If pulses are missing, the
Reset output pin is set to low. This functionality can be disabled setting WEn input to a level
low. The pulse sequence time can be ajusted within a wide range with the external capacitor
Ctw. The watchdog circuit discharges the capacitor Ctw with a constant current Icwd. If the
lower threshold Vwlth is reached, a watchdog reset is generated. To prevent this reset
condition, the microcontroller must generate a positive edge during the discharge phase of
the capacitor before the voltage reaches the threshold Vwlth.
In order to calculate the minimum time Tdis during which the microcontroller must provide
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 sawtooth
waveform at the watchdog timer capacitor Ctw.
Figure 5.
Vcw
Res
Wi
Wi
Vo
Vcr
Res
<
trr
Reset time diagram
Watchdog time diagram
(Vwhth-Vwlth) x Ctw = Icwd x Tdis
trr
twol
trd = 512 Tosc
Tosc
twop
Application information
Vrhth
Vrlth
Vout_th
Vwhth
Vwlth
Vwlth
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