MB3793 Datasheet, PDF (10/20 Pages) Fujitsu Component Limited. – POWER-VOLTAGE MONITORING IC WITH WATCHDOG TIMER

MB3793-42/30
5. Clock pulse input (Positive clock pulse)
a
CK1
b
CK2
VH
CTW
VL
The MB3793 watchdog timer monitors Clock 1 (CK1) and Clock 2 (CK2) pulses alternately. When a CK2 pulse
is detected after detecting a CK1 pulse, the monitoring time setting capacity (CTW) switches to charging from
discharging.
When two consecutive pulses occur on one side of this alternation before switching, the second pulse is ignored.
In the above figure, pulses a and b are ignored.
s OPERATION SEQUENCE
The operation sequence is explained by using Timing Chart 1.
The following item numbers correspond to the numbers in Timing Chart 1.
(1) When the power voltage (VCC) reaches about 0.8 V (VCCL), a reset signal is output.
(2) When VCC exceeds the rising-edge detection voltage (VSH), charging of power-on reset hold time setting capacitance (CTP) is started.
VSH is about 4.3 V in the MB3793-42 and 3. 07 V in the MB3793-30.
(3) When the voltage at the CTP pin setting the power-on reset hold time exceeds the threshold voltage (Vth), resetting is canceled and
the voltage at the RESET pin changes to High level to start charging of the watchdog-timer monitoring time setting capacitance (CTW).
Vth is about 3.6 V in the MB3793-42 and 2.4 V in the MB3793-30.
The power-on reset hold time (tPR) can be calculated by the following equation.
tPR (ms) ≈ A x CTP (µF)
Where, A is about 1300 in the MB3793-42 and 750 in the MB3793-30.
(4) When the voltage at the CTW pin setting the monitoring time reaches High level (VH), CTW switches to discharging from charging. VH
is about 1.24 V (reference value) in both the MB3793-42 and MB3793-30.
(5) When clock pulses are input to the CK2 pin during CTW discharging after clock pulses are input to the CK1 pin—positive-edge trigger,
CTW switches to charging.
(6) If clock pulse input does not occur at either the CK1 or CK2 clock pins during the watchdog-timer monitoring time (tWD), the CTW voltage
falls below Low level (VL), a reset signal is output, and the voltage at the RESET pin changes to Low level. VL is about 0.24 V in both
the MB3793-42 and MB3793-30.
tWD can be calculated from the following equation.
tWD (ms) ≈ B x CTW (µF) + C x CTP (µF)
Where, B is about 1500 in the MB3793-42 and 1600 in the MB3793-30. C is about 3 in the MB3793-42 and 4 in the MB3793-30; it is
much smaller than B.
Hence, when ^CC—TT—^PW ≤ 10, the calculation can be simplified as follows:
tWD (ms) ≈ B x CTW (µF)
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