MB3793-45 Datasheet, PDF (14/28 Pages) Fujitsu Component Limited. – POWER-VOLTAGE MONITORING IC WITH WATCHDOG TIMER

MB3793-45
(7) When a certain period of time tWR has passed (until the CTP pin voltage reaches or exceeds Vth again
after recharging the CTP), the MB3793 cancels the reset signal and starts operating the watchdog timer.
The watchdog timer monitor reset time tWR is set with the following equation:
tWR (ms) ^.=. D x CTP (μF)
The value of D is 100 with VCC = 5.0 V.
The MB3793 repeats operations (4) and (5) as long as the CK1/CK2 pin inputs clock pulses. If no clock
pulse is input, the MB3793 repeats operations (6) and (7).
(8) If VCC is lowered to the fall-time detected voltage (VSL) or less, the CTP pin voltage decreases and the
MB3793 outputs a reset signal (setting the RESET pin to “L” level from “H” level).
The value of VSL is 4.50 V (Typ) .
(9) When VCC reaches or exceeds VSH again, the MB3793 starts charging the CTP.
(10) When the CTP pin voltage reaches or exceeds Vth, the MB3793 cancels the reset and restarts operating
the watchdog timer. It repeats operations (4) and (5) as long as the CK1/CK2 pin inputs clock pulses.
(11) Making the inhibit pin active (setting the INH pin to “H” from “L”) forces the watchdog timer to stop
operation.
This stops only the watchdog timer, leaving the MB3793 monitoring VCC (operations (8) to (10)).
The watchdog timer remains inactive unless the inhibit input is canceled.
The inhibition (INH) pin must be connecting a voltage of more low impedance, to evade of the noise.
(12) Canceling the inhibit input (setting the INH pin to “L” from “H”) restarts the watchdog timer.
(13) The reset signal is output when the power supply is turned off to set VCC to VSL or less.
1. Equation of time-setting capacitances (CTP and CTW) and set time
tPR [ms] ^.=. A × CTP [μF]
tWD [ms] ^.=. B × CTW [μF] + C × CTP [μF]
However, when CTP/CTW ^.=. 10 or less, tWD [ms] ^.=. B × CTW [μF]
tWR [ms] ^.=. D × CTP [μF]
Values of A, B, C, and D
A
B
C
1300
1500
3
D
Remark
100
VCC = 5.0 V
Note: The width of value of tPR, tWD and tWR becomes the same ratio as width (Min, Max) of each
specification value.
2. Example (when CTP = 0.1 μF and CTW = 0.01 μF)
Symbol
VCC = 5.0 V
time
tPR
130
(ms)
tWD
15
tWR
10
14
DS04-27405-2E