MB3793-27A Datasheet, PDF (12/25 Pages) Fujitsu Component Limited. – Power Voltage Monitoring IC with Watchdog Timer

MB3793-27A
(3) When CTP has been charged for a certain period of time TPR (until the CTP pin voltage exceeds the threshold
voltage (Vth) after the start of charging), the MB3793 cancels the reset (setting the RESET pin to “H” level
from “L” level).
The Vth value is about 2.4 V with VCC = 3.3 V
The power-on reset hold time tPR is set with the following equation:
tPR (ms) ^.=. A × CTP (µF)
The value of A is about 750 with VCC = 3.3 V and about 700 with VCC = 3.0 V. The MB3793 also starts
charging the watchdog time setting capacitor (CTW).
(4) When the voltage at the watchdog timer monitor time setting pin CTW reaches the “H” level threshold voltage
VH, the CTW switches from the charge state to the discharge state.
The value of VH is always about 1.24 V regardless of the detected voltage.
(5) If the CK2 pin inputs a clock pulse (positive edge trigger) when the CTW is being discharged in the CK1-CK2
order or simultaneously, the CTW switches from the discharge state to the charge state.
The MB3793 repeats operations (4) and (5) as long as the CK1/CK2 pin inputs clock pulses with the system
logic circuit operating normally.
(6) If no clock pulse is fed to the CK1 or CK2 pin within the watchdog timer monitor time tWD due to some problem
with the system logic circuit, the CTW pin is set to the “L” level threshold voltage VL or less and the MB3793
outputs a reset signal (setting the RESET pin to “L” level from “H” level).
The value of VL is always about 0.24 V regardless of the detected voltage.
The watchdog timer monitor time tWD is set with the following equation:
tWD (ms) ^.=. B × CTW (µF)
The value of B is hardly affected by the power supply voltage; it is about 1600 with VCC = 3.0 V to 3.3 V.
(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 55 with VCC = 3.3 V and about 50 with VCC = 3.0 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 2.7 V
(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.
(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.
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