![](http://datasheet.mmic.net.cn/330000/MB3793-27A_datasheet_16435592/MB3793-27A_12.png)
12
MB3793
-27A
The V
th
value is about 2.4 V with V
CC
= 3.3 V
The power-on reset hold timer monitor time t
PR
is set with the following equation:
t
PR
(ms) ..
×
C
TP
(
μ
F)
The value of A is about 750 with V
CC
= 3.3 V and about 700 with V
CC
= 3.0 V. The MB3793 also starts
charging the watchdog timer monitor time setting capacitor (C
TW
).
(4) When the voltage at the watchdog timer monitor time setting pin C
TW
reaches the “H” level threshold voltage
V
H
, the C
TW
switches from the charge state to the discharge state.
The value of V
H
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 C
TW
is being discharged in the CK1-CK2
order or simultaneously, the C
TW
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 t
WD
due to some problem
with the system logic circuit, the CTW pin is set to the “L” level threshold voltage V
L
or less and the MB3793
outputs a reset signal (setting the RESET pin to “L” level from “H” level).
The value of V
L
is always about 0.24 V regardless of the detected voltage.
The watchdog timer monitor time t
WD
is set with the following equation:
t
WD
(ms) ..
×
C
TW
(
μ
F)
The value of B is hardly affected by the power supply voltage; it is about 1600 with V
CC
= 3.0 to 3.3 V.
(7) When a certain period of time t
WR
has passed (until the CTP pin voltage reaches or exceeds Vth again after
recharging the C
TP
), the MB3793 cancels the reset signal and starts operating the watchdog timer.
The watchdog timer monitor reset time t
WR
is set with the following equation:
t
WR
(ms) ..
TP
(
μ
F)
The value of D is 55 with V
CC
= 3.3 V and about 50 with V
CC
= 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 V
CC
is lowered to the fall-time detected voltage (V
SL
) 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 V
SL
is 2.7 V
(9) When V
CC
reaches or exceeds V
SH
again, the MB3793 starts charging the C
TP
.
(10) When the CTP pin voltage reaches or exceeds V
th
, 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 V
CC
(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 V
CC
to V
SL
or less.