MB3793-45 Datasheet, PDF (13/28 Pages) Fujitsu Component Limited. – POWER-VOLTAGE MONITORING IC WITH WATCHDOG TIMER

MB3793-45
■ OPERATION SEQUENCE
1. Positive clock pulse input
Refer to “1. Basic operation (positive clock pulse)” under “■ TIMING DIAGRAM.”
2. Negative clock pulse input
Refer to “2. Basic operation (negative clock pulse)” under “■ TIMING DIAGRAM.”
The MB3793 operates in the same way whether it inputs positive or negative pulses.
3. Clock monitoring
To use the MB3793 while monitoring only one clock, connect clock pins CK1 and CK2.
Although the MB3793 operates basically in the same way as when monitoring two clocks, it monitors the clock
signal at every other input pulse.
Refer to “3. Single-clock input monitoring (positive clock pulse)” under “■ TIMING DIAGRAM.”
4. Description of Operations
The numbers given to the following items correspond to numbers (1) to (13) used in “■ TIMING DIAGRAM.”
(1) The MB3793 outputs a reset signal when the supply voltage (VCC) reaches about 0.8 V (VCCL)
(2) If VCC reaches or exceeds the rise-time detected voltage VSH, the MB3793 starts charging the power-on
reset hold time setting capacitor CTP. At this time, the output remains in a reset state. The VSH value is
4.60 V (Typ) .
(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 3.6 V with VCC = 5.0 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 1300 with VCC = 5.0 V. The MB3793 also starts charging the watchdog timer
monitor 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) + C × CTP (μF)
The value of B is hardly affected by the power supply voltage; it is about 1500 with VCC = 5.0 V.
The value in C is about 3 which is tremendously smaller than the value in B. For this reason, it is possible
to
tWD
simplify the
(ms) ^.=. B ×
formula as
CTW (μF)
below
when
CTP/CTW
.=.
10
or
less.
DS04-27405-2E
13