HD74LV123A Datasheet, PDF (11/20 Pages) Hitachi Semiconductor – Dual Retriggerable Monostable Multivibrators

Timing diagram
A
B
CLR
Rext/
Cext
Q
Q
t_w
HD74LV123A
t _rr
t_w
t_w +t_rr
Caution in use
In order to prevent any malfunctions due to noise, connect a high frequency
performance capacitor between Vcc and GND, and keep the wiring between the
External components and Cext, Rext/Cext pins as short as possible.
Large values of Cext may cause problems when powering down the HD74LV123A
because of the amount of energy stored in the capacitor. When a system containing
this device is powered down, the capacitor may discharge from Vcc through the protection
diodes at pin 7 or pin 15.
Current through the input protection diodes must be limited to 20 mA; therefore, the turn-off
time of the Vcc power supply must not be faster than t = Vcc • Cext/(20 mA). For example,
if Vcc = 5 V and Cext = 22 µF, the Vcc supply must turn off no faster than t = (5 V) • (22 µF)/
20 mA = 5.5 ms. This is usually not a problem because power supplies are heavily filtered
and cannot discharge at this rate.
When a more rapid decrease of Vcc to zero volts occurs, the HD74LV123A may sustain damage.
To avoid this possibility, use an external calmping diode.
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