Each multivibrator of the LS221 features a negative-transition-triggered
input and a positive-transition-triggered input either of which can be used as
an inhibit input.
Pulse triggering occurs at a voltage level and is not related to the transition
time of the input pulse. Schmitt-trigger input circuitry for B input allows
jitter-free triggering for inputs as slow as 1 volt/second, providing the circuit
with excellent noise immunity. A high immunity to VCC noise is also provided
by internal latching circuitry.
Once triggered, the outputs are independent of further transitions of the
inputs and are a function of the timing components. The output pulses can be
terminated by the overriding clear. Input pulse width may be of any duration
relative to the output pulse width. Output pulse width may be varied from 35
nanoseconds to a maximum of 70 s by choosing appropriate timing
components. With Rext = 2.0 k? and Cext = 0, a typical output pulse of 30
nanoseconds is achieved. Output rise and fall times are independent of pulse
length.
Pulse width stability is achieved through internal compensation and is
virtually independent of VCC and temperature. In most applications, pulse
stability will only be limited by the accuracy of external timing components.
Jitter-free operation is maintained over the full temperature and VCC ranges
for greater than six decades of timing capacitance (10 pF to 10 ?F), and
greater than one decade of timing resistance (2.0 to 70 k? for the
SN54LS221, and 2.0 to 100 k? for the SN74LS221). Pulse width is defined
by the relationship: tw(out) = CextRext ln 2.0 ? 0.7 Cext Rext; where tW is in ns
if Cext is in pF and Rext is in k?. If pulse cutoff is not critical, capacitance up
to 1000 ?F and resistance as low as 1.4 k? may be used. The range of
jitter-free pulse widths is extended if VCC is 5.0 V and 25?C temperature.
? SN54LS221 and SN74LS221 is a Dual Highly Stable One-Shot
? Overriding Clear Terminates Output Pulse
? Pin Out is Identical to SN54/74LS1