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SN74LS122 View Datasheet(PDF) - ON Semiconductor

Part NameDescriptionManufacturer
SN74LS122 Retriggerable Monostable Multivibrators ON-Semiconductor
ON Semiconductor ON-Semiconductor
SN74LS122 Datasheet PDF : 12 Pages
1 2 3 4 5 6 7 8 9 10 Next Last
SN74LS122, SN74LS123
LS122 FUNCTIONAL TABLE
INPUTS
OUTPUTS
CLEAR
A1 A2 B1 B2 Q
Q
L
X
X
X
X
L
H
X
H
H
X
X
L
H
X
X
X
L
X
L
H
X
X
X
X
L
L
H
H
L
X
H
H
L
X
H
H
X
L
H
H
X
L
H
H
H
H
H
H
H
H
H
H
H
H
L
X
H
H
X
L
H
H
LS123 FUNCTIONAL TABLE
INPUTS
OUTPUTS
CLEAR
A
B
QQ
L
X
X
LH
X
H
X
LH
X
X
L
LH
H
L
H
H
L
H
TYPICAL APPLICATION DATA
The output pulse tW is a function of the external
components, Cext and Rext or Cext and Rint on the LS122.
For values of Cext 1000 pF, the output pulse at VCC = 5.0
V and VRC = 5.0 V (see Figures 1, 2, and 3) is given by
tW = K Rext Cext where K is nominally 0.45
If Cext is on pF and Rext is in kthen tW is in nanoseconds.
The Cext terminal of the LS122 and LS123 is an internal
connection to ground, however for the best system
performance Cext should be hard-wired to ground.
Care should be taken to keep Rext and Cext as close to the
monostable as possible with a minimum amount of
inductance between the Rext/Cext junction and the Rext/Cext
pin. Good groundplane and adequate bypassing should be
designed into the system for optimum performance to ensure
that no false triggering occurs.
It should be noted that the Cext pin is internally connected
to ground on the LS122 and LS123, but not on the LS221.
Therefore, if Cext is hard-wired externally to ground,
substitution of a LS221 onto a LS123 socket will cause the
LS221 to become non-functional.
The switching diode is not needed for electrolytic
capacitance application and should not be used on the LS122
and LS123.
To find the value of K for Cext 1000 pF, refer to Figure 4.
Variations on VCC or VRC can cause the value of K to
change, as can the temperature of the LS123, LS122.
Figures 5 and 6 show the behavior of the circuit shown in
Figures 1 and 2 if separate power supplies are used for VCC
and VRC. If VCC is tied to VRC, Figure 7 shows how K will
vary with VCC and temperature. Remember, the changes in
Rext and Cext with temperature are not calculated and
included in the graph.
As long as Cext 1000 pF and 5K Rext 260K, the
change in K with respect to Rext is negligible.
If Cext 1000 pF the graph shown on Figure 8 can be used
to determine the output pulse width. Figure 9 shows how K
will change for Cext 1000 pF if VCC and VRC are connected
to the same power supply. The pulse width tW in
nanoseconds is approximated by
tW = 6 + 0.05 Cext (pF) + 0.45 Rext (k) Cext + 11.6 Rext
In order to trim the output pulse width, it is necessary to
include a variable resistor between VCC and the Rext/Cext pin
or between VCC and the Rext pin of the LS122. Figure 10, 11,
and 12 show how this can be done. Rext remote should be
kept as close to the monostable as possible.
Retriggering of the part, as shown in Figure 3, must not
occur before Cext is discharged or the retrigger pulse will not
have any effect. The discharge time of Cext in nanoseconds
is guaranteed to be less than 0.22 Cext (pF) and is typically
0.05 Cext (pF).
For the smallest possible deviation in output pulse widths
from various devices, it is suggested that Cext be kept
1000 pF.
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