5-198
FAST AND LS TTL DATA
SN54/74LS122 • SN54/74LS123
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 Q Q
L X X L H
X H X L H
X X L L H
H L ↑
H ↓ H
↑ L H
TYPICAL APPLICATION DATA
The output pulse t
W
is a function of the external compo-
nents, C
ext
and R
ext
or C
ext
and R
int
on the LS122. For values
of C
ext
≥ 1000 pF, the output pulse at V
CC
= 5.0 V and V
RC
=
5.0 V (see Figures 1, 2, and 3) is given by
t
W
= K R
ext
C
ext
where K is nominally 0.45
If C
ext
is on pF and R
ext
is in kΩ then t
W
is in nanoseconds.
The C
ext
terminal of the LS122 and LS123 is an internal
connection to ground, however for the best system perfor-
mance C
ext
should be hard-wired to ground.
Care should be taken to keep R
ext
and C
ext
as close to the
monostable as possible with a minimum amount of inductance
between the R
ext
/C
ext
junction and the R
ext
/C
ext
pin. Good
groundplane and adequate bypassing should be designed
into the system for optimum performance to insure that no
false triggering occurs.
It should be noted that the C
ext
pin is internally connected
to ground on the LS122 and LS123, but not on the LS221.
Therefore, if C
ext
is hard-wired externally to ground, substitu-
tion of a LS221 onto a LS123 socket will cause the LS221 to
become non-functional.
The switching diode is not needed for electrolytic capaci-
tance application and should not be used on the LS122 and
LS123.
To find the value of K for C
ext
≥ 1000 pF, refer to Figure 4.
Variations on V
CC
or V
RC
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 V
CC
and V
RC
. If V
CC
is
tied to V
RC
, Figure 7 shows how K will vary with V
CC
and
temperature. Remember, the changes in R
ext
and C
ext
with
temperature are not calculated and included in the graph.
As long as C
ext
≥ 1000 pF and 5K ≤ R
ext
≤ 260K
(SN74LS122/123) or 5K ≤ R
ext
≤ 160 K (SN54LS122/123),
the change in K with respect to R
ext
is negligible.
If C
ext
≤ 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 C
ext
≤ 1000 pF if V
CC
and V
RC
are connected to the
same power supply. The pulse width t
W
in nanoseconds is
approximated by
t
W
= 6 + 0.05 C
ext
(pF) + 0.45 R
ext
(kΩ) C
ext
+ 11.6 R
ext
In order to trim the output pulse width, it is necessary to
include a variable resistor between V
CC
and the R
ext
/C
ext
pin
or between V
CC
and the R
ext
pin of the LS122. Figure 10, 11,
and 12 show how this can be done. R
ext
remote should be kept
as close to the monostable as possible.
Retriggering of the part, as shown in Figure 3, must not
occur before C
ext
is discharged or the retrigger pulse will not
have any effect. The discharge time of C
ext
in nanoseconds is
guaranteed to be less than 0.22 C
ext
(pF) and is typically 0.05
C
ext
(pF).
For the smallest possible deviation in output pulse widths
from various devices, it is suggested that C
ext
be kept
≥ 1000 pF.
