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SN74LS122, SN74LS123
Retriggerable Monostable
Multivibrators
These dc triggered multivibrators feature pulse width control by
three methods. The basic pulse width is programmed by selection of
external resistance and capacitance values. The LS122 has an internal
timing resistor that allows the circuits to be used with only an external
capacitor. Once triggered, the basic pulse width may be extended by
retriggering the gated low-level-active (A) or high-level-active (B)
inputs, or be reduced by use of the overriding clear.
•
Overriding Clear Terminates Output Pulse
•
Compensated for V
CC
and Temperature Variations
•
DC Triggered from Active-High or Active-Low Gated Logic Inputs
•
Retriggerable for Very Long Output Pulses, up to 100% Duty Cycle
•
Internal Timing Resistors on LS122
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LOW POWER SCHOTTKY
PLASTIC
N SUFFIX
CASE 646
1
14
14
1
SOIC
D SUFFIX
CASE 751A
GUARANTEED OPERATING RANGES
Symbol
V
CC
T
A
I
OH
I
OL
R
ext
C
ext
R
ext
/C
ext
Parameter
Supply Voltage
Operating Ambient
Temperature Range
Output Current
−
High
Output Current
−
Low
External Timing Resistance
External Capacitance
Wiring Capacitance at
R
ext
/C
ext
Terminal
5.0
Min
4.75
0
Typ
5.0
25
Max
5.25
70
−0.4
8.0
260
No Restriction
50
pF
16
1
Unit
V
°C
mA
mA
kW
16
1
16
1
PLASTIC
N SUFFIX
CASE 648
SOIC
D SUFFIX
CASE 751B
SOEIAJ
M SUFFIX
CASE 966
ORDERING INFORMATION
Device
SN74LS122N
SN74LS122D
SN74LS122DR2
SN74LS123N
SN74LS123D
SN74LS123DR2
SN74LS123M
SN74LS123MEL
Package
14 Pin DIP
SOIC−14
SOIC−14
16 Pin DIP
SOIC−16
SOIC−16
SOEIAJ−16
SOEIAJ−16
Shipping
2000 Units/Box
55 Units/Rail
2500/Tape & Reel
2000 Units/Box
38 Units/Rail
2500/Tape & Reel
See Note 1
See Note 1
1. For ordering information on the EIAJ version of
the SOIC package, please contact your local
ON Semiconductor representative.
©
Semiconductor Components Industries, LLC, 2006
June, 2006
−
Rev. 8
1
Publication Order Number:
SN74LS122/D
SN74LS122, SN74LS123
SN74LS123
(TOP VIEW)
(SEE NOTES 1 THRU 4)
V
CC
16
1 R
ext/ 1
C
ext
C
ext
15
14
Q
CLR Q
1
1A
2
1B
3
1
CLR
4
1Q
5
2Q
1Q
13
2Q
12
2
CLR
11
2B
10
2A
9
Q
Q
CLR
6
2
C
ext
8
7
2
GND
R
ext/
C
ext
SN74LS122
(TOP VIEW)
(SEE NOTES 1 THRU 4)
V
CC
14
R
ext/
C
ext
13
NC
12
C
ext
11
NC
10
R
int
Q
CLR Q
1
A1
NC
2
A2
3
B1
4
B2
5
CLR
6
Q
7
GND
R
int
9
Q
8
NO INTERNAL CONNECTION.
NOTES:
1. An external timing capacitor may be connected between C
ext
and R
ext
/C
ext
(positive).
2. To use the internal timing resistor of the LS122, connect R
int
to V
CC
.
3. For improved pulse width accuracy connect an external resistor between R
ext
/C
ext
and V
CC
with R
int
open-circuited.
4. To obtain variable pulse widths, connect an external variable resistance between R
int
/C
ext
and V
CC
.
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2
SN74LS122, SN74LS123
LS122 FUNCTIONAL TABLE
INPUTS
CLEAR
L
X
X
X
H
H
H
H
H
H
H
↑
↑
A1
X
H
X
X
L
L
X
X
H
↓
↓
L
X
A2
X
H
X
X
X
X
L
L
↓
↓
H
X
L
B1
X
X
L
X
↑
H
↑
H
H
H
H
H
H
B2
X
X
X
L
H
↑
H
↑
H
H
H
H
H
OUTPUTS
Q
L
L
L
L
Q
H
H
H
H
CLEAR
L
X
X
H
H
↑
LS123 FUNCTIONAL TABLE
INPUTS
A
X
H
X
L
↓
L
B
X
X
L
↑
H
H
OUTPUTS
Q
L
L
L
Q
H
H
H
TYPICAL APPLICATION DATA
The output pulse t
W
is a function of the external
components, 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
performance 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 ensure
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,
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 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, 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.
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3
SN74LS122, SN74LS123
WAVEFORMS
RETRIGGER
PULSE
B INPUT
(See Application Data)
Q OUTPUT
t
W
OUTPUT WITHOUT RETRIGGER
EXTENDING PULSE WIDTH
B INPUT
CLEAR INPUT
CLEAR PULSE
Q OUTPUT
OUTPUT WITHOUT CLEAR PULSE
OVERRIDING THE OUTPUT PULSE
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4
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