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DM74123
Operating Rules
1. An external resistor (R
X
) and external capacitor (C
X
)
are required for proper operation. The value of C
X
may
vary from 0 to any necessary value. For small time con-
stants high-grade mica, glass, polypropylene, polycar-
bonate, or polystyrene material capacitors may be
used. For large time constants use tantalum or special
aluminum capacitors. If the timing capacitors have
leakages approaching 100 nA or if stray capacitance
from either terminal to ground is greater than 50 pF the
timing equations may not represent the pulse width the
device generates.
2. When an electrolytic capacitor is used for C
X
a switch-
ing diode is often required for standard TTL one-shots
to prevent high inverse leakage current (Figure 1).
However, its use in general is not recommended with
retriggerable operation.
3. The output pulse width (T
W
) for C
X
> 1000 pF is
defined as follows:
T
W
= K R
X
C
X
(1 + 0.7/R
X
)
1. where: [R
X
is in Kilo-ohm]
[C
X
is in pico Farad]
[T
W
is in nano second]
[K
≈ 0.28]
FIGURE 1.
4. For C
X
< 1000 pF see Figure 2 for T
W
vs. C
X
family curves with R
X
as a parameter:
Pulse Width vs. R
X
and C
X
FIGURE 2.
5. To obtain variable pulse width by remote trim-
ming, the following circuit is recommended:
Note: “R
remote
” should be as close to the one-shot as possible.
FIGURE 3.
6. The retriggerable pulse width is calculated as
shown below:
T
= T
W
+ t
PLH
= K× R
X
× C
X
+ t
PLH
The retriggered pulse width is equal to the
pulse width plus a delay time period (Figure
4).
FIGURE 4.
7. Under any operating condition C
X
and R
X
must be kept as close to the one-shot device
pins as possible to minimize stray capaci-
tance, to reduce noise pick-up, and to reduce
I
× R and Ldi/dt voltage developed along their
connecting paths. If the lead length from C
X
to
pins (6) and (7) or pins (14) and (15) is greater
than 3 cm, for example, the output pulse width
might be quite different from values predicted
from the appropriate equations. A non-induc-
tive and low capacitive path is necessary to
ensure complete discharge of C
X
in each
cycle of its operation so that the output pulse
width will be accurate.
8. V
CC
and ground wiring should conform to
good high-frequency standards and practices
so that switching transients on the V
CC
and
ground return leads do not cause interaction
between one-shots. A 0.01
µF to 0.10 µF
bypass capacitor (disk ceramic or monolithic
type) from V
CC
to ground is necessary on
each device. Furthermore, the bypass capaci-
tor should be located as close to the V
CC
pin
as space permits.
Note: For further detailed device characteristics and output performance
please refer to the One-Shot Application Note, AN-366.
