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DM74LS221 Datasheet(PDF) 2 Page - Fairchild Semiconductor

Part No. DM74LS221
Description  Dual Non-Retriggerable One-Shot with Clear and Complementary Outputs
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Maker  FAIRCHILD [Fairchild Semiconductor]
Homepage  http://www.fairchildsemi.com

DM74LS221 Datasheet(HTML) 2 Page - Fairchild Semiconductor

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Functional Description
The basic output pulse width is determined by selection of
an external resistor (RX) and capacitor (CX). Once trig-
gered, the basic pulse width is independent of further input
transitions and is a function of the timing components, or it
may be reduced or terminated by use of the active low
CLEAR input. Stable output pulse width ranging from 30 ns
to 70 seconds is readily obtainable.
Operating Rules
An external resistor (RX) and an external capacitor
(CX) are required for proper operation. The value of CX
may vary from 0 to approximately 1000
µF. For small
time constants high-grade mica, glass, polypropylene,
polycarbonate, or polystyrene material capacitor may
be used. For large time constants use tantalum or spe-
cial aluminum capacitors. If timing capacitor has leak-
ages approaching 100 nA or if stray capacitance from
either terminal to ground is greater than 50 pF the tim-
ing equations may not represent the pulse width the
device generates.
When an electrolytic capacitor is used for CX a switch-
ing diode is often required for standard TTL one-shots
to prevent high inverse leakage current. This switching
diode is not needed for the DM74LS221 one-shot and
should not be used.
Furthermore, if a polarized timing capacitor is used on
the DM74LS221, the positive side of the capacitor
should be connected to the “CEXT” pin (Figure 1).
For CX >> 1000 pF, the output pulse width (tW) is
defined as follows:
where [RX is in kΩ]
[CX is in pF]
[tW is in ns]
≈ Ln2 = 0.70
The multiplicative factor K is plotted as a function of CX
for design considerations: (See Figure 4).
5. For CX < 1000 pF see Figure 3 for tW vs. CX family
curves with RX as a parameter.
To obtain variable pulse widths by remote trimming,
the following circuit is recommended: (See Figure 2).
Output pulse width versus VCC and temperatures: Fig-
ure 5 depicts the relationship between pulse width vari-
ation versus VCC. Figure 6 depicts pulse width variation
versus temperatures.
8. Duty cycle is defined as tW/T × 100 in percentage, if it
goes above 50% the output pulse width will become
shorter. If the duty cycle varies between LOW and
HIGH values, this causes output pulse width to vary, or
jitter (a function of the REXT only). To reduce jitter, REXT
should be as large as possible, for example, with
REXT = 100k jitter is not appreciable until the duty cycle
approaches 90%.
9. Under any operating condition CX and RX must be kept
as close to the one-shot device pins as possible to min-
imize stray capacitance, to reduce noise pick-up, and
to reduce I-R and Ldi/dt voltage developed along their
connecting paths. If the lead length from CX 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-inductive and low capacitive path is necessary to
ensure complete discharge of CX in each cycle of its
operation so that the output pulse width will be accu-
10. Although the DM74LS221's pin-out is identical to the
DM74LS123 it should be remembered that they are not
functionally identical. The DM74LS123 is a retrigger-
able device such that the output is dependent upon the
input transitions when its output “Q” is at the “High”
DM74LS123 to externally ground the CEXT pin for
improved system performance. However, this pin on
the DM74LS221 is not an internal connection to the
device ground. Hence, if substitution of an DM74LS221
onto an DM74LS123 design layout where the CEXT pin
is wired to the ground, the device will not function.
11. VCC and ground wiring should conform to good high-
frequency standards and practices so that switching
transients on the VCC 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 VCC to ground is necessary on each device. Fur-
thermore, the bypass capacitor should be located as
close to the VCC-pin as space permits.

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