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DM74LS123 Datasheet(PDF) 2 Page - Fairchild Semiconductor

Part No. DM74LS123
Description  Dual Retriggerable One-Shot with Clear and Complementary Outputs
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Maker  FAIRCHILD [Fairchild Semiconductor]
Homepage  http://www.fairchildsemi.com
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DM74LS123 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 may be extended by retrigger-
ing the gated active-LOW transition or active-HIGH
transition inputs or be reduced by use of the active-LOW or
CLEAR input. Retriggering to 100% duty cycle is possible
by application of an input pulse train whose cycle time is
shorter than the output cycle time such that a continuous
“HIGH” logic state is maintained at the “Q” output.
Operating Rules
1. An external resistor (RX) and an external capacitor (CX)
are required for proper operation. The value of CX 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 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 DM74LS123 one-shot and
should not be used. In general the use of the switching
diode is not recommended with retriggerable operation.
Furthermore, if a polarized timing capacitor is used on
the DM74LS123 the negative terminal of the capacitor
should be connected to the “CEXT” pin of the device
(Figure 1).
FIGURE 1.
3. For CX >> 1000 pF the output pulse width (tW) is
defined as follows:
tW = KRX CX
where [RX is in kΩ]
[CX is in pF]
[tW is in ns]
K
≈ 0.37
4. The multiplicative factor K is plotted as a function of CX
below for design considerations:
FIGURE 2.
5. For CX < 1000 pF see Figure 3 for tW vs. CX family
curves with RX as a parameter:
FIGURE 3.
6. To obtain variable pulse widths by remote trimming, the
following circuit is recommended:
FIGURE 4.
“Rremote” should be as close to the device pin as possible.
7. The retriggerable pulse width is calculated as shown
below:
T
= t
W + tPLH = K × RX × CX + tPLH
The retriggered pulse width is equal to the pulse width
plus a delay time period (Figure 5).
FIGURE 5.


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