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DM74LS123 Datasheet(PDF) 4 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Part No. DM74LS123
Description  Dual Retriggerable One-Shot with Clear and Complementary Outputs
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Maker  NSC [National Semiconductor (TI)]
Homepage  http://www.national.com
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DM74LS123 Datasheet(HTML) 4 Page - National Semiconductor (TI)

   
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Operating Rules (Continued)
5 For CX k 1000 pF see Figure 3 for TW vs CX family
curves with RX as a parameter
TLF6386 – 3
FIGURE 3
6 To obtain variable pulse widths by remote trimming the
following circuit is recommended
TLF6386 – 4
FIGURE 4
Note
‘‘Rremote’’ should be as close to the device pin as possible
7 The retriggerable pulse width is calculated as shown be-
low
T e TW a tPLH e K c RX c CX a tPLH
The retriggered pulse width is equal to the pulse width
plus a delay time period
(Figure 5)
TLF6386 – 5
FIGURE 5
8 Output pulse width variation versus VCC and tempera-
tures
Figure 6 depicts the relationship between pulse
width variation versus VCC and Figure 7 depicts pulse
width variation versus temperatures
TLF6386 – 6
FIGURE 6
TLF6386 – 7
FIGURE 7
9 Under any operating condition CX and RX must be kept
as close to the one-shot device pins as possible to mini-
mize stray capacitance to reduce noise pick-up and to
reduce I-R and Ldidt 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 accurate
10 The CEXT pins of this device are internally connected to
the internal ground For optimum system performance
they should be hard wired to the system’s return ground
plane
11 VCC and ground wiring should conform to good high-fre-
quency standards and practices so that switching tran-
sients on the VCC and ground return leads do not cause
interaction between one-shots A 001 mF to 010 mF
bypass capacitor (disk ceramic or monolithic type) from
VCC to ground is necessary on each device Further-
more the bypass capacitor should be located as close
to the VCC-pin as space permits
For further detailed device characteristics and output performance
please refer to the NSC one-shot application note AN-372
4


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