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DM74LS122 Datasheet(PDF) 4 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Part No. DM74LS122
Description  DM74LS122 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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DM74LS122 Datasheet(HTML) 4 Page - National Semiconductor (TI)

   
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Operating Rules (Continued)
The K factor is not a constant but varies with CX See
Figure 2
TLF6385 – 3
FIGURE 2
4 The switching diode required for most TTL one-shots
when using an electrolytic timing capacitor is not needed
for the ’LS122 and should not be used
5 To obtain variable pulse width by remote trimming the
following circuit is recommended
TLF6385 – 4
Note
‘‘Rremote’’ should be as close to the device pins as possible
FIGURE 3
6 The retriggerable pulse width is calculated as shown be-
low
T e TW a tPLH e 050 c RX c CX a TPLH
The retriggered pulse width is equal to the pulse width
plus a delay time period
(Figure 4)
TLF6385 – 5
FIGURE 4
7 Output pulse width variation versus VCC and operation
temperatures
Figure 5 depicts the relationship between
pulse width variation versus VCC and Figure 6 depicts
pulse width variation versus temperatures
TLF6385 – 6
FIGURE 5
TLF6385 – 7
FIGURE 6
8 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 con-
necting paths If the lead length from CX to pins (13) and
(11) 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 ca-
pacitive path is necessary to ensure complete discharge
of CX in each cycle of its operation so that the output
pulse width will be accurate
9 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 by-
pass capacitor (disk ceramic or monolithic type) from VCC
to ground is necessary on each device Furthermore 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-366
4


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