74LS121 datasheet(4/6 Pages) NSC

4 / 6 page

Operating Rules (Continued)

5 Output pulse width versus VCC and operation tempera-

tures

Figure 3 depicts the relationship between pulse

width variation versus VCC Figure 4 depicts pulse width

variation versus ambient temperature

TLF6538 – 4

FIGURE 3

TLF6538 – 5

FIGURE 4

6 The ‘‘K’’ coefficient is not a constant but varies as a

function of the timing capacitor CX Figure 5 details this

characteristic

TLF6538 – 6

FIGURE 5

7 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 c R and Ldidt voltage developed along their

connecting paths If the lead length from CX to pins (10)

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

capacitive path is necessary to ensure complete dis-

charge of CX in each cycle of its operation so that the

output pulse width will be accurate

8 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

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