DM74LS221 datasheet(5/6 Pages) NSC

5 / 6 page

Operating Rules (Continued)

8 Duty cycle is defined as TW T c 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 e 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 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 (6) and

(7) or pins (14) and (15) is greater than 3 cm for exam-

ple 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 Although the ’LS221’s pin-out is identical to the ’LS123

it should be remembered that they are not functionally

identical The ’LS123 is a retriggerable device such that

the output is dependent upon the input transitions when

its output ‘‘Q’’ is at the ‘‘High’’ state Furthermore it is

recommended for the ’LS123 to externally ground the

CEXT pin for improved system performance However

this pin on the ’LS221 is not an internal connection to

the device ground Hence if substitution of an ’LS221

onto an ’LS123 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 001 mF to 010

F 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

For further detailed device characteristics and output performance

please refer to the NSC one-shot application note AN-372

Physical Dimensions inches (millimeters)

16-Lead Small Outline Molded Package (M)

Order Number DM74LS221M

NS Package Number M16A

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