DM74LS123M datasheet(4/6 Pages) NSC

4 / 6 page

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

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