DM74LS221 datasheet(4/6 Pages) NSC

4 / 6 page

Operating Rules

1 An external resistor (RX) and an external capacitor (CX)

are required for proper operation The value of CX may

vary from 0 to approximately 1000 mF For small time

constants high-grade mica glass polypropylene polycar-

bonate or polystyrene material capacitor may be used

For large time constants use tantalum or special alumi-

num capacitors If timing capacitor has leakages ap-

proaching 100 nA or if stray capacitance from either ter-

minal to ground is greater than 50 pF the timing equations

may not represent the pulse width the device generates

2 When an electrolytic capacitor is used for CX a switching

diode is often required for standard TTL one-shots to pre-

vent high inverse leakage current This switching diode is

not needed for the ’LS221 one-shot and should not be

used

Furthermore if a polarized timing capacitor is used on the

’LS221 the positive side of the capacitor should be con-

nected to the ‘‘CEXT’’ pin (Figure 1 )

TLF6409 – 8

FIGURE 1

3 For CX ll 1000 pF the output pulse width (TW)isde-

fined as follows

TW e KRX CX

where RX is in kX

CX is in pF

TW is in ns

Ln2 e 070

4 The multiplicative factor K is plotted as a function of CX

below for design considerations

TLF6409 – 3

FIGURE 2

5 For CX k 1000 pF see Figure 3 for TW vs CX family

curves with RX as a parameter

TLF6409 – 4

FIGURE 3

6 To obtain variable pulse widths by remote trimming the

following circuit is recommended

TLF6409 – 5

Note

‘‘Rremote’’ should be as close to the one-shot as possible

FIGURE 4

7 Output pulse width versus VCC and temperatures Figure

5 depicts the relationship between pulse width variation

versus VCC Figure 6 depicts pulse width variation versus

temperatures

TLF6409 – 6

FIGURE 5

TLF6409 – 7

FIGURE 6

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