DM74121 datasheet(2/5 Pages) FAIRCHILD

Il numero della parte	DM74121

Spiegazioni elettronici	One-Shot with Clear and Complementary Outputs

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Produttore elettronici	FAIRCHILD [Fairchild Semiconductor]

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Operating Rules

1. To use the internal 2 k

Ω timing resistor, connect the

RINT pin to VCC.

2. An external resistor (RX) or the internal resistor (2 kΩ)

and an external capacitor (CX) are required for proper

operation. The value of CX may vary from 0 to any nec-

essary value. For small time constants use high-quality

mica, glass, polypropylene, polycarbonate, or polysty-

rene capacitors. For large time constants use solid tan-

talum or special aluminum capacitors. If the timing

capacitors have leakages approaching 100 nA or if

stray capacitance from either terminal to ground is

greater than 50 pF the timing equations may not repre-

sent the pulse width the device generates.

3. The pulse width is essentially determined by external

timing components RX and CX. For CX < 1000 pF see

Figure 1 design curves on tW as function of timing com-

ponents value. For CX > 1000 pF the output is defined

as:

tW = K RX CX

where [RX is in Kilo-ohm]

[CX is in pico Farad]

[tW is in nano second]

[K

≈ 0.7]

4. If CX is an electrolytic capacitor a switching diode is

often required for standard TTL one-shots to prevent

high inverse leakage current Figure 2.

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.

6. The “K” coefficient is not a constant, but varies as a

function of the timing capacitor CX. Figure 5 details this

characteristic.

7. Under any operating condition CX and RX must be kept

as close to the one-shot device pins as possible to min-

imize stray capacitance, to reduce noise pick-up, and

to reduce I X R and Ldi/dt 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 val-

ues 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 accu-

rate.

8. 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 0.01

µF to 0.10

µF 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 per-

formance please refer to the one-shot application note, AN-

366.

FIGURE 1.

FIGURE 2.

FIGURE 3.

FIGURE 4.

FIGURE 5.

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