DM74123 データシートの表示（PDF） - Fairchild Semiconductor

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DM74123

Dual Retriggerable One-Shot with Clear and Complementary Outputs

Fairchild Semiconductor 

Operating Rules

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

are required for proper operation. The value of CX may

vary from 0 to any necessary value. For small time con-

stants high-grade mica, glass, polypropylene, polycar-

bonate, or polystyrene material capacitors may be

used. For large time constants use tantalum 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 represent the pulse width the

device generates.

2. When an electrolytic capacitor is used for CX a switch-

ing diode is often required for standard TTL one-shots

to prevent high inverse leakage current (Figure 1).

However, its use in general is not recommended with

retriggerable operation.

3. The output pulse width (TW) for CX > 1000 pF is

defined as follows:

TW = K RX CX(1 + 0.7/RX)

1. where: [RX is in Kilo-ohm]

[CX is in pico Farad]

[TW is in nano second]

[K ≈ 0.28]

FIGURE 1.

4.

For CX < 1000 pF see Figure 2 for TW vs. CX

family curves with RX as a parameter:

Pulse Width vs. RX and CX

5.

To obtain variable pulse width by remote trim-

ming, the following circuit is recommended:

Note: “Rremote” should be as close to the one-shot as possible.

FIGURE 3.

6.

The retriggerable pulse width is calculated as

shown below:

T = TW + tPLH = K× RX × CX + tPLH

The retriggered pulse width is equal to the

pulse width plus a delay time period (Figure

4).

FIGURE 4.

7.

Under any operating condition CX and RX

must be kept as close to the one-shot device

pins as possible to minimize stray capaci-

tance, to reduce noise pick-up, and to reduce

I × R and Ldi/dt 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-induc-

tive 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.

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. Furthermore, the bypass capaci-

tor should be located as close to the VCC pin

as space permits.

Note: For further detailed device characteristics and output performance

please refer to the One-Shot Application Note, AN-366.

FIGURE 2.

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