Functional Description

The basic output pulse width is determined by selection of

an external resistor (RX) and capacitor (CX). Once trig-

gered, the basic pulse width may be extended by retrigger-

ing the gated active-LOW transition or active-HIGH

transition inputs or be reduced by use of the active-LOW or

CLEAR input. Retriggering to 100% duty cycle is possible

by application of an input pulse train whose cycle time is

shorter than the output cycle time such that a continuous

“HIGH” logic state is maintained at the “Q” output.

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 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. This switching

diode is not needed for the DM74LS123 one-shot and

should not be used. In general the use of the switching

diode is not recommended with retriggerable operation.

Furthermore, if a polarized timing capacitor is used on

the DM74LS123 the negative terminal of the capacitor

should be connected to the “CEXT” pin of the device

(Figure 1).

FIGURE 1.

3. For CX >> 1000 pF the output pulse width (tW) is

defined as follows:

tW = KRX CX

where [RX is in kΩ]

[CX is in pF]

[tW is in ns]

K

≈ 0.37

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

below for design considerations:

FIGURE 2.

5. For CX < 1000 pF see Figure 3 for tW vs. CX family

curves with RX as a parameter:

FIGURE 3.

6. To obtain variable pulse widths by remote trimming, the

following circuit is recommended:

FIGURE 4.

“Rremote” should be as close to the device pin as possible.

7. The retriggerable pulse width is calculated as shown

below:

T

= t

W + tPLH = K × RX × CX + tPLH

The retriggered pulse width is equal to the pulse width

plus a delay time period (Figure 5).

FIGURE 5.

www.sycelectronica.com.ar