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DM74LS123NX Scheda tecnica(PDF) 2 Page - Fairchild Semiconductor |
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DM74LS123NX Scheda tecnica(HTML) 2 Page - Fairchild Semiconductor |
2 / 8 page www.fairchildsemi.com 2 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. |
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