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DM74LS221SJ Datasheet(PDF) 2 Page - Fairchild Semiconductor |
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DM74LS221SJ Datasheet(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 is independent of further input transitions and is a function of the timing components, or it may be reduced or terminated by use of the active low CLEAR input. Stable output pulse width ranging from 30 ns to 70 seconds is readily obtainable. 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 µF. For small time constants high-grade mica, glass, polypropylene, polycarbonate, or polystyrene material capacitor may be used. For large time constants use tantalum or spe- cial aluminum capacitors. If timing capacitor has leak- ages approaching 100 nA or if stray capacitance from either terminal to ground is greater than 50 pF the tim- ing 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 DM74LS221 one-shot and should not be used. Furthermore, if a polarized timing capacitor is used on the DM74LS221, the positive side of the capacitor should be connected to the “CEXT” pin (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 ≈ Ln2 = 0.70 4. The multiplicative factor K is plotted as a function of CX for design considerations: (See Figure 4). 5. For CX < 1000 pF see Figure 3 for tW vs. CX family curves with RX as a parameter. 6. To obtain variable pulse widths by remote trimming, the following circuit is recommended: (See Figure 2). 7. Output pulse width versus VCC and temperatures: Fig- ure 5 depicts the relationship between pulse width vari- ation versus VCC. Figure 6 depicts pulse width variation versus temperatures. 8. Duty cycle is defined as tW/T × 100 in percentage, if it goes above 50% the output pulse width will become shorter. If the duty cycle varies between LOW and HIGH values, this causes output pulse width to vary, or jitter (a function of the REXT only). To reduce jitter, REXT should be as large as possible, for example, with REXT = 100k jitter is not appreciable until the duty cycle approaches 90%. 9. 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-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-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. 10. Although the DM74LS221's pin-out is identical to the DM74LS123 it should be remembered that they are not functionally identical. The DM74LS123 is a retrigger- able device such that the output is dependent upon the input transitions when its output “Q” is at the “High” state. Furthermore, it is recommended for the DM74LS123 to externally ground the CEXT pin for improved system performance. However, this pin on the DM74LS221 is not an internal connection to the device ground. Hence, if substitution of an DM74LS221 onto an DM74LS123 design layout where the CEXT pin is wired to the ground, the device will not function. 11. 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. |
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