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USC EE 459Lx - DM74LS123

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© 2000 Fairchild Semiconductor Corporation DS006386 www.fairchildsemi.comAugust 1986Revised April 2000DM74LS123 Dual Retriggerable One-Shot with Clear and Complementary OutputsDM74LS123Dual Retriggerable One-Shot with Clear and Complementary OutputsGeneral DescriptionThe DM74LS123 is a dual retriggerable monostable multi-vibrator capable of generating output pulses from a fewnano-seconds to extremely long duration up to 100% dutycycle. Each device has three inputs permitting the choice ofeither leading edge or trailing edge triggering. Pin (A) is anactive-LOW transition trigger input and pin (B) is an active-HIGH transition trigger input. The clear (CLR) input termi-nates the output pulse at a predetermined time indepen-dent of the timing components. The clear input also servesas a trigger input when it is pulsed with a low level pulsetransition (). To obtain the best trouble free operationfrom this device please read the operating rules as well asthe Fairchild Semiconductor one-shot application notescarefully and observe recommendations.Features■ DC triggered from active-HIGH transition or active-LOWtransition inputs■ Retriggerable to 100% duty cycle■ Compensated for VCC and temperature variations■ Triggerable from CLEAR input■ DTL, TTL compatible■ Input clamp diodes Ordering Code:Devices also available in Tape and Reel. Specify by appending the suffix letter “X” to the ordering code.Connection Diagram Function TableH = HIGH Logic LevelL = LOW Logic LevelX = Can Be Either LOW or HIGH↑ = Positive Going Transition↓ = Negative Going Transition = A Positive Pulse = A Negative PulseOrder Number Package Number Package DescriptionDM74LS123M M16A 16-Lead Small Outline Integrated Circuit (SOIC), JEDEC MS-012, 0.150 NarrowDM74LS123SJ M16D 16-Lead Small Outline Package (SOP), EIAJ TYPE II, 5.3mm WideDM74LS123N N16E 16-Lead Plastic Dual-In-Line Package (PDIP), JEDEC MS-001, 0.300 WideInputs OutputsCLEAR A B Q QLXXLHXHXLHXXLLHHL↑H ↓ H↑ LHwww.fairchildsemi.com 2DM74LS123Functional DescriptionThe basic output pulse width is determined by selection ofan 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-HIGHtransition inputs or be reduced by use of the active-LOW orCLEAR input. Retriggering to 100% duty cycle is possibleby application of an input pulse train whose cycle time isshorter than the output cycle time such that a continuous“HIGH” logic state is maintained at the “Q” output.Operating Rules1. An external resistor (RX) and an external capacitor (CX)are required for proper operation. The value of CX mayvary from 0 to any necessary value. For small time con-stants high-grade mica, glass, polypropylene, polycar-bonate, or polystyrene material capacitors may beused. For large time constants use tantalum or specialaluminum capacitors. If the timing capacitors haveleakages approaching 100 nA or if stray capacitancefrom either terminal to ground is greater than 50 pF thetiming equations may not represent the pulse width thedevice generates.2. When an electrolytic capacitor is used for CX a switch-ing diode is often required for standard TTL one-shotsto prevent high inverse leakage current. This switchingdiode is not needed for the DM74LS123 one-shot andshould not be used. In general the use of the switchingdiode is not recommended with retriggerable operation.Furthermore, if a polarized timing capacitor is used onthe DM74LS123 the negative terminal of the capacitorshould be connected to the “CEXT” pin of the device(Figure 1).FIGURE 1. 3. For CX >> 1000 pF the output pulse width (tW) isdefined as follows:tW = KRX CXwhere [RX is in kΩ][CX is in pF][tW is in ns]K ≈ 0.374. The multiplicative factor K is plotted as a function of CXbelow for design considerations:FIGURE 2. 5. For CX < 1000 pF see Figure 3 for tW vs. CX familycurves with RX as a parameter:FIGURE 3. 6. To obtain variable pulse widths by remote trimming, thefollowing circuit is recommended:FIGURE 4. “Rremote” should be as close to the device pin as possible.7. The retriggerable pulse width is calculated as shownbelow:T = tW + tPLH = K × RX × CX + tPLHThe retriggered pulse width is equal to the pulse widthplus a delay time period (Figure 5).FIGURE 5.3 www.fairchildsemi.comDM74LS123Operating Rules (Continued)8. Output pulse width variation versus VCC and tempera-tures: Figure 6 depicts the relationship between pulsewidth variation versus VCC, and Figure 7 depicts pulsewidth variation versus temperatures.FIGURE 6. FIGURE 7. 9. Under any operating condition CX and RX must be keptas close to the one-shot device pins as possible to min-imize stray capacitance, to reduce noise pick-up, andto reduce I-R and Ldi/dt voltage developed along theirconnecting paths. If the lead length from CX to pins (6)and (7) or pins (14) and (15) is greater than 3 cm, forexample, the output pulse width might be quite differentfrom values predicted from the appropriate equations.A non-inductive and low capacitive path is necessary toensure complete discharge of CX in each cycle of itsoperation so that the output pulse width will be accu-rate.10. The CEXT pins of this device are internally connected tothe internal ground. For optimum system performancethey should be hard wired to the system’s returnground plane.11. VCC and ground wiring should conform to good high-frequency standards and practices so that switchingtransients on the VCC and ground return leads do notcause interaction between one-shots. A 0.01 µF to 0.10µF bypass capacitor (disk ceramic or monolithic type)from VCC to ground is necessary on each device. Fur-thermore, the bypass capacitor should be located asclose to the VCC-pin as space permits.Note: For further detailed device characteristics and output per-formance please refer to the Fairchild Semiconductor one-shotapplication note AN-372.www.fairchildsemi.com 4DM74LS123Absolute Maximum Ratings(Note 1)Note 1: The “Absolute Maximum Ratings” are those values beyond whichthe safety of the device cannot be guaranteed. The device should not beoperated at these limits. The parametric values defined in the ElectricalCharacteristics tables are not guaranteed at the absolute maximum ratings.The “Recommended Operating Conditions” table will define the conditionsfor actual device operation.Recommended Operating ConditionsNote 2: TA = 25°C and VCC = 5V.Electrical


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