EDN Access--04.24.97 Circuit forms clockless, programmable one-shot

-April 24, 1997

April 24, 1997

Circuit forms clockless, programmable one-shot
John Stauber, Picker International Inc, Highland Heights, OH

Replace the timing resistor on a 74LS123 one-shot with a Howland current pump, drive the pump with a rail-to-rail voltage-output DAC, and you have a programmable one-shot with some unique features: single-supply operation, no clock required, a 25-to-1 pulse with adjustment range, and an "infinite" pulse-width capability. Figure 1 shows four such programmable one-shots having four overlapping ranges: 10 to 250 µsec, 100 to 2500 µsec, 1 to 25 µsec, and 10 to 250 msec. The following discussion refers to the top one-shot in Figure 1.

The current pump produces charging current I1, which equals (VOUTAxR14)/(R13xR15). Because this current is constant, the voltage across capacitor C11 increases at a constant rate, equal to (I1xtCHG)/C11, where tCHG is the charging time. When the LS123 triggers, the timing capacitor first discharges to approximately 0.75V, set by an internal transistor. The discharge time increases as a function of the value of C11 and equals approximately (89xC11)/VOUTA(MAX).

The discharge time remains relatively constant over the entire programmable range. Therefore, the one-shot's resultant pulse width is a function of the discharge time (tDIS) plus the time (tCHG) to charge the capacitor to 2V up from 0.75V, or

You should be aware of some caveats: First, because the pulse width, tPW, is inversely proportional to the DAC voltage, as VOUTA approaches 0V, the resultant longer pulse widths become difficult to accurately predict. The difficulty arises because, at the lower voltages, the transfer function yields greater changes in pulse widths for small changes in VOUTA. In theory, when VOUTA=0V, tPW is infinite. This infinite pulse width does, in fact, occur in practice and can be desirable in some applications. However, you cannot accurately predict the point at which tPW goes to infinity.

Empirically, it appears that the pulse width always becomes infinite at a voltage somewhere below 0.1V. Therefore, the usable range of VOUTA is between 0.1V and VOUT(MAX). You then obtain the "infinite" setting by programming the DAC to 0V. In the "infinite" mode, once the one-shot triggers, its output remains asserted until a negative pulse at the CLR input resets the circuit. Another caveat: In addition to the rail-to-rail DAC, you should use a rail-to-rail op amp to implement the Howland current pump. Also, you must match the four 10-kilohms resistors in each current pump to within ±0.1% to maintain pump accuracy. Although this technique works with any LS123 one-shot, experience shows that those from Texas Instruments have the shortest discharge times and follow the formulas most closely. (DI #2011)

Figure 1
By using a rail-to-rail DAC and a Howland current pump, you can turn a garden-variety LS123 one-shot into a single-supply, clockless, infinitely programmable one-shot circuit.

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Copyright c 1997 EDN Magazine, EDN Access . EDN is a registered trademark of Reed Properties Inc, used under license. EDN is published by Cahners Publishing Company , a unit of Reed Elsevier Inc.

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