MC74HC4538A ,Dual Precision Monostable Multivibrator(Retriggerable, Resettable)
MC74HC4538A ,Dual Precision Monostable Multivibrator(Retriggerable, Resettable)
MC74HC4538A ,Dual Precision Monostable Multivibrator(Retriggerable, Resettable)
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MC74HC4538AD ,Dual Precision Monostable Multivibrator
MC74HC4538AD ,Dual Precision Monostable Multivibrator
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MC74HC4538A
Dual Precision Monostable Multivibrator
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Prepared by: Douglas M. Buzard, Rodolfo E. Soto
IntroductionThe MC74HC4538A is a monostable multivibrator
commonly used as a one–shot, or in applications that require
a pulse width of reliable dimensions. The pulse width and
the minimum retrigger time are usually well behaved over
the suggested pulse–width range of 1μs to 1 second.
However, some customers have found that in using shorter
than recommended pulse widths the retrigger time did not
behave as it had at longer pulse widths. ON Semiconductor
has done an overall characterization of the minimum
retrigger time in an investigation of this phenomenon.
The retrigger time is applicable when the device is
triggered a second time within the period of the output pulse.
When this happens, the output pulse remains high for a
period of τ + Trr. The earliest the part can be retriggered, or
the minimum retrigger time, is the focus of this
characterization. A trigger pulse on A or B inputs before this
minimum retrigger time would be ignored.
Analysis and DataWhen used in the retriggerable mode (Figure 1), the
MC74HC4538A uses an external Rx & Cx to regulate the
output pulse width, and the minimum retrigger time (Trr).
The minimum retrigger time depends on: Time to discharge RxCx from VCC to (Vref lower=1/3
VCC) Tdischarge. This discharge occurs quickly
because external resistance, Rx, does not have any
effect on the RC time constant. The resistance in the
discharge path, as seen in Figure 2, is the
on–resistance of M3, and the interconnect resistance.
The interconnection resistance is dependent on the
polysilicon sheet resistance, the metal sheet
resistance, and the contact resistance. The
interconnection resistance is heavily process
dependent, but fortunately it is small overall and
doesn’t vary significantly from lot to lot.
The discharge time can be computed from:
Typically the value of Ri would be near 300Ω. Loop delay (Tdelay = constant) ranges from 20–60ns,
and is strongly correlated to VCC. This is the time for
the signal coming from the lower reference circuit to
reset the flip–flop, and turn off M3. The amount of
the undershoot voltage is a function of the loop delay,
and for small values of
voltage is well below the lower reference voltage. The time to charge RxCx from the undershoot voltage
back to the lower reference voltage (Vref lower). This
time is given by the RxCx transient equation:
Tcharge
where Vundershoot = (Vref lower) – Gnd. Hence the
retrigger time is given by:
