Semiconductor Components Industries, LLC, 1999
February, 2000 – Rev. 1
1 Publication Order Number:
AN1558/D
AN1558/D
Characterization of
Retrigger Time in the
HC4538A Dual Precision
Monostable Multivibrator
Prepared by: Douglas M. Buzard, Rodolfo E. Soto
Introduction
The 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 + T
rr
. 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 Data
When used in the retriggerable mode (Figure 1), the
MC74HC4538A uses an external R
x
& C
x
to regulate the
output pulse width, and the minimum retrigger time (T
rr
).
The minimum retrigger time depends on:
1) Time to discharge R
x
C
x
from V
CC
to (V
ref
lower=1/3
V
CC
) T
discharge
. This discharge occurs quickly
because external resistance, R
x
, does not have any
effect on the R
C
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:
T
discharge
+
ǒ
Ln
3
2
Ǔ
@
R
i
@
C
x
(Equation 1)
Typically the value of R
i
would be near 300Ω.
2) Loop delay (T
delay
= constant) ranges from 20–60ns,
and is strongly correlated to V
CC
. 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 capacitance the undershoot
voltage is well below the lower reference voltage.
3) The time to charge R
x
C
x
from the undershoot voltage
back to the lower reference voltage (V
ref
lower). This
time is given by the R
x
C
x
transient equation:
T
charge
+
R
x
@
C
x
@
Ln
ǒ
1
)
3
@
V
undershoot
2
@
V
CC
Ǔ
(Equation 2)
where V
undershoot
= (V
ref
lower) – Gnd. Hence the
retrigger time is given by:
(Equation 3)
T
rr
= T
discharge
+ T
delay
+ T
charge
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