Characterization of Retrigger time in the HC4538A Dual Precision Monostable Multivibrator

AN1558/D
Characterization of
Retrigger Time in the
HC4538A Dual Precision
Monostable Multivibrator
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Prepared by: Douglas M. Buzard, Rodolfo E. Soto
APPLICATION NOTE
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 + 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.
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:
Tdischarge
Ln 3
2
Ri
Cx
Typically the value of Ri would be near 300Ω.
2) 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 capacitance the undershoot
voltage is well below the lower reference voltage.
3) 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
+ Rx
@ @ ǒ) @@
Cx
Ln 1
3
Vundershoot
2
VCC
Ǔ
(Equation 2)
1) 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
February, 2000 – Rev. 1
ǒ Ǔ@ @
(Equation 1)
Analysis and Data
When 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:
 Semiconductor Components Industries, LLC, 1999
+
where Vundershoot = (Vref lower) – Gnd. Hence the
retrigger time is given by:
Trr = Tdischarge + Tdelay + Tcharge
(Equation 3)
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Publication Order Number:
AN1558/D
AN1558/D
CX
CX
RX
RISING–EDGE
TRIGGER
VCC
VCC
A = GND
Q
Q
A
B
RX
B
Q
B = VCC
FALLING–EDGE
TRIGGER
RESET = VCC
Q
RESET = VCC
Figure 1. Retriggerable Monostable Circuitry
LOGIC DETAIL
(1/2 THE DEVICE)
RxCx
UPPER
REFERENCE
CIRCUIT
–
+
Vref UPPER
VCC
VCC
OUTPUT
LATCH
LOWER
REFERENCE
CIRCUIT
M1
2 kΩ
–
+
M2
Q
Vref LOWER
M3
Q
TRIGGER CONTROL
CIRCUIT
A
C
Q
TRIGGER CONTROL
RESET CIRCUIT
CB R
B
RESET
POWER
ON
RESET
RESET LATCH
Figure 2. MC74HC4538A Logic Circuit Detail
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QUIESCENT
STATE
TRIGGER CYCLE
(A INPUT)
TRIGGER CYCLE
(B INPUT)
RESET
RETRIGGER
trr
TRIGGER INPUT A
(PIN 4 OR 12)
TRIGGER INPUT B
(PIN 5 OR 11)
TRIGGER-CONTROL
CIRCUIT OUTPUT
RX/CX INPUT
(PIN 2 OR 14)
UPPER REFERENCE
CIRCUIT
LOWER REFERENCE
CIRCUIT
RESET INPUT
(PIN 3 OR 13)
RESET LATCH
Q OUTPUT
(PIN 6 OR 10)
τ
τ
τ + trr
Figure 3. Timing Diagram
Design and Applications
The output pulse width of the HC4538A is determined by
the external timing components, Rx and Cx, and can be
represented linearly as shown in Figure 10.
The array in Table 1 was generated to make a concise
study of the behavior for the retrigger time for short pulse
widths. A sample of 10 pieces from each of 7 non–consec–
utive wafer lots were tested at each condition.
The retrigger time for external capacitance that ranges
from 3000pF < Cx < 4.7µF, Region 3 on the graphs, can be
computed by making use of the following linear equation
(Equation 4).
Trr = 10z,
where z
+ ƪ –1062.41 – (0.1236764
ƪ
@
@
@
@
(3.5256
10 *16
(Log Cx) 2
(7.9452
10 *11
Rx 2 )
(1.8339
10 *4
Rx )
) (1.13509292
Rx) ) (5.9621
@
VCC)
Table 1. Test Matrix
) (5.1513
* (171.91718
10 *5
@
@
@
Cx/Rx
10pF
100pF
220pF
1000pF
2KΩ
4.5V
4.5V
4.5V
4.5V
10KΩ
3.0V
4.5V
3.0V
4.5V
3.0V
4.5V
3.0V
4.5V
100KΩ
3.0V
4.5V
3.0V
4.5V
3.0V
4.5V
3.0V
4.5V
1MΩ
3.0V
4.5V
3.0V
4.5V
3.0V
4.5V
3.0V
4.5V
10 *12
(Log Cx)
Log Cx)
@
@
@
@ )
@ƫ
@
(Log Cx) 3 ) – (2.875
(Log Cx)
Rx )
Rx 2 )
) (0.02312176
) (4.64784302
10 8
Equation 4. Retrigger Time for 4.7µF > Cx > 3000pF
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3
10 –17
Cx )
Rx 3 )
)
(4.03306325
Log Rx)
)
@
(Log Cx) 2 )
)
AN1558/D
1.0ms
T rr (sec)
100µs
10µs
1.0µs
1MΩ
100kΩ
10KΩ
0.1µs
2KΩ
Region 1
0.01µs
1.0pF
10pF
100pF
Region 2
1000pF
Region 3
0.01µF
0.1µF
1.0µF
10µF
100µF
Cx (Farad)
Figure 4. Retrigger Time versus Timing Capacitance at VCC = 4.5V
1.0ms
T rr (sec)
100µs
10µs
1.0µs
1MΩ
100kΩ
10KΩ
0.1µs
1.0pF
Region 2
Region 1
10pF
100pF
1000pF
0.01µF
Cx (Farad)
Region 3
0.1µF
1.0µF
10µF
Figure 5. Retrigger Time versus Timing Capacitance at VCC = 3.0V
It was determined from experiment and statistical analysis
of the data that the retrigger time for small values of external
capacitance within the range of 10pF < Cx < 1000pF, Region
1, can be characterized with the following linear equation
(Equation 5).
For values of 1000pF < Cx < 3000pF, the non–linear
portion of the curves are converging. In this region, Region
2, the equation was represented by too few measurements to
generate a reasonably accurate equation. Therefore, the
equation in Region 2 will remain underived. A value may be
approximated from the graphs in Figure 4 and Figure 5.
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AN1558/D
Trr = 10z,
where z
+ ƪ –315.29624–(0.082881
ƪ
@
@
(3.984
10 *7
(4.575
10 *10
(1.36599588
@
(Log Cx) 2
@
R x)
(Log Cx) 3 )
) (3.0657
* (1.124 10*
Cx) * (1.423 10 *
Rx 2 )
10 8
@
@ )
@
@ @ *
@ ƫ@ * @
@
VCC)–(0.3146338
5
10 *12
(Log Cx)
(Log Cx)
5
10 –16
4.3277
Rx )
Rx 2 )
(94.092747
Rx 3 )–
(9.467093
Log Cx)
@
)
(Log Cx) 2 )
*
Rx )
Equation 5. Retrigger Time for 10pF < Cx < 1000pF
value of Cx increases for the same resistance, Trr increases
as it takes longer to charge the larger capacitor. For values
of Rx > 10kΩ, this increasing undershoot of Vreflower and
the resultant increase in Tcharge negates any improvement in
Trr.
At small values of Cx, the circuit capacitance will also
come into play. The size of the undershoot of Vreflower can
vary as a function of normal process variance. This will also
introduce an uncertainty into Trr for these smaller values.
The curves and regression equations here were derived
statistically and only represent the mean of the variance in
7 non–consecutive production lots.
This difference in the non–zero value of Tcharge in Region
1 can also be seen in Figure 6 and Figure 7 as the slope of Trr
becomes zero as the undershoot becomes zero.
Also, note that in Figure 8 through Figure 11, this effect
has no influence on the Output Pulse Width as the Pulse
Width is controlled by RxCx and Vrefupper.
Here, the same components of:
Trr = Tdischarge + Tdelay + Tcharge
(Equation 3)
are still represented, but have become combined by the
linear regression. The constant and VCC dependent term still
derive from the loop delay, and serve to shift the components
along the vertical axis. The major difference between this
and the larger values of Cx is twofold.
First, over all of Region 3 the undershoot is effectively 0
volts. This results in Tcharge not contributing to Trr and the
predictable minimum Trr occurring in Region 2.
Second, as we progress to smaller values of capacitance
in Region 1, Cx is too small to support Vreflower as the
charge is drained through M3. This is why the resistance of
Rx now plays a role in Trr. This condition creates the
undershoot of Vreflower and the time of Tcharge is then
controlled by the current through Rx. This is also why as the
τ = 10z,
where z
+ ƪ –1.0059363–(7.6336
ƪ
(0.8635535
@
Log Cx)
10 –3
@
VCC)
* (9.3203
@
@ @
) (0.87653815
10 *3
Log Rx
Log Rx)
)
Log Cx)
ƫ
Equation 6. Pulse Width
Also, as we have stated above, as the value of Cx decreases
in the non–linear region, the total capacitance becomes more
dependent upon internal circuit capacitance. Since the
internal circuit capacitance is process dependent, it can vary
from lot to lot, and from manufacturing site to
manufacturing site. It is for this reason that the device is not
recommended to be used in this range, as doing so would
potentially result in inconsistent performance over large
production runs. The curves represented in this applications
note were made using linear regression on a number of lots
widely separated in time, but all from the same
manufacturing site. As a result, the curves can only be
regarded as statistical means, and may not represent the
performance of any particular device the customer may
encounter.
Equation 6 is a linear regression equation for calculating
the pulse width and is also made from the data means. From
the logarithmic plots in Figure 8 through Figure 11, it can be
seen that there is no cubic dependency similar to Trr, even at
the small values of capacitance. The pulse width is
completely controlled by the relationship between RxCx and
Vrefupper. This predictability of the pulse width has tempted
some customers into trying to use the part for very short
pulse widths. Unfortunately it has also resulted in
inconsistent performance for Trr.
Summary
While smaller pulse widths and Trr values can be
achieved, selection of the external components must take
into account the introduction of undershoot of Vreflower.
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AN1558/D
1.0E–04
0.1µF
T rr (sec)
1.0E–05
0.01µF
1.0E–06
1000pF
200pF
100pF
1.0E–07
10pF
1.0E–08
1.0E+03
1.0E+04
1.0E+05
1.0E+06
RESISTANCE (Ω)
Figure 6. Retrigger Time vs Resistance at VCC = 4.5V
1.0E–03
T rr (sec)
1.0E–04
0.1µF
1.0E–05
1000pF
200pF
0.01µF
1.0E–06
100pF
10pF
1.0E–07
1.0E+04
1.0E+05
RESISTANCE (Ω)
Figure 7. Retrigger Time vs Resistance at VCC = 3.0V
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1.0E+06
AN1558/D
1.0E–02
PULSE WIDTH (sec)
1.0E–03
1.0E–04
1.0E–05
1.0E–06
1MΩ
100kΩ
10kΩ
2kΩ
1.0E–07
1.0E–11
1.0E–09
1.0E–10
1.0E–08
1.0E–07
Cx (Farads)
Figure 8. Pulse Width vs Timing Capacitance at VCC = 4.5V
1.0E–02
1.0E–04
1.0E–05
1.0E–06
1MΩ
100kΩ
10kΩ
1.0E–07
1.0E–11
1.0E–09
1.0E–10
1.0E–08
Cx (Farads)
Figure 9. Pulse Width vs Timing Capacitance at VCC = 3.0V
10s
1s
VCC = 5 V, TA = 25°C
100ms
OUTPUT PULSE WIDTH (τ )
PULSE WIDTH (sec)
1.0E–03
10ms
1ms
100µs
1 MΩ
10µs 100 kΩ
1µs
10 kΩ
1 kΩ
100ns
0.00001 0.0001
0.001
0.01
0.1
CAPACITANCE (µF)
1
10
100
Figure 10. Output Pulse Width vs Timing Capacitance
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1.0E–07
AN1558/D
1.0E–02
10pF
PULSE WIDTH (sec)
1.0E–03
100pF
1.0E–04
200pF
1000pF
1.0E–05
0.01µF
0.1µF
1.0E–06
1.0E–07
1.0E+04
1.0E+05
1.0E+06
RESISTANCE (Ω)
Figure 11. Pulse Width versus Resistance at VCC = 3.0V
1.0E–02
1.0E–03
PULSE WIDTH (sec)
10pF
1.0E–04
100pF
1.0E–05
200pF
1.0E–06
1000pF
0.01µF
1.0E–07
1.0E+03
0.1µF
1.0E+05
1.0E+04
RESISTANCE (Ω)
Figure 12. Pulse Width versus Resistance at VCC = 4.5V
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1.0E+06
AN1558/D
Notes
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Notes
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Notes
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