MC74HC4538A
CIRCUIT OPERATION
Figure 12 shows the HC4538A configured in the
retriggerable mode. Briefly, the device operates as follows
(refer to Figure 10): In the quiescent state, the external
timing capacitor, C
x
, is charged to V
CC
. When a trigger
occurs, the Q output goes high and C
x
discharges quickly to
the lower reference voltage (V
ref
Lower
[
1/3 V
CC
). C
x
then charges, through R
x
, back up to the upper reference
voltage (V
ref
Upper
[
2/3 V
CC
), at which point the
one−shot has timed out and the Q output goes low.
The following, more detailed description of the circuit
operation refers to both the logic detail (Figure 9) and the
timing diagram (Figure 10).
QUIESCENT STATE
TRIGGER OPERATION
In the quiescent state, before an input trigger appears, the
output latch is high and the reset latch is high (#1 in
Figure 10). Thus the Q output (pin 6 or 10) of the monostable
multivibrator is low (#2, Figure 10).
The output of the trigger−control circuit is low (#3), and
transistors M1, M2, and M3 are turned off. The external
timing capacitor, C
x
, is charged to V
CC
(#4), and both the
upper and lower reference circuit has a low output (#5).
In addition, the output of the trigger−control reset circuit
is low.
QUIESCENT
STATE
TRIGGER CYCLE
(A INPUT)
The HC4538A is triggered by either a rising−edge signal
at input A (#7) or a falling−edge signal at input B (#8), with
the unused trigger input and the Reset input held at the
voltage levels shown in the Function Table. Either trigger
signal will cause the output of the trigger−control circuit to
go high (#9).
The trigger−control circuit going high simultaneously
initiates two events. First, the output latch goes low, thus
taking the Q output of the HC4538A to a high state (#10).
Second, transistor M3 is turned on, which allows the
external timing capacitor, C
x
, to rapidly discharge toward
ground (#11). (Note that the voltage across C
x
appears at the
input of both the upper and lower reference circuit
comparator).
When C
x
discharges to the reference voltage of the lower
reference circuit (#12), the outputs of both reference circuits
will be high (#13). The trigger−control reset circuit goes high,
resetting the trigger−control circuit flip−flop to a low state
(#14). This turns transistor M3 off again, allowing C
x
to begin
to charge back up toward V
CC
, with a time constant t = R
x
C
x
(#15). Once the voltage across C
x
charges to above the lower
reference voltage, the lower reference circuit will go low
allowing the monostable multivibrator to be retriggered.
TRIGGER CYCLE
(B INPUT)
RESET
t
rr
RETRIGGER
7
TRIGGER INPUT A
(PIN 4 OR 12)
TRIGGER INPUT B
(PIN 5 OR 11)
9
8
24
14
11
12
13
18
13
6
16
21
23
TRIGGER-CONTROL
CIRCUIT OUTPUT
R
X
/C
X
INPUT
(PIN 2 OR 14)
3
4
15
17
V
ref
LOWER
UPPER REFERENCE
CIRCUIT
LOWER REFERENCE
CIRCUIT
RESET INPUT
(PIN 3 OR 13)
1
5
V
ref
UPPER
25
20
RESET LATCH
10
22
Q OUTPUT
(PIN 6 OR 10)
2
19
τ
τ
τ
+
t
rr
Figure 12. Timing Diagram
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