MC74HC4538A_16 Datasheet, PDF (10/14 Pages) ON Semiconductor – Dual Precision Monostable Multivibrator

MC74HC4538A
CIRCUIT OPERATION
Figure 11 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 10) and the
timing diagram (Figure 11).
QUIESCENT STATE
In the quiescent state, before an input trigger appears, the
output latch is high and the reset latch is high (#1 in Figure
11). Thus the Q output (pin 6 or 10) of the monostable
multivibrator is low (#2, Figure 11).
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.
TRIGGER OPERATION
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_xC_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.
QUIESCENT TRIGGER CYCLE
STATE
(A INPUT)
TRIGGER CYCLE
(B INPUT)
RESET
RETRIGGER
7
TRIGGER INPUT A
(PIN 4 OR 12)
TRIGGER INPUT B
(PIN 5 OR 11)
8
TRIGGER-CONTROL
CIRCUIT OUTPUT
R_X/C_X INPUT
(PIN 2 OR 14)
3
4
12
9
14
11
15
V_ref LOWER ¹³
UPPER REFERENCE
5
CIRCUIT
13
17
V_ref UPPER
18
LOWER REFERENCE
6
16
CIRCUIT
RESET INPUT
(PIN 3 OR 13)
1
RESET LATCH
Q OUTPUT
2
(PIN 6 OR 10)
10
τ
19
τ
Figure 11. Timing Diagram
t_rr
24
21 23
25
20
22
τ + t_rr
http://onsemi.com
10