HD74HC4538 Datasheet, PDF (7/16 Pages) Hitachi Semiconductor – Dual Precision Retriggerable/Resettable Monostable Multivibrators

HD74HC4538
Circuit Operation
Fig. 3 shows the HC4538 configured in the retriggerable mode. Briefly, the device operates as follows
(refer to Fig. 1): 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 references voltage (Vref Lower 1/3
V_CC). C_X then charges, through R_X, back up to the upper reference voltage (Vref 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 function diagram (Fig. 1)
and the timing diagram (Fig. 2)
Quiescent State
In the quiescent state, before an input trigger appears; the output latch is high and the reset latch is high (1
in Fig. 2). Thus the Q output (pin 6 or 10) of the monostable multivibrator is low (2 Fig. 2).
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 the upper reference circuit has a low output (5).
Transistor M4 is turned on and analog switch S1 is turned off. Thus the lower reference circuit has V_CC at
the noninverting input and a resulting low output (6).
In addition, the output of the trigger-control reset circuit is low.
Trigger Operation
The HC4538 is triggered by either a rising-edge signal as 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 three events. First, the output latch goes low, thus taking the Q
output of the HC4538 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 the upper reference circuit comparator). Third, transistor M4 is turned off and analog switch
S1 is turned on, thus allowing the voltage across C _X to also appear at the input of the 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 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). In addition, transistor M4 is turned on and analog switch S1 is turned off. Thus a high voltage level
is applied to the input of the lower reference circuit comparator, causing its output to go low (16). The
monostable multivibrator may be retriggered at any time after the trigger-control circuit goes low.
When C_X charges up to the reference voltage of the upper reference circuit (17), the output of the upper
reference circuit goes low (18). This causes the output latch to toggle, taking the Q output of the HC4538
to a low state (19), and completing the time-out cycle.
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