HD74HC4538 Datasheet, PDF (5/14 Pages) Hitachi Semiconductor – Dual Precision Retriggerable/Resettable Monostable Multivibrators

HD74HC4538
Circuit Operation
Figure 3 shows the HC4538 configured in the retriggerable mode. Briefly, the device operates as follows (refer to
figure 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 (figure 1) and the
timing diagram (figure 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 figure 2).
Thus the Q output (pin 6 or 10) of the monostable multivibrator is low (2 figure 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.
Reset Operation
A low voltage applied to the Reset pin always forces the Q output of the HC4538 to a low state.
The timing diagram illustrates the case in which reset occurs (20) while C_X is charging up toward the reference voltage
of the upper reference circuit (21). When a reset occurs, the output of the reset latch goes low (22), turning on transistor
M1. Thus C_X is allowed to quickly charge up to V_CC (23) to await the next trigger signal.
Retrigger Operation
When used in the retriggerable mode (figure 3), the HC4538 may be retriggered during timing out of the output pulse at
any time after the trigger-control circuit flip-flop has been reset (24). Because the trigger-control circuit flip-flop resets
shortly after C_X has discharged to the reference voltage of the lower reference circuit (25), the minimum retrigger time,
t_rr (Switching Waveform 1) is a function of internal propagation delays and the discharge time of C_X:
Figure 4 shows the device configured in the non-retriggerable mode.
Rev.2.00 Mar 30, 2006 page 5 of 13