HD74HC4538

7

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

occurs, the Q output goes high and C

1/3

V

C

2/3 V

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

Transistor M4 is turned on and analog switch S1 is turned off. Thus the lower reference circuit has V

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

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

comparator.

When C

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

(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

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.