M74HC4538(2008) 데이터 시트보기 (PDF) - STMicroelectronics

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M74HC4538
(Rev.:2008)

Dual retriggerable monostable multivibrator

STMicroelectronics 

M74HC4538

2

Functional description

Functional description

Standby state

The external capacitor Cx, is fully charged to VCC in the standby state. Hence, before

triggering, transistor Qp and Qn (connected to the Rx/Cx node) are both turned-off. The two

comparators that control the timing and the two reference voltage sources stop operating.

The total supply current is therefore only leakage current.

Trigger operation

Triggering occurs when:

– A is low and B has a falling edge

– B is high and A has a rising edge

After the multivibrator has been retriggered, the comparator C1 and C2 start operating and

Qn is turned on. Cx then discharges through Qn. The voltage at the node Rx/Cx external

falls.

When it reaches VREFL the output of comparator C1 becomes low. This in turn resets the

flip-flop and Qn is turned off.

At this point C1 stops functioning but C2 continues to operate.

The voltage at R/C external begins to rise with a time constant set by the external

components Rx and Cx.

Triggering the multivibrator causes Q to go high after internal delay due to the flip-flop and

the gate. Q remains high until the voltage at R/C external rises again to VREFH . At this point

C2 output goes low and G goes low. C2 stops operating. That means that after triggering

when the voltage R/C external returns to VREFH the multivibrator has returned to its

monostable state. In the case where Rx · Cx are large enough and the discharge time of the

capacitor and the delay time in the IC can be ignored, the width of the output pulse tw(out) is

as follows:

tW(OUT) = 0.72Cx • Rx

Re-triggered operation

When a second triggered pulse follows the first, its effect will depend on the state of the

multivibrator. If the capacitor Cx is being charged, the voltage level of Rx/Cx external falls to

VREFL again and Q remains high i.e. the retrigger pulse arrives in a time shorter than the

period Rx · Cx seconds, the capacitor charging time constant. If the second trigger pulse is

very close to the initial trigger pulse it is ineffective; i.e. the second trigger must arrive in the

capacitor discharge cycle to be ineffective; hence the minimum time for a second trigger to

be effective, trr (min.) depends on VCC and Cx.

Reset operation

CD is normally high. If CD is low, the trigger is not effective because Q output goes low and

trigger control flip-flop is reset. Also transistor Op is turned on and Cx is charged quickly to

VCC. Then, if CD input goes low the IC becomes waiting state both in operating and non

operating state.

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