54HC4538 データシートの表示（PDF） - STMicroelectronics

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54HC4538

DUAL RETRIGGERABLE MONOSTABLE MULTIVIBRATOR

STMicroelectronics 

BLOCK DIAGRAM

M 54/ M74H C4538

Notes : 1. Cx, Rx, Dx are external components.

2. Dx is a clamping diode.

3. The external capacitor is charged to VCC in the stand-by state, i.e. no trigger. When the supply voltage is turned off Cx is discharged

mainlythrough an internal parasitic diode (see figures). IfCx is sufficiently largeand VCC decreases rapidy, there willbesome possibility

of damaging the I.C. with a surge current or latch-up. If the voltage supply filter capacitor is large enough and VCC decrease slowly,

the surge current is automatically limited and damage the I.C. is avoided. The maximum forward current of the parasitic diode is ap-

proximately 20 mA. In cases where Cx is large the time taken for the supply voltage to fall to 0.4 VCC can be calculated as follows :

tf ≥ (VCC – 0.7) ⋅ Cx/20 mA

In cases where tf is too short an external champing diode is required to protect the I.C. from the surge current.

FUNCTIONAL DESCRIPTION

STAND-BY STATE

The external capacitor, Cx, is fully charged to VCC

in the stand-by state. Hence, before triggering, tran-

sistor 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 :

1 st) A is ”low” and B has a falling edge ;

2 nd) B is ”high” and A has a rising edge ;

After the multivibrator has been retriggered com-

parator 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, Cx.

Triggering the multivibrator causes Q to go high after

internal delay due to the flip-flop and the gate. Q re-

mains 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 at R/C external re-

turns 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 ca-

pacitor and the delay time in the I.C. can be ignored,

the width of the output pulse tw (out) is as follows :

tW(OUT) = 0.72 Cx • Rx

RE-TRIGGER OPERATION

When a second trigger pulse follows the first its ef-

fect 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.

3/14

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