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M74HC221

DUAL MONOSTABLE MULTIVIBRATOR

STMicroelectronics 

M74HC221

BLOCK DIAGRAM

(1) Cx, Rx, Dx are external components.

(2) Dx is a clamping diode.

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 di scharged mainly

trough an internal parasitic diode(see figures). If Cx is sufficiently large and Vcc decreases rapidly, there will be some 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 to the I.C. is avoided. The maximum forward current of the parasitic diode is approximately 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) x Cx/20mA

In cases where tf is too short an external clamping 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,

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 :

1 st) A is "LOW" and B has a falling edge;

2 nd) B is "HIGH" and A has a rising edge;

3 rd) A is "LOW" and B is HIGH and C1 has a

rising edge;

After the multivibrator has been retriggered

comparator C1 and C2 start operating and Qn is

turned on. Cx then discharges through Qn. The

voltage at the node R/C external falls.

When it reaches VREFL the output of comparator

C1 becomes low. This in turn reset 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 remains high until the voltage at R/C

external rises again to VREFH . At this point C2

output goes low and O goes low. C2 stop

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 I.C. can be ignored, the

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

tW(OUT) = 0.70 Cx · Rx

RESET OPERATION

CL is normally high. If CL 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. This means if CL input goes low the

IC becomes waiting state both in operating and

non operating state.

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