HC123 datasheet(3/14 Pages) STMICROELECTRONICS

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BLOCK DIAGRAM

Note :

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

(2) Dx is a clamping diode.

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

through an internal parasitic diode (see figures). If Cx is sufficiently large and VCC decreases rapidy, there will be some possibility of da-

maging 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 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 :

≥ (VCC –0.7) ⋅ 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, 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 ;

3 rd) A is low and B is high and C1 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 R/C 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

O goes low. C2 stop operating. That means that

after triggering when the voltage 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) = 0.46 Cx

⋅ Rx (HC123)

tW(OUT) =Cx

⋅ Rx (HC123A)

M54/M74HC123/123A

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