ST M74HC123 User Manual

M74HC123

DUAL RETRIGGERABLE MONOSTABLE MULTIVIBRATOR

■ HIGH SPEED :

= 23 ns (TYP.) at VCC = 6V

■ LOW POWER DISSIPATION:

STAND BY STATE : I

=4µA (MAX.) at TA=25°C

ACTIVE STATE : I

=200µA (MAX.) at V

■ HIGH NOISE IMMUNITY:

= V

NIH

■ SYMMETRICAL OUTPUT IMPEDANCE:

| = IOL = 4mA (MIN)

■ BALANCED PROPAGATION DELAYS:

≅ t

PLH

■ WIDE OPERATING VOLTAGE RANGE:

(OPR) = 2V to 6V

■ WIDE OUTPUT PULSE WIDTH RANGE :

WOUT

■ PIN AND FUNCTION COMPATIBLE WITH

= 28 % VCC (MIN.)

NIL

PHL

= 120 ns ~ 60 s OVER AT V

= 5V

= 4.5 V

74 SERIES 123

DESCRIPTION

The M74HC123 is an high speed CMOS MONOSTABLE MULTIVIBRATOR fabricated with silicon gate C There are two trigger inputs, A

MOS technology.

INPUT (negative edge) and B I NP UT (pos itive edg e). These inputs are valid for slow rising/falling signals, (tr=tf=l sec). The device may also be triggered by using the CLR

input (positive-edge) because of the Schmitt-trigger input; after triggering the output maintains the MONOSTABLE state for the time

TSSOPDIP SOP

ORDER CODES

PACKAGE TUBE T & R

DIP M74HC123B1R

SOP M74HC123M1R M74HC123RM13TR

TSSOP M74HC123TTR

period determined by the ex ternal resistor Rx a nd capacitor Cx. When Cx >

10nF and Rx > 10KΩ, the output pulse width value is approsimatively given by the formula : tW(OUT) = K · Cx · Rx.

≅ 0.45).

Taking CLR

low breaks this MONOSTABLE STATE. If the next trigger pul se oc curs du ring the MONOSTABLE period it makes the MONOSTABLE period longer. Limit for values of Cx and Rx : Cx : NO LIMIT Rx : V V

< 3.0V 5KΩ to 1MΩ

> 3.0V 1KΩ to 1MΩ

All inputs are equipped with protection circuits against static discharge and transient excess voltage.

PIN CONNECTION AND IEC LOGIC SYMBOLS

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M74HC123

INPUT AND OUTPUT EQUIVALENT CIRCUIT PIN DESCRIPTION

PIN No SYMBOL NAME AND FUNCTION

1,9 1A

2, 10 1B, 2B

3, 11

4, 12 1Q

13, 5 1Q, 2Q Outputs (Active High) 14, 6

8 GND Ground (0V)

16 Vcc Positive Supply Voltage

TRUTH TABLE

, 2A

1 CLR 2 CLR

, 2Q Outputs (Active Low)

X/CX

Trigger Inputs (Negative Edge Triggered)

Trigger Inputs (Positive Edge Triggered)

Direct Reset LOW and trigger Action at Positive Edge

External Resistor Capacitor Connection

External Capacitor Connection

External Resistor Capacitor Connection

INPUTS OUTPUTS

BCLRQQ

H H OUTPUT ENABLE

X L H L H INHIBIT H X H L H INHIBIT

L H OUTPUT ENABLE L H OUTPUT ENABLE

X X L L H INHIBIT

X : Don’t Care

NOTE

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SYSTEM DIAGRAM

M74HC123

This log i c diagram has not be used to estim at e propagation delays

TIMING CHART

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M74HC123

BLOCK DIAGRAM

(1) Cx, Rx , Dx are extern al components. (2) Dx is a clamping diode. The external capacitor is charged to Vcc in the stand-by-stat e, i.e. no trigger. When the supply vol tage is turned off Cx is discharged 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 volta ge supply filter capacitor is la rge enough and Vcc decrease s l owly, the surge cu rr ent 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 : t

> (Vcc - 0.7) x Cx/20mA

In cases where t

is too short an ext ernal clamping diode is required to prot ect the I.C. from t he surge current.

FUNCTIONAL DESCRIPTION

STAND-BY STATE The external capacit or,Cx, is f ully charged t o 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 t otal 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 Q n is turned on. Cx then discharges through Qn. The voltage at the node R/C external falls. When it reaches V

the output of comparat or

REFL

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 com ponents Rx, Cx. Triggering the multivibrator caus es 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 V

. At this point C2

REFH

output goes low and O goes low. C2 stop operating. That means that a fter triggering when the voltage R/C external returns to V

REFH

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

tW(OUT) = 0.45 Cx · Rx RE - TRIGGERED OPERATION When a second trigger pulse follows the first its

effect will depend on the state of the multivibrator. If the capacitor Cx i s being charged t he voltage level of R/C external falls to V

again and Q

REFL

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 depends on Vcc and Cx

RESET OPERATION CL is normally high. If CL is low, the trigger is not

effective because Q output goes low and trigger control f lip -f lo p is res et .

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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