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M74HC123 Datasheet(PDF) 4 Page - STMicroelectronics |
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M74HC123 Datasheet(HTML) 4 Page - STMicroelectronics |
4 / 12 page M74HC123 4/12 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) = 0.45 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 is being charged the voltage level of R/C 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 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 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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