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M74HC423 Folha de dados(PDF) 3 Page - STMicroelectronics |
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M74HC423 Folha de dados(HTML) 3 Page - STMicroelectronics |
3 / 14 page 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 : tf ≥ (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, 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 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 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) = 0.46 Cx ⋅ Rx (HC423) tW(OUT) =Cx ⋅ Rx (HC423A) M54/M74HC423/423A 3/14 |
Nº de peça semelhante - M74HC423 |
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Descrição semelhante - M74HC423 |
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