M54HC423F1R データシートの表示（PDF） - STMicroelectronics
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M54HC423F1R Datasheet PDF : 14 Pages
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BLOCK DIAGRAM
M54/M74HC423/423A
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)
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