74VHC221A 데이터 시트보기 (PDF) - Fairchild Semiconductor
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74VHC221A Datasheet PDF : 9 Pages
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Truth Table
H = HIGH Voltage Level
L = LOW Voltage Level
X: Don’t Care
Inputs
A B
H
XL
H X
L
LH
XX
= HIGH-to-LOW Transition
= LOW-to-HIGH Transition
CLR
H
H
H
H
L
Block Diagrams
Outputs
Q Q
LH
L H
LH
Function
Output Enable
Inhibit
Inhibit
Output Enable
Output Enable
Reset
Note A: Cx, Rx, Dx are external Capacitor, Resistor, and Diode, respectively.
Note B: External clamping diode, Dx;
External capacitor is charged to VCC level in the wait state, i.e. when no trigger is applied.
If the supply voltage is turned off, Cx discharges mainly through the internal (parasitic) diode. If Cx is sufficiently large and VCC drops rapidly, there will be
some possibility of damaging the IC through in rush current or latch-up. If the capacitance of the supply voltage filter is large enough and VCC drops slowly,
the in rush current is automatically limited and damage to the IC is avoided.
The maximum value of forward current through the parasitic diode is ±20 mA. In the case of a large Cx, the limit of fall time of the supply voltage is deter-
mined as follows:
tf ≥ (VCC −0.7) Cx/20 mA
(tf is the time between the supply voltage turn off and the supply voltage reaching 0.4 VCC)
In the event a system does not satisfy the above condition, an external clamping diode (Dx) is needed to protect the IC from rush current.
System Diagram
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