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74VHC221A Datasheet(PDF) 2 Page - Fairchild Semiconductor |
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74VHC221A Datasheet(HTML) 2 Page - Fairchild Semiconductor |
2 / 12 page ©1994 Fairchild Semiconductor Corporation www.fairchildsemi.com 74VHC221A Rev. 1.2 2 Connection Diagram Logic Symbol IEEE/IEC Truth Table H = HIGH Voltage Level L = LOW Voltage Level = HIGH-to-LOW Transition = LOW-to-HIGH Transition X = Don’t Care Block Diagrams 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 possibil- ity 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 para- sitic diode is ±20 mA. In the case of a large Cx, the limit of fall time of the supply voltage is determined as follows: tf ≥ (VCC – 0.7) Cx / 20mA (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 condi- tion, an external clamping diode (Dx) is needed to protect the IC from rush current. Inputs Outputs Function A B CLR QQ HH Output Enable X LHL H Inhibit HX HL H Inhibit LH Output Enable LH Output Enable XX LL H Reset |
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