TC74HC221AP datasheet(2/13 Pages) TOSHIBA

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TC74HC221AP/AF/AFN

2007-10-01

Pin Assignment

IEC Logic Symbol

Block Diagram (Note 1)(Note 2)

Note 1: Cx, Rx, Dx are external

capacitor, resistor, and diode, respectively.

Note 2: External clamping diode, Dx;

The 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 is 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 determined 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 even a system does not satisfy the above condition, an external clamping diode (Dx) is needed to

protect the IC from rush current.

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