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TC74VHC123AFT Datasheet(PDF) 6 Page - Toshiba Semiconductor |
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TC74VHC123AFT Datasheet(HTML) 6 Page - Toshiba Semiconductor |
6 / 14 page TC74VHC123,221AF/AFT/AFK 2012-02-29 6 Functional Description (1) Standby state The external capacitor (CX) is fully charged to VCC in the stand-by state. That means, before triggering, the QP and QN transistors which are connected to the RX/CX node are in the off state. Two comparators that relate to the timing of the output pulse, and two reference voltage supplies turn off. The total supply current is only leakage current. (2) Trigger operation Trigger operation is effective in any of the following three cases. First, the condition where the A input is low, and the B input has a rising signal; second, where the B input is high, and the A input has a falling signal; and third, where the A input is low and the B input is high, and the CLR input has a rising signal. After a trigger becomes effective, comparators C1 and C2 start operating, and QN is turned on. The external capacitor discharges through QN. The voltage level at the RX/CX node drops. If the RX/CX voltage level falls to the internal reference voltage VrefL, the output of C1 becomes low. The flip-flop is then reset and QN turns off. At that moment C1 stops but C2 continues operating. After QN turns off, the voltage at the RX/CX node starts rising at a rate determined by the time constant of external capacitor CX and resistor RX. Upon triggering, output Q becomes high, following some delay time of the internal F/F and gates. It stays high even if the voltage of RX/CX changes from falling to rising. When RX/CX reaches the internal reference voltage VrefH, the output of C2 becomes low, the output Q goes low and C2 stops its operation. That means, after triggering, when the voltage level of the RX/CX node reaches VrefH, the IC returns to its MONOSTABLE state. With large values of CX and RX, and ignoring the discharge time of the capacitor and internal delays of the IC, the width of the output pulse, tw (OUT), is as follows: tw (OUT) = 1.0 · CX · RX (3) Retrigger operation (TC74VHC123A) When a new trigger is applied to either input A or B while in the MONOSTABLE state, it is effective only if the IC is charging CX. The voltage level of the RX/CX node then falls to VrefL level again. Therefore the Q output stays high if the next trigger comes in before the time period set by CX and RX. If the new trigger is very close to previous trigger, such as an occurrence during the discharge cycle, it will have no effect. The minimum time for a trigger to be effective 2nd trigger, trr (min.), depends on VCC and CX. (4) Reset operation In normal operation, the CLR input is held high. If CLR is low, a trigger has no effect because the Q output is held low and the trigger control F/F is reset. Also, QP turns on and CX is charged rapidly to VCC. This means if CLR is set low, the IC goes into a wait state. |
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