HC123 datasheet(4/6 Pages) NSC

4 / 6 page

Logic Diagram

TLF5206 – 5

Theory of Operation

TLF5206 – 6

POSITIVE EDGE TRIGGER

POSITIVE EDGE RE-TRIGGER (PULSE LENGTHENING)

NEGATIVE EDGE TRIGGER

RESET PULSE SHORTENING

POSITIVE EDGE TRIGGER

CLEAR TRIGGER

FIGURE 1

TRIGGER OPERATION

As shown in

Figure 1 and the logic diagram before an input

trigger occurs the one shot is in the quiescent state with the

Q output low and the timing capacitor CEXT completely

charged to VCC When the trigger input A goes from VCC to

GND (while inputs B and clear are held to VCC) a valid trig-

ger is recognized which turns on comparator C1 and N-

channel transistor N1 j At the same time the output latch

is set With transistor N1 on the capacitor CEXT rapidly dis-

charges toward GND until VREF1 is reached At this point

the output of comparator C1 changes state and transistor

N1 turns off Comparator C1 then turns off while at the

same time comparator C2 turns on With transistor N1 off

the capacitor CEXT begins to charge through the timing re-

HC123 Datasheet(PDF) 4 Page - National Semiconductor (TI)