DM74LS123 datasheet(3/6 Pages) NSC

3 / 6 page

Switching Characteristics at VCC e 5V and TA e 25 C

RL e 2kX

Symbol

Parameters

From (Input)

CL e 15pF

Units

To (Output)

CEXT e 0 pF REXT e 5kX

CEXT e 1000 pF REXT e 10 KX

Min

Max

Min

Max

tPLH

Propagation Delay Time

AtoQ

Low to High Level Output

tPLH

Propagation Delay Time

BtoQ

Low to High Level Output

tPHL

Propagation Delay Time

AtoQ

High to Low Level Output

tPHL

Propagation Delay Time

BtoQ

High to Low Level Output

tPLH

Propagation Delay Time

Clear to Q

Low to High Level Output

tPHL

Propagation Delay Time

Clear to Q

High to Low Level Output

tWQ(Min)

Minimum Width of Pulse

AorBtoQ

200

at Output Q

tW(out)

Output Pulse Width

A or B to Q

Operating Rules

1 An external resistor (RX) and an external capacitor (CX)

are required for proper operation The value of CX may

vary from 0 to any necessary value For small time con-

stants high-grade mica glass polypropylene polycarbon-

ate or polystyrene material capacitors may be used For

large time constants use tantalum or special aluminum

capacitors If the timing capacitors have leakages ap-

proaching 100 nA or if stray capacitance from either ter-

minal to ground is greater than 50 pF the timing equations

may not represent the pulse width the device generates

2 When an electrolytic capacitor is used for CX a switching

diode is often required for standard TTL one-shots to pre-

vent high inverse leakage current This switching diode is

not needed for the ’LS123 one-shot and should not be

used In general the use of the switching diode is not

recommended with retriggerable operation

Furthermore if a polarized timing capacitor is used on the

’LS123 the negative terminal of the capacitor should be

connected to the ‘‘CEXT’’ pin of the device (Figure 1 )

TLF6386 – 8

FIGURE 1

3 For CX ll 1000 pF the output pulse width (TW)isde-

fined as follows

TW e KRX CX

where RX is in kX

CX is in pF

TW is in ns

037

4 The multiplicative factor K is plotted as a function of CX

below for design considerations

TLF6386 – 2

FIGURE 2

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