Fairchild Semiconductor DM74121N, DM74121CW Datasheet

© 1999 Fairchild Semiconductor Corporation DS006538 www.fairchildsemi.com

June 1989
Revised November 1999

DM74121 One-Shot with Clear and Complementary Outputs

DM74121
One-Shot with Clear and Complementary Outputs

General Description

The DM74121 is a mono stable mul tivibrator featuring bo th
positive and negat ive edge trigg ering with comp lementary
outputs. An internal 2kΩ timing resistor is provided for
design convenience min imizing compo nent count and l ay­out problems. this dev ice can be used with a single ex ter­nal capacitor. Inputs (A) are active- LOW trigger transition
inputs and input (B) i s and act ive-HIGH tr ansition Schmitt­trigger input that allows jitter-free triggering from inputs with
transition rates as slow as 1 volt/se cond. A high immunity
to V

CC

noise of typically 1 .5V is also provid ed by internal
circuitry at the input stage.
To obtain optimum and tro uble free opera tion please read

operating rules and one-shot application notes carefully
and observe recommendations.

Features

■ Triggered from active-HIGH transition or active-LOW
transition inputs

■ Variable pulse width from 30 ns to 28 seconds

■ Jitter free Schmitt-trigger input

■ Excellent noise immunity typically 1.2V

■ Stable pulse width up to 90% duty cycle

■ TTL, DTL compatible

■ Compensated for V

CC

and temperature variations

■ Input clamp diodes

Ordering Code:

Connection Diagram Function Table

H = HIGH Logic Level ↑ = Positive Going Transition
L = LOW Logic Level ↓ = Negative Going Transition
X = Can Be Either LOW or HIGH



= A Positive Pulse



= A Negative Pulse

Functional Description

The basic output pulse w idth is determ ined by sele ction of
an internal resistor R

INT

or an external resistor (RX) and

capacitor (C

X

). Once triggered the output pulse width is

independent of further transiti ons of th e inputs an d is func ­tion of the timing comp on ent s. P ulse w idth c an vary from a

few nano-seconds to 28 se conds by choosing a ppropriate
R

X

and CX combinations. Ther e are three trigger inputs

from the device, two n egative edge-triggering (A) inp uts,
one positive edge Schmitt-triggering (B) input.

Order Number Package Number Package Description

DM74121N N14A 14-Lead Plastic Dual-In-Line Package (PDIP), JEDEC MS-001, 0.300 Wide

Inputs Outputs

A1 A2 B Q Q

LXHLH
XLHLH
XXLLH
HHXLH
H ↓ H



↓ HH



↓↓H



LX↑



XL↑



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DM74121

Operating Rules

1. To use the internal 2 kΩ timing resistor, connect the
R

INT

pin to VCC.

2. An external resi stor (R

X

) or the interna l resisto r (2 kΩ)

and an external capacitor (C

X

) are required for prope r

operation. The value of C

X

may vary from 0 to any nec-

essary value. For small time constants use high-quality
mica, glass, polypropylene, po lycarbonate, or polysty­rene capacitors. For large time constants use solid tan­talum or special aluminum capacitors. If the timing
capacitors have leakages approaching 100 nA or if
stray capacitance from either terminal to ground is
greater than 50 pF the tim in g eq ua tions may no t re pre­sent the pulse width the device generates.

3. The pulse width is esse ntially determined by external
timing components R

X

and CX. For CX < 1000 p F see

Figure 1 design c ur ves o n t

W

as function of t i min g com-

ponents value. For C

X

> 1000 pF the output is define d
as:
t

W

= K RX C

X

where [RX is in Kilo-ohm]

[C

X

is in pico Farad]

[t

W

is in nano second]

[K ≈ 0.7]

4. If C

X

is an electrolytic ca pacitor a switching diode is

often required fo r standard TTL one-shots to pr event
high inverse leakage current Figure 2.

5. Output pulse width versus V

CC

and operation tempera -

tures: Figure 3 depicts the relationship betw een pulse
width variation versus V

CC.

Figure 4 depicts pulse

width variation versus ambient temperature.

6. The “K” coefficient is not a constant, but varies as a
function of the timing capacit or C

X

. Figure 5 details this

characteristic.

7. Under any operating condition C

X

and RX must be kept

as close to the one-shot device pins as possible to min­imize stray capacitan ce, to reduce noise pick -up, and
to reduce I X R and Ldi/dt voltage developed along
their connecting path s. If the lead length from C

X

to

pins (10) and (11) is greater than 3 cm , for example,
the output pulse width might be quite different from val­ues predicted from th e appropriate equations. A non ­inductive and low capacitive path is necessary to
ensure complete discharge of C

X

in each cycle of its

operation so that the output pulse w idth will be accu­rate.

8. V

CC

and ground wiring shou ld conform to good high-

frequency standards and practices so that switching
transients on the V

CC

and ground return le ads do not

cause interaction between one-shots. A 0.01 µF to 0.10
µF bypass capacitor (disk cer amic or monolithic type)
from V

CC

to ground is necessary on each device. Fur -

thermore, the bypass capacitor should be located as
close to the V

CC

-pin as space permits.

For further detaile d device characteristics and output per­formance please refer to the one-shot application note, AN-

366.

FIGURE 1.

FIGURE 2.

FIGURE 3.

FIGURE 4.

FIGURE 5.