74121 multivibrator Datasheet

74121 Datasheet, PDF, Equivalent

Part Number



Monostable multivibrator


Fairchild Semiconductor

Total Page 5 Pages
Download 74121 Datasheet

June 1989
Revised November 1999
One-Shot with Clear and Complementary Outputs
General Description
The DM74121 is a monostable multivibrator featuring both
positive and negative edge triggering with complementary
outputs. An internal 2ktiming resistor is provided for
design convenience minimizing component count and lay-
out problems. this device can be used with a single exter-
nal capacitor. Inputs (A) are active-LOW trigger transition
inputs and input (B) is and active-HIGH transition Schmitt-
trigger input that allows jitter-free triggering from inputs with
transition rates as slow as 1 volt/second. A high immunity
to VCC noise of typically 1.5V is also provided by internal
circuitry at the input stage.
To obtain optimum and trouble free operation please read
operating rules and one-shot application notes carefully
and observe recommendations.
s Triggered from active-HIGH transition or active-LOW
transition inputs
s Variable pulse width from 30 ns to 28 seconds
s Jitter free Schmitt-trigger input
s Excellent noise immunity typically 1.2V
s Stable pulse width up to 90% duty cycle
s TTL, DTL compatible
s Compensated for VCC and temperature variations
s Input clamp diodes
Ordering Code:
Order Number Package Number
Package Description
14-Lead Plastic Dual-In-Line Package (PDIP), JEDEC MS-001, 0.300 Wide
Connection Diagram
Function Table
Functional Description
The basic output pulse width is determined by selection of
an internal resistor RINT or an external resistor (RX) and
capacitor (CX). Once triggered the output pulse width is
independent of further transitions of the inputs and is func-
tion of the timing components. Pulse width can vary from a
A1 A2
H = HIGH Logic Level
L = LOW Logic Level
X = Can Be Either LOW or HIGH
= A Positive Pulse
= A Negative Pulse
↑ = Positive Going Transition
↓ = Negative Going Transition
few nano-seconds to 28 seconds by choosing appropriate
RX and CX combinations. There are three trigger inputs
from the device, two negative edge-triggering (A) inputs,
one positive edge Schmitt-triggering (B) input.
© 1999 Fairchild Semiconductor Corporation DS006538

Operating Rules
1. To use the internal 2 ktiming resistor, connect the
RINT pin to VCC.
2. An external resistor (RX) or the internal resistor (2 k)
and an external capacitor (CX) are required for proper
operation. The value of CX may vary from 0 to any nec-
essary value. For small time constants use high-quality
mica, glass, polypropylene, polycarbonate, or polysty-
rene capacitors. For large time constants use solid tan-
talum 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 timing equations may not repre-
sent the pulse width the device generates.
3. The pulse width is essentially determined by external
timing components RX and CX. For CX < 1000 pF see
Figure 1 design curves on tW as function of timing com-
ponents value. For CX > 1000 pF the output is defined
tW = K RX CX
where [RX is in Kilo-ohm]
[CX is in pico Farad]
[tW is in nano second]
[K 0.7]
4. If CX is an electrolytic capacitor a switching diode is
often required for standard TTL one-shots to prevent
high inverse leakage current Figure 2.
5. Output pulse width versus VCC and operation tempera-
tures: Figure 3 depicts the relationship between pulse
width variation versus VCC. 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 capacitor CX. Figure 5 details this
7. Under any operating condition CX and RX must be kept
as close to the one-shot device pins as possible to min-
imize stray capacitance, to reduce noise pick-up, and
to reduce I X R and Ldi/dt voltage developed along
their connecting paths. If the lead length from CX to
pins (10) and (11) is greater than 3 cm, for example,
the output pulse width might be quite different from val-
ues predicted from the appropriate equations. A non-
inductive and low capacitive path is necessary to
ensure complete discharge of CX in each cycle of its
operation so that the output pulse width will be accu-
8. VCC and ground wiring should conform to good high-
frequency standards and practices so that switching
transients on the VCC and ground return leads do not
cause interaction between one-shots. A 0.01 µF to 0.10
µF bypass capacitor (disk ceramic or monolithic type)
from VCC to ground is necessary on each device. Fur-
thermore, the bypass capacitor should be located as
close to the VCC-pin as space permits.
For further detailed device characteristics and output per-
formance please refer to the one-shot application note, AN-

Features DM74121 One-Shot with Clear and Compleme ntary Outputs June 1989 Revised Novemb er 1999 DM74121 One-Shot with Clear an d Complementary Outputs General Descrip tion The DM74121 is a monostable multiv ibrator featuring both positive and neg ative edge triggering with complementar y outputs. An internal 2kΩ timing res istor is provided for design convenienc e minimizing component count and layout problems. this device can be used with a single external capacitor. Inputs (A ) are active-LOW trigger transition inp uts and input (B) is and active-HIGH tr ansition Schmitttrigger input that allo ws jitter-free triggering from inputs w ith transition rates as slow as 1 volt/ second. A high immunity to VCC noise of typically 1.5V is also provided by int ernal circuitry at the input stage. To obtain optimum and trouble free operati on please read operating rules and one- shot application notes carefully and ob serve recommendations. Features s Trig gered from active-HIGH transition or active-LOW transition inputs.
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