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DM74121 Datasheet(PDF) 2 Page - Fairchild Semiconductor

Part No. DM74121
Description  One-Shot with Clear and Complementary Outputs
Download  5 Pages
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Maker  FAIRCHILD [Fairchild Semiconductor]
Homepage  http://www.fairchildsemi.com
Logo FAIRCHILD - Fairchild Semiconductor

DM74121 Datasheet(HTML) 2 Page - Fairchild Semiconductor

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Operating Rules
1. To use the internal 2 k
Ω timing resistor, connect the
RINT pin to VCC.
2. An external resistor (RX) or the internal resistor (2 kΩ)
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]
≈ 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-

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