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DM74121 One-Shot with Clear and Complementary Outputs

June 1989 Revised November 1999

DM74121 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 2kΩ timing resistor is provided for design convenience minimizing component count and layoutproblems. this device can be used with a single external capacitor. Inputs (A) are active-LOW trigger transition inputs and input (B) is and active-HIGH transition Schmitttrigger 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 optimumand 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 compatibles Compensated for VCC and temperature variations s Input clamp diodes

Ordering Code:
Order Number DM74121N Package Number N14A Package Description 14-Lead Plastic Dual-In-Line Package (PDIP), JEDEC MS-001, 0.300 Wide

Connection Diagram

Function Table
Inputs A1 L X X H H ↓ ↓ L X A2 X L X H ↓ H ↓ X L B H H L X H H H ↑ ↑ Q L L L Outputs Q H H H

H = HIGH Logic Level L = LOW Logic LevelX = Can Be Either LOW or HIGH = A Positive Pulse = A Negative Pulse

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 function of the timing components. Pulse width can vary from a

↑ =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 DM74121

Operating Rules
1. To use the internal 2 kΩ 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 necessary value. For small time constants use high-quality mica, glass, polypropylene, polycarbonate, or polystyrene capacitors. Forlarge 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 timing equations may not represent 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 1design curves on tW as function of timing components value. For CX > 1000 pF the output is defined as: t W = 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 operationtemperatures: 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 characteristic. 7. Under any operating condition CX and RX must be kept as close to the one-shot device pins as possible to...
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