NXP 74HCT123DB Monostable Multivibrator: Datasheet, Application Circuit, and Key Specifications

The NXP 74HCT123DB is a high-speed silicon-gate CMOS monostable multivibrator (one-shot) housed in a compact SSOP-16 package. It is designed to provide a precise output pulse of a determined duration in response to an input trigger signal. This device is particularly valued for its high noise immunity and low power consumption, typical of the HCT family, making it an ideal choice for interfacing between TTL and CMOS logic systems in noisy industrial or automotive environments.

A key feature of the 74HCT123 is that it contains two independent retriggerable monostable multivibrators in a single IC. Each multivibrator features active-low (A) and active-high (B) trigger inputs, offering flexibility in responding to different signal polarities. The output pulse width is primarily determined by an external resistor (R_ext) and an external capacitor (C_ext), allowing for a wide range of timing applications from microseconds to seconds. The device also provides direct clear inputs (CD), which can terminate the output pulse prematurely, adding another layer of control.

Key Specifications

Supply Voltage (VCC): 4.5 V to 5.5 V (standard 5V operation)

Logic Family: HCT (TTL-compatible CMOS inputs)

High-Level Input Voltage (VIH): min. 2.0 V

Low-Level Input Voltage (VIL): max. 0.8 V

Maximum Output Current: ±4 mA

Operating Temperature Range: -40°C to +125°C (suitable for industrial applications)

Package: SSOP-16 (DB)

Typical Propagation Delay: 15 ns

Application Circuit

A fundamental pulse generation circuit using one half of the 74HCT123 is straightforward. The output pulse width (t_w) is approximately calculated by the formula: t_w ≈ 0.28 R_ext C_ext (1 + 0.7/R_ext), where R_ext is in ohms and C_ext is in farads. For stable operation, it is recommended to use a capacitor with a value of 1 nF or greater and a resistor between 5 kΩ and 50 kΩ.

To configure a basic negative-edge-triggered one-shot:

1. Connect the B input pin to a logic HIGH state.

2. Apply the input trigger signal to the A input (active-low).

3. Connect an external timing resistor (R_ext) between the R_ext/C_ext pin and VCC.

4. Connect an external timing capacitor (C_ext) between the C_ext pin and GND.

5. The output pulse will appear on the corresponding Q output pin.

The retriggerable function allows the output pulse to be extended by applying a new trigger pulse during the active output state, which is crucial for debouncing circuits or detecting missing pulses.

Datasheet Essentials

The official datasheet from NXP Semiconductors is the ultimate source of information. Key sections to consult include:

Absolute Maximum Ratings: To avoid damaging the IC (e.g., maximum supply voltage).

DC and AC Electrical Characteristics: For detailed timing parameters, output drive capabilities, and power consumption figures.

Function Table: To understand the logic states of inputs, clear, and outputs.

Application Information: Includes graphs and formulas for determining pulse width under various conditions.

ICGOODFIND: The NXP 74HCT123DB is a versatile and robust solution for generating clean, precise timing pulses. Its TTL compatibility, wide operating temperature range, and retriggerable functionality make it a superior choice over basic 555 timer circuits for critical timing applications in digital systems, wave shaping, and noise elimination.

Keywords: Monostable Multivibrator, Retriggerable, Pulse Width, HCT Logic, Timing Circuit.