SCLS467 Data sheet

SCLS467F FEBRUARY 2003 – June 2016 SN74LV123A-Q1

PRODUCTION DATA.

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9 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.

9.1 Application Information

The SNx4LV123A device is a dual monostable multivibrator. It can be configured for many pulse width outputs and rising or falling-edge triggering. The application shown here could be used to signal separate interruptable inputs on a microcontroller when an input had a rising or falling edge.

9.2 Typical Application

Figure 11. Simplified Application Schematic

9.2.1 Design Requirements

NOTE

To prevent malfunctions due to noise, connect a high-frequency capacitor between V_CC and GND, and keep the wiring between the external components and C_ext and R_ext/C_ext terminals as short as possible.

9.2.1.1 Output Pulse Duration

The output pulse duration, t_w, is determined primarily by the values of the external capacitance (C_T) and timing resistance (R_T). The timing components are connected as shown in Figure 12.

Figure 12. Timing-Component Connections

If C_T is ≥1000 pF and K = 1.0, the pulse duration is given by Equation 1:

Equation 1. t_w = K × R_T × C_T

where

t_w = pulse duration in ns
R_T = external timing resistance in kΩ
C_T = external capacitance in pF
K = multiplier factor

if C_T is <1000 pF, K can be determined from Figure 5

Equation 1 and Figure 16 can be used to determine values for pulse duration, external resistance, and external capacitance.

9.2.1.2 Retriggering Data

The minimum input retriggering time (t_MIR) is the minimum time required after the initial signal before retriggering the input. After t_MIR, the device retriggers the output. Experimentally, it also can be shown that to retrigger the output pulse, the two adjacent input signals must be t_MIR apart, where t_MIR = 0.30 × t_w. The retrigger pulse duration is calculated as shown in Figure 13.

Figure 13. Retrigger Pulse Duration

The minimum value from the end of the input pulse to the beginning of the retriggered output must be approximately 15 ns to ensure a retriggered output (see Figure 14).

Figure 14. Input and Output Requirements

9.2.2 Detailed Design Procedure

Timing requirements:
- The pulse width must be long enough to be read by the desired output system, but short enough so that the output pulse completes prior to the next trigger event. It is recommended to make the output pulse just 10% longer than the minimum required for the output system.
Recommended input conditions:
- Slow or noisy inputs are allowed on A, B, and CLR due to Schmitt-trigger input circuitry.
- Specified high and low levels. See (V_IH and V_IL) in Recommended Operating Conditions.
Recommended output conditions:
- Load currents must not exceed the values listed in Absolute Maximum Ratings.

9.2.3 Application Curves

Operation of the devices at these or any other conditions beyond those indicated under ⁽¹⁾ is not implied.

Figure 15. Output Pulse Duration vs. External Timing Capacitance

Figure 17. Output Pulse Duration
vs External Timing Capacitance

Figure 16. Output Pulse Duration vs. External Timing Capacitance

Figure 18. Variations in Output Pulse Duration
vs Temperature