SN74LV123A: support 10s pulse output and PSPICE simulation file

Hi Alex,

Alex Xing said:

my customer have a requirement, use SN74LV123A to support 10s long pulse output, not sure does SN74LV123A support this? and any application advise here?

Yes, very long output pulses are possible, but they will require very large capacitor values. Note that there are limits on the resistor values, but not the capacitor.

In addition, the resistor should not exceed 20 kohm (for up to 125C operation) or 200kohm (for up to 85C operation) due to possible leakage current at the RC input:

I'm using a 0.5V maximum voltage drop on the resistor from the leakage current to calculate the maximum.

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Using these limits, to get a 10s pulse with up to 85C operation, then the approximate values will be:

10 = K * R * C

Using K = 1 for large capacitors (Figure 3)

R * C = 10

Selecting R = 10kohm (arbitrarily)

C = 10 / 10k = 1000uF

These would be my starting values, and then I would build the circuit with the same capacitor planned for the final version and adjust R to get the desired timing. The next step would be to test in a temperature chamber with all components to ensure that the pulse remains within the desired range for all operating temperatures.

Most likely the creator of the model forgot to update the version number when they added the wider pulse duration capabilities. There is no other version of this model.

Additionally, PSpice models for MMV circuits are no guarantee of functionality. I would highly recommend building a prototype circuit for the customer's application so they can determine the exact component values they need.

It's also important to note that these devices are not intended for precise timing applications -- if they need "more than 10 seconds" and it's ok for the pulse to be between about 10 and 20 seconds, then they can get that level of accuracy. If they need a more accurate signal, then a different solution would be better.

Thanks, Emrys, it helps a lot.