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Single Retriggerable Monostable Multivibrator product to replace SN74123

Other Parts Discussed in Thread: SN74LVC1G123, SN74123

Hi All, I have a design with 5.3 VCC to SN74123 dual retriggerable monostable multivibrators. Since I am using only one multivibrator, can you recommend a single retriggerable monostable multivibrator with a smaller footprint for me to replace SN74123? I am not sure if the products like SN74LVC1G123 can be used directly.

Thank you for your help!

  • Hi Yi Huang,

    I am sure we can find you a suitable replacement.

    You mentioned 5.3 VCC, are there more design specifications of major concern?

    Regards,

    Joe
  • Hi Joe,

    Thank you for your reply. No, there is no more specification. I used 5.3VCC and input a square waveform, whose frequency can range from 1MHz to 5MHz. I am suing Rext=20k and Cext=4.7pF to get a pulse signal out. I just want to make sure the "Schmitt-Trigger Inputs" feature in SN74LVC1G123 (I did not see this feature in the name of SN74123 ) will not cause any unexpected issues in my application.
  • Yi Huang,

    The Schmitt trigger feature should not cause any unexpected issues. Schmitt trigger inputs are very useful any time a sine wave needs to be changed into a square wave, or to clean up a noisy input signal, or the input signal has slow edges that need to be faster. For your case the input is already a square wave, so the Schmitt trigger inputs is more of an "added feature" and should be of no concern.

    Based on your Rext and Cext values, it looks like the pulse duration (tw) for the SN74123 would be about 150 ns. When compared to the datasheet for the SN74LVC1G123, these values get you to about the same pulse duration, but I can't be sure since Figure 5 in the datasheet doesn't go lower than Cext = 10pF. You might have to slightly adjust if pulse duration is a major concern for you.

    In conclusion the SN74LVC1G123 should be a fine replacement!

    If you want more information on Schmitt triggers, here is a good paper:
    www.ti.com/.../scea046.pdf

    Best Regards,

    Joe
  • Hi Joe, thank you so much for your help! I have some additional questions about the Schmitt triggers in my application, Would you advise me?

    Currently, in my application, my input is a pure AC signal with zero DC offset. The peak to peak amplitude of this sin signal can be any fixed level between 0.8V to 8V in the steady state operation, while its frequency falls into the range of 1MHz to 5MHz. The sin signal can have some slight distortion in shape.

    Right now I am using 74HCT04 to convert this sinusoidal signal to a square wave as the input of 74LS123, to minimize the variation of the sinusoidal amplitude and the distortion.

    My question is, do you think using  SN74LVC1G123 , with the help of Schmitt triggers, I can remove 74HCT04 inverter? If the Schmitt triggers can converter my input to a square waveform with the same frequency with 0 to 5V amplitude, I guess there is no need for  74HCT04 inverter, the 1nF cap and 100kOhm resistor, is that correct?

    Thank you!

  • Yi Huang,

    I want to first point out that with zero DC offset and peak-to-peak amplitudes ranging from 0.8Vp-p to 8Vp-p would mean the input to the device is going as low as -4V. These devices should not have negative voltage on the inputs. Each are meant for 0 to 5.5V maximum on the inputs. You could add a DC offset to lose the negative voltages but then you would have a maximum voltage of 8V which is above the maximum of 5.5V.

    You are right that the '74HTC04, capacitor, and resistor could be dropped. However, by doing this the signal is no longer inverted. So once removed, the signal should be tied to input A. Input B should then be tied to ground. Also, may I ask why you implemented the 100k resistor? This seems like it would bias the '74HCT04 and cause it to oscillate.

    Regards,

    Joe
  • Hi Joe, thank you for your explanation.

    I was trying to use RC as a LPF of the output of the inverter to generate a DC offset at the input of the inverter, to add the offset to the pure AC signal. According to your explanation, I think I should still keep the inverter and the RC components, instead of injecting the AC signal to the SN74LVC1G123, is this correct?

    I have another general question of the operation of this monostable multivibrator. As you know I am using a periodic pulse signal to go to "B" of the vibrator, while A is GND and CLR is high. Can I say the pulse width of my input signal is not important as far as it can generate the pulse with the same frequency at "Q"? Because the pulse width of the signal at "Q" will be determined by REXT and CEXT, the pulse width of the input signal at "B" can tolerate the variation within a reasonable range. Is this a true statement?

    Thank you!

  • Yi Huang,

    Yi Huang said:

    I was trying to use RC as a LPF of the output of the inverter to generate a DC offset at the input of the inverter, to add the offset to the pure AC signal. According to your explanation, I think I should still keep the inverter and the RC components, instead of injecting the AC signal to the SN74LVC1G123, is this correct?

    I agree that the AC signal should not be directly connected to the SN74LVC1G123. Can you please tell me more about this AC signal? Why is the AC signal going as low as 0.8 Vp-p? I only ask because even with a DC offset added, the lower voltage signals won't trigger the device.


    The SN74LVC1G123 triggers on specified voltage levels VIH and VIL below. This device operating at 5.3V means that in order to register an input as HIGH the voltage needs to hit at least 3.71V, and for a LOW the signal needs to go below 1.59V.

    Now, since the AC signal fluctuates in amplitude and frequency, I recommend conditioning the AC signal using DC bias and a comparator instead of the '74HCT04 device with the resistor and capacitor.. This would give you a square wave output that ranges from 0V to 5V, matched to the AC signal frequency. This signal could then be sent to the SN74LVC1G123. The voltage divider resistors can be chosen to achieve a DC bias. I would recommend a bias of 4V to make sure the AC signal is no longer negative. The cap for the HPF should be chosen depending on the frequency of your signal.

    Yi Huang said:

    I have another general question of the operation of this monostable multivibrator. As you know I am using a periodic pulse signal to go to "B" of the vibrator, while A is GND and CLR is high. Can I say the pulse width of my input signal is not important as far as it can generate the pulse with the same frequency at "Q"? Because the pulse width of the signal at "Q" will be determined by REXT and CEXT, the pulse width of the input signal at "B" can tolerate the variation within a reasonable range. Is this a true statement?

    The output is triggered based on the rising or falling edge of the input signal. The pulse width of the output is dependent on the Rext and Cext values not the pulse width of your input. You need to also keep in mind the pulse retrigger time. This will increase the output pulse duration if the input is triggered within a specified time after the last trigger. This is also dependent on the Rext and Cext values. Please see page 6 of the datasheet and Figure 7 on page 8 of the datasheet.

  • Hi Joe,

    Thank you very much for your detailed instruction. My AC signal comes from a wireless coil, which may suffer from the misalignment. Therefore, its amplitude may vary within a wide range.  You pointed out the item that I did not pay attention before.  I will try your proposed circuit to replace the inverter plus RC solution.

    I have another question of the description of this circuit block. If my target is to use the rising signal of the input to trigger the output signal only, and to use the REXT and CEXT to determine the pulse width, what is a proper description of the block shown below? Can I simply call it a timer? Thank you!

  • Yi Huang,

    The block you have circled is the actual Monostable Multivibrator.

    Here is a link to an App Note that can explain more: www.ti.com/.../slva720.pdf

    -Joe
  • Hi Joe,

    I am testing this sn74lvc1g123 unit and have another question. What is the equation for calculating the pulse width using REXT and CEXT? For most cases, pulse width=ln(2)*R*C, but I found this equation is not applicable to sn74lvc1g123. I checked the datasheet but only found Fig. 13 is the test results between CEXT and pulse width. Do you have the equation for this unit?

    Thank you!

  • You stated before that you will be operating at Vcc = 5.3V, so Figure 5 in the datasheet will be your best reference.

    What is your desired output pulse width?

    I don't have an equation for you, but using the figure I can get you fairly close to the desired pulse width and then we can go from there.

    -Joe