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SN74LVC1G123: Monostable with controlled pulse length by a external voltage reference

CHENGCHENG YAO
Intellectual 300 points

Part Number: SN74LVC1G123
Other Parts Discussed in Thread: LMC555, TLC555, NE555

Hello,

I am looking for pulse generator (by triggering) that has a voltage controlled pulse length. The pulse length is between 200 ns to 1 us. 

Thanks!

Chengcheng

  • 0
    TI__Mastermind 25225 points
    Hello Chengcheng,

    In theory you could use the SN74LVC1G123 as this device, but you would have to configure it slightly differently.
    Instead of connecting the negative terminal of the Cext capacitor to the Cext pin, you can connect it to your control voltage. This will change the time that the device takes to discharge and charge back up to the internal threshold of the device.
    Refer to Figure 2 in the datasheet for timing.

    That is one option, but likely a better option is to look at using something like the NE555 timer, or one of its equivalents. TLC555, LMC555 and so forth. You should be able to configure it to do what you want while having pin 5 be the control threshold voltage.

    Best,
    Michael
  • 0 in reply to Michael J Schultis
    Intellectual 300 points

    Hi Michael,

    Thank you so much for your fast response. Your suggestions are very helpful. Now I have a design in my mind using LMC55 and I would like to hear your opinion on whether it works or not,

     

    Goal: Given S1&S2, S3&S4, generate S32&42_COM and S12&12_COM using analog circuitry, as shown in Figure 1.

    • Pulse width of Scom can change based on a voltage reference (200 ns to 1 us).
    • The falling edge of S12&12_COM are aligned with and the rising edge of S3&4.

    Based on your recommendation, I plan to use two LMC555 to realize the variable pulse generation. I need to add an intermediate pulse in purple, as shown in Figure 2. This pulse is triggered by the falling edge of S1. Its pulse width is between 200 ns and 600 ns and it is controlled by the pin 5 voltage of LMC555.

    The first LMC555’s output falling edge triggers the second pulse, SCOM.  I may set the pulse width to be large enough such that it remains high until the rising edge of S3 reset it.

     

     

    Do you think it works? My main concern is the pulse width accuracy of the 555 timer. With 200 ns pulse width, the R and C can be 200 pF and 1.8 kOhm. Are those values too small? Around 20 to 30 ns of accuracy is good enough for my application.

     

    Thanks for your help!

    Chengcheng

     

  • 0 in reply to CHENGCHENG YAO
    TI__Mastermind 25225 points
    Hello Chengcheng,

    Based on what I see, it seems sound to me, however, I am not the product expert for this device.
    When you have a new question about a new part, it is recommended that you start a new thread so that the corresponding product expert is assigned to your post.

    If you copy your post into a new thread, and have the LMC555 as the part number for the post, then the product expert for that part will be able to help you directly and you will get better support for the LMC555.

    Best,
    Michael
  • 0 in reply to Michael J Schultis
    Intellectual 300 points
    Hello Michael,

    Thank you so much! I have posted a new thread, following your suggestions.

    Bests,
    Chengcheng
  • 0 in reply to CHENGCHENG YAO
    TI__Mastermind 25225 points
    Always very happy to help!

    Best,
    Michael