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TMUX1219: TMUX1219

MARKUS FRANK
Prodigy 30 points
Part Number: TMUX1219
Other Parts Discussed in Thread: TLV7031, TLV8542, TLV7032

Hello,

With reference to page 15, Figure 10 (transition time) in the datasheet of TMUX1219, I would like to know the behavior of the TMUX switch, if the control voltage ramps very slowly in time. According to Figure 10, the control voltage in this measurement setup goes from 0 to VDD in under 5 ns. Take for example the TLV854X opamp from TI, which has a slew rate of 3.5 V/ms and assume this ouput would drive the control input of the switch. What will happen when the voltage approaches the threshold of the logic control? Will the TMUX instantly switch faster then the slew rate, i.e., is it a distinct behavior, or will it take some intermediate value, following the slewrate.

Best regards,
Markus

  • 0
    Guru 268630 points

    Digital inputs switch immediately when the threshold is reached. The datasheet has no limit on the input transition time (which would imply a Schmitt-trigger input), but in any case, keeping the voltage between VIL and VIH for a long time will result in a high shoot-through current (see [FAQ] How does a slow or floating input affect a CMOS device?), so should be avoided.

    It would be a better idea to replace the opamp with a comparator (e.g., TLV7031).

  • 0
    TI__Genius 10974 points

    Hi Markus,

    We are on U.S. holiday today. We will get back to you with a response by 7/5.

    Thanks!

    Bryan

  • 0
    TI__Mastermind 26553 points

    HeyMarkus,

    Clemens response is a good one to follow here. There are no limitations and the implication here is correct. There is a very small amount of hysteresis in the control inputs that mitigates most of the effects of slow switching. That being said, there are still some sensitive spots that may still allow for shoot through and oscillation in the logic control so it's best to avoid switching slowly and between the rails.

    Thanks!
    Rami

  • 0 in reply to Rami Mooti1
    Prodigy 30 points

    Hi all,

    Many thanks for the quick feedback! I actually don't want to close this yet. I simply haven't had the time to look into it since I was involved in other things.

    It was very good information from Clemens. So, basically from what I understand, you can use the TLV7031 for a windowcomparator, but the rampup time at the outputs is much faster. This is even listed as an application in the data sheet and this is precisely what I want to do.I need to do some transient simulations with the device. SPICE data is avaialble so that is good. I will start with this now.

    Best regards,
    Markus

  • 0 in reply to Rami Mooti1
    Prodigy 30 points

    Hi all,

    Many thanks for the quick feedback! I actually don't want to close this yet. I simply haven't had the time to look into it since I was involved in other things.

    It was very good information from Clemens. So, basically from what I understand, you can use the TLV7031 for a windowcomparator, but the rampup time at the outputs is much faster. This is even listed as an application in the data sheet and this is precisely what I want to do.I need to do some transient simulations with the device. SPICE data is avaialble so that is good. I will start with this now.

    Best regards,
    Markus

  • 0 in reply to MARKUS FRANK
    TI__Genius 10974 points

    Hi Markus,

    Thanks for the update here. Let us know if you have further questions and we will be happy to assist you.

    Thanks again,

    Bryan

  • 0 in reply to Bryan Marshall
    Prodigy 30 points

    Hi Bryan,

    I have now done some simulations. I don't know the details of the circuitry inside the TLV7032 (dual configuration I'm looking at) but to me it seems to behave just like the TLV8542, but without the shortcoming of slewrate. And the bias power consumption is also in the same range. I configured it as a window comparator and it has the very steep response as expected.

    One problem and challenge I bumped into is capturing the tON time though. This is a simulation problem. When I introduce a delay for the bias source, tON to me seems that there should be another delay of 200 us after the bias ramps up (to 1.6 V) until anything happens at the output. Is this correctly understood? I can't see this behavior in my simulations with the TI SPICE data. This is actually another desired characteristic. Furthermore, in the final application the bias signal is not very steep but builds up over some time, due to an RC time constant. But for now I use a simple linear rampup source in the simulation.

    Please advise.

    Best regards,
    Markus

  • 0 in reply to MARKUS FRANK
    TI__Mastermind 24260 points

    Markus

    Thanks for your interest in our TLV7032 comparator.  I am not sure that I am completely following your question but I will attempt to explain our tON spec.  This is the amount of time for the output to reflect the conditions on the comparator input after initial power up.  So once the comparator reaches min operating voltage, it should be expected that there will be a 200us delay until the comparator exits what we call power on reset condition and starts to track the inputs.  It really depends on the ramp rate of the supply to determine if the tON time of 200us is relevant.  We test this spec with a very fast power supply ramp, so we can accurately measure tON.  However, if your supply ramp rate is slow, in the range of 10s of msec per volt, tON is essentially not noticeable.  I don't believe we model tON in our simulation model but it is something we can consider for future revisions.

    Chuck

  • 0 in reply to Chuck Sins
    Prodigy 30 points

    Hi Chuck,

    Thank you very much for the explanation and you have understood my question perfectly. In the simulation I currently have a ramp up of 10 ns from zero to 2.5 V so it is indeed very fast. In the final applcation it will be slower, but still in the range of a couple of us or so I guess there will still be some delay. I use your SPICE model and if tON is not included, this explains it all of course, that I can't see it in the simulation.

    Best regards
    Markus

  • 0 in reply to MARKUS FRANK
    TI__Mastermind 24260 points

    Sorry Markus

    I confirmed my assumption that tON isn’t modeled. If you want to model externally you would need to use external switches. Not ideal but hopefully you can get something to work. If you need further help, we can reconnect on Monday or Tuesday. 
    Chuck

  • 0 in reply to Chuck Sins
    Prodigy 30 points

    Hi Chuck,

    At this point, I believe I'm fine. I think I have come as far as I can, concerning simulations. If I wanted to simulate this, I would need to have it included in the SPICE data, since I would want to have a more realistic response for a power ramp up which I believe is difficult to realize with the switch models I have. They are too ideal. This simply has to be checked experimentally so probably I will proceed making two versions of the board, one with the TLV8542 and one with the TLV7032, to compare how they behave in real life.

    Thank you very much for your help for now. If I have more questions later, I will open up a new case.

    Best regards,
    Markus