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LM211: Operating IN- at the VCC- rail level

Steven Sherwood
Prodigy 40 points
Part Number: LM211
Other Parts Discussed in Thread: TL331, LM311, LM111

The LM211 application I have uses a single +23.5Vdc bias voltage applied to VCC+ of the LM211 comparator with the VCC- pin grounded.  For one state of the comparator, the IN+ pin of the comparator will be at 13.4Vdc and the IN- pin of the comparator will be at 0volts.  The voltage at the IN- pin will therefore violate the common mode input voltage requirement of being 0.5volts above the VCC- pin.  I have seen threads which say that the LM211 comparator output will be in the correct state as long as one of the inputs is within the required common-mode input voltage range.  So will the LM211 comparator output function properly with 0volts applied to the IN- pin, or should I make a design change to keep the IN- pin 0.5V above the VCC- pin?

  • 0
    TI__Mastermind 23395 points

    Steven

    Please allow us one more day to verify but I believe that you will need to level shift the IN- to be at least 0.5V above (VCC-).  For the LM211, if one of the inputs is within the common mode range, the other can be greater than the common mode range and the output will be the appropriate logic level.  However, that does not mean that having one input in the common mode range allows the other to be less than the common mode range.  You probably selected the LM211 for its unique output stage but if that is not necessary and you only need an open-collector output, the TL331 would be a good choice because it allows ground sensing.

    Chuck

  • 0
    Guru 140200 points

    Hi Steven,

    even if the comparator output stays in the correct state, leaving the recommended common mode input voltage range can heavily degrade the performance of comparator (output slew rate, propagation delay times, etc.). So, it's not wise to violate recommended operating conditions.

    Kai 

  • 0 in reply to Chuck Sins
    Prodigy 40 points

    The following sample circuit from the Texas Instruments LM111, LM211, LM311 datasheet, SLCS007K revised in March 2017, shows the comparator IN- pin swinging from 0V to 4.91V.  Just thought I would show this as an example of operating the IN- input at the VCC- rail level.

  • 0 in reply to Steven Sherwood
    Prodigy 40 points

    I am not sure that the picture of the sample circuit was included in the thread I posted.  I was referencing the sample circuit on sheet 16 of the SLCS007K datasheet titled "Figure 19 TTL Interface With High-Level Logic", which has a logic input which swings from 0V to 30V.

  • 0
    TI__Mastermind 49896 points

    Hello Steven,

    If you read the apps section 9.2.2.1 carefully, it states that you can violate the UPPER common mode range as long as the other input is within range. It does not mention NEGATIVE input range at all.

    Looking at the "Functional Block Diagram" schematic, grounding (typing the input to V-) either input chokes off the current mirror to the following NPN stage. You may see a trend on which way the output "flops" in this condition, but that can change over device lots and temperature.

    Our advice is to NOT ground the input and stay within the valid input range, particularly the negative limit.

    For more info, see the LM311 appnote:

    http://www.ti.com/lit/pdf/snoa642

  • 0 in reply to Paul Grohe
    Prodigy 40 points

    Paul,

    Thank you for the advice.  I thought it was interesting that section 9.2.2.1 does not discuss the effects of operating one of the inputs below the common mode voltage.  This might be useful information to add to section 9.2.2.1 in the future.  You might also consider removing "0V to 30V High-Level Logic" sample circuit in Figure 19 from the datasheet.

  • 0 in reply to Steven Sherwood
    TI__Mastermind 23395 points

    Steven

    I completely agree with your suggestions.  When we refresh the datasheet, we will make those edits.  Sorry for the confusion.  I am going to close this post since I presume you have no further questions.  If anything else comes up, please reply or start a new thread for us to respond to.

    Chuck