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LM211-EP: output BJT Saturation and input common mode clarification

Michael T Schneider
Expert 3780 points
Part Number: LM211-EP
Other Parts Discussed in Thread: TL331-EP

Team, Can you help with the below questions:

  1. Saturation Voltage of the output BJT
    1. The datasheet specifies it at 0.23, typ to 0.4, max at 8mA. Do you have data on what we should expect when operating at the 0.5mA to 1mA range? Would be nice if you had a plot of Vsat verses collector current.
  2. Input Common mode Clarification
    1. The typical specification is 13.8 at the upper end and -14.7 at the lower for +/-15V supply. Section 9.2.2.1 is somewhat confusing in its description of the input voltage ranges. In particular When IN- is higher that common mode and In+ is within…  What about if it is lower than common mode, -14.9 or –VCC?

In the past I’ve always used (IN- + IN+)/2 is the common mode voltage and as long as that voltage is within the common mode range of the part you would be good to go (basically if both inputs don’t exceed the common mode range at the same time). I want to apply a reference to the non-inverting input and sense a voltage that will start at ground and ramp up with VCC- being at ground. I should be good with this design.

It’s been a while since I’ve used a comparator that did not include VCC- in the common mode range and I would rather check.

  • +1
    TI__Mastermind 23395 points

    Michael

    Sorry for the confusion and lack of information you require in the datasheet.  Some of our older parts just do not have as much characterization data to support them.

    In regards to VOL at lower sink current levels, the values are fairly linear at low currents.

    So if we use the 8mA example with 0.23V typ and 0.4V max, we calculate output impedances of approximately 29ohms typ and 50ohms max.

    So for a 1mA sink current, VOL typ would be in the 29mV range and the max would be 50mV.

    In regards to your VCM question, your understanding of VCM is for amplifies with negative feedback, not comparators.

    In amplifiers, there is generally negative feedback causing a virtual short across the inputs.  So VCM being the average of the two inputs is valid.

    But for a comparator, each input is evaluated individually.  There is no average of the inputs for calculating VCM.

    The LM211-EP is unique in that it does not allow ground sensing.  For over temp and process variations, the VCM range only goes down to 0.5V above VEE.

    So if you need to sense all the way down to VEE, you should consider using a different comparator.

    If you have selected the LM211-EP for its speed, the closest option would be the TL331-EP which allows for ground sensing.

    Chuck