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LM211: maximum voltage allowed on LM211 input

sunil barot60
Prodigy 20 points
Part Number: LM211

We are using LM211DR in one of our products.What I noticed is max voltage rating on input pin has a note #4 that restricts input voltage to be less than or equal to VCC in case VCC is less than 15 V. We are using VCC of 5 V. Our present design is applying 12 V on input which clearly exceeds the rating you have specified. We have not received any complain from field. I want know if the note # 4 in datasheet is valid or it is carry over from old datasheet.
Another reason for me to believe that the restriction is superfluous is that ON semi makes same IC and we have it in out list as an alternative but it does not have such restriction.
 

  • 0
    Guru 140200 points
    Hi Sunil,

    common mode input voltage range is also crucial for proper performance! Have you read section 9.2.2.1 of datasheet?

    Kai
  • 0 in reply to kai klaas69
    Prodigy 20 points
    Kai,
    Section 6.1 is about "absolute maximum rating" and does not refer to any common mode voltage allowance when the note 4 clearly says as follows.
    "The magnitude of the input voltage must never exceed the magnitude of the supply voltage or ±15 V, whichever is less."
    Section 9.2.2.1 is just a design example and recommendations. Vicr it is referring to is part of electrical spec with 15 V as Vcc. We are using 5 V Vcc hence the confusion.
    Let me know if I misunderstood your comments.
  • 0
    TI__Mastermind 49956 points

    Hello Sunil,

    Note 4 is a "Whichever is less", and since 5V is less than 15, then the max is 5V.

    With a single +5V supply, the specified  input range is +0.5V to +3.5V, so you are clearly violating the input range.

    These older devices do not have ESD diode clamps to the rails, so if the rails are exceeded, they will not clamp. Bringing the input above Vcc+ reverse biases the PNP input pair and turns off the input transistor on that side.

    The On-Semi part has the same limitations (Note 2 on page 3). While there can be 30V *between* the inputs (deferentially), there is a 15V limitation on the each-input (common mode) voltage to the "midpoint" of the supplies. This becomes an issue if using large, asymmetrical supplies.

    In any case - the input voltages cannot exceed the supplies (Vcc+ or Vcc-....especially Vcc-).

    They may seem to be "working", because as long as one of the inputs is within the legal input range, at least one side of the input pair is working and the output should be correct (the input stage is forced off-balance).

    HOWEVER, this may change over temp or device variations and should not be relied upon. This is not specified behavior and reversals or oscillations could occur. .

    Note that when in this mode, the prop delay and offset specifications are out-the-window. It will have a recovery time when the input voltage finally comes within range.

    You got lucky! Officially we have to say to lower the input voltage to 3.5V or less. Comparators do not need an absolute voltage level, so adding or adjusting the attenuation can bring the input levels within legal range.