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LM6172QML-SP: Does the part have phase reversal?

Raymond Chow4
Prodigy 50 points
Part Number: LM6172QML-SP
Other Parts Discussed in Thread: OPA690, LM6172, LM6172QML, TL084

What does the output of the part look like when saturated? Does the part have phase-reversal protection?

  • 0
    TI__Genius 14540 points
    Hi,

    I will look into your questions and get back to you shortly.

    Thanks
    Christian
  • 0 in reply to Christian Yots
    TI__Genius 14540 points
    Hi,

    From what I've read about phase reversal, it is possible when the input signal exceeds the power supply rails of the device. There is no on-chip protection to prevent this. If a particular system is susceptible to this scenario, external precautions should be employed. There is no characterization data available for a saturated output as operation in this region is not recommended.

    Thanks
    Christian
  • 0 in reply to Christian Yots
    Guru 48395 points

    Hello Raymond, perhaps a little more detail would be in order -

    So the LM6172 was one of several 36V VFA devices using a high transconductance input stage to get CFA type slew rates done in Santa Clara. While we were doing similar 12V parts in Comlinear and BurrBrown, I added a triangular wave output overdrive plot to illustrate if the part had phase reversal - and of course you don't want that. the plot that is missing from these LM & LMH data sheets is shown in a similar topology device like the 12V OPA690. This is a pretty easy bench test, so answering your question would take minimal effort in the lab.

    Figure 25 in this datasheet,

    www.ti.com/.../opa690.pdf

  • 0 in reply to Michael Steffes
    Guru 48395 points

    Also Raymond, the original comment that you should not drive the input pins beyond the supply rails is correct, but not usually where phase reversal comes in - usually this is the output hitting its maximum while the inputs are still in range. Also, it is my impression but could bear verification, that phase reversal is a topological oversight - not a part to part issue. Hence, one test in the lab should give you the answer. 

  • 0 in reply to Christian Yots
    Guru 140200 points
    Hi all,

    usually, phase reversal can become an issue when the common mode input voltage range is violated.

    Unfortunately, the datasheet of LM6172QML does not specify the common mode input voltage range directly. Only the specification of common mode rejection ratio contains details about the common mode input voltage range. In this point the LM6172QML differs from the LM6172 which has a specification of common mode input voltage range in the datasheet.

    Testing can be a bit tricky. I remember that we tested the TL084 for phase reversal many years ago. When not violating the common mode input voltage range not any TL084 showed phase reversal. When violating the common mode input voltage range, on the other hand, only a fraction of TL084 showed phase reversal. So, testing a single LM6172QML which does not show phase reversal could create a false sense of security...

    Kai
  • 0 in reply to kai klaas69
    Guru 48395 points
    Thanks Kai, I have not seen this much on the parts I've worked on - normally the designers are told to work on eliminating this or they are not done. So its really these much older parts that might have an issue.
  • 0 in reply to Michael Steffes
    Guru 48395 points
    But, by the same token if you run this gain +2V/V test where the input is trying to drive the output 2X past its limits (what we do in the datasheets typically) and the device shows phase reversal - now you know. If it doesn't, you still can't be sure by your experience Kai.