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LMV358A: Violating Input Common-mode Range

yongfei qu
Prodigy 40 points
Part Number: LMV358A
Other Parts Discussed in Thread: TLV9001, TL081H, , LMV358, LMV321

The following figure is used for current limiter.  In normal, load current is small. VIN + is 4V . VIN- is greater than 4V, which exceeds Common-mode voltage range. Does this work fine?

  • 0
    Guru 275330 points

    No; outside the common-mode range, the opamp will not work correctly.

    Use a rail-to-rail opamp like the TLV9001, or one whose common-mode range includes the positive rail like the TL081H.

  • +1
    TI__Guru*** 147246 points

    Hello Yongfei,

    V4 does bring input range to the upper end of common-mode voltage range, so the current limit will work

    When not in current limit, only one input is in valid range. LMV358A claims no phase reversal. Therefore the inverting input being above range will still provide a low output to keep M1 fully on.

    Load voltage will be close to 5V - 10 * IOUT until IOUT is 100mA (VOUT 4V), then output will be 100mA. (Assuming FET on resistance << 10ohms)

  • 0 in reply to Clemens Ladisch
    Prodigy 40 points

    Hello Clemens Ladisch

    Thanks for your response. 

    My understanding is that the LMV3xxA has a similar input structure to the TLV9001, although performance is not guaranteed when the input voltage is near the VCC.

    Additional discussion:

    For additional discussion, in my application, if LMV358/LMV321 is used.

    When not in current limit, IN+ input is in valid range. Q1 is off, Q2 is on, so V2 is greater than V1，and the input stage state is determined. I think there is no phase reversal.

    When in current limit, IN+ and IN+ input are in valid range. 

    So, I think LMV358/LMV321 will work fine in my application. Is my analysis is correct?

  • 0 in reply to yongfei qu
    Guru 275330 points

    You can deduce or measure how the device behaves with out-of-range voltages. But there is no guarantee that it will always stay this way.

  • 0 in reply to yongfei qu
    TI__Guru*** 147246 points

    Yongfie,

    Keep in mind that LMV358 (post above) and LMV358A (directly below) are not the same.