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Original question:

LMV931 Latch-up?

LMV931-N: LMV931 OUTPUT LOSS

Daniel Landers
Prodigy 10 points
Part Number: LMV931-N

Hello,

We have been using LMV931MF/NOPB, Package Code A79A.  Suddenly, on a recent batch of these chips, we have been experiencing shutdown of the device.

Date code is:  *75X

Inputs are connected to a photodiode receiver HCNR201 (Broadcomm), connected as:

Cathode (HCNR201 Pin 6) to Inverting Input (pin3)
Anode (HCNR201 Pin 5) to non-inverting input (pin1)
non-inverting input (pin1) to ground connection (pin2)
inverting input connected to output via 200K FB resistor + 0.1uF FB Cap)

Since HCNR201 is an isolation amp, the rails (pin5, pin2) are powered independently from the signal that is to the inputs.

When power is off (0V pins 5,2) the output signal drives to -0.3V
When power is ON (pins 5,2) the output responds normal (specifically 1.2V to 4.2V signal)
When power is removed and the reapplied between 200ms and 800ms, which is before the output signal drops below 0V, the output will remain locked at 0V.
IF Power is reconnected <200ms (before output goes low) or >800ms (after output goes to -.3V) the signal returns to normal.

Replacing the op-amp resolves the symptom.

This seems to only apply to op-amps with date code *75X

This may or not be related, as it is on a different board and chip, but may represent conditions seen by the chip described above:
We have a possible cause, in that the external interface 5V experiences considerable noise (greater than 5V Vcm, >2Vpp at times, around 200kHz for about 1ms)
During these events, our field engineer managed to capture this event on a scope, and roughly 10-20ns after the voltage exceeded 6V, the output signal dropped to zero. and the voltage between pin1 and pin3 was about 0.5V)

I am curious if there is an issue with a recent batch of these op-amps, if there is something that is causing these to drop-out, if there is damage occuring to these op-amps due to the voltage spikes that would cause this behavior, or anything you can think of that would be the cause.

Thanks!!

  • 0
    Guru 140200 points
    Hi Daniel,

    the problem is that the signal at the LMV931 becomes -0.3V. This can push the LMV931 into latch-up.

    What happens if you decrease the feedback capacitance from 100nF to 100pF?

    Kai
  • 0 in reply to kai klaas69
    TI__Mastermind 30600 points
    Hi Daniel,

    Just so I fully understand your conditions:

    "When power is off (0V pins 5,2) the output signal drives to -0.3V
    When power is ON (pins 5,2) the output responds normal (specifically 1.2V to 4.2V signal)
    When power is removed and the reapplied between 200ms and 800ms, which is before the output signal drops below 0V, the output will remain locked at 0V.
    IF Power is reconnected <200ms (before output goes low) or >800ms (after output goes to -.3V) the signal returns to normal."

    Is the power you are referencing above the op-amp supply voltages? Is the output signal the output voltage of the op-amp or the HCNR201 device?

    My concern is that if you are removing power to the op-amp and the input signal is -0.3V, you're likely causing some current to flow and could be putting the device into electrical overstress. When you do this for long enough (more than 200ms), you can break the device. We have an excellent series of videos on this topic that I recommend you watch: training.ti.com/ti-precision-labs-op-amps

    Perhaps you could test putting a series resistance on the input pin of the op-amp to limit how much current will flow in this scenario?

    It would be quite helpful as well to see the scope shots that you mentioned as well as a schematic. You can use the "Insert Code, Attach Files and more..." link to get to a rich text editor to upload these.

    Thanks,
    Paul
  • 0
    TI__Guru*** 139976 points

    Daniel,

    Your circuit, as described, has no defined DC solution. Even with no photodiode current, the output can be any voltage and vary over time.
    You need to terminate non inverting input to some DC voltage.


    Sorry, I see that non inverting is ground. I will test some LMV931-N this week to see if I can replicate this behaviour.

  • 0 in reply to Ron Michallick
    TI__Guru*** 139976 points
    Daniel,

    Sample shipment (to me) is taking longer than usual. So results expected next week.
  • 0 in reply to Ron Michallick
    TI__Guru*** 139976 points
    Daniel,

    I tested the circuit and the inverting input and output will go below zero due to the input current when the power is off. With the power on the inverting input will actively driven to zero. Regardless of the power off time, my samples powered up well. I even tried adding some more negative current and it still powered up fine. I do not see any sensitivity in LMV931-N. A Schottky diode from inverting input to ground will limit the negative voltage to a harmless level. Noring normal power on, the diode leakage won't be an issue because the voltage across the diode is nearly zero.