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OPA2365: Regarding the phenomenon that the output of the voltage follower sticks to the V+ voltage

Koji Hayashi92
Mastermind 6970 points
Part Number: OPA2365
Other Parts Discussed in Thread: LMV951

I am using OPA2365 to configure the voltage follower circuit below.

As I raised the V+ voltage, -IN_B stuck to V+ around 1.1V to 1.5V.

I would like to know why it behaves like this.

Is this a behavior specific to this IC?

Also, please let me know of any ICs that are equivalent to OPA and do not cause this phenomenon.

Best Regards,
Koji Hayashi

  • 0
    Guru 281010 points

    The minimum supply voltage for which correct operation is guaranteed is 2.2 V.

    There are opamps with a lower minimum supply voltage (down to 0.9 V for the LMV951), but there are none that work at very low voltages.

    What is the actual problem you're trying to solve? What behaviour do you require during power up?

  • 0 in reply to Clemens Ladisch
    Mastermind 6970 points

    Thank you for reply.

    The AMP output is connected to the input of COMP, and the reference voltage of COMP is used by dividing the same voltage as V+.

    If the AMP output sticks to V+ at startup, COMP will be inverted because it will exceed the reference voltage of COMP in the subsequent stage.

    The behavior I expect from AMP is to either rise at the same voltage as +IN_B(SIM result image below), or to rise at the same voltage from1.25V(for example) without exceeding the +IN_B voltage (red line in the SIM result image below).

    Is there an IC equivalent to OPA2365 that normally operates at 5V and whose AMP output does not stick to V+ during startup?

    I believe this can be handled by changing the circuit configuration or IC, but I would like to know why the output voltage of the OPA2365 sticks at low voltages.

    Best Regards,

    Koji Hayashi

  • 0 in reply to Koji Hayashi92
    Guru 281010 points

    There are no opamps that work at very low voltages. If you have a reset signal, you could use a comparator with shutdown input, and add a pull-down resistor at the output.

    But why do you need voltage follower at all? Comparators have high-impedance inputs, so you can connect the voltage divider directly to the comparator.

  • 0 in reply to Clemens Ladisch
    Mastermind 6970 points

    Thank you for reply

    Sorry, I was explaining the circuit connections incorrectly.

    The output of the voltage follower is connected to the input of the other amplifier (+IN_A) via RC in parallel.

    It consists of a differential amplifier circuit and a voltage follower.

    When the output of the voltage follower sticks to V+, the output of the differential amplifier also sticks to V+.

    The output of the differential amplifier is input to COMP.

    　

    >There are no opamps that work at very low voltages.

    Does this mean that the output of TI products' OPAMPs may stick to the power supply at low input voltages?

    Are there any OPAMPs whose output voltage does not stick to the power supply during input voltages up to the normal operating range of 2.2V instead of the 1.25V described last time?

    　

    Best Regards,

    Koji Hayashi

  • 0 in reply to Koji Hayashi92
    TI__Expert 6050 points

    Hi Koji,

    The opamps will exhibit this sort of behavior below the minimum operating voltage because the internal circuitry of the device requires a certain voltage to completely turn on. This is why you are seeing the voltage stick to V+ in certain ranges. Normal amplifier operation can not be guaranteed below this minimum operating voltage.

    Thanks,

    Nick