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TL081H: Output Transient Capability

Jason Bonarius
Prodigy 10 points
Part Number: TL081H

I have a scenario where the TL081H will be supplied with a 18 VDC single voltage supply (Vss to GND). The output of this device goes to an external connector that is required to meet voltage transient specifications. From an LTSpice simulation it would appear as if the voltage transient on the output of the device can go as high as 24VDC for a duration of roughly 250us 

Will the device be able to withstand such a transient? The datasheet specifies input port common mode voltage maximums of Vcc+0.5V, which if it was the same for the output port, would certainly lead to damage. Does the output operate under the same limits as the input and is there a transient duration that will allow for the maximum to be uprated to a high value such as this case where is transient duration is < 250us

Regards,

Jason

  • 0
    TI__Guru**** 153186 points

    Jason,

    We need a current flow transient curve also.

  • 0 in reply to Ron Michallick
    Prodigy 10 points

    EDIT:

    Here are the output port transient voltage and current plots:

    NOTE: The maximum transient voltage has been reduced compared to the original post. I was able to add a bit of capacitance on the output to smooth the response and in doing so reduce the transient peak without compromising reponse time requirements. 

    The transient maximum is 1.2V above a supply voltage of 18VDC for 550us (longer but lower)

    The maximum current sourced from the output port peaks at 45 mA and settles within 450us.

  • 0 in reply to Jason Bonarius
    TI__Mastermind 29086 points

    Hey Jason,

    Could you possibly share the schematic?

    While adding capacitance on the output can help the transient condition, you may run into stability problems with the amplifier if not implemented properly. This depends on multiple factors, which the schematic would help answer (gain configuration, any series resistance on the output, etc.)

    Best,
    Jerry

  • 0 in reply to Gerasimos Madalvanos
    Prodigy 10 points

    Hi Jerry,

    I have attempted to attached the LTSpice simulation. I dont know if its possible. Here is a screengrab of the schematic:

    The circuit translates 3V3 logic to 28V logic. In this simulation the supply rail is set to 18V as there is a power scenario where my supply will drop to 18V and the circuit will need to continue operating. The comparitor output has a shunt 10uF capacitor which I have mentioned before as well as a 1kW 2.4kOhm MELF resistor to protect the output from a transient known as WF4. It is a 700Voc/1500Isc waveform (whichever comes first - so 700V). 

    1. I cannot use an form of diode on the output as the opamp output is dependent on the supply voltage, which for my applicaiton could vary between 12V and 36VDC, making it impossible for a diode to suit all of these voltages.

    2. I have opted for a comparitor rather than a FET based level shifter as the supply resistors in the FET based level shifter will need to handle huge power dissipations. It is not included in this sim but the circuit must maintain a high output level of > 15 VDC into a 33kOhm load when the supply is 16.8VDC.This forces me to use a smaller supply resistor in the FET based level shifter if I wish to not have a large voltage division and not satify the > 15VDC requirement, and at the same time, as the resistance comes down, the power dissipation increased to a non-feasible level.

    So - to reiterate the question: From your interpretation of the datasheet, what is the maximum voltage rating of the op amp output port? How how can the voltage go here and is it supply voltage related? Can it handle shor durations? What of the current?

  • +1 in reply to Jason Bonarius
    TI__Mastermind 29086 points

    Hey Jason,

    Usually the full protection structure is something like this.

     In this case you aren't seeing the overvoltage condition on the inputs, so the input diodes are not needed. You can use a 40V zener on the supply, keeping the op amp below the abs max supply voltage range, then the diodes on the output to protect the output body diodes from conducting excessive current.

    The body diodes aren't really meant to sink current. Usually less than a mA is not problematic, but this is not a characterized behavior, and the most robust solution is the above diagram.

    More in depth info can be found in this video series under the EOS section.

    https://www.ti.com/video/series/ti-precision-labs-op-amps.html 

    Best,
    Jerry