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Original question:

OPA994-Q1: OPA994-Q1

OPA994-Q1: Current flowing the wrong direction through V+ pin on this PSpice model

Bert Cowsky
Prodigy 10 points
Part Number: OPA994-Q1
Other Parts Discussed in Thread: OPA2991

Tool/software:

I was playing with this model and noticed some weird behavior.

When I wire up a test circuit (in PSpice for TI) current through the V+ is leaving the OpAmp, instead of flowing into it to power the output. Current is leaving both the V+ and OUT pins. It should be entering V+ and leaving through the OUT pin. I tried taking the text model and using it in a third party spice program, and noticed the same behavior.

Something appears to be wrong with the polarity of V+, in this model. Would it be possible to get an updated model with this problem fixed?

  • 0
    TI__Guru* 77521 points

    Hi Bert,  

    Bert Cowsky said:
    Current is leaving both the V+ and OUT pins. It should be entering V+ and leaving through the OUT pin.

    So far, I am unable to reproduce what you had in Tina. Please make sure the OPA2991 is operating in a linear mode. With 100ohm at the output, there is a significant voltage droop at the output and it may behave in a nonlinear mode or exhibit some strange behavior (which is possible). 

    OPA2991 E2E 04282025.TSC

    If you have other questions, please let us know. 

    Best,

    Raymond

  • 0 in reply to Raymond Zhang1
    Prodigy 10 points

    Hi Raymond,

    We were testing the op amp at high power draw, which is why I had a 100Ω load. But I retested the design with a 10kΩ load, and saw the same behavior. Initially, the opamp draws its 1.35mA quiescent current, which is how I expect it to behave. However, when my oputput current ramps up from 0mA to ~1mA, the supply current draw of the opamp DROPS down to ~0.3mA.

    How can the supply current drop, when my output current increases? This goes against my basic understanding of Kirchoff's Current law

  • +1 in reply to Bert Cowsky
    TI__Guru* 77521 points

    Hi Bert, 

    I see what you mean. The Spice model is not perfect and it may have issues - we are starting the support the general purpose op amp since last Nov., and I do notice issues under the certain scenario. 

    It will take time to correct these deficiency in the Spice model. If you need to simulate a heavy load, the best way is to use an ideal op amp for the time being. OPA2991 is able to source or sink higher current than typical op amp. Once you are satisfy with the ideal op amp simulation, you can back calculate or simulate the power dissipation and see if OPA2991 is able to meet these thermal dissipated condition for a given ambient condition. This is the best suggestion I have under the circumstance. 

    If you have other questions, please let me know. 

    Best,

    Raymond

  • 0 in reply to Raymond Zhang1
    Prodigy 10 points

    Thanks, Raymond!

    We're actually looking less at heat dissipation and more at the stability of a boost converter I'm using to powering several of these opamps under high load. Thankfully, this project isn't the most urgent task at the moment, so I will take a look at OPA2991 and do the best I can with that, for the time being.

    Appreciate the help!

    Bert Cowsky

  • 0 in reply to Bert Cowsky
    TI__Guru* 77521 points

    Hi Bert, 

    If you are working with paralleling several op amps to boost the current, please take a look at the application note. 

    https://www.ti.com/lit/ab/sboa553a/sboa553a.pdf?ts=1743061788697&ref_url=https%253A%252F%252Fwww.google.com%252F

    If you have other questions, please let me know. 

    Best,

    Raymond