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Original question:

OPA4171: OPA4171-Q1 Application issues

OPA4171: OPA4171 stability issues

Hangjie Wang
Intellectual 1010 points
Part Number: OPA4171
Other Parts Discussed in Thread: OPA4991

Hi experts,

Could you please help confirm whether OPA4171 can work stably when the load is 100+10nF? I don’t see the corresponding working condition in Figure 6-13.

In addition, can you please share how to judge the stability of an op amp? It's best to be explained by Tina simulation, thank you.

  • 0
    TI__Expert 6555 points

    Hello Hangjie,

    The plots show that the overshoot increases with cap load in the order of picoFarads and the cap load in your schematic is in order of nanoFarads which is bigger and could pose a stability problem. I suspect your configuration would experience an oscillation on its output.  

    I ran a TINA stability test on this circuit and the its phase margin(PM) is out of range of the expected PM of a stable system (45deg to 90deg). To stabilize the circuit, you can reduce  the value of the cap load. I used TINA TI to generate an output response curve that gives more details on the PM. You can do this by

    • Loading this .tsc file. It already has the ac analysis configuration:

     OPA171sim.TSC   

    • enter ctrl+alt+a
    • it should generate an AolB curve and 1/B curve
    • right clicking on the AolB curve and select PM. 

               

    I see you have an isolating resistor on the output, Riso=100ohm interacting with a Cload=10nF. There is a way to optimize the output configuration to stabilize your circuit and you can watch how to do that here: https://www.ti.com/video/4080320453001.  

    and here is a video that shows you how to perform ac analysis:https://www.ti.com/video/4080288067001 

    Regards,

    Ore.

    Hope I was able to help!

    Regards,

    Ore   

  • 0 in reply to Ore Ogundana
    TI__Intellectual 1010 points

    Hi Ore，

    Thank for your quick reply, which is very useful， do we have a suggestion for the limited Count of OPa4171 or OPA4991?

  • 0 in reply to Hangjie Wang
    TI__Mastermind 19607 points

    Hello Hangjie, 

    Could you please explain what you mean by "limited count" is this in terms of inventory? How much of OPA4171AQPWRQ1 does the customer need?

    All the best,
    Carolina 

  • 0 in reply to Caro Walter
    TI__Intellectual 1010 points

    Hello Carolina,

    Sorry for the wrong input. What I want to ask is how to confirm the maximum output load capacitance. When I adjust the capacitance to 200p, the phase margin is about 44, does this mean the max capacitance is 200p?

  • 0 in reply to Hangjie Wang
    TI__Expert 6555 points

    Hello Hangjie,

    Your initial cap load of 10nF that yielded a PM close to zero was not conducive for a stable system, and your reduced cap load that yielded a PM of 44deg is close to achieving a stable system but the recommended range is between 45deg to 90deg. You would have to implement the RIso calculation from the video to get the best values for your Riso and Cap load.

    Please confirm the cap load value you are driving in your application so the video can guide you towards having a stable system.

    Regards,

    Ore.