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OPA2607-Q1: The stability of OPA2607 with high capacitor load

A.W
Intellectual 410 points
Part Number: OPA2607-Q1
Other Parts Discussed in Thread: OPA2607, OPA607, TINA-TI

Hi

I want to use OPA2607 for current sensing in sensorless motor control application.

In the datasheet, opa x607 is not unity-stable with min-gain = 6V/V. Also the Riso is needed when high load.

I do use a Rshunt = 2mΩ ,gain = 10, Riso = 56Ω，Cload = 3.3nF, PWM frequency = 15KHz

Also, I change the opa607 tina project with my design.

Seems the minus-circuit for amplifier is not as typical feedback circuit, and the simulation for phase margin is not in the plot, I can only see the degree.

Can you help me check the TINA project, see if the parameter is Ok for my application?

Below is the plot for amplitude and phase.It seems that the phase is not the phase margin para.

How can I see the phase margin for my circuit and if it match the least 45 degree safe phase margin?

below is the link for tina9 project.

/cfs-file/__key/communityserver-discussions-components-files/14/sbomb17c.tsc

Regards

Arrow

  • +1
    TI__Intellectual 1500 points

    Hi Arrow,

    To check for the stablity  (rate of closure and phase margin measurements) in Simulation, the standard closed loop feedback configuration of the amplifier need to be opened up or broken for AC signal (DC bias is still needed for proper operation) and then a small signal is used to excite the high impedance side  where the loop was broken. This is done using a large inductor in the feedback loop and a large capacitor to couple the small AC signal. And then, AOL, 1 over beta, and loop gain curves need to be looked upon to infer the stability. 

    Your circuit has a phase margin of around 43degrees. See the below for more detail.

    You can refer to the below TI tutorial link to learn more about this method for analysing the stability.

    Spice stability Simulation: //www.youtube.com/watch?v=U6tP78BLr_A  (you can also view this video in the "Stablity>Spice simulation" section of the below Complete Opamp tutorial series)

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

    Regards

    Anant

  • 0
    Guru 140200 points

    Hi Arrow,

    as Anant has shown, the phase margin of your circuit is below 45° which means that your circuit is not very stable. It turns out that adding a phase lead capacitance of 22pF can improve the phase margin and will result in a much more stable circuit:

    arrow_opa2607.TSC

    A much better stability can also be seen from the very flat frequency response:

    Kai

  • 0 in reply to Anant Sinha
    Intellectual 410 points
    Anant Sinha said:
    43degrees

    Hi, Anant

    I try as what you suggested, and I set another group of para. of Riso = 510ohms, and Cl = 220pF,the new phase margin will be almost 60 degrees

    I think it is stable now.

    /cfs-file/__key/communityserver-discussions-components-files/14/opa2607_5F00_PhaseMargin.TSC.

    One more question, why using the voltage before Riso, not after Rsio as Vout for simulation?

    Regards

    Arrow

  • 0 in reply to kai klaas69
    Intellectual 410 points

    Hi Kai

    Is your first model is the same as Anant shows? It seems that both projects show the same result.

    Is  the stablity just involved with the phase margin when gain equals 0dB? I notice that in the plot, adding a 22pf will decrease the phase margin before gain equals 0dB, especially in 200KHz.

    By the way,can you enlighten me what frequency I should care most? Because my pwm frequency is 15kHz，should I care about the phase margin more than 100kHz ?

    It confuses me a lot.

    If I want to measure the motor current with minus 2us， should I care about the 50KHz(1/2us) as the base frequency?

    Regards

    Arrow

  • 0 in reply to A.W
    TI__Intellectual 1500 points

    Arrow,

    Since the feedback is taken directly from the output and the Riso is not in the feedback loop that is why for Loop gain curve we need to measure Voltage before Riso.

    Regards

    Anant 

  • 0 in reply to A.W
    Guru 140200 points

    Hi Arrow,

    when reducing C7 to 0pF I get the same phase margin like Anant, 45°.

    The method I use to carry out the phase stability analysis and to simulate the phase margin is similar to what is shown in the TI's training video series on stability:

    https://training.ti.com/node/1138805

    But I introduce the stimulus directly to the input pins of OPAmp and look what comes back via the feedback loop. From the frequency response and phase response I can directly determine the phase margin, without needing to use the post-processor of TINA-TI.

    Because the feedback loop is opened at the input pins, the feedback loop no longer sees the input capacitances of OPAmp ("C4", "C5" and "C6" in the simulation). Because of that the input capacitances have to be "mounted" externally.

    "L2" and "L3" close the feedback loop for DC and allow the OPAmp inputs to be properly DC biased.

    "C3" provides an AC coupling of stimulus without ruining the DC biasing of OPAmp inputs. And because the AC coupling shall be invisible even for the lowest frequencies "L2", "L3" and "C3" are chosen to be "infinitely" high.

    The advantage of this a bit more opulent method is that the complex output impedance of OPAmp is also taken into calculation.

    Keep in mind that only the phase shift at unity gain in the phase stability analysis is called "phase margin" and not the phase shift at any other frequency, because only the phase shift at unity gain can destabilize the circuit. I have discussed this here:

    https://e2e.ti.com/support/tools/simulation-hardware-system-design-tools-group/sim-hw-system-design/f/simulation-hardware-system-design-tools-forum/1087610/tida-00489-pir-amplifier-stability-analysis

    Kai

  • +1 in reply to kai klaas69
    Guru 140200 points

    And see how much the step response improves when adding the 22pF phase lead compensation cap:

    Kai