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OPA549: Questions in using OPA549

Albert Sun
Intellectual 340 points
Part Number: OPA549

Hi Team,

I am making a voltage controlled voltage source with OPA549, the following is the schematic diagram, I encountered two problems during use:

1) The power-on sequence of VIN+ and VIN- seems to have requirements: under no-load condition, the Ctrl terminal is always shorted to SGND. If VIN+ is powered on first, before VIN- is powered on, VOUT will output a voltage close to VIN+ ; Similarly, if VIN- is powered off first, VOUT will output a voltage close to VIN+. As shown below

Black: VIN+; Blue: VOUT; Rose: VIN-

But that voltage is pulled down to zero by a certain amount of resistance (in the order of hundreds of ohms). Why does the power-on and power-on sequence affect the output? This is not reflected in the D/S.

2) There will be some ringing and stable low frequency oscillation when the output is connected to no-load or capacitive load. These oscillations will also be damped by a certain amount of resistive load, but is there any other better method?

Blue: Ctrl; Black: VOUT; Green: IOUT

-A

  • 0
    TI__Guru* 93105 points

    Hi Albert,

    The OPA549 like nearly all op amps is designed to use positive and negative supplies that are applied and come up to level in the same manner, at the same time. When one supply comes up to level before the other it may force the op amp into a non-linear condition where the common-mode voltage, or another parameter's range is exceeded. Additionally, when one supply lags the other unintended, internal current paths can be formed that result in unexpected behaviors.

    Most often op amps are not characterized for power supply sequencing behaviors. The possibilities are limitless. Sometimes, more controlled start-up behaviors are had by adding power diodes or TVS diodes from each supply pin to ground. If one supply comes up earlier than the other the normally reverse-biased diode on the other supply becomes forward-biased providing a current path for the supply current.

    Regarding the ringing:

    • Is the image with the output unloaded other than an O-scope probe, or is there a capacitor at the output?
    • What are the R1 and R2 values?
    • What kind of power supply are you using, linear or switching?
    • What are the power supply bypass capacitor values?
    • Is the circuit on a PC board, or other?
    • Were the OPA549 devices obtained directly from TI, one of TI's authorized distributors, or another source?

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    Intellectual 340 points

    Hi Thomas,

    Please see my reply below. Thanks!

    Is the image with the output unloaded other than an O-scope probe, or is there a capacitor at the output?
    For the image above, there is no other cap.
    What are the R1 and R2 values? R1 = 30k, R2 = 10k, so the gain is 4.
    What kind of power supply are you using, linear or switching?
    switching power supply.
    What are the power supply bypass capacitor values?
    47uf*2(pol) + 10uf*2(MLCC) +0.1uf*1(MLCC) for either VIN+ and VIN-.
    Is the circuit on a PC board, or other? On PCB. 
    -A
  • 0 in reply to Albert Sun
    TI__Guru* 93105 points

    Hi Albert,

    Thanks for the responses regarding your OPA549 circuit. The circuit gain, supply bypassing and application circuit appear okay. A genuine TI OPA549 would be stable in the circuit and not exhibit ripple on its output waveform unless it is coming from something else in the circuit.

    • What is the frequency of the ripple seen in the O-scope image?
    • Have you observed the power supply output with the O-scope?
    • What is the frequency of the switching power supply being used to power the OPA549 circuit?
    • Do you have a linear power supply that you can temporarily test the circuit with to see if the ripple is still present?
    • Has more than one OPA549 been tested in the application?

    Thanks, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    Intellectual 340 points

    Hi Thomas,

    I need some time on bench to check it. Thanks!

    -A

  • 0 in reply to Albert Sun
    Intellectual 340 points

    Hi Thomas,

    Firstly, there is a correction for last answer. The waveform above is tested with a MLCC load (the value is about 80uF).
    I re-test an OPA549 with a 22uF/25V/X5R/0805 MLCC load.
    Ctrl has a very fast rising/falling slew-rate. The Ctrl low level = 2.5V, high level = 3.75V, so the output low level = 10V, high level = 15V. The result is shown below.
    How to explain those transient ringing (1, 3) and stable ringing (2, 4)?
    1. f=180Hz
    2. f=180Hz, Vpp=480mV
    3. f=150Hz
    4. f=150Hz, Vpp=400mV
    -A
  • 0 in reply to Albert Sun
    Guru 140200 points

    Hi Albert,

    the OPAmp oscillates because of a too high capacitive load at the output which ruins the phase margin and causes instability. See the chapter "output stage compensation" of datasheet.

    Kai

  • 0 in reply to Albert Sun
    TI__Guru 57241 points

    Hi Albert, 

    As Kai pointed out, the configuration is driving a capacitive load directly, which it will degrade the phase margin in OPA549 and cause op amp instability and oscillation at the output. 

    Please provide us the input, output and BW design requirements in the OPA549. We have to find a way to compensate the OPA549's feedback loop in order to drive the large capacitive load. 

    Please also check out the video clip shown below. 

    https://training.ti.com/ti-precision-labs-op-amps-stability-capacitive-loads?context=1139747-1139745-14685-1138805-13952

    Best,

    Raymond

  • 0 in reply to kai klaas69
    Guru 140200 points

    Hi Albert,

    please explain why you connect a such high capacitance directly at the output of OPA549. Please explain what you are planning to do.

    Kai

  • 0 in reply to kai klaas69
    TI__Guru 57241 points

    Hi Albert,

    I am going to close this inquiry. If you have additional questions, please let us know. 

    Best,

    Raymond