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TINA/Spice/OPA2544: OPA2544 Capacitive Load Drive Capability

Part Number: OPA2544

Tool/software: TINA-TI or Spice Models

Hello,

I am looking for any information on the capacitive load driving capabilities of the OPA2544, but I am not seeing anything in the datasheet.  Any additional information would be appreciated.  Thanks!

Jim

  • Hi Jim,
    I'll look into this for you.

    Regards,

    Alex Davis

    Precision Amplifier Applications - TI Tucson

  • In reply to Alexander Davis:

    Hello Alexander,

    Thanks for digging into my question.  Have you been able to find any additional information?  Let me know - thanks again!

    Jim

  • Hello Jim,

    The capacitive drive capability of the OPA2544 can be different for different application conditions. It depends on the resulting gain-phase characteristics of the particular circuit. Please provide the the following information:

    1) Do you have a schematic for your OPA2544 circuit?

    2) What are the supplies?

    4) What is the load and what are the output voltage and current requirements?

    5) How much capacitance do you need to drive with the OPA2544?

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • In reply to Thomas Kuehl:

    Thomas,

    Thanks for the questions.  My responses are provided below.

    1) Do you have a schematic for your OPA2544 circuit?

    I do not currently have a schematic for the OPA2544 circuit, but my intent is to utilize the OPA2544 in a bridge-tied-load configuration (very similar to Figure 4 in the datasheet).

    2) What are the supplies?

    The supplies for the circuit would be +/- 12V.

    3) What is the load and what are the output voltage and current requirements?

    The circuit is intended to behave like a programmable power supply.  From an output voltage standpoint, the circuit will generate DC voltages between +/- 15VDC.  The circuit will also generate arbitrary AC voltages up to 30Vpp (sinusoidal, square, etc.) at frequencies up to 50kHz.  From a loading standpoint, the maximum output current the circuit will need to supply is 500mA into a resistive load (at a minimum).

    4) How much capacitance do you need to drive with the OPA2544?

    Good question.  If the circuit is generating DC voltages, it would be desirable for the part to be stable in the presence of ~ 10uF - 100uF or so (if the circuit is powering a downstream PCBA, for instance).  If the circuit is generating AC voltages, I do realize there is an upper limit in terms of the size of the reactive load that can be driven.  There is no "hard" requirement from the customer at this point regarding reactive loads, but I would like to have a solid understanding in terms of what the part can achieve before it becomes unstable.

    Please let me know if you need any additional clarity.  I appreciate the support - thanks!

    Jim

  • In reply to Jim Hoch:

    Hi Jim,

    Thank you for the additional information about your application.

    The circuit is intended to behave like a programmable power supply.  From an output voltage standpoint, the circuit will generate DC voltages between +/- 15VDC.  The circuit will also generate arbitrary AC voltages up to 30Vpp (sinusoidal, square, etc.) at frequencies up to 50kHz.

    • You mention that your supplies are going to be +/-12 V. You wouldn't be able to achieve +/15 V with the lower voltage supplies. I would recommend supplies at least 4 V higher than the required maximum output voltage level.
    • The OPA2544 MAXIMUM OUTPUT VOLTAGE vs FREQUENCY graph on datasheet Page  5 indicates that slew rate limiting will limit the full output swing capability to a maximum frequency of about 35 kHz. Higher frequencies will begin to exhibit slew rate limiting effects. 

    We are assessing the stability of the OPA2544 in a bridged-T configuration, with its output loaded with capacitances from 10 to 100 uF. We are having to do some  work on our OPA2544 simulation model to get its open-loop output impedance (Zo) correctly set. That is an important factor in assessing the gain-phase response of the circuit. Likely, directly driving such large capacitances will result in an unstable circuit and will need to be properly compensation.

    It is going to take us a couple of days to get the stability work completed. As soon as we have the results they will post them here on the E2E for your review.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • In reply to Thomas Kuehl:

    Thomas,

    Thanks for the feedback.  Regarding the +/- 15VDC output, the circuit is implemented in a bridge-tied load configuration (there are two outputs that are out of phase).  Thus, one op-amp will generate + 7.5V and the other op-amp will generate - 7.5V for a differential voltage between the two outputs of +/- 15V.  This achieves the 4V of margin you mention above for each op-amp.

    Regarding the slew rate specification of 8V/us in the datasheet, this will be acceptable for our application (I fully understand that the high frequency harmonic content associated with a square wave, for instance, will be slew rated limited).

    Once again, thanks for investigating the capacitive load drive capabilities of the OPA2544.  For what it is worth as the analysis is performed, I would be interested in both DC and AC performance (e.g., the 10uF - 100uF capacitive load would only be present when the op-amp is driving a DC output; there would be a much smaller capacitive load when AC waveforms are being driven).

    Thanks!

    Jim

  • In reply to Jim Hoch:

    Hi Jim,

    Thanks for the clarification regarding the output voltage requirements. I now understand what you intend for it.

    The Precision Amplifiers Applications Manager, Tim Green, completed a stability analysis for your proposed OPA2544 circuit. The results are provided in the attached PowerPoint. Included in the information is a compensation scheme that provides high phase margin when driving large capacitive loads up to 100 uF. Unfortunately, the penalty for maintaining stability when driving a large capacitive load is the closed-loop bandwidth of the circuit drops to about 1.5 kHz.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

    OPA2544 Bridge.pptx

  • In reply to Thomas Kuehl:

    Thomas,

    Thanks for the information - it is very helpful.

    I have a few additional questions about the performance of the op-amp after some lab testing.  I will do my best to describe the setup, but please feel free to reach out to me if there are any questions.

    I created a rapid-prototype board for my bridge-tied load configuration.  For this test, the device was been configured to output a 50kHz sinusoid  at ~ 30Vpp into a 20 ohm (non-inductive) resistive load (see attached scope shot).  What I am seeing appears to be crossover distortion, and I am speculating that it is related to the part's ability to switch between sinking and sourcing current (I do not see any distortion when the output is unloaded).  The distortion (from a timing standpoint) is consistent across both frequency and amplitude and looks very similar to the response on page 6 of the datasheet for the "large signal" if one zooms in.  I did not see this effect in my simulations, but is this something you would expect?  Let me know - thanks!

    Jim

  • In reply to Jim Hoch:

    Hello Jim,

    Yes, you are observing crossover distortion from the OPA2544 at 50 kHz. The gain-bandwidth where the open-loop gain is 1 V/V for the OPA2544 is specified with a typical value of 1.4 MHz. Therefore, at 50 kHz there isn't much open-loop gain (Aol) remaining. You can see this in the OPEN-LOOP GAIN AND PHASE vs FREQUENCY graph shown here. 

    It appears there may be only about 24 dB (16 V/V) of Aol remaining at 50 kHz. Then, because the Aol is low at that point the benefit of negative feedback acting upon the crossover distortion is limited. If you reduce the frequency you should see the distortion decrease because the Aol is increasing at lower frequencies.

    The OPA2544 Spice model was developed in the mid-90s and uses a simple Boyle op amp model. That model actually doesn't use biploar transistors in the output stage, but instead a voltage source to provide the output voltage. That is not unusual for an op amp model developed in that era. Therefore, it will not exhibit the crossover distortion characteristic.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • In reply to Thomas Kuehl:

    Thomas,

    The time duration of the crossover distortion is consistent (~ 1 - 2us) regardless of the frequency (I specifically checked frequencies much lower than 50kHz). It simply appears to be less significant at lower frequencies because the distortion time is a small percentage of the waveform period.

    At this point, I am going to examine other parts within TI's high current op-amp family to try and minimize this effect. Thanks.

    Jim