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LMC662: Oscillation - actual vs simulation

Part Number: LMC662
Other Parts Discussed in Thread: OPA134

Customer built hardware boards based on unity gain schematic below. Oscillation was observed. To make circuit stable, a 6.8KOhm pull up resistor was added at output of Op-amp.

However, customer noted that during simulation in TINA, the exact same circuit (without 6.8KOhm pull up), does not oscillate.

Can TI explain the gap between the actual and simulation? 

In customer's own words: "In order for us to continue using this op-amp in the design, we need to be confident of the stability margin we have and be able to simulate what we measure! Could you please ask TI how they would recommend we simulate the stability of the design? We would like to do this in terms of phase margin."

  • Hello Eddie,

    The LMC662 minimum recommended supply voltage is 5V and the input common mode range only goes up to (Vs-2.4V). That would be 0.9V with a 3.3V supply.

    Try setting supply to 5V. In+ at 2.6V or less.

  • Hey Eddie, in answer to your confirm phase margin question, The TINA V11 library had the original NSM model, however, the loop gain sim for your setup showed only 2kHz GBP - not sure if that is a model issue or my issue - I dropped an OPA134 model into this and worked fine, right now, I would not trust the model and move on to a different part, 

    here is a V7 file for this, 

    This sim sets up at -2.5V on the output, might be the problem for the Aol, but it should not be doing that. 

    LMC662 unity gain follower LG sim.TSC

  • Hi Ronald,

    I am working with the customer in resolving this.

    Just to clarify, the customer is operating at 5V. The desginator is V1 and voltage is 5V please see attached image.

    And the input applied is 1.65V which is within the Vs -2.4V range.

    Thanks for the suggestion on the OPA134, but would prefer if we can resolve this.

    Thanks

  • Arun,

    In the schematic I see just 12pF and no other load. Is there more capacitance on the output or a resistive load that has an alternating current polarity (for example if the 1.65V output was used for a reference voltage for other op amps). Also useful would be a waveform of the oscillation. 

    On page 15 of the data sheet, the section labeled "AMPLIFIER TOPOLOGY" goes into detail how this amplifier is different and the sink driver and source driver have different signal paths. The 6.8k pull up resistor "fixes" this because the output current becomes unidirectional. 

    I have samples on order. If you give more details about the output load, I will test in a similar manner next week. 

  • Arun, 

    I only mentioned the OPA134 as I had TINA files open (for an article on how GBP gets confusing) and dropped into the Aol sim and it ran fine. Personally, I would not spend much time on a part that does not seem to have a good TINA model. The product page suggests some alternates, 

  • It occurred to me you may not have a TINA model - I was not able to use the one on the product folder, but TINA V11 has one in the library. This one simulates ok gain of 1, but when I tried to run an output noise with 12pF closed loop gain of 1 it crashed TINA for me, kind of a bummer as I had many other files open - won't try that again and if you do, I suggest you only have this file open.

    LMC662 unity gain follower sim.TSC