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OPA335: Oscillator circuit not working

Carlos Silva85
Expert 4915 points
Part Number: OPA335

The OPA335 model doesn't seem accurate in generating my simulation. The circuit works satisfactory on a lab setup. Please advise.

On the circuit below, R2 and R3 create a mid point supply. Initially C1 is discharged and therefore the output will go high. C1 starts charging from output through R1.

When VC1>VIN+ the output toggles low, C1 starts discharging Through R1. 

R4 provides hysteresis whenever the output changes state.

OSC_OPAMP.TSC

  • 0
    TI__Mastermind 23050 points
    Carlos,

    Tina isn't showing the oscillation because you are simulating over such a large time span without enough excitation subdivisions. Increasing the TR subdivisions in the analysis -> set analysis parameters window will make the oscillation visible. It will take a very long time to run out to 10 seconds though. You could also just run the simulation over a narrower range, 5ms to 5.5ms for example, and you should be able to see a 188kHz triangle wave at the output.
  • 0 in reply to Zak Kaye
    TI__Expert 4915 points

    Thank you Zak,

    I increased the time to see whenever the time constant was OK, I should only need to simulate a few nS to calculate the resulting frequency.
    The output should not be triangular at such a low frequency. Would you be able to advise what parameters to set on the various TR fields?

  • 0 in reply to Carlos Silva85
    TI__Mastermind 23050 points

    Carlos,

    It looks to me like you are trying to set the oscillator frequency to somewhere around 450kHz. If you zoom in to the triangular output and look at its slope you should see that the output is changing at approximately 1.65V/us. The specified slew rate of the OPA335 is only 1.6V/us. If you increase  C1 to 500pF, the output turns into a square wave and begins to actually saturate near the rails as you would expect (see attached). Based on the datasheet, I would expect the real world device to behave more like the simulation. The specified slew rate is only a typical value though so it is possible you have a device that  performs better than typical and can accommodate higher oscillation frequencies. Ultimately though, I don't believe the opa335 is a good choice for this application.