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Variation of Phase Margin at Gain Cross Over Frequency

Shihab
Expert 1205 points
Other Parts Discussed in Thread: TLE2061

Hi,

I had gone through the article titled “How system operating conditions affect CMOS op amp open-loop gain and output impedance” written by Ti application engineers.

http://industrial.embedded-computing.com/articles/how-system-operating-conditions-affect-cmos-op-amp-open-loop-gain-and-output-impedance/

For the selected op-amp in the article, I understood that the worst case unity-gain crossover frequency can vary from -33% to 67% of the nominal.

Can you also please provide an estimate as to how much the phase margin at unity gain may deviate over the operating temperature of the opamp?

This information will be useful as an input when we do a worst case stability analysis.

Thanks,

Shihab.

  • 0
    TI__Mastermind 30600 points
    Hi Shihab,

    This is a difficult estimate to provide because of the complexities around how the feedback network and load shift with temperature. But, if you design your system with 60 degrees of phase margin, that will be your stability buffer across temperature to help unsure that your system stays stable.

    If you were to look at the phase margin of the Aol curve by itself (no load) then the phase margin wouldn't change because the Aol phase curve will change with the Aol gain curve.

    Best,
    Paul
  • 0 in reply to Paul Goedeke
    Expert 1205 points

    Hi Paul,

    My question was on the phase margin variation at cross over frequency with respect to the Open Loop Gain (Aol) gragh and not on the AolBeta.

    The article tells that the nominal value of unity-gain frequency, fu is 1.8MHz.

    The article tells that "The variation of the unity-gain frequency, fu, over the operating temperature range is shown in Figure 5. Over the operating temperature, fu, can vary from 1.26 MHz to 2.75 MHz.".

    So from your reply what I understand is, even if Fu varies from 1.26 MHz to 2.75 MHz the phase margin will be constant. Is it correct?

    Thanks,

    Shihab

  • 0 in reply to Shihab
    Guru 140200 points
    Hi Shihab,

    provided the circuit is designed properly, the phase will not change very much in the range 1.26...2.75MHz.

    I guess, this thread has to do with the TLE2061 from your other thread? Why not showing us a schematic?

    Kai
  • 0 in reply to kai klaas69
    TI__Mastermind 30600 points
    Hi Shihab,

    From the perspective of the op-amp itself, if the Aol magnitude curve shifts due to temperature, then the poles must also be shifting too. This will keep the relationship between them the same and the phase margin the same. Once we add in the circuit configuration, the impact of Zo and other circuit components will make this no longer the case.

    Best,
    Paul