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OPA2365: Error Amplifier Design on PWM generation

nicolas merdrignac
Prodigy 40 points
Part Number: OPA2365

Tool/software:

Hi,

I have a question regarding the SLAU508 "TI Precision Designs" document from John Caldwell. The title of his document is "Analog Pulse Width Modulation".
I do not understand the Error Amplifier Design principle, described in the chapter A.1 . I've read the dedicated chapter several times, but i don't get the design trick with the comparator included in the feedback loop of the amplifier.

Do you have additional literrature about that function ? Could you help me to understand that function please ?

Thank you very much for your support.

Nicolas.

  • 0
    TI__Genius 16491 points

    Hello Nicolas,

    The key point of the inclusion of the comparator in the error amplifier feedback loop is the power supply rejection.  The Vtri (triangle wave) signal will directly affect Vpwm in a configuration without the comparator in the feedback loop.  When the comparator is included in the feedback loop, the OPA2635 op-amp can adjust to handle any errors or offset present in Vpwm.  If the comparator was not present, the Vpwm signal would not be sensed by the OPA2365 op-amp, resulting in a system more capable of errors in the output signal.  Additionally, with the feedback loop the Vtri signal and power supplies of the comparator do not control the average of Vpwm.  Since a comparator's output signal goes to either supply rail, the paragraph is essentially explaining how comparators have no power supply rejection.  The design of the error amplifier circuit addresses the need for power supply rejection and handling any output errors.

    I know this is a concept which can be tricky; we do have videos on power supply rejection: https://www.ti.com/video/series/precision-labs/ti-precision-labs-op-amps.html

    Look for "Power Supply Rejection" under the Op-Amps series.

    Best,

    Alec