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UC1901: UC1901 error amplifier frequency response

Nandha Kumar5
Intellectual 470 points
Part Number: UC1901

Hi, 

I was recently trying to delve deeper into understanding the working of the UC1901 IC and wanted to verify the frequency response of the error amplifier and the demodulation stage. 

To do this, without using a complete converter (to play around with the feedback values), I connected the output of the de-modulator stage to the inverting pin of the error amplifier. The non-inverting pin is connected to the ref pin. By my understanding the control loop should operate and set the bias point at 1.5V. However, this doesn't seem to happen both practically and in simulation. Can someone explain why this might be happening? Or suggest a better way to verify the frequency response of this system

Thanks

Nandha   

  • 0
    TI__Mastermind 25895 points

    Nandha,

    R7 is not connected into your circuit (there is no connection dot at the top of R7) and it is also not connected in the TINA circuit. If you connect R7, you will see different start up transient behavior. I was unable to run AC analysis on this circuit but I believe you want to see COMP changing as the INV is changing and since you are not including the UC1901 in a closed loop power converter, the best way to observe COMP and INV may be to monitor signals of interest while operating open loop.

    Regards,

    Steve M

  • 0 in reply to Steven Mappus1
    TI__Mastermind 25895 points

    Nandha,

    Have you checked out the UC1901 Average TINA model available in the product folder? This one is set up for measuring the AC response and give the result shown in Fig.1 of the data sheet.

    Steve M

  • 0 in reply to Steven Mappus1
    Intellectual 470 points

    Hi steve, 

    R7 is connected. the dot may have vanished while rearranging the circuit. Either way, I re-checked the connections and ran the simulation with no change in the results.

    Thanks 

    Nandha

  • 0 in reply to Steven Mappus1
    Intellectual 470 points

    Steve, 

    I haven't used TINA till now. Will get back after trying the software. Also does either of the SPICE or TINA model incorporate the IC behavior with the oscillator disabled? Also wanted to verify whether the IC can also be used as an amplifier for an optically coupled feedback system.

    Thanks 

    Nandha

  • 0 in reply to Nandha Kumar5
    TI__Mastermind 25895 points

    Nandha,

    Remove C3 and short CT to V2. This is what is done when Syc'ing to an external clock but if no external clock signal is applied, the oscillator is left disabled. I believe the UC1901 transient model supports this behavior.

    Regards,

    Steve M

  • 0 in reply to Steven Mappus1
    Intellectual 470 points

    Steve, 

    The oscillator disabled configuration works on the hardware but not in simulation. not sure why. Could you run the simulation with similar configuration and get back to me

    Thanks

    Nandha 

  • 0 in reply to Nandha Kumar5
    TI__Mastermind 25895 points

    Nandha

    Previously, we discussed using the average TINA model for your AC analysis but this model is without the CT pin so I'm using the transient TINA model from the UC1901 product folder. Here's a comparison between running the model as it is downloaded from the product folder without any changes vs removing C1 and shorting CT to VIN to disable the clock:

    Internal oscillator is running

    Remove C1 and short CT to VIN:

    Internal oscillator is disabled:

    Regards,

    Steve M