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TINA/Spice/LM1875: Problem using LM1875

Georgios Kefis
Intellectual 450 points
Part Number: LM1875
Other Parts Discussed in Thread: TINA-TI, , OPA564

Tool/software: TINA-TI or Spice Models

Hello,

i am trying to do a calibration and for this reason i have to use the sinus signal of a function generator. My generator can provide a signal up to 200 mA @10V p, but of course i will try to use less current if possible. Unfortunately i need a current source and i thought that the LM1875 could solve my problem since it can get the desired input signal from my generator. My "electrical load" needs current up to  +-1,2 A which corresponds to +-0,5V (that's the voltage limit) at 50 Hz. So i thought that it would not be a bad idea to use the LM 1875 which, could be connected with an electrical transformer 1/20 and then using the generator i would be able to set the voltage where i want to . For this reason i have done a simulation using TINA which unfortunately does not give me the results i would like to see. According to my application i should see 0,5Volts at 1,2A peak as the load R6 but it does not happen. Is it an issue, that has to do with my idea using the LM1875 or do i miss something regarding my simulation?? The other idea i had, is to use only the transformer but i could possibly achieve my simulation, only when i set the voltage of the generator  at 10 Vpeak, for which i am not that sure,that i could really get the expected values. That was the reason for using the LM1875. But would also a power opamp give me a solution and if so, which one ? 
Attached you can find both the scematics and the outputs of the simulation. The same circuit, without the R6 resistor, does not work at all. I understand that this IC is designed for audio applications, but i want to believe that it could also be the solution for my application, couldn't it?   


  • 0
    Guru 140200 points
    Hi Georgios,

    I have simulated your circuit with TINA-TI and it works, even without R6. But I have taken another transformer.

    Kai
  • 0 in reply to kai klaas69
    Intellectual 450 points

    Hello Kai,

    thanks a lot for your useful comments. Could you please give some extra info regarding the transformer you used and additionally , what else did you change at my circuit? The reason for using this one is that it is easy to find it using mouser.de (i am located in Germany). 
    Thank you in advance.

    Best regards,
    George

  • 0 in reply to Georgios Kefis
    Guru 140200 points

    Hi Georgios,

    here's the result of my simu:

    You see the input voltage of 1Vp, the output voltage of OPAmp VM1 of 6Vp and the output voltage across R1 VM2 of 300mVp. Also shown is the output current through R1 of 857mAp.

    In reality the transformer can introduce unwanted winding and interwinding capacitance which might need the insertion of an isolation resistance at output of OPAmp, in front of the transformer.

    georgios.TSC

    Kai

  • 0 in reply to kai klaas69
    Intellectual 450 points
    Hello Kai,

    the least i can do is to thank you, as at the end of the day, it seems to be a simulation issue. Another last question : in case i wanted to avoid the use of the transformer, would you think, that the use of the OPA 564 solve the problem as well?

    Best regards,
    George
  • 0 in reply to kai klaas69
    TI__Mastermind 24835 points

    Hi Kai,

    Thank you very much for the simulation!

    Best regards,

    Shawn Zheng

  • 0 in reply to Georgios Kefis
    TI__Mastermind 24835 points
    Hi George,
    When an audio power amplifier is driving an transformer with inadequate low-frequency performance, some issue could be caused by the saturation of the transformer core. Please find more explanation in this document: www.ti.com/.../sloa133.pdf
    Best regards,
    Shawn Zheng
  • 0 in reply to Shawn Zheng
    Intellectual 450 points

    Hello Shawn,

    thanks a lot for your comment. Having something like that also in mind, i tried to find an alternative solution, in order to avoid the transformer. The idea is to use the OPA 564  although i have not done any simulation yet. Do you believe that it would work? 
    I am looking forward to hearing from you .

    Regards,
    George

  • 0 in reply to Georgios Kefis
    TI__Mastermind 24835 points
    Hi George,
    Thank you for the reply. I'm sorry I'm not familiar with OPA564, because this forum is about audio devices. Could you please creat a new post with 'OPA564' in title in the 'General Purpose Amplifier' forum? TI application engineer for this device will give you the most professional answer for this device. Thank you!
    Best regards,
    Shawn Zheng
  • 0 in reply to Georgios Kefis
    Guru 140200 points
    Hi George,

    the OPA564 will not work because it will overheat. If I understand you right, you will drive a current of 1.2A into a load of 417mR, according to a voltage drop of 0.5V? In this case the OPAmp has to dissipate almost the full supply voltage times 1.2A, which is too much for the OPA564. See also figure 45 and 46 of datasheet.

    You should choose a Power OPAmp with much higher output current rating and improved cooling. Additonally, you could insert a resistance between output of OPAmp and load to decrease the heat dissipation of OPAmp. Keeping the supply voltages low could also help.

    Kai