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LM1875: he questions about waveform observed from the power amplifier output(LM1875)

user5884280
Prodigy 50 points
Part Number: LM1875

Currently, I amusing the power amplifier (LM1875T) that your company sells.

I am trying to amplify the amplitude by about 1.2 times by making inverting amplifier circuit using LM1875T.
We set the waveform which multiplied the signal of 200 Hz and 40 kHz to the input signal to the power amplifier.
However, when observing the waveform output from the LM1875T with an oscilloscope, it is observed that the amplification and extinction of the waveform are alternately repeated

Also, similar phenomena occurred when observed with the A / D converter, and it was also observed that the amplitude was shaken off at the moment when the waveform switched from the amplified state to the disappearing state.

As the picture shows how the waveform is amplified and disappears, you can show it if necessary.

Is there any cause for such a phenomenon?

 

Thank you.

 

  • 0
    TI__Mastermind 24835 points
    Hi Ukita-san,
    This issue occurs on your board, right? Could you please show me your SCH? And the scope screen capture for the input and output signal? It definitely helps our debug work.
    Best regards,
    Shawn Zheng
  • 0 in reply to Shawn Zheng
    Prodigy 50 points

    Hello Mr.Shawn Zheng

    Thank you very much for your reply.

    I show you SCH and A photograph of the input voltage and the output voltage observed at the same time using the A / D converter.

    I set the input voltage to about 4 V and I want to set the output voltage to 4.8V.

    As I mentioned earlier, output waveform repeats amplification and extinction.

    In the pictures of the waveform, The yellow waveform is the output voltage and the red waveform is the input voltage.

    First picture is SCH.

    Second picture is just before the amplified wave form appears.

     Third picture is just before the amplified wave form appears(zoom version).

    4th  picture is just before the amplified wave form disappears.

    Last picture is just before the amplified wave form disappears(zoom version).

    Best regards. 

  • 0 in reply to user5884280
    TI__Mastermind 24835 points
    Hello,
    So the input signal is always continuous while the output is not. Looks liken a fault is triggered, and it could be OC or OT fault. You can find more info about FAULT in section 'Current limit and SOA protection' and 'Thermal protection' on the page 7 and 8 in the datasheet.
    Best regards,
    Shawn Zheng
  • 0 in reply to Shawn Zheng
    Prodigy 50 points

    Hello,

    To refer to your advice, I attached heat sinks to the power amplifier(LM1875t).

    I was able to amplify the output wave form.

    Thank you very much.

    Ukita Yuki

  • 0 in reply to user5884280
    TI__Mastermind 24835 points
    Hi Ukita-san,
    Glad to know that! Please feel free to creat a new post if you have any other question.

    Best regards,
    Shawn Zheng