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LMC6084 oscilations

lior kadosh
Prodigy 50 points

Other Parts Discussed in Thread: LMC6084

Hi,

I have a strange problem with the LMC6084 amplifier with BDX53C darlington transistors.

When I create my circuit in PSpice it looks sable and it doesn't oscillates but the real circuit on the PCB oscillates no matter what I add to the circuit (pole or zero).

Does anyone has an idea?

Thanks,

Lior.3005.F0110TEK.TIF

  • 0
    TI__Intellectual 2630 points

    Hello Lior,

    Can you provide more details regarding the oscillation. What is the frequency, magnitude, where are you measuring it (I assume, Q1 emitter, pin 3)? A scope shot would be helpful.

    This info would help isolate the possible source of oscillation.

  • 0 in reply to Jason Seitz
    Prodigy 50 points

    I attached a scope shot in the previous post.

    I see the oscillations int the output of the amplifier and in the Q1 pin 3.

    Thanks,

    Lior.

  • 0 in reply to lior kadosh
    TI__Intellectual 2630 points

    Lior,

    Thanks for the scope shot. I saw the link in the original post but assumed incorrectly that it was the schematic.

    Magnitude is about 2V and frequency reported is ~1.8MHz "?"  which means it is not triggered or quite locked on the waveform. Looking at the period and time division though you can see frequency is about ~1.2MHz. The LMC6084 GBQ = 1.3MHz so it doesn't quite eliminate  the op amp as the source of oscillation as opposed to the higher bandwidth darlington.

    I'm still not quite sure what the source of oscialltion could be. At this point I would use standard trouble shooting techniques. The primary components that could effect the level of oscillation would be C100, R169, and R172. Increase and decrease these components and see how the oscillation is effected. Also Q1 collector (BBSE_G...) what voltage is that node at and how is it bypassed to ground?

  • 0 in reply to Jason Seitz
    Prodigy 50 points

    Thanks for your answer.

    The magnitude is 2V and the frequency is about 1MHz, I tried to replace the resistors and capacitors according to PSpice design that I did and it didn't worked, I still get the oscillations.

    The BBSE_G... node is connected to a LED and to 12V.

    Do you have any other idea?

    Thanks for your help.

    Lior.

  • 0 in reply to lior kadosh
    Prodigy 50 points

    Can anyone help me please?

  • 0 in reply to lior kadosh
    Guru 21975 points

    Iior,

    With a high current output stage as you have, there is a possibility of an unintended feedback path through power supplies or ground. Carefully consider the current path from power supply, through the LED, transistor and back to ground. Be sure that stray resistance and inductance in this current path does not create a voltage on the ground of the bypass capacitors on the op amp or the ground connection to C99.

    Regards, Bruce.

  • 0 in reply to Bruce Trump
    Prodigy 50 points

    Thanks for the answer Bruce.

    But I'm not sure that I completely understood it.

    You are impaling that parasitic inductance can create a voltage on the ground and therefor I will see the oscillations?

    What can I do to prevent it? Do I need to add resistance to the current path?

    Thanks,

    Lior