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LMH6643: Our LMH6643 Capacitance three point oscillation circuit is instability.

user1345944
Prodigy 40 points
Part Number: LMH6643

There is a LMH6643 Capacitance three point oscillation circuit like the picture below (J2 connect ot the inductance coil):

2 Problem  :

 1. Some of the LMH6643 : Heat it : oscillation,  cold state :No oscillation.

 2. Frquency of oscillation: The frquency is quite stable in office，But the frquency instability outdoors.

I don't know why it happens. Could you help me ? 

Thank you!

  • 0
    Prodigy 40 points
    Supplement: The VDDAD in picture is 5V DC.
  • 0 in reply to user1345944
    TI__Expert 7875 points
    Hi,

    If I understand correctly, you are trying to use the LMH6643 as an oscillator and it is not always oscillating and the frequency changes? What is the value of the inductor and what oscillation frequency are you trying to achieve?

    Regards,
  • 0 in reply to Jacob Freet
    Prodigy 40 points

    Hi,

      Yes, I'm trying to use the LMH6643 as an oscillator and it is not always oscillating and the frequency changes in different environment.

      The Inductor = 800uH

      The Frquency is 40kHz to 45kHz

    Thank you !

    Regards

  • 0 in reply to user1345944
    TI__Expert 7875 points
    Hi,

    My apologies for the delay in responses as I was out on vacation. I tried to simulate the stability response of your circuit using the PSPICE model and it looks like the phase of the feedback may not be inverted enough to keep the device in oscillation under all conditions. One option you could try is adding an additional phase shift with a series resistor and capacitor to ground directly on the output of the amplifier and before it connects to the feedback loop.

    Beyond that have you checked into the temperature variance on your inductor? Typically I would expect internal components to be very stable, but if the inductance is changing vs. temperature that could be parts of the reason the oscillation frequency is shifting.

    Regards,
  • 0 in reply to Jacob Freet
    Prodigy 40 points

    Hi,

    Thank you very much!

    Regards