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LM2904B: Unity gain stability

Shinya Sawamoto
Expert 5285 points
Part Number: LM2904B

Hello team,

I'd like to ask about unity gain stability for LM2904B.
Is there any limitation of the output capacitor range to gain enough phase margin in 0dB buffer use case?

Thanks in advance.
S.Sawamoto

  • 0
    TI__Genius 15410 points

    Hi Sawamoto-san,

    The LM2904B states a phase margin of 56 degrees with a capacitive load of 20pF on page 9 of the datasheet. The cap load is provided in the test conditions for the phase margin specification. Increasing the cap load will decrease the phase margin. I also recommend looking at Figure 27 in the LM2904B datasheet for a phase margin vs cap load graph. You will see that as the cap load increases the phase margin decreases. Having too low of a phase margin can cause the op amp output to oscillate leading to long settling time or unbound oscillation.

    Thank you,

    Tim Claycomb

  • 0 in reply to Tim Claycomb
    TI__Expert 5285 points

    Thanks a lot Tim.

    Allow me ask further thing about phase margin in unity gain buffer use case.
    In case of the output capacitor is much higher 0.1uF, 1uF and 10uF, the LM2904B can't be stable, is it correct?
    If yes, do you have any good option or method to cover such a large capacitance load?
    - 32V wide Vcc is required.
    - 10mA output drive capability
    - Signal speed is very slow, not needed care for fast BW.

    Thanks in advance.
    S.Sawamoto

  • 0 in reply to Shinya Sawamoto
    TI__Genius 15410 points

    Hi Sawamoto-san,

    Yes, that is correct. The LM2904B will be unstable with a capacitive load of 0.1uF, 1uF, or 10uF.

    I am not away of a device that can directly drive that large of a capacitive load. Can the customer stabilize the amplifier with an isolation resistor? Please see our TI Precision Lab Videos on Stability for more information. The videos discuss the theory of stabiliy and show multiple methods on how to stabilize an amplifier with a capacitive load.

    Thanks,

    Tim Claycomb