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LM25118: Gain margin

Nidhi P Shetty
Intellectual 735 points
Part Number: LM25118

Hello

In the document  www.ti.com/.../snvu065 selected bandwidth is assumed on what basis?
For system stability what should be the gain margin value if gain margin is negative?
Whether gain margin is to be more negative? or less negative?
For example whether system with -22db gain margin is more stable or one with -55db is more stable?
Please help me out in this concern.

Regards
Nidhi P Shetty

  • 0
    TI__Guru* 78070 points
    Hi Nidhi,

    I would suggest you to take a college course on control system because these questions cannot be easily explained here in e2e. Just a top level criteria based on control theory regarding the Gain Margin: it must be GM>6dB, or in your term -6dB. GM is the gain below unity gain (0dB), and the difference is the margin. No margin means your gain is at 0dB when the phase crosses 0deg. So when people say GM they usually do not mention the minus sign, or you may cause confusion in discussion/communication. Owing to component variations, engineers usually target a GM of >10dB to count for the possible variations. Being stable is being stable, there is no concept of being more stable. So more GM does not not mean more stable. To get more GM, you may sacrifice the other parameter, the Phase Margin, or the loop may look weird. Please refer to a good text book on the control theory and you will understand better.

    Thanks,
    Youhao