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LM2663: noise reduction

Allan Fan
Expert 7050 points
Other Parts Discussed in Thread: LM2663, LM2662, LM2776

Team,

Customer tested LM2663 EVM from 5Vin to -5Vout@90mA. But seeing spur -46.33dBm@91.6KHz.(Specification: in band noise -80dBm). Any suggestion to reduce it?

Regards,

Allan

  • 0
    TI__Guru**** 191445 points

    Looks like that is the switching frequency and its harmonic.  You might try changing the switching frequency.

  • 0
    TI__Intellectual 1120 points

    Hi Allan,

    As JohnTucker advised, changing switching frequency would be one way to do it. You can slow down the switching frequency by adding capacitance between OSC pin and GND.


    Alternatively, adding more output capacitance reduces output ripple, but it could be that it is not enough to meet specification.

    Regards,

    Juha

  • 0 in reply to Juha Karttunen
    TI__Expert 7050 points

    Hi Juha,

    I thought to decrease output ripple might try to increase switching frequency but adding external capacitor is to slow down instead. The typical performance characteristics shows the relationship between external cap and the OSC frequency but that seems only for LM2662? How about LM2663?  

    Customer also tried to add LC filter at output but still seeing noise over specification. Any other suggestions?

    Regards,

    Allan

  • 0 in reply to Allan Fan
    TI__Intellectual 1120 points
    Hi Allan,

    You are correct, frequency can be adjusted only downwards. I think figure in datasheet you are referring to, can be used for LM2663 also. FC = V+ case is effectively same as LM2663's fixed 150kHz oscillator frequency.

    Another possibility could be to use LM2776 instead. It has 2MHz oscillator frequency.

    Regards,
    Juha