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LM2663: Output current noise

Yusuke Mukuno
Expert 7395 points
Part Number: LM2663

HI expert,

My customer have an issue about output current noise with LM2663.

Configuration : Positive doubler

output current waveform

Diode(D1) :  Customer try to change Diode from SS355VMTE-17(Vf=1.2V) to RB520SM-30 (Vf=0.58V) Because, the datasheet is recommended low Vf diode.

Then this noise is little bit improved. Could you please let me know the reason for it? 

Also, Could you please let me know your recommendation for diode Vf (max)?

Noise frequency  : around 75kHz

I think, It is switching frequency, right?

Datasheet is described Fsw = 27.5kHz(min), 75kHz(Typ). Could you please let me know the maximum value?

Also, what parameters does this variation depend on? (Temp/ Vin/ unit variation/ something like that)

Do you have any idea for the avoiding current noise?

Thanks

Muk 

  • 0
    TI__Guru 65170 points

    Hello Yusuke,

    You mean the spikes that are on top of the output current waveform, correct?

    Which type of capacitors are you using?

    According note 6 below the electrical characteristics, the oscillator frequency of LM2663 is 150kHz so the switching frequency is 75kHz. The switching frequency can change.by adding a capacitor on pin OSC to GND. See Figures 8 and 9 in the datasheet.

  • 0 in reply to Brigitte
    TI__Expert 7395 points

    Brigitte-san,

    > You mean the spikes that are on top of the output current waveform, correct? : Yes, I mean the spike.

    >Which type of capacitors are you using? :  Ceramic capacitors

    Is this noise affected from the characteristics of the diode? Also, Could you please let me know your recommendation for diode Vf (max)?

    >According note 6 below the electrical characteristics, the oscillator frequency of LM2663 is 150kHz so the switching frequency is 75kHz. The switching frequency can change.by adding a capacitor on pin OSC to GND. See Figures 8 and 9 in the datasheet.

    Customer would like to know the factor for spec of switching frequency variation. Also, Test condition for Fsw spec is OSC=Open

    I found Figure9 to 12. it is meaning, the switching frequency variation is depend on Temp and power supply. (Of course, unit variation)

    Do you have any idea for another factors?

    Thanks

    Muk

  • 0 in reply to Yusuke Mukuno
    TI__Guru 65170 points

    Hello Muk,

    I would recommend to add a small high-frequency capacitor parallel to the output capacitor close to the IC which is still capacitive at the frequency of the noise spikes (the rise and fall times of the spikes are important, not the switching frequency). The output capacitor used at the moment is most probable already inductive at this frequency.

    I do not expect that the diode influences these spikes. The forward voltage is not too important as it only makes sure that the difference from VIN to VOUT is not getting too big at startup. Just use one diode and not 2 in series.

    The series resistance of the flying capacitor does, so a resistor in series there could be helpful, but this reduces the efficiency. 

    If there is no test condition given in the test condition column, please take the test condition at the top of the list. This is the generic test condition. In the test condition column only changes to the top condition are given.

    I do not expect more factors.

  • 0 in reply to Brigitte
    TI__Expert 7395 points

    Hello Brigitte,

    I  requested adding small capacitor like 0.1uF/0.01uF for output capacitor and flying capacitor.

    They tried to add 0.1uF. But, the noise was not improved.

    I will try to suggest the series resistance of the flying capacitor.

    Thanks

    Muk

  • 0 in reply to Yusuke Mukuno
    TI__Guru 65170 points

    Hello Yusuke,

    Did they add small high frequency caps to the flying cap as well as the output? If yes, they only should add the high-frequency cap to the output. And this small cap needs to be as close to the IC as possible.

  • 0 in reply to Brigitte
    TI__Guru 65170 points

    Hello Yusuke,

    I expect that you have been able to solve this issue, so I will close the thread now. If it is not solved, please post below or start a new thread if this one is already locked.