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TPS65130: Ringing issue with -12V

Kristi khondkar
Prodigy 30 points
Part Number: TPS65130


We are seeing ringing with the -12V Supply for our board. With 5V input this  +/-12V feed supply voltage to biasing Op-amps to other boa. Schematic attached. Any help would be appreciated.

  • 0
    TI__Expert 6330 points
    Hi Kristi,

    I have notified our expert regarding this topic. Please expect a response by 1/29/19.

    Thanks,
    Aaron
  • 0
    TI__Mastermind 18550 points
    Hello Kristi,

    Negative converter feedback pin (FBN) can be sensitive to noise coupled through feed-forward capacitor C31. Can you try increasing value of resistor R21 to 50K ohm or higher to see if this improves ringing?

    Kind Regards,
    Liaqat
  • 0 in reply to Liaqat Khan
    Prodigy 30 points

    Hi Liaqat,

    Thanks for your reply. I tried changing the R21 to 50K but still can see the ringing, see below. 

    The particulars of our application are: Circuit board with mixed signal A/D, -55 +125C operation, the load is typically 30-50mA depending on various configurations, but there is minimal load variation during normal operation. This ringing coupling up with McAsp Lines and Showing up as a glitch massing up the communication protocol.

    Thanks,

    Kristi

  • 0 in reply to Kristi khondkar
    TI__Mastermind 18550 points
    Hello Kristi,

    Are you sure that this waveform is from the inverting connverter output (at C41)? I believe what you have sown in your reply is switch node waveform for negative converter at L3/D2 junction. Can you please double check -12V output at C41?

    Another thing I noticed in your schematics is that values of feed-forward capacitors C30 and C31 calculated using equations 11 and 12 in the datasheet should be ~4.7pF each and not 6.8pF and 12pF shown on your schematics.

    Kind Regards,
    Liaqat
  • 0 in reply to Liaqat Khan
    Prodigy 30 points
    Yes the wave form that i have was indeed the switch node waveform for negative converter at L3/D2 junction, when the switch is close there is no ringing, but when open the ringing show up, question is there any non conduction mode happening that stage?
    I was also wondering why the ringing does not show up in +12v between L2/D1? Its looks pretty clean.
    I will try your next suggestion and see if that help.

    Regards.
    Kristi
  • 0 in reply to Kristi khondkar
    TI__Mastermind 18550 points
    Hello,

    This is normal behavior of the device at light load because negative converter goes to dis-continuous conduction mode at light load. Positive converter stays in continuous conduction mode even at lighter loads. Please see description in the datasheet under section 7.1 "Overview". If you increase load to around 80mA, this ringing will go away and the device will operate in continuous conduction mode.
    If this ringing is causing an issue in your system, you could try running the negative converter in power save mode by tying PSN pin high. This will put the negative converter in PFM mode at light loads where switching will occur in bursts to maintain regulation. This can likely put noise spectrum outside of you band of interest. Hope this explanation is helpful.
    Kind Regards,
    Liaqat