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TPS62933P: Output oscillates while current probe is inserted

keita.shoji
Prodigy 210 points
Part Number: TPS62933P
Other Parts Discussed in Thread: TPS54302

Hi,

I'm seeing unusual behavior while current probe is inserted at the output of DC-DC converter. Once I remove the probe, output waveform become normal. It is also not oscillating with constant current load. I checked loop gain without current probe and it looked fine with 55deg of phase margin. I attached several waveforms below for reference.

Model of current probe is Tektronix TCP202.

Do you have any idea why the converter is oscillating?

Best regards,

Figure1. Output oscillation

Figure 2. Loop gain

  • 0
    TI__Genius 15117 points

    Hello Keita,

    Did you measure input current or other buck circuits current and found similar issue?

    Please post your schematic and layout, I will review.

  • 0 in reply to Miranda Gu
    Prodigy 210 points

    Hello,

    Several TPS62933P are used on the board, but only one is having issue of oscillation. I haven't measured input current yet. If I disconnect this buck circuits from the board and replace it with similar circuit with TPS54302, it still have same issue.

    Further testing revealed that oscillation was in sync with spread spectrum frequency (Fsw/128), and peaks at lower PWM frequency. I'm not sure how this is related to oscillation, but same behavior is also observed on TPS54302.

    Please review the schematic and layout attached below.

    Best regards,

  • 0 in reply to keita.shoji
    TI__Expert 4360 points

    Hi Keita

    The Miranda is OOO, I will check it and give you a feedback tomorrow, thanks!

  • 0 in reply to Xueliang Zhang
    TI__Expert 4360 points

    Hi Keita

    The schematic looks well, one concern of layout that the FB trace and inductor are too close, which will result in lot of SW noise being coupled to FB trace. It might cause the IC's unstable. have you tried other oscilloscopes and probes? thanks!