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TPS630242: Output Noise

John Fedak
Expert 6715 points
Part Number: TPS630242

Hi Team,

Good morning, my customer is using the TPS630242 in their design and seeing a lot of noise on the output:

The ripple is about 9-11mV and there is a 60 kHz sub-harmonic oscillation. I haven’t looked at the inductor current which would tell me if indeed these are sub-harmonic oscillations.

Attached are some scope captures where I measured the ripple at the output filter caps, no load. I set the PFM/PWM pin low and then high and initially didn’t see any difference in the waveforms. I then changed the inductor value to a 4.7uH and the converter did not like this. I was seeing 0.5A drawn from the input continuously. I then replaced back the original 1uH inductor and measured the ripple again. This time, the PFM/PWM pin did place the IC in PFM mode when low and in PWM mode when high. However, the sub-harmonic oscillations are there.

No Load Forced PWM:

No Load PFM:

*Updated Images*

Interested for your thoughts and please let me know if there are any questions.

Regards,

~John

  • 0
    TI__Expert 7925 points
    Hello John,

    There should certainly be difference in the ripple between PFM and PWM mode. In PFM mode, the ripple is 30 mVpp typically, 50 mVpp worst case. In PWM mode, the ripple is depends on the Cout. On the EVM it is well below 10 mV.
    - Is the PFM/PWM ripple too high for the customer's application?
    - Could you share the schematic/specs and the PCB layout?
    - Did you measure the ripple using an active probe?

    Best regards,
    Milos
  • 0 in reply to Milos
    TI__Expert 6715 points
    Hi Milos,

    I have sent over the schematics/layout in an email - the bigger concern than the noise is the 60kHz Oscillation.

    Regards,
    ~John
  • 0 in reply to John Fedak
    Prodigy 150 points

    Hi Milos,

    Can someone please take a look at the schematic and layout. We have two different designs using this same part. Both designs exhibit a 60kHz sub-harmonic oscillation. That's  the first issue. Second, the ripple in forced PWM is still huge, over 10mV. I've placed 186uF worth of output filter caps and no different from 60uF. I've also more than tripled the input cap from 10uF to 32uF. In an attempt to reduce the output ripple, I increased the inductor value to 8.2uH but this converter did not like it at all- the part would consume 500mA from the input supply. I know the spec. mentions the max. value is limited to 1.3uH.

    It seems this converter is displaying symptoms of instability. There is no mention in the DS the need for ESR at the output caps.

    Thank you,

    Jorge

  • 0 in reply to Jorge De La Rocha
    TI__Expert 7925 points
    Hi John, Jorge,

    As we are continuing discussion in private, I will close this thread.

    Best regards,
    Milos