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LM5122 2-phase waveform jitter on slave chip - master OK.

Phil Cowett
Prodigy 50 points
Other Parts Discussed in Thread: LM5122EVM-2PH, LM5122

We are using a pair of LM5122s in a 2 phase master-slave configuration. Although our design is higher power with external 12V power supplies for gate drive, push-pull gate drive boosters, your LM5122EVM-2PH exhibits the same symptoms, although not as severe. The problem is switching waveform jitter on the slave side only. On our design, we found that a 470pF cap on the sync line between the master and slave chips helped some, it caused the duty cycle of the sync waveform to deviate from 50% duty cycle. Our design has a significant current imbalance between the 2 phases of about 35%. Your EVM exhibits only a 5% - 10% current imbalance. Any help would be appreciated because our customer is screaming at us for a fix! Thank you in advance for your help!

  • 0
    TI__Mastermind 49001 points

    Hi
    In order to have well balanced performance;
    Slope1 value should be the same as Slope2.
    Inductance of two inductors should be close each other.
    AGND1 should be tied well to AGND2.
    Regards,
    EL

  • 0 in reply to EricLee
    Prodigy 50 points

    Thank you EricLee.

    The LM5122 does not have any pins labeled Cramp or Rramp. It does have a pin labeled SLOPE, and the resistors from the two pins are identical. We have taken pains to match the 2 inductor values. I assume AGND1 and AGND2 refer to the AGND pins on each of the two chips.

    Is your reply is specific to the LM5122 or is it a general comment that applies to many PWM chips?


    I was thinking that Driving the chips from an external 2-phase oscillator would help. What do you think?

    Regards,

    Phil Cowett

    Duryea Technologies

  • 0 in reply to Phil Cowett
    TI__Mastermind 49001 points
    Hi
    Have you check the switching frequency ? If one board runs at lower frequency than the other, then they will not be balanced. Otherwise, I don't expect it will show a better performance with external synchronization clock.
    Regards,
    EL
  • 0 in reply to EricLee
    TI__Mastermind 49001 points
    Hi
    I corrected. I meant Slope1 and Slope2.
    Regards,
    EL
  • 0 in reply to EricLee
    Prodigy 50 points
    Hi,
    Both chips are on the same board, and we are using the same synchronization circuit as on the TI LM5122EVM-2PH. It is not possible for them to operate at different frequencies. Would it help if I sent you a schematic? If so, can it be kept confidential? Our board operates at 100KHz, while your EVM operates at 250KHz, but we are not trying to synchronize your board with ours.

    Please note that the greater problem is the slave chip PWM waveform jitter. I do not believe you have addressed this. Also, the fact that your EVM exhibits the same problem suggests to me that there is a design flaw in the LM5122 chip which affects its use in 2-phase applications.
    Regards,
    Phil Cowett
  • 0 in reply to Phil Cowett
    TI__Mastermind 49001 points

    Hi Phil

    You posted to an EXTERNAL FORUM. This content will be visible to anyone who visits the forum website and findable through internet search engines. If you want, please leave your e-mail here and I will contact you.

    Waveforms are useful for me to investigate the issue. It is hard to guess the behavior only with a schematic.

    Please capture primary side inductor current , slave side inductor current, COMP together.

    Have you check the frequency? if the frequency of slave board is different than master's, there can be an unbalance issue.

    Regards,
    EL

  • 0 in reply to EricLee
    Prodigy 50 points
    Hi Eric Lee,
    My email is phil.cowett@verizon.net. Capturing waveforms is difficult because I am working in Virginia and Duryea is in Pennsylvania, about a 5 hour drive. However, there is a simpler approach. Because the TI LM5122EVM-2PH exhibits the same problem with jitter on Phase 2 (slave), I would be satisfied if you could obtain one of these boards and observe the phase 2 waveform yourself. We have tested it at half load and at full load. The EVM has a fixed 28V output with a 9V-20V input range. It exhibits the jitter only on Phase 2 (slave) at an input around 12-13V. The schematic and layout are in the Users Guide here: www.ti.com/.../getliterature.tsp. The EVM has essentially the same synchronization circuit as ours does, which is traceable back to the LM5122 data sheet.
    I am focusing on the slave phase jitter and not the phase current imbalance. I am confident we can fix the current imbalance ourselves once the jitter problem is fixed. We need to be certain that the LM5122 does not have some inherent problems. At this point we believe it does.
    Both phases operate at the same frequency both on your design and ours.
    Regards,
    Phil Cowett
  • 0 in reply to Phil Cowett
    Prodigy 50 points

    I haven't heard from anyone in a while, so we are beginning a new design which does not use the LM5122. We do not have a choice. There is just too much risk in using this chip. If someone can resolve the obvious issues with this chip, as was demonstrated in TI's Eval board, in the next few days, we may reconsider our decision not to use the LM5122 chip.