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LM25085 issue regarding with Jitter

Denis liu
Prodigy 225 points
Other Parts Discussed in Thread: LM25085

Design for:

Input voltage: 24V

Output voltage: 15V

Output current: 0-4A

Output cap 3300uF, ESR value is 28mOhm.

 

it seems that the schematic and test waveform can't be uploaded.

 

Issue is that when have light load, the Jitter is obvious and cause current limit at every duty cycle.

but when with middle load or heavy load, the Jitter issue disappear.

The question is that

  1. Will the Jitter issue be accepted cause output voltage be abnormal(the test value show voltage ripple is accepted)
  2. is the frequent current limit is a issue to LM25085 itself(the Voltage spike of mosfet and diode has big margin)
  3. If change the output cap to be 470uF 5mOhm, will it help? If we use smaller RrCr, the jitter issue is improved while bigger output voltage ripple.
  • 0
    Prodigy 225 points
    Will the Jitter issue be accepted or Jitter will cause output voltage be abnormal(the test value show voltage ripple is accepted)
  • 0 in reply to Denis liu
    TI__Mastermind 31580 points
    Hi Dan,

    I have moved your post to the non-isolated dc/dc forum where one of my colleagues from that BU will answer your question.

    Regards

    Peter
  • 0 in reply to Peter Meaney
    TI__Genius 13700 points

    Hello Dan,

    First how are you trying to upload the schematic and scope capture?

    Without this information it will be hard to fine tune a solution to the problem.

    The way that you describe what is going in your system with a large amount of PWM jitter at light load which is fixed by increasing the injected ripple to the feedback pin you have narrowed in where I think that we need to look. What are you Cr, Rr, Cc values and are yo using a Cff capacitor?

    There are a number of paths forward that I would like to work through but having the schematic is the firt step in that process.

  • 0 in reply to Ryan May
    Prodigy 225 points

    LM25085 issue.docx

    hi, Ryan,

    attachment is our schematic and test waveforms.

  • 0 in reply to Denis liu
    Prodigy 225 points
    there is a fuse and a diode between the node +15V_CAP and +15V_FB
  • 0 in reply to Denis liu
    Prodigy 225 points
    there is a harness about 100mm long from the output to the load which containing 10pcs 100uF cap.
  • 0 in reply to Denis liu
    TI__Genius 13700 points

    Hello Dan,

    Thank you for the schematic. There are a number of things that I have questions about.

    What is the purpose of R212?

    Why do you have a high-side snubber made up of R206 and C103?

    What the values for C70 and C71?

    What are the voltage ratings on D1 and Q1? Part numbers would be great.

    Are you using C95 as a feed forward capacitor? If so that could be passing a bunch of the ripple from the ripple injection circuit forward to the output.

    There are a number of other questions that I have but first I want to understand a few of the things you have on your schematic to see if we can remove some things that might be putting unwanted signals to cause these problems.

    Looking through the scope captures that you have it looks like the LM25085 is triggering erratically. We can address this by working through a few of the issues and dialing in your ripple injection circuit.

  • 0 in reply to Denis liu
    TI__Genius 11490 points
    Hi Dan,

    Looking at the .doc report, it appears that the jitter issue is at heavy load. Is that correct? The description says 'jitter at light load'. Please clarify.
    Please also provide the following information:
    1. FET part number.
    2. Intended frequency of operation (in CCM).
    3. Also pease label the curves as it is very difficult to figure out the channels from the pictures.

    Thanks and regards,
    Vijay
  • 0 in reply to Ryan May
    Prodigy 225 points
    hi, Ryan,
    1. the purpose of R212 is to reduce the noise to GS of the mosfet Q1; as the Q1 current is small, we can try to remove it.
    2. the snubber made up of R206 and C103 is helpful to reduce the spike of Q1.
    3. C70 is 47nF and C71 is unpopulated
    4.C95 is unpopulated
    5. the mosfet Q1 is SPP18P06PG; D1 is SB860; both 60V rating
  • 0 in reply to Vijay86929
    Prodigy 225 points
    hi, Vijay,
    the jitter issue is serious at light load.
    the FET is SPP18P06PG.
    intended frequency is 200kHz.
    I will re-labeled the curves in the report later.