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LM3481: LM3481QMM/NOPB: Frequency Issue

DEVARAJAN RAJENDRAN
Prodigy 10 points
Part Number: LM3481

Hello Team, 

I am using the LM3481QMM/NOPB in a Flyback DCM topology. In Frequency adjust pin I have connected two resistors of values 100K & 51K in series ; Both resistors have a tolerance of 1%.  So my controller should operate in a frequency range of 139KHz to 142KHz. Additionally a +/-15% Tolerance is considered as controller internal tolerance.  So frequency range will become 118KHz to 163KHz.

At ambient and cold (-40°C) temperatures, the operating frequency is within 118KHz and 163KHz. But at High Temperature (130°C), the frequency goes up to 180KHz. i.e It exceeds the estimated maximum limit of 163KHz.

To ensure this is not because of the degrading of FA resistor, test is done in three different boards. But in all boards, the same issue is observed at high temperature. So it is understood that the issue is with controller.

Could you please clarify whether the Controller Tolerance of +/- 15% is accurate or is it higher than 15% ?

Also please let me know other possibilities which could be a potential reason for this issue.

Thanks in advance  

  • 0
    TI__Mastermind 47921 points

    Hi Devarajan,

    thank you for using the E2E forum.

    The datasheet gives the switching frequency variation for the the range from -40 to 125 Degree.
    If you extrapolate this for 130 Degree, you will get a +16.6 %

    Additionally have you also considered the resistance variation due to temperature?

    Best regards,

     Stefan

  • 0 in reply to Stefan Schauer
    Prodigy 10 points

    Hello Stefan,

    Thanks for your prompt response.

    Yes I have considered the resistance variation due to temperature and considered the overall tolerance of resistor as 2.6%. So Frequency range shall be 136.93KHz to 143.96KHz.

    Adding 16.6% ICs tolerance shall widen this range to 109.54KHz to 172.76KHz.

    My doubt is the Temperature (-40° to 125°) you specified is junction temperature. But 130°C I mentioned is ambient temp; So obviously my Junction Temperature will be still higher. So even if I assume worst case, Junction temperature can raise up to 164°C (because IC undergo Thermal shutdown at 165°C) , the extrapolated tolerance value will 20%. So frequency range for 20% tolerance will be 109KHz to 173KHz.

    But in my measurements, I see the frequency goes up to 180KHz (~7KHz higher). So I doubt the tolerance at 125°C might be more than 15%.

    Could you please clarify whether this understanding is correct?

    Also could you please share me the snap from datasheet / other technical notes mentioning the IC's internal tolerance as +/- 15%. Because I don't find this in the datasheet from google; I just found this 15% tolerance from our old design document.

  • 0 in reply to DEVARAJAN RAJENDRAN
    TI__Mastermind 47921 points

    Hi Devarajan,

    I will try to get the characterization data of the LM3481 for a deeper analysis but that can take some days.

    Best regards,

     Stefan

  • 0 in reply to Stefan Schauer
    Prodigy 10 points

    Hello Stefan,

    Yeah please do the needful.

    One more query is, this issue is seen in one configuration board but in other board (with same circuit without any changes) this issue is not seen. The only difference between both boards is, in first board the IC is assembled in top side and in other board it is mounted in bottom side; Due to this the Heat dissipation seen by the IC will differ in both boards and so issue is seen in one but not in other. Else second possible reason I suspect is, both ICs are received in different batches; It is not necessary that all ICs exhibit same tolerance, it differs batch to batch and the value mentioned in datasheet is maximum. 

    So I am trying to find the root cause with the above understanding. Please let me know if there are some other possible reasons.

  • 0 in reply to DEVARAJAN RAJENDRAN
    TI__Mastermind 47921 points

    Hi Devarajan,

    i also just measured this on an EVM without seeing any issues.

    Can you swap the LM3481 between the two boards and check again:

    Does the issue stay with the board or does it go with the IC?

    Best regards,

     Stefan

  • 0 in reply to Stefan Schauer
    Prodigy 10 points

    Hello Stefan,

    Yeah thanks. I have passed your suggestion to our team.

    Will try to check asap and come back. 

  • 0 in reply to DEVARAJAN RAJENDRAN
    TI__Mastermind 47921 points

    Hi Devarajan,

    I have not seen an update for the last 14 days, so I assume the questions are/or answered and the issue is solved.
    I close this thread now. If there is still something open reply and the thread will get open again.
    If you have any other question or of the thread gets locked, please open a new one.

    Clicking the Resolved Button also helps us to maintain this forum.

    Best regards,
    Stefan

  • +1 in reply to Stefan Schauer
    Prodigy 10 points

    Actually Sorry we are little busy in other activity and not yet verified. As of now May be I will mark it as RESOLVED. If I see some issues after performing the test as per Stefan's suggestion, I will come back in new thread. Thanks