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Original question:

LM5009: Frequency variation

LM5009: discontinuous operating frequency

K.Hamamoto
Guru 12155 points
Part Number: LM5009

Hi

According to datasheet, the frequency in discontinuous operating mode is described at bellow.

The variation of frequency is similar to the variation of Ton as you mentioned at the previous post.

How about in discontinuous operating ? ( How much is the variation in DCM? Is that same as CCM?)

Best Regards,

Koji Hamamoto

  • 0
    TI__Mastermind 31070 points
    Hello Koji,

    In DCM the frequency will fold-back to much lower frequencies. The TON is relatively fixed. The off time will depend on the load, the HS fet will not turn on until VFB= VREF. This is totally load dependent and effective switch frequency can be very low as a result.

    Hope this helps?
  • 0 in reply to David Baba
    Guru 12155 points

    Hi David,

    Thank you for your reply. Yes,I understand the frequency in DCM is depended on the load as it mentioned in equation 1 at page 8.

    So that, in my understanding the frequency in DCM is not much variation if the load is same at each sample. (It is similar the variation of Ton.)

    Is my understanding correct?

    On our customer board, the frequency of a sample is 22kHz in DCM.

    (In my calculation it is 21kHz. It almost same as the calculation)

    The another sample is 7kHz. I think this sample (freq=7kHz in DCM) is abnormal.(it may a broken sample.)

    Best Regards,

    Koji Hamamoto

  • 0 in reply to K.Hamamoto
    TI__Mastermind 31070 points
    Hello Koji,

    The calculation is an approximation, there is nothing wrong with your customer board have variation of frequency in different samples.

    Hope this helps?
  • 0 in reply to David Baba
    Guru 12155 points
    Hi David,

    Thank you for your prompt reply.

    OK. So the point is, in DCM there are large varition is allowed in different sample even if the load is same. correct?
    (22kHz and 7kHz the both DCM freq are allowed in difference sample.)

    Best Regards,
    Koji Hamamoto
  • 0 in reply to K.Hamamoto
    TI__Mastermind 31070 points
    Hello Koji,

    You are correct.

    Hope this helps?