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UCC28060 and SLUA625

Persa
Prodigy 200 points
Other Parts Discussed in Thread: UCC28060

Hi,

on document SLUA625, page 17,  maximum on time is calculated to be 7.12uSec. it demands that KTL be 1.5x10^-6 as Tonmax=KTx4.825 from UCC28060 datasheet page 17 eq5. based on KTL=(RTSET/133k)*4 (UCC28060 datasheet  page 18) then RTSET must be around 50K. it is 160K on schematic Fig 15 on SLUA625. What is the source of this discrepancy?

Also on page 17 above figure 11, how has the on time of 1.46uS been driven?

Thanks

  • 0
    TI__Mastermind 31580 points
    Hi Persa,

    I will ask one of the applications engineers to take a look at this.

    Regards

    Peter
  • 0
    TI__Mastermind 34465 points
    The on time is set by the resistor value on TSET (100k in schematic) and the voltage on error amp output.
    The design is capable of having a max on time of about 14uS but it wont reach this level
    because the error amp will not reach its max output.

    The equation in the reference design are saying that the on time can reach 7.12uS.
    I believe these equation are based on a 60W load required on the output.
    SLUA625 seems to make an assumption for a min switching frequency and I don't see where this number comes from except by a desire to have the switching frequency at 65kHz or above. This helps with the transformer design but I think it is sort of misleading, as you have discovered.

    Hopes this helps.

    John Griffin
  • 0 in reply to John Griffin1
    Prodigy 200 points
    I have no problem with the selection of 65kHz. the on time is determined by equation 11 on page 13 of app note. as per last paragraph of page 14, k varies from 1.1 to 3.4, so Ton must vary between about 7uSec to 3.5uSec. I wonder where the 1.46uSec mentioned above fig 11 is coming from.
    The other problem I have is mentioned on my post. the value of RTSET that must be around 50kHz, not 160k. This is a key parameter.
    Thanks
  • 0 in reply to Persa
    Prodigy 200 points

    I am looking forwad to receiving a response to my May 5 post.

    Thanks