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UCC28251 / Prebiased Start Up

K.Yaita
Guru 29720 points
Other Parts Discussed in Thread: UCC28251, UCC28250

Hi Team,

The datasheet shows the following about "Prebiased Start Up".



Is there timing chart of OUTA/OUTB and SRA/SRB in prebias start up?

Best Regards,
Yaita / Japan disty

  • 0
    TI__Expert 3200 points
    Hi Yaita-san,

    The timing diagram would show an increasing duty cycle for the OUTA/OUTB and SRA/SRB outputs. You can think of the timing diagram similar to Figure 26 where if more then the one switching cycle is drawn, the duty cycle would increase until the design reaches steady-state. The starting duty cycle is described in sections 9.1.2.1 and 9.1.2.2. Does this help you?
  • 0 in reply to Brian Johnson96
    Guru 29720 points

    Hi Brian-san,

    I appreciate for your support.

    Is there waveform of Vout(output voltage of DCDC converter) when VDD happens to drop below UVLO and then VDD recover? 
    The datasheet shows UCC28251 has the function of prebias operation, so my customer wants to see the waveform.

    Best regards,
    Yaita

  • 0 in reply to K.Yaita
    TI__Genius 14580 points

    Hi, Yaita,

    I have looked through D/S and User guide, I didn't find the waveforms of primary side  and secondary side during start up. I will see if I can find any from other group.

    I wonder if the controller is located on the primary side or secondary side?

    Regarding to UVLO. SS is discharged during UVLO. the output  voltage goes to soft start procedure with prebias.

    Thanks,
    Sean

  • 0 in reply to Sean Xu
    Guru 29720 points

    Hi Sean-san,

    Thank you for your support.
    I will wait for your feedback.

    >I wonder if the controller is located on the primary side or secondary side?
    My customer assumes secondary side control.

    Best Regards,
    Yaita

  • 0 in reply to K.Yaita
    TI__Intellectual 1550 points
    Hi Yaita,

    Here is an application note (www.ti.com/.../slaa477.pdf) regarding the pre-biased startup with UCC28250 (predecessor of UCC28251) located on the secondary side. See if it helps.

    Regards,
    Wangxin
  • 0 in reply to K.Yaita
    TI__Intellectual 1550 points
    It seems the link is not accessible. You can find it by searching "Designing UCC28250 as a Secondary Side Control for
    Output Turn-On with a Pre-Bias Condition".