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Generate AM335x DDR3 VREF

g.f.
Guru 15520 points
Other Parts Discussed in Thread: TPS51200

Hi,

I have a question about AM335x DDR3 VREF.

While using termination regulator(like TPS51200) for VTT,
is it okay to generate VREF from 1KΩ1% resistor divider other than regulator?
Or must generate VREF from regulator?

best regards,
g.f.

  • 0
    TI__Guru 62855 points

    You should use the TPS51200 REFOUT to source the AM335x DDR_VREF and DDR3 VREF inputs.  This sets the AM335x and DDR3 input thresholds to the same volate as VTT since the TPS51200 sources the same voltage to REFOUT and VTT.  The input thresholds may have a small voltage offset from the VTT voltage if you use a separate voltage divider to source the AM335x DDR_VREF and DDR3 VREF inputs.

    This may not cause a problem if the voltage offset is small enough, but is not the preferred way to source the AM335x DDR_VREF and DDR3 VREF inputs.

    Why do you want to do this since it requires another voltage divider?

    Regards,
    Paul

  • 0 in reply to peaves
    Guru 15520 points

    Hi, Paul

    Thank you for the answer.

    Sorry, I'm not requiring another voltage divider.
    I just wanted to know because of the following:

    With C6678EVM, DDR3 VTT and VREF was source by different voltage divider.
    VTT was source from regulator and VREF was source by resistor divider.

    Please let me check that my understanding is correct.

    If using VTT, termination regulator is recommended
    and need to consider the voltage offset.
    Therefore, VTT and VREF should source from the regulator.

    If not using VTT, no termination regulator is recommended
    and no need to consider the voltage offset.
    Therefore, VREF should source from resistor divider.

    best regards,
    g.f.

  • 0 in reply to g.f.
    TI__Guru 62855 points

    Yes, that is correct.

    Regards,
    Paul

  • 0 in reply to peaves
    Guru 15520 points

    Thank you so much.

    best regards,
    g.f.