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Original question:

TPS650864: Looking for a PMIC solution for the Xilinx Ultrascale+ part XCKU11P-1FFVE1517I

TPS650864: Buck 6 usage

chethan md
Intellectual 860 points
Part Number: TPS650864
Other Parts Discussed in Thread: TPS650861

Hi i will be mostly using TPS650864 in my design:

in the application diagram, Buck6 is typically used to power DDR and VTT LDO for DDR reference.

In my application, sine i use just 1Gb DDR i can as well use any of Buck3, Buck4, OR buck5 which have limited current of 3A--which is enough. I need the Bucks with external FETs for some other High current applications.

So, can i use any of Buck3, Buck4 or Buck5 for DDR and then tie the output of this buck to PVIN_VTT for the VTT_LDO.

Can i do something like this?

  • 0
    TI__Genius 12735 points

    Hi Chethan, 

    VTT LDO is internally referenced based on the ouput voltage of Buck6 so it is not as simple as changing the input source for VTT LDO. You would have two options:

    1. Ensure Buck6 is operating at the same output voltage as whichever buck converter you plan use to power VTT LDO. This should keep the referencing correct and would allow the high current capabilities of Buck6 to be used on something else.

    2. Use a resistor divider circuit on VTT LDO feedback pin (VTTFB) to change the output voltage VTT LDO is supplying. This would only work if Buck6 is supplying a lower voltage than your DDR memory requires. You would connect Buck3/4/5 to the input of VTT LDO and use the resistor divider to decrease the voltage at the feedback pin, which would cause VTT LDO to increase the output voltage. 

    Neither of these use cases have been validated and the device is not guaranteed to function as expected. In either case you may want to consider using TPS650861 instead of the TPS650864x devices. TPS650861 is a user programmable version of the TPS650864 devices and is the same silicon. This would allow you to change voltages on the different bucks as needed. 

    Best regards,

    Layne J

  • 0 in reply to Layne J
    Intellectual 860 points

    okay thanks, i understood the complication now.