There is no requirement to use a separate regulator to supply Vref for Micron's DDR SDRAM. However, because Vref is the reference voltage for all single-ended inputs, any noise due to sharing the regulator with other I.C.s on a board or by using a voltage divider from the VDD supply, will directly impact the noise margin on those inputs. Many multi-drop systems already have a designated voltage regulator for DDR memory. Other systems that incorporate point-to-point memory typically use a simple voltage divider resistor network between VDD and VSS.  System designers should evaluate the priorities and trade-offs for each particular system and use the power supply scheme that is optimal for the system.