BF-548 Power Tracking IC ADP1823

Hi,

In BF-548 EZ-Kit Lite, Pg-17  2.5V for DDR and 5V for USB are generated from ADP1823,  2.5V is tracking VREG5, whereas 5V USB is not tracking anything.

Why is it done so as blackfin does not have any power sequencing requirement, can i use separate switcher to generate 2.5V for DDR and DDR Controller?

Thanks