A Package-on-Package (PoP) configuration is common in phones to reduce the foot print area of the memory and processor packages. In feature phones, a flash memory package is stacked on top of a baseband processor as shown in Smartphone's, a DRAM-Flash memory package is on top of an application processor. Some Smartphone's have both kinds of PoPs. Some constraints imposed on the design of components for mobile devices are the size of the handheld device, safety for handling, heat dissipation, and in-system electromagnetic interference. This paper discusses challenges in designing the next generation low power DRAM subsystem operating at multi-gigabits per second. A new mobile DRAM interface that can meet the challenges and some test data are presented.