The smartphone single-cell design has been through a number of revisions. It has been thoroughly tested and is in mass-market production. Preserving this known-good design reduces risk involved in redesign that normally would delay time-to-market. In a two or three cell Li-Ion stacked system, the voltage is nominally 7.2 V or 10.8 V. The higher voltage means a smaller difference between the battery stack output voltage and the voltage required to drive the forward voltage of the WLEDs, thus reducing losses in the boost converter.
This multi-cell battery stack configuration has trade-offs. Analogous to the way boosts are less efficient at high output-to-input voltage ratios, buck converters are less efficient at higher input-to-output voltage ratios and efficiency of converters connected to the processor, memory and IO may all suffer. Additionally, this higher voltage cannot be connected directly to most of the other circuitry in the system.
For example, the inter-integrated circuit (I 2C) bus used to communicate from integrated circuit (IC) to IC inside the cell phone typically operates at a maximum of 5 V. The single-cell power management unit likely has a 6 V input voltage limit The low