We all want our smart phone (or other portable, battery powered device) to charge as fast as possible.  But, the current path from the USB port or wall adapter through the charger IC to the battery includes various resistive elements that drop voltage and therefore waste power.  For example, the adapter cord, internal PCB traces, switches inside the charger IC and the physical battery connectors all have finite resistances.  Through proper design and component selection, the portable device maker can design to account for these power losses and still extract maximum power from the USB port or adapter.  To provide even more security against adapter collapse and allow the use of third adapters, battery chargers with the input voltage dynamic power management (VIN-DPM) feature, actively monitor their input voltage and reduce the charger's input current limit to prevent adapter collapse. (See Figure 2 below of a charger profile using TI's latest state-of-the-art battery charger ICs with DPM feature).

The article, "Extracting Maximum Power from the Supply When Charging a Battery" (TI Analog Applications Journal) explains how to design the charging system to achieve the maximum power from the adapter despite the undesired resistances between the supply and battery.  

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