OVERVIEW
The following overview references various industry standards. It is always recommended to consult the latest standard
to ensure the most recent and accurate information.
Rechargeable portable equipment requires an external power source to charge its batteries. USB ports are convenient
locations for charging because of an available 5-V power source. Universally accepted standards are required to
ensure host and client-side devices meet the power management requirements. Traditionally, USB host ports following
the USB 2.0 Specification must provide at least 500 mA to downstream client-side devices. Because multiple USB
devices can be attached to a single USB port through a bus-powered hub, it is the responsibility of the client-side
device to negotiate the power allotment from the host to guarantee the total current draw does not exceed 500 mA. In
general, each USB device can subsequently request more current, which is granted in steps of 100 mA up 500 mA total.
The host may grant or deny the request based on the available current.
Additionally, the success of the USB technology makes the micro-USB connector a popular choice for wall adapter
cables. This allows a portable device to charge from both a wall adapter and USB port with only one connector.
One common difficulty has resulted from this. As USB charging has gained popularity, the 500-mA minimum defined by
the USB 2.0 Specification or 900 mA defined in the USB 3.0 Specification, has become insufficient for many handsets,
tablets and personal media players (PMP) which have a higher rated charging current. Wall adapters and car chargers
can provide much more current than 500 mA or 900 mA to fast charge portable devices. Several new standards have
been introduced defining protocol handshaking methods that allow host and client devices to acknowledge and draw
additional current beyond the 500 mA (defined in the USB 2.0 Specification) or 900 mA (defined in the USB 3.0
Specification) minimum while using a single micro-USB input connector.
The devices support four of the most common protocols:
USB Battery Charging Specification, Revision 1.2 (BC1.2)
lChinese Telecommunications Industry Standard YD/T 1591-2009
 Divider mode
1.2 V on both D+ and D– lines