VL812 View Datasheet(PDF) - Unspecified
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VL812 Datasheet PDF : 18 Pages
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Rapid Charging over USB in Detail
VL812 USB 3.0 Hub Controller (B1)
3 Concepts of Rapid Charging over USB:
Rapid Charging over USB enables charging of devices at rates in excess of baseline USB
standards. The current limit of USB 2.0 is 500mA for configured devices, and the current limit of
USB 3.0 is 900mA for configured devices. Depending on the device, Rapid Charging
implementations typically feature current limits between 1000mA to 2000mA.
It is the Host/Hub’s responsibility to advertise Rapid Charging capabilities, and it is the
Device’s responsibility to recognize and determine the Rapid Charging capabilities of the
Host/Hub port it is connected to. Since Rapid Charging over USB allows charging at rates in
excess of USB spec, this detection mechanism is necessary to ensure safe and reliable operation,
and can prevent situations where a device requests more current than what a host/hub port can
supply. Also, this implies that Rapid Charging will only occur when both Host/Hub and Device
supports it.
The rate at which a device charges is dependant upon the device. This means that the
device must determine the host/hub port’s capabilities to determine which charging mode to use.
Also, the rate at which a device charges can vary depending on the status of the device. For
example, some devices only charge at their maximum rate when the battery is nearly depleted.
When the battery is nearly full, they may switch to a trickle-charge mode. The Host/Hub
rapid-charging port has no control over this behavior.
Supported USB Charging Modes
SDP – Standard Downstream Port
This is a typical USB 2.0 or USB 3.0 port and does not explicitly support Rapid USB Charging. SDP is
constrained to the current limits as defined in the USB 2.0 or USB 3.0 spec which are 500mA and 900mA
respectively. While the actual current limit is enforced by the polyfuse or power-switch providing
current-limiting functionality for the downstream port, most USB devices will not draw more than 500mA
or 900mA under USB 2.0 or USB 3.0 modes.
CDP – Charging Downstream Port
CDP is defined in the USB Battery Charging Specification 1.2 and enables devices that are able to
correctly recognize CDP to simultaneously function as a USB device while drawing up to 1.5A for Rapid
Charging when connected to the downstream port of a USB Host or Hub that advertises CDP capability.
DCP – Dedicated Charging Port
DCP is defined in the USB Battery Charging Specification 1.2 and has been in use on an unofficial basis
prior to the official USB Battery Charging Specification. DCP is a dedicated charging mode, so when a
device is charging under DCP, regular USB operations such as data transfer to the device are not
supported.
Special Modes
Various vendors such as Apple, RIM, Motorola, etc may employ different detection mechanisms
compared to other USB devices and thus, may enter Rapid Charging under the previously mentioned
charging modes. VL811+ supports an auto detection mechanism that provides charging for the majority
of devices.
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