Belkin XM Commander Especificações descarregar pdf (Página 48)

REF: BB_SRM_xM BeagleBoard-xM System

Reference Manual

Revision A2

Page 48 of 164

8.3.1 USB DC Source

The USB specification requires that the current consumed prior to enumeration be limited

to 100mA @ 5V (500mW). The 5V DC from the USB is routed through the TPS2141

switch to insure that this requirement is met as uncharged capacitors on the BeagleBoard

can exhibit a large current drain during start up that could exceed this requirement. The

TPS2141 is a USB 2.0 Specification-compatible IC containing a dual-current limiting

power switch and an adjustable low dropout regulator (LDO). Both the switch and LDO

limit inrush current by controlling the turn on slew rate. The dual-current-limiting feature

of the switch allows USB peripherals to utilize high-value capacitance at the output of the

switch, while keeping the inrush current low.

During turn on, the switch limits the current delivered to the capacitive load to less than

100 mA. When the output voltage from the switch reaches about 93% of the input

voltage, the switch current limit increases to 800mA (minimum), at which point higher

current loads can be turned on. The higher current limit provides short circuit protection

while allowing the peripheral to draw maximum current from the USB bus.

When in the USB powered mode and no DC supply is connected, the TPS2141 is

enabled, allowing the power to be supplied to the board from the OTG port through the

integrated switch inside the TPS2141.

8.3.2 Wall Supply Source

A wall supply can be used to provide power to the board. A regulated 5V DC supply of at

least 1A is required and a rating of 3A is preferred, assuming that the USB ports and

expansion headers are likely to be used. It needs to have a 2.1mm plug with a center hot

configuration. If you are using the USB HUB or Ethernet interface, additional current is

required. In the event that a higher DC load is required due to the addition of a

Daughtercard or if all the USB host ports need to supply the full 500mA per port, a

higher current supply can be used. The maximum current should not exceed 3A.

8.3.3 DC Source Control

Unlike when powering from the USB OTG port, in the case of the DC voltage, the

current limiting is not required. As long as the DC supply is not connected, the switch for

the USB is enabled. When the DC supply is plugged in, the switch is disabled because the

ground is removed from pin 5 of the TPS2141. This insures that the 5V from the USB is

not connected by disabling the internal FET. In the case where there is no USB plugged

in, there is no 5V available to be routed so the removal of the pullup in pin 5 has no

affect.

When in the DC mode of operation, the USB OTG can be used in the Host or Client

modes. The TPS65950 will be responsible for handling the supply of the VBUS_5V0 rail

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Belkin XM Commander Especificações Página 48