BeagleBoard XM C2, Reference Manual

Page: 51 / 163

REF: BB_SRM_xM
BeagleBoard-xM System
Reference Manual
Revision C2
Page 51 of 163
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 .
7.3.2 Wall Supply Source
A wall supply can be used to provide power to the board. A regulated 5V DC supply of at
least 2A 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.
7.3.3 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
in the OTG or Host modes. As this is limited to 100mA, a powered hub must be used to
support peripherals on the OTG port.
It is possible to provide 5V via the expansion connectors as would be the case from a
daughter card to prevent you from having to have two DC supplies. You should be
careful in doing this. If you plan to use the USB OTG port, you will need to place an
unconnected connector into the DC power jack to insure that the DC from the OTG port
is not shorted to the 5V supplied via the expansion connector. There is a signal called
nUSB_PWR which if a logic level “1” (5V) indicates that there is 5V supplied by the
USB OTG port, it is plugged in, and the DC dummy jack is installed. This condition
could be used on the daughtercard to know that it is OK to supply power onto the
expansion bus to power the board. If this signal is low, then that indicates that there is no