There is sufficient current capacity available from the 3.3-V LDO to power not only the entire MSP430 but also other components in the system, via the VUSB pin.
If the device is to always be connected to USB, then perhaps no other power system is needed. If it only occasionally connects to USB and is battery-powered otherwise, then sourcing system power via the 3.3-V LDO takes the power burden away from the battery. Alternatively, if the battery is rechargeable, the recharging can be driven from VUSB.
Self-Powered Devices
Some applications may be self-powered, in that the VUSB power is supplied externally. In these cases, the 3.3-V LDO would be disabled (VUSBEN = 0). For proper USB operation, the voltage on VBUS can still be detected, even while the 3.3-V LDO disabled, by setting USBDETEN = 1. When VBUS rises above the USB power brownout level, low voltage detection becomes enabled. When VBUS rises further above the launch voltage VLAUNCH, the voltage on VBUS is detected.
Current Limitation and Overload Protection
The 3.3-V LDO features a current limitation to protect the transceiver during shorted-cable conditions. A short or overload condition – that is, when the output of the LDO becomes current-limited to IDET. This is reported to software via the VUOVLIFG flag. See device ce-specific data sheet for value e of IDET.
If this event occurs, it means USB operation may become unreliable, due to insufficient power supply. As a result, soft the ware may wish to cease USB operation. If the OVLAOFF bit is set, to USB the operation is automatically terminated by clearing VUSBEN.
During overload conditions, VUSB and V18 drop below their nominal output voltage. In power scenarios where DVCC is exclusively supplied from VUSB, repetitive system restarts may be triggered as long as a short or overload condition exists. For this reason, firmware should avoid re-enabling USB after detection of an overload on the previous power session, until the cause of failure can be identified. Ultimately, it is the user's responsibility to ensure that the current drawn from VBUS does not exceed IDET.
The VUOVLIFG flag can be used to indicate an overcurrent condition on the 3.3-V LDO.
When an overcurrent condition is detected, VUOVLIFG = 1. In addition to the VUOVLIFG being set, an interrupt is also generated when VUOVLIE = 1. The USB power system brownout circuit is supplied from VBUS or DVCC, whichever carries the higher voltage.