The USB power system incorporates dual LDO regulators (3.3 V and 1.8 V) that allow the entire MSP430 device to be powered from 5-V VBUS when it is made available from the USB host. Alternatively, the power system can supply power only to the USB module, or it can be unused altogether, as in a fully self-powered device. The block diagram is shown in Figure 42- 2
The 3.3-V LDO receives 5 V from VBUS and provides power to the transceiver, as well as the VUSB pin. Using this setup prevents the relatively high load of the transceiver and PLL from loading a local system power supply if used. Thus it is very useful in battery-powered devices.
The 1.8-V LDO receives power from the VUSB pin – which is to be sourced either from the internal 3.3-V LDO or externally – and provides power to the USB PLL and transceiver. The 1.8-V LDO in the USB module is not related to the LDO that resides in the MSP430 Power Management Module (PMM). The inputs and outputs of the LDOs are shown in Figures 42-2. VBUS, VUSB, and V18 need to be connected to external capacitors. The V18 pin is not intended to source other components in the system, rather it exists solely for the attachment of a load capacitor.