Often you would like to use a micro controller to control a load that draws a much higher current than the micro controller can source or sink through its digital I/O pins. An example of this would be driving a motor, a high current LED. Or you need to switch a much higher voltage for example a 10W resistive heater might require a 10V source and a current of 1A.
A quick look at the Data Sheet for the MSP430G2553, pg. 24 reveals that you should not try to source or sink a combined current more than 48mA across all the I/O pins.
If you wish to drive a high current or voltage output then you are going to need to use an external power supply and an external MOSFET to switch the current to the load. The figure below shows the typical circuit that is employed for this task. The high side of the load is connected to a power supply that is able to supply the voltage and current that the load requires. An n-type MOSFET is then used as a switch that can be controlled by the micro controller. When choosing the MOSFET you need to ensure that it can handle the Voltage and current that the load requires. For example a FDP7030BL n-type Logic MOSFET will allow for a maximum drain source voltage (Vds) of 30V and a drain current of 60A. For smaller drain currents you could use a BS170, Vds = 60V , Id = 500mA. These devices are known as a logic MOSFET as their gate input is suitable for driving directly from a 5 or 3 Volt logic device such as a micro controller with Vgs for switching around 2V. MOSFETs are particularly suitable for this application because the high impedance of the gate input means that virtually no current is drawn from the micro controller. When using the circuit below it is important that the maximum drain source voltage exceeds that of the power supply that is attached to the high side of the load as when the MOSFET is switched off by applying a logic low to the gate input the voltage at the drain input and thus Vds will equal the power supply voltage. When choosing the power supply voltage it is also important to consider the voltage drop (Vds) that occurs when the MOSFET is switched on by applying a logic high to the gate input. For the FDP7030BL the on state resistance between the drain and the source (Rds) is of the order of 10mΩ.
When driving inductive or capacitive loads, such as the motor, it is important to include a diode in parallel with the load to provide a path for the reverse current to flow to protect the MOSFET. Note the ground of this circuit and the micro controller most be common, i.e. connected.