7.2.1.1.1 Power the MSP430 Microcontroller

Several versions of the TPS715A-NM are ideal for powering the MSP430 microcontroller. Table 2 shows potential applications of some voltage versions.

The TPS715A-NM family of devices offers many output voltage versions to allow the supply voltage to be optimized for the MSP430, thereby minimizing the supply current consumed by the MSP430.

7.2.1.2.1 External Capacitor Requirements

Although not required, a 0.047-μF or larger input bypass capacitor, connected between IN and GND and located close to the device, is recommended to improve transient response and noise rejection of the power supply as a whole. A higher-value input capacitor may be necessary if large, fast-rise-time load transients are anticipated and the device is located several inches from the power source.

The TPS715A-NM device requires an output capacitor connected between OUT and GND to stabilize the internal control loop. Any capacitor (including ceramic and tantalum) greater than or equal to 0.47 μF properly stabilizes this loop. The X7R- or X5R-type capacitors are recommended because these capacitors have a wider temperature specification and lower temperature coefficient, but other types of capacitors can be used.

7.2.1.2.2 Dropout Voltage (V_DO)

Generally speaking, the dropout voltage often refers to the voltage difference between the input and output voltage (V_DO = V_IN – V_OUT). However, in the Electrical Characteristics table, V_DO is defined as the V_IN – V_OUT voltage at the rated current, where the pass-FET is fully enhanced in the ohmic region of operation and is characterized by the classic R_DS(on) of the FET. V_DO indirectly specifies a minimum input voltage above the nominal programmed output voltage at which the output voltage is expected to remain within its accuracy boundary. If the input falls below this V_DO limit (V_IN < V_OUT + V_DO), then the output voltage decreases to follow the input voltage.

Dropout voltage is always determined by the R_DS(on) of the main pass-FET. Therefore, if the LDO operates below the rated current, then the V_DO for that current scales accordingly. R_DS(on) can be calculated using Equation 1.

Equation 1.

7.2.2.1.1 Setting V_OUT for the TPS715A01 Adjustable LDO

The TPS715A-NM family of devices contains an adjustable-version, the TPS715A01 device, that sets the output voltage using an external resistor divider as shown in Figure 20. The output voltage operating range is 1.2 V to 15 V, and is calculated using Equation 2.

Equation 2.

where

• V_REF = 1.205 V (typical)

Choose resistors R1 and R2 to allow approximately 1.5-μA of current through the resistor divider. Lower value resistors can be used for improved noise performance, but consume more power. Avoid higher resistor values because leakage current into or out of FB across R1, R2 creates an offset voltage that is proportional to V_OUT divided by V_REF. The recommended design procedure is to choose R2 = 1 MΩ to set the divider current at 1.5 μA, and then calculate R1 using Equation 3.

Equation 3.

Figure 20 shows this configuration.