This design example is powered by a rechargeable battery that can be a building block in many portable applications. Noise-sensitive portable electronics require an efficient, small-size solution for the power supply. Traditional LDOs are known for low efficiency in contrast to low-input, low-output voltage (LILO) LDOs, such as the TPS7A15. Using a bias rail in the TPS7A15 allows the device to operate at a lower input voltage, thus reducing the voltage drop across the pass transistor and maximizing device efficiency. The low voltage drop allows the efficiency of the LDO to approximate that of a DC/DC converter. Equation 8 calculates the efficiency for this design.

Equation 8 reduces to Equation 9 because the design example load current is much greater than the quiescent current of the bias rail.

For this design example, the 0.9-V output version (TPS7A1509) is selected. A nominal 1.05-V input supply comes from a DC/DC converter connected to the battery. Use a minimum 1.0-μF input capacitor to minimize the effect of resistance and inductance between the 1.05-V source and the LDO input. Use a minimum 2.2-μF output capacitor for stability and good load transient response.

The dropout voltage (VDO) is less than 80 mV maximum at a 0.9-V output voltage and 400-mA output current, so there are no dropout issues with a minimum input voltage of 1.0 V and a maximum output current of 200 mA. In addition, the TPS7A15 is designed to meet key specifications so long as the input voltage is at least 100 mV greater than the output voltage.