A 3.3 V output LDO has a dropout voltage specified as max 300 mV (at 500 mA output current). What is the minimum input voltage needed to maintain the output voltage while supplying 500 mA?

Answer: A. 3.6 V

Dropout voltage is the minimum voltage difference required between input and output to maintain the output voltage. Adding the max of 300 mV to the 3.3 V output, the input must be 3.6 V or higher. Dropout voltage increases with output current and, in many cases, at higher temperature, so use the value specified under the current and temperature conditions of your application. For example, if a lithium-ion battery's voltage falls to 3.5 V, the output can no longer hold 3.3 V and begins to drop. 3.0 V is the mistake of subtracting the dropout voltage instead of adding it.