The smart start-up circuits basically include a sensor, two stacked PMOS transistors, two stacked NMOS transistors, and a feedback line. If the sensing voltage does not reach the expected voltage compared to the midpoint voltage of the sensor, the output voltage of the sensor turns on the corresponding transistor, which provides a current to its output until the voltage at feedback reaches the midpoint voltage. The time to reach the midpoint voltage at a load is simply equal to the charge stored at the load divided by the current, which can be scaled by a device aspect ratio of the transistor. Consequently, all smart start-up circuits provide an initial output voltage level closer to the output voltage level that reaches the equilibrium according to schedule.