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 variable start - up circuits provide an initial output voltage level closer to the output voltage level that reaches the equilibrium according to schedule .