An object is to reduce degradation of circuit operation and to reduce the area of the entire circuit. A power source circuit is provided with a first terminal to which first voltage is input; a second terminal to which second voltage is input; a comparator being connected to the first terminal and the second terminal and comparing the first voltage and the second voltage; a digital circuit averaging, integrating, and digital pulse width modulating a first digital signal output from the comparator; a PWM output driver amplifying a second digital signal output from the digital circuit; and a smoothing circuit smoothing the amplified second digital signal.