Constant current circuit used for ring oscillator and charge pump circuit

A ring oscillator having a plurality of inverters connected in a shape of a ring and generating a clock signal comprising

• a plurality of separate constant current sources each provided to one of the inverters to supply a constant drive current to the corresponding one of the inverters, and
• a reference constant current source for feeding a reference constant current which determines current values of the plurality of separate constant current sources, wherein
• the reference constant current source has a characteristic that a steep slope of change with respect to a source voltage on a lower voltage side becomes a small slope on a higher voltage side, and
• wherein the reference constant current source comprises:
• a first current path configured by a series connection between a resistance and a diode-connected transistor, and placed between a power source and ground, and
• a second current path configured by a series connection between at least two diode-connected transistors connected in series and a resistance having a resistance value smaller than that of the resistance in the first current path, and placed between a power source and ground, wherein, in the reference constant current source
• a current of the second current path is drawn from a junction point between the resistance and the transistor in the first current path, and
• when the source voltage is low and a current flows only in the first current path, the current running through the first current path is fed as the reference constant current, whereas when the source voltage is high and the current flows also in the second current path in addition to the first current path, the current obtained by subtracting the current of the second current path from the current of the first current path is fed as the reference constant current.