Variable delay circuit, and differential voltage-controlled ring oscillator using the same, and pll using the oscillator

Abstract

An inventive voltage-controlled ring oscillator provides a relatively stable oscillation frequency even with a lower power supply voltage. The oscillator includes an odd number of inverters and a variable delay circuit connected in a ring. The variable delay circuit comprises first and second control terminals to which first and second control signals that determine the amount of delay are applied; and a switching circuit including a first and second switching elements each comprising a MOS transistor. The variable delay circuit further includes an off control circuit, responsive to an input signal from the inverter portion, for turning off one of the first and second switching elements by shorting a gate-source path of the one switching element; and a current control circuit, responsive to an off state of the second switching element, for causing the first control signal to control a first current flowing through the first switching element by providing a conductive path between the first control terminal and a gate of the first switching element, and responsive to an off state of the first switching element, for causing the second control signal to control a second current flowing through the second switching element by providing a conductive path between the second control terminal and a gate of the second switching element.