Patent ID: 7391274

Claim:
A method to achieve a constant delay time of ring oscillators requiring a low operating voltage is comprising the steps of: provide a chain ring oscillator comprising a system current source circuit comprising three branches, wherein a first branch comprises a resistance element and a constant voltage circuit comprising an operational amplifier, a bandgap reference circuit, providing input for said operational amplifier, and a transistor and each of the second and third branches comprises a PMOS transistor, an NMOS transistors, a system voltage supply V CC , and n stages, wherein each of the stages of said ring oscillator has a first and a second current source and an inverter circuit and wherein a single-ended output of the inverter circuit of the n-th stage is fed back to the inverter circuit of a first stage; use said constant voltage circuit to generate in said first branch a first constant current, which is defined by a voltage generated by said constant voltage circuit and said resistance element; mirror said first constant current to said PMOS transistor of said second branch of said system current source circuit in order to generate a second constant current; mirror said second constant current from said NMOS transistor of the second branch to said NMOS transistor of said third branch in order to generate a third constant current and to each of said second current sources of each of said stages of said ring oscillator; and mirror said third constant current from the PMOS transistor of the third branch to each of said first current sources of each of said stages of said ring oscillator.