Delay locked-loop circuit for reducing load of variable delay unit at high-frequency operation and locking external clock signal stably

Abstract

Disclosed is a delay locked-loop circuit comprising a phase detector for detecting a phase difference between an external clock signal and an internal clock signal, a delay unit controller for generating a control signal in response to output of the phase detector, and a variable delay unit for delaying the external clock in response to the control signal to synchronize the internal clock with the external clock, the variable delay unit comprising a first group of delay cells used at or above a predetermined frequency, a second group of delay cells used with the first group of delay cells at or below a predetermined frequency, switch transistors for connecting/disconnecting the first group of delay cells and the second group of delay cells to/from a first output line and a second output line of the variable delay unit, respectively, in response to the control signal, and a switch for connecting/disconnecting the first output line to/from the second output line in response to a delay use signal representing the use of one of the delay cells in the first group.