hereinafter , a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings . fig2 is an embodiment of a dll circuit according to the present invention . the clock buffers 211 and 212 are internal buffers that receive external clocks / clk and clk . the clock signal / clk ( pronounced as “ clock bar ” or as “ clock complement ”) is an inverted form of the clock signal , clk . the clock signals / clk and clk , which have passed through the clock buffers 211 and 212 respectively , are indicated by internal clock signals fclkt 2 and rclkt 2 . unlike fig1 , it should be noted that the clock buffers 211 and 212 in fig2 are controlled by an inverted signal ckeb of a clock enable signal cke . the signal ckeb is a low level or logic zero when a memory device is in normal operation mode . the signal ckeb is a high level or logic 1 , when the memory device is in a power - down mode . for reference , the clock buffers 211 and 212 are disabled when the device enters into power - down mode . a delay circuit or delay line 213 receives the internal clock signal fclkt 2 at an input end of the delay line 213 in order to delay the internal clock signal fclkt 2 for a predetermined period of time . the delay circuit / delay line 214 receives the internal clock signal rclkt 2 at an input end of the delay line 214 and delays the internal clock signal rclkt 2 for a predetermined period of time . for reference , the delay times in the delay lines 113 and 114 are varied by a delay line controller 217 as will be described later . a replica delay unit 215 receives an output signal from the delay line 214 and provides a fixed delay time that nearly coincides with the sum of a delay time t 1 of the clock buffer 211 and a delay time t 2 of a dll driver 218 . a phase comparator 216 compares the phase of the internal clock signal rclkt 2 , which is an output signal of the buffer 212 , with a phase of an output signal fb_clk of the replica delay unit 215 . the delay line controller 217 controls the delay time of the delay lines 213 and 214 in response to the output signal of the phase comparator 216 . dll drivers 218 and 219 receive the output signals of the delay lines 213 and 214 . the dll drivers also output the internal dll signals fclk_dll and rclk_dll . the controller 220 receives a signal ckeb and a signal fb_clk and outputs a signal cke_dll to the phase comparator 216 to control operation of the phase comparator 216 . when a signal ckeb for entering the power - down mode is applied at a high level , the controller 220 outputs the signal cke_dll to the phase comparator at a low level to block operation of the phase comparator 216 ( refer to fig3 ). that is , unlike conventional cases , the phase comparator 216 according to the present invention is disabled by the controller 220 upon entering into the power - down mode . an example of the controller 220 depicted in fig2 is illustrated in fig3 . fig3 is an example of a controller suggested in this invention . as illustrated in the drawing , a controller is comprised of a d flip - flop 31 and a nor gate 32 . the input terminal ( in ) of d flip - flop 31 receives the signal ckeb , and the clock terminal ( clk ) receives the signal fb_clk . the nor gate 32 receives the signal ckeb and an output signal cked_d of the d flip - flop 31 , and an output signal cke_dll of the nor gate 32 controls operation of the phase comparator 216 shown in fig2 . for reference , the phase comparator 216 is disabled when the output signal cke_dll of the nor gate 32 is a low level . hereinafter , an operation of the embodiment suggested in fig2 will be divided for explanation into normal operation mode and power - down mode . first , the operation of a dll circuit in normal operation mode will be described . in normal operation mode , the signal ckeb is low level and therefore the clock buffers 211 and 212 are in enable state and the controller 220 is in disable state . since the controller 220 is in disable state , the operation of a circuit of fig2 is same as the operation of a typical dll circuit . in other words , internal clock signals fclkt 2 and rclkt 2 output from the clock buffers 211 and 212 pass through the delay lines 213 and 214 respectively to be applied to dll drivers 218 and 219 . an output signal of the delay line 214 is also applied to the replica delay unit 215 . the phase comparator 216 compares a phase difference between an output signal fb_clk of the replica delay unit 215 and an output signal rclkt 2 of the clock buffer 212 . the delay line controller 217 controls the delay times of the delay lines 213 and 214 in response to an output signal of the phase comparator 216 . the above - mentioned operation will be repeated until the phases of the signals rclkt 2 and fb_clk applied to the phase comparator 216 coincide with each other within error range . next , the operation of a dll circuit in power - down mode will be described . upon entering the power - down mode , an inverted signal ckeb of the clock enable signal is changed to a high level . in this case , the clock buffers 211 and 212 are disabled by the signal ckeb . when both clock buffers 211 and 212 are in a disable state , the power consumed in a dll circuit of fig2 can be reduced . upon entering into the power - down mode , since the signal ckeb is at a high level , an output signal cke_dll of the controller 220 is at a low level ( refer to fig3 .) when the signal cke_dll is low level , the phase comparator 216 will be in the disabled state . unlike conventional cases , therefore , the power consumption of the phase comparator 216 can be also reduced . next , upon exiting from power - down mode , an inverted signal ckeb of the clock enable signal is changed to a low level . therefore , the clock buffers 211 and 212 will be changed from disable state to enable state . concerning this , the phase comparator 216 of this invention will operate after a predetermined period of time passes since the signal ckeb of low level is applied ( in this regard , the operation is greatly different from the prior art ). concerning this it will be more specifically described in detail . in conventional cases , upon exiting from power - down mode , the phase comparator operates immediately , thereby causing a malfunction . this malfunction is generated because an abnormal signal fb_clk is applied . in the invention disclosed and claimed herein , however , phase comparator operation 216 is controlled by the controller 220 . concerning this , as illustrated in fig3 , an embodiment of the controller will be described . as described above , since an output signal cke_dll of the controller 32 maintains low level just prior to exiting from power - down mode , the phase comparator 216 is in disable state . upon exiting from the power - down mode , the signal ckeb is changed to a low level . an output signal ckeb_d of the d flip - flop 31 will be changed to low level after a signal fb_clk is applied to a clock terminal . therefore , after an output signal rclkt 2 of the clock buffer 212 , which is enabled upon exiting from power - down mode , passes through the delay line 214 and the replica delay unit 215 to be applied to a clock terminal of the d flip - flop 31 , the output signal ckeb_d of d flip - flop 31 becomes low level . as a result , an output signal cke_dll of the controller becomes high level after the signal rclkt 2 passes through the delay line 214 and the replica delay unit 215 to be applied to a clock terminal of d flip - flop 31 . as described above , when the output signal cke_dll of the controller becomes high level , the phase comparator is enabled to operate . therefore , a malfunction caused by operating the phase comparator upon exiting from power - down mode can be prevented . fig4 is another embodiment of a dll circuit according to the present invention . as can be seen in fig4 , there is provided a controller 420 for controlling a delay line controller 417 . clock buffers 411 and 412 are internal buffers for receiving external clocks / clk and clk . the clock buffers 411 and 412 also receive the signals ckeb . the clock signal / clk is an inverted signal of the clock signal clk . the clock signals / clk and clk which have passed through the clock buffers 411 and 412 respectively are indicated by internal clock signals fclkt 2 and rclkt 2 . as illustrated in fig2 , it should be noted that the clock buffers 411 and 412 in fig4 are controlled by an inverted signal ckeb of a clock enable signal cke . the signal ckeb is low level when a memory device is in normal operation mode , and the signal ckeb is high level when it is in power - down mode . for reference , the clock buffers 411 and 412 become disable state upon entering into power - down mode . a first delay line 413 receives the internal clock signal fclkt 2 at the input end of the delay line 413 in order to delay the signal fclkt 2 for a predetermined period of time . a second delay line 414 receives the internal clock signal rclkt 2 at the input end of the delay line 414 to delay the rclkt 2 signal for a predetermined period of time . for reference , the delay times in the delay lines 413 and 414 are varied by a delay line controller 417 as will be described later . a replica delay unit 415 receives an output signal of the delay line 414 . the replica delay unit 415 is a delay unit having a fixed delay time , which nearly coincides with the sum of a delay time t 1 of the clock buffer 411 and a delay time t 2 of a dll driver 418 . a phase comparator 416 compares a phase of the internal clock signal rclkt 2 , which is an output signal of the buffer 412 , with a phase of an output signal fb_clk of the replica delay unit 415 . the delay line controller 417 controls the delay time of the delay lines 413 and 414 in response to the output signal of the phase comparator 416 . dll drivers 418 and 419 receive the output signals of the delay lines 413 and 414 to output internal dll signals fclk_dll and rclk_dll . the controller 420 receives a signal ckeb and a signal fb_clk and outputs a signal cke_dll for controlling an operation of the delay line controller 417 . when a signal ckeb for noftifying power - down mode entry is applied at high level , the controller 220 outputs the signal cke_dll at low level to block an operation of the delay line controller 417 . in the embodiment illustrated in fig4 , a wrong phase detection result is output from the phase comparator 416 upon exiting from a power - down mode as in the prior to be applied to the delay line controller . however , for the delay line controller 417 according to the present invention , the delay line controller 417 is enabled after the signal fb_clk is normally applied . therefore , the possibility of generating a malfunction as in the prior art is reduced or eliminated . fig5 is an embodiment of a controller illustrated in fig4 . the controller of fig5 includes latches 51 , 52 and 53 , inverters 54 and 56 , a nor gate 55 , and transmission switches 57 a , 57 b and 57 c . when a signal fb_clk , which is a delay signal of an internal clock signal rclkt 2 , is changed to high level for the first time after exiting from power - down mode , a clock signal ckeb will be stored in latch 51 . when a signal fb_clk is changed to low level after a half cycle of the internal clock signal rclkt 2 , the signal ckeb stored in latch 51 will be stored in or shifted to latch 52 . next , when the signal fb_clk is changed again to high level after a half cycle of the internal clock signal rclkt 2 , the signal ckeb stored in the latch 52 passes into latch 53 and an inverter 54 to be applied to a nor gate 55 . an output signal of the inverter 54 is “ ckeb_d .” the nor gate 55 receives the signal ckeb and the output signal ckeb_d of the inverter 54 , and the delay line controller 417 will be enabled when an output signal cke_dll of the nor gate 55 is high level . as a result , it is seen that the delay line controller is enabled after one cycle of a clock signal clk or / clk . as seen in fig4 and 5 , a malfunction of the dll circuit can be prevented by delaying a timing of operating a delay line controller upon exiting from power - down mode . according to the present invention , in power - down mode , the power consumed in a dll circuit can be reduced , and furthermore a malfunction of the dll circuit which may be generated upon exiting from power - down mode can be prevented in advance . the embodiments described above and depicted in the figures are merely examples of the invention . the true scope of which is defined and determined by the appurtenant claims .