hereinafter , preferred embodiments of the present invention will be described with reference to the accompanying drawings . in the following description of the present invention , a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear . referring to fig3 , a complex terminal according to an embodiment of the present invention roughly includes a main battery 100 supplying electric power , a phone module 200 performing the function of a portable phone , and a pda module 300 performing the function of a personal information terminal or pda . the phone module 200 includes a terminal power supply 202 and a terminal controller 204 . the terminal power supply 202 transfers electric power supplied from the main battery 100 to devices in the phone module 200 , such as the terminal controller 204 . the terminal controller 204 controls general operation of the phone module 200 . the pda module 300 includes a pda power supply 302 , a first switch 304 , a first voltage sensor 306 , a second voltage sensor 308 , a second switch 316 , a charger 312 , an auxiliary battery 314 , a third switch 310 , a dc / dc converter 318 , a pda controller 320 , and a storage unit 322 . the pda power supply 302 outputs the electric power supplied from the main battery 100 after adjusting the voltage level of the electric power to a required input voltage level of the pda controller 320 . the first switch 304 is located between the pda power supply 302 , the pda controller 320 , and 381 , so as to connect the pda power supply 302 and the pda controller 320 or the pda controller 320 and the dc / dc converter 318 . the first voltage sensor 306 senses the voltage of the electric power supplied from the main battery 100 . when the sensed voltage is higher than a first voltage ( e . g ., 3 . 0 v ), the first voltage sensor 306 directs the first switch 304 to connect the pda controller 320 with the pda power supply 302 so that the pda controller 320 can receive electric power from the main battery 100 . the first voltage has a magnitude capable of operating the pda controller 320 , for example , 3 . 0 v . in contrast , when the sensed voltage is lower than the first voltage , the first voltage sensor 306 directs the first switch 304 to connect the dc / dc converter 318 and the pda controller 320 so that the pda controller 320 can receive electric power from the auxiliary battery 314 . further , when the sensed voltage is lower than the first voltage , the first voltage sensor 306 sends an interrupt signal to the pda controller 320 . the signal indicates that the pda controller 320 cannot receive the electric power from the main battery 100 . the interrupt signal causes the pda controller 320 to enter into a sleep mode . when the charger 312 is connected with the main battery 100 , the charger 312 charges the auxiliary battery 314 by means of the electric power supplied from the main battery 100 . the third switch 310 is located between the main battery 100 and the charger 312 , so as to connect or disconnect the main battery 100 and the charger 312 . the auxiliary battery 314 is charged through the charger 312 . when the auxiliary battery 314 has been charged , it can replace the main battery 100 in supplying electric power to the pda controller 320 . the second switch 316 is located between the auxiliary battery 314 and the dc / dc converter 318 , so as to connect or disconnect the auxiliary battery 314 and the dc / dc converter 318 . when the dc / dc converter 318 has been connected with the auxiliary battery 314 , the dc / dc converter 318 outputs the electric power supplied from the auxiliary battery 314 after adjusting the voltage level of the electric power to a required input voltage level of the pda controller 320 . the second voltage sensor 308 senses the voltage of the electric power supplied from the main battery 100 . when the sensed voltage becomes a second voltage ( e . g ., 3 . 2 v ), the second voltage sensor 308 directs the third switch 310 to disconnect the main battery 100 and the charger 312 from each other , thereby preventing the auxiliary battery 314 from being discharged toward the main battery 100 . further , when the sensed voltage becomes the second voltage , the second voltage sensor 308 directs the second switch 316 to connect the auxiliary battery 314 with the dc / dc converter 318 and adjusts the voltage level of the electric power supplied from the auxiliary battery 314 to a required input voltage level of the pda controller 320 . the second voltage is higher than the first voltage by a difference , which is sufficient for normal operation of the pda controller 320 before the pda controller 320 receives the interrupt signal and enters into the sleep mode . the pda controller 320 controls general operation of the pda module 300 . the pda controller 320 receives electric power from the main battery 100 when the voltage of the main battery 100 is higher than the first voltage and from the auxiliary battery 314 when the voltage of the main battery 100 is lower than or equal to the first voltage . when the pda controller 320 has received the interrupt signal from the first voltage sensor 306 , which notifies that the pda controller 320 cannot receive the electric power from the main battery 100 , the pda controller 320 stops all the processes currently in progress , enters into the sleep mode ( power - saving mode ), and stores data of the complex terminal . a power control apparatus of a complex terminal , according to an embodiment of the present invention , operates the dc / dc converter 318 before the interrupt signal is inputted to the pda controller 320 . this is different from a power control apparatus of a conventional complex terminal . as a result , the voltage of the electric power supplied through the dc / dc converter 318 from the auxiliary battery 314 can be instantly applied to the pda controller 320 . fig4 is a graph showing the voltage applied to the pda module 300 , which changes according to time when the main battery 100 is discharged , according to an embodiment of the present invention . fig5 is a graph showing the voltage applied to the pda module 300 , which changes according to time when the main battery 100 is attached to or separated from the complex terminal , according to an embodiment of the present invention . fig6 is a graph showing the voltage applied to the pda controller 320 according to an embodiment of the present invention , which changes according to time when the main battery 100 is discharged , attached to , or separated from the complex terminal . hereinafter , a method according to an embodiment of the present invention will be described in detail , which can reduce a phenomenon in which the voltage applied to the pda controller 320 becomes unstable when the main battery 100 is discharged , or assembled with or separated from the complex terminal . in fig4 , t 1 represents a time point at which the voltage applied from the main battery 100 to the pda module 300 begins to be lowered due to discharge of the main battery 100 . in fig5 , t 1 represents a time point at which the voltage applied from the main battery 100 to the pda module 300 begins to be lowered due to separation of the main battery 100 from the complex terminal . in fig4 and 5 , t 2 represents a time point at which the voltage applied to the pda module 300 becomes the second voltage ( e . g ., 3 . 2 v ) due to discharge or separation of the main battery 100 , and t 3 represents a time point at which the voltage applied to the pda module 300 becomes the first voltage ( e . g ., 3 . 0 v ). when the voltage of the electric power supplied from the main battery 100 reaches the second voltage while decreasing as described above , that is , at the time point t 2 , the second voltage sensor 308 directs the third switch 310 to disconnect the main battery 100 and the charger 312 from each other , thereby interrupting charge of the auxiliary battery 314 . further , at the time point t 2 , the second voltage sensor 308 directs the second switch 316 to connect the auxiliary battery 314 and the dc / dc converter 318 , thereby enabling the electric power to be supplied from the auxiliary battery 314 to the dc / dc converter 318 . further , at the time point t 3 at which the electric power supplied from the main battery 100 becomes lower than the first voltage , the first voltage sensor 306 transfers an interrupt signal to the pda controller 320 , which notifies that the main battery 100 cannot supply the electric power . further , at the time point t 3 , the first voltage sensor 306 directs the first switch 304 to connect the dc / dc converter 318 and the pda controller 320 . herein , the interval between t 2 and t 3 is several milliseconds . that is , a power control apparatus according to the present invention connects the auxiliary battery 314 and the dc / dc converter 318 with each other by means of the second switch 316 several milliseconds before the pda controller 320 receives the interrupt signal . therefore , at the time point t 3 , the pda controller 320 can instantly receive the electric power through the dc / dc converter 318 simultaneously while receiving the interrupt signal . as a result , instability of the voltage applied to the pda controller 320 can be reduced as shown in fig6 in the complex terminal according to the present invention , in comparison with the conventional complex terminal . according to experiments , when the pda controller 320 was realized by a dragonball - cpu , it took 800 μs for the pda controller 320 to enter into the sleep mode after the interrupt signal was inputted . a time delay in the operation of the second voltage sensor 308 was about 100 μs , and a time delay in the operation of the dc / dc converter 318 was about 10 μs . therefore , the interval between t 2 and t 3 is sufficient for normal application of the voltage to the pda controller 320 before the pda controller 320 enters into the sleep mode after receiving the interrupt signal . in the power control apparatus of a complex terminal according to the present invention as described above , the dc / dc converter is operated before an interrupt signal for entering the pda controller into the sleep mode is outputted to the pda controller . this is different from the operation of the power control apparatus of the typical complex terminal . therefore , the present invention can reduce the instability of the voltage applied to the pda controller , which may exist due to time it takes for the dc / dc converter to receive the electric power from the auxiliary battery and to supply the electric power to the pda controller . that is , in the complex terminal according to the present invention , while electric power source is switched from the main battery to the embedded auxiliary battery , the dc / dc circuit for converting the voltage of the auxiliary battery is operated more rapidly , so that the voltage applied to the pda module can be stabilized and thus loss of data in the complex terminal can be reduced . while the invention has been shown and described with reference to certain preferred embodiments thereof , it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims .