some preferred embodiments of the present invention will be described in detail in the following . however , beside the detailed description , the present invention can also be applied widely in other embodiments and the scope of the present invention is only limited by the appended claims . the main spirit of the present invention is , instead of the ordinal chip - enable pin , the led driving ic utilizes a pin which is set for the other signals or purposes as the input node of dimming control signals , thereby reducing the pin &# 39 ; s numbers to have smaller package of the ic . fig2 demonstrates an led driving circuit with a preferred embodiment of the present invention . unlike the conventional led driving circuit shown in fig1 , the present led driving ic 20 excludes a chip - enable pin en . the dimming control signals generated from the general purpose input / output ( gpio ) module 28 are firstly inputted to the pin cn of the charge pump 22 , and then transferred to the chip - enable / dimming control module 24 . in the other words , the pin cn replaces the original chip - enable pin en and becomes the input node for receiving dimming control signals . before the detail descriptions about the present led driving circuit shown in fig2 , a brief operation principle of a charge pump is described in first . fig3 a depicts a simple diagram of a charge pump . in addition to a capacitor c , there are further two sets of switches 30 and 32 that controlled by two non - overlapped switching signals φ 1 and φ 2 ( referring to fig3 b ), wherein each set of switches are closed while the switching signals ( φ 1 or φ 2 ) change to high level . when the switching signal φ 1 changes to high , the first set of switches 30 are closed and the second set of switches 32 are open , therefore an external power source across the vin and the gnd terminal may charge the capacitor c . when the switching signals φ 2 changes to high ( the switching signal φ 1 stays low ), the first set of switches 30 are opened and the second set of switches 32 are closed , therefore the capacitor c discharges to the vout terminal to drive loads ( for example , the leds ). generally , frequencies of the switching signals φ 1 and φ 2 are scaled in mhz , and the charge pump couples current from the input terminal vin to the output terminal vout by continuing switching the connections of the capacitor c . referring to fig4 a , for conciseness of the specification , the charge pump 22 and the transfer capacitor cf shown in fig2 could be implemented by the charge pump shown in fig3 a . as mentioned above , dimming control signals are inputted through the pin cn and then transferred to the chip - enable / dimming control module 24 , and the chip - enable / dimming control module 24 detects voltage of the pin cn to retrieve the dimming control signals . the resistor r is used to limit the current outputted from the gpio 28 . since the gpio 28 has smaller current - driving force than the external power source , the pin cn is responsive of the across voltage of the transfer capacitor cf as under normal operation of the charge pump 22 . hence , the chip - enable / dimming control module 24 could not detect and sample the inputted dimming control signals directly . in order to make the voltage of the pin cn being responsive of the dimming control signals , a method is proposed in the present invention . referring to fig4 a , by temporally closing the switches 40 and 42 , the pin cn will be floated and therefore be responsive of the inputted dimming control signals . fig4 b depicts the timing relationship between of the circuitry shown in fig4 a . in side the driving ic 20 , a checking signal chk is generated periodically , which could stop the switching signal φ 1 and φ 2 for one or several time interval . during the period that both the switching signal φ 1 and φ 2 are low , the chip - enable / dimming control module 24 detects and samples the voltage of the pin cn , and therefore retrieves the dimming control signals inputted from the gpio 28 . the checking signal chk could be generated by the chip - enable / dimming control module 24 or an internal clock ( not shown in the drawing ) of the driving ic 20 , and it &# 39 ; s not limited in the present invention . when the switching signals φ 1 and φ 2 are both low and thereafter the switches 40 and 42 are both off , since one terminal of the transfer capacitor cf ( pin cp ) is open , so the pin cn becomes a high impedance node and is floating . it results in the gpio 28 being able to pull the voltage of the pin cn up to high ( or low , according to the dimming control signal ). in the other words , while the switches 40 and 42 are both off , the voltage of the pin cn could be pulled up rapidly to have the same voltage with the dimming control signals because the pin cn is currently a high impedance , thereby making the chip - enable / dimming control module 24 being able to detect and sample the dimming control signals . with higher frequency of the checking signal chk , the sampled dimming control signals are closer to the original signal waveform , so that the chip - enable / dimming control module 24 could control the current sink 26 or on / off of the driving ic 20 itself for dimming the leds accordingly . hence , the chip - enable pin en could be reduced while packaging the driving ic , and the present invention may achieves the object of the invention for reducing the pin &# 39 ; s numbers , decreasing the volume of the ic and the manufacturing cost . furthermore , fig5 a depicts an embodiment led driving circuit that applies the present invention which includes : a charge pump 22 , a chip - enable / dimming control module 24 and an internal clock 58 , wherein the pin cn is directly connected to the chip - enable / dimming control module 24 by reference to the transmission line 562 . the difference to the prior art is , not only the switching signals φ 1 and φ 2 , the internal clock 58 also generates the checking signal chk for periodically stopping the switching signals φ 1 and φ 2 . moreover , as well as another embodiment shown in fig5 b , the checking signal chk could be generated by the chip - enable / dimming control module 24 instead , as well as the previous descriptions . while applying the led driving ic of fig5 a or 5 b to the led driving circuit shown in fig2 , the dimming control signals could be inputted through the pin cn directly . in view of the foregoing , the main spirit of the dimming method is : the method temporally switches a specific pin of the driving ic to high impedance , by which the gpio pulls the voltage of the specific pin up and therefore be responsive of the dimming control signals . hence , it should be understood that the present invention does not limit the specific pin to the pin cn only , but the pin cp or anyone else is also possible . in further , the present invention not only applies to the white led driving circuit described in foregoing paragraphs but also the other applications . if the designer could find and temporally switch a pin of an ic to high impedance , a control signal could be inputted even without a specific pin for the control signal . of course , the condition lies on there is no influence on the normal operation of the other signals that inputted / outputted through that specific pin . although the description discloses the preferred embodiment herein , it is not limit the spirit of the invention . it is intended that the specification and examples to be considered as exemplary only , with a true scope and spirit of the invention being indicated by the following claims .