in the following , embodiments of the present invention will be described with reference to the accompanying drawings . fig2 is a circuit diagram showing the construction of a first embodiment of a charge control circuit according to the present invention . the charge control circuit of fig2 includes an input voltage detecting circuit 31 and a drive circuit 32 as main elements , and is provided in the battery pack 13 shown in fig1 . the input voltage detecting circuit 31 includes resistors r 1 and r 2 , an amplifier 35 , and a reference - voltage source 36 . the drive circuit 32 includes an nmos transistor 37 . in the input voltage detecting circuit 31 , the input voltage v + of the battery pack 13 is divided by the resistors r 1 and r 2 , and a divided potential va is supplied to the non - inverted input terminal of the amplifier 35 . the inverted input terminal of the amplifier 35 receives a reference potential from the reference - voltage source 36 . the amplifier 35 generates an output responsive to a difference between the divided potential va and the reference potential for provision to the gate node of the nmos transistor 37 of the drive circuit 32 . the drain node of the nmos transistor 37 is connected to the gate node of the pmos transistor 21 . with this provision , a potential at the gate node of the pmos transistor 21 is controlled according to the level of the input voltage v +. as the input voltage v + increases , the output potential of the amplifier 35 rises , resulting in a drop in the gate potential of the pmos transistor 21 . this reduces the on resistance of the pmos transistor 21 . since the charger 11 ( see fig1 ) performs constant - current charging by use of the current detecting resistor 14 , a drop in the on resistance of the pmos transistor 21 causes a drop in the input voltage v +. in this manner , the input voltage v + is controlled to drop in response to an increase in the input voltage v +, thereby settling at a predetermined voltage . conversely , a drop in the input voltage v + results in a drop in the output potential of the amplifier 35 , thereby raising the gate potential of the pmos transistor 21 . this increases the on resistance of the pmos transistor 21 . since the charger 11 ( see fig1 ) performs constant - current charging by use of the current detecting resistor 14 , an increase in the on resistance of the pmos transistor 21 causes a rise in the input voltage v +. in this manner , the input voltage v + is controlled to rise in response to a drop in the input voltage v +, thereby settling at a predetermined voltage . in this manner , the present invention controls the on resistance of the pmos transistor 21 in response to the input voltage v + such that the on resistance takes any value between a fully conductive state and a fully non - conductive state . in the related - art construction shown in fig1 , the control of the pmos transistor 21 uses only two states , i . e ., an “ on ” state or an “ off ” state . in the present invention , the control of the pmos transistor 21 is performed continuously between the “ on ” state and the “ off ” state . this makes it possible to achieve feedback control that stabilizes the input voltage v + at a predetermined voltage level . at the time of overcharging , the overcharge detecting signal changes to high , making the nmos transistor 33 conductive . this makes the nmos transistor 37 of the drive circuit 32 nonconductive , so that the gate of the pmos transistor 21 coupled to the input voltage v + through a resistor r 0 changes to high . as a result , the pmos transistor 21 becomes nonconductive , suspending charging . although the pmos transistor 21 for charge control is nonconductive at the time of overcharging , a parasitic diode 34 permits discharging . fig3 is a diagram for explaining the charging operation of the charge control circuit shown in fig2 . in fig3 , the horizontal axis represents time t . as a charging operation continues , the battery voltage of the lithium - ion battery 24 gradually goes up . to cancel such a voltage rise , a charge control operation by the input voltage detecting circuit 31 and the drive circuit 32 lowers the gate voltage and on resistance ron of the pmos transistor 21 . with this , the ds voltage that is a voltage between the drain and source of the pmos transistor 21 gradually drops . fig4 is a circuit diagram showing the construction of a second embodiment of the charge control circuit according to the present invention . in fig4 , the same elements as those of fig2 are referred to by the same numerals , and a description thereof will be omitted . a charge control circuit of fig4 includes an nmos transistor 40 in addition to the construction of the charge control circuit of fig2 . the nmos transistor 40 receives a standby signal at its gate node , and couples the gate node of the pmos transistor 21 to a ground when a standby signal is high . this standby signal becomes high when the load of a power supply is small in an apparatus to which the battery pack 13 is attached . if the apparatus to which the battery pack 13 is attached is a notebook personal computer , for example , the load of a power supply is small in the standby mode of the notebook personal computer since operations are suspended in most of the core electronic circuits . in this standby mode , the standby signal becomes high , thereby coupling the gate node of the pmos transistor 21 to a ground and thus making the pmos transistor 21 fully conductive . in the second embodiment shown in fig4 , the pmos transistor 21 is fully turned on when the load of a power supply ( i . e ., the load of the charger 11 ) is small , thereby achieving a charging process with a massive current . this makes it possible to shorten a charge time . fig5 is a circuit diagram showing the construction of a third embodiment of the charge control circuit according to the present invention . in fig5 , the same elements as those of fig2 are referred to by the same numerals , and a description thereof will be omitted . a charge control circuit of fig5 includes a current detecting resistor rs and a current detector 50 in addition to the construction of the charge control circuit of fig2 . with the provision of the current detector 50 , the drive circuit 32 is changed to a drive circuit 32 a . since the gate node of the nmos transistor 37 of the drive circuit 32 a is connected to the output of the current detector 50 , the output of the amplifier 35 of an input voltage detecting circuit 31 a is coupled to the gate node of the nmos transistor 37 via an nmos transistor 38 intervening therebetween . due to this change , further , the inverted input terminal and non - inverted input terminal of the amplifier 35 are switched with each other . the current detector 50 includes an amplifier 51 and a reference - voltage source 52 . a voltage responsive to a current flowing through the lithium - ion battery 24 is generated as voltage vb at one end of the current detecting resistor rs . this potential vb is supplied to an inverted input terminal of the amplifier 51 . the non - inverted input terminal of the amplifier 51 receives a reference potential from the reference - voltage source 52 . the amplifier 51 generates an output responsive to a difference between the potential vb and the reference potential for provision to the gate node of the nmos transistor 37 of the drive circuit 32 a . the drain node of the nmos transistor 37 is connected to the gate node of the pmos transistor 21 . the gate node of the nmos transistor 37 is further connected to the drain node of the nmos transistor 38 . as a charge current increases , the voltage vb proportional to the charge current goes up , resulting in a drop in the output voltage of the amplifier 51 . in response , the gate potential of the pmos transistor 21 rises , thereby increasing the on resistance of the pmos transistor 21 . this causes the charge current to decrease . in this manner the charge current is controlled to drop in response to an increase in the charge current , thereby settling at a predetermined current amount . as a charge current decreases , the voltage vb proportional to the charge current goes down , resulting in a rise in the output voltage of the amplifier 51 . in response , the gate potential of the pmos transistor 21 falls , thereby decreasing the on resistance of the pmos transistor 21 . this causes the charge current to increase . in this manner the charge current is controlled to rise in response to a drop in the charge current , thereby settling at a predetermined current amount . in the embodiment describe above , the current detector 50 detects a charge current running through the lithium - ion battery 24 , and the drive circuit 32 a controls the on resistance of the pmos transistor 21 in response to the detected charge current . this eliminates a need for the constant - current - based control of the charger 11 . further , the input voltage detecting circuit 31 a detects the input voltage v +, and the drive circuit 32 a controls the on resistance of the pmos transistor 21 in response to the detected input voltage v +. this makes it possible to keep constant the input voltage v + as in the first embodiment . fig6 is a circuit diagram showing the construction of a fourth embodiment of the charge control circuit according to the present invention . in fig6 , the same elements as those of fig2 are referred to by the same numerals , and a description thereof will be omitted . a charge control circuit of fig6 includes a differential amplification circuit 61 , a multiplication circuit 62 , a current monitoring circuit 63 , an amplifier 64 , an nmos transistor 65 , and a current detecting resistor rs in addition to the construction of the charge control circuit of fig2 . further , an input voltage detecting circuit 31 a is provided in place of the input voltage detecting circuit 31 . in the input voltage detecting circuit 31 a , the inverted input terminal and non - inverted input terminal of the amplifier 35 are switched with each other compared to the amplifier 35 of the input voltage detecting circuit 31 . the differential amplification circuit 61 includes resistors r 3 through r 6 and an amplifier 67 . the non - inverted input terminal of the amplifier 67 receives a potential that is obtained by dividing the input voltage v + of the battery pack 13 by the resistor r 5 and the resistor r 6 . the inverted input terminal of the amplifier 67 receives a potential responsive to the potential vb appearing at one end of the pmos transistor 21 . the potential vb is a potential appearing on the opposite side to the v + side among the source node and drain node of the pmos transistor 21 . with this provision , the amplifier 67 outputs a potential responsive to a difference between v + and vb , i . e ., a potential responsive to a voltage drop across the pmos transistor 21 . the output of the amplifier 67 is supplied to the multiplication circuit 62 . the multiplication circuit 62 includes resistors r 7 through r 10 , transistors q 1 through q 6 , voltage sources v 1 and v 2 , and a constant current source 68 . as described above , the potential responsive to a difference between v + and vb is input into the multiplication circuit 62 from the amplifier 67 of the differential amplification circuit 61 . further , the multiplication circuit 62 receives a potential responsive to a charge current irs from the amplifier 69 of the current monitoring circuit 63 . the amplifier 69 detects a voltage drop produced by the charge current irs flowing through the current detecting resistor rs . the multiplication circuit 62 obtains a product of the charge current irs and the potential difference between v + and vb so as to calculate electric power consumed by the pmos transistor 21 . an output vc of the multiplication circuit 62 indicative of this electric power is supplied to the inverted input terminal of the amplifier 64 . the non - inverted input terminal of the amplifier 64 is connected to a reference - voltage source vth 2 . as the electric power vc consumed at the pmos transistor 21 increases , the output of the amplifier 64 drops , resulting in a rise in the gate voltage of the pmos transistor 21 . consequently , the on resistance of the pmos transistor 21 increases , thereby decreasing the charge current . as the electric power vc consumed at the pmos transistor 21 decreases , the output of the amplifier 64 rises , resulting in a drop in the gate voltage of the pmos transistor 21 . consequently , the on resistance of the pmos transistor 21 decreases , thereby increasing the charge current . with this provision , it is possible to keep constant the electric power consumed at the pmos transistor 21 , thereby suppressing excess heat generation at the pmos transistor 21 . further , the input voltage v + is kept constant in the same manner as in the first embodiment shown in fig2 . in the fourth embodiment shown in fig6 , the gate node of the nmos transistor 37 of the drive circuit 32 is connected to the output of the amplifier 64 , the output of the amplifier 35 of the input voltage detecting circuit 31 a is coupled to the gate node of the nmos transistor 37 via an nmos transistor 65 intervening therebetween . due to this change , further , the inverted input terminal and non - inverted input terminal of the amplifier 35 are switched with each other . moreover , the circuit construction of the multiplication circuit 62 is illustrated as an example , and is not intended to be limiting . any circuit construction may be used as long as it achieves a multiplication function . fig7 is a circuit diagram showing the construction of a fifth embodiment of the charge control circuit according to the present invention . in fig7 , the same elements as those of fig4 are referred to by the same numerals , and a description thereof will be omitted . in addition to the construction of the charge control circuit of fig4 , a charge control circuit of fig7 includes a pmos transistor 71 , a parasitic diode 72 , and resistors r 11 and r 12 . moreover , the output of the drive circuit 32 is supplied to the gate node of the pmos transistor 71 for the precharge purpose rather than to the gate node of the pmos transistor 21 . during a normal charging process , the pmos transistor 21 is in a nonconductive state . in this case , the input voltage detecting circuit 31 and the drive circuit 32 control the on resistance of the pmos transistor 71 in such manner that the on resistance may take any value between the conductive state and the nonconductive state , thereby charging the lithium - ion battery 24 while keeping the input voltage v + constant . when the load of the power supply is small , i . e ., when the load of the charger 11 is small ( as in the cases of current - consumption saving mode ), the standby signal is changed to high so as to perform charging with a massive current . this shortens the charge time . in the construction described above , the pmos transistor 21 that can only be set to either a fully conductive state or a fully nonconductive state is provided separately from the mos transistor 71 that can be set to any state in a range between a fully conductive state and a fully nonconductive state . this makes it possible to use an mos transistor suitable for a desired operation . if only the pmos transistor 21 is provided , the turning on of the pmos transistor 21 when the voltage of the lithium - ion battery 24 is very low causes an excessive current to flow , which may destroy the pmos transistor 21 . in the construction of fig7 , the presence of the pmos transistor 71 provides a function to precharge the lithium - ion battery 24 before the pmos transistor 21 is turned on . this provides an advantage in that excessive heating of the pmos transistor 21 is prevented . that is , the pmos transistor 71 provides dual functions , i . e ., the function to stabilize the input voltage v + according to the invention and the function to precharge the lithium - ion battery 24 . fig8 is a circuit diagram showing the construction of a sixth embodiment of the charge control circuit according to the present invention . in fig8 , the same elements as those of fig5 are referred to by the same numerals , and a description thereof will be omitted . in a charge control circuit of fig8 , a current monitoring circuit 80 is provided in place of the current detector 50 , compared with the construction of the charge control circuit of fig5 . the current monitoring circuit 80 receives voltages appearing at opposite ends of the current detecting resistor rs and a voltage vd appearing at one end of a thermistor 81 . the thermistor 81 and a resistor r 15 are connected in series , and are situated between a voltage vth 3 and a ground potential . the thermistor 81 has such characteristics that its resistance decreases at high temperature , and increases at low temperature . as the pmos transistor 21 generates heat by consuming electric power , the resistance of the thermistor 81 changes , causing a change in the voltage vd . the current monitoring circuit 80 includes amplifiers 82 and 83 and a reference - voltage source 84 . the amplifier 83 detects a voltage drop produced by the charge current flowing through the current detecting resistor rs , and supplies a potential ve responsive to the amount of the charge current to the inversed input terminal of the amplifier 82 . a first non - inverted input terminal of the amplifier 82 receives a reference voltage from the reference - voltage source 84 , and a second non - inverted input terminal receives the potential vd responsive to the resistance of the thermistor 81 . among the two non - inverted inputs of the amplifier 82 , one having a lower potential is given priority for comparison with ve . that is , vd is compared with ve if vd is smaller than the reference potential , and the reference potential is compared with ve if the reference potential is smaller than vd . at low temperature the resistance of the thermistor 81 is large , so that the potential vd is higher than the reference potential . in this case , ve is compared with the reference potential , resulting in the same operation as in the case of the construction shown in fig5 . the charge current can thus be kept constant . also , control by the input voltage detecting circuit 31 a and the drive circuit 32 a keeps the input voltage v + constant . as the pmos transistor 21 generates heat and becomes high temperature , the resistance of the thermistor 81 drops , resulting in the potential vd being lower than the reference potential . in this case , ve is compared with vd . as vd becomes lower than ve due to a temperature rise , the output of the amplifier 82 drops , which increases the gate voltage of the pmos transistor 21 . consequently , the on resistance of the pmos transistor 21 becomes large , with a resultant decrease in the charge current . conversely , as vd becomes higher than ve due to a temperature drop , the output of the amplifier 82 rises , which decreases the gate voltage of the pmos transistor 21 . consequently , the on resistance of the pmos transistor 21 becomes small , with a resultant increase in the charge current . in this manner , the charge current is decreased in response to a temperature rise , and is increased in response to a temperature drop . this makes it possible to ensure a sufficient charge current while keeping the heat generation of the pmos transistor 21 below a predetermined temperature . further , the present invention is not limited to these embodiments , but various variations and modifications may be made without departing from the scope of the present invention .