Patent Publication Number: US-2015067364-A1

Title: Information processing apparatus and power supply monitoring circuit

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2013-177045, filed on Aug. 28, 2013, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to an information processing apparatus and a power supply monitoring circuit. 
     BACKGROUND 
     Some mobile information processing apparatuses (hereinafter, simply referred to as “mobile devices”), such as notebook personal computers (PC) or mobile phones, include multiple electrical power supply sources, such as alternating current (AC) adapters or batteries. Furthermore, the mobile devices that include multiple electrical power supply sources each include a power supply monitoring circuit that identifies an electrical power supply source that supplies electrical power to a central processing unit (CPU) or other information processing units. 
     The power supply monitoring circuit that identifies an electrical power supply source monitors a state of the electrical power supply source. Then, the power supply monitoring circuit controls the operation of the CPU or a miscellaneous system in accordance with the state of the electrical power supply source. Furthermore, the power supply monitoring circuit switches multiple electrical power supply sources in accordance with the state of the electrical power supply sources. 
     Furthermore, in recent years, in order to reduce electrical power consumption and to speed up a response at the time of operation performed by an operator, information processing units, such as CPUs or the like, include various modes, such as an electrical power saving mode or a turbo mode. 
     The electrical power saving mode mentioned here is a mode in which, by reducing the throughput of an information processing unit, electrical power consumption is reduced as much as possible by decreasing a clock or a power supply voltage. With the electrical power saving mode, low electrical power consumption can be implemented. 
     Furthermore, the turbo mode mentioned here is a mode in which, even though electrical power consumption is increased, a process performed by an information processing unit can be implemented at high speed by increasing a clock or a power supply voltage. In the turbo mode, by promptly making a response to the operation performed by an operator or by promptly making an intermittent response to a device, such as a hard disk drive (HDD), it is possible to reduce stress placed on the operator. In other words, by using the turbo mode, it is possible to improve a response speed. 
     A mobile device switches modes in accordance with an amount of the task to be processed or with a temperature inside the device. For example, when no task or some task is present, a mobile device is running in the electrical power saving mode. When an amount of task is sharply increased by an operation performed by an operator, the mobile device shifts to the turbo mode. If the temperature inside the device rises to a certain level or more due to the operation performed in the turbo mode, the mobile device shifts to the electrical power saving mode in order to prevent the device from being destructed due to heating by the device. Furthermore, if an amount of task is decreased during the operation in the turbo mode, the mobile device also shifts to the electrical power saving mode. 
     Furthermore, in recent years, due to pursuing a response speed and an increase in the operating time of a battery, electrical power consumed in the turbo mode is increased by a factor of two or more when compared with a case in which the electrical power saving mode is used. 
     Accordingly, in accordance with an increase in the electrical power consumed in the turbo mode, it is preferable to increase the rated current of an external power supply, such as an AC adapter, that supplies electrical power to a mobile device. The “rated” mentioned here is a value that is determined, by a manufacturer, as an upper limit of the current that can be output by the external power supply under a specific condition. However, in practice, because the rated current is set by taking into consideration a margin of the operation, the external power supply can output a current that exceeds the rated current. Furthermore, the upper limit of the current that can be output may sometimes vary due to the effect of the environment, such as individual difference between various parts included in an information processing apparatus, an input voltage, a temperature at the vicinity of the device, and the like. 
     As a method that controls electrical power that is input from an external power supply, there is a conventional technology that determines, when a current supplied from an AC adapter is equal to or greater than a threshold, that this state is an excess current; that reduces a clock of a CPU; and that reduces a current to be consumed. Furthermore, there is a conventional technology that reduces an output voltage when a temperature rise is detected in an AC adapter; that reduces a clock of a CPU in response to a change in a voltage; and that reduces a current to be consumed. 
     Patent Document 1: Japanese Laid-open Patent Publication No. 2004-133646 
     Patent Document 2: Japanese Laid-open Patent Publication No. 2009-225610 
     However, because the electrical power consumption is increased when the turbo mode is used, the current from an electrical power supply source, such as an external power supply or a battery, is increased. Consequently, a voltage drop is increased in a path from the electrical power supply source to the CPU or to a miscellaneous system. Furthermore, if the electrical power consumption is increased, the impedance of the main body of the device that includes the CPU or the miscellaneous system is decreased. When the impedance is decreased, a protection circuit in an external power supply operates and thus the voltage that is output from the external power supply drops. 
     As described above, if the output voltage from the external power supply drops and falls below a threshold that is used to identify the external power supply, the power supply monitoring circuit erroneously detects that an AC adapter is disconnected and thus the electrical power supply from the external power supply may possibly be interrupted. The interruption of the power supply due to the erroneous detection affects the operation mode of the mobile device. Furthermore, if an output voltage from the external power supply falls below the threshold, the power supply monitoring circuit determines that the external power supply is newly connected; therefore, identification of the external power supply may possibly be changed. Because a special task is needed for identifying the external power supply, the throughput of the mobile device may possibly be reduced due to a repeatedly performed identification process. Furthermore, due to the repeatedly performed identification process, an operator may possibly and erroneously determine that a failure has occurred in the mobile device. 
     SUMMARY 
     According to an aspect of an embodiment, an information processing apparatus includes: an external power supply; a secondary battery; a power supply monitoring circuit; and an information processing unit, wherein the information processing unit is operated by electrical power supplied from the external power supply or the secondary battery, and the power supply monitoring circuit includes a first determining unit that determines that a voltage supplied from the external power supply is equal to or greater than a voltage threshold, a second determining unit that determines that a current supplied from the external power supply is equal to or greater than a current threshold, and an electrical power saving control circuit that drops, when the first determining unit determines that the supplied voltage is lower than the voltage threshold and the second determining unit determines that the supplied current is equal to or greater than the current threshold, electrical power consumed by the information processing unit. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating an information processing apparatus; 
         FIG. 2  is a circuit diagram illustrating a voltage determining comparator; 
         FIG. 3  is a schematic diagram illustrating an output from the voltage determining comparator; 
         FIG. 4  is a circuit diagram illustrating a current determining comparator; 
         FIG. 5  is a schematic diagram illustrating an output from the current determining comparator; 
         FIG. 6  is a block diagram illustrating an electrical power saving control circuit; 
         FIG. 7  is a schematic diagram illustrating an output from a NOR circuit; 
         FIG. 8  is a circuit diagram illustrating a power supply switching comparator according to a first embodiment; 
         FIG. 9  is a schematic diagram illustrating an output from the power supply switching comparator according to the first embodiment; 
         FIG. 10  is a flowchart illustrating the flow of electrical power saving control performed by an AC adapter identifying circuit according to the first embodiment; 
         FIG. 11  is a sequence diagram illustrating the circuit operation performed when an unauthorized AC adapter having a low output voltage is connected in the first embodiment; 
         FIG. 12  is a sequence diagram illustrating the circuit operation performed when an AC adapter is connected in a state in which a battery is running in the first embodiment; 
         FIG. 13  is a sequence diagram illustrating the circuit operation performed when, in the first embodiment, the capacity of the AC adapter exceeds after a mode is shifted to the turbo mode while the AC adapter is being connected; 
         FIG. 14  is a sequence diagram illustrating the circuit operation performed when the AC adapter is disconnected in the first embodiment; 
         FIG. 15  is a circuit diagram illustrating a power supply switching comparator according to a second embodiment; 
         FIG. 16  is a schematic diagram illustrating an output from a power supply switching comparator according to the second embodiment; 
         FIG. 17  is a sequence diagram illustrating the circuit operation performed when, in the second embodiment, the capacity of the AC adapter exceeds after a mode is shifted to the turbo mode while the AC adapter is being connected; 
         FIG. 18  is a sequence diagram illustrating the circuit operation performed when the AC adapter is disconnected in the second embodiment; and 
         FIG. 19  is a block diagram illustrating an example of the hardware configuration of a notebook PC. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Preferred embodiments of the present invention will be explained with reference to accompanying drawings. The information processing apparatus and the power supply monitoring circuit disclosed in the present invention are not limited to the embodiments described below. In particular, a description will be given of an example of a notebook PC that includes an AC adapter and will be given of an example of an AC adapter identifying circuit mounted on the notebook PC; however, any mobile device may also be used as long as the mobile device can receive a supply of electrical power from an external power supply. 
     [a] First Embodiment 
       FIG. 1  is a block diagram illustrating an information processing apparatus. As illustrated in  FIG. 1 , the information processing apparatus according to the first embodiment includes a notebook PC  1  and an AC adapter  2 . The solid lines that connect each unit illustrated in  FIG. 1  represent electrical power supply lines that are used to send electrical power. Furthermore, the broken lines that connect each unit represent signal lines that are used to send signals. 
     The AC adapter  2  is an external power supply that supplies electrical power to the notebook PC  1 . A protection circuit  23  is mounted on the AC adapter  2 . The AC adapter  2  mentioned here corresponds to an example of an “external power supply”. 
     The protection circuit  23  is a circuit that drops, if an amount of electrical power that exceeds the capacity of the AC adapter  2  is supplied from the AC adapter  2 , a voltage output from the AC adapter  2  in order to avoid damage or degradation of internal parts in the AC adapter  2 . 
     Specifically, when the impedance of a CPU  20  and a miscellaneous system  21  falls below a threshold, the protection circuit  23  decreases an output voltage from the AC adapter  2 . Then, if the impedance of the CPU  20  and the miscellaneous system  21  is recovered to a level equal to or greater than the threshold, the protection circuit  23  releases a drop in the output voltage of the AC adapter  2 . 
     The notebook PC  1  includes an AC adapter identifying circuit  10 , an AC adapter connector  11 , a battery connector  12 , a battery  13 , a boot up power supply circuit  14 , a CPU power supply circuit  15 , a miscellaneous system power supply circuit  16 , the CPU  20 , and the miscellaneous system  21 . Furthermore, the notebook PC  1  includes field effect transistor (FET) switches  201  and  202 , an AC adapter current sensing resistor  18 , and a battery rectifier diode  19  on the electrical power supply line that connects the AC adapter connector  11  and the CPU  20  and the miscellaneous system  21 . Hereinafter, the transmission path for the electrical power from the AC adapter connector  11  is referred to as an AC adapter line  17 . 
     Furthermore, the notebook PC  1  receives a supply of electrical power from the AC adapter  2 , in addition to the battery  13 . 
     The CPU  20  is an arithmetic processing unit that performs an arithmetic processing. The CPU  20  operates in response to receiving electrical power supplied from the AC adapter  2  or the battery  13  via the CPU power supply circuit  15 . The CPU  20  and the miscellaneous system  21  mentioned here correspond to an example of an “information processing unit”. 
     Furthermore, the CPU  20  has a function of controlling the electrical power saving mode. During the time for which the CPU  20  receives an input of a High voltage signal from an electrical power saving control circuit  104 , which will be described later, to a control terminal in the electrical power saving mode, the CPU  20  shifts each of the units, such as the CPU  20 , the miscellaneous system  21 , or the like, to the electrical power saving mode. 
     The miscellaneous system  21  is a system that is used in various processes performed in, for example, a memory, a hard disk, or the like in the notebook PC  1 . The miscellaneous system  21  operates in response to receiving a supply of electrical power from the AC adapter  2  or the battery  13  via the miscellaneous system power supply circuit  16 . The CPU  20  and the miscellaneous system  21  mentioned here correspond to an example of an “information processing unit”. Furthermore, in a description below, for convenience of description, there may be a case in which, a description will sometimes be given, for an electrical power supplied to the CPU  20  and the miscellaneous system  21 , by omitting the CPU power supply circuit  15  and the miscellaneous system power supply circuit  16 . 
     The AC adapter connector  11  is a connecting terminal of the AC adapter  2  in the notebook PC  1 . The AC adapter  2  is connected to the AC adapter connector  11 . The AC adapter connector  11  receives an electrical power supply from the AC adapter  2 . Then, the AC adapter connector  11  outputs the electrical power supplied from the AC adapter  2  to the AC adapter line  17 . In the following, the voltage of the electrical power that has been output from the AC adapter  2  is referred to as an “AC adapter voltage”. Furthermore, the current of the electrical power that has been output from the AC adapter  2  is referred to as an “AC adapter current”. 
     Furthermore, the electrical power output from the AC adapter connector  11  is input to a voltage determining comparator  101 . Furthermore, the AC adapter voltage output from the AC adapter connector  11  is input, as a signal, to the voltage determining comparator  101  and a power supply switching comparator  103 . 
     The battery  13  is a built-in power supply installed inside the notebook PC  1 . The battery  13  supplies the electrical power stored in the battery  13  to each of the units in the notebook PC  1 . The battery  13  and the AC adapter  2  can simultaneously be connected to the notebook PC  1 . The output voltage of the battery  13  is set lower than that of the AC adapter voltage. Consequently, if electrical power is supplied from the AC adapter  2 , the electrical power consumed by the battery  13  can be reduced. The battery  13  corresponds to an example of a “secondary battery”. 
     In a description below, the CPU power supply circuit  15  and the miscellaneous system power supply circuit  16  are referred to as an “onboard power supply” and the AC adapter  2  and the battery  13  are referred to as an “electrical power source”, which are distinguished. 
     The battery connector  12  is a connecting terminal of the battery  13 . The electrical power supply line extending from the battery connector  12  is connected to the electrical power supply line that extends from the FET switch  202  towards the CPU power supply circuit  15  and the miscellaneous system power supply circuit  16 . The battery connector  12  receives an electrical power supply from the battery  13 . Then, the battery connector  12  outputs the electrical power supplied from the battery  13  toward the connection point on the electrical power supply line extending from the FET switch  202  to the CPU  20  and the miscellaneous system  21 . Furthermore, on the electrical power supply line extending from the battery connector  12 , the battery rectifier diode  19  is arranged. 
     The boot up power supply circuit  14  receives an electrical power supply from the AC adapter  2  when the FET switches  201  and  202 , which will be described later, are turned on. Furthermore, the boot up power supply circuit  14  receives an electrical power supply from the AC adapter  2  or the battery  13  when the FET switches  201  and  202  are turned off. Specifically, the boot up power supply circuit  14  always receives a supply of electrical power. 
     The boot up power supply circuit  14  supplies electrical power to a current determining comparator  102  and the power supply switching comparator  103 , which will be described later. Here, in the first embodiment, a description has been given of only the electrical power supplied from the boot up power supply circuit  14  to both the current determining comparator  102  and the power supply switching comparator  103 ; however, the boot up power supply circuit  14  may also supply electrical power to another member. 
     On the AC adapter line  17  extending from the AC adapter connector  11 , the FET switches  201  and  202  and the AC adapter current sensing resistor  18  are arranged. 
     The FET switches  201  and  202  are turned on and off in response to the control performed by the power supply switching comparator  103 , which will be described later. 
     Specifically, if the FET switches  201  and  202  receive an input of a signal with a High voltage (hereinafter, simply be referred to as a “High signal”) from the power supply switching comparator  103 , the FET switches  201  and  202  are turned on. If the FET switches  201  and  202  are turned on, the electrical power output from the AC adapter  2  is supplied to the CPU  20  and the miscellaneous system  21  passing through the AC adapter current sensing resistor  18  via the FET switches  201  and  202 . 
     In contrast, if the FET switches  201  and  202  receive an input of a signal with Low voltage (hereinafter, simply be referred ti as a “Low signal”) from the power supply switching comparator  103 , the FET switches  201  and  202  are turned off. If the FET switches  201  and  202  are turned off, the electrical power output from the AC adapter  2  is not supplied to the CPU  20  and the miscellaneous system  21  because the AC adapter line  17  is cut off. Furthermore, the AC adapter current does not flow through the AC adapter current sensing resistor  18  either. In this case, the electrical power output from the battery  13  is supplied to the CPU  20  and the miscellaneous system  21 . 
     The AC adapter identifying circuit  10  includes the voltage determining comparator  101 , the current determining comparator  102 , the power supply switching comparator  103  and the electrical power saving control circuit  104 . The AC adapter identifying circuit  10  mentioned here corresponds to an example of a “power supply monitoring circuit”. 
     In the following, the voltage determining comparator  101  will be described with reference to  FIG. 2 .  FIG. 2  is a circuit diagram illustrating a voltage determining comparator. The voltage determining comparator  101  mentioned here corresponds to an example of a “first determining unit”. 
     The voltage determining comparator  101  includes resistors  111  and  112  on the signal path to which an AC adapter voltage as a signal is input from the AC adapter line  17 . Furthermore, the end portion at the opposite side from the resistor  112 , to which the AC adapter voltage is input, on the signal path is connected to ground. 
     Furthermore, the signal path is branched off between the resistors  111  and  112  and is connected to a comparator  114 . The resistors  111  and  112  divide the signal from the AC adapter line  17 . Then, the signal divided by the resistors  111  and  112  is supplied to the comparator  114 . For example, the signal with the voltage of {2/(R1+R2)}×V1 is input to the comparator  114 , where the resistance of the resistor  111  is R1 (Ω), the resistance of the resistor  112  is R2 (Ω), and the voltage of an input signal from the AC adapter line  17  is V1 (V). 
     The comparator  114  is a comparator with open drain output. The comparator  114  is driven by electrical power supplied from the AC adapter line  17 . 
     The signal line on the output side of the comparator  114  is connected to the electrical power saving control circuit  104 . Furthermore, the signal line that connects the comparator  114  to the electrical power saving control circuit  104  is connected to the boot up power supply circuit  14  via a resistor  115 . The resistor  115  is a pull-up resistor. 
     The comparator  114  is not driven when the AC adapter  2  is not connected to the AC adapter connector  11 . In such a case, the signal that is input to the electrical power saving control circuit  104  is a signal with the electric potential of the resistor  115 . Specifically, if the AC adapter  2  is not connected to the AC adapter connector  11 , the voltage determining comparator  101  inputs the High signal to the electrical power saving control circuit  104 . 
     If the AC adapter  2  is connected to the AC adapter connector  11 , the comparator  114  receives an input of the signal that is divided by the resistors  111  and  112 . Furthermore, the comparator  114  receives an input of the reference voltage from a reference voltage  113 . 
     The comparator  114  compares the voltage of the signal divided by the resistors  111  and  112  with the reference voltage and outputs, if the voltage of the signal divided by the resistors  111  and  112  is equal to or greater than the reference voltage, the High signal to the electrical power saving control circuit  104 . Furthermore, if the voltage of the signal divided by the resistors  111  and  112  is lower than the reference voltage, the comparator  114  outputs the Low signal to the electrical power saving control circuit  104 . 
     At this point, if the AC adapter  2  operates normally, the voltage of the signal divided by the resistors  111  and  112  becomes equal to or greater than the reference voltage. Consequently, if the AC adapter  2  operates normally, the voltage determining comparator  101  outputs the High signal to the electrical power saving control circuit  104 . In contrast, if a voltage drop becomes large when the AC adapter  2  operated in the turbo mode or if an AC adapter voltage drops because the protection circuit  23  is operated, a state occurs in which the voltage of the signal divided by the resistors  111  and  112  falls below the reference voltage. Accordingly, in such a case, the voltage determining comparator  101  outputs the Low signal to the electrical power saving control circuit  104 . 
     In other words, the voltage determining comparator  101  can determine whether an AC adapter voltage drops by using the voltage of the signal output from the reference voltage  113  as a threshold. Specifically, by adjusting the reference voltage  113 , the voltage determining comparator  101  can determine whether the AC adapter voltage falls below the electrical power threshold that is the reference for determining whether the AC adapter voltage is unusually low. Specifically, by setting the reference voltage  113  such that the voltage threshold is lower than the lower limit of the AC adapter voltage in the rated state of the AC adapter  2  by a predetermined value, the voltage determining comparator  101  can determine whether the AC adapter  2  is deviated from the rated state. For example, the reference voltage  113  is set such that the voltage threshold is lower than the lower limit of the AC adapter voltage in the rated state of the AC adapter  2  by 1 (V). However, it is preferable to set the voltage threshold greater than the voltage value that is used by the power supply switching comparator  103 , which will be described later, to turn off the FET switches  201  and  202 . By setting the reference voltage  113  in this way, the electrical power saving control circuit  104 , which will be described later, can solve a problem by shifting to the electrical power saving mode before the power supply switching comparator  103  detects an abnormality of the AC adapter  2  and disconnects a supply of electrical power. 
     The output from the voltage determining comparator  101  described above can be schematically represented by the diagram illustrated in  FIG. 3 .  FIG. 3  is a schematic diagram illustrating an output from the voltage determining comparator. In the column entitled “output” illustrated in  FIG. 3 , symbol “H” indicates an output of the High signal and symbol “L” indicates an output of the Low signal. 
     When the AC adapter voltage is V, if V is equal to or greater than a voltage threshold, the voltage determining comparator  101  outputs the High signal. Furthermore, if V is lower than the voltage threshold, the voltage determining comparator  101  outputs the Low signal. Furthermore, if the AC adapter  2  is not connected, the state at that time is V=0; therefore, the voltage determining comparator  101  outputs the High signal. 
     In the following, the current determining comparator  102  will be described with reference to  FIG. 4 .  FIG. 4  is a circuit diagram illustrating a current determining comparator. The current determining comparator  102  mentioned here corresponds to a “second determining unit”. 
     An amplifier  121  and a comparator  123  in the current determining comparator  102  are driven by the electrical power supplied from the boot up power supply circuit  14 . 
     The amplifier  121  is a differential amplifier. A voltage on the input side and the voltage on the output side of the AC adapter current sensing resistor  18  are input to the amplifier  121 . Then, the amplifier  121  amplifies the difference between the input voltages and then outputs the amplified voltage to the comparator  123 . 
     The comparator  123  is a push-pull output comparator. The comparator  123  receives an input of a voltage from the amplifier  121 . Furthermore, the comparator  123  receives an input of the reference voltage from a reference voltage  122 . 
     The comparator  123  compares the voltage input from the amplifier  121  with the reference voltage. If the voltage input from the amplifier  121  is equal to or greater than the reference voltage, the comparator  123  outputs the Low signal. Furthermore, if the voltage input from the amplifier  121  is lower than the reference voltage, the comparator  123  outputs the High signal. 
     If the current flowing through the AC adapter current sensing resistor  18  increases, the difference between the voltage on the input side and the voltage on the output side of the AC adapter current sensing resistor  18  becomes large. Specifically, if the AC adapter current with a value equal to or greater than a predetermined value that is taken on the basis of the reference voltage  122  is flowing through the AC adapter current sensing resistor  18 , the comparator  123  outputs the Low signal. Furthermore, if the AC adapter current with a value less than the predetermined value is flowing through the AC adapter current sensing resistor  18 , the comparator  123  outputs the High signal. Specifically, by adjusting both the amplification factor of the amplifier  121  and the voltage output from the reference voltage  122 , the current determining comparator  102  can determine whether the AC adapter current is greater than the current threshold. Furthermore, the reference voltage  122  is preferably set such that the current threshold satisfies an appropriate condition. For example, the current threshold may also be the lower limit value of the AC adapter current that is conceivably flowing when the AC adapter  2  is connected to the AC adapter connector  11  and is running. Furthermore, the current threshold may also be the value of the current flowing when the load applied to the CPU  20  is equal to or greater than a predetermined value. 
     The output of the current determining comparator  102  described above can be schematically represented by the diagram illustrated in  FIG. 5 .  FIG. 5  is a schematic diagram illustrating an output from the current determining comparator. In the column entitled “output” illustrated in  FIG. 5 , symbol “H” indicates an output of the High signal and symbol “L” indicates an output of the Low signal. 
     When the AC adapter current is I, if I is equal to or greater than the current threshold, the current determining comparator  102  outputs the Low signal. Furthermore, if I is lower than the current threshold, the current determining comparator  102  outputs the High signal. 
     In the following, the electrical power saving control circuit  104  will be described with reference to  FIG. 6 .  FIG. 6  is a block diagram illustrating an electrical power saving control circuit. The electrical power saving control circuit  104  includes a NOR circuit  141  and a one-shot circuit  142 . 
     The NOR circuit  141  receives an input of a signal from each of the voltage determining comparator  101  and the current determining comparator  102 . Then, the NOR circuit  141  outputs logical NOR of the two input signals to the one-shot circuit  142 . 
       FIG. 7  is a schematic diagram illustrating an output from a NOR circuit. The NOR circuit  141  outputs the Low signal only when the output of the voltage determining comparator  101  and the output of the current determining comparator  102  are the High signal. In other conditions, the NOR circuit  141  outputs the Low signal. 
     Specifically, the NOR circuit  141  outputs the High signal only when the AC adapter voltage is lower than the voltage threshold and the AC adapter current is greater than the current threshold. In other words, the NOR circuit  141  outputs the High signal when, in the state in which an appropriate AC adapter  2  is connected, the AC adapter voltage is lower than the lower limit of the AC adapter voltage in the rated state of the AC adapter  2 . In such a case, it is assumed that the NOR circuit  141  determines that the voltage of the electrical power supplied to the AC adapter  2  is too high. 
     The one-shot circuit  142  receives an input of a signal from the NOR circuit  141 . If the High signal is input from the NOR circuit  141 , the one-shot circuit  142  continues to output the High signal to the control terminal in the electrical power saving mode in the CPU  20  for a certain time period. 
     In the following, the power supply switching comparator  103  will be described with reference to  FIG. 8 .  FIG. 8  is a circuit diagram illustrating a power supply switching comparator according to a first embodiment. The power supply switching comparator  103  mentioned here corresponds to an example of a “power supply switching circuit”. 
     The power supply switching comparator  103  is a circuit that selects one of the AC adapter  2  and the battery  13  as the electrical power source. The power supply switching comparator  103  includes resistors  131  and  132  on the signal path to which an AC adapter voltage as a signal is input from the AC adapter line  17 . Furthermore, the end portion at the opposite side from the resistor  132 , to which the AC adapter voltage is input, arranged on the signal path is connected to ground. 
     Furthermore, the signal path is branched off between the resistors  131  and  132  and is connected to a comparator  134 . The resistors  131  and  132  divide the signal from the AC adapter line  17 . Then, the signal divided by the resistors  131  and  132  is supplied to the comparator  134 . 
     The comparator  134  is a push-pull output comparator. The comparator  134  is driven by the electrical power supplied from the boot up power supply circuit  14 . 
     The signal line on the output side of the comparator  134  is connected to the FET switches  201  and  202 . 
     The comparator  134  receives an input of the signal divided by the resistors  131  and  132 . Furthermore, the comparator  134  receives an input of the reference voltage from a reference voltage  133 . 
     The comparator  134  compares the voltage of the signal divided by the resistors  131  and  132  with the reference voltage and outputs, if the voltage of the signal divided by the resistors  131  and  132  is equal to or greater than the reference voltage, the High signal to the FET switches  201  and  202  as a power supply switching signal. Furthermore, if the voltage of the signal divided by the resistors  131  and  132  is lower than the reference voltage, the comparator  134  outputs the Low signal to the FET switches  201  and  202  as a power supply switching signal. 
     At this point, if the connected AC adapter  2  is appropriate, the voltage of the signal divided by the resistors  131  and  132  is equal to or greater than the reference voltage. Consequently, if the appropriate AC adapter  2  is connected, the power supply switching comparator  103  outputs a High power supply switching signal to the FET switches  201  and  202 . In contrast, if an inappropriate adapter is connected as the AC adapter  2 , a state in which the voltage of the signal divided by the resistors  131  and  132  falls below the reference voltage occurs. Accordingly, in such a case, the power supply switching comparator  103  outputs a Low power supply switching signal to the FET switches  201  and  202 . 
     In other words, the power supply switching comparator  103  can determine whether the AC adapter  2  is an appropriate adapter by using the voltage of the signal output from the reference voltage  133  as the threshold. Specifically, by setting the reference voltage  133  such that the value of the reference voltage  133  is lower than the lower limit of the AC adapter voltage in the rated state of the appropriate AC adapter  2  by a predetermined value, the power supply switching comparator  103  can determine whether the AC adapter  2  is appropriate for the adapter. For example, the reference voltage  133  is set such that the switching threshold, which is used for the reference for determining whether the AC adapter  2  is appropriate, is lower than the lower limit of the AC adapter voltage in the rated state of the AC adapter  2  by 1.4 (V). However, as described before, the switching threshold is preferably lower than the voltage threshold that is used by the voltage determining comparator  101  as the reference. 
     The output of the power supply switching comparator  103  described above can be schematically represented by the diagram illustrated in  FIG. 9 .  FIG. 9  is a schematic diagram illustrating an output from the power supply switching comparator according to the first embodiment. In the column entitled “power supply switching signal” illustrated in  FIG. 9 , symbol “H” indicates an output of the High power supply switching signal and symbol “L” indicates an output of the Low power supply switching signal. 
     When the AC adapter voltage is V, if V is equal to or greater than the switching threshold, the power supply switching comparator  103  outputs the High power supply switching signal. In this case, the FET switches  201  and  202  are turned on (ON). Furthermore, if V is lower than the switching threshold, the power supply switching comparator  103  outputs the Low power supply switching signal. In this case, the FET switches  201  and  202  are turned off (OFF). 
     In the following, the overall flow of electrical power saving control performed by the AC adapter identifying circuit  10  according to the first embodiment will be described with reference to  FIG. 10 .  FIG. 10  is a flowchart illustrating the flow of electrical power saving control performed by an AC adapter identifying circuit according to the first embodiment. 
     The current determining comparator  102  determines whether the AC adapter current is equal to or greater than the current threshold (Step S 1 ). If the AC adapter current is smaller than the current threshold (No at Step S 1 ), the current determining comparator  102  outputs the High signal. Specifically, the output of the current determining comparator  102  becomes High (Step S 2 ). Then, the current determining comparator  102  returns to Step S 1 . 
     In contrast, if the AC adapter current is equal to or greater than the current threshold (Yes at Step S 1 ), the current determining comparator  102  outputs the Low signal. Specifically, the output of the current determining comparator  102  becomes Low (Step S 3 ). 
     Then, the voltage determining comparator  101  determines whether the AC adapter voltage is smaller than the voltage threshold (Step S 4 ). If the AC adapter voltage is equal to or greater than the voltage threshold (No at Step S 4 ), the voltage determining comparator  101  outputs the High signal. Specifically, the output of the voltage determining comparator  101  becomes High (Step S 5 ). Then, the voltage determining comparator  101  returns to Step S 4 . 
     In contrast, if the AC adapter voltage is smaller than the voltage threshold (Yes at Step S 4 ), the voltage determining comparator  101  outputs the Low signal. Specifically, the output of the voltage determining comparator  101  is Low (Step S 6 ). 
     The NOR circuit  141  receives an input of the Low signal from both the voltage determining comparator  101  and the current determining comparator  102 . Then, the output of the NOR circuit  141  becomes High (Step S 7 ). 
     The one-shot circuit  142  receives an input of the High signal from the NOR circuit  141 . Then, the one-shot circuit  142  allows the electrical power saving mode control signal to be High for a certain time period (Step S 8 ). In response to the state in which the electrical power saving mode control signal becomes High, the CPU  20  shifts to the electrical power saving mode. 
     When the CPU  20  shifts to the electrical power saving mode, the electrical power consumed by the CPU  20  and the miscellaneous system  21  drops. In response to this state, the output voltage of the AC adapter  2  drops. In such a case, the impedance of the CPU  20  and the miscellaneous system  21  increases. For example, when the protection circuit  23  is running, if the impedance exceeds the threshold, a drop in the voltage performed by the protection circuit  23  is released. Then, the voltage determining comparator  101  determines whether the AC adapter voltage is equal to or greater than the voltage threshold (Step S 9 ). If the AC adapter voltage is smaller than the voltage threshold (No at Step S 9 ), because a drop in the voltage performed by the protection circuit  23  has not been released, the voltage determining comparator  101  outputs the Low signal. Specifically, the output from the voltage determining comparator  101  becomes Low (Step S 10 ). In this case, the voltage determining comparator  101  returns to Step S 9 . 
     In contrast, if the AC adapter voltage is equal to or greater than the voltage threshold (Yes at Step S 9 ), because a drop in the voltage performed by the protection circuit  23  has been released, the voltage determining comparator  101  outputs the High signal. Specifically, the output from the voltage determining comparator  101  becomes High (Step S 11 ). 
     Because the High signal is input from the voltage determining comparator  101 , the electrical power saving control circuit  104  outputs the Low signal as the electrical power saving mode control signal. Specifically, the electrical power saving mode control signal becomes Low (Step S 12 ). When the CPU  20  receives an input of the Low signal as the electrical power saving mode control signal, the CPU  20  releases the electrical power saving mode. 
     In the following, the operation of each of the circuits performed when the AC adapter  2 , which is an unauthorized adapter having a low output voltage, is connected to the notebook PC  1  in the first embodiment will be described with reference to  FIG. 11 .  FIG. 11  is a sequence diagram illustrating the circuit operation performed when an unauthorized AC adapter having a low output voltage is connected in the first embodiment. 
     A graph  31  illustrated in  FIG. 11  is a graph that indicates the variation in the AC adapter voltage. A graph  32  is a graph that indicates the variation in the AC adapter current. A graph  33  is a graph that indicates the variation in the battery voltage that is the voltage supplied from a battery. A graph  34  is a graph that indicates the variation in the battery current that is the current supplied from the battery. A graph  35  is a graph that indicates the variation in the input voltage output to the CPU power supply circuit  15  and the miscellaneous system power supply circuit  16 . A graph  36  is a graph that indicates the variation in the power supply switching signal. A graph  37  is a graph that indicates the variation in the output of the voltage determining comparator  101 . A graph  38  is a graph that indicates the variation in the output from the current determining comparator  102 . A graph  39  is a graph that indicates the output of the NOR circuit  141 . A graph  40  is a graph that indicates the variation in the electrical power saving mode control signal. A graph  41  is a graph that indicates the variation in the power supply switching threshold. 
     In all of the graphs  31  to  39 , an elapsed time is represented by the horizontal axis. Furthermore, in the graphs  31 ,  33 ,  35 , and  41 , the vertical axis indicates the voltage value. Furthermore, in the graphs  32  and  34 , the vertical axis indicates the current value. Furthermore, in the graphs  36  to  40 , the vertical axis indicates High/Low of the signal. Furthermore, in the graphs  31  to  39 , the broken line indicates the reference value. 
     Furthermore, if the broken line overlaps with the reference value in a graph, in order to easy to understand, the target line indicated by the graph is upwardly shifted from the reference value. For example, in the graph  39 , because the output of the NOR circuit  141  is always Low and overlaps with the reference value, the output line is indicated such that the line does not overlap with the reference value. 
     The reference value of the AC adapter voltage is 0 (V). Furthermore, because the AC adapter  2  is not connected at first, the AC adapter voltage is 0 (V) as indicated by the graph  31 . Then, at a timing  311 , the AC adapter  2 , which is unauthorized, is connected to the AC adapter connector  11 . At this point, it is assumed that the voltage in the rated state of the appropriate AC adapter  2  is 18 to 21 (V). In the following, a description will be given in a case in which the voltage of the connected unauthorized AC adapter  2  is 16 (V). In such a case, after the AC adapter  2  is connected, as indicated by the graph  31 , the AC adapter voltage becomes 16 (V). 
     Furthermore, because the connected AC adapter  2  is an unauthorized adapter, the current does not flow and, as indicated by the graph  32 , the AC adapter current maintains 0 (A). 
     Furthermore, as indicated by the graph  33 , the battery voltage is always 12.6 (V). 
     In this case, as indicated by the graph  41 , the power supply switching threshold is 16.6 (V) and the AC adapter voltage of 16 (V) is lower than the power supply switching threshold. Consequently, as indicated by the graph  36 , the power supply switching signal output from the power supply switching comparator  103  is the Low signal before and after the AC adapter  2  is connected. Accordingly, the FET switches  201  and  202  remain to be turned off. 
     Because the FET switches  201  and  202  are always turned off, the electrical power from the battery  13  is supplied to both the CPU  20  and the miscellaneous system  21  and, furthermore, the battery current maintains, as indicated by the graph  34 , the value of 1.5 (A). 
     In this case, as indicated by the graph  35 , the voltage input to each of the CPU power supply circuit  15  and the miscellaneous system power supply circuit  16  is 12.6 (V) that is the same as the battery voltage. 
     The voltage threshold of the voltage determining comparator  101  is 17 (V) that is greater than the power supply switching threshold. However, because the AC adapter  2  is an unauthorized AC adapter, the comparator  114  is not driven. Consequently, the voltage is increased by the resistor  115  and the output of the voltage determining comparator  101  is always High, as indicated by the graph  37 . 
     Furthermore, it is assumed that the current threshold of the current determining comparator  102  is 1.5 (A). In this case, as indicated by the graph  32 , because the AC adapter current is 0 (A), the output of the current determining comparator  102  is always High as indicated by the graph  38 . 
     The High signal is input to the NOR circuit  141  from both the voltage determining comparator  101  and the current determining comparator  102 . Consequently, as indicated by the graph  39 , the Low signal is maintained for the output of the NOR circuit  141 . 
     Because the output of the NOR circuit  141  is Low, as indicated by the graph  40 , Low is maintained in the electrical power saving mode control signal that is output from the electrical power saving control circuit  104 . Specifically, the CPU  20  maintains the current state without shifting to the electrical power saving mode. 
     In the following, the operation of each of the circuits when the AC adapter  2  is connected to the notebook PC  1  in the state in which the battery  13  is running in the first embodiment will be described with reference to  FIG. 12 .  FIG. 12  is a sequence diagram illustrating the circuit operation performed when an AC adapter is connected in a state in which a battery is running in the first embodiment. In the graphs  31  to  41  illustrated in  FIG. 12 , the vertical axis, the horizontal axis, and the reference value indicate the same as those used in  FIG. 11 . 
     Because the AC adapter  2  is not connected at first, as indicated by the graph  31 , the AC adapter voltage is 0 (V). Then, at a timing  312 , the appropriate AC adapter  2  is connected to the AC adapter connector  11 . Then, because the connected AC adapter  2  is an appropriate adapter, after the AC adapter  2  is connected, as indicated by the graph  31 , the AC adapter voltage becomes 18 to 21 (V). 
     Furthermore, as indicated by the graph  33 , the battery voltage is always 12.6 (V). 
     In this case, as indicated by the graph  41 , because the power supply switching threshold is 16.6 (V), the AC adapter voltage of 18 to 21 (V) is greater than the power supply switching threshold. Consequently, as indicated by the graph  36 , the power supply switching signal output from the power supply switching comparator  103  shifts from Low to High at a timing  361 . Consequently, the state of the FET switches  201  and  202  shifts from OFF to ON at the timing  361 . 
     As indicated by the graph  32 , because the FET switches  201  and  202  are turned on at a timing  321 , the AC adapter current rises up to 2 A at the timing  321  due to the inrush current input to an input capacitor, such as the CPU power supply circuit  15 , of an onboard power supply. Then, the AC adapter current remains at 1 (A) after a timing  322 . 
     As indicated by the graph  34 , because the FET switches  201  and  202  are turned on at a timing  341 , the battery current gradually drops at the timing  341  and then finally drops to 0 (A). 
     In this case, as indicated by the graph  35 , the input voltage to the CPU power supply circuit  15  and the miscellaneous system power supply circuit  16  starts to increase, at a timing  351 , from 12.6 (V), which is the same voltage as the battery voltage, up to 18 to 21 (V), which is the same voltages as the AC adapter voltage. 
     Before the FET switches  201  and  202  are turned on, because the AC adapter  2  is not connected, the comparator  114  is not driven. Consequently, the voltage is increased by the resistor  115  and thus the output of the voltage determining comparator  101  becomes High until a timing  371 , as indicated by the graph  37 . Then, the AC adapter voltage becomes 18 to 21 (V), which is greater than the voltage threshold of 17 (V); therefore, the output of the voltage determining comparator  101  remains High after the timing  371 . 
     In this case, as indicated by the graph  32 , because the AC adapter current is 1 (A) and is less than the current threshold of 1.5 (A), the output of the current determining comparator  102  is always High as indicated by the graph  38 . However, it is assumed that the current determining comparator  102  does not detect an instantaneous rise in the current due to the inrush current. 
     The High signal is input to the NOR circuit  141  from both the voltage determining comparator  101  and the current determining comparator  102 . Consequently, as indicated by the graph  39 , the Low signal is maintained in the output of the NOR circuit  141 . 
     Because the output of the NOR circuit  141  is Low, as indicated by the graph  40 , Low is maintained in the electrical power saving mode control signal that is output from the electrical power saving control circuit  104 . Specifically, the CPU  20  maintains the current state without shifting to the electrical power saving mode. 
     In the following, the operation of each of the circuits when the capacity of the AC adapter  2  exceeds after the mode is shifted to the turbo mode while the AC adapter  2  is being connected to the notebook PC  1  in the first embodiment will be described with reference to  FIG. 13 .  FIG. 13  is a sequence diagram illustrating the circuit operation performed when, in the first embodiment, the capacity of an AC adapter exceeds after a mode is shifted to the turbo mode while the AC adapter is being connected. In the graphs  31  to  41  illustrated in  FIG. 13 , the vertical axis, the horizontal axis, and the reference value indicate the same as those used in  FIG. 11 . 
     Because the AC adapter  2  has already been connected, the AC adapter voltage is 19 (V) at first as indicated by the graph  31 . Furthermore, as indicated by the graph  32 , the AC adapter current is 1 (A) at first. 
     Then, at the timing  322  indicated by the graph  32 , the CPU  20  starts a process that has a high processing load. Because the load applied to the CPU  20  increases, the AC adapter current starts to increase at the timing  322 . Then, as indicated by the graph  38 , if the AC adapter current increases and exceeds the current threshold, the output of the current determining comparator  102  shifts from the High to Low at a timing  381 . 
     Because the processing load is high, the CPU  20  shifts to the turbo mode. The AC adapter current further increases at a timing  323  at which the mode is shifted to the turbo mode. Then, if the impedance of the CPU  20  and the miscellaneous system  21  falls below the predetermined value, the protection circuit  23  determines that the capacity of the AC adapter  2  exceeds and then drops the AC adapter voltage. Consequently, as indicated by the graph  31 , the AC adapter voltage drops at a timing  313 . In this case, as indicated by the graph  35 , the input voltage to each of the CPU power supply circuit  15  and the miscellaneous system power supply circuit  16  also drops at a timing  352 . 
     Then, as indicated by the graph  37 , at a timing  372  at which the AC adapter voltage drops and falls below the voltage threshold, the output of the voltage determining comparator  101  shifts from High to Low. At this point, it is assumed that the AC adapter voltage drops to 16.8 (V). 
     In this case, as indicated by the graph  41 , the power supply switching threshold is 16.6 (V) and the AC adapter voltage is equal to or greater than the power supply switching threshold. Accordingly, as indicated by the graph  36 , the power supply switching signal remains High. Specifically, the FET switches  201  and  202  remain to be turned on. 
     At this point, the Low signal is input to the NOR circuit  141  from both the voltage determining comparator  101  and the current determining comparator  102 . Accordingly, as indicated by the graph  39 , the output of the NOR circuit  141  is changed from Low to High at a timing  391 . Then, as indicated by the graph  40 , the one-shot circuit  142  in the electrical power saving control circuit  104  outputs a High electrical power saving mode control signal for a certain time period after a timing  401 . In response to this state, the CPU  20  shifts to the electrical power saving mode. 
     Because the mode is shifted to the electrical power saving mode, the impedance of the CPU  20  and the miscellaneous system  21  increases. Consequently, as indicated by the graph  32 , the AC adapter current drops after a timing  324 . Then, the AC adapter current remains at 1 (A). Thereafter, as indicated by the graph  38 , at a timing  382  at which the AC adapter current falls below the current threshold, the output of the current determining comparator  102  shifts from Low to High. 
     When the impedance of the CPU  20  and the miscellaneous system  21  exceeds the threshold, the protection circuit  23  releases the drop in the AC adapter voltage. Consequently, as indicated by the graph  31 , the AC adapter voltage increases after a timing  314 . In response to this state, as indicated by the graph  35 , the input voltage to the CPU power supply circuit  15  and the miscellaneous system power supply circuit  16  starts to increase after a timing  353 . Then, the AC adapter voltage and the input voltage to the CPU power supply circuit  15  and the miscellaneous system power supply circuit  16  remain at 19 (V). 
     Then, as indicated by the graph  37 , when the AC adapter voltage exceeds the voltage threshold, the output of the voltage determining comparator  101  shifts from Low to High at a timing  373 . 
     In this state, as indicated by the graph  39 , the output of the NOR circuit  141  shifts from High to Low at a timing  392  at which the output of the voltage determining comparator  101  is changed to High. 
     As described above, by shifting the CPU  20  and the miscellaneous system  21  to the electrical power saving mode without immediately interrupting the electrical power supplied from the AC adapter  2  even if the capacity of the AC adapter  2  exceeds, the electrical power supply can be maintained while the electrical power consumption is reduced and the electrical power supply is controlled to be within the capacity of the AC adapter  2 . 
     In the following, the operation of each of the circuits when the AC adapter  2  is disconnected from the notebook PC  1  in the first embodiment will be described with reference to  FIG. 14 .  FIG. 14  is a sequence diagram illustrating the circuit operation performed when the AC adapter is disconnected in the first embodiment. In the graphs  31  to  41  illustrated in  FIG. 14 , the vertical axis, the horizontal axis, and the reference value indicate the same as those used in  FIG. 11 . 
     Because the AC adapter  2  is connected at first, as indicated by the graph  31 , the AC adapter voltage is 19 (V). Then, at a timing  315 , the AC adapter  2  is disconnected from the AC adapter connector  11 . When the AC adapter  2  is disconnected, the AC adapter voltage drops to 0 (V). 
     As indicated by the graph  32 , the AC adapter current is 2 (A) because a process that has a processing load is being performed at first. Then, the AC adapter current becomes 0 (A) at a timing  325  at which the AC adapter  2  is disconnected. 
     At this point, the AC adapter voltage is 0 (V) and is lower than the power supply switching threshold. Consequently, as indicated by the graph  36 , the power supply switching signal that is output from the power supply switching comparator  103  shifts from High to Low at a timing  362 . Consequently, the FET switches  201  and  202  are turned off from the on state at the timing  362 . 
     As indicated by the graph  33 , the battery voltage is always 12.6 (V). 
     After the FET switches  201  and  202  are turned off, because an input capacitor, such as the CPU power supply circuit  15  or the like, of an onboard power supply is discharged, a supply of the electrical power from the battery  13  is delayed. Consequently, as indicated by the graph  34 , the battery current increases up to 3 (A) at a timing  342  that is little later the timing  362  at which the FET switches  201  and  202  are turned off. 
     At this point, as indicated by the graph  35 , the input voltage to the CPU power supply circuit  15  and the miscellaneous system power supply circuit  16  starts to drop, at a timing  354 , from the AC adapter voltage of 19 (V) to 12.6 (V) that is the same voltage as the battery voltage. 
     Furthermore, before the FET switches  201  and  202  are turned off, because the AC adapter voltage is 19 (V) and is equal to or greater than the voltage threshold of 17 (V), as indicated by the graph  37 , the output of the voltage determining comparator  101  becomes High. When the AC adapter is disconnected, the electrical power supply to the comparator  114  stops and thus the comparator  114  stops. Consequently, the voltage is increased by the resistor  115  and, as indicated by the graph  37 , the output of the voltage determining comparator  101  becomes High. 
     Furthermore, before the FET switches  201  and  202  are turned off, because the AC adapter current is 2 (A) and thus is equal to or greater than the current threshold of 1.5 (A), as indicated by the graph  38 , the output of the current determining comparator  102  becomes Low. Thereafter, as indicated by the graph  38 , when the AC adapter  2  is disconnected, the AC adapter current becomes 0 (A) and thus the output of the current determining comparator  102  shifts from Low to High at a timing  383 . 
     The High signal is input to the NOR circuit  141  from the voltage determining comparator  101 . Consequently, as indicated by the graph  39 , the Low signal is maintained for the output from the NOR circuit  141 . 
     Because the output of the NOR circuit  141  is Low, as indicated by the graph  40 , the electrical power saving mode control signal that is output from the electrical power saving control circuit  104  is maintained at Low. Specifically, the CPU  20  maintains the current state without shifting to the electrical power saving mode. 
     As described above, the power supply monitoring circuit according to the first embodiment shifts the CPU  20  and the miscellaneous system  21  in an information processing apparatus to the electrical power saving mode without immediately interrupting the electrical power supply from the AC adapter even when the supplied electrical power exceeds the capacity of the AC adapter. Consequently, when an electrical power supply exceeds the capacity of the AC adapter, the electrical power supply from the AC adapter can be maintained while the electrical power consumed by the CPU  20  and the miscellaneous system  21  is reduced and the electrical power supply is controlled to be within the capacity of the AC adapter. 
     Furthermore, if an unauthorized AC adapter is connected, the power supply monitoring circuit according to the first embodiment detects this state and thus does not allow a supply of electrical power from the AC adapter. Consequently, the reliability of the information processing apparatus can be maintained. 
     Accordingly, an external power supply can be efficiently used up to the upper limit of the capacity of the electrical power supply while the reliability is maintained. 
     [b] Second Embodiment 
     In the following, an information processing apparatus according to a second embodiment will be described. The information processing apparatus according to the second embodiment differs from the first embodiment in that a power supply switching threshold is changed. A notebook PC that is the information processing apparatus according to the second embodiment is also illustrated in the block diagram in  FIG. 1 . In a description below, descriptions of the units having the same functions as those performed by the units in the first embodiment will be omitted. 
       FIG. 15  is a circuit diagram illustrating a power supply switching comparator according to a second embodiment. In the power supply switching comparator  103  according to the second embodiment, a resistor  135  and a FET switch  136  are added to the power supply switching comparator  103  according to the first embodiment. 
     The resistor  135  is arranged on the output side of the resistor  132 . Furthermore, the end portion on the output side of the resistor  135  is connected to ground. 
     Furthermore, the FET switch  136  is arranged on a path that is arranged to bypass both ends of the resistor  135 . Then, the FET switch  136  receives an input of the current determining comparator  102 . When the input of the current determining comparator  102  is the High signal, the FET switch  136  is turned on. Furthermore, when the input of the current determining comparator  102  is the Low signal, the FET switch  136  is turned off. 
     When the FET switch  136  is turned on, the comparator  134  receives an input of a signal divided by the resistors  131  and  132 . Furthermore, when the FET switch  136  is turned off, the comparator  134  receives a signal divided by the resistors  131 ,  132 , and  135 . 
     Here, it is assume that the resistor  131  is R1 (Ω), the resistor  132  is R2 (Ω), the resistor  135  is R3 (Ω), and the AC adapter voltage is V1 (V). In such a case, when the FET switch  136  is turned on, the comparator  134  receives the input of the signal with the voltage of {R2/(R1+R2)}×V1 (V). In contrast, when the FET switch  136  is turned off, the comparator  134  receives an input of the signal with the voltage of {(R2+R3)/(R1+R2+R3)}×V1 (V). 
     Specifically, when the FET switch  136  is turned off, the comparator  134  receives an input of a signal with a voltage that is higher than that received when the FET switch  136  is turned on. 
     At this point, if the AC adapter current is equal to or greater than the current threshold, because the output of the current determining comparator becomes Low, the FET switch  136  is turned off. Furthermore, if the AC adapter current is smaller than the current threshold, because the output from the current determining comparator  102  becomes High, the FET switch  136  is turned on. 
     Specifically, when the AC adapter current is equal to or greater than the current threshold, the comparator  134  receives an input of a signal with a voltage that is higher than that received when the AC adapter current is smaller than the current threshold. 
     The comparator  134  compares the voltage of the divided signal with the reference voltage. If the voltage of the divided signal is equal to or greater than the reference voltage, the comparator  134  outputs the High signal as the power supply switching signal to the FET switches  201  and  202 . Furthermore, if the voltage of the divided signal is less than the reference voltage, the comparator  134  outputs the Low signal as the power supply switching signal to the FET switches  201  and  202 . 
     Then, if the AC adapter current is equal to or greater than the current threshold, because the comparator  134  receives an input of a voltage that is higher than that input when the AC adapter current is smaller than the current threshold, it is more difficult for the comparator  134  to output the Low signal when the AC adapter current is equal to or greater than the current threshold compared with a case in which the AC adapter current is smaller than the current threshold. Specifically, the power supply switching threshold used when the AC adapter current is equal to or greater than the current threshold is lower than the power supply switching threshold used when the AC adapter current is smaller than the current threshold. 
     For example, when the current threshold is set to the current value that is obtained when a process with a predetermined amount processing load is performed, the state in which the AC adapter current is equal to or greater than the current threshold indicates the state in which the CPU  20  performs a process with a predetermined amount of processing load in response to receiving an electrical power supply from the AC adapter  2 . Specifically, if the AC adapter  2 , which is an appropriate adapter, is connected and if a process with a certain amount of high processing load is performed, the power supply switching threshold needs to be decreased. 
     The reason for this is as follows. Namely, because the AC adapter  2  is appropriate, an unauthorized AC adapter does not need to be detected. However, because the processing load of the CPU  20  or the miscellaneous system  21  is high, there is a high possibility that the mode has been shifted to the turbo mode. Accordingly, without interrupting the electrical power supply from the AC adapter  2  as much as possible, the electrical power supplied from the AC adapter  2  needs to be reduced, with priority, within the capacity of the AC adapter  2  by shifting the mode to the electrical power saving mode. Hereinafter, the power supply switching threshold used when the AC adapter current is equal to or greater than the current threshold is referred to as an “electrical power saving priority threshold”. Furthermore, the power supply switching threshold used when the AC adapter current is smaller than the current threshold is referred to as a “safety priority threshold”. The electrical power saving priority threshold mentioned here corresponds to an example of a “first threshold”. Furthermore, the safety priority threshold mentioned here corresponds to an example of a “second threshold”. 
     However, the electrical power saving priority threshold preferably be a value smaller than the voltage determining threshold. The reason for this is to avoid, before the mode is shifted to the electrical power saving mode, an interruption of a supply of electrical power from the AC adapter  2 . Furthermore, the safety priority threshold is preferably a value greater than the voltage determining threshold. The reason for this is to interrupt, when the unauthorized AC adapter  2  is connected, a supply of electrical power from the AC adapter  2  before the mode is shifted to the electrical power saving mode. 
     The output from the power supply switching comparator  103  described above can be schematically represented by the diagram illustrated in  FIG. 16 .  FIG. 16  is a schematic diagram illustrating an output from a power supply switching comparator according to the second embodiment. In the column entitled “output of the current determining comparator  102 ” and “power supply switching signal” illustrated in  FIG. 16 , symbol “H” indicates an output of a High power supply switching signal and symbol “L” indicates an output of a Low power supply switching signal. 
     When the AC adapter current is I and the AC adapter voltage is V, if I is equal to or greater than the current threshold, the current determining comparator  102  outputs the Low signal and the FET switch  136  is turned off. In this case, the power supply switching threshold to be used is the electrical power saving priority threshold. Accordingly, if V is equal to or greater than the electrical power saving priority threshold, the power supply switching comparator  103  outputs the High power supply switching signal. In this case, the FET switches  201  and  202  are turned on (ON). Furthermore, if V is lower than the electrical power saving priority threshold, the power supply switching comparator  103  outputs a Low power supply switching signal. In this case, the FET switches  201  and  202  are turned off (OFF). 
     In contrast, if I is smaller than the current threshold, the current determining comparator  102  outputs the High signal and the FET switch  136  is turned on. In this case, the power supply switching threshold to be used is the safety priority threshold. Consequently, if V is equal to or greater than the safety priority threshold, the power supply switching comparator  103  outputs the High power supply switching signal. In this case, the FET switches  201  and  202  are turned on (ON). Furthermore, if V is less than the safety priority threshold, the power supply switching comparator  103  outputs a Low power supply switching signal. In this case, the FET switches  201  and  202  are turned off (OFF). 
     In the following, a description will be given of the circuit operation in accordance with the state of the AC adapter. Here, a description will be given of a case in which, between the power supply switching thresholds, the safety priority threshold is 17.6 (V) and the electrical power saving priority threshold is 16.6 (V). 
     First, a description will be given of a case in which the AC adapter  2 , which is unauthorized adapter with a low output voltage, is connected to the AC adapter connector  11 . In this case, the operation is the same as that performed in the first embodiment described with reference to  FIG. 11 . However, because the output of the current determining comparator  102  is High, the FET switch  136  is turned on and the power supply switching threshold becomes 17.6 (V) that is the safety priority threshold. Specifically, the reference for determining whether the unauthorized AC adapter  2  is connected becomes stricter than that used in the first embodiment. In this case, because the threshold is greater than that used in the first embodiment, similarly to the first embodiment, the AC adapter voltage is lower than the safety priority threshold. Consequently, the power supply switching comparator  103  remains to turn off the FET switches  201  and  202 . Accordingly, the CPU  20  and the miscellaneous system  21  continue to receive the electrical power supply from the battery  13 . 
     In the following, a description will be given of a case in which the AC adapter  2  is connected in the state in which the battery  13  is running. In this case, the operation is the same as that performed in the first embodiment described with reference to  FIG. 12 . However, also in this case, because the output of the current determining comparator  102  is High, the FET switch  136  is turned on and the power supply switching threshold becomes 17.6 (V) that is the safety priority threshold. Specifically, the reference for determining whether the unauthorized AC adapter is connected becomes stricter than that used in the first embodiment. 
     In the following, the operation of each of the circuits when the capacity of the AC adapter  2  exceeds after the mode is shifted to the turbo mode while the AC adapter  2  is being connected to the notebook PC  1  in the second embodiment will be described with reference to  FIG. 17 .  FIG. 17  is a sequence diagram illustrating the circuit operation performed when, in the second embodiment, the capacity of the AC adapter exceeds after a mode is shifted to the turbo mode while the AC adapter is being connected. In the graphs  31  to  41  illustrated in  FIG. 17 , the vertical axis, the horizontal axis, and the reference value indicate the same as those used in  FIG. 11 . 
     In this case, the circuit operations indicated by the graphs  31  to  40  are the same as those illustrated in  FIG. 13 . Accordingly, a description will be given of a shift of the power supply switching threshold indicated by the graph  41 . 
     Because the AC adapter  2  has already been connected, as indicated by the graph  31 , the AC adapter voltage is 19 (V) at first. Furthermore, as indicated by the graph  32 , the AC adapter current is 1 (A) at first. In this case, the AC adapter current does not exceed the current threshold and, as indicated by the graph  38 , the output of the current determining comparator  102  is High. In this case, as indicated by the graph  41 , the FET switch  136  is turned on and the power supply switching threshold is 17.6 (V) that is the safety priority threshold. 
     Then, the CPU  20  starts to perform a process having a high load. Because the load applied to the CPU  20  becomes high, the AC adapter current starts to increase. Then, as indicated by the graph  38 , when the AC adapter current increases and exceeds the current threshold, the output of the current determining comparator  102  shifts from High to Low. In response to this state, as indicated by the graph  41 , the power supply switching threshold is changed to 16.6(V), at a timing  411 , that is the electrical power saving priority threshold. 
     Thereafter, the Low signal is input to the NOR circuit  141  from both the voltage determining comparator  101  and the current determining comparator  102 . Thus, as indicated by the graph  39 , the output of the NOR circuit  141  shifts from Low to High. Then, as indicated by the graph  40 , the one-shot circuit  142  in the electrical power saving control circuit  104  outputs a High electrical power saving mode control signal for a certain time period after the High signal is received from the NOR circuit  141 . In response to this state, the CPU  20  shifts to the electrical power saving mode. 
     Then, because the CPU  20  shifts to the electrical power saving mode, the impedance of the CPU  20  and the miscellaneous system  21  increase. Accordingly, as indicated by the graph  32 , the AC adapter current drops. Thereafter, the AC adapter current remains at 1 A. Then, at a timing at which the AC adapter current falls below the current threshold, as indicated by the graph  38 , the output of the current determining comparator  102  shifts from Low to High. In response to this state, as indicated by the graph  41 , at a timing  412 , the power supply switching threshold is reset to 17.6 (V) that is the safety priority threshold. 
     In the following, the operation of each of the circuits when the AC adapter  2  is disconnected from the notebook PC  1  in the second embodiment will be described with reference to  FIG. 18 .  FIG. 18  is a sequence diagram illustrating the circuit operation performed when the AC adapter is disconnected in the second embodiment. In the graphs  31  to  41  illustrated in  FIG. 18 , the vertical axis, the horizontal axis, and the reference value indicate the same as those used in  FIG. 11 . In this case, a description will be given of a case in which the CPU  20  and the miscellaneous system  21  each performs a process that has a high load and the AC adapter current exceeds the current threshold. 
     As indicated by the graph  32 , the AC adapter current is 2 (A) at first because a process that has a high processing load is being performed. In this case, as indicated by the graph  38 , the AC adapter current exceeds the current threshold of 1.5 (A), the output of the current determining comparator  102  is Low. Consequently, the FET switch  136  is turned off and, as indicated by the graph  41 , the power supply switching threshold is 16.6 (V) that is the electrical power saving priority threshold. 
     Then, the AC adapter current becomes 0 (A) at the timing at which the AC adapter  2  is disconnected. In this case, because the AC adapter current falls below the current threshold of 1.5 (A), as indicated by the graph  38 , the output from the current determining comparator  102  shifts from Low to High. Consequently, the FET switch  136  is turned on and, as indicated by the graph  41 , the power supply switching threshold is changed to 17.6 (V) that is the safety priority threshold at a timing  413 . 
     As described above, the power supply monitoring circuit according to the second embodiment determines, by using electrical power saving priority threshold when the AC adapter current is equal to or greater than the current threshold, whether electrical power source needs to be shifted and determines, by using the safety priority threshold when the AC adapter current is less than the current threshold, whether the electrical power source needs to be shifted. For example, it is assumed that the current threshold is set to a current of a process with a predetermined amount of load is being performed. In such a case, the safety priority threshold is used when the CPU  20  and the miscellaneous system  21  perform or do not perform a process with the load equal to or less than the predetermined amount. Consequently, a connection of an unauthorized AC adapter can be more reliably detected. Thus, the reliability of the information processing apparatus can be maintained. 
     Furthermore, when the CPU  20  and the miscellaneous system  21  perform the process with the load equal to or greater than the predetermined amount, the information processing apparatus is shifted to the electrical power saving mode in order to avoid a risk while continuing an electrical power supply from the AC adapter by using the electrical power saving priority threshold. Consequently, the AC adapter can be used up to the limit of the capacity of the electrical power supply. 
     Accordingly, the power supply monitoring circuit according to the second embodiment can efficiently use an external power supply up to the limit of the capacity of the electrical power supply while surely maintaining the reliability. 
     (Hardware Configuration) 
     In the following, an example of the hardware configuration of the notebook PC  1  will be described with reference to  FIG. 19 .  FIG. 19  is a block diagram illustrating an example of the hardware configuration of a notebook PC. 
     The notebook PC  1  includes a CPU  20 , a memory  901 , a liquid crystal panel  902 , an external monitor connector  903 , a chip set  904 , a built-in hard disk  905 , an optical disk drive  906 , and a keyboard controller/keyboard  907 . Furthermore, the notebook PC  1  includes a glide point  908 , an audio codec/speaker  909 , a basic input output system (BIOS) read only memory (ROM)  910 , and a universal system bus (USB) connector  911 . Furthermore, the notebook PC  1  includes a power supply circuit  912  and a battery  13 . 
     The CPU  20  is connected to the memory  901 , the liquid crystal panel  902 , the external monitor connector  903 , and the chip set  904  via a bus. An external monitor  913  may also be connected to the external monitor connector  903 . 
     The built-in hard disk  905 , the optical disk drive  906 , the keyboard controller/keyboard  907 , the glide point  908 , the audio codec/speaker  909 , the BIOS ROM  910 , and the USB connector  911  are connected to the chip set  904 . An external storage device/mouse  914  or the like may also be connected to the USB connector  911 . 
     Each of the devices connected to the memory  901 , the liquid crystal panel  902 , and the chip set  904  is an example of the miscellaneous system  21  illustrated in  FIG. 1 . 
     The power supply circuit  912  includes each of the circuits other than the CPU  20 , the miscellaneous system  21 , and the battery  13  illustrated in  FIG. 1 . For example, the power supply circuit  912  includes the AC adapter identifying circuit  10 , the AC adapter connector  11 , the battery connector  12 , the boot up power supply circuit  14 , the CPU power supply circuit  15 , and the miscellaneous system power supply circuit  16 . Furthermore, the power supply circuit  912  includes the AC adapter line  17 , the AC adapter current sensing resistor  18 , the battery rectifier diode  19 , the FET switches  201  and  202 , and the like. The power supply circuit  912  supplies electrical power to each of the units enclosed by the broken line  930 . Furthermore, the power supply circuit  912  sends an electrical power saving mode switching signal to the CPU  20 . 
     According to an aspect of an embodiment of the information processing apparatus and the power supply monitoring circuit disclosed in the present invention, an advantage is provided in that the capacity of an electrical power supply of external connection power supply can be sufficiently used while reliability is maintained. 
     All examples and conditional language recited herein are intended for pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.