Abstract:
Provided is a power supply switching circuit capable of efficiently supplying a desired voltage among a plurality of voltages to a load. In the case of a P-type semiconductor substrate, N-type MOS transistors are provided between a load and an AC adapter and between the load and a battery, and hence no parasitic diode exists between the load and the AC adapter or the battery, resulting in no current path due to the parasitic diode. Thus, when the AC adapter and the battery are connected to the power supply switching circuit, the N-type MOS transistor is turned off, whereby the current path between the battery and the load is cut off completely and the N-type MOS transistor is turned on. Accordingly, the battery cannot supply a voltage to the load while only the AC adapter can supply a voltage to the load.

Description:
RELATED APPLICATIONS 
   This application claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. JP2007-335808 filed on Dec. 27, 2007, the entire content of which is hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a power supply switching circuit for switching power supply voltages to be supplied to a load. 
   2. Description of the Related Art 
   Portable electronic devices and the like are designed to operate with a plurality of power supplies like an AC adapter and a battery. In general, a voltage of the AC adapter is higher than a voltage of the battery. When the AC adapter is connected to a rechargeable electronic device, a power supply voltage is supplied from the AC adapter to a load. In this case, a rectifier diode disposed between the power supply and the load prevents a current from flowing from the AC adapter to the battery. 
   However, the above-mentioned circuit may cause a power loss because the voltage supplied from the battery to the load is dropped by a forward voltage drop of the rectifier diode. 
   In order to solve the problem described above, a power supply switching circuit illustrated in  FIG. 4  is proposed. The power supply switching circuit of  FIG. 4  includes a P-type MOS transistor instead of the rectifier diode. More specifically, a P-type MOS transistor M 1  is disposed between an AC adapter  23  and a load  28 , and a P-type MOS transistor M 2  is disposed between a battery  20  and the load  28  (see, for example, Japanese Patent Application Laid-open No. 2006-254672). 
   However, the conventional power supply switching circuit illustrated in  FIG. 4  has a problem that if a semiconductor substrate is of P type, a higher voltage between a voltage of the AC adapter  23  and a voltage of the battery  20  may be supplied to the load  28  because of a parasitic diode existing in the P-type MOS transistor. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in view of the above-mentioned problem, and it is an object thereof to provide a power supply switching circuit capable of effectively supplying a desired voltage among a plurality of voltages to a load. 
   In order to solve the above-mentioned problem, the present invention provides a power supply switching circuit including: a first power source terminal to which a first power supply is connected; a second power source terminal to which a second power supply is connected; a first MOS transistor of a second conductive type which is formed on a semiconductor substrate of a first conductive type, and includes a drain connected to the first power source terminal and a source connected to a load terminal; a second MOS transistor of the second conductive type which is formed on the semiconductor substrate of the first conductive type, and includes a drain connected to the second power source terminal and a source connected to the load terminal; a first power supply detection circuit for detecting that the first power supply is connected to the first power source terminal, and delivering a detection signal; a control circuit for delivering, upon receiving the detection signal, a control signal to a gate of the first MOS transistor and a gate of the second MOS transistor; a step-up circuit for stepping up one of a voltage of the first power supply and a voltage of the second power supply, to deliver a stepped-up voltage; a first level shifter which is disposed between the control circuit and the gate of the first MOS transistor and performs a level shift operation on the control signal to the stepped-up voltage; and a second level shifter which is disposed between the control circuit and the gate of the second MOS transistor and performs a level shift operation on the control signal to the stepped-up voltage. 
   According to the power supply switching circuit of the present invention, in the case of the first conductive type semiconductor substrate, the second conductive type MOS transistor is disposed as a switch circuit between the load and one of the first power supply and the second power supply. Therefore, no parasitic diode exists between the load and one of the first power supply and the second power supply, whereby there exists no current path due to the parasitic diode. 
   Therefore, the power supply switching circuit of the present invention can supply the load effectively with a desired voltage among a plurality of voltages. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
       FIG. 1  is a diagram illustrating a power supply switching circuit according to a first embodiment of the present invention; 
       FIG. 2  is a diagram illustrating a power supply switching circuit according to a second embodiment of the present invention; 
       FIG. 3  is a diagram illustrating a power supply switching circuit according to a third embodiment of the present invention; and 
       FIG. 4  is a diagram illustrating a conventional power supply switching circuit. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, with reference to the attached drawings, embodiments of the present invention are described. 
   First Embodiment 
   In the first place, a structure of a power supply switching circuit according to a first embodiment of the present invention is described.  FIG. 1  is a diagram illustrating the power supply switching circuit of the first embodiment of the present invention. 
   Here, a power supply switching circuit  101  is formed on a semiconductor substrate of P type as a first conductive type. 
   The power supply switching circuit  101  includes N-type MOS transistors  106  and  107 , level shifters  108  and  109 , diodes  110  and  111 , an AC adapter detection circuit  112 , a control circuit  113 , and a step-up circuit  114 . The power supply switching circuit  101  is connected to an AC adapter  102 , a battery  103 , a capacitor  104 , and a load  105 , for example, as a peripheral circuit. 
   A first power source terminal for connecting to the AC adapter  102  is connected to a drain of the N-type MOS transistor  106 , an anode of the diode  110  and an input terminal of the AC adapter detection circuit  112 . A second power source terminal for connecting to the battery  103  is connected to a drain of the N-type MOS transistor  107  and an anode of the diode  111 . A gate of the N-type MOS transistor  106  is connected to an output terminal of the level shifter  108 , and a source thereof is connected to a load terminal. A gate of the N-type MOS transistor  107  is connected to an output terminal of the level shifter  109 , and a source thereof is connected to the load terminal for connecting to the load  105 . An output terminal of the AC adapter detection circuit  112  is connected to a first input terminal of the control circuit  113 . Each of cathodes of the diodes  110  and  111  is connected to an input terminal of the step-up circuit  114  and a power source terminal of the control circuit  113 . An output terminal of the step-up circuit  114  is connected to the capacitor  104 . A power source terminal of the level shifter  108  is connected to an output terminal of the step-up circuit  114 , and an input terminal thereof is connected to a first output terminal of the control circuit  113 . A power source terminal of the level shifter  109  is connected to the output terminal of the step-up circuit  114 , and an input terminal thereof is connected to a second output terminal of the control circuit  113 . 
   Next, an operation of the power supply switching circuit of the first embodiment is described. 
   &lt;The case where the AC adapter  102  and the battery  103  are connected to the power supply switching circuit  101 &gt; 
   When the AC adapter detection circuit  112  detects that the AC adapter  102  is connected to the power supply switching circuit  101 , the AC adapter detection circuit  112  delivers a detection signal to the control circuit  113 . Then, the control circuit  113  delivers a high level signal to the level shifter  108  and delivers a low level signal to the level shifter  109 . The level shifter  108  performs a level shift operation on the high level signal based on a stepped-up voltage and charges accumulated in the capacitor  104 , and delivers the level-shifted high level signal to the gate of the N-type MOS transistor  106 . Therefore, the N-type MOS transistor  106  is turned on, whereby a voltage of the AC adapter  102  is supplied to the load  105 . The level shifter  109  delivers the low level signal to the gate of the N-type MOS transistor  107 . Therefore, the N-type MOS transistor  107  is turned off, and hence a voltage of the battery  103  is not supplied to the load  105 . 
   A higher voltage between the voltage of the AC adapter  102  and the voltage of the battery  103  is supplied to the step-up circuit  114  via the diode  110  or the diode  111 . Here, a current due to the supplied voltage does not flow into the AC adapter  102  or the battery  103  because of the diode  110  or the diode  111 . The step-up circuit  114  performs a step up operation based on the supplied voltage, and delivers the stepped-up voltage to the level shifters  108  and  109 . In addition, the step-up circuit  114  delivers the stepped-up voltage to the capacitor  104 , whereby the charges are accumulated in the capacitor  104 . 
   &lt;The case where only the battery  103  is connected to the power supply switching circuit  101 &gt; 
   When the AC adapter detection circuit  112  detects that the AC adapter  102  is not being connected to the power supply switching circuit  101 , the AC adapter detection circuit  112  delivers a detection signal to the control circuit  113 . Then, the control circuit  113  delivers the low level signal to the level shifter  108  and delivers the high level signal to the level shifter  109 . The level shifter  108  delivers the low level signal to the gate of the N-type MOS transistor  106 . Therefore, the N-type MOS transistor  106  is turned off. The level shifter  109  performs the level shift operation on the high level signal based on the stepped-up voltage and the charges accumulated in the capacitor  104 , and delivers the level-shifted high level signal to the gate of the N-type MOS transistor  107 . Therefore, the N-type MOS transistor  107  is turned on, whereby the voltage of the battery  103  is supplied to the load  105 . 
   The voltage of the battery  103  is supplied to the step-up circuit  114  via the diode  111 . Here, a current due to the supplied voltage does not flow into the first power source terminal because of the diode  110 . The step-up circuit  114  performs the step up operation based on the supplied voltage, so that the stepped-up voltage is delivered to the level shifters  108  to  109 . In addition, the step-up circuit  114  delivers the stepped-up voltage to the capacitor  104 , whereby the charges are accumulated in the capacitor  104 . 
   Here, the control circuit  113  is designed to have a delay time after the N-type MOS transistor  106  is turned off until the N-type MOS transistor  107  is turned on, and a delay time after the N-type MOS transistor  107  turned off until the N-type MOS transistor  106  is turned on. Therefore, the N-type MOS transistors  106  and  107  are not turned on at the same time, and hence the AC adapter  102  and the battery  103  are not short-circuited, whereby the AC adapter  102  does not charge the battery  103 , for instance. In addition, the step-up circuit  114  stops the step up operation when the stepped-up voltage becomes a predetermined voltage, and hence a power loss is decreased. 
   In addition, the level-shifted high level signal in the level shifter  108  and the N-type MOS transistor  106  has a voltage higher than a voltage determined by adding a threshold voltage of the N-type MOS transistor  106  to the voltage of the AC adapter  102 . Therefore, the gate voltage of the N-type MOS transistor  106  corresponds to the level-shifted high level signal, and hence a voltage between the gate and the source of the N-type MOS transistor  106  is secured even when the N-type MOS transistor  106  is turned on so that the source voltage of the N-type MOS transistor  106  becomes substantially equal to the drain voltage thereof. Therefore, a voltage drop is hardly generated between the source and the drain of the N-type MOS transistor  106 , and thus the voltage of the AC adapter  102  can be supplied to the load  105  with little voltage drop. Thus, the power loss is reduced. The same is true for the level shifter  109  and the N-type MOS transistor  107 . 
   According to the power supply switching circuit of the first embodiment, in the case of the P-type semiconductor substrate, the N-type MOS transistor is disposed between the load  105  and the AC adapter  102  or the battery  103 . Therefore, no parasitic diode exists between the load  105  and the AC adapter  102  or the battery  103 , whereby there exists no current path due to the parasitic diode. Therefore, if the AC adapter  102  and the battery  103  are connected to the power supply switching circuit  101 , the N-type MOS transistor  107  is turned off while the N-type MOS transistor  106  is turned on. Thus, only the AC adapter  102  can supply the voltage to the load  105 . 
   In addition, there exists no current path due to a parasitic diode. Therefore, a parasitic bipolar transistor is prevented from being turned on by a current flowing in a parasitic diode, whereby a power loss due to the parasitic bipolar transistor can be reduced. 
   In addition, when the N-type MOS transistor  106  or the N-type MOS transistor  107  is turned on, even if a large parasitic capacitance exists in the gate of the N-type MOS transistor  106  or  107 , the parasitic capacitance is charged by the charges accumulated in the capacitor  104 . Therefore, the stepped-up voltage delivered from the step-up circuit  114  is hardly dropped due to the parasitic capacitance. 
   Note that the power supply switching circuit  101  may be formed on an N-type substrate and the N-type MOS transistors  106  and  107  may be replaced with P-type MOS transistors. 
   In addition, if drive capabilities of the N-type MOS transistors  106  and  107  increase, ON resistance values of the N-type MOS transistors  106  and  107  are decreased, with the result that the power losses in the N-type MOS transistors  106  and  107  are reduced. 
   In addition, the capacitor  104  may be disposed inside a semiconductor device on which the power supply switching circuit  101  is mounted or may be disposed outside the semiconductor device. 
   Second Embodiment 
   Next, a power supply switching circuit  101  according to a second embodiment of the present invention is described. Here, as a condition of a peripheral device, it is supposed that the load  105  includes a capacitor (not shown) for stabilizing power supply, and that the voltage of the AC adapter  102  is higher than the voltage of the battery  103 . 
     FIG. 2  is a diagram illustrating the power supply switching circuit of the second embodiment. The power supply switching circuit of the second embodiment includes a comparator  215  in addition to the power supply switching circuit of the first embodiment. 
   A power source terminal of the comparator  215  is connected to the cathodes of the diodes  110  and  111 , a noninverting input terminal thereof is connected to the drain of the N-type MOS transistor  107 , an inverting input terminal thereof is connected to the source of the N-type MOS transistor  107 , and an output terminal thereof is connected to a second input terminal of the control circuit  113 . 
   Next, an operation of the power supply switching circuit of the second embodiment is described. 
   &lt;The case where only the battery  103  is connected to the power supply switching circuit  101 &gt; 
   The comparator  215  compares the voltage of the battery  103  (first power source terminal voltage) with a voltage to be supplied to the load  105  (load terminal voltage). Then, if the load terminal voltage is higher than or equal to the first power source terminal voltage, the comparator  215  controls the control circuit  113  to turn off the N-type MOS transistor  107 . In addition, if the load terminal voltage is lower than the first power source terminal voltage, the comparator  215  controls the control circuit  113  to turn on the N-type MOS transistor  107 . 
   Here, when the AC adapter  102  is detached from the power supply switching circuit  101 , the voltage of the load  105  is the voltage of the AC adapter  102  due to the capacitor for stabilizing power supply. In other words, the voltage of the load  105  is higher than the voltage of the battery  103 . On this occasion, according to the power supply switching circuit of the second embodiment described above, the comparator  215  controls the control circuit  113  to turn off the N-type MOS transistor  107 . Therefore, a current does not flow back from the load  105  to the battery  103 . 
   Third Embodiment 
   Next, a structure of a power supply switching circuit  101  according to a third embodiment of the present invention is described.  FIG. 3  is a diagram illustrating the power supply switching circuit of the third embodiment. 
   The power supply switching circuit of the third embodiment includes a pull-down circuit  316  in addition to the power supply switching circuit of the first embodiment. 
   A power source terminal of the pull-down circuit  316  is connected to the first power source terminal to which the AC adapter  102  is connected, and an output terminal thereof is connected to a second input terminal of the control circuit  113 . 
   Next, an operation of the power supply switching circuit of the third embodiment is described. 
   &lt;The case where only the battery  103  is connected to the power supply switching circuit  101 &gt; 
   The first power source terminal for connecting to the AC adapter  102  is opened, and hence it is easily affected by noise or a leak current. Therefore, a detection error may occur in the AC adapter detection circuit  112 . 
   However, according to the power supply switching circuit of the third embodiment, even if noise or a leak current occurs at the first power source terminal, the AC adapter detection circuit  112  can operate stably without a detection error because the first power source terminal is pulled down to a ground potential via the pull-down circuit  316 . 
   Note that even if the AC adapter  102  is connected to the power supply switching circuit  101 , a discharge capability of the pull-down circuit  316  is sufficiently lower than a power supply capability of the AC adapter  102 . Therefore, the voltage of the AC adapter  102  is sufficiently supplied to the load  105 . 
   In addition, it is possible to adopt a structure in which the pull-down circuit  316  is provided with a switch circuit so that the pull-down circuit  316  is connected to the power supply switching circuit  101  when the AC adapter  102  is detached therefrom. According to this structure, when the AC adapter  102  is connected to the power supply switching circuit  101 , the pull-down circuit  316  is disconnected, whereby a power loss due to the pull-down circuit  316  can be eliminated.