Power supply control circuit

A power supply control circuit is disclosed. The power supply control circuit controls a current which is supplied from a power source to a load. The power supply control circuit includes a reverse current detecting circuit which detects a reverse current flowing from the load to the power source and a reverse current preventing circuit which disconnects a line between the power source and the load when the reverse current is detected by the reverse current detecting circuit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a power supply control circuit for controlling a current which is supplied from a power source to a load by controlling a transistor disposed between the power source and the load.

2. Description of the Related Art

Recently, many mobile electronic instruments have been used, and the mobile electronic instrument is driven by a secondary battery such as a lithium-ion battery. The secondary battery installed in the mobile electronic instrument is charged from a DC (direct current) power source by connecting to the DC power source via an AC adaptor or a USB port.

In order to control a charging current, a charging control circuit is disclosed. The charging control circuit charges a secondary battery by controlling a current which is supplied from a DC power source to the secondary battery by being connected between the DC power source and the secondary battery such as the lithium-ion battery (refer to Patent Documents 1 and 2).

FIG. 2is a circuit diagram including a charging control circuit.

InFIG. 2, a DC voltage is applied to a charging control circuit11from a DC power source12. The DC voltage is smoothened by a capacitor C1and is applied to the source of a control transistor M1. The control transistor M1is a p channel MOS transistor and its drain is connected to a secondary battery13via a current detecting resistor Rs. A starting rheostat R1is connected between the source and the gate of the control transistor M1. A capacitor C2is connected to the secondary battery13in parallel and smoothens a voltage applied to the secondary battery13.

The control transistor M1controls a current which is supplied to the secondary battery13from the DC power source12by being controlled corresponding to a control signal from a control circuit21. A voltage at both the ends of the current detecting resistor Rs is applied to the control circuit21. The control circuit21controls the control transistor M1so that a voltage applied to the secondary battery13becomes a constant voltage or a current applied to the secondary battery13becomes a constant current.

Between the drain and the back gate of the control transistor M1, a body diode D1is formed as a plastic diode so that a current from the secondary battery13to the DC power source12becomes a forward current. Therefore, when the voltage of the secondary battery13becomes larger than that of the DC power source12, a current flows reversely from the secondary battery13to the DC power source12via the body diode D1.

In order to prevent the reverse current, a diode (not shown) is disposed in series between the DC power source12and the secondary battery13so that a current from the DC power source12to the secondary battery13becomes a forward current (refer to Patent Document 1). Further, in order to prevent the reverse current, a voltage applied to the back gate of the control transistor M1is controlled (refer to Patent Document 2).

However, when the diode is disposed in series between the DC power source12and the secondary battery13, a voltage drop caused by the forward current in the diode occurs. Consequently, the charging efficiency is decreased.

SUMMARY OF THE INVENTION

The present invention provides a power supply control circuit which prevents a reverse current from a load to a power source.

According to one aspect of the present invention, there is provided a power supply control circuit for controlling a current which is supplied from a power source to a load. The power supply control circuit includes a reverse current detecting circuit which detects a reverse current flowing from the load to the power source and a reverse current preventing circuit which disconnects a line between the power source and the load when the reverse current is detected by the reverse current detecting circuit.

According to another aspect of the present invention, the reverse current detecting circuit detects the reverse current based on a difference between a voltage of the power source and a voltage of the load, and detects the reverse current when the voltage of the load is smaller than the voltage of the power source.

According to another aspect of the present invention, the reverse current detecting circuit includes a subtraction circuit which outputs the difference between the voltage of the power source and the voltage of the load, and a comparison circuit which compares the output from the subtraction circuit and a reference voltage and outputs a comparison result.

According to another aspect of the present invention, the reverse current preventing circuit is disposed between the power source and the control transistor and includes a reverse current preventing transistor in which a body diode is formed, and the reverse direction of the body diode is a direction from the control transistor side to the power source side.

According to another aspect of the present invention, the reverse current preventing circuit includes a driving circuit which drives the reverse current preventing transistor by using an output from the reverse current detecting circuit, and a power source is supplied to the driving circuit from a connection point of the reverse current preventing transistor and the control transistor.

According to another aspect of the present invention, the power supply control circuit further includes a control transistor disposed between the power source and the load, and the current which is supplied from the power source to the load is controlled by controlling the control transistor.

According to an embodiment of the present invention, a reverse current flowing from a load to a power source is detected and a line between the power source and the load is disconnected when the reverse current is detected. Therefore, the reverse current which flows from the load to the power source can be prevented.

Other advantages and further features of the present invention will become apparent from the following detailed description when read in connection point with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, an embodiment of the present invention is described.

FIG. 1is a circuit diagram including a power supply control circuit according to the embodiment of the present invention. In the embodiment of the present invention, the detailed description of the same elements described inFIG. 2is omitted.

As shown inFIG. 1, a power supply control circuit101includes a power control IC111, a current detecting resistor Rs, and capacitors C1and C2. The power supply control circuit101is disposed between a DC power source102and a load103.

In the power supply control circuit101, the power control IC111includes a reverse current detecting circuit121and a reverse current preventing circuit122. That is, the power supply control circuit101newly provides the reverse current detecting circuit121and the reverse current preventing circuit122.

The reverse current detecting circuit121includes a subtraction circuit131and a comparison circuit132.

The subtraction circuit131includes resistors R11through R14and a differential amplifier141, and outputs a signal in which a voltage at the terminal Tout is subtracted from a voltage at the terminal Tin. The signal output from the subtraction circuit131becomes positive when the voltage at the terminal Tin is larger than that at the terminal Tout, that is, in a state that a current flows from the DC power source102to the load103. On the other hand, the signal output from the subtraction circuit131becomes negative when the voltage at the terminal Tin is smaller than that at the terminal Tout, that is, in a state that a current flows from the load103to the DC power source102. The signal output from the subtraction circuit131is supplied to the comparison circuit132.

The comparison circuit132includes a reference voltage source151, resistors R21and R22, and a comparator152, and compares a reference voltage generated by the reference voltage source151and the resistors R21and R22with the output (signal) from the subtraction circuit131. The output from the comparison circuit132becomes a low level when the output from the subtraction circuit131is smaller than the reference voltage, that is, in a reverse current flowing (detecting) state. On the other hand, the output from the comparison circuit132becomes a high level when the output from the subtraction circuit131is larger than the reference voltage, that is, in a non-reverse current flowing (detecting) state. The values of the resistors R11through R14and the resistors R21and R22are adjusted so that a reverse current can be detected even in a state that the voltage at the load103is relatively smaller than that at the DC power source102. The values of the resistors R11through R14and the resistors R21and R22are adjusted by using, for example, a laser trimming method.

The reverse current preventing circuit122includes transistors M11and M12, a resistor R31, and inverters161and162.

The transistor M11is an n channel MOS transistor and the output from the comparison circuit132of the reverse current detecting circuit121is supplied to its gate. The source of the transistor M11is connected to the ground terminal Tgnd, and the drain of the transistor M11is connected to the source of the control transistor M1via the resistor R31. One end of the resistor R31is connected to a connection point of the source of the transistor. M12and the source of the control transistor M1, and the other end of the resistor R31is connected to a connection point of the drain of the transistor M11and the input of the inverter161.

The connection point of the drain of the transistor M11and the resistor R31is connected to the gate of the transistor M12via the inverters161and162. The drain of the transistor M12is connected to the terminal Tin, and its source is connected to the source of the control transistor M1. In the transistor M12, a body diode D12is formed.

The transistor M12is driven and switched by a driving circuit composed of the transistor M11, the resistor R31, and the inverters161and162. The driving circuit obtains driving power from the connection point of the source of the control transistor M1and the source of the transistor M12. Even when the transistor M12is turned off, the driving circuit obtains driving power from the load103via the body diode D1of the control transistor M1and can drive the transistor M12.

The transistor M11is turned off when the output from the reverse current detecting circuit121is a low level, that is, when the reverse current is detected. When the transistor M11is turned off, the input of the inverter161becomes a high level. With this, the output from the inverter162becomes a high level and the gate of the transistor M12becomes a high level. Then, the transistor M12is turned off. When the transistor M12is turned off, the terminal Tin is disconnected from the source of the control transistor M1. At this time, in the body diode D12of the transistor M12, the terminal Tin is in the reverse direction viewed from the source of the control transistor M1; therefore, a reverse current from the source of the control transistor M1to the terminal Tin can be prevented.

The transistor M11is turned on when the output from the reverse current detecting circuit121is a high level, that is, when the reverse current is not detected. When the transistor M11is turned on, the input of the inverter161becomes a low level. With this, the output from the inverter162becomes a low level and the gate of the transistor M12becomes a low level. Then, the transistor M12is turned on. When the transistor M12is turned on, the terminal Tin is connected to the source of the control transistor M1, and a current flows from the terminal Tin to the source of the control transistor M1. That is, the current flows from the terminal Tin to the terminal Tout via the control transistor M1.

As described above, according to the embodiment of the present invention, in a state that a voltage at the terminal Tin is larger than that at the terminal Tout and a current can be supplied to the load103from the DC power source102, the transistor M12is turned on and the current can be supplied from the DC power source102to the load103with a low loss. On the other hand, in a state that a voltage at the terminal Tin is smaller than that at the terminal Tout and a current can be supplied from the load103to the DC power source102, the transistor M12is turned off and the current cannot be supplied from the load103to the DC power source102by the body diode D12in the transistor M12. That is, a reverse current from the load103to the DC power source102can be prevented.

According to an embodiment of the present invention, since the values of the resistors R11through R14, and R21and R22can be finely adjusted by a laser trimming method, the reverse current can be prevented.

In addition, when it is defined that the voltage at the terminal Tin is Vin and the voltage at the terminal Tout is Vout, the reverse current preventing condition is not simply determined as Vin<Vout. That is, the condition is determined as (Vin+α)<Vout, in this case, for example, α=100 mV. Therefore, the reverse current can be prevented by the above condition.

Further, the DC power source102is not limited to the AC adaptor or the USB port connected to a power source line. A secondary battery such as a lithium-ion battery, a nickel-cadmium battery, and so on can be used as the DC power source102.

In addition, as the load103, a rechargeable battery (secondary battery) such as a lithium-ion battery, a nickel-cadmium battery, and so on can be used.

Further, the present invention is not limited to the embodiment, but variations and modifications may be made without departing from the scope of the present invention.

The present application is based on Japanese Priority Patent Application No. 2005-282445 filed on Sep. 28, 2005, with the Japanese Patent Office, the entire contents of which are hereby incorporated by reference.