Charging and discharging control circuit and charging type power supply device

Provided is a charging and discharging protection circuit realizing low current consumption in an overcurrent detection state, easy calculation of an automatic return impedance, and high usability. A pull-down circuit for pulling down an overcurrent detection terminal to a VSS terminal is connected in series between the overcurrent detection terminal and a switching circuit. The switching circuit is connected in series between the pull-down circuit and the VSS terminal.

This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. JP2006-219195 filed Aug. 11, 2006, 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 charging and discharging control circuit for controlling the charging and discharging of a secondary battery and to a charging type power supply device including the charging and discharging control circuit.

2. Description of the Related Art

A charging type power supply device using a secondary battery includes, in order to protect the secondary battery, a charging and discharging control circuit for detecting the overcharging and overdischarging of the secondary battery and an overcurrent flowing into a load to control the charging and discharging of the secondary battery. In order to protect the secondary battery and reduce current consumption, the charging and discharging control circuit has been designed in various ways and a circuit as described in JP 2002-238173 A has been proposed.

FIG. 7shows a conventional charging type power supply device.

In the conventional charging type power supply device, a current flows from a secondary battery into a load103connected between external terminals105and106through a switching circuit102. When a voltage at an overcurrent detection terminal113connected with the external terminal106becomes higher than an overcurrent detection voltage, a charging and discharging control circuit210controls to turn OFF the switching circuit102. This state is referred to as overcurrent detection state.

In the overcurrent detection state, an N-channel transistor251of a pull-down circuit219and an N-channel transistor252of a switching circuit220are turned ON. Then, the overcurrent detection terminal113is pulled down to a VSS terminal112through a resistor253. After the charging and discharging control circuit210enters the overcurrent detection state, when the load103is disconnected from the external terminals105and106, a voltage at the overcurrent detection terminal113approaches a VSS voltage. When the voltage at the overcurrent detection terminal113becomes lower than the overcurrent detection voltage, the charging and discharging control circuit210is released from the overcurrent detection state to turn ON the switching circuit102.

The above-mentioned circuit operation is referred to as automatic return operation. An impedance between the external terminals105and106at the time of automatic return is referred to as automatic return impedance.

In an overcharging state in which a charger104is connected between the external terminals105and106and a secondary battery101has a voltage higher than a predetermined voltage value, the charging and discharging control circuit210controls to turn OFF the switching circuit102. This state is referred to as overcharging detection state.

In the overcharging detection state, the voltage at the overcurrent detection terminal113becomes lower than the VSS voltage by the charger104. Therefore, the charging and discharging control circuit210controls to turn OFF the N-channel transistor252of the switching circuit220, thereby preventing a charging current from flowing through the resister253and a parasitic diode254of a pull-down circuit219.

However, the conventional charging type power supply device has a problem in that the current consumption is increased by a phenomenon as described below.

FIG. 8is a cross sectional view showing the pull-down circuit and the switching circuit in the conventional charging and discharging control circuit210. In the overcurrent detection state, the overcurrent detection terminal113is pulled up to a VDD terminal111through the external terminal106, the load103, and the external terminal105. Therefore, there is the following problem. A base current flows from the overcurrent detection terminal113into a P-well of the N-channel transistor252. Then, a parasitic bipolar transistor501is turned ON and a current flows from the VDD terminal111to the VSS terminal112through the resistor253, thereby increasing the current consumption of the charging and discharging control circuit.

For the automatic return, it is necessary to reduce the base current to a value at which the parasitic bipolar transistor501is not turned ON. That is, it is necessary to increase the automatic return impedance. However, the switching circuit and the pull-down circuit as described above have a problem in that the calculation of the automatic return impedance is complicated because the automatic return impedance is nonlinearly changed by a voltage of the secondary battery101.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to solve the problems and to provide a charging and discharging protection circuit realizing low current consumption in an overcurrent detection state, easy calculation of an automatic return impedance, and high usability.

In order to achieve the object, according to the present invention, a charging and discharging control circuit includes

an overcharging detection circuit for monitoring a voltage of a secondary battery to detect whether or not the secondary battery is in an overcharging state, a switching circuit for controlling a connection between the secondary battery and an external terminal,

an overcurrent detection circuit for monitoring a current flowing into the switching circuit based on a voltage at an overcurrent detection terminal to detect an overcurrent,

a charging and discharging control circuit for controlling a switching of the switching circuit based on a signal from each of the detection circuits,

a pull-down circuit controlled by the charging and discharging control circuit, for pulling down the overcurrent detection terminal in an overcurrent detection state,

a charger detection circuit, and

a switching circuit for disconnecting the pull-down circuit with a VSS terminal in response to a signal from the charger detection circuit when a charger is connected between the pull-down circuit and the VSS terminal.

According to the present invention, it is possible to provide a charging and discharging control circuit realizing low current consumption in the overcurrent detection state, easy calculation of an automatic return impedance, and high usability, and a charging type power supply device including the charging and discharging protection circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Embodiment

FIG. 1is a circuit block diagram showing a charging type power supply device according to a first embodiment of the present invention.

A charging type power supply device100includes a secondary battery101which is chargeable and dischargeable, a switching circuit102serving as a current adjusting means capable of adjusting charging and discharging currents, external terminals105and106, and a charging and discharging control circuit110for controlling the switching circuit102. A load103such as a mobile telephone or a charger104is connected between the external terminals105and106.

The charging and discharging control circuit110includes a VDD terminal111and a VSS terminal112which are connected with the secondary battery101, an output terminal114which is a control terminal of the switching circuit102, an overcurrent detection terminal113and an overcurrent detection circuit117which are used for overcurrent detection, an overcharging detection circuit115, an internal control circuit116for controlling the switching circuit102based on signals from the detection circuits, a charger detection circuit118, a pull-down circuit119, and a switching circuit120. The overcharging detection circuit115includes a comparator121, a reference voltage circuit122, and resistors123and124. The overcurrent detection circuit117includes a comparator131and a reference voltage circuit132. The charger detection circuit118includes a constant current circuit141and an N-channel transistor142. The pull-down circuit119includes an N-channel transistor151, a parasitic diode154, and a resistor153. The switching circuit120includes an N-channel transistor152.

In the charging type power supply device having the above-mentioned structure, when the load103is connected between the external terminals105and106, a current corresponding to the load flows into the switching circuit102. In an overcurrent state in which a voltage at the overcurrent detection terminal113connected with the external terminal106is higher than a voltage of the reference voltage circuit132, a detection signal is output from the comparator131to the internal control circuit116. The internal control circuit116outputs a control signal to the switching circuit102through the output terminal114to stop the discharging. This state is referred to as overcurrent detection state. In some cases, the internal control circuit116generates a control signal delayed by a predetermined delay time.

In the overcurrent detection state, the N-channel transistor151of the pull-down circuit119and the N-channel transistor152of the switching circuit120are turned ON. Then, the overcurrent detection terminal113is pulled down to the VSS terminal112through the resistor153. Therefore, after the charging and discharging control circuit110enters the overcurrent detection state, when the load103is disconnected from the external terminals105and106, the voltage at the overcurrent detection terminal113approaches a voltage at the VSS terminal. When the voltage at the overcurrent detection terminal113becomes lower than the voltage of the reference voltage circuit132, the overcurrent detection state is released. This operation is referred to as automatic return operation and an impedance between the external terminals105and106at the time of automatic return is referred to as automatic return impedance.

When the charger104is connected between the external terminals105and106, a detection signal is output from the comparator121to the internal control circuit116in an overcharging state in which a voltage obtained by dividing a voltage of the secondary battery101by the resistors123and124is higher than a voltage of the reference voltage circuit122. The internal control circuit116outputs a control signal to the switching circuit102through the output terminal114to stop the charging. This state is referred to as overcharging detection state. In some cases, the internal control circuit116generates a control signal delayed by a predetermined delay time.

In the overcharging detection state, the voltage at the overcurrent detection terminal113becomes lower than a voltage at the VSS terminal112by the charger104, so the N-channel transistor142is turned ON to pull down a drain of the N-channel transistor142which is pulled up in the constant current circuit141to the voltage at the overcurrent detection terminal113. Therefore, the N-channel transistor152of the switching circuit120is turned OFF to prevent a charging current from flowing through the resister153and the parasitic diode154of the pull-down circuit119. In some cases, the output terminal114and the switching circuit102are provided for each of charging and discharging.

FIG. 2is a cross sectional view showing the pull-down circuit and the switching circuit in the first embodiment.

In the overcurrent detection state, the overcurrent detection terminal113is pulled up to the VDD terminal111by the load103. At this time, the N-channel transistor151is being turned ON, so a current flows into the N-channel transistor152through the resistor153. However, the N-channel transistor152is also being turned ON, so a voltage of a P-well thereof becomes equal to the voltage at the VSS terminal112. Therefore, a parasitic bipolar transistor is not turned ON, with the result that the current consumption of the charging and discharging control circuit does not increase.

Because the parasitic bipolar transistor is not turned ON, the automatic return impedance is simply expressed by the following expression.
RZ=(VDD/VREF−1)×RPD(Expression 1)

where RZ denotes the automatic return impedance, VDD denotes the voltage of the secondary battery101, VREF denotes the voltage of the reference voltage circuit132, and RPD denotes a resistance value of the pull-down circuit119.

Therefore, according to the first embodiment as described above, it is possible to provide a charging and discharging control circuit realizing low current consumption in an overcurrent detection state, easy calculation of an automatic return impedance, and high usability, and a charging type power supply device including the charging and discharging protection circuit.

Second Embodiment

FIG. 3is a circuit block diagram showing a charging type power supply device according to a second embodiment.

A connection relationship in a pull-down circuit319is different from that in the pull-down circuit119of the charging type power supply device according to the first embodiment. That is, the overcurrent detection terminal113is connected with a resistor353. The resistor353is connected with an N-channel transistor351and a parasitic diode354. The other structures and detection operations are identical to those of the charging type power supply device according to the first embodiment.

FIG. 4is a cross sectional view showing the pull-down circuit and the switching circuit in the second embodiment.

In the overcurrent detection state, the overcurrent detection terminal113is pulled up to the VDD terminal111by the load103. At this time, the N-channel transistor351is being turned ON, so a current flows into the N-channel transistor152through the resistor353. However, the N-channel transistor152is also being turned ON, so a voltage of a P-well becomes equal to the voltage at the VSS terminal112. Therefore, a parasitic bipolar transistor is not turned ON, with the result that the current consumption of the charging and discharging control circuit does not increase.

Thus, the same effect as that in the first embodiment can be obtained. The P-well can be commonly used for the pull-down circuit and the switching circuit as shown inFIG. 4, so a circuit area can be reduced.

Third Embodiment

FIG. 5is a circuit block diagram showing a charging type power supply device according to a third embodiment.

A structure of a pull-down circuit419is different from that of the pull-down circuit119of the charging type power supply device according to the first embodiment. That is, the pull-down circuit419is composed only of an N-channel transistor451and a parasitic diode454. In stead of the resistor153, a size of the N-channel transistor451is adjusted to obtain a predetermined resistance value. The other structures and detection operations are identical to those of the charging type power supply device according to the first embodiment.

FIG. 6is a cross sectional view showing the pull-down circuit and the switching circuit in the third embodiment.

In the overcurrent detection state, the overcurrent detection terminal113is pulled up to the VDD terminal111by the load103. At this time, the N-channel transistor451is being turned ON, so a current flows into the N-channel transistor152. However, the N-channel transistor152is also being turned ON, so a voltage of a P-well becomes equal to the voltage at the VSS terminal112. Therefore, a parasitic bipolar transistor is not turned ON, with the result that the current consumption of the charging and discharging control circuit does not increase.

Thus, the same effect as that in the first embodiment can be obtained. The P-well can be commonly used for the pull-down circuit and the switching circuit as shown inFIG. 6, so a circuit area can be reduced.