CN100593277C - Charging circuit for spare battery - Google Patents

Abstract

A charging circuit for a backup battery, comprising: a constant voltage circuit part for outputting one of a plurality of predetermined constant voltages and charging the backup battery by supplying the constant voltage thereto; a detection circuit part for detecting the voltage of the backup battery a battery voltage; and a control circuit portion for controlling selection of a constant voltage in response to the detected battery voltage. Another charging circuit includes: a constant current circuit part for outputting one of two predetermined constant currents to the backup battery; a constant voltage circuit part for charging the backup battery by supplying a predetermined constant voltage thereto; a battery first detection circuit part for detecting the battery voltage of the backup battery; a charge current detection circuit part for outputting a predetermined charge end signal; and a charge control circuit part for stopping the constant current circuit part and the constant voltage circuit part when receiving the charge end signal operation.

Description

Charging circuit for spare battery

This application is a divisional application of an invention patent application with an application date of September 12, 2002, an application number of 02809443.3, and an invention title of "charging circuit for a backup battery".

technical field

The present invention relates generally to a charging circuit for a rechargeable backup battery, and more particularly to a charging circuit for a backup battery capable of rapid charging and avoiding the generation of noise in a frequency band that would be detrimental to devices using the charging circuit, such as mobile phones have negative impacts.

Background technique

As a charging method of a lithium ion battery, when generally divided, a constant current/constant voltage charging method and a pulse charging method are used. In the constant current/constant voltage charging method, the charging time can be shortened by increasing the charging current to the Li-ion battery and making the charging constant voltage applied to the Li-ion battery slightly higher than the full voltage of the battery. However, when a Li-ion battery is overcharged, it is possible to degrade the performance of the battery. On the other hand, the pulse charging method does little damage to the battery due to the use of reactive cycles during charging of the Li-ion battery.

As the above-mentioned pulse charging method, there are the following three methods.

The first method, as described in Japanese Laid-Open Patent Application No. 6-113474, is to complete charging when the voltage in the reactive period reaches a predetermined voltage.

The second method is to create conditions for starting charging and stopping charging, and to repeat the starting and terminating of charging under the conditions. When the duration of the charging suspension period is equal to or greater than a predetermined time, or when the ratio of the charging period to the charging suspension period exceeds a predetermined value, the charging is terminated. For example, during charging, when the voltage of the battery reaches the first voltage, the charging is stopped; and when the voltage drops to the second voltage, the charging is resumed.

The third method, as described in Japanese Laid-Open Patent Application No. 7-336908, is to alternately repeat charging with a high-level voltage and a low-level voltage, and when the charging current of the low-level voltage is equal to or less than a predetermined current value , to end charging.

However, in the first method described above, there is a problem that the charging time becomes longer than that of the constant current/constant voltage method. Also, in the above-mentioned second method, its charging time is shortened to some extent as compared with the constant current/constant voltage method. However, since each of the charging period and the charging suspension period drastically changes between the charging start and just before the charging end, the switching frequency of the charging period and the charging suspension period varies within a wide range. Therefore, there is a problem of generating noise over a wide frequency band.

In addition, in the third method described above, since a current detection device for detecting a charging current of a low-level voltage is required, a current detection element is inserted in series in the charging circuit. Thus, there is a problem of power loss. Also, the value of the current sense resistor must be large enough to detect when the charging current is zero. Therefore, there is another problem in that the loss of electric energy becomes large, and a more complicated circuit is required.

Also, in mobile wireless communication devices such as mobile phones, backup batteries are often used as a power source. In particular, lithium-ion batteries have a high energy density per unit area and per unit mass. Therefore, it is possible to make a device including a lithium ion battery small and light. When charging a lithium-ion battery, a constant-voltage charging method that maintains the battery voltage constant and a constant-current/constant-voltage charging method that performs constant-voltage charging after constant-current charging are utilized. In the charging circuit, regardless of the method used, charging is terminated by detecting that the charging current during constant voltage charging is equal to or less than a predetermined full charging current.

Next, a conventional charging circuit for a backup battery will be described. Figure 4 shows a conventional charging circuit for a backup battery. In FIG. 4, the charging circuit includes: AC adapter 110; adapter detection circuit 112 for detecting whether the AC adapter 110 has been connected; battery voltage detection circuit 116 for detecting the voltage of the spare battery 114 to be charged; constant voltage The circuit 118 is used to perform constant voltage charging of the backup battery 113; the charging current detection circuit 122 is used to detect the charging current flowing into the backup battery 114; the resistor R1 is used to cause the passing charging current to generate a voltage drop; the diode D1 is used to A current is prevented from flowing into the AC adapter 110 from the backup battery 114 ; and a charge control circuit 124 for performing drive control of the constant voltage circuit 118 . AC adapter 110 is connected to terminal 130 . The constant voltage circuit 118 includes a constant voltage generating circuit 140 generating a reference voltage BE1, a control transistor M1, and an operational amplifier A1. In addition, the charging current detection circuit 122 includes a constant voltage generation circuit 142 for generating a reference voltage BE2 and an operational amplifier A2. Furthermore, the adapter detection circuit 112 includes a constant voltage generating circuit 144 generating a reference voltage BE3 and an operational amplifier A3. The resistor R1 is connected between the AC adapter 110 and the control transistor M1. The diode D1 is connected between the control transistor M1 and the backup battery 114 .

Next, the operation of the charging circuit will be described. When the AC adapter 110 is connected to the charging circuit via the terminal 130, and when the voltage of the AC adapter 110 is equal to or greater than a predetermined value, the adapter detection circuit 112 outputs a predetermined signal Sg1 to the charging control circuit 124. In addition, the battery voltage detection circuit 116 backs up the battery voltage of the battery 114, and outputs a battery voltage signal Sg2. When the signal Sg1 is input from the adapter detection circuit 112, the charge control circuit 124 starts to operate, and outputs a predetermined charge control signal Sg5 to the constant voltage circuit 118. When the charging control signal Sg5 is input, the constant voltage circuit 118 starts constant voltage charging of the backup battery 114 . During charging, diode D1 blocks current from back-up battery 114 from flowing back to AC adapter 110 via control transistor M1 and resistor R1. The charging current flows through the resistor R1 to generate a voltage drop, and the generated voltage is applied to the charging current detection circuit 122 . When the charging current detection circuit 122 detects that the charging current is less than a predetermined value according to the input voltage, the charging current detection circuit 122 transmits a predetermined charging end signal Sg6 to the charging control circuit 124 . When the charging end signal Sg6 is input to the charging control circuit 124, the charging control circuit 124 outputs the charging control signal Sg5, and stops the operation of the constant voltage circuit 118.

As mentioned above, in order to detect the charging current, the conventional charging circuit uses the resistor R1. However, at the beginning of charging, the charging current is relatively high and produces a significant voltage drop. Therefore, the heat generation of the resistor R1 becomes high. In addition, the energy loss due to heat generation is also large. In order to reduce this heating and energy waste, try to make the resistance value of the resistor R1 very small. However, by performing constant voltage charging, the detected current at the end of charging is small, and since the voltage drop across resistor R1 is low, the input bias voltage of the operational amplifier A1 used to detect the resulting voltage cannot be ignored . In other words, there is a problem that the accuracy of detecting the charging current becomes low. Also, there is a problem that since operational amplifiers having a small configuration voltage are expensive, manufacturing costs increase when they are used.

Also, a problem arises when a large current is supplied to the backup battery at the start of charging in the case where the backup battery is in an over-discharged state. Therefore, with this charging circuit, it is impossible to charge a backup battery that is in an over-discharged state.

Contents of the invention

A general object of the present invention is to provide an improved and useful charging circuit for backup batteries which solves the above-mentioned problems.

A more specific object of the present invention is to provide a simple charging circuit for a spare battery which enables to shorten the charging time while avoiding the generation of noise in the frequency band which would negatively affect the device using the charging circuit.

Another aspect of the present invention is to provide a charging circuit capable of accurately detecting the fully charged state of a spare battery while generating less heat and loss.

Another and more specific object of the present invention is to provide a charging circuit capable of reducing manufacturing costs.

Still another object of the present invention is to provide a charging circuit that can also charge a backup battery that is in an over-discharged state.

In order to achieve the above object, according to one aspect of the present invention, there is provided a charging circuit for a backup battery, comprising: a constant voltage circuit part, adapted to select and output one of a plurality of predetermined constant voltages in response to an input control signal, and charging the backup battery by supplying a selected constant voltage thereto; a detection circuit portion for detecting a battery voltage of the backup battery; and a control circuit portion for responding to the detected battery voltage from said detection circuit portion, Controlling selection of a constant voltage applied by the constant voltage circuit portion, when the battery voltage of the backup battery is equal to or less than a first constant voltage, the control circuit portion makes the constant voltage The circuit portion charges the backup battery, and when the battery voltage of the backup battery exceeds the first constant voltage, charges the backup battery by alternately supplying thereto a predetermined second constant voltage and a predetermined third constant voltage, said The third constant pressure is lower than the second constant pressure.

According to the above aspects of the present invention, since constant voltage charging is performed prior to pulse charging, it is possible to charge the backup battery with a large current. In addition, charging can be performed by switching from/to a high-level constant voltage to/from a low-level constant voltage at a constant period (switching period) even after the start of pulse charging. Therefore, since the charging current continues, the charging time can be shortened. At the same time, it is also possible to set the switching cycle to a frequency that does not adversely affect the device using the charging circuit.

Furthermore, according to another aspect of the present invention, when the battery voltage of the backup battery exceeds a predetermined charging end voltage, the control circuit portion detects the end of charging of the backup battery, and performs a predetermined charging end operation while causing the constant voltage circuit portion to charge to the backup battery. The battery provides a third normal pressure.

According to the above aspects of the present invention, overcharging can be effectively avoided.

In addition, according to another aspect of the present invention, the second constant voltage may be equal to the first constant voltage.

According to the above aspects of the present invention, it is possible to simplify the circuit and charge the backup battery without damaging the backup battery.

In addition, according to another aspect of the present invention, the second constant pressure may be greater than the first constant pressure.

According to the above aspects of the present invention, it is possible to shorten the charging time without damaging the backup battery by making the high-level voltage during pulse charging slightly larger than the full charging voltage.

In addition, according to another aspect of the present invention, the charging circuit may further include a load circuit part for connecting a load in parallel with the backup battery according to the third constant voltage output from the constant voltage circuit part.

According to the above aspect of the present invention, it is possible to stabilize the battery voltage of the backup battery charged at the third constant voltage during pulse charging. Therefore, detection errors of the end of charging can be reduced. In addition, the flexibility of the pulse charging cycle can be increased. Also, the period can be set to a frequency that does not adversely affect the device using the charging circuit.

According to another aspect of the present invention, the constant voltage circuit part may include: a constant voltage generating circuit for generating and outputting a first constant voltage, a second constant voltage and a third constant voltage; a voltage switching circuit for Part of the control signal, selects and outputs one of the first constant voltage, the second constant voltage and the third constant voltage output from the constant voltage generating circuit; the voltage comparator is used to compare the constant voltage output from the data voltage switching circuit and a battery voltage of the backup battery, and output a comparison signal according to the comparison result; a control transistor for transmitting a current from a predetermined DC power source to the backup battery according to the comparison signal; and a diode for blocking the current A predetermined DC power source flows from the backup battery via the control transistor.

According to the above aspects of the present invention, it is possible to charge a spare battery by switching from constant voltage charging to pulse charging and a simple circuit structure.

In addition, according to another aspect of the present invention, there is provided a charging circuit for charging a backup battery; the charging circuit includes: a constant current circuit part, connected in series between an external DC power supply and the backup battery, and responding to an input control signal, to A backup battery outputs one of the first and second constant currents; a constant voltage circuit portion connected in parallel with the constant current circuit portion, and charges the backup battery by supplying a predetermined constant voltage thereto; a battery voltage detection circuit portion for Detecting and outputting the battery voltage of the backup battery; the charging current detection circuit part, when the output current of the constant voltage circuit part stops, outputs a predetermined charging end signal; and the charging control circuit part, when the charging end signal is input, stops the operation of the constant current circuit section and the constant voltage circuit section, wherein, when the battery voltage of the backup battery is lower than a predetermined voltage, the charging control circuit section outputs a control signal to the constant current circuit section so that the constant The current circuit portion outputs a first constant current, and when the battery voltage of the backup battery is equal to or greater than a predetermined voltage, the charging control circuit portion outputs a control signal to the constant current circuit portion so that the constant current circuit portion outputs a second constant current. constant current, the second constant current is greater than the first constant current.

Furthermore, according to another aspect of the present invention, there is provided a charging circuit for charging a spare battery, the charging circuit comprising: a constant voltage circuit part connected between an external DC power supply and the spare battery, and voltage to charge the backup battery; the battery voltage detection circuit part is used to detect and output the battery voltage of the backup battery; the charging current detection circuit part is used to output when the current output from the constant voltage circuit part becomes a predetermined value a predetermined charging end signal; and a charging control circuit section for stopping the operation of the constant voltage circuit section when the predetermined charging end signal is input, the constant voltage circuit section including: a constant voltage generating circuit for generating and outputting a predetermined constant voltage; a voltage comparator for comparing the battery voltage of the backup battery with the predetermined constant voltage, and outputting a comparison signal indicating a comparison result; and a control transistor for directing current from an external direct current according to the comparison signal indicating the comparison result The power supply is transmitted to the backup battery, and the charging current detection circuit part is used to detect the comparison signal output from the voltage comparator, and by determining that the current passed by the control transistor is a predetermined value based on the detected comparison signal to output a predetermined charging end signal.

According to the above aspect of the present invention, charging is terminated by detecting the charging current output from the constant voltage circuit without using a resistor. Therefore, there is no heat generation and no energy loss due to the absence of electrical resistance. Accordingly, the fully charged state of the backup battery can be detected with high accuracy.

Also, according to the above aspect of the present invention, in the case where the battery voltage of the backup battery is lower than a predetermined voltage, the backup battery can be charged with an amount of current suitable for the situation. Therefore, it is possible to charge the backup battery in the overdischarged state. In addition, the charging circuit described above can be realized while limiting the increase in circuit size. Therefore, manufacturing cost can be reduced.

Other aspects, features and advantages of the present invention will become more apparent when the following detailed description is read in conjunction with the accompanying drawings.

Description of drawings

FIG. 1 shows a structural diagram of a charging circuit for a backup battery according to a first embodiment of the present invention;

FIG. 2 shows a timing diagram of the operation of the charging circuit 1 in FIG. 1;

FIG. 3 shows a flowchart explaining the operation of the charging control circuit 6 in FIG. 1;

Figure 4 shows a diagram of a conventional charging circuit;

Fig. 5 shows a charging circuit according to a second embodiment of the present invention;

Fig. 6 A shows the change figure of the voltage of the backup battery with the charging time of the circuit shown in Fig. 5;

Fig. 6 B shows the change diagram of the charging current of the backup battery with the charging time of the circuit shown in Fig. 5;

Fig. 6 C shows the change diagram of the gate voltage of the pMOS transistor with the charging time of the circuit shown in Fig. 5;

Figure 7 shows an optional bidirectional transistor diagram;

Figure 8 shows a charging circuit according to a third embodiment of the present invention;

Fig. 9 A has shown the change diagram of the battery voltage of the backup battery with the charging time of the circuit shown in Fig. 8;

Fig. 9B shows the change diagram of the charging current with the charging time of the circuit shown in Fig. 8;

Fig. 9 C shows the change diagram of the gate voltage of the pMOS transistor with the charging time of the circuit shown in Fig. 8; And

Fig. 10 shows another discharge current according to the third embodiment of the present invention.

Detailed ways

Next, a detailed description will be given of a first embodiment of the present invention with reference to the drawings.

(first embodiment)

FIG. 1 shows a structural diagram of a charging circuit for a backup battery according to a first embodiment of the present invention. It should be noted that Fig. 1 shows an example of a charging circuit for a lithium-ion battery of a mobile phone.

In Fig. 1, the charging circuit 1 of the backup battery comprises: an adapter detection circuit 2 for outputting a predetermined signal when the power supply voltage from an AC adapter 10 as a DC power supply is equal to or greater than a predetermined value; a battery voltage detection circuit 3 for for detecting and outputting a positive voltage Vb (hereinafter referred to as "battery voltage") Vb of a lithium ion battery 11 used as a backup battery; and a constant voltage circuit 4 for charging the lithium ion battery 11 at a constant voltage.

In addition, the charging circuit 1 includes: a constant current circuit 5 for pre-charging the lithium-ion battery 11 with a predetermined constant current; a charging control circuit 6 for responding to signals from the adapter detection circuit 2 and from the battery voltage detection circuit 3 The detected voltage causes the constant voltage circuit 4 to charge the Li-ion battery 11 by the pulse charging method, and the constant current circuit 5 to perform pre-charging; and the load circuit 7 is connected to the Li-ion battery 11 in parallel.

In addition, the constant voltage circuit 4 includes a constant voltage generating circuit 21 , a voltage switching circuit 22 , an operational amplifier 23 , a control transistor 24 , a diode 25 and a gate control circuit 26 . The constant voltage generating circuit 21 generates and outputs three predetermined constant voltages E1 to E3. The voltage switching circuit 22 selects one of the constant voltages E1 to E3 from the constant voltage generating circuit 21 in accordance with a control signal from the charging control circuit 6, and outputs the selected voltage as a reference voltage Vr. The operational amplifier 23 works as a voltage comparator, and the control transistor 24 , which is a PMOS transistor, controls the charging current from the AC adapter 10 applied to the lithium-ion battery 11 . The gate control circuit 26 controls the operation of the control transistor 24 according to the output signal from the operational amplifier 23 . In addition, the charging control circuit 6 functions as a control circuit. The constant voltage E1 corresponds to the first constant voltage, the constant voltage E2 corresponds to the second constant voltage, and the constant voltage E3 corresponds to the third constant voltage.

The control transistor 24 , the diode 25 and the lithium-ion battery 11 are connected in series between the power terminal 31 and the ground, so that the charging current is supplied to the lithium-ion battery 11 . The AC adapter 10 is used to power the power supply terminal 31 . In a case where the voltage of the power supply terminal 31 is lower than the battery voltage Vb of the lithium ion battery 11 , the diode 25 prevents current from flowing back from the lithium ion battery 11 to the AC adapter 10 .

The voltage switching circuit 22 selects one of the constant voltages E1 to E3 according to the voltage switching signal Ss from the charging control circuit 6 , and outputs the selected constant voltage to the inverting input terminal of the operational amplifier 23 . The battery voltage Vb of the lithium ion battery 11 is supplied to the non-inverting input terminal of the operational amplifier 23 . The input terminal of the operational amplifier 23 is connected to the gate of the control transistor 24 via the gate control circuit 26 . In addition, the driving of the operational amplifier 23 is controlled by a control signal from the charging control circuit 6 .

On the other hand, the load circuit 7 is a series circuit including a resistor 35 and an NMOS transistor 36 . A resistor 35 and an NMOS transistor 36 are connected in series between the positive electrode and ground. The NMOS transistor operates according to the constant voltage selected by the voltage switching circuit 22 . The resistor 35 acts as a load of the control transistor 24 of the constant voltage circuit 4 when the NMOS transistor 36 is turned on. The relationship between the constant voltages E1 to E3 satisfies the condition E2=E1>E3. When the voltage switching circuit 22 selects the constant voltage E3 as the reference voltage Vr in response to the voltage switching signal Ss, the NMOS transistor 36 is turned on. When the constant voltage E1 or E2 is selected as the reference voltage Vr, the NMOS transistor 36 is turned off and assumed to be in a cut-off state.

FIG. 2 shows a timing chart of the operation of charging circuit 1 shown in FIG. 1 ; a description will be given of an example of the operation of each part in FIG. 1 with reference to FIG. 2 .

First, when the AC adapter 10 supplies power and a predetermined signal is input from the adapter detection circuit 2, the charging control circuit 6 is activated. The battery voltage detection circuit 3 detects the battery voltage Vb of the lithium ion battery 11 , and outputs the detected voltage value to the charging control circuit 6 .

In the case that the battery voltage Vb of the lithium ion battery 11 is equal to or less than the predetermined value V1, the charge control circuit 6 activates the constant current circuit 5 to start precharging the lithium ion battery 11 with a precharge charging current. In addition, at this time, the charge control circuit 6 stops the operation of the operational amplifier 23 , thereby preventing current from flowing into the lithium ion battery 11 via the control transistor 24 .

For example, when the lithium ion battery 11 is a 4.2V lithium ion battery, the above-mentioned predetermined value V1 may be set to about 2.5V. This is because a problem may occur when the lithium ion battery 11 is suddenly charged with a large current while it is in an overdischarged state. Precharging of the lithium ion battery 11 is performed so that the charging current decreases when charging starts. The precharge current Ip is a current used for precharging, and is generally set at about several mA to several tens of mA.

When the battery voltage Vb of the lithium-ion battery 11 increases to a predetermined value V1, the charging control circuit 6 determines that the lithium-ion battery 11 is a normal battery, ends the precharging performed by the constant current circuit 5, and outputs the voltage switching signal Ss, thereby Charging is switched from pre-charging to constant-voltage charging by the constant-voltage circuit 4 . Also, when precharging, the operation of the constant voltage circuit 4 is stopped, and the diode 25 prevents current from flowing from the lithium ion battery 11 to the AC adapter 10 .

When the precharging is completed, the charging control circuit 6 causes the voltage switching circuit 22 to select the constant voltage E1 through the voltage switching signal Ss. The selected constant voltage E1 is output to the inverting input terminal of the operational amplifier 23 as the reference voltage Vr. The output voltage of the constant voltage circuit 4 becomes a constant voltage E1, and the lithium ion battery 11 is charged with the constant voltage E1. FIG. 2 shows the charging current when the lithium ion battery 11 is charged with a constant voltage E1. A constant current limited by the current capacity of the AC adapter 10 or the control transistor 24 is output from the constant voltage circuit 4 as the charging current Ic.

When the battery voltage Vb of the lithium ion battery 11 gradually increases and reaches the same voltage E1 as the output voltage of the constant voltage circuit 4, the charging control circuit 6 performs operation control of the constant voltage circuit 4, thereby charging the lithium ion battery by the pulse charging method. 11 Charge. In addition, the constant voltage E1 can be set to 4.2V, which is the full charge voltage of the Li-ion battery.

The pulse charging method is a method of repeatedly switching the output voltage of the constant voltage circuit 4 from/to the constant voltage E2 to/from the constant voltage E3 at a predetermined cycle to charge the lithium ion battery 11 . When the voltage of the lithium-ion battery 11 reaches the voltage E1, the charging control circuit 6 outputs the voltage switching signal Ss to the voltage switching circuit 22, so that the voltage switching circuit 22 selects the constant voltage E3, and the output voltage of the constant voltage circuit 4 is set to be a constant voltage E3. The constant voltage E3 is lower than the constant voltage E1. However, the voltage setting of the constant voltage E3 should be such that a sufficient charging current Ic can be output to the lithium ion battery 11 after the charging method is switched to the pulse charging method. For example, in the case of the lithium ion battery, the constant voltage E3 may be set to 4.0V to 4.1V.

Next, after the predetermined time T1 elapses, since the charging control circuit 6 outputs the voltage control signal Ss to the voltage switching circuit 22 so that the voltage switching circuit 22 selects the constant voltage E3, the charging control circuit 6 outputs the voltage switching signal Ss to the voltage switching circuit 22, so that the voltage switching circuit 22 selects the constant voltage E2. The voltage switching circuit 22 selects and outputs the constant voltage E2, so that the output voltage of the constant voltage circuit 4 becomes the constant voltage E2. The constant voltage E2 may be set to have the same voltage as the constant voltage E1, or set to a voltage slightly greater than the constant voltage E1, eg, about 0.1V. In addition, it should be noted that FIG. 2 shows, for example, the situation where the constant voltage E2 is greater than the constant voltage E1 .

In the case where the constant voltage E2 is set to be the same voltage as the constant voltage E1 , it is impossible to supply an overvoltage to the lithium ion battery 11 . Therefore, there is no risk of damage to the ion battery 11 . Also, since the constant voltage E2 is set to the same voltage as the constant voltage E1, the circuit is simplified. However, one drawback is that the charging time becomes a bit longer. In the case that the constant voltage E2 is set slightly greater than the constant voltage E1, the charging time can be shortened. At the same time, due to the pulse charging method, the possibility of damaging the lithium-ion battery can also be reduced.

Next, after the predetermined time T1 elapses, since the charging control circuit 6 outputs the voltage switching signal Ss to the voltage switching circuit 22, the voltage switching circuit 22 selects the constant voltage E2. The charging control circuit 6 outputs the voltage switching signal Ss to the voltage switching circuit 22 so that the voltage switching circuit 22 selects the constant voltage E3 again. The voltage switching circuit 22 selects and outputs the constant voltage E3 again, so that the output voltage of the constant voltage circuit 4 becomes the constant voltage E3. In this way, the charging control circuit 6 causes the constant voltage circuit 4 to alternately output the constant voltages E2 and E3 at a constant cycle until the charging of the lithium ion battery 11 is completed.

It can be seen from FIG. 2 that immediately after the charging method is switched to the pulse charging method, since the charging current Ic is limited by the current capacity of the AC adapter 10 or the control transistor 24, no matter whether the output voltage of the constant voltage circuit 4 is constant Voltage E3 or constant voltage E2, the charging current Ic is approximately constant. However, when the lithium ion battery 11 is charged, the charging current Ic during charging with the constant voltage E3 gradually decreases. In addition, when the battery voltage Vb of the lithium ion battery 11 becomes equal to or greater than the constant voltage E3 when the lithium ion battery 11 is charged, the charging current Ic does not flow when charged with the constant voltage E3. This method is similar to the conventional pulse charging method of repeating charging and stopping charging. In this charging method, damage to the lithium-ion battery can be avoided, and the service life of the lithium-ion battery 11 can be extended.

When the lithium ion battery is further charged, and when the battery voltage Vb of the lithium ion battery 11 charged with the constant voltage E3 exceeds the predetermined charge end voltage Ve, the charging control circuit 6 determines that the lithium ion battery 11 is fully charged, and stops the operation of the operational amplifier 23. operation, thereby stopping the operation of the constant voltage circuit 4, and stopping the charging operation to the lithium ion battery 11.

When the voltage switching circuit 22 selects the constant voltage E3, the NMOS transistor 36 of the load circuit 7 is turned on. The resistor 35 acts as a load of the constant voltage circuit 4 when the NMOS transistor 36 is turned on. However, when the output voltage of the constant voltage circuit 4 is switched from the constant voltage E2 to the constant voltage E3, the time required for the battery voltage Vb of the lithium ion battery to reach a stable voltage can be shortened. In addition, the time required for comparison with the charging end voltage Ve performed by the charging control circuit 6 can also be shortened. Therefore, the time for charging the lithium ion battery 11 with the constant voltage E3 can be set shorter. Thus, it is possible to increase the flexibility of setting the charging cycle of the pulse charging to a frequency that does not affect the device using the charging circuit.

FIG. 3 shows a flowchart explaining the operation of charging control circuit 6 in FIG. 1 . Referring to FIG. 3 , a description will be given of the flow of operation of the charging control circuit 6 . It should be noted that the processing of each step is performed by the charging control circuit 6 unless otherwise described.

In FIG. 3 , first, step S1 detects whether the voltage of the power supply terminal 31 is equal to or greater than a predetermined voltage based on a signal input from the adapter detection circuit 2 . If it cannot be detected that the voltage at the power supply terminal 31 is equal to or greater than the predetermined voltage (NO in step S1), step S1 is repeated. If the voltage of the power supply terminal 31 is detected to be equal to or greater than the predetermined voltage (Yes in step S1), step S2 determines whether the battery voltage Vb of the lithium ion battery 11 detected by the battery voltage detection circuit 3 exceeds a predetermined value V1.

In step S2, if the battery voltage Vb of the lithium ion battery 11 is equal to or less than the predetermined value V1 (NO in step S2), then step S3 activates the constant current circuit 5, thereby precharging the lithium ion battery, and the process returns to step S2. On the contrary, in step S2, if the battery voltage Vb of the lithium ion battery 11 exceeds the predetermined value V1 (Yes in step S2), then step S4 activates the operational amplifier 23, and at the same time makes the voltage switching circuit 22 select the constant voltage E1, and at the same time The voltage E1 performs constant-voltage charging of the lithium-ion battery 11 .

After that, step S5 determines whether the battery voltage Vb of the lithium ion battery 11 exceeds the constant voltage E1. If the battery voltage Vb of the lithium ion battery 11 is equal to or lower than the constant voltage E1 (NO in step S5), step S5 is repeated. On the contrary, in step S5, if the battery voltage Vb of the lithium ion battery 11 exceeds the constant voltage E1 (Yes in step S5), then step S6 makes the voltage switching circuit 22 select the constant voltage E3, and makes the constant voltage circuit 4 use the constant voltage E3 charges the lithium-ion battery 11 .

Next, since charging with the constant voltage E3 has started, step S7 determines whether or not a predetermined time T1 has elapsed. If the predetermined time T1 has not elapsed (NO in step S7), charging with the constant voltage E3 is continued until the predetermined time T1 has elapsed. Also, in step S7, if the predetermined time T1 has elapsed (YES in step S7), the process proceeds to step S8. Step S8 determines whether the battery voltage Vb is equal to or greater than a predetermined end-of-charge voltage Ve. If the battery voltage Vb is equal to or greater than the charge end voltage Ve (YES in step S8), the charge of the lithium ion battery 11 ends, and the processing also ends.

Further, in step S8, if the battery voltage Vb is smaller than the end-of-charge voltage Ve (NO in step S8), the process proceeds to step S9. Step S9 causes the voltage switching circuit 22 to select the constant voltage E2, and causes the constant voltage 4 to charge the lithium-ion battery 11 with the constant voltage E2. Next, since charging with the constant voltage E2 has started, step S10 determines whether or not time T1 has elapsed. If the predetermined time T1 has elapsed (NO in step S10), charging with the constant voltage E2 is continued until the predetermined time T1 has elapsed. Also, in step S10, if the predetermined time T1 has elapsed (YES in step S10), the process proceeds to step S6.

As described above, the charging circuit according to the first embodiment of the present invention precharges the lithium ion battery 11 with the precharging current Ip from the constant current circuit 5 when the battery voltage Vb is equal to or less than the predetermined value V1. When the battery voltage Vb exceeds a predetermined value V1 , the charging circuit performs constant-voltage charging with a constant voltage E1 from the constant-voltage circuit 4 . When the battery voltage Vb is equal to the constant voltage E1, the charging circuit performs constant voltage switching control on the voltage switching circuit 22 to perform pulse charging so that the constant voltages E2 and E3 are alternately output from the constant voltage circuit 4 at a constant cycle. Therefore, by adding a simple circuit, when charging a lithium-ion battery, the charging time can be shortened, and it is also possible to avoid generation of noise in a frequency band that is affected by a device using the charging circuit.

Next, a description will be given of a second embodiment of the present invention with reference to the drawings.

(second embodiment)

Fig. 5 shows a charging circuit according to a second embodiment of the present invention. In Fig. 5, the charging circuit includes: AC adapter B10 for providing charging current; adapter detection circuit 12 for detecting the connection of AC adapter B10; battery voltage detection circuit 16 for detecting the voltage of backup battery 14; constant voltage The circuit 18 is used to perform constant voltage charging on the backup battery 14; the constant current circuit 20 is used to provide a constant current to the backup battery 14; the gate voltage detection circuit B22 is used to detect the voltage of the control terminal of the control transistor M1; the diode D1 , for preventing current from flowing into the AC adapter B10 from the backup battery B24; and a charge control circuit B24 for performing drive control of the constant voltage circuit 18 and the constant current circuit 20. An AC adapter B10 is connected to terminal 30 . The constant voltage circuit 18 includes a constant voltage generating current 40 for generating a reference voltage BE1, a control transistor M1, and an operational amplifier A1. The gate voltage detection circuit B22 includes a constant voltage generation circuit 42 for generating a reference voltage BE2 and an operational amplifier A2. The adapter detection circuit 12 includes a constant voltage generating circuit 44 for generating a reference voltage BE3 and an operational amplifier A3. In addition, a diode D1 is connected between the control transistor M1 and the backup battery 14 . Diode D1 prevents current from flowing from backup battery 14 into AC adapter B10 via control transistor M1. Also, in FIG. 5, the control transistor M1 is shown as a p-channel metal oxide semiconductor field effect transistor (hereinafter referred to as "pMOS transistor").

Next, the operation of the charging circuit according to the second embodiment will be given. When the AC adapter B10 as a power source of the charging circuit is connected to the charging circuit via the terminal 30, and the voltage of the input terminal of the operational amplifier A3 connected to the terminal 30 is equal to or greater than a predetermined reference voltage BE3, the adapter detection circuit 12 sends a predetermined signal Sg1 to Charging control circuit B24. In addition, the battery voltage detection circuit 16 detects the battery voltage of the backup battery 14, generates a battery voltage signal Sg1, and outputs the signal to the charging control circuit B24. When the signal Sg1 is input, the charging control circuit B24 is activated. When the battery voltage signal Sg2 is input, the charge control circuit B24 outputs a constant current control signal Sg3 to the constant current circuit 20 . When the constant current control signal Sg3 is input, the constant current circuit 20 is activated. The constant current circuit 20 includes two internal power sources, and is capable of outputting one of two currents in the direction indicated by I _B in FIG. 5 . When the charging control circuit B24 detects that the battery voltage of the spare battery 14 is lower than the predetermined voltage BV1 according to the input battery voltage signal Sg2, the charging control circuit B24 outputs the constant current switching signal Sg4 and the constant current control signal Sg3 to the constant current circuit 20 . Since the battery voltage of the backup battery 14 is lower than BV1, that is, the backup battery 14 is in an over-discharged state, there will be problems when the backup battery 14 is suddenly charged with a large current, so this is done in order to reduce the charging current. However, when the constant current switching signal Sg4 is input to the constant current circuit 20, the constant current circuit 20 outputs a current with a current value BI1. In the case of a lithium ion battery, the voltage BV1 is set to about 2.5V, and generally, the current value BI1 ranges from several mA to several tens of mA. As described above, when the constant current control signal Sg2 is output to the constant current circuit 20, charging of the backup battery 14 starts.

The charge control circuit B24 determines that the backup battery is a normal battery, and when the backup battery 14 is charged with a current value of BI1, the constant current value switching signal Sg4 is output to the constant current circuit 20, and the charge control circuit B24 outputs the constant current value switching signal Sg4 to the constant current circuit 20. The battery voltage signal Sg2 provided by the voltage detection circuit 16 detects that the battery voltage of the backup battery 14 has reached a predetermined voltage BV1. Therefore, the constant current circuit 20 outputs a current value BI2 greater than the current value BI1 to the backup battery 14 . The current value BI2 is equal to the full charge current flowing into the backup battery 14 when the constant voltage charging ends. Also, the charging control circuit B24 outputs the charging control signal Sg5 to the constant voltage circuit 18 , thereby activating the constant voltage circuit 18 . The constant voltage circuit 18 outputs charging current to the backup battery 14 in the direction _BIC shown in FIG. 5 . Subsequently, the backup battery 14 is charged by the current output by the constant voltage circuit 18 and the constant current circuit 20 .

Afterwards, when the battery voltage of the backup battery 14 further increases and reaches a voltage BV2 approximately equal to the reference voltage BE1 of the constant voltage circuit 18, the battery voltage of the backup battery 14 no longer increases, remains constant, and only the charging current gradually decreases. At this time, the operational amplifier A1 compares the battery voltage of the backup battery 14 with the reference voltage BE1, and the operational amplifier A1 supplies a positive gate voltage (control voltage) to the gate (control terminal) of the pMOS transistor M1 according to the difference. The higher the battery voltage of the backup battery 14, the higher the supplied gate voltage becomes. Therefore, the drain current is gradually limited. That is, the charging current applied to the backup battery 14 gradually decreases. In the case of a lithium ion battery, the voltage BV2 is set to approximately 4.2V. As the voltage increases further, problems arise due to the separation of the lithium metal within the backup battery 14 . Even in the conventional constant current constant voltage charging circuit, when the charging voltage of the backup battery 14 reaches the voltage BV2, the constant current charging is switched to the constant voltage charging. Also, ideally, when the battery voltage of backup battery 14 reaches voltage BV2, the total charging current starts to decrease simultaneously. However, there is some time lag depending on the progress of chemical reactions within the battery.

6A, 6B, 6C show diagrams of the above operation. FIG. 6A shows changes in battery voltage of the backup battery with charging time. FIG. 6B shows changes in charging current with charging time. In addition, FIG. 6C shows a change in the gate voltage of the pMOS transistor M1 with charging time. 6B shows the change of the current A (indicated by a thick line) output by the constant current circuit 20, the change of the charging current B output from the constant voltage circuit 18, and The total charging current C obtained by the current output by the constant voltage circuit 18 . 6A and 6B, backup battery 14 is charged with a current having a current value BI1 output from constant current circuit 20 until the voltage reaches BV1 (until charging time t1). When the battery voltage of the backup battery 14 reaches BV1, the constant current circuit 20 outputs a charging current with a current value of BI2, and the constant voltage circuit 18 also starts to output a charging current. The charging current output from the constant voltage circuit 18 is a current that is initially limited by the current capacity of the AC adapter B10 or the current capacity of the pMOS transistor M1, so their current capacities are relatively small. FIG. 6B shows, for example, the charging current in the case where the current capacity of the AC adapter B10 is relatively small. The backup battery 14 is charged by the current output by the constant voltage circuit 18 and the constant current circuit 20, and thus, the battery voltage of the backup battery 14 increases, and reaches a predetermined voltage BV2.

When a certain time elapses after the battery voltage of backup battery 14 reaches predetermined value BV2, the gate voltage of pMOS transistor M1 starts to gradually increase, and the current output from constant voltage circuit 18 starts to gradually decrease in response to this increase. However, as shown in FIG. 6C, at the charging time t2, the gate voltage of the pMOS transistor M1 increases close to the AC adapter voltage. At this time, the pMOS transistor of the constant voltage circuit 18 is turned off, and the charging current output from the constant voltage circuit 18 is stopped. In other words, the total charging current is only the current having the current value BI2 output from the constant current circuit 20 .

In the charging circuit according to this embodiment, since the current value BI2 is set equal to the full charging current value, it can be considered that charging ends when the pMOS transistor M1 of the constant voltage circuit 18 is turned off, and there is only A current of the current value BI2 flows into the backup battery 14 .

Therefore, if the reference voltage BE2 of the gate voltage detection circuit B22 is set so that a lower voltage obtained by dropping the reference voltage BE2 from the voltage of the AC adapter B10 is equal to the gate voltage when the pMOS transistor M1 is turned off, then, when the control transistor When M1 is cut off, that is, when the gate voltage of the PMOS transistor M1 input to an input terminal of the operational amplifier A2 is equal to the voltage of the AC adapter B10 by a voltage of the reference voltage BE2, the gate voltage detection circuit B22 outputs to the charging control circuit B24 Charging end signal Sg6. As described above, the gate voltage detection circuit B22 detects that a predetermined current flows into the backup battery 14 by detecting the gate voltage of the pMOS transistor M1. Therefore, the gate voltage detection circuit B22 can be referred to as a charging current detection circuit. When the charging end signal Sg6 is input to the charging control circuit B24, the charging control circuit B24 outputs the charging control signal Sg5 and the constant current control signal Sg3 to the constant voltage circuit 18 and the constant current circuit 20, respectively, and stops the operation of both circuits.

In the charging circuit according to this embodiment, a resistor for detecting charging current is not required. Thus, there is no heating or energy loss due to resistance. Therefore, it is possible to accurately detect the fully charged state. Furthermore, the current value of the current output from the constant current circuit 20 can be selected from among different current values. Therefore, even an over-discharged battery or the like can be charged without adding a new circuit.

In addition, in the charging circuit according to this embodiment, the gate voltage detection circuit B22 sets the voltage lowered by the reference voltage BE2 from the voltage of the AC adapter B10 to be equal to the pMOS transistor M1 off using the constant voltage generation circuit 42 that generates the reference voltage BE2. when the gate voltage. However, this is the same thing as setting the charge end voltage equal to the gate voltage when the pMOS transistor M1 is turned off by using the constant voltage generation circuit 42 that generates the charge end voltage.

Also, it should be noted that a pMOS transistor M1 is used as the control transistor M1 in FIG. 5 , however, even when a double-base PNP transistor as shown in FIG. 7 is used, a similar effect can be obtained. In this case, the reference voltage BE2 of the gate voltage detection circuit B22 may be set such that the voltage lowered by the reference voltage BE2 from the voltage of the AC adapter B10 is equal to the base voltage when the double base PNP transistor is turned off.

(third embodiment)

FIG. 8 shows a charging circuit diagram of the backup battery 14 according to the third embodiment of the present invention. In FIG. 8, the same components as those corresponding to those in FIG. 5 are denoted by the same reference numerals, and descriptions thereof are omitted. The charging circuit according to the third embodiment includes, in addition to the charging circuit shown in FIG. 5 : a current control circuit 50 for controlling the charging current output from the pMOS transistor M1 ; and a load resistor R2 . In addition, a diode D3 is connected between the operational amplifier A1 of the constant voltage circuit 18 and the pMOS transistor M1. The current control circuit 50 includes a constant voltage generating circuit 46 , an operational amplifier A4 and a diode D2 . One end of the load resistor R2 is grounded, and the other end is connected to the gate terminal of the pMOS transistor M1.

9A, 9B and 9B respectively show a change in the battery voltage of the backup battery 14, a change in the charging current, and a change in the gate voltage of the pMOS transistor M1 with the charging time. 9B shows the current A (indicated by a thick line) output from the constant current circuit 20, the charging current B output from the constant voltage circuit 18, and the charging current B output from the constant voltage circuit 18 by adding the current output from the constant current circuit 20 The total charging current obtained on the current. Until the battery voltage of the backup battery 14 reaches the predetermined voltage BV1 (before the charging time becomes t1), the charging circuit according to the third embodiment operates similarly to the charging circuit according to the second embodiment. The charging control circuit B24 outputs a constant current value switching signal Sg4 to the constant current circuit 20 when detecting that the battery voltage of the backup battery 14 has reached a predetermined value BV1 based on the battery voltage signal Sg2 output from the battery voltage detection circuit 16 . Accordingly, the constant current circuit 20 outputs a current value BI2 larger than the current value BI1 to the backup battery 14 . Furthermore, the charge control circuit B24 outputs a charge control signal Sg5 to the constant voltage circuit 18 and the current control circuit 50 so as to activate the constant voltage circuit 18 and the current control circuit 50, respectively.

First, since the battery voltage of the backup battery is still very low, the output of the operational amplifier A1 of the constant voltage circuit 18 is approximately 0V. On the other hand, the operational amplifier A4 of the current control circuit 50 compares the gate voltage of the pMOS transistor M1 with the voltage dropped by the reference voltage BE4 from the voltage of the AC adapter B10 (the voltage of the terminal 30), and outputs the voltage, so the pMOS transistor M1 The gate voltage of the transistor M1 is maintained constant and equal to the voltage dropped by the reference voltage BE4 from the voltage of the AC adapter B10. At this time, the diode D3 of the constant voltage circuit 18 prevents current from flowing into the operational amplifier A1 from the gate terminal of the pMOS transistor M1. Ultimately, the gate voltage of the pMOS transistor M1 is maintained constant, and the drain current of the pMOS transistor M1, that is, the charging current output from the constant voltage circuit 18 is constant at the current value BI3.

However, due to the performance of the pMOS transistor M1, there is a case where a predetermined drain current does not occur even if a predetermined gate voltage is applied. Therefore, as shown in FIG. 8, by configuring the load resistor R2, precise adjustment of the gate voltage is performed to generate a predetermined drain current. As described above, the backup battery 14 is charged by the constant current with the current value BI2 and the drain current with the current value BI3.

When the battery voltage of the backup battery 14 increases and reaches the predetermined voltage BV2, the output voltage of the operational amplifier A1 of the constant voltage circuit 18 increases, and current starts flowing from the operational amplifier A1 to the gate terminal of the pMOS transistor M1 via the diode D3. Accordingly, the gate voltage of the pMOS transistor M1 increases. However, the output of the operational amplifier A4 of the constant current control circuit 50 drops to approximately 0V. Accordingly, current stops flowing from the operational amplifier A4 to the gate of the pMOS transistor M1 via the diode D2. When the gate voltage of the pMOS transistor M1 increases, the drain current output from the pMOS transistor M1 decreases. When the backup battery 14 is further charged, the gate voltage of the pMOS transistor M1 is further increased, and the pMOS transistor M1 is turned off. At this time, the current flowing into the backup battery 14 has a current value BI2 equal to the full charge current that flows into the backup battery 14 at the end of the constant voltage charging.

When the reference voltage BE2 of the gate voltage detection circuit B22 is set so that the voltage of the reference voltage BE2 dropped from the voltage of the AC adapter B10 is equal to the gate voltage when the pMOS transistor M1 is turned off, the gate voltage detection is performed when the control transistor M1 is turned off. The circuit B22 outputs a charging end signal Sg6 to the charging control circuit B24. When the charging control signal Sg6 is input to the charging control circuit B24, the charging control circuit B24 outputs the charging control signal Sg5 and the constant current control signal Sg3 to the constant voltage circuit 18 and the constant current circuit 20, respectively, and stops the operations of both circuits.

In the charging circuit according to this embodiment, even immediately after the constant voltage circuit 18 is driven, a predetermined gate voltage can be applied to the pMOS transistor M1. Therefore, it is possible to supply the backup battery 14 with a predetermined constant current that does not depend on the current capacity of the AC adapter B10 or the current capacity of the pMOS transistor M1. Therefore, even immediately after the constant voltage circuit 18 is driven, it is possible to supply the backup battery 14 with a charging current having an appropriate current value that does not damage the backup battery 14 .

In addition, in the charging circuit according to this embodiment, a resistance for detecting charging current is not required. Therefore, there is no heating or energy loss due to resistance. Thus, it is possible to accurately detect the fully charged state of the backup battery. In addition, the current value of the current output from the constant current circuit 20 can be selected from different current values. Therefore, it is possible to charge even an over-discharged battery or the like without adding a new circuit.

Furthermore, in the charging circuit according to the third embodiment, by using the constant voltage generation circuit 42 that generates the reference voltage BE2, the gate voltage detection circuit B22 sets the voltage dropped by the reference voltage BE2 from the voltage of the AC adapter B10 to be equal to the pMOS Base voltage when transistor M1 is off. This is the same thing as setting the charge end voltage equal to the gate voltage when the pMOS transistor M1 is turned off by using a constant voltage generating circuit that generates the charge end voltage. In addition, by using the constant voltage generation circuit 46 that generates the reference voltage BE4, the current control circuit sets a voltage lowered by the reference voltage BE4 from the voltage of the AC adapter B10 equal to the gate voltage of the pMOS transistor M1 when outputting a predetermined constant current. However, this is the same thing as setting a certain control voltage equal to the gate voltage of the pMOS transistor M1 that outputs a predetermined constant current by using a constant voltage generating circuit that generates the control voltage.

Also, in FIG. 8, the control transistor M1 is a pMOS transistor. However, a similar effect can also be obtained using a double-base PNP transistor as shown in FIG. 7 . In this case, the reference voltage BE2 of the gate voltage detection circuit B22 may be set so that the voltage by the reference voltage BE2 from the voltage of the AC adapter B10 is equal to the base voltage when the double base PNP transistor is turned off. In addition, the reference voltage BE4 of the reference voltage 46 of the constant voltage generation circuit may be set so that the voltage dropped by the reference voltage BE4 from the voltage of the AC adapter B10 is equal to the base voltage of the double base PNP transistor outputting a predetermined constant current.

In addition, in the charging circuit according to this embodiment, the current control circuit 50 maintains the gate voltage of the pMOS transistor M1 constant, and makes a predetermined constant current flow to the backup battery 14 via the pMOS transistor M1 . However, another structure may also be used as long as a predetermined constant current can be made to the backup battery 14 via the pMOS transistor M1. A similar effect can be obtained even in such a case. However, when the load resistor R2 is configured as shown in FIGS. 8 and 10, it is possible to easily adjust the value of the gate voltage applied to the pMOS transistor M1. For example, even in the case where the pMOS transistor M1 is replaced by another pMOS transistor M1 of a different manufacturer, the gate voltage can be simply adjusted according to the performance of the pMOS transistor M1. Therefore, a predetermined constant current can be made to flow to the backup battery 14 regardless of the performance of the pMOS transistor M1.

In addition, in the charging circuit of FIG. 8 , the constant current circuit 20 may be a constant current circuit that only outputs the current value BI1. FIG. 10 shows a charging circuit diagram in the above case. The constant current circuit 20 has a single current source for outputting a current with a current value of BI1, and the constant current circuit 20 is controlled by a constant current control signal Sg3 output from the charging control circuit B24.

In the case where the battery voltage of the backup battery 14 is lower than the predetermined voltage BV1, since the charging control circuit B24 inputs the constant current control signal Sg3, the constant current circuit 20 is activated, and the backup battery 14 is charged only with a current whose current value is BI1. . When the charging control circuit detects that the battery voltage of the spare battery 14 reaches the predetermined voltage BV1 according to the battery voltage signal Sg2 output by the battery voltage detection circuit 16, the charging control circuit B24 sends a constant current control signal Sg3 to the constant current circuit 20, thereby stopping the constant current Operation of Circuit 20 . Also, the charge control circuit B24 outputs a charge control signal Sg5 to activate the constant voltage circuit 18 and the current control circuit 50 . The operations of the current control circuit 50 and the constant voltage circuit 18 are the same as those of the corresponding parts in FIG. 8 .

In the charging circuit of FIG. 10 , the reference voltage BE5 of the gate first detection circuit B22 is different from the reference voltage BE2 of the charging circuit in FIGS. 4 and 8 . This reference voltage BE5 is set so that the voltage dropped by the reference voltage BE5 from the voltage of the AC adapter 110 is the same as the voltage applied to the gate terminal of the pMOS transistor M1, so that the drain current of the pMOS transistor M1 is equal to the current value I2. Therefore, charging is charging from constant current charging to constant voltage charging. When the gate voltage of the pMOS transistor M1 increases and reaches a voltage lower than the voltage of the AC adapter B10 by the reference voltage BE5, the gate voltage detection circuit B22 outputs a charging end signal Sg6 to the charging control circuit B24. When the charging end signal Sg6 is input, the charging control circuit B24 outputs a charging control signal Sg5 to the constant voltage circuit 18 and the constant current control circuit 50, thereby stopping their operations.

In the charging circuit shown in FIG. 10, the constant current circuit 20 may include a single current source. The circuit size is thereby reduced. As a result, manufacturing costs are reduced.

The invention is not limited to the specifically disclosed embodiments, and changes and modifications may be made without departing from the scope of the invention.

This application is based on Japanese priority application No. 2001-279823 filed on September 14, 2001 and application No. 2001-287039 filed on September 20, 2001, the entire contents of which are incorporated herein by reference.

Claims (2)

1. A charging circuit for charging a spare battery, comprising: The constant current circuit part is connected in series between the external DC power supply and the backup battery, and outputs one of the first and second constant currents to the backup battery in response to an input control signal; a constant voltage circuit section connected in parallel to the constant current circuit section, and charges a backup battery by supplying it with a predetermined constant voltage; The battery voltage detection circuit part is used to detect and output the battery voltage of the backup battery; a charging current detection circuit section that outputs a predetermined charging end signal when a predetermined current flows into said backup battery; and a charge control circuit section that stops operations of the constant current circuit section and the constant voltage circuit section when a charge end signal is input, Wherein, when the battery voltage of the backup battery is lower than a predetermined voltage, the charging control circuit part outputs a control signal to the constant current circuit part, so that the constant current circuit part outputs the first constant current, and when the battery of the backup battery When the voltage is equal to or greater than a predetermined voltage, the charging control circuit part outputs a control signal to the constant current circuit part so that the constant current circuit part outputs a second constant current which is greater than the first constant current. 2. A charging circuit for charging a spare battery, comprising: The constant current circuit part is connected in series between the external DC power supply and the backup battery, and outputs one of the first and second constant currents to the backup battery in response to an input control signal; a constant voltage circuit section connected in parallel to the constant current circuit section, and charges a backup battery by supplying it with a predetermined constant voltage; The battery voltage detection circuit part is used to detect and output the battery voltage of the backup battery; a gate voltage detection circuit section that outputs a predetermined charging end signal when the control transistor is turned off; and a charge control circuit section that stops operations of the constant current circuit section and the constant voltage circuit section when a charge end signal is input, Wherein, when the battery voltage of the backup battery is lower than a predetermined voltage, the charging control circuit part outputs a control signal to the constant current circuit part, so that the constant current circuit part outputs the first constant current, and when the battery of the backup battery When the voltage is equal to or greater than a predetermined voltage, the charging control circuit part outputs a control signal to the constant current circuit part so that the constant current circuit part outputs a second constant current which is greater than the first constant current.

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