# Patent

## Title
Battery charger for preventing charging currents from overshooting during mode transition and method thereof

## Abstract

A battery charger for charging a battery through controlling a charging regulation circuit is provided. The battery charger includes a constant voltage mode controller, a current sensing unit, and a reference voltage generator. The constant voltage mode controller is for comparing a battery voltage of the battery with a first reference voltage to generate a regulation signal, and utilizing the regulation signal to control the charging regulation circuit to regulate a charging current applied to the battery. The current sensing unit is for monitoring the charging current to generate an error signal. The reference voltage generator is for setting the first reference voltage according to the error signal. By adding a voltage generator to the battery charger, the overshoot charging current will be reduced and the mode transition will become smooth.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This continuation application claims the benefit of co-pending U.S. patent application Ser. No. 10/904,772, filed on Nov. 29, 2004 and included herein by reference.


BACKGROUND
The present invention relates to a battery charger, and more particularly, to a battery charger and related method for preventing the charging current from overshooting during the charging mode transition.
In a battery charging system for a lithium-ion (Li-ion) battery, a constant current (CC) mode is adopted to apply a high current to an exhausted battery to activate a rapid charging operation. When the battery is charged to a desired voltage level, the battery charging system switches to a constant voltage (CV) mode to maintain the battery at this desired voltage level. Since there exists an internal resistor in the battery, the battery is not fully-charged at the end of the CC mode. The voltage drop on the internal resistor makes the battery voltage be higher than it really is in the CC mode. After entering the CV mode, the battery voltage will be kept at the desired voltage level. In other words, the battery will be kept charging until the charging current becomes zero. When the charging current is zero, there is no voltage drop on the internal resistor and the battery is fully-charged to the desired voltage level.
The CC charging mode cannot be applied to the battery once the battery reaches the desired voltage level because the energy storage capacity of the battery would exceed the nominal rating, leading to AC adaptor, battery and charging system damage. However, the CC mode needs to be used during the first part of the charging operation in order to minimize overall charging time, i.e., the time for charging the battery with the CC mode must be maximized. Therefore, a proper transition between two charging modes is crucial to the battery charging system's performance.
Please refer to FIG. 1, which shows a related art battery charger 100. The related art battery charger 100 is used to charge a battery 150, and includes a charging regulation circuit 110, a comparator 120, a constant current (CC) mode controller 130, and a constant voltage (CV) mode controller 140, wherein the comparator 120 can be a hysteresis comparator for stabilizing the charging mode. As shown in FIG. 1, it is well-known that the battery 150 is equivalent to a series connection of a resistor Rint and a capacitor Cint, and the charging regulation circuit 110 is connected to a power supplier (not shown). The comparator 120 is used to compare a battery voltage Vbat of the battery 150 with a reference signal Vref—1 to check whether the battery voltage Vbat is below a threshold. If the battery voltage Vbat is lower than the reference signal Vref—1, the comparator 120 sends out a non-enabling signal D′ to switch on switches SW1, SW3, and to switch off switches SW2, SW4, and then the battery 150 is charged in the CC mode.
In the CC mode the charging regulation circuit 110 is controlled to provide the battery 150 with a constant charging current. As shown in FIG. 1, the charging regulation circuit 110 is configured by a PMOS transistor 111 to regulate the required charging current. The CC mode controller 130 includes a sensor 135, which monitors the charging current flowing through a resistor R. After measuring the charging current flowing through the resistor R, the sensor 135 outputs a voltage V1, which corresponds to the voltage drop across the resistor R, into an operational amplifier 132. For example, if the detected voltage drop is 160 mV, the sensor 135 converts the voltage drop into a voltage level of 160 mV. The operational amplifier 132 sends out a regulation signal S1 to adjust the gate voltage of the PMOS 111 for regulating and stabilizing the charging current outputted from the power supplier, which is further explained as follows. Here, the charging regulation circuit 110, the resistor R, the sensor 135, and the operational amplifier 132 form a closed loop; the voltage V1 acts as a feedback signal. As shown in FIG. 1, the operational amplifier 132 determines the voltage level of the regulation signal S1 by comparing the incoming voltage V1 with a reference signal Vref—2. Assume the charging current during the CC mode is designed to be 10 mA, and the resistance of the resistor R is a known value 50Ω. It is clear that if the charging regulation circuit 110 successfully outputs the desired charging current 10 mA, the voltage drop cross the resistor R will be 500 mV. Therefore, the reference signal Vref—2 is set to 500 mV for checking whether the current flowing through the resistor R has the desired current value. If the voltage V1 is greater than the reference signal Vref—2, the regulation signal S1, which has a higher voltage level amplified by the operational amplifier 132, controls the charging regulation circuit 110 to reduce the charging current; however, if the voltage V1 is less than the reference signal Vref—2, the regulation signal S1, which has a lower voltage level amplified by the operational amplifier 132, controls the charging regulation circuit II 0 to increase the charging current. As a result, the battery 150 receives a stable charging current.
The battery charger 100 turns into the CV mode from the CC mode when the battery voltage Vbat is at a termination voltage level, that is, a reference voltage Vref—3. When the battery charger 100 is charging in the CC mode, the comparator 120 keeps comparing the battery voltage Vbat with the reference voltage Vref—3. When the battery voltage Vbat is not less than the reference voltage Vref—3, the comparator 120 sends out the enabling signal D to change the on/off states of the switches. Therefore, switches SW1, SW3 are switched off, and switches SW2, SW4 are switched on. As a result, the battery charger 100 enters the CV mode. In the CV mode, the comparator 120 keeps comparing the battery voltage Vbat with the reference voltage Vref—1. When the battery voltage Vbat is lower than the reference voltage Vref—1, the battery charger 100 returns to the CC mode.
In the CV mode the charging regulation circuit 110 charges the battery 150 to the termination voltage level and the battery charger 100 maintains the battery voltage Vbat at the termination voltage level. In the CV mode, the CV mode controller 140 acts as a regulator to regulate the charging current. An operational amplifier 142 within the CV mode controller 140 compares the battery voltage Vbat with a reference voltage Vref—3, and sends out a regulation signal S2 to control the gate voltage of the PMOS 111 for further tuning the charging current. Similar to the CC mode, the CV mode also forms a closed loop including the charging regulation circuit 110, the resistor R, and the operational amplifier 142. In order to stabilize the battery voltage Vbat at the reference voltage Vref—3, the operational amplifier 142 compares the reference voltage Vref—3 with the battery voltage Vbat to decide how to regulate the charging current. In other words, the gate voltage of the PMOS transistor 111 is precisely adjusted by the regulation signal S2 when the battery voltage Vbat deviates from the reference voltage Vref—3. As a result, the battery 150 is steadily charged at the constant battery voltage Vbat.
The battery charger 100 will enter the CC mode again when the battery voltage Vbat is lower than the reference voltage Vref—1, for example, fully-charged battery 150 is removed and a new exhausted battery is connected, which is explained as follows. During charging in the CV mode, the operational amplifier 142 controls the charging regulation circuit 110, while the comparator 120 keeps comparing the battery voltage Vbat with the reference signal Vref—1, which is lower than the reference voltage Vref—3. When a fully-charged battery is taken away and an exhausted battery is connected to the battery charger 100, the battery voltage Vbat becomes low. If the battery voltage Vbat is lower than the reference signal Vref—1, the comparator 120 sends out a non-enabling signal D′ to initialize the battery charger 100, setting it into the CC mode wherein switches SW1, SW3 are on and switches SW2, SW4 are off. Therefore, the battery charger 100 again provides the exhausted battery with a constant charging current.
As mentioned above, there are two modes, the CC mode and the CV mode, selectively used by the battery charger 100; hence there is a transition between these two modes. However, because the comparators 120 and the operational amplifier 142 are not perfectly matched due to well-known manufacturing variations, the transition between these two modes may not be very smooth, and this situation could cause an overshoot charging current to damage an AC adaptor, the battery charger 100 or the battery 150. For example, assume that the constant charging current during the CC mode is 800 mA, the reference signal Vref—1 is 4.1V, and the reference voltage Vref—3 is 4.2V, which is the same as the termination voltage. Ideally, the transition occurs when the battery voltage Vbat is equal to 4.2V, and then the CV mode closed loop is enabled to maintain the battery voltage Vbat at 4.2V. As a result, the charging current would still be kept at 800 mA at the moment of entering the CV mode, and the transition between two modes would therefore be very smooth. However, practically, because the comparator 120 and the operational amplifier 142 are not matched in their characteristics, the voltage at which the transition occurs and the voltage at which the CV mode loop tries to maintain are likely to be different. That is, if the comparator 120 abnormally sends out an enabling signal D when Vbat is still less than 4.2V, for example 4.1V, and then the operational amplifier 142 will sends out a regulation signal S2 to maintain the battery voltage Vbat at 4.2V by further increasing the charging current by controlling the PMOS 111. The charging current at the situation is larger than the current in the CC mode. Therefore, an overshoot charging current capable of damaging the AC adaptor, the battery charger 100 or the battery 150 is likely to be induced.
With this in mind, it is desirable to provide a battery charger which can minimize the overshoot charging current to prevent the AC adaptor, the battery charger and the battery from being damaged.
SUMMARY
One of the objectives of the present invention is therefore to provide a battery charger for preventing an overshoot charging current to solve the above problem.
According to an exemplary embodiment of the present invention, a battery charger for charging a battery through controlling a charging regulation circuit is disclosed. The battery charger includes a constant voltage mode controller, a current sensing unit, and a reference voltage generator. The constant voltage mode controller is for comparing a battery voltage of the battery with a first reference voltage to generate a regulation signal, and utilizing the regulation signal to control the charging regulation circuit to regulate a charging current applied to the battery. The current sensing unit is for monitoring the charging current to generate an error signal. The reference voltage generator is coupled to the constant voltage mode controller and the current sensing unit, and is for setting the first reference voltage according to the error signal. Therefore, by adding a voltage generator to the battery charger, the overshoot charging current will be reduced and the mode transition will become smooth.
In addition, the present invention further discloses a battery charger including a constant current mode controller, a constant voltage mode controller, a first switch, a second switch, a third switch, a fourth switch, and a comparator. The constant current mode controller is configured to monitor a charging current applied to a battery to generate a first regulation signal, and utilize the first regulation signal to control a charging regulation circuit to regulate the charging current for maintaining the charging current at a predetermined current level. The constant voltage mode controller is configured to compare a battery voltage of the battery with a first reference voltage to generate a second regulation signal, and utilize the second regulation signal to control the charging regulation circuit to regulate the charging current. The first switch is for transmitting the first regulation signal generated from the constant current mode controller to the charging regulation circuit when switched on. The second switch is for transmitting the second regulation signal generated from the constant voltage mode controller to the charging regulation circuit when switched on. The comparator is for comparing the battery voltage with either the first reference voltage or a second reference voltage, wherein the first reference voltage is higher than the second reference voltage. The third switch is for transmitting the first reference voltage to the comparator when switched on. The fourth switch is for transmitting the second reference voltage to the comparator when switched on. The comparator switches on the first witch and the third switch and switches off the second switch and the fourth switch when the battery voltage is lower than the second reference voltage; and the comparator switches off the first witch and the third switch and switches on the second switch and the fourth switch when the battery voltage is not lower than the second reference voltage.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram illustrating a battery charger according to the related art.
FIG. 2 is a diagram illustrating a battery charger according to an embodiment of the present invention.
FIG. 3 is a circuit diagram of a Digital-to-Analog Converter (DAC) shown in FIG. 2.
FIG. 4 is a diagram illustrating a battery charger according to another embodiment of the present invention.



DETAILED DESCRIPTION
In order to reduce the overshoot charging current, an adjusting mechanism can be applied to control the battery voltage Vbat. Please refer to FIG. 2, which shows a battery charger 200 according to the present invention. In this embodiment, the battery charger 200 includes the charging regulation circuit 110 formed by the PMOS transistor 111 the comparator 120, the CC mode controller 130, the CV mode controller 140, and a reference voltage generator 210 to charge a battery 150, wherein the comparator 120 can be a hysteresis comparator for stabilizing the charging mode. Compared with the related art battery charger 100 shown in FIG. 1, the battery charger 200 is very similar to the battery charger 100, and the only difference lies in that the battery charger 200 contains an extra reference voltage generator 210, which is electrically connected to the operational amplifier 142, to properly tune the reference voltage Vref—3. Further, the reference voltage generator 210, as shown in FIG. 2, is electrically connected to an output node of the operational amplifier 132 through a switch SW5, and is electrically connected to the comparator 120 through the switch SW3. Please note that the functionality and operation of each of the charging regulation circuit 110, the comparator 120, the CC mode controller 130, and the CV mode controller 140 has been described above, and the lengthy description is omitted for brevity.
When the battery charger 200 is in the CC mode, the switches SW1, SW3 are on and switches SW2, SW4, SW5 are off, and the comparator 120 compares the battery voltage Vbat with the reference voltage Vref—3. In the CC mode, the reference voltage Vref—3 is set to a termination value controlled by the reference voltage generator 210. When the battery voltage Vbat is not less than the reference voltage Vref—3, 4.2V for Li-ion battery for example, the comparator 120 sends out the enabling signal D to switch off SW1, SW3 and switch on SW2, SW4, SW5, and the battery charger 200 hence enters the CV mode. The operational amplifier 142 tries to maintain the battery voltage Vbat at the reference voltage Vref—3, i.e., 4.2V, and meanwhile the operational amplifier 132 serves as a pure comparator since the switch SW1 is now off and the operational amplifier 132 is not included in the closed loop mentioned above. The operational amplifier 132 compares the voltage V1 with the reference signal Vref—2 to determine how to adjust the charging current, wherein the voltage V1 indicates a current level of the charging current flowing through the resistor R. For example, if the resistor R is 0.2Ω, and the constant charging current of the CC mode is 800 mA, the reference signal Vref—2 should be 160 mV, which is equal to 0.2Ω×800 mA. When the battery charger 200 transits form CC mode to CV mode, if the transient charging current is higher than 800 mA, the voltage V1 is greater than 160 mV. As a result, the comparator 132 sends an error signal D1 having a positive voltage level to the reference voltage generator 210. The error signal D1 is further received by an analog-to-digital converter (ADC) 211. In the present invention, the ADC 211 contains a sampler 212, which is activated by the enabling signal D and is triggered by an external clock signal CLK, for example, a 10 kHz clock, and a shift register 214. After the sampler 212 samples the error signal D1, the error signal D1 is converted from an analog signal to a digital shift signal R/L to right-shift/left-shift the bits stored in the shift register 214.
In the present invention the shift register 214 has 9 bits, where only one bit is set to “1” and other bits hold “0” for defining a control code (for example, “000000001”, “000000010”, . . . , or “100000000”). This type of control code is called a “1 of 9 code”. Then, the stored control code is transferred to a digital-to-analog converter (DAC) 216.
The DAC 216 converts the received digital control code into the reference voltage Vref—3. In this embodiment, the DAC 216 can be simply implemented by a multiplexer (MUX). Please refer to FIG. 3, which shows the inner circuitry of the DAC 216. The DAC 216 has nine inputs corresponding to nine reference levels ranging equally from 3.9V to 4.2V. The control code outputted from the shift register 214 determines which of the 9 inputs is selected and then outputted to be the third voltage reference Vref—3. For example, as shown in FIG. 3, the control code is “001000000”, so the reference voltage Vref—3 will be “4.2−2ΔV”, where ΔV is equal to (4.2−3.9)/8.
Generally speaking, the control code is initialized with “100000000”, so when the battery charger 200 works during the CC mode or in the beginning of the CV mode, the reference voltage Vref—3 is equal to 4.2V. During the CV mode, if the sampler 212 samples a high level from the incoming error signal D1, that is, if the current level of the charging current is higher than the desired 800 mA, the shift signal R/L right-shifts the bit “1” to form a new control word “010000000”. At this time, the reference voltage Vref—3 is decreased by ΔV, so the battery voltage Vbat is held at a lower voltage level and the charging current decreases accordingly. Moreover, if the sampler 212 later samples a low level from the incoming error signal D1, that is, if the current level of the charging current is lower than 800 mA, the shift signal R/L left-shifts the bit “1” bit, so the reference voltage Vref—3 is increased by ΔV, leading to an increase of the charging current.
According to another embodiment of the present invention, please refer to FIG. 4, the battery charger 400, compared with the battery charger 200, has an additional current sensing unit 410 for taking charge of the work that the CC mode controller 130 does in the CV mode. The current sensing unit 410 has a sensor 415 for sensing the current flowing through the resistor R and an operational amplifier 412 for comparing the voltage V1 with the reference signal Vref—2 to determine how to adjust the charging current. In this case, the operational amplifier 132 of the CC mode controller 130 is used purely for generating the regulation signal S1 in the CC mode, while the operational amplifier 412 of the current sensing unit 410 is used purely for generating the error signal D1 in the CV mode. Although there is a small difference between battery chargers 200 and 400 due to the additional current sensing unit 410, however, the functions of other elements and the operating principle of the battery charger 400 are the same as those mentioned in FIG. 2.
In summary, by gradually and adequately increasing or decreasing the reference voltage Vref—3, the unwanted over-current charging originally affecting the battery charger during the mode transition can be greatly reduced. As a result, for a large range of mismatch that may be between the comparator 120 and the operational amplifier 142, the overshoot of the charging current can be almost totally prevented by adding the adjusting mechanism to the related art battery charger 100.
Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.

## Claims

1. A battery charger for charging a battery through controlling a charging regulation circuit, comprising:
a constant voltage mode controller, configured for comparing a battery voltage of the battery with a first reference voltage to generate a first regulation signal, and for utilizing the first regulation signal to control the charging regulation circuit to regulate a charging current applied to the battery;
a current sensing unit, configured for monitoring the charging current to generate an error signal, wherein the current sensing unit comprises:
a sensor, for deriving a voltage level indicative of a current level of the charging current; and
a comparator, coupled to the sensor, for comparing the voltage level with a second reference voltage to generate the error signal; and

a reference voltage generator, coupled to the constant voltage mode controller and the current sensing unit, for setting the first reference voltage according to the error signal.

2. The battery charger of claim 1, wherein the sensor is arranged to derive the voltage level by measuring a voltage drop across a resistor due to the charging current, and the reference voltage generator adjusts the first reference voltage when the error signal indicates that the voltage level is higher or lower than the second reference voltage.

3. The battery charger of claim 2, wherein the reference voltage generator initially sets the first reference voltage by a termination value of the battery voltage; and when the error signal indicates that the voltage level is higher than the second reference voltage, the reference voltage generator reduces the first reference voltage from the termination value to a new value.

4. The battery charger of claim 3, wherein after the first reference voltage is reduced from the termination value to the new value: when the first reference voltage is not currently set by the termination value and the error signal indicates that the voltage level is higher than the second reference voltage, the reference voltage generator decreases the first reference voltage; and when the first reference voltage is not currently set by the termination value and the error signal indicates that the voltage level is lower than the second reference voltage, the reference voltage generator increases the first reference voltage.

5. The battery charger of claim 1, wherein the reference voltage generator comprises:
an analog-to-digital converter (ADC), for converting the error signal generated from the current sensing unit into a control code; and
a digital-to-analog converter (DAC), coupled to the ADC, for converting the control code into the first reference voltage.

6. The battery charger of claim 5, wherein the ADC comprises:
a sampler, coupled to the current sensing unit, for generating a sampling result by sampling the error signal; and
a shift register, coupled to the sampler, for right-shifting bits of the control code stored therein when the sampling result has a first logic value, and left-shifting bits of the control code when the sampling result has a second logic value different from the first logic value.

7. The battery charger of claim 5, wherein the DAC comprises:
a multiplexer, for selecting one of a plurality of reference levels according to the control code stored in the shift register and outputting a selected reference level as the first reference voltage.

8. The battery charger of claim 1, wherein when the battery charger operates under a constant current mode, the current sensing unit acts as a constant current mode controller configured for generating the error signal serving as a second regulation signal to the charging regulation circuit rather than to the reference voltage generator; and when the battery charger operates under a constant voltage mode, the current sensing unit generates the error signal to the reference voltage generator rather than to the charging regulation circuit.

9. The battery charger of claim 8, further comprising:
a comparator, for comparing the battery voltage with either the first reference voltage or a second reference voltage lower than the first reference voltage, thereby controlling the battery charger to charge the battery under the constant current mode or the constant voltage mode, wherein the constant current mode controller is allowed to control the charging regulation circuit when the battery voltage is lower than the second reference voltage; and the constant voltage mode controller is allowed to control the charging regulation circuit when the battery voltage is not lower than the first reference voltage.

10. The battery charger of claim 9, wherein the comparator is a hysteresis comparator.

11. The battery charger of claim 1, wherein the constant voltage mode controller and the current sensing unit are active when the battery charger operates under a constant voltage mode, and the battery charger further comprises:
a constant current mode controller, being active when the battery charger operates under a constant current mode and configured to monitor the charging current applied to the battery to generate a second regulation signal and utilize the second regulation signal to control the charging regulation circuit to regulate the charging current for maintaining the charging current at a predetermined current level.

12. The battery charger of claim 10, further comprising:
a comparator, for comparing the battery voltage with either the first reference voltage or a second reference voltage lower than the first reference voltage, thereby controlling the battery charger to charge the battery under the constant current mode or the constant voltage mode, wherein the constant current mode controller is allowed to control the charging regulation circuit when the battery voltage is lower than the second reference voltage; and the constant voltage mode controller is allowed to control the charging regulation circuit when the battery voltage is not lower than the first reference voltage.

13. The battery charger of claim 12, wherein the comparator is a hysteresis comparator.

14. A battery charger for charging a battery through controlling a charging regulation circuit, comprising:
a constant current mode controller, configured to monitor a charging current applied to the battery to generate a first regulation signal, and utilize the first regulation signal to control the charging regulation circuit to regulate the charging current for maintaining the charging current at a predetermined current level;
a constant voltage mode controller, configured to compare a battery voltage of the battery with a first reference voltage to generate a second regulation signal, and utilize the second regulation signal to control the charging regulation circuit to regulate the charging current;
a first switch, for transmitting the first regulation signal generated from the constant current mode controller to the charging regulation circuit when switched on;
a second switch, for transmitting the second regulation signal generated from the constant voltage mode controller to the charging regulation circuit when switched on;
a comparator, for comparing the battery voltage with either the first reference voltage or a second reference voltage, wherein the first reference voltage is higher than the second reference voltage;
a third switch, for transmitting the first reference voltage to the comparator when switched on; and
a fourth switch, for transmitting the second reference voltage to the comparator when switched on;
wherein the comparator switches on the first switch and the third switch and switches off the second switch and the fourth switch when the battery voltage is lower than the second reference voltage; and the comparator switches off the first switch and the third switch and switches on the second switch and the fourth switch when the battery voltage is not lower than the second reference voltage.

15. The battery charger of claim 14, wherein the comparator is a hysteresis comparator.

16. The battery charger of claim 14, wherein the constant current mode controller comprises: a sensor, for deriving a voltage level indicative of a current level of the charging current; and a comparator, coupled to the sensor, for comparing the voltage level with a third reference voltage to generate the first regulation signal.

17. The battery charger of claim 16, wherein the sensor is arranged to derive the voltage level by measuring a voltage drop across a resistor due to the charging current.

18. A battery charger for charging a battery through controlling a charging regulation circuit, comprising:
a constant voltage mode controller, configured for comparing a battery voltage of the battery with a first reference voltage to generate a first regulation signal, and for utilizing the first regulation signal to control the charging regulation circuit to regulate a charging current applied to the battery;
a current sensing unit, configured for monitoring the charging current to generate an error signal; and
a reference voltage generator, coupled to the constant voltage mode controller and the current sensing unit, for setting the first reference voltage according to the error signal, wherein when the battery charger operates under a constant current mode, the current sensing unit acts as a constant current mode controller configured for generating the error signal serving as a second regulation signal to the charging regulation circuit rather than to the reference voltage generator; and when the battery charger operates under a constant voltage mode, the current sensing unit generates the error signal to the reference voltage generator rather than to the charging regulation circuit.