Battery charger for preventing both overshoot charging currents and overcharged battery voltage during mode transitions and method thereof

1. A battery charger for charging a battery, comprising:

a charging regulation circuit having an input terminal coupled to a power supplier, an output terminal coupled to a node N 1 , and a control terminal coupled to a node N 3 , wherein the charging regulation circuit outputs a charging current whose amount is regulated based on a first regulation signal at the control terminal;

an operational amplifier having a positive input terminal coupled to a node N 2 which is further coupled to the battery, a negative input terminal coupled to a first reference voltage, and an output terminal connected to a node N 4 and outputting the first regulation signal at node N 4 , the operational amplifier outputting a first level of an enabling signal when a battery voltage of the battery is equal to or greater than the first reference voltage;

a first switch unit for connecting a node N 6 to either the node N 4 or a node N 5 according to the enabling signal;

a second switch unit for connecting or disconnecting the node N 4 to/from the node N 3 according to the enabling signal; and

a comparator having a positive input terminal coupled to the node N 2 , a negative input terminal coupled to a second reference voltage, and an output terminal coupled to a node N 5 , the comparator outputting a second level of the enabling signal when the battery voltage is less than the second reference voltage, wherein the second reference voltage is less than the first reference voltage and the second level of the enabling signal is different from the first level of the enabling signal,

wherein the enabling signal is a signal at the node N 6 , the voltage at the node N 2 reflects the battery voltage, and the battery is connected between the node N 2 and ground.

2. The battery charger of claim 1 , wherein when the enabling signal is at the first level, the first switch unit connects the node N 6 to the node N 4 and the second switch unit disconnects the node N 4 from the node N 3 .

3. The battery charger of claim 1 , wherein when the enabling signal is at the second level, the second switch unit connects the node N 4 to the node N 3 .

4. The battery charger of claim 1 , wherein when the enabling signal is at the second level, the first switch unit connects the node N 6 to the node N 5 .

5. The battery charger of claim 1 , wherein the battery charger further comprises:

a current sensing unit having a first input terminal coupled to the node N 1 , a second input terminal coupled to a reference signal, a first output terminal coupled to the node N 2 , a second output terminal coupled to a node N 7 , wherein the current sensing unit senses a current level on a path from the node N 1 to the node N 2 and outputs a second regulation signal at the node N 7 based on the current level and the reference signal; and

a third switch unit for electrically connecting or disconnecting the node N 3 to the node N 7 according to the enabling signal.

6. The battery charger of claim 5 , wherein when the enabling signal is at the first level, the node N 3 is connected to the node N 7 .

7. The battery charger of claim 6 , wherein when the voltage at node N 2 is lower than the second reference voltage, the enabling signal is at the first level.

8. The battery charger of claim 1 , wherein the battery is a Li-ion battery.