Battery charging circuit and battery charger
View Patent ↗A battery charging circuit and a battery charger that stabilizes operation when switching between charging modes. The battery charging circuit includes first and second transistors that form a current mirror circuit with an output transistor. The source terminal of the first transistor is connected to a first resistor, and the source terminal of the second transistor is connected to a second resistor. Each source terminal is connected to a switch circuit, which controls switching between a trickle charge mode and a fast charge mode. The supply of current to the first and second resistors from the discrete transistors reduces the difference in phase lag resulting from the CR time constant and stabilizes operation in the trickle charge and the fast charge modes.
1. A battery charging circuit comprising:
an output transistor that supplies a battery with charging current;
first and second transistors, each having a control terminal, which is connected to the control terminal of the other transistor and that of the output transistor, and a current input terminal, which is connected to the current input terminal of the other transistor and that of the output transistor;
a first terminal that connects a current output terminal of the first transistor to a first resistor, which determines a charging current in a trickle charge mode;
a second terminal that connects a current output terminal of the second transistor to a second resistor, which determines a charging current in a fast charge mode;
a selective switch circuit that selectively outputs voltage from the first and second terminals;
a first error amplifier that compares an output from the switch circuit with a current restriction reference voltage, which is for restricting current; and
a current amount determination unit that determines a gate voltage of the output transistor based on an output of the error amplifier;
wherein the first transistor is larger in size than the second transistor.
2. The battery charging circuit according to claim 1 , wherein the switch circuit is connected to a mode switching circuit that detects voltage of the battery and outputs a switching signal when the voltage of the battery reaches a mode switching reference voltage; and
the switch circuit changes connection from the first terminal to the second terminal in accordance with the switching signal.
3. The battery charging circuit according to claim 1 , further comprising:
a first switch circuit connected between the current output terminal of the first transistor and the first terminal; and
a second switch circuit connected between the current output terminal of the second transistor and the second terminal;
wherein the first and second switch circuits are switched in synchronism with the selection of the selective switch circuit.
4. The battery charging circuit according to claim 1 , further comprising:
a resistor having one end connected to a current output terminal of the output transistor and another end connected to ground; and
a second error amplifier that compares a divisional voltage of the resistor with a voltage restriction reference voltage;
wherein the current amount determination unit determines the gate voltage based on the comparisons of the first and second error amplifiers.
5. A battery charger comprising:
an output transistor that supplies a battery with charging current;
first and second transistors, each having a control terminal, which is connected to the control terminal of the other transistor and that of the output transistor, and a current input terminal, which is connected to the current input terminal of the other transistor and that of the output transistor;
a first resistor connected to a current output terminal of the first transistor to determine a charging current in a trickle charge mode;
a second resistor connected to a current output terminal of the second transistor to determine a charging current in a fast charge mode;
a mode switching circuit that detects voltage of the battery and outputs a switching signal when the voltage of the battery reaches a mode switching reference voltage;
a selective switch circuit that selectively outputs current output terminal voltage of the first transistor or current output terminal of the second transistor in accordance with the switching signal;
a first error amplifier that compares an output from the switch circuit with a current restriction reference voltage, which is for restricting current; and
a current amount determination unit that determines a gate voltage of the output transistor based on an output of the error amplifier;
wherein the first transistor is larger in size than the second transistor.