Charging apparatus and charging current detecting circuit thereof
A charging apparatus and a charging current detecting circuit thereof are disclosed in which it is possible to implement an electric circuit with little voltage loss by using an operational amplifier of low cost. A detecting resistor is disposed at the power voltage side of the downstream side of a power supply. The charging apparatus further includes an auxiliary power means for generating an auxiliary voltage to supplement the voltage limited to the range of an input voltage of the operational amplifier.
1. A charging apparatus comprising:
a power supply that supplies a charging current to a secondary battery;
a detecting resistor that detects the charging current, the detecting resistor being disposed at a power voltage side of a downstream side of the power supply;
an operational amplifier that applies a voltage from the power supply;
a control transistor that controls the supply of the charging current, wherein an emitter terminal of the control transistor is connected to the downstream side of the power supply with the detecting resistor therebetween, a collector terminal of the control transistor is connected to the positive electrode of a secondary battery, and a base terminal of the control transistor is connected to an output terminal of the operational amplifier, and the negative electrode of the secondary battery is connected to a ground terminal of the power supply; and
a charging current detecting circuit, which comprises the detecting resistor and an auxiliary power generator that generates an auxiliary voltage to supplement a voltage limited to the range of an input voltage of the operational amplifier,
wherein one end of the detecting resistor connects the positive electrode of the power supply, and the other end of the detecting resistor connects an inverting input terminal of the operational amplifier, and a non-inverting input terminal of the operational amplifier connects the positive electrode of the power supply, and
wherein the auxiliary power generator comprises a signal generator connected to a power supply terminal of the operational amplifier that generates a signal containing an alternating current component; an AC coupling capacitor that AC-couples a signal output from the signal generator; a first diode that clamps the AC-coupled signal to the power voltage; and a second diode disposed at a downstream side of the AC-coupling capacitor and first diode that rectifies a signal; and, the cathode of the first diode and the anode of the second diode connected to each other, and the cathode of the second diode and the power supply terminal of the operational amplifier are connected to each other.