Battery charging unit

A battery charging circuit, particularly useful in a laptop computer system, includes a voltage input terminal coupled to a dc voltage (V+). A first switch (11) couples the voltage input terminal to an output (14) coupled to the battery through a current sense resistor (15). A first current sink (19) which is on when the charger circuit is on draws current from the input side of the current sense resistor (15) through a first resistor (17) to develop a first voltage (V2). This voltage (V2) and a voltage (V3) coupled through a second resistor (21) from the battery side of the sense resistor (15) are inputs to a comparator (25), the output of which controls the first switch (11). A second current sink (23), when on, draws current through the second resistor (21) causing the first switch (11) to be held off, turning the charging circuit off. An ON/OFF terminal, to which a variable duty cycle square wave is supplied, controls the turning on and off of the first (19) and second (21) current sinks, with the length of the duty cycle controlling the average current supplied to the battery.