A battery charger (100) suitable for sub-miniaturization and connection to
a wall transformer power supply (20) to charge a battery (30). The battery
charger (100) features a switch (130) that controls flow of current from
the transformer (20) either to output terminals for charging the battery
(30) or to ground, a voltage regulator (120), a microprocessor (110), a
current sensing resistor (150) and a Schottky diode (140). The
microprocessor (110) is coupled to the switch (130) to control whether the
switch is open or closed. The secondary leakage inductance of the wall
transformer (20) is exploited to control charging of the battery. The
microprocessor (110) is programmed to initiate a charging mode comprising
oscillation between a conduction interval and a flyback interval. A
charging pulse is delivered to the battery (30) during the flyback
interval.