Abstract: Regulation of the output supply voltage to a load powered by a rechargeable battery of a portable apparatus, typically a telephone, is advantageously implemented by exploiting the switching STEP-DOWN REGULATOR of an in-built battery charger. The regulator circuit configures itself in function of the voltage level at the regulator input to retain an unmodified constant current and constant voltage battery charger function as long as a sufficiently high voltage source is connected to the input. Otherwise the battery voltage is applied to the input of the regulator and configuring means modify automatically the partition ratio of an output voltage sensing divider of the voltage-mode control loop of the charger, isolate the battery pole from the output and disable the current-mode control loop of the charger.