The telephone subscriber lines provide -48V voltage, but the equipment on the customer premises is usually operated from a lower voltage. The circuit using MAX638 illustrates a conversion of this available voltage to 5V required in modems, telephone test sets, and other portable systems connected to subscriber (household) telephone lines.
This circuit (Figure 1) is not battery-powered in the usual sense. Its power comes from a - 48V lead-acid battery in a remote central office of the telephone system. The circuit is handy for use in modems, telephone test sets, and other portable systems connected to subscriber (household) telephone lines. It is also useful in telemetry systems.