MAX638 Datasheet, PDF (1/4 Pages) Maxim Integrated Products – +5V/Adjustable CMOS Step-down Switching Regulator

POWER-SUPPLY CIRCUITS
Apr 30, 1999
48V-to-5V DC-DC Converter Using MAX638
Switching Step-Down Regulator Borrows Power
from Phone Lines
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.