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.
Figure 1. This circuit draws line power in the off-hook condition and delivers as much as
250mW at the 5V output, while maintaining normal voice (or data) communications over the
phone lines.
For systems operating on 250mW or less, the circuit eliminates need for battery packs or ac
adapters by drawing power from ordinary phone jacks. Built into peripheral equipment such
as PCMCIA modem cards, the circuit can spare the main battery in a portable computer.
The current available from subscriber lines in the on-hook state is virtually zero. In the offhook state, however, current is limited only by the sum of impedances in the central-office
battery and the intervening telephone wires. Regulations such as Part 68 of the FCC rules do
not restrict the current a subscriber can draw; you can even short out the line (which is what
the hold button does). There is one condition: the off-hook current must exceed 20mA to
ensure activation of a network-access relay in the central office.
For maximum power transfer, your impedance should match that of the line, but line
impedance varies greatly, according to distance from the central office. Impedance matching
is also in conflict with the need to draw at least 20mA. The 12V zener clamp, however,
provides a termination that works well for line impedances up to 1700Ω. This line impedance
sharply restricts the power available to you (Figure 2).
Figure 2. Available power from the Figure 1 circuit declines steeply with line impedance.
Because the circuit allows normal telephone communications while drawing power, it must
block the switching noise (generated by IC1) from entering the hybrid transformer (not
shown). Q1 and C1 perform this function by simulating an inductor that chokes off the noise
currents. And, because the circuit sinks about 35mA regardless of the 5V load current, a
"wet" hybrid transformer (one able to sink loop current) is not required. Instead, the hybrid
can be a "dry" type with a smaller, lighter core.
More Information
MAX638: QuickView -- Full (PDF) Data Sheet -- Free Samples