下载
Maxim > Design Support > Technical Documents > Reference Designs > Amplifier and Comparator Circuits > APP 714
Maxim > Design Support > Technical Documents > Reference Designs > Battery Management > APP 714
Maxim > Design Support > Technical Documents > Reference Designs > Communications Circuits > APP 714
Keywords: current sense, high side current sense, output overcurrent protection, load current monitor,
high side amp, current sense amp
REFERENCE DESIGN 714 INCLUDES: Tested Circuit Schematic Description
Isolated Power Supplies for Telecom Applications
Jul 22, 2002
Abstract: Simple easy to use high side current sense amps protects power supplies as a result of
irregular supply voltage caused by short-circuit conditions.
To ensure compliance with all international standards, the designer of an isolated power supply for
telecom applications needs to take certain facts into account: The input voltage is never 110V or 220V
from the mains, but the lower voltage of a lead-acid battery (-12V, -24V, or -48V) that is charged by
means of the mains through rectification (Figure 1). (Batteries back up the telecom system in case of
failure in the mains supply.) To combat corrosion due to ion flow, the input voltage is always negative,
with the positive terminal connected to ground.
Figure 1. In this block diagram of the backup architecture used in telecom systems, a battery block is
kept charged by the mains rectifier. In the event of a mains failure, it supplies the system to prevent
transmission interruptions.
ISDN and other systems powered by a remote source often operate with a higher input voltage: up to
120VDC. (Losses due to line resistance are proportional to the square of supply current, so the lower
supply current associated with higher input voltage allows a longer transmission line.) By powering the
telecom phone through the cable as well as the local mains, such systems eliminate the need for a
backup battery at the cable's user end. When required, the cable therefore carries both transmission
signals and supply current (Figure 2).
Page 1 of 14
