Isolated Data Conversion
Almost any serial-interface device is a candidate for
operation across an isolation barrier; Figure 6 illustrates
one example. The MAX176 analog-to-digital converter
(ADC) operates from +5V and -12V supplies, provided
by the multiple-tapped secondary and linear regulators.
This circuit easily supplies several hundred milliwatts of
additional isolated power for signal conditioning, multi-
plexing, or sensors. A +12V supply can be generated
by adding two more diodes from the ends of the sec-
ondary, and a -5V supply can be generated by con-
necting additional diodes to the
1
⁄
4
and
3
⁄
4
tap points on
the secondary. The MAX845 supplies sufficient power
for almost any Maxim ADC.
Telephone-Subscriber-Line Power Supply
The standard telephone system is placed in the “off
hook” state by placing a load on the line to signal the
central office that service is requested. Normally, most of
this power is wasted in a load resistor, but some systems
can benefit from utilizing this free power. Figure 7 shows
one way to transform the wasted telephone power to an
isolated, regulated 5V at currents up to 50mA.
Because the telephone line is a high-impedance
source, there can be a start-up problem with any DC-
to-DC converter; when the line voltage is low during
start-up, the frequency can be too low for the trans-
former, causing it to saturate. This excess saturation
current can keep the voltage from climbing to normal
operating levels. Thus the purpose of Q1, Q2, and the
associated resistors is to ensure that the MAX845
remains in the shutdown mode until the voltage is high
enough to allow proper operation.
Isolated 4mA to 20mA Analog Interface
The 4mA to 20mA current loop is widely used in the
process-control industry for transducer and actuator
control signals. These signals are commonly referred to
a distant ground that may be at a considerably higher
voltage with respect to the local ground. The circuit in
Figure 8 generates an isolated 4mA to 20mA current
from a 5V supply.
Isolated RS-485 Data Interface
The MAX845 power-supply transformer driver also pro-
vides isolated power for RS-485 data-interface applica-
tions. The application circuit of Figure 9 combines the
MAX845 with a low-dropout linear regulator, a trans-
former, several high-speed optocouplers, and a Maxim
RS-485 interface device.
Isolated RS-232 Data Interface
The MAX845 is ideal for isolated RS-232 data-interface
applications requiring more than four transceivers. Its
750mW output power capability enables it to drive 10
transceivers simultaneously. Figure 10 shows the typi-
cal application circuit for a complete 120kbps isolated
RS-232 data interface. This figure also shows how the
Sharp PC417 optocouplers can be replaced by the
lower-cost Quality Technologies 4N25 devices to
achieve data transfer rates up to 19.2kbps.
MAX845
Isolated Transformer Driver
for PCMCIA Applications
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