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MAX845

Isolated Transformer Driver for PCMCIA Applications

Maxim Integrated 

Isolated Transformer Driver

for PCMCIA Applications

5V

6

VCC

IN

0.33µF

1

D1

MAX845

N.C.

3

FS

4

SD

GND1 GND2

2

7

8

D2

N.C.

HALO

TGM-030P3

5

MBR0520L*

0.33µF

78L05

IN

OUT

GND

*1N914 POSSIBLE FOR LOWER CURRENTS

Figure 5. Low-Noise Supply

-5V

100mA

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.

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