Datasheet MAX845 (Maxim) - 7

Isolated Transformer Driver for PCMCIA Applications MAX845
6 1 5V V D1 CC IN 0.33µF MAX845 MBR0520L* 78L05 3 N.C. FS IN OUT GND 0.33µF 4 8 SD D2 HALO GND1 GND2 N.C. -5V TGM-030P3 100mA 2 7 5 *1N914 POSSIBLE FOR LOWER CURRENTS Figure 5. Low-Noise Supply
Isolated Data Conversion
remains in the shutdown mode until the voltage is high Almost any serial-interface device is a candidate for enough to allow proper operation. operation across an isolation barrier; Figure 6 illustrates one example. The MAX176 analog-to-digital converter
Isolated 4mA to 20mA Analog Interface
(ADC) operates from +5V and -12V supplies, provided The 4mA to 20mA current loop is widely used in the by the multiple-tapped secondary and linear regulators. process-control industry for transducer and actuator This circuit easily supplies several hundred milliwatts of control signals. These signals are commonly referred to additional isolated power for signal conditioning, multi- a distant ground that may be at a considerably higher plexing, or sensors. A +12V supply can be generated voltage with respect to the local ground. The circuit in by adding two more diodes from the ends of the sec- Figure 8 generates an isolated 4mA to 20mA current ondary, and a -5V supply can be generated by con- from a 5V supply. necting additional diodes to the 1⁄4 and 3⁄4 tap points on
Isolated RS-485 Data Interface
the secondary. The MAX845 supplies sufficient power The MAX845 power-supply transformer driver also pro- for almost any Maxim ADC. vides isolated power for RS-485 data-interface applica-
Telephone-Subscriber-Line Power Supply
tions. The application circuit of Figure 9 combines the The standard telephone system is placed in the “off MAX845 with a low-dropout linear regulator, a trans- hook” state by placing a load on the line to signal the former, several high-speed optocouplers, and a Maxim central office that service is requested. Normally, most of RS-485 interface device. this power is wasted in a load resistor, but some systems
Isolated RS-232 Data Interface
can benefit from utilizing this free power. Figure 7 shows The MAX845 is ideal for isolated RS-232 data-interface one way to transform the wasted telephone power to an applications requiring more than four transceivers. Its isolated, regulated 5V at currents up to 50mA. 750mW output power capability enables it to drive 10 Because the telephone line is a high-impedance transceivers simultaneously. Figure 10 shows the typi- source, there can be a start-up problem with any DC- cal application circuit for a complete 120kbps isolated to-DC converter; when the line voltage is low during RS-232 data interface. This figure also shows how the start-up, the frequency can be too low for the trans- Sharp PC417 optocouplers can be replaced by the former, causing it to saturate. This excess saturation lower-cost Quality Technologies 4N25 devices to current can keep the voltage from climbing to normal achieve data transfer rates up to 19.2kbps. operating levels. Thus the purpose of Q1, Q2, and the associated resistors is to ensure that the MAX845
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