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MAX253 Datasheet(PDF) 11 Page - Maxim Integrated Products |
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MAX253 Datasheet(HTML) 11 Page - Maxim Integrated Products |
11 / 16 page Transformer Driver for Isolated RS-485 Interface ______________________________________________________________________________________ 11 10µF 5V MAX253 D1 D2 GND1 GND2 VCC 1 8 6 27 1N5817 SD 4 1N5817 1CT:5CT* 49.9k Ω 7 6 3 2 4 7 6 3 2 4 10k Ω 24.9 Ω 2N3904 MAX480 MAX480 5V 0.1V to 0.5V ISO 5V 78L05 IL300 4 3 2 1 5 6 24V UNREGULATED 49.9k Ω RL 0k Ω to 1kΩ 2N3904 *SEE TABLE 2 ISOLATION BARRIER VIN Figure 8. Typical 4mA to 20mA Application Circuit Output-Ripple Filtering A simple lowpass pi-filter (Figures 6 and 7) can be added to the output to reduce output ripple noise to about 10mVp-p. The cutoff frequency shown is 21kHz. Since the filter inductor is in series with the circuit output, minimize its resistance so the voltage drop across it is not excessive. Isolated 4mA to 20mA Analog Interface The 4mA to 20mA current loop is a standard analog signal range that 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. An analog signal in the range of 0.1V to 0.5V is applied to the first MAX480 to generate a signal current in the range of 20µA to 100µA. This low-level signal is trans- ferred across the barrier by the Siemens IL300 linear optocoupler. This device is unique in that it corrects the dominant nonlinearity present in most optocou- plers—the LED efficiency variation. The IL300 is really two optocouplers in the same package sharing the same LED; one detector is across the isolation barrier, the other is on the same side as the LED (Figure 8). The lat- ter detector is used to generate a feedback signal identi- cal to the signal on the isolated side of the barrier. The current signal transferred across the barrier is converted back to a voltage that matches the input in the 100mV to 500mV range. This voltage is then transformed to the final 4mA to 20mA current signal range by the second MAX480, Darlington stage, and the 20 Ω resistor. Isolated ADC Almost any serial-interface device is a candidate for operation across an isolation barrier; Figure 10 illus- trates one example. The MAX176 analog-to-digital converter (ADC) operates from +5V and -12V supplies, provided by the multiple-tapped secondary and linear regulators. If some additional isolated power is needed for signal conditioning, multiplexing, or possibly for a |
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