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MC1488MG Datenblatt(PDF) 6 Page - ON Semiconductor |
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MC1488MG Datenblatt(HTML) 6 Page - ON Semiconductor |
6 / 11 page MC1488 http://onsemi.com 6 APPLICATIONS INFORMATION The Electronic Industries Association EIA−232D specification details the requirements for the interface between data processing equipment and data communications equipment. This standard specifies not only the number and type of interface leads, but also the voltage levels to be used. The MC1488 quad driver and its companion circuit, the MC1489 quad receiver, provide a complete interface system between DTL or TTL logic levels and the EIA−232D defined levels. The EIA−232D requirements as applied to drivers are discussed herein. The required driver voltages are defined as between 5.0 and 15 V in magnitude and are positive for a Logic “0” and negative for a Logic “1.” These voltages are so defined when the drivers are terminated with a 3000 to 7000 W resistor. The MC1488 meets this voltage requirement by converting a DTL/TTL logic level into EIA−232D levels with one stage of inversion. The EIA−232D specification further requires that during transitions, the driver output slew rate must not exceed 30 V per microsecond. The inherent slew rate of the MC1488 is much too fast for this requirement. The current limited output of the device can be used to control this slew rate by connecting a capacitor to each driver output. The required capacitor can be easily determined by using the relationship C = IOS x DT/DV from which Figure 14 is derived. Accordingly, a 330 pF capacitor on each output will guarantee a worst case slew rate of 30 V per microsecond. 1000 10 100 1.0 10 333 pF 30 V/ ms Figure 14. Slew Rate versus Capacitance for ISC = 10 mA C, CAPACITANCE (pF) 10,000 100 1,000 1.0 The interface driver is also required to withstand an accidental short to any other conductor in an interconnecting cable. The worst possible signal on any conductor would be another driver using a plus or minus 15 V, 500 mA source. The MC1488 is designed to indefinitely withstand such a short to all four outputs in a package as long as the power supply voltages are greater than 9.0 V (i.e., VCC q 9.0 V; VEE p − 9.0 V). In some power supply designs, a loss of system power causes a low impedance on the power supply outputs. When this occurs, a low impedance to ground would exist at the power inputs to the MC1488 effectively shorting the 300 W output resistors to ground. If all four outputs were then shorted to plus or minus 15 V, the power dissipation in these resistors would be excessive. Therefore, if the system is designed to permit low impedances to ground at the power supplies of the drivers, a diode should be placed in each power supply lead to prevent overheating in this fault condition. These two diodes, as shown in Figure 15, could be used to decouple all the driver packages in a system. (These same diodes will allow the MC1488 to withstand momentary shorts to the ±25 V limits specified in the earlier Standard EIA−232B.) The addition of the diodes also permits the MC1488 to withstand faults with power supplies of less than the 9.0 V stated above. Figure 15. Power Supply Protection to Meet Power Off Fault Conditions VCC 14 MC1488 VEE 7 MC1488 14 MC1488 14 7 1 17 1 The maximum short circuit current allowable under fault conditions is more than guaranteed by the previously mentioned 10 mA output current limiting. Other Applications The MC1488 is an extremely versatile line driver with a myriad of possible applications. Several features of the drivers enhance this versatility: 1. Output Current Limiting − this enables the circuit designer to define the output voltage levels independent of power supplies and can be accomplished by diode clamping of the output pins. Figure 16 shows the MC1488 used as a DTL to MOS translator where the high level voltage output is clamped one diode above ground. The resistor divider shown is used to reduce the output voltage below the 300 mV above ground MOS input level limit. |
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