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MC74172B Datasheet(PDF) 9 Page - ON Semiconductor |
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MC74172B Datasheet(HTML) 9 Page - ON Semiconductor |
9 / 14 page MC75172B MC75174B http://onsemi.com 9 APPLICATIONS INFORMATION Description The MC75172B and MC75174B are differential line drivers designed to comply with EIA−485 Standard (April 1983) for use in balanced digital multipoint systems containing multiple drivers. The drivers also comply with EIA−422−A and CCITT Recommendations V.11 and X.27. The drivers meet the EIA−485 requirement for protection from damage in the event that two or more drivers attempt to transmit data simultaneously on the same cable. Data rates in excess of 10 MBPS are possible, depending on the cable length and cable characteristics. A single power supply, 5.0 V, ±5%, is required at a nominal current of 60 mA, plus load currents. Outputs Each output (when active) will be a low or a high voltage, which depends on the input state and the load current (see Table 1, 2 and Figures 7 to 10). The graphs apply to each driver, regardless of how many other drivers within the package are supplying load current. Table 1. MC75172B Truth Table Data Input Enables Outputs EN EN Y Z H L H L X H H X X L X X L L H H L H L Z L H L H Z Table 2. MC75174B Truth Table Data Input Enable Outputs Y Z H L X H H L H L Z L H Z H = Logic high, L = Logic low, X = Irrelevant, Z = High impedance The two outputs of a driver are always complementary. A “high” output can only source current out, while a “low” output can only sink current (except for short circuit current − see Figure 16). The outputs will be in the high impedance mode when: the Enable inputs are set according to Table 1 or 2; a) VCC is less than 1.5 V; the junction temperature exceeds the trip point of the thermal shutdown circuit (see below). When in this condition, the output’s source and sink capability are shut off, and only leakage currents will flow (see Figures 13, 14). Disabled outputs may be taken to any voltage between −7.0 V and 12 V without damage. b) c) The drivers are protected from short circuits by two methods: Current limiting is provided at each output, in both the source and sink direction, for shorts to any voltage within the range of 12V to −7.0V, with respect to circuit ground (see Figure 16). The short circuit current will flow until the fault is removed, or until the thermal shutdown circuit activates (see below). The current limiting circuit has a negative temperature coefficient and requires no resetting upon removal of the fault condition. a) A thermal shutdown circuit disables the outputs when the junction temperature reaches 150 °C, ±20°C. The thermal shutdown circuit has a hysteresis of ≈ 12°C to prevent oscillations. When this circuit activates, the output stage of each driver is put into the high impedance mode, thereby shutting off the output currents. The remainder of the internal circuitry remains biased. The outputs will become active once again as the IC cools down. b) Driver Inputs The driver inputs determine the state of the outputs in accordance with Tables 1 and 2. The driver inputs have a nominal threshold of 1.2 V, and their voltage must be kept within the range of 0 V to VCC for proper operation. If the voltage is taken more than 0.5 V below ground, excessive currents will flow, and proper operation of the drivers will be affected. An open pin is equivalent to a logic high, but good design practices dictate that inputs should never be left open. The characteristics of the driver inputs are shown in Figure 15. This graph is not affected by the state of the Enable pins. Enable Logic Each driver’s outputs are active when the Enable inputs (Pins 4 and 12) are true according to Tables 1 and 2. The Enable inputs have a nominal threshold of 1.2 V and their voltage must be kept within the range of 0 V to VCC for proper operation. If the voltage is taken more than 0.5 V below ground, excessive currents will flow, and proper operation of the drivers will be affected. An open pin is equivalent to a logic high, but good design practices dictate that inputs should never be left open. The Enable input characteristics are shown in Figure 15. Operating Temperature Range The minimum ambient operating temperature is listed as −40 °C to meet EIA−485 specifications, and 0°C to meet EIA−422−A specifications. The higher VOD required by EIA−422−A is the reason for the narrower temperature range. |
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