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MC75174BP Datasheet(PDF) 9 Page - ON Semiconductor |
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MC75174BP Datasheet(HTML) 9 Page - ON Semiconductor |
9 / 13 page MC75172B, MC75174B http://onsemi.com 9 The maximum ambient operating temperature (applicable to both EIA−485 and EIA−422−A) is listed as 85 °C. However, a lower ambient may be required depending on system use (i.e. specifically how many drivers within a package are used) and at what current levels they are operating. The maximum power which may be dissipated within the package is determined by: PDmax + T Jmax –T A RqJA RqJA = package thermal resistance (typical 70 °C/W for the DIP package, 85°C/W for SOIC package); TJmax = max. operating junction temperature, and TA = ambient temperature. where: Since the thermal shutdown feature has a trip point of 150 °C, ±20°C, TJmax is selected to be 130°C. The power dissipated within the package is calculated from: = {[(VCC − VOH) • IOH] + VOL • IOL)} each driver = + (VCC • ICC) VCC = the supply voltage; VOH, VOL are measured or estimated from Figures 7 to 10; ICC = the quiescent power supply current (typical 60 mA). PD where: As indicated in the equation, the first term (in brackets) must be calculated and summed for each of the four drivers, while the last term is common to the entire package. Example 1: TA = 25 °C, IOL = IOH = 55 mA for each driver, VCC = 5.0 V, DIP package. How many drivers per package can be used? Maximum allowable power dissipation is: PDmax + 130 °C * 25°C 70 °C W + 1.5 W Since the power supply current of 60 mA dissipates 300 mW, that leaves 1.2 W (1.5 W − 0.3 W) for the drivers. From Figures 7 and 9, VOL [1.75 V, and VOH [3.85 V. The power dissipated in each driver is: {(5.0 − 3.85) • 0.055} + (1.75 • 0.055) = 160 mW. Since each driver dissipates 160 mW, the four drivers per package could be used in this application. Example 2: TA = 85 °C, IOL = 27.8 mA, IOH = 20 mA for each driver, VCC =5.0 V, SOIC package. How many drivers per package can be used? Maximum allowable power dissipation is: PDmax + 130 °C * 85°C 85 °C W + 0.53 W Since the power supply current of 60 mA dissipates 300 mW, that leaves 230 mW (530 mW − 300 mW) for the drivers. From Figures 8 and 10 (adjusted for VCC = 5.0 V), VOL [1.38 V, and VOH [4.27 V. The power dissipated in each driver is: {(5.0 − 4.27) • 0.020} + (1.38 • 0.0278) = 53 mW Since each driver dissipates 53 mW, the use of all four drivers in a package would be marginal. Options include reducing the load current, reducing the ambient temperature, and/or providing a heat sink. System Requirements EIA−485 requires each driver to be capable of transmitting data differentially to at least 32 unit loads, plus an equivalent DC termination resistance of 60 W, over a common mode voltage of −7.0 to 12 V. A unit load (U.L.), as defined by EIA−485, is shown in Figure 17. Figure 17. Unit Load Definition 1.0 mA V 5.0 V −3.0 V −7.0 V 12 V −0.8 mA Reprinted from EIA−485, Electronic Industries Association, Washington,DC. I A load current within the shaded regions represents an impedance of less than one U.L., while a load current of a magnitude outside the shaded area is greater than one U.L. A system’s total load is the sum of the unit load equivalents of each receiver’s input current, and each disabled driver’s output leakage current. The 60 W termination resistance mentioned above allows for two 120 W terminating resistors. Using the EIA−485 requirements (worst case limits), and the graphs of Figures 7 and 9, it can be determined that the maximum current an MC75172B or MC75174B driver will source or sink is [65 mA. System Example An example of a typical EIA−485 system is shown in Figure 18. In this example, it is assumed each receiver’s input characteristics correspond to 1.0 U.L. as defined in Figure 17. Each “off” driver, with a maximum leakage of ±50 mA over the common mode range, presents a load of [0.06 U.L. The total load for the active driver is therefore 8.3 unit loads, plus the parallel combination of the two terminating resistors (60 W). It is up to the system software to control the driver Enable pins to ensure that only one driver is active at any time. |
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