MC75172B Datasheet, PDF (9/12 Pages) ON Semiconductor – QUAD EIA-485 LINE DRIVERS

MC75172B MC75174B
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
TJmax–TA
R_qJA
where:
R_θJA = 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.
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:
PD
where:
= {[(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).
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:
+ ^*ń + 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:
+ ^*ń + 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 60Ω, 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
I
1.0 mA
–7.0 V ^{–3.0 V}
V
5.0 V
12 V
–0.8 mA
Reprinted from EIA–485, Electronic Industries Association,
Washington,DC.
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 60Ω termination resistance
mentioned above allows for two 120Ω 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 µA 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
(60Ω). It is up to the system software to control the driver
Enable pins to ensure that only one driver is active at any
time.
Termination Resistors
Transmission line theory states that, in order to preserve
the shape and integrity of a waveform traveling along a cable,
the cable must be terminated in an impedance equal to its
characteristic impedance. In a system such as that depicted
in Figure 18, in which data can travel in both directions, both
physical ends of the cable must be terminated. Stubs, leading
to each receiver and driver, should be as short as possible.
Leaving off the terminations will generally result in
reflections which can have amplitudes of several volts above
VCC or below ground. These overshoots and undershoots
can disrupt the driver and/or receiver operation, create false
data, and in some cases damage components on the bus.
MOTOROLA ANALOG IC DEVICE DATA
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