MC75174B Motorola, MC75174B Datasheet - Page 9

• Manufacturer Part Number: MC75174B
• Description: (MC75172B / MC75174B) QUAD EIA-485 LINE DRIVERS
• Manufacturer: Motorola

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:
where:
150 C, 20 C, T Jmax is selected to be 130 C. The power
dissipated within the package is calculated from:
PD
where:
must be calculated and summed for each of the four drivers,
while the last term is common to the entire package.
V CC = 5.0 V, DIP package. How many drivers per package
can be used?
300 mW, that leaves 1.2 W (1.5 W – 0.3 W) for the drivers.
From Figures 7 and 9, V OL
power dissipated in each driver is:
Since each driver dissipates 160 mW, the four drivers per
package could be used in this application
driver, V CC = 5.0 V, SOIC package. How many drivers per
package can be used?
300 mW, that leaves 230 mW (530 mW – 300 mW) for the
drivers. From Figures 8 and 10 (adjusted for V CC = 5.0 V),
V OL
each driver is:
drivers in a package would be marginal. Options include
MOTOROLA ANALOG IC DEVICE DATA
The maximum ambient operating temperature (applicable
Since the thermal shutdown feature has a trip point of
As indicated in the equation, the first term (in brackets)
Example 1: T A = 25 C, I OL = I OH = 55 mA for each driver,
Maximum allowable power dissipation is:
Since the power supply current of 60 mA dissipates
{(5.0 – 3.85) 0.055} + (1.75 0.055) = 160 mW.
Example 2: T A = 85 C, I OL = 27.8 mA, I OH = 20 mA for each
Maximum allowable power dissipation is:
Since the power supply current of 60 mA dissipates
{(5.0 – 4.27) 0.020} + (1.38 0.0278) = 53 mW
Since each driver dissipates 53 mW, the use of all four
[
1.38 V, and V OH
PD max
PD max
PD max
= {[(V CC – V OH ) I OH ] + V OL I OL )} each driver
=
V CC = the supply voltage;
V OH , V OL are measured or estimated from
Figures 7 to 10;
I CC = the quiescent power supply current
(typical 60 mA).
+ (V CC I CC )
R JA = package thermal resistance (typical
70 C/W for the DIP package, 85 C/W for SOIC
package);
T Jmax = max. operating junction
temperature, and
T A = ambient temperature.
+
+
+
T
130
130 C
Jmax
R
70 C W
85 C W
q
C
JA
–T
[
*
*
[
4.27 V. The power dissipated in
A
25 C
85 C
1.75 V, and V OH
+
+
1.5 W
0.53 W
[
MC75172B MC75174B
3.85 V. The
reducing the load current, reducing the ambient temperature,
and/or providing a heat sink.
System Requirements
data differentially to at least 32 unit loads, plus an equivalent
DC termination resistance of 60 , 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.
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
mentioned above allows for two 120 terminating resistors.
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
System Example
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 A over the
common mode range, presents a load of
total load for the active driver is therefore 8.3 unit loads, plus
the parallel combination of the two terminating resistors
(60 ). 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
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.
reflections which can have amplitudes of several volts above
V CC 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.
Reprinted from EIA–485, Electronic Industries Association,
Washington,DC.
EIA–485 requires each driver to be capable of transmitting
A load current within the shaded regions represents an
Using the EIA–485 requirements (worst case limits), and
An example of a typical EIA–485 system is shown in
Transmission line theory states that, in order to preserve
Leaving off the terminations will generally result in
–7.0 V
Figure 17. Unit Load Definition
–3.0 V
1.0 mA
[
65 mA.
–0.8 mA
I
5.0 V
termination resistance
[
0.06 U.L. The
12 V
9
V