MC75172B MC75174B

http://onsemi.com

10

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

RqJA

70

°C/W for the DIP package, 85°C/W for

SOIC package);

temperature, and

where:

Since the thermal shutdown feature has a trip point of

150

dissipated within the package is calculated from:

Figures 7 to 10;

(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.

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.

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.

per package can be used?

Maximum allowable power dissipation is:

PDmax +

130

85

+ 0.53 W

Since the power supply current of 60 mA dissipates

300 mW, that leaves 230 mW (530 mW − 300 mW) for the

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

Ω, 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

Ω termination resistance

mentioned above allows for two 120

Ω 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 (60

Ω). It is up to the system software

to control the driver Enable pins to ensure that only one

driver is active at any time.