MC75172B Ver la hoja de datos (PDF) - ON Semiconductor

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Fabricante

MC75172B

Quad EIA−485 Line Drivers with Three−State Outputs

ON Semiconductor 

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

RqJA

where:

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.

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:

PDmax +

130°C * 25°C

70°CńW

+ 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°CńW

+

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 60W, 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.

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.

Figure 17. Unit Load Definition

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 60W termination resistance

mentioned above allows for two 120W 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 (60W). 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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