MC75172B, MC75174B

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8

APPLICATIONS INFORMATION

Description

The MC75172B and MC75174B are differential line

drivers designed to comply with EIA−485 Standard (April

1983) for use in balanced digital multipoint systems

containing multiple drivers. The drivers also comply with

EIA−422−A and CCITT Recommendations V.11 and X.27.

The drivers meet the EIA−485 requirement for protection

from damage in the event that two or more drivers attempt

to transmit data simultaneously on the same cable. Data

rates in excess of 10 MBPS are possible, depending on the

cable length and cable characteristics. A single power

supply, 5.0 V,

±5%, is required at a nominal current of

60 mA, plus load currents.

Outputs

Each output (when active) will be a low or a high voltage,

which depends on the input state and the load current (see

Table 1, 2 and Figures 7 to 10). The graphs apply to each

driver, regardless of how many other drivers within the

package are supplying load current.

Table 1. MC75172B Truth Table

Data Input

Enables

Outputs

EN

EN

Y

Z

H

L

H

L

X

H

H

X

X

L

X

X

L

L

H

H

L

H

L

Z

L

H

L

H

Z

Table 2. MC75174B Truth Table

Data Input

Enable

Outputs

Y

Z

H

L

X

H

H

L

H

L

Z

L

H

Z

H = Logic high, L = Logic low, X = Irrelevant, Z = High impedance

The two outputs of a driver are always complementary.

A “high” output can only source current out, while a “low”

output can only sink current (except for short circuit current

− see Figure 16).

The outputs will be in the high impedance mode when:

the Enable inputs are set according to Table 1 or 2;

a)

the junction temperature exceeds the trip point of

the thermal shutdown circuit (see below). When in

this condition, the output’s source and sink

capability are shut off, and only leakage currents

will flow (see Figures 13, 14). Disabled outputs may

be taken to any voltage between −7.0 V and 12 V

without damage.

b)

c)

The drivers are protected from short circuits by two

methods:

Current limiting is provided at each output, in both

the source and sink direction, for shorts to any

voltage within the range of 12 V to −7.0 V, with

respect to circuit ground (see Figure 16). The short

circuit current will flow until the fault is removed, or

until the thermal shutdown circuit activates (see

below). The current limiting circuit has a negative

temperature coefficient and requires no resetting

upon removal of the fault condition.

a)

A thermal shutdown circuit disables the outputs

when the junction temperature reaches 150

°C,

±20°C. The thermal shutdown circuit has a

hysteresis of

≈ 12°C to prevent oscillations. When

this circuit activates, the output stage of each driver

is put into the high impedance mode, thereby

shutting off the output currents. The remainder of the

internal circuitry remains biased. The outputs will

become active once again as the IC cools down.

b)

Driver Inputs

The driver inputs determine the state of the outputs in

accordance with Tables 1 and 2. The driver inputs have a

nominal threshold of 1.2 V, and their voltage must be kept

voltage is taken more than 0.5 V below ground, excessive

currents will flow, and proper operation of the drivers will

be affected. An open pin is equivalent to a logic high, but

good design practices dictate that inputs should never be

left open. The characteristics of the driver inputs are shown

in Figure 15. This graph is not affected by the state of the

Enable pins.

Enable Logic

Each driver’s outputs are active when the Enable inputs

(Pins 4 and 12) are true according to Tables 1 and 2.

The Enable inputs have a nominal threshold of 1.2 V and

for proper operation. If the voltage is taken more than 0.5 V

below ground, excessive currents will flow, and proper

operation of the drivers will be affected. An open pin is

equivalent to a logic high, but good design practices dictate

that inputs should never be left open. The Enable input

characteristics are shown in Figure 15.

Operating Temperature Range

The minimum ambient operating temperature is listed as

−40

°C to meet EIA−485 specifications, and 0°C to meet

EIA−422−A is the reason for the narrower temperature range.