MC75172B MC75174B

8

MOTOROLA ANALOG IC DEVICE DATA

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

simultaneoulsy 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

Enables

Outputs

Data Input

EN

EN

Y

Z

H

H

X

H

L

H

L

H

H

X

X

H

L

L

H

L

H

H

X

X

L

L

H

H

L

H

L

X

X

L

L

H

L

L

H

L

X

X

L

L

H

L

Z

H

Z

Table 2. MC75174B Truth Table

Outputs

Data Input

Enable

Y

Z

H

H

H

L

L

X

H

L

L

Z

H

Z

X

L

Z

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)

VCC is less than 1.5 V;

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

within the range of 0 V to VCC 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 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

their voltage must be kept within the range of 0 V to VCC 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 specifications. The higher VOD required by

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