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)
V
CC
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 12V to −7.0V, 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 V
CC
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 V
CC
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 V
OD
required by
EIA−422−A is the reason for the narrower temperature range.
