MC75172B Datasheet, PDF (8/12 Pages) ON Semiconductor – QUAD EIA-485 LINE DRIVERS

MC75172B MC75174B
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
L
H
X
L
H
H
X
L
H
L
L
X
L
L
H
X
L
H
Z
Z
Table 2. MC75174B Truth Table
Outputs
Data Input
H
L
X
Enable
H
H
L
Y
Z
H
L
L
H
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:
a) the Enable inputs are set according to Table 1 or 2;
b) VCC is less than 1.5 V;
c) 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.
The drivers are protected from short circuits by two
methods:
a) 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.
b) A thermal shutdown circuit disables the outputs when
the junction temperature reaches 150°C, ±ā20°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.
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.
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