MC1488
APPLICATIONS INFORMATION
The Electronic Industries Association EIA–232D specification
should be placed in each power supply lead to prevent overheating in
this fault condition. These two diodes, as shown in Figure 13, could be
used to decouple all the driver packages in a system. (These same
diodes will allow the MC1488 to withstand momentary shorts to the
± 25 V limits specified in the earlier Standard EIA–232B.) The
addition of the diodes also permits the MC1488 to withstand faults
with power supplies of less than the 9.0 V stated above.
details the requirements for the interface between data processing
equipment and data communications equipment. This standard
specifies not only the number and type of interface leads, but also the
voltagelevels to be used. The MC1488 quad driver and its companion
circuit, the MC1489 quad receiver, provide a complete interface
system between DTL or TTL logic levels and the EIA–232D defined
levels. The EIA–232D requirements as applied to drivers are
discussed herein.
The required driver voltages are defined as between 5.0 and 15 V
in magnitude and are positive for a Logic “0” and negative for a Logic
“1.” These voltages are so defined when the drivers are terminated
with a 3000 to 7000 Ω resistor. The MC1488 meets this voltage
requirement by converting a DTL/TTL logic level into EIA–232D
levels with one stage of inversion.
TheEIA–232D specification further requires that during transitions,
the driver output slew rate must not exceed 30 V per microsecond.
The inherent slew rate of the MC1488 is much too fast for this
requirement. The current limited output of the device can be used to
control this slew rate by connecting a capacitor to each driver output.
The required capacitor can be easily determined by using the
Figure 13. Power Supply Protection
to Meet Power Off Fault Conditions
V
CC
14
14
14
MC1488
MC1488
MC1488
relationship C = I
x ∆T/∆V from which Figure 12 is derived.
OS
Accordingly, a 330 pF capacitor on each output will guarantee a
worst case slew rate of 30 V per microsecond.
Figure 12. Slew Rate versus Capacitance
7
1
7
1
7
1
for I
= 10 mA
SC
V
EE
1000
100
The maximum short circuit current allowable under fault conditions
is more than guaranteed by the previously mentioned 10 mA output
current limiting.
30 V/µs
Other Applications
The MC1488 is an extremely versatile line driver with a myriad of
possible applications. Several features of the drivers enhance this
versatility:
1. Output Current Limiting – this enables the circuit designer to
define the output voltage levels independent of power supplies and
can be accomplished by diode clamping of the output pins. Figure 14
shows the MC1488 used as a DTL to MOS translator where the high
level voltage output is clamped one diode above ground. The
resistor divider shown is used to reduce the output voltage below the
300 mV above ground MOS input level limit.
10
333 pF
1.0
1.0
10
100
C, CAPACITANCE (pF)
1,000
10,000
The interface driver is also required to withstand an accidental
short to any other conductor in an interconnecting cable. The worst
possible signal on any conductor would be another driver using a
plus or minus 15 V, 500 mA source. The MC1488 is designed to
indefinitely withstand such a short to all four outputs in a package as
2. Power Supply Range – as can be seen from the schematic
drawing of the drivers, the positive and negative driving elements of
the device are essentially independent and do not require matching
power supplies. In fact, the positive supply can vary from a minimum
7.0 V (required for driving the negative pulldown section) to the
maximum specified 15 V. The negative supply can vary from
approximately – 2.5 V to the minimum specified – 15 V. The MC1488
will drive the output to within 2.0 V of the positive or negative supplies
aslongasthecurrentoutputlimitsarenotexceeded. Thecombination
of the current limiting and supply voltage features allow a wide
combination of possible outputs within the same quad package. Thus
if only a portion of the four drivers are used for driving EIA–232D
lines, the remainder could be used for DTL to MOS or even DTL to
DTL translation. Figure 15 shows one such combination.
long as the power supply voltages are greater than 9.0 V (i.e., V
CC
– 9.0 V). In some power supply designs, a loss of
9.0 V; V
EE
systempower causes a low impedance on the power supply outputs.
When this occurs, a low impedance to ground would exist at the
power inputs to the MC1488 effectively shorting the 300 Ω output
resistors to ground. If all four outputs were then shorted to plus or
minus 15 V, the power dissipation in these resistors would be
excessive. Therefore, if the system is designed to permit low
impedances to ground at the power supplies of the drivers, a diode
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MOTOROLA ANALOG IC DEVICE DATA
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