IC-WE Datasheet, PDF (6/10 Pages) IC-Haus GmbH – 3-CHANNEL 75 Ω LINE DRIVER

iC-WE
3-CHANNEL 75 Ω LINE DRIVER
Rev D1, Page 6/10
APPLICATIONS INFORMATION
Line drivers for automation & control equipment connect digital signals with TTL or CMOS levels to 24 V systems
via cables. Due to possible short-circuits, the drivers are current-limited and lock out in the event of over-
temperature.
The maximum permissible signal frequency depends on
32
the capacitive load of the outputs (cable length) or the
28
consequential power dissipation in the iC-WE.
24
VB = 30V
Except for saturation voltages, the maximum output vol-
20
A=High
tage corresponds to supply voltage VB when the output is
16
open. Fig. 1 shows the typical DC output characteristic of
12
a driver as a function of the load. The differential output
8
resistance is about 75 Ω in broad ranges.
4
Every open-circuited input is set to high level by an
internal pull-up current source; an additional inter-
0
connection with VCC enhances the interference
immunity. An input can be set to low level in response to
a short-circuit or a resistance (<7.5 kΩ) to GND.
0 50 100 150 200 250 300 350 400 450 500
Load Current [ mA ]
Fig. 1: Influence of load on output voltage
LINE EFFECTS
In PLC systems, data transmission with 24 V signals is
generally conducted without a line termination with the
characteristic impedance. A mismatched line end pro-
duces reflections which travel back and forth if there is no
line adapter at the driver end either. The transmission is
disrupted in case of high-speed pulse trains.
In the iC-WE, signal reflection is prevented by an integra-
ted characteristic impedance adapter, as shown in Fig. 2.
Fig. 2: Reflections due to open line end
During a pulse transmission the amplitude at the output of
the iC-WE initially only increases to about one half the
level of supply voltage VB since the internal resistance of
the driver and the line characteristic impedance form a
voltage divider. A wave with this amplitude is injected into
the line and experiences a total reflection at the high
impedance end of the line following a delay based on the
length of the cable. The open or high impedance
terminated end of the line exhibits a voltage maximum
with double amplitude since outgoing and reflected wave
are superimposed.
Fig. 3: Pulse transmission and transit times
Following a further delay the reflected wave also increases the driver output to twice the amplitude of the wave
initially injected, possibly capped by the integrated diode suppressor circuit. The integrated characteristic
impedance adaption in the iC-WE prevents another reflection and the voltage achieved is maintained along and
at the end of the line.
A mismatch between the iC-WE and the line influences the level of the initially injected wave and produces
reflections at the driver end. The output signal may have a number of graduations. Nonetheless, lines with
characteristic impedances in the range 40 to 150 Ω permit satisfactory transmissions.
Fig. 3 shows the transmission of a short pulse of 1.5 µs. The signal delay to the end of the cable (here 100 m)
is markedly longer than the transit time in the iC-WE driver.