iC-DL
3-CHANNEL DIFFERENTIAL LINE DRIVER
Rev B1, Page 6/9
DESCRIPTION
Line drivers for control engineering couple TTL- or
CMOS-compatible digital signals with 24 V systems via
cables. The maximum permissible signal frequency is
dependent on the capacitive load of the outputs (ca-
ble length) or, more specifically, the power dissipation
in iC-DL resulting from this. To avoid possible short
circuiting the drivers are current-limited and shutdown
with excessive temperature.
When the output is open the maximum output voltage
corresponds to supply voltage VB (with the exception
of any saturation voltages). Figure 2 gives the typical
DC output characteristic of a driver as a function of the
load. The differential output resistance is typically 75
Ω
over a wide voltage range.
40
36
32
28
ever, further reflection of back travelling signals is pre-
vented by an integrated impedance network, as shown
in Figure 3.
Figure 3: Reflections caused by a mismatched line
termination
During a pulse transmission the amplitude at the iC-
DL output initially only increases to half the value of
supply voltage VB as the internal driver resistance and
characteristic line impedance form a voltage divider. A
wave with this amplitude is coupled into the line and
experiences after a delay a total reflection at the high-
impedance end of the line. At this position, the re-
flected wave superimposes with the transmitted wave
and generates a signal with the double wave amplitude
at the receiving device.
VB = 40 V
VE = hi
V(A) [V]
24
20
16
12
VB = 24 V
8
4
0
0
100
200
300
400
500
- I(A) [mA]
Figure 2: Load dependence of the output voltage
(High-side stage)
Each open-circuited input is set to low by an internal
pull-down current source; an additional connection to
GND increases the device’s immunity to interference.
The inputs are TTL- and CMOS-compatible. Due to
their high input voltage range, the inputs can also be
set to high-level by applying VCC or VB.
LINE EFFECTS
In PLC systems data transmission using 24 V sig-
nals usually occurs without a matched line termina-
tion. A mismatched line termination generates reflec-
tions which travel back and forth if there is also no line
adaptation on the driver side of the device. With rapid
pulse trains transmission is disrupted. In iC-DL, how-
Figure 4: Pulse transmission and transit times
After a further delay, the reflected wave also increases
the driver output to the full voltage swing. iC-DL’s inte-
grated impedance adapter prevents any further reflec-
tion and the achieved voltage is maintained along and
at the termination of the line.
A mismatch between iC-DL and the transmission line
influences the level of the signal wave first coupled
into the line, resulting in reflections at the beginning
of the line. The output signal may then have a num-
ber of graduations. Voltage peaks beyond VB or below
GND are capped by integrated diodes. By this way,
transmisssion lines with a characteristic impedance
between 30 and 140
Ω
permit proper operation.
ICHAUS [ IC-HAUS GMBH ]
