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.

0

0

4

8

12

16

20

24

28

32

36

40

100

200

300

400

500

- I(A) [mA]

VB = 40 V

VE = hi

VB = 24 V

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-

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.

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.