iC-HX
preliminar y
Rev A1, Page 8/11
3-CHANNEL DIFFERENTIAL COLD LINE DRIVER
Power dissipation in the driver occurs with each switch-
ing edge when over the double signal run time the in-
ternal resistor forms a voltage divider with the charac-
teristic line impedance and is proportional to the length
of the connected line and the switching frequency. If
the internal resistor is perfectly matched to the charac-
teristic line impedance, the voltage divider generates
half the supply voltage at the line input, only supplying
the full voltage when an echo occurs. iC-HX exploits
this behavior of the open line in order to reduce the
power dissipation in the driver. A switch is triggered
by applying the halved low-impedance supply voltage,
buffered with capacitors, to the line input and termi-
nated by applying the internal resistor shortly before
the echo occurs. Power dissipation occurs regardless
of the length of the connected line in the time between
the application of the resistor to the line and the begin-
ning of the echo. In order to control this process iC-HX
must recognize the length of the connected line. The
line is measured using an integrated procedure which
evaluates the line echo. This principle of power dis-
sipation reduction only functions when a single wave
travels along the line. The maximum transmission fre-
quency with a reduced power dissipation is directly
proportional to the line length. If the transmission fre-
quency is too high for the line length, iC-xSwitch is no
longer used, resulting in increased power dissipation in
the driver. The required halved supply voltage is gen-
erated internally in the chip and must be buffered by
capacitors. On a rising edge current flows from the ca-
pacitor into the line and back into the capacitor on a
falling edge. With the differential operation of two lines
the currents flow from one line to the other and back
again.
Figure 5 shows the three switches, the integrated re-
sistor to match the characteristic line impedance and
the connected line. VB is the positive power supply
and VB/2 is the half of it. The control of the switches
depends on the input signals of the device and the
length of the connected line. With all enable-signals
at lo-level the output A is high impedance (tristate).
VB
at the beginning (A) and end (B) of the line at intervals
t1 to t8. Figure 6 shows operation without iC-Xswitch.
Power dissipation P
D
(HX) occurs at intervals t1 to t4
and t5 to t8. Figure 7 describes operation with iC-
xSwitch; power dissipation P
D
(HX) occurs between t3
and t4 and t7 and t8. The mean power dissipation is
significant for the warming of the device, which is pro-
portional to the duty cycle. This results in a reduced
power dissipation (at the same frequency), meaning
there is less power dissipation with a shorter line or
through the use of iC-xSwitch with a long line, for ex-
ample.
V(E)
V(A)
V(B)
ENHi
ENLo
ENxS
PD(HX)
Time
t1 t2
t4
t5 t6
t8
Figure 6: Power dissipation P
D
(HX) without iC-
xSwitch
V(E)
V(A)
V(B)
ENHi
ENLo
ENHi
HiSwitch
Line
ENxS
PD(HX)
ENLo
LoSwitch
ENxS
xSwitch
Time
t1 t2 t3 t4
t5 t6 t7 t8
VB/2
Figure 7: Power dissipation P
D
(HX) with iC-xSwitch
Figure 5: Circuit diagram with switches and line
Figures 6 and 7 show the input signal V(E), the switch
trigger signals derived from this and the voltage curve
An example for the power dissipation is given in figure
the iC-HX behaves like the iC-DL.
ICHAUS [ IC-HAUS GMBH ]
