MTC-20172-PC AMI Semiconductor, Inc., MTC-20172-PC Datasheet - Page 38

• Manufacturer Part Number: MTC-20172-PC
• Description: S Interface Circuit for ISDN (SIC)
• Manufacturer: AMI Semiconductor, Inc.

High-Ohmic State when
not Powered.
The S-bus drivers of the SIC will be in
a high-ohmic state when not powered,
i.e. when the bus is active, but the SIC
is not powered. This is required to
fulfil the CCITT requirements on output
impedance in the TE (or LT-T) position,
in parallel with other SICs. (This is
achieved by using only n-type devices
for the complete driver stage, because
push-pull CMOS drivers always have
parasitic diodes to both power supply
and ground. These diodes limit the
voltage between the SXP and SXN
pins to less than 1.2V, by shorting the
differential voltage via the power pins.
On the S-bus, the amplitude of the
voltage would drop to 0.6 volts, while
the CCITT I.430, 8.5.1.2 requires
currents below 0.6 mA when signals
with peak amplitudes of 1.2 V are
applied on the S-bus).
5.2.4 Ternary Driver:
Compatibility and Evolution
On the SIC, the ternary drivers require
two series resistors of around 34Ω
between with the balanced output
drivers. The 34Ω includes the real part
of the impedance of the sending
transformer. When an MTC-2072 SIC
is replaced by the MTC-20172 SIC,
the extra resistors must be added to
the PCB.
Originally the MTC-2072 SIC drivers
were current sources, driving a known
7.5 mA in the transformer, with a
simple voltage limitation. In the MTC-
2072 SIC, a reference resistor (Rref) is
used to generate the nominal 7.5 mA
current.
The voltage drive with 80Ω in series
limits overshoots on single pulses,
compared to the older high-ohmic
current drive. However, the peak-
voltage on the bus goes to 140%,
when multiple S components are
driving the bus simultaneously.
Note also that the 80Ω series
Fig. 5.4 : Pulse Mask 400 Ohm
28
MTC-20172
Data Sheet & Reference Manual
Rev. 1.1 February 1997
resistance could be considered
necessary to fulfill the CCITT
requirements on the driver output
impedance when transmitting a zero,
although the actual measurement
technique specified by the CCITT is
questionable, when the SIC drives the
specified nominal 400Ω impedance.
(CCITT I.430 8.5.1.2).
5.2.5 S-bus Transmitter Timing