I
NTEGRATED
C
IRCUITS
D
IVISION
CPC5622
10 www.ixysic.com R03
3. Using LITELINK
As a full-featured telephone line interface, LITELINK
performs the following functions:
• DC termination and V/I slope control
• AC impedance control
• 2-wire to 4-wire conversion (hybrid)
• Current limiting
• Ringing detect signalling reception
• Caller ID signalling reception
• Switch hook
LITELINK can accommodate specific application
features without sacrificing basic functionality or
performance. Application features include, but are not
limited to:
• High transmit power operation
• Pulse dialing
• Ground start
• Loop start
• Parallel telephone off-hook detection (line intrusion)
• Battery reversal detection
• Line presence detection
• World-wide programmable operation
This section of the data sheet describes LITELINK
operation in standard configuration for usual
operation. IXYS Integrated Circuits Division offers
additional application information on-line (see Section 5
on page 14) for the following topics:
• Circuit isolation considerations
• Optimizing LITELINK performance
• Data Access Arrangement architecture
• LITELINK circuit descriptions
• Surge protection
• EMI considerations
Other specific application materials are also
referenced in this section as appropriate.
3.1 Switch Hook Control (On-hook
and Off-hook States)
LITELINK operates in one of two conditions, on-hook
and off-hook. In the on-hook condition the telephone
line is available for calls. In the off-hook condition the
telephone line is engaged. The OH
control input is
used to place LITELINK in one of these two states.
With OH
high, LITELINK is on-hook and ready to
make or receive a call. Also while on-hook,
LITELINK’s ringing detector and CID amplifiers are
both active.
Asserting OH
low causes LITELINK to answer or
originate a call by entering the off-hook state. In the
off-hook state, loop current flows through LITELINK.
3.2 On-hook Operation: OH=1
The LITELINK application circuit leakage current is
less than 10 A with 100 V across ring and tip,
equivalent to greater than 10 M on-hook resistance.
3.2.1 Ringing Signal Reception via the
Snoop Circuit
In the on-hook state (OH not asserted), an internal
multiplexer engages the snoop circuitry. This circuit
simultaneously monitors the telephone line for two
conditions; incoming ringing signal and caller ID data
bursts.
Refer to the application schematic diagram (see Figure
2 on page 6). C7 (C
SNP-
) and C8 (C
SNP+
) provide a
high-voltage isolation barrier between the telephone
line and SNP- and SNP+ input pins of the LITELINK
while coupling AC signals to the snoop amplifier. The
snoop circuit “snoops” the telephone line continuously
while drawing no dc current. In the LITELINK, the
incoming ringing signals are compared to a reference
level. When the ringing signal exceeds the preset
threshold, the internal comparators generate the
RING
and RING2 signals which are output from
LITELINK at pins 9 and 10, respectively. Selection of
which output to use is dependent upon the support
logic responsible for monitoring and filtering the
ringing detect signals. To reduce or eliminate false
ringing detects this signal should be digitally filtered
and qualified by the system as a valid ringing signal. A
logic low output on RING
or RING2 indicates that the
LITELINK ringing signal detect threshold has been
exceeded. In the absence of any incoming ac signal
the RING
and RING2 outputs are held high.
The CPC5622 RING
output signal is generated by a
half-wave ringing detector while the RING2
output is
generated by a full-wave ringing detector. A half-wave
ringing detector’s output frequency follows the
frequency of the incoming ringing signal from the
Central Office (CO) while a full-wave ringing detector’s
output frequency is twice that of the incoming signal.
Because RING
is the output of a half-wave detector, it
