CPC7508 Datasheet, PDF (9/14 Pages) Clare, Inc. – Line Card Access Switch

2. Functional Description
CPC7508
2.1 Introduction
The CPC7508 LCAS provides the necessary test
access functions for line card interfaces supported by
ringing SLICs in contemporary Fiber In The Loop
(FITL), Fiber To The Home (FTTH) applications and
Voice over Internet Protocol (VoIP).
These applications have a different working
environment than standard traditional Digital Loop
Carrier (DLC) equipment and therefore have unique
requirements. Two significant differences are the
diverse supply voltages and the interface to ringing
SLICs.
The once common 5V supply is generally not available
in the modern short loop products made feasible with
the advent of broadband services. To support
applications where a 5V supply is not available but a
12V supply is, the V_DD input power specification for
the CPC7508 has been set accordingly.
Ringing SLICs have replaced the customary standard
SLIC and ringing relay configuration and for the LCAS
to be compatible with a ringing SLIC. the LCAS
internal protection circuits have been removed. This
was essential so as not to clip the ringing waveform
output from the ringing SLIC.
Traditional test access is provided by two pair of test
access switches, TEST_IN and TEST_OUT,
employing the TEST_IN & OUT state. Supplementing
the traditional test access switches is the TEST
BRIDGE switch pair capable of cross connecting the
test busses thereby providing the means to validate
the status of SW1 and SW2, the Break Switches.
2.2 Description
2.2.1 Overview
Because the CPC7508 LCAS utilizes solid-state
switch construction to implement the switching
functions this means no impulse noise is generated
when switching large line potentials. To ensure proper
voice performance the linear break switches SW1 and
SW2 have exceptionally low R_ON and excellent
matching characteristics. The switches have a
minimum open contact breakdown voltage of 320 V
and a minimum contact to ground breakdown voltage
of 225 V at +25°C, sufficiently high with proper
protection to prevent breakdown in the presence of a
transient fault condition.
Integrated into the CPC7508 is a dynamic active
current limit, a DC current limit and a thermal
shutdown mechanism to provide protection for the
SLIC during a fault condition. Positive and negative
lightning surge currents are reduced by the dynamic
current limiting circuitry and excess power-cross
potentials are restricted by the DC current limit and
thermal shutdown circuits.
To protect the CPC7508 from an over-voltage fault
condition, use of a secondary protector is required.
The secondary protector must limit the voltage seen at
it’s tip and ring line terminals to a level below the
maximum breakdown voltage of the switches. With
proper selection of the secondary protector, a line
interface circuit using the CPC7508 will meet all
relevant ITU, LSSGR, TIA/EIA and IEC protection
requirements.
The CPC7508 operates from a single +12.5 V supply
giving the device extremely low power consumption in
any state.
State control is via TTL logic-level compatible inputs
so no additional driver circuitry or level translators are
required. TTL compatible inputs make state control of
the CPC7508 with low voltage logic devices possible.
2.2.2 Logic States
The CPC7508 provides eight distinct states enabling
sufficient configurations to satisfy most design
requirements. They are defined below.
• Talk. Break switches SW1 and SW2 closed, all other
switches open. This provides a path between the
ringing SLIC and the drop allowing communication
and signalling to pass between the subscriber and
the network.
• Test_IN. Test switches SW5 and SW6 closed, all
other switches open. In this state the SLIC, CODEC
and digital carrier performance can be tested via the
Test In or Channel Test bus
• Test_OUT. Test switches SW3 and SW4 closed and
all other switches open. This state provides the
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