CPC7591 Datasheet, PDF (10/19 Pages) Clare, Inc. – Line Card Access Switch

CPC7591
2. Functional Description
2.1 Introduction
The CPC7591 has three states:
• Talk. Line break switches SW1 and SW2 closed,
ringing switches SW3 and SW4 open.
• Ringing. Ringing switches SW3 and SW4 closed,
line break switches SW1 and SW2 open.
• All-off. All switches open.
See “Truth Table” on page 9 for more information.
The CPC7591 offers break-before-make and
make-before-break switching from the ringing state to
the talk state with simple TTL level logic input control.
Solid-state switch construction means no impulse
noise is generated when switching during ringing
cadence or ring trip, eliminating the need for external
zero-cross switching circuitry. State-control is via TTL
logic-level input so no additional driver circuitry is
required. The linear line break switches SW1 and
SW2 have exceptionally low R_ON and excellent
matching characteristics. The ringing switch, SW4,
has a minimum open contact breakdown voltage of
465 V at +25°C, sufficiently high with proper protection
to prevent breakdown in the presence of a transient
fault condition (i.e., passing the transient on to the
ringing generator).
Integrated into the CPC7591 is an over-voltage
clamping circuit, active current limiting, and a thermal
shutdown mechanism to provide protection to the
SLIC during a fault condition. Positive and negative
lightning surge currents are reduced by the current
limiting circuitry and hazardous potentials are diverted
away from the SLIC via the protection diode bridge or
the optional integrated protection SCR. Power-cross
potentials are also reduced by the current limiting and
thermal shutdown circuits.
To protect the CPC7591 from an overvoltage fault
condition, the use of a secondary protector is required.
The secondary protector must limit the voltage seen at
the T_LINE and R_LINE terminals to a level below the
maximum breakdown voltage of the switches. To
minimize the stress on the solid-state contacts, use of
a foldback or crowbar type secondary protector is
highly recommended. With proper selection of the
secondary protector, a line card using the CPC7591
will meet all relevant ITU, LSSGR, TIA/EIA and IEC
protection requirements.
The CPC7591 operates from a single +5 V supply
only. This gives the device extremely low idle and
active power consumption with virtually any range of
battery voltage. The battery voltage used by the
CPC7591 has a two fold function. For protection
purposes it is used as a fault condition current source
for the internal integrated protection circuitry.
Secondly, it is used as a reference so that in the event
of battery voltage loss, the CPC7591 will enter the
all-off state.
2.2 Under Voltage Switch Lock Out Circuitry
2.2.1 Introduction
Smart logic in the CPC7591 now provides for switch
state control during both power up and power loss
transitions. An internal detector is used to evaluate the
V_DD supply to determine when to de-assert the under
voltage switch lock out circuitry with a rising V_DD and
when to assert the under voltage switch lock out
circuitry with a falling V_DD. Any time unsatisfactory low
V_DD conditions exist, the lock out circuit overrides user
switch control by blocking the information at the
external input pins and conditioning internal switch
commands to the all-off state. Upon restoration of
V_DD, the switches will remain in the all-off state until
the LATCH input is pulled low.
The rising V_DD switch lock-out release threshold is
internally set to ensure all internal logic is properly
biased and functional before accepting external switch
commands from the inputs to control the switch states.
For a falling V_DD event, the lock-out threshold is set to
assure proper logic and switch behavior up to the
moment the switches are forced off and external
inputs are suppressed.
To facilitate hot plug insertion and system power up
state control, the LATCH pin has an integrated weak
pull up resistor to the V_DD power rail that will hold a
non-driven LATCH pin at a logic high state. This
enables board designers to use the CPC7591 with
FPGAs and other devices that provide high
impedance outputs during power up and logic
configuration. The weak pull up allows a fan out of up
to 32 when the system’s LATCH control driver has a
logic low minimum sink capability of 4mA.
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