CPC7581 Datasheet, PDF (8/12 Pages) Clare, Inc. – Line Card Access Switch

CPC7581
Functional Description
Introduction
The CPC7581 has three states:
• Idle/talk state (line break switches SW1, and SW2
closed, ringing switches SW3, SW4 open)
• Ringing state (line break switches SW1, and SW2
open, ringing switches SW3, SW4 closed)
• All Off state (line break switches SW1, and SW2
open, ringing switches SW3, SW4 open)
The CPC7581 offers break-before-make and make-
before-break switching with simple logic level input con-
trol. Solid state switch construction means no impulse
noise is generated when switching during ring cadence
or ring trip, thus eliminating the need for external “zero
cross” switching circuitry. State control is via logic level
input so no additional driver circuitry is required. The line
break switches SW1 and SW2 are linear switches that
have exceptionally low RDSON and excellent matching
characteristics. The ringing access switch SW4 has a
breakdown voltage rating of >480V which is sufficiently
high, with proper protection, to prevent breakdown in the
presence of a transient fault condition.
Integrated into the CPC7581 is a diode bridge/SCR
clamping circuit, current limiting and thermal shutdown
mechanism to provide protection to the SLIC device
during a fault condition. Positive and negative surges
are reduced by the current limiting circuitry and steered
to ground via diodes and the integrated SCR. Power
cross transients are also reduced by the current limiting
and thermal shutdown circuits. Please note that only
the CPC7581BA has the integrated protection SCR.
To protect the CPC7581 from an overvoltage fault con-
dition, use of a secondary protector is required. The
secondary protector must limit the voltage seen at the
tip and ring terminals to a level below the max break-
down voltage of the switches. To minimize the stress on
the solid-state contacts, use of a foldback or crowbar
type secondary protector is recommended. With proper
selection of the secondary protector, a line card using
the CPC7581 will meet all relevant ITU, LSSGR, FCC
or UL protection requirements.
The CPC7581 operates from a +5V supply only. This
gives the device extremely low idle and active power dis-
sipation and allows use with virtually any range of battery
voltage. A battery voltage is also used by the CPC7581
as a reference for the integrated protection circuit. In the
event of a loss of battery voltage, the CPC7581 will enter
an “all off” state.
Switch Timing
The CPC7581 provides, when switching from the ringing
state to the idle/talk state, the ability to control the timing
when the ringing access switches SW3 and SW4 are
released relative to the state of the line break switches
SW1 and SW2 using simple logic level input. This is
referred to a “make before break” or “break before make”
operation. When the line break switch contacts (SW1,
SW2) are closed (or made) before the ringing access
switch contact (SW3, SW4) is opened (or broken), this is
referred to a “make-before-break” operation. “Break-
before-make” operation occurs when the ringing access
contact (SW3, SW4) is opened (broken) before the line
break switch contacts (SW1, SW2) are closed (made).
With the CPC7581 the “make before break” and “break
before make” operations can easily be selected by apply-
ing logic level inputs to pin 10 of the device.
The logic sequences for either mode of operation are
given in Tables 5 and 6. Logic states and explanations
are given in Table 8.
Ring Access Switch Zero Cross Current Turn Off
After the application of a logic input to turn SW4 off, the
ring access switch is designed to delay the change in
state until the next zero crossing. Once on, the switch
requires a zero current cross to turn off and therefore
should not be used to switch a pure DC signal. The
switch will remain in the on state no matter what logic
input until the next zero crossing. For proper operation,
pin 12 (R_Ring) should be connected using proper imped-
ance to a ring generator or other AC source. These
switching characteristics will reduce and possibly elimi-
nate overall system impulse noise normally associated
with ringing access switches. The attributes of ringing
access switch SW4 may make it possible to eliminate
the need for a zero cross switching scheme. A minimum
impedance of 300Ω in series with the ring generator is
recommended.
Power Supplies
Both a +5V supply and battery voltage are connected
to the CPC7581. CPC7581 switch state control is pow-
ered exclusively by the +5V supply. As a result, the
CPC7581 exhibits extremely low power dissipation dur-
ing both active and idle states.
The battery voltage is not used for switch control but
rather as a reference by the integrated secondary pro-
tection circuitry. The integrated SCR is designed to trig-
ger when pin 2 (T_BAT) or pin 15 (R_BAT) drops 2 to 4V
below the battery. This trigger prevents a fault induced
overvoltage event at the T_BAT or R_BAT nodes.
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Rev. 2