CPC7581
2. Functional Description
relevant ITU, LSSGR, TIA/EIA and IEC protection
requirements.
2.1 Introduction
The CPC7581 has three states:
The CPC7581 operates from a +5 V supply only. This
gives the device extremely low idle and active power
consumption and allows use with virtually any range of
battery voltage. 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 enters the all-off state.
• 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 CPC7581 offers break-before-make and
make-before-break switching from the ringing state to
the talk state with simple logic-level input control.
Solid-state switch construction means no impulse
noise is generated when switching during ring
cadence or ring trip, 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
2.2 Switch Logic
The CPC7581 provides, when switching from the
ringing state to the talk state, the ability to control the
release timing of the ringing switches SW3 and SW4
relative to the state of the line break switches SW1
and SW2 using simple logic-level input. This is called
make-before-break or break-before-make operation.
When the line break switch contacts (SW1 and SW2)
are closed (or made) before the ringing switch
contacts (SW3 and SW4) are opened (or broken), this
is called make-before-break operation.
linear switches that have exceptionally low R and
ON
excellent matching characteristics. The ringing switch
SW4 has a breakdown voltage rating of 465V @ 25°C.
This is 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).
Break-before-make operation occurs when the ringing
contacts (SW3 and SW4) are opened (broken) before
the line break contacts (SW1 and SW2) are closed
(made).
Integrated into the CPC7581 is a over voltage
clamping circuit, active current limiting, and a 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 hazardous potentials are steered to
ground via diodes and, in CPC7581xA and
CPC7581xC parts, an integrated SCR. Power-cross
potentials are also reduced by the current limiting and
thermal shutdown circuits.
To use make-before-break ringing switch release
timing, de-assert IN
during ringing. This
RINGING
causes the operational sequence shown in “Make-
Before-Break Operation Logic Table (Ringing to Talk
Transition)” on page 11 to occur.
To protect the CPC7581 from an overvoltage fault
condition, the 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
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
recommended. With proper selection of the secondary
protector, a line card using the CPC7581 will meet all
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