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R06

CPC7581

2 Functional Description

2.1 Introduction

The CPC7581 has three states:

ringing switches SW3 and SW4 open.

line break switches SW1 and SW2 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

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).

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 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

relevant ITU, LSSGR, TIA/EIA and IEC protection

requirements.

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.

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.

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).

To use make-before-break ringing switch release

causes the operational sequence shown in “Make-

Before-Break Operation Logic Table (Ringing to Talk

Transition)” on page 11 to occur.