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8

CPC7581

Rev. 2

Functional Description

Introduction

The CPC7581 has three states:

closed, ringing switches SW3, SW4 open)

open, ringing switches SW3, SW4 closed)

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

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 300

Ω 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

below the battery. This trigger prevents a fault induced

overvoltage event at the T