CPC7691

R01

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13

Ringing to Talk Logic Sequence: Break-Before-Make

Logic states and explanations are provided in the “Introduction” on page 9.

2.8 Ringing Switch Zero-Cross Current Turn Off

After the application of a logic change to cease

ringing, the ringing switch (SW4), designed to delay

deactivation until a zero current event, will remain

active until a current zero-crossing occurs. Once on

(active) the ringing switch requires a zero current

event to turn off and therefore should not be used to

switch a DC-only signal. This zero current switching

characteristic will reduce and possibly eliminate

overall system impulse noise normally associated with

ringing switches. See IXYS Integrated Circuits

Division application note AN-144, Impulse Noise Benefits

of Line Card Access Switches for more information. The

attributes of ringing switch SW4 may make it possible

to eliminate the need for a zero-cross switching

scheme. A minimum impedance of 300

 in series with

the ringing generator is recommended.

2.9 Power Supplies

Both a +5V supply and battery voltage are connected

to the CPC7691. Switch state control is powered

exclusively by the +5V supply. As a result, the

CPC7691 exhibits extremely low power consumption

during active and idle states.

2.10 Internal Protection

The CPC7691 uses a combination of current limited

break switches, a diode bridge/SCR clamping circuit,

and a thermal shutdown mechanism to protect the

SLIC device or other associated circuitry from damage

during line transient events such as lightning and fault

conditions such as power induction and power-cross.

2.10.1 Current Limiting Function

If a lightning strike transient occurs when the device is

in the talk state, the current passed from the line to the

internal integrated protection circuitry is restricted by

the dynamic current limit response of the active break

switches. For instance, during the talk state when a

1000V 10x1000

s lightning pulse (GR-1089-CORE)

is applied to the line though a properly clamped

will be a pulse having a typical magnitude of 2.5A with

a duration less than 0.5

s.

Whenever a power-cross fault occurs with the device

in the talk state, the current passed though the break

switches SW1 and SW2 on to the integrated

protection circuit is limited by the DC current limit

response of the break switches. The DC current limit

specified over temperature ranges from 80mA to

425mA and has a negative temperature coefficient. As

a result, if the device is subjected to extended heating

due to a power-cross fault condition, the measured

temperature increases. If the device temperature rises

sufficiently, the thermal shutdown mechanism will

activate, and the device will enter the All-Off state.

2.10.2 Diode Bridge/SCR

During a positive potential transient or other fault

limited by the break switches, are conducted through

State

Latch

Timing

Break

Switches

Ringing

Return

Switch

(SW3)

Ringing

Switch

(SW4)

Ringing

1

0

Z-

Off

On

On

All-Off

1

0

Hold this state for one-half of the ringing cycle.

SW4 waiting for zero current to turn off.

Off

Off

On

All-Off

0

Zero current has occurred. SW4 has opened

Off

Off

Off

Talk

0

Z

Close break switches

On

Off

Off