CPC7581

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R05

return to normal operation. If the transient has not

passed, current will flow at the value allowed by the

dynamic DC current limiting of the switches and

heating will begin again, reactivating the thermal

shutdown mechanism. This cycle of entering and

exiting the thermal shutdown mode will continue as

long as the fault condition persists. If the magnitude of

the fault condition is great enough, the external

secondary protector could activate and shunt all

current to ground.

value of 300

μA biased from V

Clare recommends the use of an open-drain type

current to ground during normal operation when the

all-off state is not required.

2.5 Ringing Switch Zero-Cross

Current Turn Off

After the application of a logic input to turn SW4 off,

the ringing 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 the logic

input until the next zero crossing. These switching

characteristics will reduce and possibly eliminate

overall system impulse noise normally associated with

ringing switches. See Clare 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

Ω in series with the ring generator is

recommended.

2.6 Power Supplies

Both a +5 V supply and battery voltage are connected

to the CPC7581. CPC7581 switch state control is

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

the CPC7581 exhibits extremely low power dissipation

during both active and all-off states.

The battery voltage is not used for switch control but

rather as a supply for the integrated secondary

protection circuitry. The integrated SCR is designed to

2.7 Battery Voltage Monitor

battery voltage. If system battery voltage is lost, the

CPC7581 immediately enters the all-off state. It

remains in this state until the battery voltage is

restored. The device also enters the all-off state if the

system battery voltage goes more positive than –10 V,

and remains in the all-off state until the battery voltage

goes more negative than –15 V. This battery monitor

feature draws a small current from the battery (less

than 1

μA typical) and adds slightly to the device’s

overall power dissipation.

Due to the nature of the internal protection circuitry,

operate, but offers no transient protection. The supply

supply voltage applied to the line driver device.

2.8 Protection

2.8.1 Diode Bridge/SCR

The CPC7581 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. During a

positive transient condition, the fault current is

conducted through the diode bridge to ground via

ground. During a negative transient of 2 to 4 V more

negative than the battery, the SCR conducts and faults

In order for the SCR to crowbar (or foldback), the on

voltage (see “Protection Circuitry Electrical

Specifications” on page 9) of the SCR must be less

negative than the battery reference voltage. If the

battery voltage is less negative than the SCR on

trigger current, the SCR will not crowbar.

For power induction or power-cross fault conditions,

the positive cycle of the transient is clamped to a diode

drop above ground and the fault current directed to

ground. The negative cycle of the transient will cause

the SCR to conduct when the voltage exceeds the