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12

Rev. E

CPC7583

(TSD) as a control are 0 which forces the device to the “all

off state” or float which allow logic inputs to remain active.

This may require use of an open collector buffer.

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. These switching characteristics will reduce and

possibly eliminate overall system impulse noise normally

associated with ringing access switches. The attributes of

ringing access switch 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 CPC7583. CPC7583 switch state control is powered

exclusively by the +5V supply. As a result, the CPC7583

exhibits extremely low power dissipation during both active

and idle states.

Battery Voltage Monitor

The CPC7583 also uses the voltage reference to monitor

battery voltage. If battery voltage is lost, the CPC7583 will

immediately enter the “all off” state and remain in this state

until the battery voltage is restored. The device will also

enter the “all off” state if the battery voltage rises above –

10V and will remain there until the battery voltage drops

below –15V. This battery monitor feature draws a small

current from the battery (<1

µA) and will add slightly to the

device’s overall power dissipation.

Protection

Diode Bridge/SCR

The CPC7583 uses a combination of current limited break

switches, a diode bridge/SCR clamping circuit and a ther-

mal shutdown mechanism to protect the SLIC device or

other associated circuitry from damage during line tran-

sient events such as lightning. During a positive transient

condition, the fault current is conducted through the diode

bridge and to ground. During a negative transient of two or

four volts more negative than the battery, the SCR con-

ducts and faults are shunted to ground via the SCR and

diode bridge.

Also, in order for the SCR to crowbar or foldback, the on

voltage (see Table 11) of the SCR must be less negative

than the battery reference voltage. If the battery voltage is

less negative the SCR on voltage, the SCR will not crow-

bar, however it will conduct fault currents to ground.

For power induction or power cross fault conditions, the

positive cycle of the transient is clamped to the diode drop

above ground and the fault current directed to ground. The

negative cycle of the transient will cause the SCR to con-

duct when the voltage exceeds the battery reference volt-

age by two to four volts, steering the current to ground.

Current Limiting function

If a lightning strike transient occurs when the device in the

talk/idle state, the current is passed along the line to the

integrated protection circuitry and limited by the dynamic

current limit response of break switches SW1 and SW2.

When a 1000V 10x1000 pulse (LSSGR lightning) is ap-

plied to the line though a properly clamped external pro-

be a pulse with a typical magnitude and duration of 2.5A

and < 0.5ms.

If a power cross fault occurs with device in the talk/idle

state, the current is passed though the break switches SW1

and SW2 on to the integrated protection circuit and is lim-

ited by the dynamic DC current limit response of the two

break switches. The DC current limit, specified over tem-

perature, is between 80mA and 400mA and the circuitry

has a negative temperature coefficient. As a result, if the

device is subjected to extended heating due to power cross

will decrease as the device temperature increases. If the

device temperature rises sufficiently, the temperature shut-

down mechanism will activate and the device will default to

the “all off” state.

Temperature Shutdown

The thermal shutdown mechanism will activate when the

device temperature reaches a minimum of 110

ϒC placing

the device in the “all off” state regardless of logic input.

During this thermal shutdown mode, pin 13 (TSD) will read

0V. Normal output of TSD is +V

DD

.

If presented with a short duration transient such as a light-

ning event, the thermal shutdown feature will not typically

activate. But in an extended power cross transient, the

device temperature will rise and the thermal shutdown will

activate forcing the switches to an “all off” state. At this

will drop to zero. Once the device enters thermal shut-

down it will remain in the “all off” state until the temperature

of the device drops below the activation level of the ther-

mal shutdown circuit. This will return the device to the state

prior to thermal shutdown. 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