CPC7581(2001) 查看數據表（PDF） - Clare Inc => IXYS

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CPC7581
(Rev.:2001)

Line Card Access Switch

Clare Inc => IXYS 

CPC7581

Battery Voltage Monitor

The CPC7581 also uses the voltage reference to moni-

tor battery voltage. If battery voltage is lost, the CPC7581

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 volt-

age drops below –15V. This battery monitor feature

draws a small current from the battery (< 1µA typ.) and

will add slightly to the device’s overall power dissipation.

Protection

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 and to ground. Voltage is

clamped to the diode drop above ground. During a neg-

ative transient of two to four volts more negative than

the battery, the SCR conducts 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 7) 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

crowbar, 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 conduct when the voltage exceeds the bat-

tery reference voltage 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 light-

ning) is applied to the line though a properly clamped

external protector, the current seen at pins 2 (TBAT) and

pin 15 (RBAT) will 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 limited by the dynamic DC current limit response

of the two break switches. The DC current limit, specified

over temperature, is between 80mA and 425mA and the

circuitry has a negative temperature coefficient. As a

result, if the device is subjected to extended heating due

to power cross fault, the measured current at pin 2 (TBAT)

and pin 15 (RBAT) will decrease as the device tempera-

ture increases. If the device temperature rises sufficient-

ly, the temperature shutdown 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°C

placing the device in the “all off” state regardless of logic

input. During this thermal shutdown mode, pin 8 (TSD)

will read 0V. Normal output of TSD is +VDD

If presented with a short duration transient such as a

lightning event, the thermal shutdown feature will not

typically activate. But in an extended power cross tran-

sient, the device temperature will rise and the thermal

shutdown will activate forcing the switches to an “all off”

state. At this point the current measured at pin 2 (TBAT)

and pin 15 (RBAT) will drop to zero. Once the device

enters thermal shutdown it will remain in the “all off”

state until the temperature of the device drops below

the activation level of the thermal shutdown circuit. This

will return the device to the state prior to thermal shut-

down. 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, reactivat-

ing 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.

External Protection Elements

The CPC7581 requires only one overvoltage secondary

protector on the loop side of the device. The integrated

protection feature described above negates the need

for protection on the line side. The purpose of the sec-

ondary protector is to limit voltage transients to levels

that do not exceed the breakdown voltage or input-out-

put isolation barrier of the CPC7581. A foldback or

crowbar type protector is recommended to minimize

stresses on the device.

Consult Clare’s application note, AN-100, “Designing

Surge and Power Fault Protection Circuits for Solid

State Subscriber Line Interfaces” for equations related

to the specifications of external secondary protectors,

fused resistors and PTCs.

Rev. 2

www.clare.com

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