CPC7695 datasheet(21/25 Pages) IXYS

21 / 25 page

INTEGRATED CIRCUITS DIVISION

CPC7695

R01

www.ixysic.com

Although battery power is not used for switch control, it

is required to supply trigger current for the integrated

internal protection circuitry SCR during fault

conditions. This integrated SCR is designed to

activate whenever the voltage at TBAT or RBAT drops

2V to 4V below the applied voltage on the VBAT pin.

Because the battery supply at this pin is required to

source trigger current during negative overvoltage

fault conditions at tip and ring, it is important that the

net supplying this current be a low impedance path for

high speed transients such as lightning. This will

permit trigger currents to flow enabling the SCR to

activate and thereby prevent a fault induced negative

overvoltage event at the TBAT or RBAT nodes.

2.10 Protection

2.10.1 Current Limiting Function

If a lightning strike transient occurs when the device is

in theTalk state, the current is passed along the line to

the integrated protection circuitry and restricted by the

dynamic current limit response of the active switches.

During theTalk state when a 1000V 10x1000

S pulse

(GR-1089-CORE lightning) is applied to the line

though a properly clamped external protector, the

current seen at TLINE or RLINE will be a pulse with a

typical magnitude of 2.5A and a duration of less than

0.5

s.

If a power-cross fault occurs with the device in theTalk

state, the current is passed though the break switches

SW1 and SW2 on to the integrated protection circuit

but 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 a power-cross fault condition,

the measured current into TLINE or RLINE will decrease

as the device 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

The CPC7695 uses a combination of current limited

break switches, a diode bridge/SCR clamping circuit,

and a thermal shutdown mechanism to protect the

SLIC device and 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

FGND. Voltage is clamped to a diode drop above

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

negative than the voltage source at VBAT, the SCR

conducts and faults are shunted to FGND via the SCR.

Devices utilizing the diode bridge will steer negative

transients to VBAT.

In order for the SCR to crowbar or foldback, the SCR’s

on-voltage (see “Protection Circuitry Electrical

Specifications” on page 14) must be less than the

applied voltage at the VBAT pin. If the VBAT voltage is

less negative than the SCR on-voltage, or if the VBAT

supply is unable to source the 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 is directed to

ground. The negative cycle of the transient will cause

the SCR to conduct when the voltage exceeds the

VBAT reference voltage by two to four volts, steering

the fault current to ground.

Note: The CPC7695xB does not contain a protection

SCR but instead utilizes a diode bridge to clamp both

polarities of a fault transient. These diodes pass the

charge of negative fault potentials to the VBAT pin.

2.10.3 Thermal Shutdown

The thermal shutdown mechanism will activate when

the device die temperature reaches a minimum of

110°C, placing the device in the All-Off state

regardless of INRINGING, INTESTin and INTESTout logic

inputs. During thermal shutdown events the TSD pin

will output a logic low with a nominal 0V level. A logic

high is output from the TSD pin during normal

operation with a typical output level equal to VDD.

If presented with a short duration transient such as a

lightning event, the thermal shutdown feature will

typically not activate. But in an extended power-cross

event, the device temperature will rise and the thermal

shutdown mechanism will activate forcing the switches

to the All-Off state. At this point the current measured

into TLINE or RLINE will drop to zero. Once the device

enters thermal shutdown it will remain in the All-Off

state until the temperature of the die drops below the

deactivation level of the thermal shutdown circuit. This

permits the device to return to normal operation. If the

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