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CPC7581 Datasheet(PDF) 9 Page - Clare, Inc. |
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CPC7581 Datasheet(HTML) 9 Page - Clare, Inc. |
9 / 12 page CPC7581 www.clare.com 9 Rev. 2 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 (< 1 µA 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 (T BAT) and pin 15 (R BAT) 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 (T BAT) and pin 15 (R BAT) 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 +V DD 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 (T BAT) and pin 15 (R BAT) 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. |
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