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ISL5571AIB 데이터시트(PDF) 11 Page - Intersil Corporation |
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ISL5571AIB 데이터시트(HTML) 11 Page - Intersil Corporation |
11 / 12 page 11 with the integrated secondary protection, features offered by the ISL5571A, the application circuit will pass the AC power- cross and lightning immunity tests of the following regulatory requirements: • GR 1089-CORE •ITU-T K.20 This section will discuss the issues that must be considered when designing an external protection circuit for use with the ISL5571A. The external protection circuitry should be designed to limit the peak voltages on the TLine and RLine terminals of the ISL5571A. The most potentially stressful condition concerning the ISL5571A is low level power-cross when the ISL5571A switches are closed. Under this condition, the external protection circuitry limits the voltage and corresponding power dissipation until the ISL5571A thermal shutdown circuitry opens the switches. The external protector chosen for the TLine terminal must limit at a maximum of 130V thereby limiting the power stress on the Ring Return switch (SW3). The protector chosen for the RLine terminal must limit at a maximum of 220V thereby limiting the power stress on the Line Break switch (SW2). The 220V break-over voltage of the protector on RLine is large enough to not interfere with the AC ring signal during ringing. Texas Instruments and Teccor Electronics have designed specific parts to protect solid state line card access switches. The following protectors are recommended: Texas Instruments Part Number TISPL758LF3D(TLine and RLine) Teccor Electronics Part Number P1200SC (TLine) and P2000SC (RLine) Refer to the above company’s data sheets for information on their parts and reference designs for protection of solid state line card access switches, see Related Literature section on Page One. Break-Before-Make Operation The ISL5571A device inherently has a Break-before-Make condition between the following switches: a. Between the Line Break switch SW1 and the Ring Return switch SW3 during the transition from the Idle state to the Power Ringing state. b. Between the Line Break switch SW2 and the Ring Access switch SW4 during the transition from the Idle state to the Power Ringing state. Make-Before-Break Operation The ISL5571A device could exhibit a Make-before-Break condition between the Line Break switch SW2 and the Ring Access switch SW4 during the transition from the Power Ringing state to the Idle / Talk state. There is a period of time that can be as much as 25ms (1/2 cycle of the 20Hz ring signal) when both SW2 and SW4 will both be on (closed). This occurs because SW4 is an SCR and requires a zero current crossing to turn off. Protection SRC Latch-Up In the Make-before-Break condition, when transiting from the Power Ringing state to the Idle/ Talk state, during the negative cycle of the ring generator it is possible for enough current to flow that the protection SCR will turn on. This will result in shorting the ring generator and the Ring terminal of the SLIC to ground. When either SW2 or SW4 turns off, the Ring terminal of the SLIC will remain shorted to ground unless the output current limit of the SLIC is less than the holding current (110mA) of the protection SCR. The current limit of most SLICs are set well below the 110mA minimum holding current of the ISL5571A protection SCR and therefore should not be a concern. Another method to prevent latch-up of the protection SCR would be to strobe the TSD pin of the ISL5571A. as discussed in Break-Before-Make section. Ring Access Switch Quiescent Current During Ringing The Ring Access switch (SW4) is a silicon control rectifier type switch (SCR). During power ringing, the Ring Access switch will draw a nominal 2mA of current from the ring generator. This current is called IRING QUIESCENT and is equal to: IRING QUIESCENT = IRING GEN - IRING LOAD. System designers need to ensure that this additional current can be provided by the ring generator. Glossary of Acronyms AC = Alternating Current BIMOS = Bipolar Metal-Oxide Semiconductor CO = Central Office CODEC = CODer-DECoder DC = Direct Current DLC = Digital Loop Carrier DAML = Digitally Added Main Line EMI = Electromagnetic Interference ESD = Electrostatic Discharge FITL = Fiber in the Loop HFC = Hybrid Fiber Coax ICs = Integrated Circuits LCAS = Line Card Access Switch PBX = Private Branch Exchange REN = Ring Equivalency Number MOSFET = Metal-Oxide Semiconductor Field-Effect Transistor SCR = Silicon Control Rectifier SLIC = Subscriber Line Interface Circuit SMDs = Surface Mount Devices SOIC = Small Outline Integrated Circuit TSD = Thermal ShutDown TTL = Transistor-Transistor Logic ISL5571A |
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