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ISL5586 Datasheet(PDF) 10 Page - Intersil Corporation |
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ISL5586 Datasheet(HTML) 10 Page - Intersil Corporation |
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10 / 20 page  10 FN4924.2 November 3, 2004 The slope of the feed characteristic and the battery voltage define the maximum loop current on the shortest possible loop as the short circuit current ISC. The term ILIM is the programmed current limit, 1760/RIL. The line segment IA represents the constant current region of the loop current limit function. The maximum loop resistance for a programmed loop current is defined as RKNEE. When RKNEE is exceeded, the device will transition from constant current feed to constant voltage, resistive feed. The line segment IB represents the resistive feed portion of the load characteristic. Power Dissipation The power dissipated by the SLIC in the Forward Active mode while on hook is strictly a function of the quiescent currents for each supply. Off hook power dissipation is increased above the quiescent power dissipation by the DC load. If the loop length is less than or equal to RKNEE, the device is providing constant current (IA), and the power dissipation is calculated using Equation 26. If the loop length is greater than RKNEE , the device is operating in the constant voltage, resistive feed region. The power dissipated in this region is calculated using Equation 27. Since the current relationships are different for constant current versus constant voltage, the region of device operation is critical to valid power dissipation calculations. Reverse Active Mode Overview The reverse active mode (RA, 011) provides the same functionality as the forward active mode. On hook transmission, DC loop feed, and voice transmission are supported. Loop supervision is provided by the switch hook detector. The device may be operated from either high or low battery. When in the Reverse Active mode the Tip and Ring DC voltage characteristics exchange roles. That is, Ring is typically 4V below ground and Tip is typically 4.5V more positive than battery. Silent Polarity Reversal Changing from forward active to reverse active or vice versa is referred to as polarity reversal. Many applications require control of the polarity reversal transition time. Requirements range from minimizing cross talk to protocol signalling. The SLIC uses an external low voltage capacitor, CPOL, to set the reversal time. The capacitor is isolated from the AC loop so that loop stability is not influenced by its selection. Once CPOL is set, the reversal time will remain nearly constant over various load conditions. The internal circuitry used to set the polarity reversal time is shown in Figure 7. During Forward Active the switch is open and the current from source I1 charges the external timing capacitor CPOL. The internal resistor provides a clamping function for the voltage at the POL node. When the Reverse Active mode is initiated the switch closes and the difference current (I2-I1) discharges the timing capacitor. The voltage at the POL node drives one side of a transistor differential pair which forces the Forward or Reverse condition on the Tip and Ring amplifiers. The forward/reverse transition time is given by Equation 28, where ∆time is the required reversal time. Polarized capacitors may be used for CPOL. The low voltage at the POL pin and minimal voltage excursion in the order of ±0.75V, are well suited for polarized capacitors. FIGURE 6. ILOOP VERSUS RLOOP LOAD CHARACTERISTIC RLOOP (Ω) RKNEE ILIM ISC IA IB 2RP ISC ILIM VTR OC () 2RPILIM – 1.1e4 ------------------------------------------------------ + = (EQ. 21) IA ILIM VTR OC () RLOOPILIM – 1.1e4 -------------------------------------------------------------- + = (EQ. 22) RKNEE VTR OC () ILIM ------------------------ = (EQ. 23) IB VTR OC () RLOOP ------------------------ = (EQ. 24) PFAQ VBH I × BHQ VBL IBLQ × VCC ICCQ × ++ = (EQ. 25) PFA IA () PFA Q () VBLxIA () RLOOPxI2A () – + = (EQ. 26) PFA IB () PFA Q () VBLxIB () RLOOPxI2B () – + = (EQ. 27) CPOL ∆time 75000 ---------------- = (EQ. 28) ISL5586 |
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