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ISL5585FCR Datasheet(PDF) 9 Page - Intersil Corporation |
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ISL5585FCR Datasheet(HTML) 9 Page - Intersil Corporation |
9 / 22 page 9 detection of ring trip or programmed off hook loop current, whichever is greater. To allow for proper ring trip operation, the transient current limit setting should be set at least 25% higher than the peak ring trip current setting. Setting the transient current 25% higher should account for programming tolerances of both the ring trip threshold and the transient current limit. If loop current is larger than ring trip current (low REN applications) then the transient current limit should be set at least 35% higher than the loop current setting. The slightly higher offset accounts for the slope of the loop current limit function. Attention to detail should be exercised when programming the transient current limit setting. If ring trip detect does not occur while ringing, then re-examine the transient current limit and ring trip threshold settings. DC Loop Feed The feedback mechanism for monitoring the DC portion of the loop current is the loop detector. A low pass filter is used in the feedback to block voice band signals from interfering with the loop current limit function. The pole of the low pass filter is set by the external capacitor CDC. The value of the external capacitor should be 4.7 µF, 6.3V rated polarized or non-polarized capacitor. Most applications will operate the device from low battery while off hook. The DC feed characteristic of the device will drive Tip and Ring towards half battery to regulate the DC loop current. For light loads or Long Loops, Tip will be near -4V and Ring will be near VVBL + 5V. Figure 2 shows the DC feed characteristic in terms of tip to ring voltage and loop current. The point on the y-axis labeled VTR(OC) is the open circuit Tip to Ring voltage and is defined by the feed battery voltage in Equation 6. Figure 3 illustrates the actual loop current for a given set of loop conditions. The loop conditions are determined by the low battery voltage and the DC loop impedance. The DC loop impedance is the sum of the protection resistance, copper resistance (ohms/foot) and the telephone off hook DC resistance. 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. Process variations in the ISL5585 effect the ILIM and 11.11k Ω slope in Equation 8. All units are tested with: a 300 Ω load across tip and ring, VBAT=-24V and ILIM set to 25mA. Equation 8 can be used to predict the ideal current at this setting (25.76mA). All units are tested to be within ±8.5% of this ideal value (23.57mA to 27.95mA). The maximum loop impedance for a programmed loop current is defined as RKNEE. When RKNEE is exceeded, the device will transition from constant current feed to constant voltage or resistive feed. The line segment IB represents the resistive feed portion of the load characteristic Impedance Matching The impedance of the device is programmed with the external component RS. RS is the gain setting resistor for the Transmit Amplifier that provides impedance matching. If complex impedance matching is required, then a complex network can be substituted for RS. The feedback mechanism for monitoring the AC portion of the loop current consists of two amplifiers, the Sense Amplifier (SA) and the Transmit Amplifier (TA). The AC feedback signal is used for impedance synthesis. A detailed model of the AC feed back loop is shown in Figure 4. FIGURE 2. DC FEED CHARACTERISTIC m = ( ∆VTR/∆IL) = 11.1kΩ ILOOP (mA) ILIM VTR(OC) LONG LOOP SHORT LOOP V TR OC () V BL 9 – = (EQ. 6) FIGURE 3. ILOOP vs. RLOOP LOAD CHARACTERISTIC RLOOP (Ω) RKNEE ILIM ISC IA IB 2RP LONG LOOP SHORT LOOP m=11.11k m=Vtr(oc)/Rloop CONSTANT CURRENT CONSTANT VOLTAGE OR RESISTIVE FEED I SC I LIM V TR OC () 2RPILIM – 11.11k ------------------------------------------------------ + = (EQ. 7) I A I LIM V TR OC () RLOOPILIM – 11.11k -------------------------------------------------------------- + = (EQ. 8) R KNEE V TR OC () I LIM ------------------------ = (EQ. 9) I B V TR OC () R LOOP ------------------------ = (EQ. 10) ISL5585 |
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