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HC5518XEVAL1 Datasheet(PDF) 11 Page - Intersil Corporation |
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HC5518XEVAL1 Datasheet(HTML) 11 Page - Intersil Corporation |
11 / 20 page 11 FN4831.14 December 18, 2006 Forward Active Overview The forward active mode (FA, 001) is the primary AC transmission mode of the device. On hook transmission, DC loop feed and voice transmission are supported during forward active. Loop supervision is provided by either the switch hook detector (E0 = 1) or the ground key detector (E0 = 0). The device may be operated from either high or low battery for on- hook transmission and low battery for loop feed. On-Hook Transmission The primary purpose of on hook transmission will be to support caller ID and other advanced signalling features. The transmission over load level while on hook is 3.5VPEAK. When operating from the high battery, the DC voltages at Tip and Ring are MTU compliant. The typical Tip voltage is -4V and the Ring voltage is a function of the battery voltage for battery voltages less than -60V as shown in Equation 19. Loop supervision is provided by the switch hook detector at the DET output. When DET goes low, the low battery should be selected for DC loop feed and voice transmission. Feed Architecture The design implements a voltage feed current sense architecture. The device controls the voltage across Tip and Ring based on the sensing of load current. Resistors are placed in series with Tip and Ring outputs to provide the current sensing. The diagram below illustrates the concept. By monitoring the current at the amplifier output, a negative feedback mechanism sets the output voltage for a defined load. The amplifier gains are set by resistor ratios (RA, RB, RC) providing all the performance benefits of matched resistors. The internal sense resistor, RCS, is much smaller than the gain resistors and is typically 20 Ω for this device. The feedback mechanism, KS, represents the amplifier configuration providing the negative feedback. 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. 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, Tip will be near -4V and Ring will be near VVBL + 4V. The following diagram shows the DC feed characteristic. 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. The curve of Figure 5 determines 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 ( Ω/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. The maximum loop impedance for a programmed loop current is defined as RKNEE. V RING V BH 4 + = (EQ. 19) FIGURE 4. VOLTAGE FEED CURRENT SENSE DIAGRAM + - + - VIN VOUT RC RCS RL RB RA KS FIGURE 5. DC FEED CHARACTERISTIC m = ( ΔVTR/ΔIL) = 11.1kΩ ILOOP (mA) ILIM VTR(OC) V TR OC () V BL 8 – = (EQ. 20) FIGURE 6. ILOOP vs RLOOP LOAD CHARACTERISTIC RLOOP (Ω) RKNEE ILIM ISC IA IB 2RP I SC I LIM V TR OC () 2RPILIM – 1.1e4 ------------------------------------------------------ + = (EQ. 21) I A I LIM V TR OC () RLOOPILIM – 1.1e4 -------------------------------------------------------------- + = (EQ. 22) R KNEE V TR OC () I LIM ------------------------ = (EQ. 23) HC55185 |
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