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HC55185ECRZ Datasheet(PDF) 9 Page - Intersil Corporation |
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HC55185ECRZ Datasheet(HTML) 9 Page - Intersil Corporation |
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9 / 20 page  9 FN4831.14 December 18, 2006 Design Equations Loop Supervision Thresholds SWITCH HOOK DETECT The switch hook detect threshold is set by a single external resistor, RSH. Equation 3 is used to calculate the value of RSH. The term ISH is the desired DC loop current threshold. The loop current threshold programming range is from 5mA to 15mA. GROUND KEY DETECT The ground key detector senses a DC current imbalance between the Tip and Ring terminals when the ring terminal is connected to ground. The ground key detect threshold is not externally programmable and is internally fixed to 12mA regardless of the switch hook threshold. RING TRIP DETECT The ring trip detect threshold is set by a single external resistor, RRT. IRT should be set between the peak ringing current and the peak off hook current while still ringing. In addition, the ring trip current must be set below the transient current limit, including tolerances. The capacitor CRT, in parallel with RRT, will set the ring trip response time. Loop Current Limit The loop current limit of the device is programmed by the external resistor RIL. The value of RIL can be calculated using Equation 5: The term ILIM is the desired loop current limit. The loop current limit programming range is from 15mA to 45mA. Impedance Matching The impedance of the device is programmed with the external component RS. RS is the gain setting resistor for the feedback amplifier that provides impedance matching. If complex impedance matching is required, then a complex network can be substituted for RS. RESISTIVE IMPEDANCE SYNTHESIS The source impedance of the device, ZO , can be calculated in Equation 6. The required impedance is defined by the terminating impedance and protection resistors as shown in Equation 7. 4-WIRE TO 2-WIRE GAIN The 4-wire to 2-wire gain is defined as the receive gain. It is a function of the terminating impedance, synthesized impedance and protection resistors. Equation 8 calculates the receive gain, G42. When the device source impedance and protection resistors equals the terminating impedance, the receive gain equals unity. 2-WIRE TO 4-WIRE GAIN The 2-wire to 4-wire gain (G24) is the gain from tip and ring to the VTX output. The transmit gain is calculated in Equation 9. When the protection resistors are set to zero, the transmit gain is -6dB. TRANSHYBRID GAIN The transhybrid gain is defined as the 4-wire to 4-wire gain (G44). When the protection resistors are set to zero, the transhybrid gain is -6dB. COMPLEX IMPEDANCE SYNTHESIS Substituting the impedance programming resistor, RS, with a complex programming network provides complex impedance synthesis. The reference designators in the programming network match the evaluation board. The component RS has a different design equation than the RS used for resistive impedance synthesis. The design equations for each component are provided below. R SH 600 I SH ⁄ = (EQ. 3) R RT 1800 I RT ⁄ = (EQ. 4) R IL 1760 I LIM ------------- = (EQ. 5) R S 133.3 Z O () = (EQ. 6) Z O Z L 2R P – = (EQ. 7) G 42 2 Z L Z O + 2RP + ZL ------------------------------------------ ⎝⎠ ⎜⎟ ⎛⎞ – = (EQ. 8) G 24 Z O Z O + 2 RP + ZL ------------------------------------------ ⎝⎠ ⎜⎟ ⎛⎞ – = (EQ. 9) G 44 Z O Z O 2R P Z L ++ --------------------------------------- ⎝⎠ ⎜⎟ ⎛⎞ – = (EQ. 10) FIGURE 2. COMPLEX PROGRAMMING NETWORK 2-WIRE NETWORK R1 R2 C2 PROGRAMMING NETWORK RSeries RParallel CParallel R Series 133.3 R 1 2R P () – () × = (EQ. 11) R Parallel 133.3 R 2 × = (EQ. 12) C Parallel C 2 133.3 ⁄ · = (EQ. 13) HC55185 |
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