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ISL5585FCM 데이터시트(PDF) 8 Page - Intersil Corporation |
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ISL5585FCM 데이터시트(HTML) 8 Page - Intersil Corporation |
8 / 22 page 8 Design Equations Switch Hook Detect The switch hook detect threshold is set by a single external resistor, RSH. Equation 1 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 (40k Ω < RSH<120kΩ). 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 3. The term ILIM is the desired loop current limit. The loop current limit programming range is from 15mA to 45mA (39k Ω < RIL<117kΩ). Transient Current Limit The drive current capability of the output tip and ring amplifiers is programmed by an external resistor RTL. This output current limit is separate from the DC loop current limit function. The current limit circuit works in both the source and sink direction, with an internally fixed offset to prevent the current limit functions from turning on simultaneously. The current limit function is provided by sensing line current and reducing the voltage drive to the load when the externally set threshold is exceeded, hence forcing a constant source or sink current. Transient Source Current Programming The source current is externally programmed as shown in Equation 4. For example a source current limit setting of 95mA is programmed with a 18.7k Ω (RTL) resistor connected from the TL pin of the device to ground. This setting determines the maximum amount of current which flows from Tip to Ring during an off hook event until the DC loop current limit responds. In addition this setting also determines the amount of current which will flow from Tip or Ring when external battery faults occur. Transient Sink Current Programming The sink current limit is internally offset 20% higher than the externally programmed source current limit setting. If the source current limit is set to 95mA, the sink current limit will be 114mA. This setting will determine the maximum current that flows into Tip or Ring when external ground faults occur. Understanding Transient Current Limit Each tip and ring amplifier is designed to limit source current and sink current. Figure 1 shows the functionality of the circuit for the case of limiting the source current. A similar diagram applies to the sink current limit with current polarity changed accordingly. During normal operation, the error current (IERR) is zero and the output voltage is determined by the signal current (ISIG) multiplied by the 200K feedback resistor. With the current polarity as shown for ISIG, the output voltage moves positivewith respect to half battery. Assuming the amplifier output is driving a load at a more negative potential, the amplifier output will source current. During excessive output source current flow, the scaled output current (IO/K) exceeds the reference current (IREF) forcing an error current (IERR). With the polarity as shown the error current subtracts from the signal current, which reduces the amplifier output voltage. By reducing the output voltage the source current to the load is decreased and the output current is limited. Setting the Proper Transient Current Limit Since this feature programs the maximum output current of the device, the setting must be high enough to allow for R SH 600 I SH ⁄ = (EQ. 1) R RT 1800 I RT ⁄ = (EQ. 2) R IL 1760 I LIM ------------- = (EQ. 3) R TL 1650 I SRC ------------- = (EQ. 4) I SNK 1.20 I SRC × = (EQ. 5) FIGURE 1. CURRENT LIMIT FUNCTIONAL DIAGRAM TIP or RING + - IREF = 1.21/TL IO IO/K IERR ISIG 20 200K VB/2 ISL5585 |
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