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HC55185CIM Datasheet(PDF) 8 Page - Intersil Corporation |
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HC55185CIM Datasheet(HTML) 8 Page - Intersil Corporation |
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8 / 20 page  8 FN4831.14 December 18, 2006 Functional Improvements In addition to parametric improvements, internal circuit changes and application circuit changes have been made to improve the overall device functionality. Off Hook Interface The transient behavior of the device in response to mode changes has been significantly improved. The benefit to the application is reduction or more likely elimination of DET glitches when off hook events occur. In addition to internal circuit modifications, the change of CFB value contributes to this functional improvement. Transient Current Limit The drive current capability of the output amplifiers is determined by an externally programmable output current limit circuit which is separate from the DC loop current limit function and programmed at the pin TL. 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. SOURCE CURRENT PROGRAMMING The source current is externally programmed as shown in Equation 1. For example a source current limit setting of 50mA is programmed with a 35.6k Ω resistor connected from pin 16 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. 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 50mA, the sink current limit will be 60mA. This setting will determine the maximum current that flows into Tip or Ring when external ground faults occur. FUNCTIONAL DESCRIPTION Each amplifier is designed to limit source current and sink current. The diagram below 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 positive with 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. DETERMINING THE PROPER SETTING Since this feature programs the maximum output current of the device, the setting must be high enough to allow for 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 applica- tions) 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. R TL 1780 I SRC ------------- = (EQ. 1) I SNK 1.20 I SRC × = (EQ. 2) FIGURE 1. CURRENT LIMIT FUNCTIONAL DIAGRAM TIP or RING + - IREF = 1.21/TL IO IO/K IERR ISIG 20 200k VB/2 HC55185 |
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