Patent Application
20080080701
The present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like references indicate similar elements and in which:
The subscriber loop 190 communicates analog data signals (e.g., voiceband communications) as well as subscriber loop “handshaking” or control signals. The subscriber loop state is often specified in terms of the tip 192 and ring 194 portions of the subscriber loop.
The SLIC is typically expected to perform a number of functions often collectively referred to as the BORSCHT requirements. BORSCHT is an acronym for “battery feed,” “overvoltage protection,” “ring,” “supervision,” “codec,” “hybrid,” and “test.” The term “linefeed” will be used interchangeably with “battery feed”. Modern SLICs may have battery backup, but the supply to the subscriber line is typically not actually provided by a battery.
The ring function, for example, enables the SLIC to signal the subscriber equipment 160. In one embodiment, subscriber equipment 160 is a telephone. Thus, the ring function enables the SLIC to ring the telephone.
In the illustrated embodiment, the BORSCHT functions are distributed between a signal processor 120 and a linefeed driver 130. Signal processor 120 is responsible for at least the ring control, supervision, codec, and hybrid functions. Signal processor 120 controls and interprets the large signal subscriber loop control signals as well as handling the small signal analog voiceband data and the digital voiceband data.
In one embodiment, signal processor 120 is an integrated circuit. The integrated circuit includes sense inputs for both a sensed tip and a sensed ring signal of the subscriber loop. The integrated circuit generates subscriber loop linefeed driver control signal in response to the sensed signals. The signal processor has relatively low power requirements and can be implemented in a low voltage integrated circuit operating in the range of approximately 5 volts or less.
Signal processor 120 receives subscriber loop state information from linefeed driver 130 as indicated by tip/ring sense 116. The signal processor may alternatively directly sense the tip and ring as indicated by tip/ring sense 118. This information is used to generate linefeed driver control 114 signals for linefeed driver 130. Analog voiceband 112 data is bi-directionally communicated between linefeed driver 130 and signal processor 120. In an alternative embodiment, analog voiceband signals are communicated downstream to the subscriber equipment via the linefeed driver but upstream analog voiceband signals are extracted from the tip/ring sense 118.
SLIC 110 includes a digital network interface 140 for communicating digitized voiceband data to the digital switching network of the public switched telephone network (PSTN). The SLIC may also include a processor interface 150 to enable programmatic control of the signal processor 120. The processor interface effectively enables programmatic or dynamic control of battery control, battery feed state control, voiceband data amplification and level shifting, longitudinal balance, ringing currents, and other subscriber loop control parameters as well as setting thresholds including ring trip detection and off-hook detection threshold.
Linefeed driver 130 maintains responsibility for battery feed to tip 192 and ring 194. The battery feed and supervision circuitry typically operate in the range of 40-75 volts. In some implementations the ringing function is handled by the same circuitry as the battery feed and supervision circuitry. In other implementations, the ringing function is performed by higher voltage ringing circuitry (75-150 Vrms).
Linefeed driver 130 modifies the large signal tip and ring operating conditions in response to linefeed driver control 114 provided by signal processor 120. This arrangement enables the signal processor to perform processing as needed to handle the majority of the BORSCHT functions. For example, the supervisory functions of ring trip, ground key, and off-hook detection can be determined by signal processor 120 based on operating parameters provided by tip/ring sense 116.
The linefeed driver receives a linefeed supply VBAT for driving the subscriber line for SLIC “on-hook” and “off-hook” operational states. An alternate linefeed supply (ALT VBAT) may be provided to handle the higher voltage levels (75-150 Vrms) associated with ringing.
VBAT may be provided as a fixed supply level. Typically VBAT is shared among a plurality of SLICs. Each SLIC is associated with its own subscriber line. The line conditions may vary greatly from one subscriber line to another. One subscriber line may be considerably shorter than another, for example. Shorter length subscriber loops require less power to drive. VBAT, however, is selected to accommodate a worst-case scenario for driving the subscriber line. Excess power must be dissipated by the SLIC. Excess power results in an increased thermal load that may be problematic when the increased thermal load is carried by an integrated circuit.
In one embodiment, the signal processor 220 is fabricated as a low voltage complementary metal oxide semiconductor (CMOS) integrated circuit. In one embodiment, the linefeed driver 230 is fabricated as a higher voltage CMOS integrated circuit to support the higher power requirements associated with driving the subscriber line. The linefeed driver may alternatively be fabricated as a bipolar or bi-CMOS integrated circuit.
VBAT is typically around −48 volts. The signal processor relies on a power supply of VDD. In the illustrated embodiment, the magnitude of VBAT is much greater than the magnitude of VDD (i.e., |VBAT|>>|VDD|).
Although a tracking battery supply might be used to provide no more VBAT than necessary to meet the operational needs of a specific SLIC, practical implementations such a tracking battery supply would be required for each SLIC. A tracking battery supply (VBAT) per device may not be an economical architecture for a large number of SLICs. Thus practical implementations may tend to result in a shared fixed power supply (VBAT) for a plurality of SLICs.
One disadvantage of a shared fixed power supply architecture is that excess power is generated and must be dissipated as heat or otherwise wasted for each SLIC not using a power supply level optimized for its particular operational state or for its particular line conditions.
For example, the power supply must be capable of supporting the worst-case scenario such as a maximum subscriber line length provided for by specification. In the event the subscriber line is considerably shorter than the maximum expected length, the SLIC will be required to absorb the excess power. The resulting additional thermal load can be problematic for integrated circuits of the SLIC. Instead of a tracking battery supply for each SLIC, a power offload component is provided to dissipate excess power resulting from the battery supply.
A power offload element is provided in order to offload the power that would otherwise have to be dissipated by the linefeed driver 230. In the illustrated embodiment, the power offload element 280 is a bipolar junction transistor (i.e., “BJT” or “bipolar”) QREG. In an alternative embodiment, other power offload elements such as a field effect transistor (FET) 205 may be used. The power offload element is responsive to a control signal for varying the amount of power offloaded.
Generally, the amount of power required for the SLIC is dependent upon the operational state as well as the specific characteristics of the subscriber line associated with the SLIC. The amount of power offloading is regulated in accordance with these concerns. In particular, a target linefeed supply is calculated and the control signal varies the supply drop 286 from the power supply VBAT 288 to match the linefeed supply (LS 284) to the target linefeed supply (LS 254).
In the illustrated embodiment, power offloading control circuitry is distributed across the linefeed driver 230 and the signal processor 220. Based upon the line condition 248 of the subscriber line, the digital signal processor (DSP 250) computes a target linefeed supply level 254 for the subscriber line driver 270. The linefeed supply (LS 284) is used by driver 270 to drive the subscriber line 290. The linefeed supply level is sensed using RSENSE 282 and sense amplifier 262 to provide a sensed linefeed supply 256 as feedback.
The sensed linefeed supply 256 and target linefeed supply 254 are provided to error amplifier 260 to generate an error signal 232. In the illustrated embodiment, the error amplifier has analog inputs. Accordingly, the target linefeed supply level provided by the DSP is converted to an analog signal using a digital-to-analog converter (DAC 252) to generate a corresponding analog target linefeed supply 254.
Error signal 232 is a control signal for the power offload element 280. Instead of driving the power offload element directly, however, the error signal is provided to a pre-driver 274 for generating the control signal 276 for the power offload element. The pre-driver efficiently interfaces the error signal 232 to the power offload element (BJT, MOS, etc.) without compromising control loop dynamics. The pre-driver provides any necessary voltage-current conversion, scaling, and level shifting, but is otherwise responsive to the error signal. In one embodiment, the pre-driver resides with the linefeed driver 230 rather than the signal processor 220. In the illustrated embodiment, the pre-driver provides a voltage stand-off for the signal processor.
The control signal 276 varies in accordance with the error signal 232. The control signal is provided to the base of transistor QREG. LS 284 is moved relative to VBAT 288 by varying the error/control signal. The pre-driver provides a voltage standoff between the signal processor and the power offload element. When power offloading is disabled, the supply drop contributed by the power offload element is relatively negligible such that LS is approximately the same as VBAT.
Referring to
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For example, the SLIC initiates ringing and thus can readily regulate how well power offloading tracks the ringing waveform. An off-hook state, however, is initiated by the subscriber equipment rather than the SLIC. The SLIC must first determine that the change in conditions of the subscriber line accurately represents a transition from on-hook to off-hook. As a result, the control algorithm might incorporate delay or debounce features that are unnecessary for ringing. The debounce feature requires the state to be maintained for a period of time before power offloading tracks the waveform. The delay feature time-shifts the tracking.
The amount of supply overhead (i.e., proximity of tracking to waveform) may also change depending upon the state. Distortion of voiceband communications may be highly undesirable, however, a distorted ringing signal may be little more than an annoyance. In addition to supply level overhead, maximum or minimum supply levels may be set. In general the control algorithm executed by the DSP may rely upon state-specific parameters including: delay, debounce interval, overhead, upper supply level, lower supply level, etc.
Some subscriber line states are relatively low-power states. Power offloading may not be necessary in these states. For example, if the subscriber equipment is “on-hook” and the SLIC is not ringing the subscriber equipment, then no power offloading is necessary. In one embodiment, no power offloading except when the subscriber line state is one of a pre-defined set of power offload enabled states.
In the preceding detailed description, the invention is described with reference to specific exemplary embodiments thereof. Various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.
