(12) United States Patent
`(10) Patent N0.:
`US 6,301,358 B1
`
`Chen et al.
`(45) Date of Patent:
`Oct. 9, 2001
`
`U5006301358B1
`
`(54) DUAL-SLOPE CURRENT BATTERY-FEED
`CIRCUIT
`
`0 806 859
`98 21875
`
`............................. H04M/19/00
`11/1997 (EP)
`5/1998 (W0) ........................... H04M/19/00
`
`(75)
`
`Inventors: Robert K. Chen, North Andover, MA
`(Clng);Ul;1eter ‘l' H' Knollman, Arvada,
`'
`(
`)
`(73) Assignee: Avaya Technology Corp., Miami
`Lakes FL (US)
`’
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U’S‘C’ 1540)) by 0 days.
`
`( >1 ) Notice:
`
`(21) APPL N05 09/087,216
`-
`.
`,
`(29)
`Filed‘
`May 29 1998
`
`Int. C1.7 .................................................... H04M 11/00
`(51)
`
`(52) U:S- 0-
`379/413; 379/400
`(58) Fleld of Search ..................................... 379/413, 400,
`379/399
`
`7
`(56)
`
`.
`References Clted
`U S PATENT DOCUMENTS
`.
`.
`12/1983 Embree et a1.
`........................ 179/:77
`4,419,542
`
`10/:1984 Al‘“ 6? 3L ~~~~~~~~~~~ 179/70
`424762350
`431988 Mchellliet all‘
`379/413
`$973294” 1:
`
`$133: £3622? et a'
`:32}:
`5’331’33 *
`[’1998 Apfel e151""""
`379/413
`5’737’411 *
`
`5/1998 Akhteruzzaman
`379/413
`5,754,644 *
`.......................... 379/413
`5:854:839 * 12/1998 Chen et a1.
`FOREIGN PATENT DOCUMENTS
`
`OTHER PUBLICATIONS
`Pariani, A. et al: “SLIC chip set adapts to different line
`lengths”, Electronic Engineering, vol. 62, N0. 766, Oct. 1,
`1990, pp. 59—60, 62, XP000165395, ISSN: 0013—4902.
`U. S. Patent Application, Akhteruzzaman 5, Serial No.
`08/672,190, “Method For Customizing Operation Of ALine
`Interface Circuit In A Telecommunications Networ ”, Filed
`Jun. 27> 1996
`* cited by examiner
`
`Primary Examiner—Stella W00
`(74) Attorney, Agent, or Firm—David Volejnicek
`
`ABSTRACT
`(57)
`Adual-supply line-interface circuit (100) uses a —48V power
`supply (VBAT1) to drive long subscriber loops (120) and uses
`a —28V power supply (VBAIZ) to drive short subscriber
`loops. For intermediate-length loops, a dual-slope current-
`feed profile (FIG. 4) is employed to limit the line-Circuit’s
`power dissipation. The line-interface circuit operates in an
`apparent constant-current mode, generating about 40 mA of
`differential line current using the lOW power supply, 11p to 3
`threshold line voltage of about 25V, which is equal to the
`low power supply voltage minus required overhead. For
`longer loops, the line-interface circuit switches to a second
`constant—current mode, generating about 22 mA of differ—
`ential current using the high power supply, which maintains
`the loop current constant until it drops to the 48V resistive-
`feed value.
`
`0 559 336 A2
`
`9/1993 (EP) ............................. H04M/19/08
`
`14 Claims, 3 Drawing Sheets
`
`- VBAT2
`
`
`
`
`
`IMPEDANCE— Von
`
`
`
`MATCHING ,_°
`
`BUFFER
`
`
`DIFFERENTIAL
`
`EL
`123
`LEV
`CURRENT
`
`L2 SHIFT
`
`SENSOR
`
`IMPEDANCE-
`
`
`MATCHING
`
`BUFFER
`RING 102 .
`
`
`
`
`V3111
`
`VBATZ
`
`
`
`'VBAT1
`
`
`
`NETWORK-l N l —2005
`
`

`

`US. Patent
`
`Oct. 9, 2001
`
`Sheet1,0f3
`
`US 6,301,358 B1
`
`
`
`
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`
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`

`

`US. Patent
`
`Oct. 9, 2001
`
`Sheet 2 0f3
`
`US 6,301,358 B1
`
`FIG. 2
`
`’
`
`v
`
`I BATZ
`
`200 L’m 202 '
`
`
`
`FIG. 3
`
`34
`
`IPROG
`(HA)
`
`0
`
`0
`
`2.5V MIN 2.8V
`
`3.1V MAX
`
`Vcrz— V9102
`
`FIG. 4
`
`4o
`
`ITR
`(mA)
`
`22
`
`o
`
`
`
`o
`
`24.9v
`
`25.5v
`
`39.5v 41v
`
`45
`
`VTR
`(VTIP - VRING)
`
`

`

`US. Patent
`
`Oct. 9, 2001
`
`Sheet 3 0f3
`
`US 6,301,358 B1
`
`vcc
`
`
`
`

`

`US 6,301,358 B1
`
`1
`DUAL-SLOPE CURRENT BATTERY-FEED
`CIRCUIT
`
`TECHNICAL FIELD
`
`This invention relates generally to analog telephone line
`interface circuits, and specifically to the battery feed circuits
`of such line interface circuit.
`
`BACKGROUND OF THE INVENTION
`
`Conventional analog telephone line-interface circuits,
`also known as analog port circuits, require a 48VDC power
`supply for operation and for reliable signaling on long
`subscriber loops (telephone lines). Long loops have a high
`resistance relative to short loops, and therefore require a
`relatively high voltage to drive them. The circuit which
`couples the DC power to the telephone line is known as a
`battery-feed circuit. Even though battery-feed circuits com-
`monly employ current-limiting and limit loop current to
`42mA, 2W of power can be dissipated by the line-interface
`circuit. This high power dissipation limits the number of
`line-interface circuits that can be integrated on a single
`integrated-circuit device (a “chip”), as well as the number of
`telephone lines that can be served by a single 48V power
`supply.
`To reduce power dissipation, the art has employed dual-
`supply line-interface circuits. These circuits employ a sec-
`ond power supply having a voltage lower than the high-
`voltage (48V) power supply, for powering short subscriber
`loops.
`
`SUMMARY OF THE INVENTION
`
`In order to reduce even further the power dissipated by a
`dual—supply line—interface circuit, a dual—slope current—limit
`profile is employed for operation of the line-interface circuit
`to effect current limiting. The second power supply prefer-
`ably operates at 28V, which can be generated from the
`high-voltage (48V) supply via a DC-to-DC converter. This
`significantly increases the supply current that is made avail-
`able by the line-interface circuit to short subscriber loops,
`and thus significantly increases the number of short sub-
`scriber loops which the power supply can handle. For
`example, assuming 90% efficiency of the converter,
`the
`supply current and the short-loop-handling capacity of the
`power supply are increased by 50%. The 48V supply is still
`used directly to drive long loops. For intermediate-length
`loops, the dual—slope current—feed profile is employed to
`limit the line-interface circuit’s power dissipation. The line-
`interface circuit operates in an apparent constant-current
`mode using the low power supply up to a threshold line
`voltage which is equal
`to the low power supply voltage
`minus required overhead. For longer loops, the line-interface
`circuit switches to a second constant-current mode which is
`
`substantially lower than the constant current for the shorter
`loops, which maintains the loop current constant until the
`loop current drops to the 48V resistive-feed value (the
`minimum value required to drive a telephony device con-
`nected to the loop).
`Generally according to the invention, a line—interface
`circuit for connecting to an analog telephone line that
`comprises a pair of leads (e.g.,
`tip and ring leads) has a
`battery-feed circuit that monitors line voltage across the pair
`of leads and substantially maintains line current flowing
`between the leads at one of two substantially constant
`values. When the line voltage is exceeded by a first threshold
`voltage (e.g., ~25V), the battery-feed circuit maintains the
`
`10
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`15
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`30
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`35
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`40
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`45
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`50
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`65
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`2
`line current at a first substantially-constant value (e.g., 40
`mA). When the line voltage exceeds a second threshold
`voltage (e. g., ~25 .5V), the battery-feed circuit maintains the
`line current at a second substantially-constant value (e.g., 22
`mA). If the two thresholds are not one and the same, the
`battery—feed circuit preferably varies the line current
`between the first and the second values as the line voltage
`varies between the first and the second thresholds.
`Preferably, the line current monitored by the battery-feed
`circuit is differential current between the two leads. More
`specifically according to a preferred embodiment of the
`invention,
`the battery-feed circuit comprises a driver for
`driving (powering) the line which uses a first power supply
`of dual power supplies to drive the line while the line current
`is at the first current value, and uses a second power supply
`of the dual power supplies to drive the line while the line
`current is at the second value. The dual power supplies
`operate at voltages of significantly different magnitudes—
`for example, the first power supply operates at —28VDC and
`the second power supply operates at —48VDC.
`Illustratively, the battery-feed circuit includes a current-
`feedback loop that includes a constant-current supply that
`generates a constant current for driving the feedback loop to
`produce a constant current of one of the first and the second
`current values on the line. The feedback loop further
`includes a variable-current supply that generates a variable
`current that combines with the constant current generated by
`the constant-current supply to drive the feedback loop. The
`variable current varies with the line voltage to cause the
`feedback loop to produce the constant current of the one
`current value on the line when the line voltage is exceeded
`by the first threshold value, and to cause the feedback loop
`to produce a constant current of another of the first and
`second current values on the line when the line voltage
`exceeds the second threshold value. The variable current
`
`further illustratively causes the feedback loop to produce a
`line current that varies between the first and the second
`current values as the line voltage varies between the first and
`the second threshold values, and vice versa.
`In one implementation, a line-interface circuit for con-
`necting to an analog phone line comprising a pair of leads
`has a battery-feed circuit that powers the line from one of a
`pair of power supplies operating at significantly different
`voltages. The battery—feed circuit comprises a pair of
`drivers, each driving a different one of the pair of leads and
`each sensing voltage on the different one of the pair of leads.
`One driver uses a first one of the pair of power supplies to
`drive the line while the differential current on the leads of the
`line is at a first value, and uses a second one of the pair of
`power supplies to drive the line while the differential current
`is at a second value. The two power supplies operate at
`voltages of significantly different magnitude. The battery-
`feed circuit also includes a differential-current sensor for
`
`sensing the differential current flowing between the pair of
`leads and generating a first voltage representative of the
`differential current. The first voltage is used to control a
`second voltage at a junction. The battery-feed circuit further
`includes a transconductance amplifier that drives the one of
`the pair of drivers. It has an input connected to the junction.
`Avariable—current source generates a variable current at the
`junction as a function of line voltage in order to create a
`variable said second voltage at the junction. The net effect is
`that
`the differential-current sensor,
`the variable-current
`generator, the transconductance amplifier, and the one driver
`form a current-feedback loop that maintains the differential
`current at a substantially constant first value when the line
`voltage is below the first threshold value, and maintains the
`
`

`

`US 6,301,358 B1
`
`3
`differential current at a substantially constant second value
`significantly smaller than the first value when the line
`voltage is above the second threshold value, greater than the
`first threshold value.
`
`These and other advantages and features of the invention
`will become more apparent from the following description
`of an illustrative embodiment of the invention considered
`
`together with the drawing.
`BRIEF DESCRIPTION OF THE DRAWING
`
`FIG. 1 is a partial circuit—and—block diagram of a tele—
`phone line-interface circuit
`that embodies an illustrative
`example of the invention;
`FIG. 2 is a partial circuit diagram of a variable-current
`supply of the telephone line-interface circuit of FIG. 1;
`FIG. 3 is a diagram of the operational characteristic of the
`variable-current supply of FIG. 2;
`FIG. 4 is a diagram of the operational characteristic of the
`telephone line-interface circuit of FIG. 1; and
`FIG. 5 is a circuit diagram of an amplifier of the telephone
`line-interface circuit of FIG. 1.
`
`DETAILED DESCRIPTION
`
`FIG. 1 shows those portions of a telephone line-interface
`circuit 100 that are relevant to an understanding of this
`invention. Circuit 100 is illustratively an L7500-series or an
`L8500-series subscriberline-interface circuit
`(SLIC)
`integrated-circuit device of Lucent Technologies Inc. The
`SLIC utilizes a voltage-feed current-sense architecture,
`wherein a pair of voltage sources feed the DC power as well
`as the voice-band signal to a telephone line 120, and the
`signal from the far end (e.g., a telephone) is sensed by a
`differential-current-sense circuit that is connected in series
`
`with line 120. The impedances which the SLIC presents to
`line 120 can be synthesized by the gain around the feedback
`loop.
`Circuit 100 includes a pair of amplifiers AT 103 and AR
`104 that are connected through a differential-current sensor
`105 to the tip lead 101 and the ring lead 102, respectively,
`of telephone line 120 and deliver current
`thereto. The
`delivered current enables the telephone switching system to
`detect
`the presence and status of equipment (e.g., a
`telephone) connected to telephone line 120. Circuit 100 also
`couplcs audio signals from line 9 to telephone line 120 and
`from telephone line 120 to line L1.
`Power amplifiers 103 and 104 are voltage-mode opera-
`tional amplifiers operating in unity-gain configuration to
`transmit onto line 120 audio signals supplied to their posi-
`tive inputs by transmit line L2 through a level-shift circuit
`123. Tip lead 101 provides negative feedback to amplifier
`AT 103, while ring lead 102 provides negative feedback to
`amplifier AR 104. The positive input of amplifier 103 is
`connected through an impedance—matching buffer 115 to a
`voltage source VCFl, which in this example provides
`approximately ~2 VDC. The positive input of amplifier 104
`is connected through an impedance-matching buffer 116 to
`a voltage VCF2~ VCF2 is produced by forcing a current
`generated by a current supply 125 into a resistor 114 that is
`connected to the VBAD (—48 VDC) supply rail. Illustratively,
`the current output by current supply 125 is 50 yA and
`resistor 114 is 100 ksz, so VCF2 is —43 VDC ('—48V+50
`,u.A*100 kQ) when the loop current in line 120 is zero.
`Amplifiers 103 and 104 supply VCF1 and VCR to tip and
`ring leads 101 and 102, respectively.
`Differential current sensor 105 detects the difference in
`
`current flowing on leads 101 and 102 and puts out an
`
`4
`indication of that difference to a negative input of an
`amplifier AX 106. A positive input of amplifier 106 is
`connected to ground. Amplifier 106 amplifies the difference
`indication by a magnitude determined by a feedback resistor
`107 which connects the output VHR of amplifier 106 back to
`the negative input of amplifier 106.
`In this illustrative
`example, with no loop current flowing in line 120, output
`VITR of amplifier 106 is at 0V. With loop current flowing in
`line 120 in the normal direction (from tip lead 101 to ring
`lead 102), output VITR of amplifier 106 is negative. The
`transimpedance gain from the differential loop current to
`Vme is about 250V per one Ampere of differential current.
`The output VITR of amplifier 106 drives signal line V1112 121.
`Line VITR 121 is connected to audio receive line L1 through
`a DC-blocking capacitor 122. Line V1712 121 is also con-
`nected through a current-limiting resistor 108 to a junction
`124 with the output of a current supply 109. Current supply
`109 is connected to the supply rail VCC, which in this
`example is +5 VDC, and outputs a constant current of 75 MA
`to junction 124 in this example.
`Junction 124 is connected to a transconductance stage
`111—113 which includes an operational amplifier 111, a PNP
`transistor 112, and a resistor 113. Junction 124 is connected
`to a positive input of operational amplifier 111. The output
`of operational amplifier 111 is connected to the base of
`transistor 112. The emitter of transistor 112 is connected to
`
`the negative input of operational amplifier 111, and through
`resistor 113 to ground. The collector of transistor 112 is
`connected to VCF’Z’ If the voltage at junction 124 is positive,
`then the current output from the collector of transistor 112 is
`zero. However, if the voltage at junction 124 is negative,
`then the current output from the collector of transistor 112 is
`equal to the voltage at junction 124 divided by resistor 113.
`The current from the collector of transistor 112 is fed into
`
`resistor 114 and therethrough to VBATl' The voltage gain
`from junction 124 to VCF2 is inverting (a gain of —50 in this
`example) for junction 124 having negative voltages. For
`junction 124 having a voltage of zero or a positive voltage,
`the gain is zero. The transimpedance gain from the loop
`current of line 120 to Vme 121 is 250 V/A, as stated earlier.
`Then the input impedance which circuit 100 presents to line
`120 is 12.5 kg (250 V/A*50). This is the impedance value
`when circuit 100 is in loop-current-limiting mode.
`The voltage at junction 124 is determined by the voltage
`on line VITR 121, resistor 108, and current supply 109. As
`stated earlier, line VITR 121 is at 0V when the loop current
`is at zero; hence, the voltage at junction 124 is positive. As
`the loop current flows, as stated earlier, voltage on line VITR
`121 becomes negative. The loop current for which junction
`124 becomes 0 VDC is the current limit for the SLIC.
`As described so far,
`line circuit 100 is conventional.
`According to the invention, however, by varying the current
`supplied to junction 124, the current limit of circuit 100 can
`be changed. Junction 124 is also connected to the input of a
`second current supply 110. Current supply 110 is driven by
`a voltage VBATI, which in this example is —48 VDC, and
`sinks a variable current 11mm; from junction 124, which in
`this example varies from 0 to 34 yA. Hence, the net current
`at junction 124 is a variable current of 41 to 75 ,uA. The
`amount of current sinked by current supply 110 is a function
`of the difference between a voltage VBAIZ> which in this
`example is —28 VDC, and Van. Both of these voltages are
`connected to current supply 110.
`FIG. 2 shows the structure of relevant parts of variable
`current supply 110. An NPN transistor 200 has its collector
`connected to junction 124, its base connected through a
`voltage supply 220 to VBAD, and its emitter connected to the
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`

`

`US 6,301,358 B1
`
`5
`base of a second NPN transistor 201. Voltage supply 220
`keeps the base of transistor 200 at about 2.8 VDC above
`VBATZ‘ The collector of transistor 201 is connected to
`junction 124, and its emitter is connected to an input of a
`diode 203. Aresistor 202 connects the base of transistor 201
`to its emitter. Together, transistors 200 and 201 and resistor
`202 form a Darlington pair.
`In a symmetrical configuration, an NPN transistor 210 has
`its collector connected to ground,
`its base connected to
`VCF2> and its emitter connected to the base of a second NPN
`transistor 211. The collector of transistor 211 is connected to
`ground, and its emitter is connected to an input of a diode
`213. A resistor 212 connects the base of transistor 211 to its
`
`emitter. Together, transistors 210 and 211 and resistor 212
`also form a Darlington pair.
`The outputs of diodes 203 and 213 are respectively
`connected to the collectors of NPN transistors 205 and 207,
`and are interconnected by a resistor 204. The bases of
`transistors 205 and 207 are connected to a biasing voltage
`source VNR1> which is adjusted to cause each transistor 205
`and 207 to draw 17 yA of current. The emitters of transistors
`205 and 207 are respectively connected across resistors 206
`and 208 to VBATl‘
`The operation of variable current supply 110 is as follows.
`When VCF2_VBAI2 is less than 2.8V—the voltage at the
`base of transistor 200—transistors 210 and 211 are turned off
`
`and transistors 200 and 201 are turned on and conducting the
`34 MA that are being drawn by transistors 205 and 207 away
`from junction 124, thereby resulting in 41 yA of current
`across resistor 108. When VCF2_VBAI2 is more than the
`2.8V at the base of transistor 200, transistors 200 and 201 are
`turned off and not conducting current from junction 124
`while transistors 210 and 211 are turned on and conducting
`from ground (and not from junction 124) the 34 uA that are
`being drawn by transistors 205 and 207. This results in the
`full 75 yA of current output by current source 109 across
`resistor 108. When VCF2_ BA” is substantially at 2.8V,
`transistors 200 and 201 and 210 and 211 are partially on,
`resulting in a narrow transition region where between 0 and
`34 yA are being conducted by current source 110 away from
`junction 124.
`The operational characteristic of current supply 110 is
`shown in FIG. 3. While the voltage difference VCF2_VBAT2
`is below a first threshold of about 2.5V, supply 110 sinks 34
`,uA of current. Above this threshold in the vicinity of 2.8V,
`supply 110 sinks current
`in proportion to the voltage
`difference, up to a second threshold of about 3.1 V, at which
`point supply 110 sinks no current. Beyond the second
`threshold, supply 110 continues to sink no current.
`The resulting c1] rrent-limiting operation of line circuit 100
`of FIG. 1 is as shown in FIG. 4 and described below. While
`
`line 120 is not in use, the voltage VTR between tip lead 101
`and ring lead 102 (where VTR=VCF2=VCF1) is about 41V, the
`current ITR from tip lead 101 to ring lead 102 is zero, the
`differential current on leads 101 and 102 of telephone line
`120 is also zero, so the voltage on Vme line 121 is 0, and the
`current produced by current supplies 109 and 110 at junction
`124 is 41 yA (i.e., 75 ,uA—34 yA), which produces a 5V drop
`across resistor 108, i.e., a 5V level at junction 124, thereby
`turning off high-gain stage cascade 111—113. With cascade
`111—113 turned off, current supply 125 and resistor 114 keep
`VCsz at about —43V. This produces a difference of about
`—15V between chz and VBATZ, which (see FIG. 3) causes
`current generator 110 to sink 34 yA of current from junction
`124.
`
`When line 120 comes into use (e.g., a telephone goes “off
`hook” on line 120) VTR begins to drop, and when it drops to
`
`6
`about 41V, loop current begins to flow in line 120. The loop
`current in line 120 increases to about 22 mA as VTR drops
`to about 39.5V. At this point, line VITR 121 is sufficiently
`negative so that junction 124 is at OVDC (41 ,uA*133
`kQ/250), high-gain cascade 111—113 turns on and limits the
`loop current in line 120 to about 22 mA as Vm drops further.
`When VTR drops to about 25 .5V, IFROG current output by
`circuit 110 starts to decrease from 34 yA to zero. The net
`current flow output of junction 124 to resistor 108 is
`increased from 41 ”A to 75 yA as VTR drops further to 24.9V.
`Any further decrease in VTR does not result in increased
`current output from junction 124 into resistor 108; therefore,
`the loop current in line 120 stays at a relatively constant
`value of about 40 mA.
`
`In order to take full advantage of this DC feed profile for
`power-feeding efficiency, amplifier AR 104 must be modi-
`fied from its traditional three-stage configuration. FIG. 5
`shows such a simplified voltage-mode operational amplifier.
`Essentially, the modification involves adding a fourth stage
`comprising a current-steering transistor and a diode to the
`amplifier output. The first stage, comprising a current source
`500 and transistors 502—505,
`is a transconductance
`amplifier, which outputs a current at junction 508 into the
`base of a transistor 506. The second stage, comprising a
`current source 501 and the transistor 506,
`is a common-
`emitter amplifier, which takes the output current from the
`first stage and beta-multiplies it
`to its collector output,
`junction 509. A Miller capacitor 507 connected between
`junctions 508 and 509 compensates the operational amplifier
`to ensure stable unity gain. The third stage is a push-pull
`amplifier, comprising transistors 510 and 511, which pro-
`vides the drive capability to the output load. In order to take
`advantage of VBAIZ being a lower supply voltage than
`VBATl, a current-steering transistor 512 is incorporated in
`the design. It works in the following manner. If Vom—VBATZ
`is greater than 2.5V, transistor 512 is in its active mode, and
`the load current sink from junction 509 flows to VBATZ
`through a diode 513.
`Only a small fraction of current (1/(1+beta)) of the load
`current flows into the emitter of transistor 511 and to VBAH.
`If VOUT—VBArz is less than 2.5V, transistor 512 is in satu—
`ration and cannot support the load current with high beta; the
`load flows through the base-emitter junction of transistor
`512 into the emitter of transistor 511 and to VBAH. The
`threshold of 2.5V is controlled by the forward-on voltage of
`diode 513 and the internal collector resistance of transistor
`512 times the worst-case loop current. This 2.5V threshold
`is also incorporated into the design of circuit 110 to ensure
`that, when the load current is steered from VBATZ to VBATl’
`the tip and ring current limit has already reached 22 mA,
`thereby minimizing the SLIC chip internal power dissipa-
`tion.
`
`Of course, various changes and modifications to the
`illustrative embodiment described above will be apparent to
`those skilled in the art. For example, the circuitry can be
`implemented from active components having an opposite
`polarity to that shown. Also, the circuitry can be imple-
`mented using different circuit
`technologies or circuit
`designs. Such changes and modifications can be made
`without departing from the spirit and the scope of the
`invention and without diminishing its attendant advantages.
`It is therefore intended that such changes and modifications
`be covered by the following claims.
`What is claimed is:
`
`1. A line-interface circuit for connecting to an analog
`telephone line comprising a pair of leads having a battery-
`feed circuit including circuitry that monitors line voltage
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`10
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`15
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`30
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`35
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`4O
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`45
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`50
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`55
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`60
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`65
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`US 6,301,358 B1
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`7
`across the pair of leads and further including circuitry that
`maintains line current flowing between the leads at a sub-
`stantially constant first current value when the line voltage
`is exceeded by a first threshold value and maintains the
`current at a substantially constant second current value that
`is significantly smaller than the first current value when the
`line voltage exceeds a second threshold value,
`including
`circuitry that monitors diflerential said current flowing
`between the leads to keep said current substantially constant
`at the first and the second current values.
`2. The line-interface circuit of claim 1 wherein the first
`threshold value and the second threshold value are substan-
`tially the same.
`3. The line-interface circuit of claim 1 wherein the second
`threshold value exceeds the first threshold value and the
`battery circuit varies the current from the first current value
`to the second current value as the line voltage changes from
`the first threshold value to the second threshold value, and
`vice versa.
`4. The line-interface circuit of claim 1 whose battery-feed
`circuit includes a current-feedback loop having a constant-
`current supply generating a constant current that drives the
`current-feedback loop to produce a constant current of one
`of the first and the second current values on the line, and the
`current-feedback loop further has a variable-current supply
`generating a variable current
`that
`is combined with the
`constant current generated by the constant-current supply to
`drive the loop and that varies with the line voltage,
`the
`variable current causing the current-feedback loop to pro-
`duce the constant current of the one current value on the line
`when the line voltage is exceeded by the first threshold value
`and causing the current-feedback loop to produce a constant
`current of another of the first and the second current values
`on the line when the line voltage exceeds the second
`threshold value.
`5. The line-interface circuit of claim 1 wherein the vari-
`able current further causes the current-feedback loop to
`produce a line current on the line that varies between the first
`and the second current values as the line voltage varies
`between the first and the second threshold values, and vice
`versa.
`6. The line-interface circuit of claim 1 wherein the
`battery-feed circuit comprises a driver for powering the line,
`the driver using a first of a pair of power supplies to drive
`the line while the line current is at the first current value and
`using a second of the pair of power supplies to drive the line
`while the line current is at the second value, the first power
`supply operating at a voltage of significantly greater mag-
`nitude that a voltage at which the second power supply
`operates.
`7. The line-interface circuit of claim 6 wherein the driver
`sinks line current from the line to a —28V power supply
`while the line current is at the first current value and sinks
`current from the line to a —48V power supply while the line
`current is at the second current value.
`8. The line-interface circuit of claim 7 wherein the first
`current value is about 40mA, and the second current value
`is about 22 mA.
`9. A line-interface circuit for connecting to an analog
`telephone line comprising a pair of leads, having a battery-
`feed circuit that powers the line from one of a pair of power
`supplies operating at significantly different voltages and that
`comprises
`a pair of drivers each driving a different one of the pair of
`leads and each sensing voltage on the different one of
`the pair of leads,
`a differential-current sensor sensing a differential current
`flowing between the pair of leads and generating a first
`voltage representative of the differential current,
`the
`first voltage controlling a second voltage at a junction,
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`8
`a transconductance amplifier driving one of the pair of
`drivers and having an input connected to the junction,
`a variable-current source generating a variable current at
`the junction as a function of line voltage to create a
`variable said second voltage at the junction so that the
`differential—current sensor, the variable—current source,
`the transconductance amplifier, and the one driver form
`a current-feedback loop that maintains the diflerential
`current at a substantially constant first value when the
`line voltage is below a first threshold value and main—
`tains the differential current at a substantially constant
`second value significantly smaller than the first value
`when the line voltage is above a second threshold value
`greater than the first threshold,
`the one driver using a first of the pair of power supplies
`to drive the line while the diflerential current is at the
`first value and using a second of the pair of power
`supplies to drive the line while the differential current
`is at the second value, the first power supply operating
`at a voltage of significantly greater magnitude than a
`voltage at which the second power supply operates.
`10. The linc-intcrfacc circuit of claim 9 wherein the
`variable-current source comprises
`a constant-current first source generating a constant cur-
`rent at the junction to create the second voltage at the
`junction that causes the feedback loop to maintain the
`differential current at one of the constant first and
`second values, and
`a variable-current second source generating a variable
`current at the junction as a function of line voltage
`which, when combined with the constant current gen-
`erated by the constant current source, creates the sec-
`ond voltage at the junction that causes the feedback
`loop to maintain the differential current at the first value
`when the line voltage is below the first threshold and to
`maintain the differential current at the second value
`when the line voltage is above the second threshold.
`11. The line-interface circuit of claim 10 wherein the
`second source causes the feedback loop to vary the diifer-
`ential current between the first and the second value when
`the line voltage varies between the first and the second
`thresholds.
`12. A line-interface circuit for connecting to an analog
`telephone line comprising a pair of leads, having a battery-
`feed circuit including circuitry that monitors line voltage
`across the pair of leads and further including circuitry that
`maintains line current flowing between the leads at a sub-
`stantially constant first current value when the line voltage
`is exceeded by a first threshold value and maintains the
`current at a substantially constant second current value that
`is significantly smaller than the first current value when the
`line voltage exceeds a second threshold value, and the
`battery feed circuit further including a driver for powering
`the line, the driver using a first of a pair of power supplies
`to drive the line while the line current is at the first current
`value and using a second of the pair of power supplies to
`drive the line while the line current is at the second value,
`the first power supply operating at a voltage of significantly
`greater magnitude that a voltage at w