Patent Publication Number: US-RE42237-E

Title: Active mode line voltage regulator for ringing subscriber line interface circuit

Description:
FIELD OF THE INVENTION 
     The present invention relates in general to telecommunication systems and subsystems therefor, and is particularly directed to a new and improved mechanism for limiting the DC voltage applied to a tip/ring amplifier of a subscriber line interface circuit (SLIC) to a voltage that is allowed to comply with minimum on-hook battery requirements, yet prevents a sustained excessively high (and potentially dangerous) voltage from being applied to the SLIC. 
     BACKGROUND OF THE INVENTION 
     Subscriber line interface circuits (SLICs) are employed by telecommunication service providers to interface a communication wireline pair with subscriber equipment, including both voice and data communication devices. In order to be interfaced with a variety of telecommunication circuits including circuits providing digital codec functionality, the transmission channels of the SLIC must conform with a very demanding set of performance requirements, including but not necessarily limited to accuracy, linearity, low noise, filtering, insensitivity to common mode signals, low power consumption, and ease of impedance matching programmability. In this regard, the DC voltage parameters of a ringing SLIC are governed by both the operational requirements of the device to which the SLIC is coupled (such as the minimum on-hook voltage (e.g., on the order of 40 VDC) required by a facsimile machine or modem), as well as telecommunication industry safety standards (that currently limit the allowable sustained DC voltage to a value of 56.5 VDC). 
     A reduced complexity illustration of a conventional multi-current control-based circuit amplifier architecture for complying with this requirement is diagrammatically illustrated in  FIG. 1 , as comprising a (Tip/Ring) amplifier  10  having its non-inverting (+) input  11  coupled to a voltage dividing node  21  of a voltage divider  20 . The voltage divider is formed of a pair of equal valued (R) resistors  22  and  23 , that connect a DC battery voltage (VBAT) to ground (GND). The amplifier  10  has an inverting (−) input  12  coupled to an output node  13  by way of a feedback (value R) resistor  14 . The inverting (−) input  12  of the amplifier is further coupled to a current source  31 , which may be configured as a current mirror, and is operative to supply a current corresponding to that sensed flowing through the voltage divider  20 , or I=VBAT/2R. 
     In order to constrain the amplifier input voltage within prescribed operational limits (e.g., the above referenced 56.5 VDC value) irrespective of the value of the battery voltage VBAT, the inverting (−) input  12  of amplifier  10  is further coupled to a plurality of current source/sink circuits  32  and  33 . The current mirror (sink)  32  sinks an equal and opposite polarity current I=VBAT/2R from the inverting polarity (−) input node  12 , so that current source/sink pair  31 / 32  effectively provide a pair of currents at the inverting (−) input node  12  that are complementary to those provided at the non-inverting (+) input node  11 , by way of the voltage divider  20 . An additional current mirror  33  is used to controllably supply the amplifier&#39;s inverting polarity (−) node  12  with an auxiliary, compensation current derived in accordance with MTU specifications and designated in  FIG. 1  as current I=Vmtu/R. 
     Typically, this auxiliary current is generated by sensing the current through resistors  22  and  23 , and then comparing the sensed current to a threshold current reference value. The difference between these two currents is applied to current mirror  33 , which produces the auxiliary current I=Vmtu/R. Unfortunately, such a multi-current source based regulation scheme not only dissipates substantial power, but is prone to introducing voltage regulation component-based noise into the voice path of the SLIC. 
     SUMMARY OF THE INVENTION 
     Pursuant to the present invention, these drawbacks are effectively obviated by dispensing with the MTU-based current compensation circuitry, and instead making a relatively simple circuit modification to the battery voltage supply path, so as to couple a voltage regulator circuit between the battery voltage terminal and the input nodes of the SLIC&#39;s tip/ring amplifiers. For present day ringing SLICs, the parameters of the voltage regulator are such as to limit its output voltage to 56.5 VDC. Thus, a battery voltage VBAT having any value less than 56.5 VDC will be replicated as such for application to the amplifier input, while a voltage at or above 56.5 VDC is limited to a value of 56.5 VDC. 
     In a first embodiment, the voltage regulator circuit is installed between the battery supply terminal VBAT and the battery input end of the voltage divider, to a central node of which of the tip/ring amplifier is coupled. In a second embodiment, the battery input end of the voltage divider is coupled directly to the battery terminal, while the voltage regulator is coupled to the central node to which the amplifier input is coupled. In each embodiment, the current mirror that drives the complementary input node of the amplifier is referenced to the current flowing through that portion of the voltage divider subject to the regulated voltage Vreg, so that the mirrored current is not affected by excessive battery voltage swing. To prevent noise from being introduced into the voice path of the amplifier from the regulated DC supply circuitry, a low pass filter that passes only DC supply energy (for example, one having an upper frequency cut-off on the order to 2-8 Hz), may be incorporated into the current mirror circuitry. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  diagrammatically illustrates a reduced complexity illustration of a conventional multi-current control-based voltage regulation circuit for a tip/ring amplifier of a subscriber line interface circuit; 
         FIG. 2  is a reduced complexity diagram of a first embodiment of the active mode line voltage regulator for a tip/ring amplifier of a ringing subscriber line interface circuit of the invention; 
         FIG. 3  shows a second embodiment of the active mode line voltage regulator of the invention; and 
         FIG. 4  shows the application of the embodiment of the invention of  FIG. 3  to both tip and ring amplifiers of a ringing subscriber line interface circuit. 
     
    
    
     DETAILED DESCRIPTION 
     Before detailing the active mode subscriber line interface circuit voltage regulator of the present invention, it should be observed that the invention resides primarily in a prescribed arrangement of conventional communication circuits and voltage regulation components therefor. As a consequence, this arrangement has been shown in the drawings by readily understandable block diagrams and associated circuit diagrams, which depict only those specific details that are pertinent to the present invention, so as not to obscure the disclosure with particulars apparent to those skilled in the art having the benefit of the description herein, whereby the invention may be more readily understood. 
     Attention is now directed to  FIG. 2 , which is a reduced complexity diagram of a first embodiment of the active mode line voltage regulator for a respective tip/ring amplifier of a ringing subscriber line interface circuit of the invention. (It is to be understood that the polarities shown in  FIG. 2  may be reversed, without a loss in generality.) Similar to the conventional circuit of  FIG. 1 , a respective (Tip/Ring) amplifier  10  has its non-inverting (+) input  11  coupled to the voltage dividing node  21  of voltage divider  20  formed of a pair of equal valued (R) series-coupled resistors  22  and  23 . A first end  25  of the voltage divider is referenced to a prescribed voltage (shown as GND) and a second end  24  thereof is coupled to receive a prescribed DC voltage (derived from the battery voltage (VBAT)). Also, as in  FIG. 1 , the inverting polarity (−) input  12  of the amplifier  10  is coupled to output node  13  by way of a feedback resistor  14  (having a value R), and to a (current mirror-configured) current source  41 . 
     In accordance with the invention, rather than the current source  41  driving the amplifier input (−) node  12  with a current based upon the actual battery voltage VBAT, and having to couple that node to an additional, substantial power-dissipating set of voltage regulation current sources as described above, voltage regulation is realized by a relatively simple modification of the input path from battery VBAT to the voltage divider  20  for the (+) input node  11 . In particular, a voltage regulator circuit (of conventional configuration)  50  is installed between the battery terminal VBAT and the second end  24  of the voltage divider  20 . 
     For the presently discussed example of a ringing SLIC, the parameters of the voltage regulator  50  are such as to limit its output voltage to 56.5 VDC. Thus, a battery voltage VBAT having any value less than 56.5 VDC would be replicated as such at the input terminal end  24  of the voltage divider  20 , while a voltage at or above 56.5 VDC would be limited to a value of 56.5 VDC at node  24 . Thus, the current through the voltage divider  20  and therefore the current supplied to amplifier input node  12  by the current source  41  is based upon the regulated voltage Vreg output by the voltage regulator  50 , rather than upon the battery voltage VBAT. In particular, the current supplied by current source  41  is the current I=Vreg/2R. 
     In order to prevent noise from being introduced into the voice path of the amplifier, a low pass filter that passes only DC supply energy (for example, one having an upper frequency cut-off on the order to 2-8 Hz), may be incorporated into the current mirror circuitry, through which the current flowing through the voltage divider  20  is sensed and mirrored by way of current mirror  41  into the inverting node  12  of the amplifier  10 . Such a low pass filter (LPF) is shown at  43  in the input path of the current mirror  41 . 
     Rather than couple the voltage regulator  50  between the battery VBAT and the voltage divider  20 , as in the first embodiment of  FIG. 2 , the terminal end  24  of the voltage divider  20  may be coupled directly to the battery terminal as in  FIG. 1 , and the voltage regulator  50  coupled directly to the node  21  to which the amplifier (+) input  11  is coupled, as shown in FIG.  3 . In this embodiment, fluctuations in the battery voltage VBAT above Vreg only affect the differential voltage across resistor  23 . The current mirror  41  is referenced to the current flowing through resistor  22  of the voltage divider  20 , so that its value (I=Vreg/R) is not affected by an excessive battery voltage swing and remains defined by Vreg, but with a single resistor divisor (the value of resistor  22 ). 
       FIG. 4  shows the application of the embodiment of the invention of  FIG. 3  to both tip and ring amplifiers of a ringing subscriber line interface circuit. Here, respective Tip and Ring amplifiers  10 T and  10 R have their non-inverting (+) inputs  11 T and  11 R coupled in common to the reference node  21  of the voltage divider  20 . The inverting polarity (−) input  12 T of the Tip amplifier  10 T is coupled to its output node  13 T by feedback resistor  14 T and to a current source  41 T. Similarly, the inverting polarity (−) input  12 R of the Ring amplifier  10 R is coupled to its output node  13 R by a feedback resistor  14 R and to a current source  41 R. As in the embodiment of  FIG. 3 , the current mirrors  41 T and  41 R are referenced to the current flowing through the resistor  22  of the voltage divider  20 , and generate opposite polarity currents I=Vreg/R, as shown.  FIG. 4  also shows respective low pass filters  43 T and  43 R incorporated with the current mirror circuits  41 T and  41 R, to prevent high frequency noise from being introduced into the tip and ring amplifiers, as described above. 
     As will be appreciated from the foregoing description, circuit complexity and power consumption drawbacks of conventional MTU-based current compensation circuitry of ringing SLIC architectures are effectively obviated by coupling a voltage regulator circuit between the battery voltage terminal and the input nodes of the SLIC&#39;s tip/ring amplifiers. Since the current mirror that drives the complementary input node of the amplifier is referenced to the current flowing through that portion of the voltage divider subject to the regulated voltage Vreg, the mirrored current will not be affected by excessive battery voltage swing. Also, incorporating a low pass filter into the current mirror circuitry prevents the introduction of noise from the DC supply circuitry into the voice path of the amplifier. 
     While we have shown and described several embodiments in accordance with the present invention, it is to be understood that the same is not limited thereto but is susceptible to numerous changes and modifications as known to a person skilled in the art, and we therefore do not wish to be limited to the details shown and described herein, but intend to cover all such changes and modifications as are obvious to one of ordinary skill in the art.