Abstract:
A signal line driving circuit with power control for selectively reducing internal power dissipation when driving an external load. While driving the external load with a constant current the output voltage generated across such load is monitored. If the load impedance decreases sufficiently to cause the output voltage to fall below a predetermined threshold value and, therefore, cause the voltage across the signal line driving circuit to increase, the magnitude of the power supply voltage is automatically reduced, thereby reducing the voltage across the signal line driving circuit. Such a signal line driving circuit is particularly advantageous as a subscriber line interface circuit (SLIC). As the subscriber goes from an on-hook condition to an off-hook condition and if the subscriber loop is sufficiently short (or low in impedance), a lower power supply voltage is used to minimize the power dissipation of the SLIC while still maintaining the required subscriber loop current.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to signal line driving circuits for providing output signals with approximately constant output currents, and in particular, to subscriber line interface circuits for telephone lines. 
     2. Description of the Related Art 
     A subscriber line interface circuit (SLIC) is used for each pair of wires (“tip” and “ring”) forming a subscriber telephone line and is responsible for conveying both incoming and outgoing signals (e.g., voice, facsimile and data) while providing necessary power and impedance matching. Such circuit is typically located in the central office and is analog in its function, although digital versions (e.g., ISDN) are being used more often. 
     When the subscriber telephone is on-hook the DC output current, neglecting any leakage impedances, is substantially zero. When the subscriber loop goes into an off-hook state, industry standards require a DC output current through the subscriber loop to be approximately constant, e.g., within the nominal range of 30-40 milliamperes (mA). This DC current provides power to the subscriber telephone circuitry, such as the digital keypad. The impedance of the subscriber loop will depend upon the particular telephone, or telephones, connected to the loop, as well as the transmission length of the loop itself. Therefore, the SLIC must be designed to provide the nominal required DC output current for this range of loop impedances while still providing for transmission of the AC signals (voice, facsimile, etc.). 
     Based upon the foregoing, the following typical scenarios will be encountered. With an output current of 30 mA, a nominal DC power supply voltage of 50 volts for the SLIC, and a subscriber loop impedance of 1000 ohms (resistive), the output voltage presented to the subscriber loop will be 30 volts (=30 mA×1000 ohms). Accordingly, 20 volts will be dropped across the SLIC output circuitry, thereby resulting in approximately 0.6 watts (=30 mA×20 volts) of power dissipation in the output circuitry of the SLIC. However, if the subscriber loop impedance is instead only 500 ohms, then the output voltage becomes 15 volts (=30 mA×500 ohms). This results in 35 volts being dropped across the output circuitry of the SLIC, which, in turn, results in an internal power dissipation of approximately 1.05 watts (=30 mA×35 volts). Hence, it can be seen that, depending upon the subscriber loop impedance, a wide variance in the internal power dissipation of the SLIC can be encountered. Such a wide variance in internal power dissipation imposes substantial design constraints for the SLIC, and prevents such SLIC from performing at maximum efficiency. 
     A number of attempts to minimize the internal power dissipation of the SLIC have included such techniques as using an external resistor (connected in series with the power supply) for dissipating the excess power and using a switching voltage regulator. However, both techniques have significant disadvantages. Simply relocating the power dissipation to an external resistor does not improve overall efficiency of the system, and while a switching voltage regulator may improve power efficiency, significant switching noise can be induced into the subscriber loop. 
     Accordingly, it would be desirable to have a technique by which internal power dissipation can be automatically reduced by maximizing power efficiency and avoiding any introduction of signal noise. 
     SUMMARY OF THE INVENTION 
     A signal line driving circuit with self-controlled internal power dissipation in accordance with the present invention minimizes internal power dissipation while maximizing overall power efficiency and avoiding introduction of extraneous signal noise into the system. 
     In accordance with one embodiment of the present invention, a signal line driving circuit with power control for selectively reducing internal power dissipation when driving an external load includes a signal driver circuit and a power control circuit. The signal driver circuit is configured to connect and provide an output signal to an external impedance and to receive a source current and an input signal which corresponds to such output signal and in accordance therewith provide such output signal and a control signal which varies in relation to such output signal. The output signal includes an output current which is approximately constant and an output voltage which varies in relation to the external impedance and output current. The power control circuit, coupled to the signal driver circuit, is configured to connect to a plurality of voltage sources and receive therefrom a plurality of source voltages and to receive the control signal and in accordance therewith convey the source current from one of the plurality of voltage sources to the signal driver circuit. 
     In accordance with another embodiment of the present invention, a method of driving an external load via a signal line while selectively reducing power dissipation includes the steps of: 
     connecting to an external impedance; 
     connecting to a plurality of voltage sources; 
     applying an output signal to the external impedance; 
     receiving a source current and an input signal which corresponds to the output signal and in accordance therewith generating the output signal and a control signal which varies in relation to the output signal, wherein the output signal includes an output current which is approximately constant and an output voltage which varies in relation to the external impedance and the output current; 
     receiving a plurality of source voltages from the voltage sources; and 
     receiving the control signal and in accordance therewith conveying the source current from one of the plurality of voltage sources. 
     These and other features and advantages of the present invention will be understood upon consideration of the following detailed description of the invention and the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a functional block diagram of a signal interface system using a signal line driving circuit with self-controlled internal power dissipation in accordance with one embodiment of the present invention. 
     FIG. 2 is a more detailed functional block and schematic diagram of the signal line driving circuit portion of the system of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     (While the following discussion is in the context of a subscriber line interface circuit (SLIC) for telephone signals, it should be recognized that the underlying principles of the present invention can be applied to other forms of circuits which provide output signals with substantially constant currents and for which self-control of internal power dissipation is desired.) 
     Referring to FIG. 1, a bi-directional signal interface system  10  using a signal line driving circuit with self-controlled internal power dissipation in accordance with one embodiment of the present invention includes a driver stage  12 , a receiver stage  14 , a power control stage  16  and multiple power sources  18 , interconnected substantially as shown. The driver stage  12  provides an output signal  13  to an output node  20  for conveyance to an external signal line  22 . The output node  20  also conveys from the external signal line  22  an input signal  21  which is processed by the receiver  14 . The output signal  15  from the receiver  14  is fed back to the driver stage  12 . As is well known in the art of SLICs, this allows for duplex operation of the external signal line  22  by subtracting out the receiver output signal  15  from the input signal  11  to the driver stage  12 . 
     The driver stage  12  provides a feedback signal  17   a  to the power control stage  16  which provides DC power  17   b  to the driver stage  12 . Based upon the feedback signal  17   a,  the power control stage  16  selects one of multiple DC voltages  19  from the DC power sources  18 . For example, when the load impedance presented via the external signal line  22  is low, based upon the output signal  13  current, the output voltage presented to the output node  20  is low. When this output voltage becomes low enough that the resulting voltage drop across the output of the driver stage  12  (Vdriver=Vsource−Voutput) exceeds a predetermined threshold (Vdriver&gt;Vthreshold), the feedback signal  17   a  can instruct the power control stage  16  to select a lower power supply voltage  19  from another power source  18  (discussed in more detail below). 
     Referring to FIG. 2, the driver  12  and power control  16  stages in accordance with one embodiment of the present invention are shown in more detail. In this embodiment, as would be typical for a SLIC, the external circuit is a differential circuit. Accordingly, the output node  20  includes two nodes  20   a,    20   b  and the external signal line  22  includes two lines  22   a,    22   b.  In the context of a SLIC, these two signal lines  22   a,    22   b  form the subscriber loop. 
     The driver stage  12  includes a transconductance stage  30  which differentially receives the input voltage signals  11 ,  15  and generates two intermediate current signals  31   a,    31   b  which are buffered by output driver amplifiers  32   a,    32   b.  These amplifiers  32   a,    32   b  provide the approximately constant output current signals  13   a,    13   b  (discussed in more detail below) to the output nodes  20   a,    20   b.  (As discussed above, the output nodes  20   a,    20   b  also receive the incoming signals  21   a,    21   b  from the subscriber loop  22   a,    22   b  which are processed by the receiver  14  in accordance with well known principles.) These amplifiers  32   a,    32   b  are powered by positive and negative power sources. The positive power source V+ is typically zero for telecommunications applications. The negative power source V− is a negative supply. 
     The power control stage  16  includes a differential amplifier  40  which drives a pass transistor connected between a diode  44   a  and the primary power source  18   a  which provides a negative power supply voltage Vbat. The input signal to the differential amplifier  40 , which is the feedback, or control, signal  17   a  from the transconductance stage  30 , is filtered by a low pass filter formed by the series resistor  46  (e.g., 500 kilohms) and shunt capacitor  48  (e.g., 220 nanofarads). Connected across the resistor  46  is a “speed up” circuit  50  which, as discussed in more detail below, selectively reduces the overall resistance so as to speed up the change in the voltage across the capacitor  48 . 
     Another diode  44   b  is used to connect another power source  18   b  having another negative voltage Vbatr to the same node  17   c  as that to which the first diode  44   a  is connected. It is this node  17   c  which provides the negative power supply voltage  17   b  for the output driver amplifiers  32   a,    32   b.  The first power supply voltage Vbat is the more negative voltage (e.g., −56 volts), while the second power supply voltage Vbatr is a less negative voltage (e.g., −30 volts). 
     As the impedance of the subscriber loop  22   a,    22   b  reduces, such as when the subscriber goes off-hook, the differential voltage Vab at the loop nodes  20   a,    20   b  also decreases. This voltage Vab is sensed by the receiver  14  (which typically has a high input impedance relative to the impedance of the subscriber loop and the output impedances of the driver amplifiers  32   a,    32   b ). Accordingly, the receiver output voltage  15 , which corresponds to the subscriber loop node voltage Vab, also decreases. This, in turn, causes the control voltage  17   a  from the transconductance stage  30  to also decrease. This control voltage  17   a  is generally proportional to the receiver output voltage  15  (which in turn, is generally proportional to the subscriber loop node voltage Vab), plus some amount of overhead voltage Voh necessary for the driver amplifiers  32   a,    32   b  to operate. 
     As this voltage  17   a  decreases further, the differential amplifier  40  gradually causes the pass transistor  42  to turn off. If the control voltage  17   a  decreases sufficiently, the transistor  42  becomes cut off and the supply current for the negative supply terminals of the driver amplifiers  32   a,    32   b  is then drawn through the second diode  44   b  from the second power supply  18   b  instead of through the first diode  44   a  from the first power supply  18   a.  Since this second power supply  18   b  has a reduced voltage Vbatr, the voltage dropped across the driver amplifiers  32   a,    32   b  is reduced, thereby reducing the internal power dissipation of the driver amplifiers  32   a,    32   b.  Since the output current  13   a,    13   b  is maintained approximately constant, the signal provided to the subscriber is remains unaffected. Hence, the transition between power supply voltages is not dependent upon the actual voltage values of the power supplies  18  and occurs without introducing any signal noise. 
     As noted above, the output current signals  13   a,    13   b  are approximately constant. More specifically, the output current signals  13   a,    13   b  are approximately constant for a given impedance Zloop of the subscriber loop  22   a,    22   b.  Hence, for example, if the impedance Zloop (e.g., the resistance Rloop) of the subscriber loop  22   a,    22   b  increases (e.g., the loop becomes longer) and becomes less negligible with respect to the subscriber loop feed resistance Rfeed (e.g., 150 ohms) within the output circuits (not shown) of the driver amplifiers  32   a,    32   b,  then the magnitude Iloop of the output current signals  13   a,    13   b  will decrease in accordance with the relationship Iloop=(Vbat−Voh)/(Rloop+Rfeed). However, provided that the impedance Zloop of the subscriber loop  22   a,    22   b  remains constant, the magnitude Iloop of the output current signals  13   a,    13   b  will also remain constant. 
     Furthermore, it should be recognized that notwithstanding the similar directions of the arrows for the output current signals  13   a,    13   b  in FIG. 2, the directions of such current signals  13   a,    13   b  are opposite to one another. In other words, if output current  13   a  is flowing out to subscriber loop leg  22   a  via node  20   a,  then output current  13   b  is flowing in from subscriber loop leg  22   b  via node  20   b,  and vice versa. 
     During transient signals in the loop  22   a,    22   b,  such as on/off-hook transient signals or dialling, the voltage provided to the loop nodes  20   a,    20   b  must be allowed to slew quickly enough to avoid impacting the dial pulsed distortion parameters. This is achieved by shunting the resistor  46  in the input filter for the differential amplifier  40 . A transient detection circuit elsewhere in the system (not shown) generates a trigger signal  51  which closes a switch  52  within the speed of circuit  50 . This places a shunting resistor  54  in parallel with the original resistor  46  to reduce the overall resistance value by a sufficiently significant amount (e.g., by a factor of 100). This allows the circuit to reject speech signals during normal transmission and yet quickly slew in response to normal transient signals. 
     Based upon the foregoing, it should be recognized that although the power control stage  16  has been discussed in terms of switching between two power sources, it is possible to design another power control stage which, in conformance with the foregoing discussion, can select between more than two power sources. For example, by duplicating the combination of filter circuit  46 ,  48 , differential amplifier  40 , transistor  42  and diode  44   a  and connecting such duplicate circuits between other power sources having voltages with values intermediate to voltages Vbat and Vbatr, it is possible to provide for multiple stepped reductions in the power supply voltage provided to the driver amplifiers  32   a,    32   b.  This would allow the power dissipation of the driver amplifiers  32   a,    32   b  to be maintained within a fairly narrow power range. 
     Further based upon the foregoing, it should also be recognized that the principles of the presently claimed invention are not limited to use with circuits using negative power supplies or only bipolar technologies, but can also be applied to circuits using positive power supplies or other device technologies as well, such as metal oxide semiconductor (MOS). 
     Various other modifications and alterations in the structure and method of operation of this invention will be apparent to those skilled in the art without departing from the scope and spirit of the invention. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. It is intended that the following claims define the scope of the present invention and that structures and methods within the scope of these claims and their equivalents be covered thereby.