Abstract:
Control circuit and method for regulating the supply voltage of an SLIC circuit ( 15 ), having a monitoring circuit ( 36 ) for calculating an SLIC supply voltage which is required to reach a nominal supply current (I nom ) for an analogue terminal ( 2 ) which is connected to the SLIC circuit ( 15 ); and having a DC/DC converter ( 53 ), which is controlled by the monitoring circuit ( 36 ), for converting a battery voltage (V batt ) to the calculated SLIC supply voltage.

Description:
RELATED ART 
       FIG. 1  shows an interface for connecting an analog terminal, in particular an analog telephone, to a digital switching system according to the prior art. 
     The analog terminal is connected via two telephone lines to an SLIC circuit which is integrated in the interface. The SLIC circuit (SLIC: Subscriber Line Interface Circuit) carries out what are referred to as the BORSCHT tasks, namely: 
     Battery feeding, that is to say remote power supply feed to the terminal, 
     Overvoltage protection, 
     Ringing, that is to say calling the subscriber terminal, 
     Signaling, 
     Coding, that is to say PCM coding/decoding, and 
     Hybrid testing, or testing and measurement. 
     The SLIC circuit contains battery switching devices for applying various battery voltages V batt1 , V batt2 , . . . . The SLIC circuit is connected via a signal receiving line RC and a signal transmitting line TX as well as respectively associated ground lines to a Codec circuit. The SLIC circuit matches the signal which is transmitted on the four lines of the Codec circuit to the analog telephone connecting lines. The Codec circuit receives a PCM-coded digital signal via a data line for transmission of digital data. The interface also contains an FSK circuit (FSK: Frequency Shift Keying) for telephone number display and a DTMF circuit (DTMF: Dual Tone Multi Frequency), which decodes the transmitted tones during a telephone call produced by the analog terminal, so that this dialing information is transmitted for switching. 
     The power supply for the analog terminal is provided by the SLIC circuit. The SLIC circuit in this case uses a connected battery voltage V batt  to produce a feed voltage V feed  for application to the two telephone connecting lines. This feed voltage V feed  produces a supply current for supplying the terminal. In this case:
 
Δ U=V   battery   −V   DROP   −V   line   −V   telephone   (1)
 
where ΔU is a voltage drop, which is not constant, within the SLIC circuit,
 
     V DROP  is a constant voltage drop caused by an internal resistance in the SLIC circuit, 
     V line  is the voltage drop on the telephone connecting line, 
     and V telephone  is the constant telephone supply voltage. 
     In this case:
 
 V   DROP   =I   feed   ·R   internal   (2)
 
where:
 
     I feed  is the feed current emitted from the SLIC circuit to the telephone connecting line, and 
     R internal  is the constant internal resistance of the SLIC circuit. 
     The voltage V line  applied to the telephone connecting line is not constant, and depends on the length, and hence on the resistance, of the telephone line:
 
 V   line   =I   feed   ·R   line   (3)
 
     A greater or lesser voltage is thus dropped on the SLIC circuit depending on the line length L and hence on the line resistance R line :
 
 P   loss   =ΔU·I   feed   =ΔR   line   ·I   feed   2   (4)
 
     The power loss P loss  produced in the SLIC circuit produces heat which must be dissipated to the environment. In order to prevent overheating of the interface, and hence an adverse effect on its functions, measures are therefore taken for cooling of and heat dissipation from the SLIC circuit. As the battery voltage V battery  rises, the power loss P loss  in the SLIC circuit increases. If, conversely, the battery voltage V battery  is too low, it is impossible for the SLIC circuit to provide sufficient power to supply the terminal. 
     In the conventional interface according to the prior art, as is illustrated in  FIG. 1 , a switching device is therefore provided in the SLIC circuit, and switches between two or more battery voltages V batt  depending on the telephone connecting line length L. One disadvantage of switching between different battery voltages V batt  is that the circuitry complexity for generation of different battery voltages V batt  is relatively high, and this results in only a minor reduction in the power loss P loss  within the SLIC circuit. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is therefore to provide a method and an apparatus for regulating the supply voltage of an SLIC circuit, in which the power loss produced in the SLIC circuit is minimal. 
     According to the invention, this is achieved by a method having the features specified in patent claim  1 , and by a control circuit having the features specified in patent claim  5 . 
     The invention provides a method for regulating an SLIC supply voltage with minimum power loss, in which case the method has the following steps, namely: 
     application of a telephone connecting line measurement feed voltage to a telephone line by means of an SLIC circuit, 
     measurement by a monitoring circuit of a telephone line current which flows via the telephone connecting line when the measurement feed voltage is applied, 
     calculation by the monitoring circuit of the required SLIC supply voltage for the SLIC circuit as a function of the measured telephone connecting line current, 
     in which case, with the calculated SLIC supply voltage, the SLIC circuit applies to the telephone line a telephone line feed voltage which produces a predetermined nominal supply current for a terminal which is connected to the telephone connecting lines, and 
     production of the calculated SLIC supply voltage for application to the SLIC circuit. 
     In one preferred embodiment of the method according to the invention, the SLIC supply voltage for the SLIC circuit is calculated by: 
     calculation of the line resistance of the telephone connecting line as a function of the measured current flowing via the telephone connecting line and of the applied telephone connecting line measurement feed voltage, 
     calculation of a required telephone connecting line measurement feed voltage, required to produce the predetermined nominal supply current in the calculated line resistance, 
     calculation of an internal voltage drop in the SLIC circuit as a function of an internal resistance of the SLIC circuit and of the predetermined nominal supply current, and by 
     calculation of the required SLIC supply voltage for the SLIC circuit by addition of the calculated telephone connecting line feed voltage and the calculated internal voltage drop in the SLIC circuit. 
     The SLIC supply voltage is preferably produced by DC/DC conversion of a battery voltage which is applied to a DC/DC converter. 
     In one preferred embodiment of the method according to the invention, the battery voltage which is applied to the DC/DC converter is pulse-width-modulated by means of a control signal in the monitoring circuit in order to produce the SLIC supply voltage. 
     The invention furthermore provides a control circuit for regulating the supply voltage of an SLIC circuit, having a monitoring circuit for calculating an SLIC supply voltage which is required to achieve a nominal supply current, 
     and having a DC/DC converter circuit, which is controlled by the monitoring circuit, for converting a battery voltage to the calculated SLIC supply voltage. 
     The DC/DC converter circuit preferably has a controllable switching device, to which the battery voltage is applied, and has an induction circuit, which is connected to the controllable switching device, for producing the SLIC supply voltage. 
     In one preferred embodiment of the control circuit according to the invention, the controllable switching device is a transistor. 
     The transistor is preferably a field-effect transistor. 
     The gate connection of the field-effect transistor is in this case preferably connected via a control line to the monitoring circuit, with the battery voltage being applied to the source connection and with the induction circuit being connected to the drain connection. 
     In one particularly preferred embodiment of the control circuit according to the invention, the battery voltage which is applied to the source connection of the field-effect transistor is passed on by means of a control signal, which is received at the gate connection of the field-effect transistor via the control line from the monitoring device, in order to emit a pulse-width-modulated DC voltage signal via the drain or source connection to the induction circuit. 
     The induction circuit preferably uses mutual induction to produce the SLIC supply voltage from the received pulse-width-modulated DC voltage signal. 
     The DC/DC converter circuit preferably has a rectifier circuit. 
     In one further preferred embodiment of the control circuit according to the invention, the DC/DC converter circuit has a smoothing circuit for smoothing the SLIC supply voltage. 
     The monitoring circuit is preferably connected to a Codec circuit. 
     In a first embodiment of the control circuit according to the invention, the monitoring circuit is integrated in the SLIC circuit. 
     In an alternative embodiment of the control circuit according to the invention, the monitoring circuit is integrated in the Codec circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the method according to the invention and of the control circuit according to the invention for regulating an SLIC supply voltage will be described in the following text with reference to the attached figures in order to explain the features that are essential to the invention. In the figures: 
         FIG. 1  shows an interface for connection of an analog subscriber terminal to a digital telephone network according to the prior art; 
         FIG. 2  shows an interface circuit for connection of an analog terminal to a digital switching system, which interface circuit contains a control circuit according to the invention; 
         FIG. 3  shows a preferred embodiment of the control circuit according to the invention; 
         FIG. 4  shows a flowchart of one preferred embodiment of the method according to the invention for regulating the SLIC supply voltage; 
         FIG. 5  shows a diagram to explain the method of operation of the control circuit according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 2  shows a block diagram of an interface circuit  1  for connection of an analog terminal  2  to at least one data line  3  for transmission of digital signals. The interface circuit  1  contains a connection  4  for connection of the interface circuit  1  to the data transmission line  3 . Voice signal information and data information are transmitted in digital form via the data transmission line  3 . The connection  4  is connected via a data frame synchronization line  5  (Frame Sync), a clock line  6 , a data receiving line  7 , a data transmitting line  8  and a reference ground line  9  to a Codec circuit  10 . The digital data applied is decoded by the Codec circuit  10 , and is converted to analog signals. The Codec circuit  10  is connected via a receiving line  11 , a receiving ground line  12 , a transmitting line  13  and a transmitting ground line  14  to an SLIC circuit  15  which is provided in the interface circuit  1 . 
     An FSK circuit  16  (FSK: Frequency Shift Keying) is connected via a line  17  to the receiving line  11 . A DTMF circuit  18  is connected via a line  19  to the transmitting line  13 . The FSK circuit produces the telephone number to be displayed on the terminal  2  when a telephone call is received. Conversely, when an outgoing telephone call is made from the terminal  2 , the DTMF circuit  18  produces the telephone number to be displayed on the receiver. 
     The Codec circuit  10 , the FSK circuit  16  and the DTMF circuit  18  are supplied with a supply voltage V DD  of, for example, 5 V via a supply voltage line  20  at a supply voltage connection  21 . The SLIC circuit  15  has two output connections  22 ,  23 , which are connected to two telephone connecting lines  24 ,  25  for connection of the analog subscriber terminal  2 . The SLIC circuit (SLIC: Subscriber Line Interface Circuit) converts the signal which is present on the four lines  11  to  14  into a two-wire telephone signal. At the same time, the SLIC circuit  15  supplies a telephone supply voltage and a supply current via the telephone connecting lines  24 ,  25  to the analog subscriber terminal  2 . 
     In the preferred embodiment shown in  FIG. 2 , the Codec circuit  10  has a control connection  26 , which is connected via a control line  27  to a control input  28  of the control circuit  29  according to the invention. The control circuit  29  receives the battery voltage V batt , which is applied to one input  32  of the interface circuit  1 , via a battery voltage input connection  30  and a voltage line  31 . As can be seen from  FIG. 2 , only one battery voltage V batt  is required, so that the circuit complexity for producing the battery voltage is minimal. 
     The control circuit  29  furthermore has an output  33  for emitting the SLIC supply voltage produced by the control circuit  29  via a supply voltage line  34  to a supply voltage connection  35  of the SLIC circuit  15 . 
       FIG. 3  shows a preferred embodiment of the control circuit  29  according to the invention. The control circuit  29  contains a monitoring circuit  36 , which is connected via an internal signal line  37  to the control input  28  of the control circuit  29 . The monitoring circuit  36  emits a control signal via a control line  38  to a gate connection  39  of a field-effect transistor  40 . The field-effect transistor  40  has a source connection  41 , which is connected via a line  42  to the battery supply voltage connection  30  of the control circuit  29 . The field-effect transistor  40  also has a drain connection  43 , which is connected via an internal line  44  to a first coil  45  in an induction circuit  46 . The induction circuit  46  has a second coil  47 , which is inductively coupled to the first coil  45  and whose first connection is connected via a rectifier diode  48  and a signal line  49  to the supply voltage output connection  33  of the control circuit  29  for emitting the SLIC supply voltage. The second connections of the two coils  45 ,  47  of the induction circuit  46  are connected via a ground line  50  to a ground connection  51  of the control circuit  29 . The control circuit  29  furthermore contains a capacitor  52 , which is connected in parallel with the second coil  47  of the induction circuit  46 , in order to smooth the emitted SLIC supply voltage. 
     The controllable field-effect transistor  40  and the induction circuit  46  together form a controllable DC/DC converter circuit  53 . The DC/DC converter circuit  53  is used to convert the battery voltage V battery , which is applied to the connection  30 , to a desired SLIC supply voltage, which is emitted at the connection  33  to the SLIC circuit  15  shown in  FIG. 2 . 
     The required SLIC supply voltage is calculated by the monitoring circuit  36  contained in the control circuit  29 . The monitoring circuit  36  contains a calculation unit for this purpose. Depending on the calculated SLIC supply voltage that is required, the monitoring circuit  36  calculates a pulse-width-modulation ratio (7 WVM), and emits a control signal for pulse-width modulation of the battery voltage V batt  which is present at the source connection  41 , via the control line  38  to the gate connection  39 . The pulse-width-modulated DC voltage signal which is applied to the drain connection  43  is passed via the line  44  to the first coil  45  in the induction circuit  46 . Mutual induction results in the required supply voltage being produced in the second coil  47  of the induction circuit  46 , and this is rectified by the rectifier diode  48 . The capacitor  52  smooths the required supply voltage that is produced. 
       FIG. 4  shows a flowchart of a particularly preferred embodiment of the method according to the invention for regulating an SLIC supply voltage. 
     In an initialization step S 0 , a feed voltage V feed0  is supplied from the SLIC circuit  15  to the telephone connecting lines  24 ,  25 . 
     In step S 1 , the Codec circuit  10  measures the actual current I act  flowing on the telephone connecting lines  24 ,  25 , and the appropriate control information data is emitted via the line  27  to the monitoring circuit  36  that is contained in the control circuit  29 . The monitoring circuit  36  contains a memory apparatus, in which a predetermined required supply current for analog subscriber terminals  2  is stored. 
     A check is carried out in step S 2  to determine whether the measured actual current I act  corresponds to the required supply current I nom  for the subscriber terminal  2 . If this is the case, the process returns to step S 1  in order to measure the actual current I act  once again. 
     If the actually measured current flowing via the telephone connecting lines  24 ,  25  differs from the required supply current I nom , a check is carried out in step S 3  to determine whether the actually measured current I act  is less than the required supply current I nom . If this is the case, the SLIC supply voltage is increased in step S 4 . 
     If it is found in step S 3  that the current actually flowing on the telephone connecting lines  24 ,  25  to the subscriber terminal  2  is greater than the desired, predetermined supply current I nom , the SLIC supply voltage is correspondingly reduced in step S 5 . 
     The required SLIC supply voltage is calculated as follows. First of all, depending on the current I act  actually flowing and as measured in step S 1 , the line resistance R line  of the telephone connecting lines  24  and the resistance of the telephone R telephone    25  are calculated as a function of the measured current I act  and the telephone line feed voltage V feed  applied by the SLIC circuit  15  to the connections  22 ,  23 . This is used to calculate the required telephone line feed voltage V feed nom , for which the predetermined nominal supply current I nom  is produced by the monitoring device  36  within the control circuit  29 . Furthermore, the internal voltage drop V DROP  in the SLIC circuit  15  is calculated as a function of a known, constant internal resistance R internal  of the SLIC circuit  15  and the predetermined nominal supply current I nom . The monitoring circuit  36  then calculates the required supply voltage to be applied to the supply voltage connection  35  for the SLIC circuit  15 , as a function of the required telephone line feed voltage V feed nom  and the internal voltage drop V DROP  in the SLIC circuit  15 , by adding the required telephone line feed voltage V feed nom  and the constant internal voltage drop V DROP . 
     The monitoring circuit  36  uses the calculated required supply voltage for the SLIC circuit  15  to calculate a pulse-width-modulation ratio PWMV for driving the gate connection  39  via the control line  38 . The width of the pulses in the pulse-width-modulated DC voltage signal at the drain connection  43  of the field-effect transistor  40  is increased in order to increase the supply voltage for the SLIC circuit  15 , as emitted by the output  33  from the induction circuit  46 . Conversely, the supply voltage for the SLIC circuit  15  is reduced by reducing the width of the pulses in the pulse-width-modulated DC voltage signal at the drain connection  43  of the field-effect transistor  40 . 
     The monitoring circuit  36  in the control circuit  29  can be integrated in the Codec circuit  10 , in a first embodiment. In an alternative embodiment, the monitoring circuit  36  is integrated in the SLIC circuit  15 . Integration of the monitoring circuit  36  in the Codec circuit  10  offers the advantage that the monitoring circuit can be constructed in the same way as the Codec circuit  10  using CMOS technology, and thus itself requires only a low supply voltage V DD . The supply voltage for the SLIC circuit is dynamically matched to the line length L of the telephone connecting lines  24 ,  25 . 
       FIG. 5  shows a diagram to explain the method of operation of the control method according to the invention. 
     If a feed voltage V feed  of magnitude U 0  is emitted by the SLIC circuit  15 , in the initialization step S 0 , to the telephone connecting lines  24 ,  25 , whose resistance is R line , the current I feed  that is fed in corresponds precisely to the required nominal supply current I supply nom . 
     If, in a case A, the telephone connecting lines  24 ,  25  are relatively long, the application of the original measurement voltage U 0  leads to an excessively low feed current I 0a  flowing via the telephone connecting lines  24 ,  25 . Thus, in order that the desired supply current flows, the control circuit  29  increases the SLIC supply voltage in such a manner that a feed voltage V feed  with a value U A  is applied to the output connections  22 ,  23  of the SLIC circuit  15 . 
     If, conversely, in a case B, the telephone connecting lines  24 ,  25  are very short, then an excessively high feed current I 0b  flows when a measurement voltage U 0  is applied. In a corresponding manner, the control circuit  29  reduces the supply voltage from the SLIC circuit  15  in such a manner that the SLIC circuit  15  emits a lower feed voltage V feed  of magnitude U B  at the output connections  22 ,  23 . 
     The required nominal supply current for the subscriber terminal  2  is defined at a fixed value in every country, and is typically in a range between 20 and 50 mA. 
     The supply voltages set at the input connection  35  of the SLIC circuit  15  by the control circuit  29  are typically in the range from −15 V to −60 V with respect to ground. 
     Since the voice signals are isolated from the high battery voltages, this results in an improvement in the voice quality and a reduction in the production costs. Signal crosstalk, temperature changes and increases in the applied battery voltage have no effect on the sensitive transducers within the Codec circuit  10  when using the control circuit according to the invention. 
     Minimizing the power loss that occurs in the SLIC circuit  15  simplifies the cooling and dissipation of the heat that is produced, thus allowing the production costs for the interface circuit  1  to be reduced. Since only one battery voltage V batt  need be produced for application to the connection  32  of the control circuit  1 , this avoids further circuitry complexity. 
     The supply voltage for the SLIC circuit  15  is matched to the actual telephone connection characteristics, and the sharing of the Codec circuit and the SLIC circuit in an integrated control circuit  29  allows thermal decoupling, and possibly also crosstalk between analog and digital signals, to be avoided. 
     LIST OF REFERENCE SYMBOLS 
     
         
           1  Interface circuit 
           2  Analogue terminal 
           3  Data line 
           4  Connection 
           5  Frame synchronization line 
           6  Clock line 
           7  Receiving line 
           8  Transmitting line 
           9  Earth line 
           10  Codec circuit 
           11  Receiving line 
           12  Receiving earth line 
           13  Transmitting line 
           14  Transmitting earth line 
           15  SLIC circuit 
           16  FSK circuit 
           17  Line 
           18  DTMF circuit 
           19  Line 
           20  Supply voltage line 
           21  Supply connection 
           22  SLIC circuit output 
           23  SLIC circuit output 
           24  Telephone connecting line 
           25  Telephone connecting line 
           26  Connection 
           27  Line 
           28  Input 
           29  Control circuit 
           30  Battery voltage connection 
           31  Line 
           32  Connection 
           33  Control circuit output 
           34  Supply voltage line 
           35  Supply connection 
           36  Monitoring circuit 
           37  Line 
           38  Control line 
           39  Gate connection 
           40  Field-effect transistor 
           41  Source connection 
           42  Line 
           43  Drain connection 
           44  Line 
           45  Coil 
           46  Induction circuit 
           47  Coil 
           48  Rectifier diode 
           52  Smoothing capacitor 
           53  DC/DC converter