This application claims priority under 35 U.S.C. §§119 and/or 365 to 9903116-3 filed in Sweden on Sep. 3, 1999; the entire content of which is hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The invention relates generally to subscriber line interface circuits and more specifically to a method and an arrangement for shortening the recovery time of a line driver in a subscriber line interface circuit after an overvoltage on its output terminal. 
     BACKGROUND OF THE INVENTION 
     In subscriber line interface circuits (SLICs), line drivers connected to tip and ring wires of respective transmission lines, have to be turned off when the voltage on the tip or ring wire comes close to the negative supply voltage of the line drivers. 
     The line drivers are turned off in order to avoid an uncontrolled function that could lead to the destruction of the line drivers. 
     When turned off, the line drivers are put in a high-impedance state with no ability to sink or source current. This may lead to that the line drivers are stuck in this state for a longer or shorter time. 
     An example of a situation when this can occur is when a charged line is connected to the SLIC after a ringing sequence. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to eliminate these problems with the known line drivers. 
     This is obtained by the method and the arrangement in accordance with the invention by initiating a current generation before the turn-off function is activated with the purpose of “pulling” the tip and/or ring wires, i.e. actually the voltage of the tip and/or the ring wires, into the active operating range of the line drivers. 
     Hereby, the recovery time of the line drivers after an overvoltage will be considerably shortened. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will be described more in detail below with reference to the appended drawing on which 
     FIG. 1A shows a known line driver connected to a wire of a transmission line, 
     FIG. 1B is a diagram illustrating a recovery of the line driver in FIG. 1A after an overvoltage on its output terminal, 
     FIG. 2A shows the line driver in FIG. 1A with an arrangement according to the invention, and 
     FIG. 2B is a diagram illustrating the recovery of the line driver in FIG. 2A after an overvoltage on its output terminal. 
    
    
     DESCRIPTION OF THE INVENTION 
     FIG. 1A shows a known line driver  1  in a SLIC  2 , which is not shown as to the rest. 
     The line driver  1  is connected with its output terminal  3  to one wire  4  of a two-wire transmission line to a load, e.g. a subscriber station (not shown). 
     The supply voltage terminals of the line driver  1  are connected between ground GND and a negative supply voltage UB. 
     The input terminal of the line driver  1  is connected to means (not shown) for generating input signals IN. 
     FIG. 1B is a diagram illustrating how an overvoltage on the output terminal  3  of the line driver  1  in FIG. 1A is handled by the line driver  1 . By overvoltage is meant a voltage that, in the embodiment in FIG. 1B, is more negative than a turn-off or threshold voltage UT at which the line driver  1  is turned off when the overvoltage appears, and at which the line driver  1  is turned on again when the overvoltage has disappeared in a manner known per se. 
     As mentioned above in the introductory portion, an overvoltage can appear on a wire  4  when the transmission line after a ringing sequence is connected to the SLIC  2 . 
     In normal operation, the voltage on the wire  4  is supposed to be U 4 . 
     At time t 1 , an overvoltage that is more negative than the threshold voltage UT, appears. 
     In a manner known per se, as mentioned above, the line driver  1  is instantaneously turned off when the voltage equals the threshold voltage UT at time t 1  in this case, and stays turned off as long as the overvoltage is present. 
     Suppose that the overvoltage ends at time t 2 . Then, the line driver  1  starts to recover in an uncontrolled manner, mainly due to internal leakage currents. At time t 3 , the output voltage of the line driver  1  is supposed to equal UT. At that time t 3 , the line driver  1  is turned on again in a manner known per se as indicated above, entering its active state, and recovers quickly in a controlled manner to the voltage U 4  at time t 4 . 
     Thus, the recovery process is terminated at time t 4 . This means that the recovery time is equal to the time between times t 2  and t 4  in the line driver  1  in FIG.  1 A. 
     Components, voltages and times in FIGS. 2A and 2B that are identical to those in FIGS. 1A and 1B have been provided with identical reference characters. 
     In FIG. 2A, the line driver  1  is provided with an arrangement according to the invention for shortening its uncontrolled recovery time after an overvoltage on its output terminal  3 , exceeding the threshold voltage UT. 
     In the embodiment illustrated in FIG. 2A, this arrangement comprises a voltage-to-current converter  5 . 
     The converter  5  is connected with one input terminal  6  to the output  3  of the line driver  1 , with its other input terminal  7  to the supply voltage UB via a voltage source  8  providing a threshold voltage UT′, and with its output terminal  9  to the output  3  of the line driver  1  as shown in FIG.  2 A. 
     The threshold voltage UT′ is more positive than the threshold voltage UT as apparent from FIG.  2 B. In view of the fact that both threshold voltages UT′ and UT are fixed relative to the supply voltage UB, the difference between the threshold voltages UT′ and UT is constant regardless of variations of the supply voltage UB. 
     The voltage-to-current converter  5  is adapted to generate an output current I that is proportional to the voltage difference between the output voltage UO of the line driver  1  and the supply voltage UB plus the threshold voltage UT′ up to a maximum current value. The maximum value of the output current I of the converter  5  will not be reached until UO&lt;UT. 
     In accordance with the invention, this current generation always starts before the turn-off function of the line driver  1  is activated at the threshold voltage UT. 
     Hereby, it is avoided that the line driver  1  is stuck in an uncontrolled state 
     In FIG. 2B, as in FIG. 1B, when the overvoltage equals the threshold voltage UT at time t 1 , the line driver  1  is turned off and stays turned off until the overvoltage disappears at time t 2 . 
     However, already when the voltage difference UO−UB&lt;UT′, the voltage-to-current converter  5  begins to generate the current I. 
     Due to this controlled output current I of the converter  5  to the output  3  of the line driver  1 , i.e. to the transmission line  4 , the voltage of the output terminal  3  of the line driver  1  starts to increase in a controlled manner towards the threshold voltage UT after that the overvoltage disappears at time t 2 . 
     When the voltage at the output  3  of the line driver  1  equals UT, the line driver  1  is turned on in a manner known per se as indicated above, and its output voltage quickly recovers to the voltage U 4  at time t 4  as in FIG.  1 B. 
     When the voltage difference UO−UB&gt;UT′ at time t 3 ′, the voltage-to-current converter  5  stops generating the current I. 
     Instead of the current generating device in the form of the voltage-to-current converter  5  as described with reference to FIGS. 2A and 2B, a comparator (not shown) can be used to sense the voltage difference, and a current generator (not shown) controlled by the output signal of the comparator, can be used to supply the current I to the output  3  of the line driver  1 . 
     As should be apparent from the above, by generating the current I in accordance with the invention, the voltage of the output of the line driver will be “pulled” into the active operating range of the line driver, and its the recovery will be quicker than in line drivers known so far.
To shorten the recovery time of a line driver ( 1) after an overvoltage on its output terminal ( 3), a current generating device ( 5) senses a voltage difference between the output voltage of the line driver ( 1) and its supply voltage (UB) plus a threshold voltage (UT'), and supplies a current (I) that is proportional to the sensed voltage difference up to a maximum current to the output ( 3) of the line driver ( 1) to raise its potential to thereby shorten its recovery time.