BACKGROUND OF THE INVENTION In a subscriber line interface circuit driving a two-wire line, the line voltage as well as the line current is a function of the line load, i. e. the resistance of the telephone set and the line itself.

When the line is open, the line is supplied with a maximum voltage at the same time as the current is zero. In order for the line circuit to operate, it must have access to a sup- ply voltage which is somewhat higher than the required line voltage.

When the line is closed, i. e. when the line is loaded, the line voltage will decrease while the line current will increase depending on the sum of the line resistance and the load resistance.

The difference between the supply voltage and the line voltage will be applied across the line circuit through which the line current flows. This causes power consumption in the line circuit.

The power consumption in the line circuit will be highest for short lines. i. e. for a low total value of the resistances of the line and the load.

In many cases, the power consumption or power loss is accepted in the line circuit.

In some cases, a DC/DC converter can be used for each line to keep the maximum power consumption low. The output voltage from the respective converter is continu- ously controlled in order to be adapted to the instant line load. However, the use of such converters causes disturbing radiation and increased complexity of the circuits.

From WO 96/15617, it is known to use a number of analog series regulators corres- ponding to the number of driving voltages to minimize the power consumption by switching the supply voltage to the supply voltage having an absolute value which for the moment is the lowest adequate supply voltage.

When using line circuits with the above switching arrangement with e. g. two supply voltages, the voltage having the lower absolute value is set to a fixed predetermined value for which the least power consumption is expected. This predetermined value is usually set for all line interface boards. The voltage having the higher absolute value, is usually the battery voltage.

SUMMARY OF THE INVENTION The object of the invention is to further minimize the power consumption of line cir- cuits on line interface boards.

This is generally attained in accordance with the invention by adaptively adapting the supply voltage in response to the actual line voltages.

Hereby, it will be possible to further reduce the power consumption in line circuits on line interface boards.

BRIEF DESCRIPTION OF THE DRAWING The invention will be described more in detail below with reference to the appended drawing on which the single figure shows an embodiment of an arrangement according to the invention.

DESCRIPTION OF THE INVENTION The figure shows a line interface board 1 having a number of line circuits LC 1, LC2...

LCn where n can be equal to e. g. 16.

In a manner known per se, two-wire lines are connected to the line circuits LC1, LC2 ... LCn, and in the embodiment shown, the lines are supposed to be terminated by re- spective impedances LD1, LD2... LDn.

In the respective line circuit, the respective wire of the lines is driven by amplifiers LA1, LA2... LAn and LB1, LB2... LBn, respectively.

In accordance with the invention, these amplifiers are supplied in such a manner that the total power consumption of the line circuits LC1, LC2... LCn on the line interface board 1, is kept as low as possible.

In the embodiment shown, the line circuits LC1, LC2... LCn are supposed to be sup- plied either with a fixed battery voltage VBAT or a variable voltage VB of a lower absolute value than VBAT.

In this connection, it should be pointed out that also VBAT may be variable.

In the illustrated embodiment of the invention, a switch Sl, S2... Sn is provided in each line circuit LCI, LC2... LCn to switch the supply voltage between VBAT and VB. These switches may of course equally well be located external to the line circuits.

A controllable DC/DC converter 2 is provided on the line interface board 1 to generate VB from VBAT under control of signals from a processor 3.

The processor 3 is adapted to control the switches S 1, S2... Sn as will be described below.

In the illustrated embodiment of the invention, each line circuit LC1, LC2... LCn is provided with a circuit LV1, LV2... LVn for measuring the line voltage of the respec- tive line circuit LC 1, LC2... LCn. The line voltage measured by the respective circuit LV1, LV2... LVn is provided as input signals to the processor 3. It is to be understood that the circuits for measuring the line voltage equally well may be located external to the line circuits.

On the basis of measured line voltages, the processor 3 is adapted to prognosticate a number of possible values for the supply voltage VB. Also, the processor 3 is adapted to determine, from the measured line voltages as obtained from the circuits LV1, LV2 ... LVn, whether or not the respective line circuit LC1, LC2... LCn can be supplied with the respective possible value prognosticated for the supply voltage VB.

For the line circuits which can be supplied with the possible values prognosticated for the supply voltage VB, the processor 3 is adapted to calculate the power consumption of those line circuits for each of the possible values of VB.

For the line circuits which can not be supplied with the possible values prognosticated for the supply voltage VB, the processor 3 is adapted to calculate the power consump- tion of those line circuits using the fixed battery voltage VBAT.

For each of the possible values for the supply voltage VB, the processor 3 is adapted to sum the calculated power consumptions of the line circuits on the line interface board 1.

Preferably, only the power consumptions of active line circuits are calculated and summed.

To select the value of the supply voltage VB to be used, the processor 3 is adapted to select that value that gives the lowest total power consumption of the active line cir- cuits on the line interface board 1.

To bring the line circuits on the line interface board 1 to choose as supply voltage ei- ther the battery voltage VBAT or the selected value of the supply voltage VB, as gen- erated by the converter 2 in response to an input signal from the processor 3, the proc- essor 3 is adapted to control the switches S 1, S2... Sn to the position where either VBAT or the selected value of VB is supplied to the respective line circuit.

A practical example will be described below.

Suppose that on the line interface board 1, there are three active line circuits having line voltages 13,40 and 23 V, respectively. The line current is constant and supposed to be 0, 027 A.VBAT = 48 V.

In order for a line circuit to be supplied by VB, it is supposed that VB S : measured line voltage + 6 V.

VB is fixed to 28 V Before the line interface board has been installed, the line voltages for all line interface boards has been investigated. It is supposed that the optimal value of VB has been found to be 28 V. Consequently, VB is set to 28 V on all line interface boards.

One of the active line circuits can be supplied with VB = 28 V, while the other two line circuits must be supplied with VBAT = 48 V.

Thus, the power consumption in the three active line circuits will be equal to (28-13) x 0,027 + (48-40) x 0,027 + (48-23) x 0,027 = 1,296 W.

VB is adaptive Three possible values for VB is prognosticated by adding 6V to the measured line voltages, i. e. 13,40, and 23 V, respectively.

Thus, VB is prognosticated to the three following possible values: <BR> <BR> VB1=19V13+6V<BR> <BR> VB2-29V23+6V13+6V<BR> <BR> VB3 =46V2 40+6V223 +6V2 13 +6V.

For VB 1 = 19 V, only one line circuit can be supplied with this supply voltage, while the other two line circuits have to be supplied with VBAT = 48 V.

Thus, the power consumption will be equal to (19-13) x 0,027 + (48-40) x 0,027 + (48- 23) x 0,027 = 1,053 W on the line interface board.

For VB2 = 29 V, two of the line circuits can be supplied with this supply voltage, while the third line circuit have to be supplied with VBAT = 48 V.

Thus, the power consumption on the line interface board will be equal to (29-13) x 0,027 + (48-40) x 0,027 + (29-23) x 0,027 = 0,81 W.

For VB3 = 46 V, all three active line circuits can be supplied with that supply voltage.

In this case the power consumption of the three active line circuits will be equal to (46- 13) x 0,027 + (46-40) x 0,027 + (46-23) x 0,027 = 1,674 W.

As apparent from the above, both VB 1 = 19 V and VB2 = 29 V result in a lower power consumption in comparison with having VB constantly fixed to 28 V.

Since VB2 = 29 V gives the lowest power consumption, in this example, the processor 3 is adapted to select that voltage as the voltage VB to be generated by the converter 2.

Then, the processor 3 controls the switches Sl, S2... Sn to the position where either VBAT or VB is supplied to the respective line circuit LC 1, LC2... LCn.