Control circuit for controlling a DC supply voltage for a terminal

A control device for controlling a supply voltage which can be switched between a constant line current operation and a constant line voltage operation, wherein a line current, which consists of a direct current for supplying the terminal and of an alternating current for transmitting information, flows via a subscriber line of a terminal, comprising:

• (a) a sensing circuit configured to sense an analogue input voltage (VSENSE) dependent on the line current;
• (b) a subtractor which, in the constant line current operation, is configured to subtract an analogue feedback voltage dependent on an adjustable nominal direct-current value from the sensed input voltage for generating an analogue difference voltage;
• (c) an analogue/digital converter configured to convert the generated analogue difference voltage into a digital difference voltage value;
• (d) a digital low-pass filter configured to filter a sequence of control error values out of the generated sequence of difference voltage values;
• (e) a controller configured to generate a control value in dependence on the filtered-out sequence of control error values;
• and
• (f) a first digital/analogue converter which, in the constant line current operation, is configured to convert the control value in each case generated by the controller into the analogue direct voltage for supplying the terminal.