| 摘要 |
<p>A telephone subscriber loop interface circuit ensures that the loop current, even when a short length of cable is used, does not exceed about 50mA. A transformer (5) has a winding connected to the telephone line (2) and a split winding (6) connected to TIP and RING sides of the subscriber loop in the conventional manner. One half (6a) of the split winding is, as is conventional, connected to ground through a first resistor (8) but the second half of the split winding is connected through a second resistor (11) and a current limit circuit to a negative battery terminal. The current limit circuit is formed of a low ON resistance power MOS FET (12) and a control circuit which has a current sensing resistor (13) in series with the MOS FET (12). When no loop current is flowing the total resistance of the second resistor (11), current sensing resistor (13) and source to drain resistance of the MOS FET (12) equals the resistance of the first resistor. However, when loop current flows, the control circuit controls the gate of the MOS FET (12) thereby increasing the source to drain resistance and maintaining the current no greater than a predetermined value.</p> |
