| 摘要 |
PURPOSE:To obtain a ground-fault detecting circuit suited for formation of the semiconductor integrated circuit, by generating the voltage proportional to the battery line current based on the reference voltage set previously and then comparing the voltage with the voltage proportional to the ground current. CONSTITUTION:When the hook switch of the telephone TEL is turned on, the loop current flows from the ground G' and via the ground line B, TEL, battery line A and battery VBB each. And if the line A has a ground-fault with the resistance RG' due to occurrence of the fault, the ground current flows in addition to the loop current. And thus the battery line current IA becomes larger than the ground line current IB. The current/voltage converting circuit consisting of the transistor TrQ0 and resistances R10 plus R11 generates the voltage proportional to the current IA via the base of the TrQ1 and based on the reference power source VREF. And the voltage proportional to the current IB is generated at the base of the TrQ2 via the resistance R10'. The TrQ1 and Q2 from the voltage comparator. And when the current IA becomes larger than the current IB by the source VREF and by an amount equivalent to the current value set previously, the TrQ1 is turned on to give an inversion to the output of the voltage comparator. Thus the ground-fault detection is carried out. |
