Patent Publication Number: US-RE36179-E

Title: Switching circuit for selecting an output signal from plural input signals

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a switching circuit of the monolithic IC type. 
     FIG. 2 shows a circuit diagram of the conventional switching circuit. Input terminals 1 and 2 receive different voltages V1 and V2, respectively. Either of the received voltages is outputted from an output terminal 5 through corresponding switching transistors 3 and 4. The voltages V1 and V2 are inputted into a comparator 6. The comparator 6 produces an output which is concurrently applied to a gate of the switching transistor 4 and to a gate of the other switching transistor 3 through an inverter 7. 
     In a condition where V1&gt;V2, the comparator 6 produces an output having a high level so that the switching transistor 4 is turned off or placed in a non-conductive switching state. On the other hand, the output of the inverter 7 is held at a low level so that the other switching transistor 3 is turned on or placed on a conductive switching state. Consequently in this condition, the output terminal 5 provides the voltage V1. In a condition where V1&lt;V2, the switching states of the switching transistors 3 and 4 are reversed so that the output terminal 5 provides the voltage V2. The comparator 6 and the inverter 7 are powered from the output terminal 5. 
     However, in this circuit construction, it is impossible to turn off both of the switching transistors so as to place the output terminal 5 in a floating state. The reason is that the switching transistor of the MOS type is structurally accompanied with a parasitic diode having an anode coupled to either of the input terminals 1 and 2 and a cathode coupled to the output terminal 5. Therefore, the output terminal 5 receives through the parasitic diode a voltage equal to the higher one of the input voltages minus the voltage drop across the parasitic diode. This voltage is effective to operate the comparator 6 and the inverter 7 to enable the same to turn on and off the switching transistors 3 and 4 to thereby constitute a current path having a voltage drop smaller than the voltage drop across the parasitic diode. 
     For the above described reason, the output terminal 5 necessarily provides the higher one of the voltages V1 and V2, thereby causing a problem that the floating state cannot be realized. 
     SUMMARY OF THE INVENTION 
     In order to solve the above noted problem of the prior art, an object of the present invention is to provide an improved switching circuit responsive to an external signal to place its output terminal in a floating state. 
     In realization of the aforesaid object, a pair of the switching transistors are integrated in the form of MOS transistors on a substrate, and their source region and drain region are electrically separated from the substrate. Further, a diode is formed between each of the input terminals and the substrate. Moreover, controlling circuits such as a comparator are powered through the substrate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of the inventive switching circuit; and 
     FIG. 2 is a circuit diagram of the conventional switching circuit. 
    
    
     DETAILED DESCRIPTION OF EMBODIMENT 
     Hereinafter, an embodiment of the invention is described in detail with reference to the drawings. FIG. 1 shows a circuit diagram of the monolithic switching circuit according to the invention. A pair of input terminals 1 and 2 are connected to an output terminal 5 through switching transistors 3 and 4, respectively. The input terminals 1 and 2 are connected also to a comparator 6 in manner similar to the conventional circuit. According to the invention, advantageously both the source and drain regions of the switching transistors 3 and 4 are electrically separated from a substrate on which the switching transistors 3 and 4 are formed. Further, the substrate is connected to a diode 8 having an anode coupled to the input terminal 1, and the substrate is also connected to another diode 9 having an anode coupled to the input terminal 2. Moreover, the substrate is also connected to a power supply line which is connected to .Iadd.a power source terminal 12 connected to .Iaddend.the comparator 6 and a gate circuit 10. The gate circuit 10 has one input terminal connected to a control terminal 11 receptive of a control signal and another input terminal connected to an output terminal of the comparator 6. The comparator 6, gate circuit 10 and control terminal 11 comprise a control circuit which, as described below, controls the switching states of the switching transistors 3, 4. 
     In operation, when voltages V1 and V2 are applied to the input terminals 1 and 2, respectively, the comparator 6 and the gate circuit 10 are placed in the operative state since electric power is supplied thereto through the diodes 8 and 9, respectively. At this time, the supplied electric power source voltage .Iadd.applied to the power source terminal 12 .Iaddend.is equal to the higher one of the voltages V1 and V2 minus a voltage drop developed across the corresponding diode. Then, the comparator 6 and gate circuit 10 operate according to the relative magnitude difference between the pair of voltages V1 and V2 to selectively turn on and off the switching transistors 3 and 4. In such operation, the control terminal 11 must be held at the high level. 
     On the other hand, when the control terminal 11 is switched to the low level, the output of the gate circuit 10 is turned to the high level so that the switching transistor 3 is turned off. If the condition V1&gt;V2 is held at this time, the other switching transistor 4 is also turned off to thereby place the output terminal 5 in the floating state. 
     The inventive switching circuit can be constructed in the form of a monolithic IC integrated with all of the components including a comparator, a gate circuit and diodes, in the form of MOS transistors. Further, the diode is preferable composed of a Shottky barrier diode having a relatively small voltage drop in the forward direction in order to improve performance of the switching circuit. 
     As described above, according to the present invention, a diode is formed between each input terminal and a substrate in which the switching transistor is integrated in the form of a MOS transistor, and control circuits such as a comparator are powered through the substrate. By such a construction, the pair of switching transistors can be concurrently turned off in response to an external control signal so as to place the output terminal in the floating state in the switching circuit.