Patent ID: 12228957

PREFERRED EMBODIMENT OF THE PRESENT INVENTION

Hereinafter, embodiments of the disclosure are described with reference to the drawings. Note that, the following embodiments do not limit the scope of the disclosure, and configurations obtained by selectively combining multiple examples are also included in the disclosure.

“Overall Configuration”

FIG.1is a diagram showing a circuit of an output detection circuit according to an embodiment. Here, in the circuit according to the embodiment, a metal-oxide-semiconductor field-effect transistor (MOSFET) is used as a transistor.

In an N-type transistor M1, the drain is connected to an input end IN, the source is connected to ground gnd, and a signal LS_gate is supplied to the gate. A load to be driven is connected to the input end IN, and a current iin flowing through the load is supplied to the input end IN. Therefore, the current iin flows from the input end IN toward the ground gnd via the transistor M1. The transistor M1is referred to as the output FET.

The current iin is an output current to be detected, and changes from a positive-direction current to a negative-direction current. In addition, a resistance when the transistor M1is ON, that is, an ON resistance, is represented by Ron. Therefore, a voltage vin of the input end IN is expressed by vin=Ron*in.

The gate of an N-type transistor M2is commonly connected to the gate of the transistor M1. The drain of the transistor M2is connected to the input end IN. A current source gm3is connected to the source of the transistor M2. The current source gm3makes a current flow toward the input end IN. Therefore, the transistor M2makes the current flow from the source thereof toward the drain. The current source gm3can be constituted by a transistor through which a current from a power source is made to flow. The transistor M2makes a mirror current corresponding to a negative-direction current of the transistor M1flow, and thus the transistor M2is referred to as the negative mirror FET.

A connection point between the source of the transistor M2and the current source gm3is connected to a positive input end of an operational amplifier op2. That is, an upstream side voltage of the transistor M2is supplied to the positive input end of the operational amplifier op2. A negative input end of the operational amplifier op2is connected to the ground. That is, a source side voltage of the transistor M1is supplied to the negative input end of the operational amplifier op2. An output end of the operational amplifier op2is connected to a control end of the current source gm3. An output voltage of the operational amplifier op2is converted into a current by the current source gm3. The operational amplifier op2is referred to as the second operational amplifier.

The operational amplifier op2operates in a manner that the source of the transistor M2becomes the ground gnd. Therefore, the transistor M2makes a current having the same current density as the transistor M1(a current corresponding to an area ratio with the transistor M1) flow. When an ON resistance of the transistor M2is set to r1and the current flowing through the transistor M2is set to ino, the voltage vin of the input end IN is expressed by vin=r1*ino.

The gate of an N-type transistor M3is also commonly connected to the gate of the transistor M1. A current source gm1is connected to the drain of the transistor M3. The source of the transistor M3is connected to the ground gnd. The current source gm1makes a current flow toward the ground gnd. Therefore, the transistor M3makes the current flow from the drain toward the source. The current source gm1can be constituted by a transistor through which a current from the power source is made to flow. The transistor M3makes a mirror current corresponding to a positive-direction current of the transistor M1flow, and thus the transistor M3is referred to as the positive mirror FET.

A connection point between the drain of the transistor M3and the current source gm1is connected to a positive input end of an operational amplifier opt. That is, an upstream side voltage of the transistor M3is supplied to the positive input end of the operational amplifier op1. A negative input end of the operational amplifier op1is connected to the input end IN via an N-type transistor M4. That is, a drain side voltage of the transistor Mt is supplied to the negative input end of the operational amplifier op1. In addition, the gate of the transistor M4is commonly connected to the gates of the transistors M1, M2, and M3. The operational amplifier op1is referred to as the first operational amplifier.

Besides, an output end of the operational amplifier opt is connected to a control end of the current source gm1.

Here, the current source gm1makes the current flow toward the ground gnd, and in a case that the current in at the input end IN flows out from the input end IN toward the load side, that is, in a negative direction, no current flows through the transistor M3. The gate of the transistor M4is connected to the LS_gate similarly to the gate of the transistor M1, and is ON when the transistor Mt is ON and is OFF when the transistor Mt is OFF. The transistor M4is not necessary when the operational amplifier op1is an ideal circuit. However, when the transistor M1is OFF, the voltage vin may be at a high potential, and in this case, the transistor M4is turned off, thereby preventing the input of the operational amplifier op1from reaching a high potential, and preventing the operational amplifier op1from being destroyed.

In a case that the current in at the input end IN flows into the input end IN toward the load side, that is, in a positive direction, the operational amplifier opt operates in a manner that the positive input end is at the same voltage as the input end IN, and a current having the same current density as the transistor M1(a current corresponding to an area ratio with the transistor M1) flows through the transistor M3.

When an ON resistance of the transistor M3is set to r1and the current flowing through the transistor M3is set to ipo, the voltage at the positive input end of the operational amplifier opt is expressed by r1*ipo, and the operational amplifier opt operates in a manner that the voltage vin of the input end IN becomes r1*ipo.

In this way, a current that is proportional to the negative current in flowing through the transistor M1flows through the transistor M2, and a current that is proportional to the positive current in flowing through the transistor M1flows through the transistor M3.

The control end of the current source gm3and a control end of a current source gm4are connected to the output end of the operational amplifier op2. The current source gm4makes a current which is the same as that of the current source gm3but in an opposite direction flow. For example, the current source gm4may be formed by the same transistor as that of the current source gm3, the same current as that of the current source gm3may be generated in the current source gm4, and an inversion circuit may be used for the generated current to reverse a current direction. As the inversion circuit, various types of circuits can be used.

The control end of the current source gm1and a control end of a current source gm2are connected to the output end of the operational amplifier op1. The current source gm2makes the same current as that of the current source gm1flow. For example, the current source gm1and the current source gm2can be constituted by a current mirror. The output voltage of the operational amplifier op2is converted into a current by the current sources gm1and gm2.

A connection point between the current source gm2and the current source gm4is a current output end iout. As described above, the current source gm2makes the positive output current ipo flow, and the current source gm4makes the negative output current ino flow, and thus the current ipo flows out of the current output end iout and the current ino is drawn in from the current output end.

A negative input end of an operational amplifier op3is connected to the current output end iout. A reference voltage vref0is supplied to a positive input end of the operational amplifier op3, an output end of the operational amplifier op3is connected to an output end sout, and feedback is performed to the negative input end via a resistor R1. Therefore, the output end sout is at a voltage that fluctuates within voltages of io*R1by using the reference voltage vref0as a reference. Therefore, an output proportional to the current iin in both positive and negative directions is obtained as a detection value. The operational amplifier op3is referred to as the third operational amplifier.

FIG.2is a diagram showing an output current io from the current output end Iout. In this way, the current ipo is 0 when the current iin is negative, and increases according to the current iin when the current iin is positive. Regarding the current ino, the negative current negatively increases when the current iin is negative, and the current ino becomes 0 when the current iin is positive. Therefore, regarding the output current io, the current ino is output when the current iin is negative and the current ipo is output when the current iin is positive. Therefore, the output current io changes according to the current iin regardless of whether the current iin is positive or negative.

“Variation Configuration”

FIG.3is a circuit diagram showing a configuration of a variation example. The operational amplifier op3inFIG.1is omitted, the output end sout is directly connected to the current output end iout, and is connected, via the resistor R1, to a reference voltage source outputting the reference voltage vref0. By setting the resistor R1to have a relatively large resistance, the current flowing through the resistor R1can be reduced, and the center voltage of the output end sout can be set to the reference voltage vref0. That is, the output end sout outputs the current io with the reference voltage vref0as a center.

FIG.4shows a configuration in which the linearity near a connection point between currents ipo and ino is improved.FIG.5is a diagram showing the state of currents ipo and ino near the 0 point.

In the foregoing example, the current source gm2is constituted by a transistor Mp3, and the current source gm4is constituted by a transistor Mn4. In this case, in the diagram, a leakage current as represented by IMp3and IMn4is generated in the transistors Mp3and Mn4. That is, the transistor cannot be completely turned off, and some amount of current flows even when the transistor is turned off.

In the example shown inFIG.4, a transistor Mp4is arranged between the transistor Mp3and the current output end iout, and a transistor Mn3is arranged between the transistor Mn4and the current output end iout.

In addition, a help circuit HP is interposed between the operational amplifier opt, the operational amplifier op2, and the current sources gm2and gm4.

The gate of a P-type transistor Mp1is connected to the gate of the P-type transistor Mp3. The source of the transistor Mp1may be connected to the same power source (at a high voltage) as the transistor Mp3. The drain of the transistor Mp1is connected to the source of the P-type transistor Mp2, and the drain of the transistor Mp2is connected to a connection point between the transistor Mn3and the transistor Mn4.

The gate of an N-type transistor Mn2is connected to the gate of the N-type transistor Mn4. The source of the transistor Mn2may be connected to the same power source (at a low voltage) as the transistor Mn4. The drain of the transistor Mn2is connected to the source of the transistor Mn1, and the drain of the transistor Mn2is connected to a connection point between the transistor Mp3and the transistor Mp4.

In this configuration, when the current in is positive, the amount of leakage current of the transistor Mn4is supplied via the transistors Mp1and Mp2. On the other hand, when the current in is negative, the leakage current of the transistor Mp3is drawn out via the transistors Mn1and Mn2.

Moreover, the gates of the transistors Mp2and Mp4are connected to a power source vbp, and the transistors Mp2and Mp4are turned on when the current in is positive. The gates of the transistors Mn1and Mn3are connected to a power source vbn, and are turned on when the current in is negative.

By the circuit shown inFIG.4, the leakage current of the transistors Mp3and Mn4is compensated, and then the linearity of the output current can be improved.