Current sense amplifier

Abstract

Easily set the offset voltage of the post-amplifier. [Solution] The system includes a preamplifier 10 that obtains a preamplifier positive output vc and a preamplifier negative output vb according to the difference between the upper and lower voltages of a current sensing resistor R0, and a postamplifier 12 that has a positive input terminal to which the preamplifier positive output is input and a negative input terminal to which the preamplifier negative output is input, and obtains a postamplifier output vout. The postamplifier 12 is an input path to the positive input terminal. to The positive input resistor to be placed R4b And the negative input resistor where the input path to the negative input terminal is located. R4a And, post-amp output end The system includes a feedback resistor R5 positioned in the return path from the negative input terminal to the negative input terminal, and a resistor network connecting the positive input terminal to a reference power supply and ground, which supplies an offset voltage obtained by resistance division of the voltage of the reference power supply to the positive input terminal, and whose combined resistance value corresponds to the resistance value of the feedback resistor.

Description

Technical Field

[0001] The present disclosure relates to a current sense amplifier that detects a current flowing through a current detection resistor.

Background Art

[0002] In a drive circuit that drives a load such as a motor, a drive transistor that controls the drive current is used. In order to control the drive current of the motor, it is necessary to detect the current flowing through the drive transistor, and a current sense amplifier is used to detect the drive current of the motor.

[0003] Here, as the drive transistor, a power MOSFET is often used. However, since this power MOSFET conducts a large current, it is relatively large and is often used externally attached to a semiconductor substrate constituting the drive circuit.

[0004] In such a case, a current detection resistor is connected in series with the power MOSFET outside the semiconductor substrate, and the drive current of the motor can be detected by detecting the voltage drop across the current detection resistor with a current sense amplifier in the semiconductor substrate.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] In such a current sense amplifier, a single-ended output at a desired offset voltage is desired, and at that time, it is desired that noise and the like are as little as possible.

Means for Solving the Problems

[0007] A current sense amplifier according to one embodiment of the present disclosure includes a preamplifier having a preamplifier positive input terminal to which the upper voltage of a current sensing resistor is input, and a preamplifier negative input terminal to which the lower voltage of the sensing resistor is input, and which obtains a preamplifier positive output at the positive output terminal and a preamplifier negative output at the negative output terminal according to the difference between the upper voltage and the lower voltage; a postamplifier having a postamplifier negative input terminal to which the preamplifier negative output is input, and a postamplifier having a postamplifier positive input terminal to which the preamplifier positive output is input, and which obtains a postamplifier output according to the difference between the preamplifier positive output and the preamplifier negative output, wherein the postamplifier has the preamplifier negative output input to the negative input terminal of a second operational amplifier, and the preamplifier positive output The system includes: a second operational amplifier to which a force is input and which obtains a second operational amplifier output based on the difference between these forces; a second operational amplifier positive input resistor arranged in the input path to the second operational amplifier positive input terminal; a second operational amplifier negative input resistor arranged in the input path to the second operational amplifier negative input terminal; a second operational amplifier feedback resistor arranged in the feedback path from the second operational amplifier output to the second operational amplifier negative input terminal; and a resistor network connecting the second operational amplifier positive input terminal to a reference power supply and ground, wherein the combined resistance value from the second operational amplifier positive input terminal to the reference power supply and ground corresponds to the resistance value of the second operational amplifier feedback resistor.

[0008] The resistor network includes a setting resistor, one end of which is connected to the positive input terminal of the second operational amplifier, and a divider resistor, which obtains a voltage divided at the midpoint of a plurality of series-connected resistors placed between a reference power supply and ground, wherein the other end of the setting resistor is preferably connected to the midpoint.

[0009] The system includes an insertion resistor inserted between the other end of the setting resistor and the midpoint, a parallel resistor with one end connected to ground, and a switch that selectively connects either the connection point between the setting resistor and the insertion resistor or the other end of the parallel resistor to the midpoint, wherein the switch can change the voltage supplied to the other end of the setting resistor while maintaining the resistance value from the other end of the setting resistor to the reference power supply and ground.

[0010] The second operational amplifier feedback resistor and the setting resistor can have their resistance values ​​changed, and when the resistance value of the second operational amplifier feedback resistor is changed, the resistance value of the setting resistor is changed so that the combined resistance value of the resistor network is maintained to be the same as the resistance value of the second operational amplifier feedback resistor. Furthermore, the current sense amplifier according to this disclosure is a current sense amplifier that receives a pair of input signals corresponding to the voltage drop across a current sensing resistor and obtains an amplifier output, and includes a positive input terminal into which the positive side signals of the pair of input signals are input, a negative input terminal into which the negative side signals of the pair of input signals are input, an output terminal that outputs an amplifier output corresponding to the difference between the positive side signals and the negative side signals, a positive side input resistor arranged in the input path to the positive input terminal, a negative side input resistor arranged in the input path to the negative input terminal, a feedback resistor arranged in the return path from the output terminal to the negative input terminal of the second operational amplifier, and a resistor network connecting the positive input terminal to a reference power supply and ground, wherein the combined resistance value from the positive input terminal to the reference power supply and ground corresponds to the resistance value of the second operational amplifier feedback resistor. [Effects of the Invention]

[0011] According to the post-amplifier described herein, the output offset voltage can be set by a resistor network, eliminating the need to use components that cause noise interference, such as buffer amplifiers.

[0012] Furthermore, by making the resistance values ​​of the resistor network changeable via a switch, the offset voltage of the post-amplifier can be adjusted without changing the gain. [Brief explanation of the drawing]

[0013] [Figure 1] This is a circuit diagram showing the configuration of a current sense amplifier according to an embodiment. [Figure 2] This figure shows the common-mode operation of the operational amplifier opa1 in the preamplifier 10. [Figure 3]This diagram shows the detailed configuration of the op-amp opa1. [Figure 4] This figure shows the configuration when p-channel transistors Mp3 and Mp4 are used instead of n-channel transistors Mn1 and Mn2 in the configuration of Figure 3. [Modes for carrying out the invention]

[0014] The embodiments of this disclosure will be described below with reference to the drawings. The embodiments described below are not limiting to this disclosure, and configurations formed by selectively combining multiple examples are also included in this disclosure.

[0015] "Overall structure" Figure 1 is a circuit diagram showing the configuration of a current sense amplifier according to an embodiment. The current sense amplifier 100 detects the current flowing through a current sensing resistor R0 connected in series with the load. Here, the load is, for example, a motor, and the current flowing through the load, for example, the motor's drive current, is controlled by a drive transistor connected in series with the load and the current sensing resistor R0. For this drive current control, for example, PWM control is used. As the transistor, for example, a MOSFET (metal-oxide-semiconductor field-effect transistor) is used.

[0016] The current sense amplifier 100 includes two blocks: a preamplifier 10 and a postamplifier 12. In this example, the current sense amplifier 100 is formed as a semiconductor integrated circuit within a semiconductor substrate, while the current sensing resistor R0 is externally mounted to the semiconductor substrate and connected in series with the drive transistor and load outside the semiconductor substrate.

[0017] The voltages across the current sensing resistor R0 (upper voltage vip, lower voltage vin) are input to the positive input terminal and negative input terminal of the preamplifier 10, respectively. The preamplifier 10 outputs the positive output voltage vb and the negative output voltage vc, which correspond to the difference between the upper voltage vip and the lower voltage vin, respectively.

[0018] The upper voltage vip is input to the negative input terminal (-) of the operational amplifier opa1 through the negative input resistor R1a. The lower voltage vin is input to the positive input terminal (+) of the operational amplifier opa1 through the positive input resistor R1b. The operational amplifier opa1 is referred to as the first operational amplifier.

[0019] The positive output terminal and the negative input terminal of the operational amplifier opa1 are connected by the negative feedback resistor R3a, and the negative output terminal and the positive input terminal are connected by the negative feedback resistor R3b. A pair of outputs of the operational amplifier opa1 directly become the outputs of the preamplifier 10. Let the voltage of the preamplifier negative output vb be vb, and the voltage of the preamplifier positive output vc be vc. The preamplifier negative output vb is input to the postamplifier negative input terminal of the postamplifier 12, and the preamplifier positive output vc is input to the postamplifier positive input terminal of the postamplifier 12, respectively.

[0020] In this embodiment, the negative input terminal of the operational amplifier opa1 is connected to the reference power supply vref that outputs the reference voltage vref through the first input adjustment resistor R2a, and the positive input terminal is connected through the second input adjustment resistor R2b. Note that the power supply that supplies the reference voltage vref to the operational amplifier opa1 is called a constant voltage source.

[0021] In this way, the preamplifier 10 is a fully differential amplifier, and the gain is determined by the resistance values of the resistors R1a, R1b, R3a, and R3b described above. Also, the DC voltages of the negative input terminal and the positive input terminal of the operational amplifier opa1 are set by the resistance values of the input adjustment resistors R2a and R2b.

[0022] The postamplifier 12 has an operational amplifier opa2 inside. A pair of outputs of the preamplifier 10, namely the preamplifier negative output vb and the preamplifier positive output vc, are respectively input to a pair of inputs of the operational amplifier opa2. That is, the preamplifier outputs vb and vc are input to the negative input terminal (-) and the positive input terminal (+) of the operational amplifier opa2 through the negative-side input resistor R4a and the positive-side input resistor R4b, respectively. The operational amplifier opa2 is referred to as the second operational amplifier.

[0023] The opa2 operational amplifier is a single-ended operational amplifier with one output, providing a single voltage output, the post-amplifier output vout. This post-amplifier output vout then becomes the output signal of the current sense amplifier 100. The opa2 operational amplifier provides a single post-amplifier output vout that is suitable for input to an ADC (analog-to-digital converter).

[0024] A feedback resistor R5 is placed in the feedback path from the output terminal to the negative input terminal of the operational amplifier opa2. In this example, the resistance value of the feedback resistor R5 is variable. Depending on the circuit specifications, a feedback resistor R5 with a different resistance value can be used.

[0025] The positive input terminal of op-amp opa2 is connected to the reference power supply vref and ground GND of the reference voltage vref via the resistor network 14. One end of the setting resistor R6 in the resistor network 14 is connected to the positive input terminal of op-amp opa2. In this example, the resistance value of the setting resistor R6 is variable. Depending on the circuit specifications, the setting resistor R6 can be set to a value corresponding to the resistance value of the feedback resistor R5.

[0026] The other end of the setting resistor R6 is supplied with a set of switchable resistors, through which multiple voltages are supplied, which are obtained by dividing the reference voltage vref.

[0027] Specifically, the other end of the setting resistor R6 is connected via the insertion resistor R7 to the midpoint of two divider resistors R8a and R8b, which are connected in series between the reference voltage vref and ground GND. In addition, the other end of a parallel resistor R9, one end of which is connected to ground GND, is connected to contact s1 of the switch SW, and the connection point of the setting resistor R6 and the insertion resistor R7 is connected to contact s0 of the switch SW. The switch SW can switch the connection point of divider resistors R8a and R8b to either contact s1 or contact s0.

[0028] When the switch SW selects contact s0, the other end of setting resistor R6 is directly connected to the connection point of divider resistors R8a and R8b, and the voltage obtained by dividing the reference voltage vref by divider resistors R8a and R8b is supplied to the other end of setting resistor R6.

[0029] When contact s1 is selected by switch SW, the other end of setting resistor R6 is connected to the connection point of divider resistors R8a and R8b via insertion resistor R7, and a parallel resistor R9 is connected in parallel to divider resistor R8b. That is, the other end of setting resistor R6 is connected to the reference power supply vref via insertion resistor R7 and divider resistor R8a, and is also connected to ground GND by the parallel connection of insertion resistor R7, divider resistor R8b and parallel resistor R9.

[0030] "Preamplifier Operation" <Adjusting the input voltage va> Figure 2 shows the common-mode operation of the operational amplifier opa1 in the preamplifier 10.

[0031] In this example, the input vicm (vicm=(vip+vin) / 2) is input to the negative input terminal of opa1, and a single output vocm (vocm=(vb+vc) / 2) is obtained. The resistances of each path are assumed to be R1=R1a=R1b, ​​R2=R2a=R2b, and R3=R3a=R3b. This allows us to simulate the operation of opa1 when there is no voltage between the positive and negative input terminals.

[0032] Without the input adjustment resistor R2, the input voltage va of the op-amp opa1 should be the voltage between vicm and vocm. If we set the resistor R3 = 2 * R1 and the voltages vicm = -2V and vocm = 2V, then the input voltage va = -0.67V < 0V, as shown below.

[0033] va=vicm*R3 / (R1+R3)+vocm*R1 / (R1+R3) =(-2V)*2 / 3+2*1 / 3=-0.67V<0V

[0034] Furthermore, if we set the input adjustment resistor R2 to R2 = 2 * R1 and the reference voltage to vref = 3V, then the input voltage va = 0.25V > 0V, as shown below. va=(-2 / 3)V*2 / (2 / 3+2)+3V*2 / 3 / (2 / 3+2) =0.25V>0V

[0035] In this way, by adjusting the resistance value of the input adjustment resistor R2, the input voltage va of the operational amplifier opa1 can be adjusted to the desired range.

[0036] Therefore, in the configuration shown in Figure 1, even if the lower voltage vin of the current sensing resistor R0 is a negative voltage, the input voltage va of the operational amplifier opa1 can be made positive by adjusting the resistance values ​​of the input adjustment resistors R2a and R2b.

[0037] Common Mode Feedback Figure 3 shows the detailed configuration of the operational amplifier opa1. As shown, the upper voltage vip and the lower voltage vin are input to the positive and negative input terminals of the input gain stage gm, respectively. The input gain stage gm outputs a positive current output iop and a negative current output ion to the two inputs according to the difference.

[0038] The positive current output iop is connected to the drain of p-channel transistor Mp1. The source of p-channel transistor Mp1 is connected to the positive power supply VDD of the power supply voltage VDD. The negative current output ion is connected to the drain of p-channel transistor Mp2. The source of p-channel transistor Mp2 is connected to the positive power supply VDD.

[0039] The drain of the p-channel transistor Mp1 is connected to the gate of the n-channel transistor Mn1. The drain of the n-channel transistor Mn1 is connected to the power supply vdd, and its source is connected to a current source ib1 that carries current ib1, as well as to the positive output vop that outputs the preamplifier negative output vb.

[0040] The drain of the p-channel transistor Mp2 is connected to the gate of the n-channel transistor Mn2. The drain of the n-channel transistor Mn2 is connected to the power supply vdd, and its source is connected to the current source ib2, which carries current ib2, and also to the negative output von, which outputs the preamplifier positive output vc.

[0041] The gates of the p-channel transistors Mp1 and Mp2 are connected in common, to which the positive output vop is connected via pull-up resistor Rb1, and the negative output von is connected via pull-up resistor Rb2.

[0042] Therefore, a voltage corresponding to the positive current output iop of the input gain stage gm is output from the source of the n-channel transistor Mn1 to the positive output vop. Also, a voltage corresponding to the negative current output ion of the input gain stage gm is output from the source of the n-channel transistor Mn2 to the negative output von.

[0043] Here, the gate voltage of p-channel transistors Mp1 and Mp2 when they are ON is the source voltage minus the gate-source voltage vgson of p-channel transistors Mp1 and Mp2 when they are ON, i.e., vdd-vgson. Since the gates of p-channel transistors Mp1 and Mp2 are connected to the positive output vop and the negative output von by pull-up resistors Rb1 and Rb2, the output common voltage is (vop+von) / 2 = vdd-vgson.

[0044] Thus, the output common voltage is close to the power supply voltage vdd and is determined in accordance with the power supply voltage vdd. Therefore, the adjustment range can be made relatively large, making it easy to correct the input common voltage to the op-amp opa1 using input adjustment resistors R2a and R2b, and allowing the input common voltage of the op-amp opa1 to be set relatively high.

[0045] In particular, this example employs native-type or depletion-type transistors for the n-channel transistors Mn1 and Mn2. Because native-type or depletion-type transistors have a small on-time gate-source voltage vgson, the drain-source voltages of the p-channel transistors Mp1 and Mp2 can be set to values ​​necessary and sufficient for operation.

[0046] Figure 4 shows the configuration when p-channel transistors Mp3 and Mp4 are used instead of n-channel transistors Mn1 and Mn2 in the configuration of Figure 3.

[0047] In this configuration, a current source ib1 is placed between the source of p-channel transistor Mp3 and the power supply vdd, and the drain of p-channel transistor Mp3 is connected to ground GND. A current source ib2 is also placed between the source of p-channel transistor Mp4 and the power supply vdd, and the drain of p-channel transistor Mp4 is connected to ground GND. The source of p-channel transistor Mp3 is connected to the positive output vop, and the source of p-channel transistor Mp4 is connected to the negative output von. In this case, p-channel transistors Mp3 and Mp4 operate according to the output of the input gain stage gm, producing the positive output vop and the negative output von. The common output voltage of the positive output vop and the negative output von is (vop + von) / 2 = vdd - vgson. Therefore, a relatively high input common voltage can be set in the operational amplifier opa1. The potential of the output of the input gain stage gm is the power supply voltage vdd - 2vgson, allowing the drain-source voltage of p-channel transistors Mp1 and Mp2 to be set to a value necessary and sufficient for operation.

[0048] "Post-amp operation" Returning to Figure 1, let's explain the operation of the post-amplifier 12. The post-amplifier 12 is a single-ended amplifier that obtains a single output voltage vout depending on the difference between the input voltages vb and vc. The amplification factor and offset voltage are set by the resistance values ​​of several resistors surrounding the operational amplifier opa2.

[0049] First, to achieve typical operation, we assume that the resistors are R8=R8a=R8b and R4=R4a=R4b.

[0050] The gain of post-amplifier 12 is R5 / R4, and in this case, the combined resistance between the positive input terminal of op-amp opa2 and the power supply and ground at the other end of the resistor network should be equal to R5. Note that this combined resistance is calculated assuming that the reference power supply vref and ground GND are shorted. The reason is that the reference power supply vref is assumed to be connected to an ideal power supply with a 0-ohm resistor, and when considering resistance values, the ideal power supply is calculated as 0V.

[0051] Therefore, if s0 is selected in switch SW, R5 = R6 + R8 / 2 This is the result.

[0052] In this case, the offset voltage vd of the output of op-amp opa2 relative to ground GND is: vd = vref * R8 / (R8 + R8) = vref / 2 This is the result.

[0053] On the other hand, if s1 is selected in switch SW, R5 = R6 + R7 + (R8 / 2) / / R9 That is the case. The mark " / / " indicates a parallel connection.

[0054] Here, the conditions under which s0 is selected in switch SW must be maintained. R8 / 2 = R7 + (R8 / 2) / / R9 That is the case. The resistance values ​​of the parallel connection ( / / ) of R8 / 2 and R9 are: (R8 / 2) / / R9=(R8 / 2*R9) / [(R8 / 2)+R9)] Therefore, R7 = R8 * R8 / [2 * (R8 + 2 * R9)] This is the result.

[0055] If you want to set the offset voltage vd = vref / 8 when s1 is selected in switch SW, (R9 / / R8) / (R9 / / R8+R8)=1 / 8 And, R9 = R8 / 6, R7=R8*R8 / [2*(R8+2*R9)]=(3 / 8)*R8 You can set the resistance values ​​of resistors R9 and R7 as shown above.

[0056] Thus, according to this embodiment, the offset voltage of the post-amplifier 12 can be set to two appropriate values ​​(vref / 2, vref / 8) by switching the switch SW.

[0057] In particular, the offset voltage can be set by adjusting the resistance values ​​of resistors R7, R8a, R8b, and R9, independently of resistors R5 and R6, i.e., separate from the gain setting. Furthermore, the gain of op-amp op2 can be set by changing the resistance values ​​of resistors R5 and R6. In Figure 1, resistors R5 and R6 are represented as variable resistors, illustrating that the gain can be changed.

[0058] When the gain of post-amplifier 12 varies from 5 to 40 times, R5 = 5*R4 to 40*R4, and R6 + R8 / 2 = 5*R4 to 40*R4. When R6 = 0, R8 / 2 = 5*R4, and R8 = 10*R4. In other words, the resistance value of resistor R8 can be increased up to 10*R4, providing a wide range of choices for setting the offset voltage.

[0059] "Effects of the Embodiment" A post-amp resistor network with a voltage setting switch can eliminate buffer amplifiers that may introduce offset or noise.

[0060] In the preamplifier 10, the input voltage of the operational amplifier opa1 can be adjusted by setting the resistance value of the input adjustment resistor R2 (R2a, R2b). Even if the input to the current sense amplifier 100 is a negative voltage, the operational amplifier opa1 can raise its input voltage above ground GND by setting the input adjustment resistor R2.

[0061] By connecting the output stage of the preamplifier 10 to the gate of a p-channel transistor whose source is connected to the power supply, the output voltage can be made close to that of the power supply, making it easier to set the common-mode input voltage of the preamplifier 10.

[0062] According to the post-amplifier described herein, the output offset voltage can be set by a resistor network, eliminating the need to use components that cause noise interference, such as buffer amplifiers.

[0063] By making the resistance value of the resistor network connected to the positive input terminal of the post-amplifier changeable via a switch, the offset voltage of the post-amplifier can be adjusted without changing the gain. [Explanation of Symbols]

[0064] 10 preamplifiers, 12 post-amplifiers, 14 resistor networks, 100 current sense amplifiers.

Claims

1. A preamplifier having a positive input terminal to which the upper voltage of a current sensing resistor is input, and a negative input terminal to which the lower voltage of the sensing resistor is input, and obtaining a positive output at the positive output terminal and a negative output at the negative output terminal according to the difference between the upper voltage and the lower voltage, A post-amplifier has a post-amplifier negative input terminal to which the negative output of the preamplifier is input, and a post-amplifier positive input terminal to which the positive output of the preamplifier is input, and obtains a post-amplifier output corresponding to the difference between the positive output of the preamplifier and the negative output of the preamplifier. Includes, The aforementioned post-amplifier is The negative output of the preamplifier is input to the negative input terminal of the second operational amplifier, and the positive output of the preamplifier is input to the positive input terminal of the second operational amplifier, and the output of the second operational amplifier is obtained based on the difference between these two. The second operational amplifier positive input resistor is placed in the input path to the positive input terminal of the second operational amplifier, The second operational amplifier negative input resistor is placed in the input path to the negative input terminal of the second operational amplifier, A second operational amplifier feedback resistor is placed in the feedback path from the output of the second operational amplifier to the negative input terminal of the second operational amplifier, A resistor network connecting the positive input terminal of the second operational amplifier to a reference power supply and ground, wherein the combined resistance value from the positive input terminal of the second operational amplifier to the reference power supply and ground corresponds to the resistance value of the feedback resistor of the second operational amplifier, including, Current sense amplifier.

2. A current sense amplifier according to claim 1, The resistor network includes a setting resistor, one end of which is connected to the positive input terminal of the second operational amplifier, and a divider resistor, which obtains a voltage divided at the midpoint of a plurality of series-connected resistors placed between a reference power supply and ground, the other end of which is connected to the midpoint. Current sense amplifier.

3. A current sense amplifier according to claim 2, An insertion resistor is inserted between the other end of the aforementioned setting resistor and the aforementioned midpoint, One end is connected to ground in a parallel resistor, A switch that selectively connects either the connection point between the setting resistor and the insertion resistor, or the other end of the parallel resistor, to the intermediate point. The switch includes the ability to change the voltage supplied to the other end of the setting resistor while maintaining the resistance value from the other end of the setting resistor to the reference power supply and ground. Current sense amplifier.

4. A current sense amplifier according to claim 3, The second operational amplifier feedback resistor and the setting resistor have adjustable resistance values. When the resistance value of the second operational amplifier feedback resistor is changed, the combined resistance value of the resistor network is maintained at the same value as the resistance value of the second operational amplifier feedback resistor by changing the resistance value of the setting resistor. Current sense amplifier.

5. A current sense amplifier that receives a pair of input signals corresponding to the voltage drop across a current sensing resistor and obtains an amplifier output, The positive input terminal to which the positive side signal of the pair of input signals is input, The negative input terminal to which the negative side signal of the pair of input signals is input, An output terminal that outputs an amplifier output corresponding to the difference between the positive signal and the negative signal, A positive input resistor is placed in the input path to the positive input terminal, A negative input resistor is placed in the input path to the negative input terminal, A feedback resistor is placed in the feedback path from the output terminal to the negative input terminal of the second operational amplifier, A resistor network connecting the positive input terminal to a reference power supply and ground, wherein the combined resistance value from the positive input terminal to the reference power supply and ground corresponds to the resistance value of the second operational amplifier feedback resistor, including, Current sense amplifier.

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