Current sense amplifier

Abstract

To improve the temperature dependence in a current sense amplifier.SOLUTION: A current sense amplifier measures a voltage drop in a drive transistor LS so as to measure the current flowing through the drive transistor LS. The current sense amplifier includes: a preamplifier pre-amp, to which a voltage across the drive transistor LS is inputted, and which obtains a positive output and a negative output that correspond to a voltage difference of the inputted voltage across the drive transistor; and a switch sw, which connects the input end of a common mode voltage vcm and the negative output, the common mode voltage vcm serving as the operation reference of the preamplifier pre-amp. A change of the positive output caused by turning on / off the switch sw can be detected.SELECTED DRAWING: Figure 1

Description

Technical Field

[0006] ,

[0001] The present disclosure relates to detecting the current flowing through a driving transistor. do It relates to a current sense amplifier.

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. Further, in order to control the drive current, it is necessary to detect the current flowing through the drive transistor, and a current sense amplifier is used to detect the current flowing through the drive transistor. In order to achieve excellent motor control, the current sense amplifier requires high speed, high precision, and low temperature drift. <​​​​​​​​​​​​​​​​​​​​​​​​​​​​​

[0007] When offset trimming is performed correctly and the input offset is zero, the input bias current flows evenly, so no output current from the input stage flows to the current-voltage converter in the subsequent circuit. On the other hand, if there is an offset, the error current remaining after adjusting the center voltage to the target value can cause temperature drift. [Means for solving the problem]

[0008] The current sense amplifier according to this disclosure is a current sense amplifier that measures the voltage drop in a drive transistor and measures the current flowing through the drive transistor, wherein the drive transistor The device includes an attenuator connected to the positive and negative terminals, which obtains positive and negative outputs corresponding to the voltage drop in the drive transistor; a first feedback resistor connecting the positive output terminal and the negative input terminal; and a second feedback resistor connecting the negative output terminal and the positive input terminal, wherein the positive output of the attenuator is input to the negative input terminal, and the negative output of the attenuator is input to the positive input terminal. The input and input The difference between the positive and negative outputs of the attenuator. in accordance with The positive output is connected to the negative output terminal, and the negative output is connected to the positive output terminal. A preamplifier, an input terminal of a common-mode voltage that serves as the operating reference for the preamplifier, and Positive output end A switch to connect them, A post-amplifier receives the positive and negative outputs of the preamplifier and obtains an output corresponding to the difference between the positive and negative outputs of the preamplifier. Includes, The attenuator consists of the on-resistance of a transistor, and the first and second feedback resistors consist of standard resistors. ru. [Effects of the Invention]

[0010] The preamplifier's offset can be detected by the change in output when the switch is turned on or off. Therefore, the offset voltage can be reduced by trimming the preamplifier. This reduces the temperature dependence of the current sense amplifier. [Brief explanation of the drawing]

[0011] [Figure 1] This is a circuit diagram showing the configuration of a current sense amplifier according to an embodiment. [Figure 2] This figure shows examples of set resistance values ​​for each resistor in the circuit shown in Figure 1. [Figure 3] This diagram shows the change in output vout when the switch sw is turned on. [Figure 4] This figure shows the temperature characteristics of vout for various trimming settings. [Figure 5] This is an example of a preamplifier configured with two single-ended operational amplifiers, OPA1-1 and OPA1-2. [Modes for carrying out the invention]

[0012] 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.

[0013] "Overall structure" Figure 1 is a circuit diagram showing the configuration of a current sense amplifier according to an embodiment.

[0014] The drive transistor LS is a transistor that supplies drive current to the load. In this example, it is composed of an n-channel MOSFET (metal-oxide-semiconductor field-effect transistor), with the drain connected to the load and the source connected to ground (gnd). Here, the current flowing through the drive transistor LS is denoted as "iin" and the on-resistance as "Rs".

[0015] The voltage sense amplifier according to this embodiment detects the drive current flowing through the drive transistor LS from the voltage drop (voltage across its terminals) when the drive transistor LS is turned on.

[0016] The drain of the drive transistor LS is connected to the sense terminal (sense), and the source is connected to the ground terminal (gnd).

[0017] The sense terminal sense corresponding to the drain and source of the drive transistor LS and the ground terminal gnd are input to the attenuator att. The attenuator att consists of three resistors Ra, Rb, and Rc. One end of the resistor Ra is connected to the sense terminal sense, and one end of the resistor Rc is connected to the ground terminal gnd. Then, the other end of the resistor Ra and the ground terminal gnd are connected by the resistor Rb. The other ends of the resistor Ra and the resistor Rc are a pair of output terminals of the attenuator att. The resistors Ra, Rb, and Rc are composed of transistors with substantially the same temperature characteristics as the drive transistor LS, and a control signal similar to the gate of the drive transistor LS is supplied to the gate of each transistor.

[0018] In this way, by using transistors of the same type as the drive transistor LS for the elements of the attenuator att, the current passing through the attenuator att becomes a value in which a component proportional to the current flowing through the drive transistor LS and the feedback current flowing through the resistors Rd and Re in the pre - amp are superimposed. Note that the displacement of the current passing through the attenuator att is proportional to the current flowing through the drive transistor LS. This is because the DC component flowing through the resistors Rd and Re is determined by the common - mode voltage vcm, and the change amount is equivalent to the current generated in the attenuator att proportional to the LS current.

[0019] A pair of outputs from the attenuator att are input to the pre - amp pre - amp. The output terminal from the resistor Ra is connected to the negative input terminal of the first operational amplifier OPA1, and the output terminal from the resistor Rc is connected to the positive input terminal of the first operational amplifier OPA1. The positive output end (negative output with respect to the current iin) is connected to the negative input terminal via the feedback resistor Rd. Also, the negative output end (positive output with respect to the current iin) of the first operational amplifier OPA1 is connected to the positive input terminal via the feedback resistor Re. Also, the common - mode voltage vcm is supplied to the common - mode voltage input terminal of the first operational amplifier OPA1.

[0020] The operational amplifier OPA1 amplifies a pair of inputs (input voltages) according to the resistance values ​​of the feedback resistors Rd and Re, using the common-mode voltage Vcm as the operating reference voltage (average voltage of the positive and negative outputs). The voltages of the pair of outputs are vbn and vbp, and the common-mode voltage Vcm = (vbn + vbp) / 2.

[0021] The input terminal and the negative input terminal of this common-mode voltage are connected by a switch (sw). In other words, by turning on the switch (sw), the positive output terminal is fixed at vcm.

[0022] A preamplifier is Negative out A voltage difference output corresponding to the voltage drop across the drive transistor LS is obtained between the power terminal and the positive output terminal.

[0023] The negative and positive output terminals of the preamplifier (pre-amp) are input to the postamplifier (post-amp). The negative output terminal of the preamplifier (pre-amp) is connected to the negative input terminal of the second operational amplifier (OPA2) via resistor Rf. The output terminal of the second operational amplifier (OPA2) is connected to the negative input terminal of the second operational amplifier (OPA2) via feedback resistor Ri. Furthermore, a reference voltage vref is supplied to the negative input terminal of the second operational amplifier (OPA2) (the connection point of resistors Rf and Ri) via resistor Rh.

[0024] The positive output terminal of the preamplifier is connected to the positive input terminal of the second operational amplifier OPA2 via resistor Rg. A reference voltage vref is supplied to the positive input terminal of the second operational amplifier OPA2 via resistor Rj.

[0025] Furthermore, the output terminal of the second operational amplifier OPA2 becomes the output terminal of the detection signal, output vout.

[0026] Therefore, the pair of inputs of the operational amplifier OPA2 are referenced to a reference voltage vref, and the difference between the pair of inputs is amplified according to the resistance value of resistor Ri and output.

[0027] Figure 2 shows examples of set resistance values ​​for each resistor in the circuit shown in Figure 1. In this example, Resistance Ra=n*r1 Resistance Rb=r1 Resistance Rc=r1 Resistance Rd=n*R1 Resistance Re=(n+1)R1 Resistance Rf=R2 Resistance Rg=R2 Resistance Rh=m*R2 Resistance Ri=R3 Resistance Rj=m*R2 It will be set to this.

[0028] In this example, the coefficient "n" is the attenuation index in the attenuator att. For high current measurements, a large value for n is used. The coefficient "m" defines the ratio of (vref-vout) to (vout-vcm) when iin=0A. In this configuration, when the drive current iin = 0A, vbp = vbn = vcm, and to minimize errors, if the current flowing through the feedback resistor Ri (=R3) of the operational amplifier OPA2 is set to 0, the potential difference between the positive and negative input terminals of the operational amplifier OPA2 becomes 0V. Therefore, vc=vout It will become. When the drive current iin operates symmetrically in positive and negative directions, and vout changes from 0V to vref, it is desirable to set vout = vref / 2 when iin = 0A. In this case, when iin = 0A, vc = vref / 2. Furthermore, the negative input terminal voltage of the operational amplifier OPA2 is vc=vcm*m*R2 / [(1+m)R2]+vref*R2 / [(m+1)R2] =vcm*m / (m+1)+vref / (m+1) That is the case. Therefore, vcm = vc * (m + 1) / m - vref / m And so, Substitute vc = vref / 2, vcm = vref * (m + 1) / (2m) - vref / m =vref*(m-1) / (2m) This is the result.

[0029] In such a current sense amplifier, the gain of the current iin versus output voltage vout is defined by the resistor network in the pre-amp and post-amp. Therefore, the gain of the current sense amplifier can be trimmed by adjusting the resistance values ​​of these resistors.

[0030] Here, if the current iin = 0A flowing through the drive transistor LS, the difference between the two outputs of the preamplifier should be 0V. However, if there is an offset voltage (vos) in the preamplifier, even if the current iin = 0, vbp - vbn will not be 0 depending on the offset voltage vos.

[0031] If the settings are as shown in the diagram, vbp - vbn = (1 + (n + 1) * R1 / r1) vos This is the result.

[0032] Thus, if the offset voltage vos is not zero, this offset voltage can cause temperature drift. In other words, the cause of temperature drift is that the feedback resistors Re.Re (i.e., R1) of the operational amplifier OPA1 and the resistors Ra, Rb, and Rc (i.e., r1) of the attenuator att are uncorrelated with respect to the temperature coefficient. This is because the resistor R1 of the operational amplifier OPA1 is composed of a standard resistor, and the resistor r1 of the attenuator att is composed of the on-resistance of the transistor (FET). However, when the offset voltage vos = 0, the change in the ratio of resistors R1 and r1 due to temperature does not affect vbp-vbn. Therefore, in this embodiment, the offset voltage vos of the preamplifier pre-amp is adjusted to 0V.

[0033] The current sense amplifier according to this embodiment has a switch sw, and when this switch sw is turned on, vbn = vcm. Also, vcm = (vbn + vbp) / 2.

[0034] Therefore, when the switch sw is ON and an offset voltage vos exists, both vbp and vbn are shifted according to half of vos.

[0035] In this embodiment, a voltage shift corresponding to the magnitude of the offset voltage vos can be detected by switching the switch sw on and off. Therefore, while checking the output shift caused by switching the switch sw on and off, the offset voltage vos can be brought closer to 0 using the offset adjustment function of the preamplifier pre-amp. Specifically, this adjustment is performed by trimming the input offset adjustment circuit provided in the input stage of the operational amplifier OPA1. Such trimming can also be achieved by using resistors, currents, or transistors of different sizes.

[0036] Figure 3 shows the change in output vout when the switch sw is turned on. Each line represents a different setting of the offset adjustment trimming circuit in OPA1 for a preamplifier (pre-amp) with an offset voltage. In this example, the second setting from the top shows the smallest change in output vout when the switch sw is turned on or off. Therefore, this setting of trimming is appropriate.

[0037] Furthermore, various known trimming techniques can be employed, such as changing the bias voltage to the positive and negative input terminals of the preamplifier.

[0038] Figure 4 shows the temperature characteristics of vout when using each of the trimming settings described above. The second setting from the top, which was considered optimal in Figure 2, shows the smallest change due to temperature, indicating that this setting can reduce temperature dependence.

[0039] "Other configuration examples" Figure 5 shows an example where the preamplifier is configured with two single-ended first operational amplifiers, OPA1-1 and OPA1-2.

[0040] In this example, the output of the first operational amplifier OPA1-1 is connected to the negative input terminal via the feedback resistor Rk.

[0041] On the other hand, the positive input terminal of the first operational amplifier OPA1-1 is output directly through resistor Rw. The output of the first operational amplifier OPA1-1 and the output side of resistor Rw form a pair of outputs, which are connected by divider resistors Rt and Ru. The connection point of these divider resistors Rt and Ru is input to the negative input terminal of the first operational amplifier OPA1-2. The common-mode voltage vcm is input to the positive input terminal of the first operational amplifier OPA1-2, and the output of the operational amplifier OPA is connected to the connection point of resistor Rw and divider resistor Ru. The common-mode voltage vcm is obtained by voltage division across resistors Rm and Rn connected between the reference voltage csaref and ground and. Here, operational amplifier OPA1-2 is an inverting amplifier of the output of operational amplifier OPA1-1. With the common-mode voltage vcm as the reference, when vbn is negative it outputs a positive value to vbp, and when vbn is positive it operates in the opposite way. The amplitude is determined by the ratio of resistors Rt and Ru. In this example, the resistance values ​​of both resistors Rt and Ru are R7, so it operates as an inverting amplifier with a gain of 1. The comparison result of op-amp OPA1-2 is then fed back to the positive input terminal of op-amp OPA1-1 via Rw.

[0042] Furthermore, in the negative feedback path of the post-amp, resistors R4 and R5 and the switch LOWGAIN_SW are connected in series in parallel with the feedback resistor Ri. By turning on the switch LOWGAIN_SW, the resistance value of the feedback resistor can be reduced, thereby reducing the gain of the second operational amplifier OPA2.

[0043] In this configuration, OPA1-2 always maintains a common-mode voltage of vcm, which may be helpful for fast settling.

[0044] And in this example, Resistance Ra=r1 Resistance Rb=r2 Resistance Rc=r2 r1:r2=4:1 Resistance Rk=4*R1 Resistance Rw=5*R1 Resistance Rf=R2 Resistance Rg=R2 Resistance Rh=2*R2 Resistance Rj=2*R2 Resistance Ri=R3 Resistance Rt=R7 Resistance Ru=R7 Resistance Rm=3*R6 Resistance Ru=R6 It will be set to this.

[0045] <When the switch is off> Assuming iin=0, sw=off, vcm=vcsaref / 4, r1=4*r2, the voltages of the pair of outputs of the attenuator att are va, va+vos, the output voltage of the first operational amplifier OPA1-1 is vbn, the output voltage of the resistor Rw (output voltage of OPA1-2) is vbp, and the output of the second operational amplifier OPA2 is sout, the voltages at each point are as follows. vcm = (vbp + vbn) / 2 va + vos = vbp * r² / (r² + 5 * R¹) va=vbn*(r1 / / r2) / (r1 / / r2+4*R1)=vbn*r2 / (r2+5*R1) ( (Note that the symbol [ / / ] indicates parallel connection.) vos=(vbp-vbn)*r² / (r²+5*R1) vbp-vbn=vos*(r2+5*R1) / r2=vos*(1+5*R1 / r2) vbp + vbn = 2 * vcm vbp = vcm + vos*(r² + 5*R1) / (2*r²) sout(OFF)=csaref / 3+(2 / 3)*[vcm+vos*(r2+5*R1) / (2*r2)]+(vbp-vbn)*R3 / R2 <When the switch is ON> vbn=vcm va = vcm * r² / (r² + 5 * R¹) va + vos = vbp * r² / (r² + 5 * R¹) vcm*r2 / (r2+5*R1)+vos=vbp*r2 / (r2+5*R1) vbp = vcm + vos*(r² + 5*R1) / r² vbp - vbn = vos * (r² + 5 * R¹) / r² vbp+vbn=2*vcm+vos*(r2+5*R1) / r2 vbp = vcm + vos*(r² + 5*R1) / r² sout(ON) = csaref / 3 + (2 / 3)*[vcm + vos*(r2+5*R1) / r2] + (vbp - vbn)*R3 / R2

[0046] <Difference due to switch on / off> Δsout=sout(ON)-sout(OFF)=(2 / 3)*vos*(r2+5*R1) / (2*R2)=vos*(r2+5*R1) / (3*R2)

[0047] Thus, the difference Δsout between the ON and OFF states of the switch sw is proportional to vos.

[0048] <Specific numerical examples> An example of specific settings used in an actual application is shown below.

[0049] The on-resistance Rs of the drive transistor LS is approximately 8mΩ, and the ratio of the output voltage sout to the current iin of the current sense amplifier, sout / iin, can be set to approximately 1.5V / 30A.

[0050] in this case, Total current-to-voltage gain = 1.5V / (8mohm * 30A) = 6.25. Current across resistor Rb ir2 = 169uA, The gain of the post-amp is R3 / R2 = 4.5 The output difference of the preamplifier (Vbp - Vbn) is 660mV. Preamplifier feedback resistor reference value R1 = 390 ohms The resistance values ​​of the attenuator att, Rb and Rc, are r2 = 700 ohms.

[0051] <Trimming> In the configuration shown in Figure 5, a variable power supply is provided before the positive and negative input terminals of the operational amplifier OPA1-1. This configuration allows for adjustment of the bias voltage by resistor trimming. This trimming allows for adjustment of the offset voltage vos of the preamplifier.

[0052] Furthermore, the resistor Rn used to determine the common-mode voltage Vcm of op-amp OPA1-2 and the resistor Rj in the reference voltage vref connection path are made into trimmable variable resistors. This allows adjustment for offsets occurring in op-amps other than OPA1-1, such as OPA1-2 and OPA2, and offsets occurring in the output sout due to errors in the resistance ratios of resistors Rf, Rh, Rg, and Rj.

[0053] Additionally, trimming the post-amp resistors Ri, R4, and R5 allows you to change the trimming gain and other parameters.

[0054] "Other configurations" By implementing the preamp output stage with an NMOS source follower, the common-mode voltage of the preamp output can be brought close to ground level, allowing a relatively large current to flow through the attenuator block.

[0055] To adjust the output of the post-amp to the midpoint of the reference voltage vref, it is best to connect each input terminal of the operational amplifier inside the post-amp to the reference voltage vref via a resistor.

[0056] To make the common-mode voltage vcm of the preamp output proportional to the reference voltage vref, the output voltage of the post-amp will also be proportional to the reference voltage.

[0057] When the switch is turned on, the post-amp output voltage shifts proportionally to the offset voltage of the pre-amp input stage, allowing it to be measured for trimming without touching the sensitive node. [Explanation of Symbols]

[0058] LS: drive transistor, sw: switch, OPA: operational amplifier, R,r: resistors.

Claims

1. A current sense amplifier that measures the voltage drop across a drive transistor and measures the current flowing through the drive transistor, An attenuator connected to the positive and negative sides of the drive transistor, which obtains positive and negative outputs corresponding to the voltage drop in the drive transistor, A preamplifier having a first feedback resistor connecting the positive output terminal and the negative input terminal, and a second feedback resistor connecting the negative output terminal and the positive input terminal, wherein the positive output of the attenuator is input to the negative input terminal, and the negative output of the attenuator is input to the positive input terminal, and a positive output corresponding to the difference between the positive and negative outputs of the input attenuator is output to the negative output terminal and a negative output is output to the positive output terminal, A switch connects the input terminal of the common-mode voltage that serves as the operating reference for the preamplifier to the positive output terminal, A post-amplifier receives the positive and negative outputs of the preamplifier and obtains an output corresponding to the difference between the positive and negative outputs of the preamplifier. Includes, The attenuator consists of the on-resistance of a transistor, and the first and second feedback resistors consist of standard resistors. Current sense amplifier.

2. A current sense amplifier according to claim 1, The attenuator uses a transistor with the same characteristics as the drive transistor. Current sense amplifier.

3. A current sense amplifier according to claim 1, The negative output vbn of the preamplifier is connected to the input terminal of the reference voltage vref via the first divider resistor Rf and the second divider resistor Rh, the positive output vbp of the preamplifier is connected to the input terminal of the reference voltage vref via the third divider resistor Rg and the fourth divider resistor Rj, the connection point of the first divider resistor Rf and the second divider resistor Rh is connected to the negative input terminal of the post-amplifier, and the connection point of the third divider resistor Rg and the fourth divider resistor Rj is connected to the positive input terminal of the post-amplifier. Current sense amplifier.

4. A current sense amplifier according to claim 3, The resistance values ​​of the first divider resistor Rf and the third divider resistor Rg are made the same, and the resistance values ​​of the second divider resistor Rh and the fourth divider resistor Rj are made the same, and the output of the post-amplifier when the current flowing through the drive transistor is 0 is set to 1 / 2 of the reference voltage vref. Current sense amplifier.

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