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]

[0001] The present disclosure relates to a current sense amplifier for detecting a current flowing through a drive transistor. [Background technology]

[0002] In a drive circuit that drives a load such as a motor, a drive transistor is used to control the drive current. 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. To achieve excellent motor control, the current sense amplifier needs to be fast, highly accurate, and have low temperature drift.

[0003] In addition, it is desirable for the current sense amplifier to operate with low power consumption. Therefore, a configuration in which a current detection resistor is connected in series with the drive transistor is not desirable. It is desirable to detect the current by the voltage drop caused by the on-resistance (Rdson) of the power MOSFET used as the drive transistor.

[0004] However, since the on-resistance Rdson changes nonlinearly depending on the current flow and temperature, it is difficult to detect the current with high accuracy using the on-resistance.

[0005] In particular, offset trimming in current sense amplifiers is very important in compensating for temperature dependency because if offset trimming is not performed accurately, it will cause temperature drift at 0A current, which will be difficult to calibrate in subsequent user applications. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 7-113826 Summary of the Invention [Problem to be solved by the invention]

[0007] When offset trimming is performed correctly and the input offset is zero, the input bias current flows in a balanced manner, so the output current from the input stage does not flow to the current-to-voltage converter in the subsequent circuit. On the other hand, if there is an offset, the error current that remains after adjusting the center voltage to the target value may cause temperature drift. [Means for solving the problem]

[0008] The current sense amplifier of the present disclosure is a current sense amplifier that measures the voltage drop across a drive transistor to measure the current flowing through the drive transistor, and includes a preamplifier that receives the voltage across the drive transistor and obtains positive and negative outputs corresponding to the voltage difference between the input voltages across the drive transistor, and a switch that connects an input terminal of a common mode voltage that serves as the operating reference for the preamplifier and the negative output, and can detect changes in the positive and negative outputs by turning the switch on and off.

[0009] It is also preferable that the amplifier further includes a post-amplifier to which the positive and negative outputs of the pre-amplifier are input and which obtains an output according to the difference in input voltages, and that the change in the positive and negative outputs caused by turning the switch on and off can be detected as a change in the output of the post-amplifier. Effect of the Invention

[0010] The offset of the preamplifier can be detected by the change in output when the switch is turned on and off. Therefore, the offset voltage can be reduced by trimming the preamplifier. This reduces the temperature dependency of the current sense amplifier. [Brief description of the drawings]

[0011] [Figure 1] 1 is a circuit diagram showing a configuration of a current sense amplifier according to an embodiment; [Diagram 2] 2 is a diagram showing an example of set resistance values ​​of resistors in the circuit of FIG. 1. [Diagram 3] FIG. 13 is a diagram showing a change in output vout by turning on a switch sw. [Figure 4] FIG. 13 is a diagram showing the temperature characteristics of vout for various trimming settings. [Diagram 5] In this example, the pre-amp is configured with two single-ended first operational amplifiers OPA1-1 and OPA1-2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. Note that the following embodiments do not limit the present disclosure, and configurations obtained by selectively combining multiple examples are also included in the present disclosure.

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

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

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

[0017] The sense terminal sense, which corresponds to the drain and source of the drive transistor LS, and the ground terminal gnd are input to the attenuator att. The attenuator att is composed 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. The other end of the resistor Ra is connected to the ground terminal gnd by the resistor Rb. The other end of the resistor Ra and the other end of the resistor Rc form a pair of output terminals of the attenuator att. The resistors Ra, Rb, and Rc are composed of transistors with almost the same temperature characteristics as the drive transistor LS, and the gates of each transistor are supplied with the same control signal as the gate of the drive transistor LS.

[0018] In this way, by using the same type of transistor as the drive transistor LS for the attenuator att element, the current passing through the attenuator att is a value that is a composite of 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 preamplifier pre-amp. 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 is equivalent to the current generated in the attenuator att that is proportional to the LS current.

[0019] A pair of outputs from the attenuator att are input to the pre-amplifier 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 (negative output with respect to the current iin) of the first operational amplifier OPA1 is connected to the negative input terminal via a feedback resistor Rd. In addition, the negative output (positive output with respect to the current iin) of the first operational amplifier OPA1 is connected to the positive input terminal via a feedback resistor Re. In addition, a 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 and outputs a pair of inputs (input voltages) according to the resistance values ​​of the feedback resistors Rd and Re, using a 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 voltages vbn and vbp, and the common-mode voltage Vcm=(vbn+vbp) / 2.

[0021] The input terminal of this common mode voltage and the negative input terminal are connected by a switch sw. That is, by turning on the switch SW, the positive output terminal is fixed to vcm.

[0022] The pre-amplifier pre-amp obtains an output of a voltage difference between its negative input / output terminal and its positive output terminal, the voltage difference corresponding to the voltage drop of the drive transistor LS.

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

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

[0025] The output terminal of the second operational amplifier OPA2 serves as the output terminal for the output vout which is the detection signal.

[0026] Therefore, the operational amplifier OPA2 uses the reference voltage vref as a reference for a pair of inputs, and amplifies and outputs the difference between the pair of inputs in accordance with the resistance value of the resistor Ri.

[0027] FIG. 2 is a diagram showing an example of set resistance values ​​of each resistor in the circuit of FIG. 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 is set to.

[0028] In this example, the coefficient "n" is the attenuation exponent for the attenuator att. For high current measurements, a large value is used for n. Also, 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 the current flowing through the feedback resistor Ri (=R3) of the operational amplifier OPA2 is set to 0 in order to minimize error, the potential difference between the positive and negative input terminals of the operational amplifier OPA2 will be 0V. Therefore, vc=vout become. If the drive current iin operates symmetrically between positive and negative and vout varies from 0V to vref, it is desirable to set vout=vref / 2 when iin=0A. In this case, when iin=0A, vc=vref / 2. In addition, the negative input 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) It is. Therefore, vcm=vc*(m+1) / m-vref / m And then, Substituting vc=vref / 2, vcm=vref*(m+1) / (2m)-vref / m =vref*(m-1) / (2m) It becomes.

[0029] The gain of such a current sense amplifier, iin vs. output voltage vout, is defined by the resistor network in the pre-amplifier pre-amp and post-amplifier 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 flowing through the drive transistor LS is 0 A, the difference between the pair of outputs of the preamplifier pre-amp should be 0 V. However, if there is an offset voltage (vos) in the preamplifier pre-amp, vbp-vbn will not be 0 according to the offset voltage vos even if the current iin is 0.

[0031] If the settings are as shown in the figure, vbp-vbn=(1+(n+1)*R1 / r1)vos It becomes.

[0032] Thus, if the offset voltage vos is not 0, there is a possibility that temperature drift will occur due to this offset voltage. That is, the cause of the temperature drift is that the feedback resistance Re.Re (i.e., R1) of the operational amplifier OPA1 and the resistances Ra, Rb, and Rc (i.e., r1) of the attenuator att are uncorrelated in terms of temperature coefficient. This is because the resistance R1 of the operational amplifier OPA1 is composed of a standard resistor, and the resistance r1 of the attenuator att is composed of the on-resistance of a transistor (FET). However, when the offset voltage vos=0, the change in the ratio of the resistance R1 and the resistance 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 switch sw is turned ON and an offset voltage vos is present, both vbp and vbn shift according to half vos.

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

[0036] FIG. 3 is a diagram showing the change in output vout when the switch sw is turned on. A separate line shows the case where the setting value of the trimming circuit for offset adjustment of OPA1 is changed 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 and off. Therefore, it can be seen that this setting trimming should be adopted.

[0037] It should be noted that various known trimming methods can be adopted, such as changing the bias voltage to the positive and negative input terminals of the pre-amplifier pre-amp.

[0038] Figure 4 shows the temperature characteristics of vout when the above trimming settings are used. The second setting from the top, which is considered optimal in Figure 2, shows the smallest change due to temperature, and this setting can reduce temperature dependency.

[0039] "Other configuration examples" FIG. 5 shows an example in which the pre-amplifier "pre-amp" 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 a feedback resistor Rk.

[0041] On the other hand, the positive input terminal of the first operational amplifier OPA1-1 is directly outputted through the resistor Rw. The output of the first operational amplifier OPA1-1 and the output side of the resistor Rw form a pair of outputs, and are connected between them by the dividing resistors Rt and Ru. The connection point of the dividing resistors Rt and Ru is inputted to the negative input terminal of the first operational amplifier OPA1-2. The common mode voltage vcm is inputted 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 the resistor Rw and the dividing resistor Ru. The common mode voltage vcm is obtained by voltage division of the resistors Rm and Rn connected between the reference voltage csaref and the ground agnd. Here, the operational amplifier OPA1-2 is an inverting amplifier of the output of the operational amplifier opa1-1. Based on the common mode voltage vcm, when vbn is negative, it outputs a positive to vbp, and when vbn is positive, it operates in the opposite manner. The amplitude is determined by the ratio of resistors Rt and Ru. In this example, the resistance values ​​of resistors Rt and Ru are both 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] In addition, in the negative feedback path of the post-amplifier post-amp, resistors R4 and R5 and a 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 to reduce the gain of the second operational amplifier OPA2.

[0043] In such a configuration, OPA1-2 will always keep the common-mode voltage at vcm, which may help with 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 is set to.

[0045] <When the switch is off> If we assume that iin = 0, sw = off, vcm = vcsaref / 4, r1 = 4*r2, the voltages va and va + vos of the pair of outputs of the attenuator att, the output voltage vbn of the first operational amplifier OPA1-1, the output voltage of resistor Rw (the output voltage of OPA1-2) vbp, and the output sout of the second operational amplifier OPA2, the voltages at each point will be 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 a 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] In this way, the difference Δsout between when the switch sw is ON and when it is OFF is proportional to vos.

[0048] <Specific numerical examples> An example of specific setting values ​​for an actual application is shown below.

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

[0050] in this case, Total current-to-voltage gain Total_gain=1.5V / (8mohm*30A)=6.25, Resistor Rb current ir2 = 169uA, Post-amp amplification factor R3 / R2=4.5 Pre-amp output difference Vbp-vbn=660mV The resistance standard value of the feedback resistor of the preamplifier is R1 = 390 ohms Attenuator att resistors Rb and Rc resistance value r2 = 700 ohms

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

[0052] In addition, the resistor Rn for determining the common mode voltage Vcm of the operational amplifier OPA1-2 and the resistor Rj of the reference voltage vref connection path are made into trimmable variable resistors, which makes it possible to adjust the offset of the operational amplifiers OPA1-2 and OPA2 that occur other than the operational amplifier OPA1-1, and the offset that occurs in the output sout due to an error in the resistance ratio of the resistors Rf, Rh, Rg, and Rj.

[0053] Moreover, the trimming gain and the like can be changed by trimming the resistors Ri, R4, and R5 of the post-amplifier post-amp.

[0054] "Other Configurations" The pre-amp output stage is realized by an NMOS source follower, and the common-mode voltage of the pre-amp output can be made close to the GND level so that a relatively large current can flow in the attenuator att block.

[0055] In order to adjust the output of the post-amplifier post-amp at the midpoint of the reference voltage vref, each input terminal of the operational amplifier inside the post-amplifier post-amp may be connected to the reference voltage vref via a resistor.

[0056] Since the common-mode voltage vcm of the pre-amp output is made proportional to the reference voltage vref, the output voltage of the post-amp is proportional to the reference voltage.

[0057] When the switch sw is turned on, the post-amp output voltage shifts proportional to the offset voltage of the pre-amp input stage, so it can be measured for trimming without touching sensitive nodes. [Explanation of symbols]

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

Claims

1. A current sense amplifier that measures a voltage drop across a drive transistor to measure a current flowing through the drive transistor, a preamplifier that receives a voltage between both ends of the drive transistor and obtains a positive output and a negative output according to a voltage difference between the input voltages; a switch for connecting an input terminal of a common mode voltage that is an operating reference of the preamplifier and the negative output; Including, A change in the positive output caused by turning the switch on and off can be detected. Current Sense Amplifier.

2. 2. A current sense amplifier according to claim 1, a post-amplifier that receives the positive and negative outputs of the pre-amplifier and obtains an output according to a difference between the input voltages; A change in the positive output caused by turning on and off the switch can be detected as a change in the output of the post-amplifier. Current Sense Amplifier.

3. 2. A current sense amplifier according to claim 1, an attenuator using a transistor having the same characteristics as the driving transistor is provided in a stage preceding the preamplifier; Current Sense Amplifier.

4. 3. A current sense amplifier according to claim 2, The post-amplifier is supplied with a reference voltage that serves as the operating standard thereof, and a relationship between the output of the post-amplifier when the current flowing through the drive transistor is zero and the reference voltage is set by a combination of resistors inserted between the input terminal of the reference voltage and the input terminal of the common mode voltage. Current Sense Amplifier.

5. 5. A current sense amplifier according to claim 4, The output of the post-amplifier is set to 1 / 2 of the reference voltage. Current Sense Amplifier.

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