Current sense amplifier

Abstract

The invention is to improve temperature dependency in current sense amplifiers. A current sense amplifier measures a voltage drop in a drive transistor (LS) and measures a current flowing through the drive transistor (LS). The current sense amplifier includes: a pre-amp to which a voltage across the driving transistor (LS) is input and which obtains a positive output and a negative output corresponding to a voltage difference between the input voltages across the driving transistor (LS); and a switch (sw) that connects an input terminal of a common mode voltage (vcm), which is an operation reference for the preamplifier (pre-amp), and the negative output. A change in the positive output caused by switching on / off the switch (sw) can be detected.

Description

Technical Field

[0001] The present invention relates to a current sensing amplifier for detecting a current flowing in a driving transistor. Background Art

[0002] In the drive circuit of a load such as a motor, a drive transistor is used to control the drive current. In addition, in order to control the drive current, it is necessary to detect the current flowing in the drive transistor, and a current sensing amplifier is used to detect the current flowing in the drive transistor. In order to achieve excellent motor control, the current sensing amplifier must meet the requirements of high speed, high precision, and low temperature drift.

[0003] In addition, it is desirable that the current sensing amplifier operates with low power consumption. Therefore, it is not appropriate to connect the current detection resistor in series with the driving transistor. It is more ideal to detect the current through the voltage drop generated by the on-resistance (Rdson) of the power MOSFET (metal-oxide-semiconductor field-effect transistor) used as the driving transistor.

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

[0005] Offset trimming in current sense amplifiers is especially important to compensate for temperature dependency. The reason is that if the offset trimming is not done accurately, it will cause temperature drift at 0A current, which is difficult to correct later by the user application system.

[0006] [Prior art literature]

[0007] [Patent Document]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 7-113826 Summary of the invention

[0009] [Problems to be solved by the invention]

[0010] When offset trimming is performed correctly and the input offset is zero, the input bias current flows steadily, so the output current from the input stage does not flow to the current-to-voltage converter of the subsequent circuit. On the other hand, when there is an offset, the error current remaining after the center voltage is adjusted to the target value may cause temperature drift.

[0011] [Technical means to solve the problem]

[0012] The current sensing amplifier of the present invention determines the voltage drop in the driving transistor and measures the current flowing in the driving transistor, including: a preamplifier, which inputs the voltage between the two ends of the driving transistor and obtains positive output and negative output corresponding to the voltage difference between the input voltage between the two ends; a switch, which connects the input end of the common mode voltage that serves as the action reference of the preamplifier and the negative output; and can detect the changes in the positive output and negative output caused by turning on / off the switch.

[0013] In addition, the current sensing amplifier of the present invention may also include a post-amplifier, which inputs the positive output and negative output of the pre-amplifier to obtain an output corresponding to the input voltage difference. The current sensing amplifier can detect the changes in the positive output and negative output caused by turning on / off the switch as the output changes of the post-amplifier.

[0014] [Effects of the invention]

[0015] The offset of the preamplifier can be detected by the output change when the switch is turned on / off. Therefore, the offset voltage can be reduced by trimming the preamplifier. Therefore, the temperature dependence of the current sense amplifier can be reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 : is a circuit diagram showing the configuration of a current sense amplifier according to an embodiment.

[0017] Figure 2 Yes means Figure 1 A diagram showing an example of a set resistance value of each resistor in a circuit.

[0018] Figure 3 : is a graph showing the change of the output vout caused by turning on the switch sw.

[0019] Figure 4 It is a graph showing the temperature characteristics of vout when various trimming settings are set.

[0020] Figure 5 This is an example in which a pre-amplifier pre-amp is formed by two single-ended amplifiers, namely, a first operational amplifier OPA1-1 and a first operational amplifier OPA1-2. DETAILED DESCRIPTION

[0021] Hereinafter, embodiments of the present invention will be described below with reference to the accompanying drawings. Note that the following embodiments do not limit the present invention, and configurations obtained by selectively combining a plurality of examples are also included in the present invention.

[0022] Overall composition

[0023] Figure 1: is a circuit diagram showing the configuration of a current sense amplifier according to an embodiment.

[0024] The driving transistor LS is a transistor that supplies a driving current to a load. In this example, the driving transistor LS is composed of an n-channel MOSFET (metal-oxide-semiconductor field-effect transistor), whose drain is connected to the load and whose source is connected to the ground electrode gnd. Here, the current flowing in the driving transistor LS is represented as "iin" and the on-resistance is represented as "Rs".

[0025] The voltage sense amplifier of the present embodiment detects the drive current flowing through the drive transistor LS based on the voltage drop (voltage between both ends) when the drive transistor LS is turned on.

[0026] The drain of the driving transistor LS becomes the sensing terminal sense, and the source becomes the ground terminal gnd.

[0027] The sensing terminal sense and the ground terminal gnd of the driving transistor LS, which are equivalent to the drain and source, 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 sensing terminal sense, and one end of the resistor Rc is connected to the ground terminal gnd. In addition, the other end of the resistor Ra and the ground terminal gnd are connected by a resistor Rb. The other end of the resistor Ra and the other end of the resistor Rc become a pair of output terminals of the attenuator att. The resistors Ra, Rb, and Rc are composed of transistors with temperature characteristics roughly the same as those of the driving transistor LS, and the gate of each transistor is supplied with the same control signal as the gate of the driving transistor LS.

[0028] In this way, by using the same type of transistor as the driving transistor LS as the element of the attenuator att, the current flowing through the attenuator att becomes a value obtained by superimposing a component proportional to the current flowing in the driving transistor LS and the feedback current flowing in the resistor Rd, Re in the preamplifier pre-amp. In addition, the displacement of the current flowing through the attenuator att is proportional to the current flowing in the driving transistor LS. The reason is that the DC (Direct Current) component flowing in the resistor Rd, Re is determined by the common mode voltage vcm, and the change is equal to the current generated in the attenuator att which is proportional to the LS current.

[0029] A pair of outputs from the attenuator att is input to the preamplifier 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 relative to the current iin) of the first operational amplifier OPA1 is connected to the negative input terminal via the feedback resistor Rd. In addition, the negative output (positive output relative to the current iin) of the first operational amplifier OPA1 is connected to the positive input terminal via the feedback resistor Re. In addition, the common mode voltage vcm is supplied to the common mode voltage input terminal of the first operational amplifier OPA1.

[0030] The operational amplifier OPA1 uses the common mode voltage Vcm as the operating reference voltage (average voltage of positive and negative outputs) for a pair of inputs (input voltages), increases and outputs the voltage corresponding to the resistance value of the feedback resistors Rd and Re. The voltage of the pair of outputs is the voltage vbn, vbp, and the common mode voltage Vcm = (vbn + vbp) / 2.

[0031] Furthermore, the input terminal of the common mode voltage is connected to the negative input terminal via the switch sw. That is, by turning on the switch SW, the positive output terminal is fixed to vcm.

[0032] The pre-amplifier pre-amp obtains an output having a voltage difference between its negative output terminal and positive output terminal corresponding to the voltage drop of the driving transistor LS.

[0033] The negative output terminal and the positive output terminal of the preamplifier pre-amp are input to the post-amplifier 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 the resistor Rf. In addition, the output terminal of the second operational amplifier OPA2 is connected to the negative input terminal of the second operational amplifier OPA2 via the feedback resistor Ri. Furthermore, the reference voltage vref is supplied to the negative input terminal (the connection point of the resistor Rf and the resistor Ri) of the second operational amplifier OPA2 via the resistor Rh.

[0034] The positive output terminal of the preamplifier pre-amp is connected to the positive input terminal of the second operational amplifier OPA2 via the resistor Rg. The reference voltage vref is supplied to the positive input terminal of the second operational amplifier OPA2 via the resistor Rj.

[0035] Furthermore, the output terminal of the second operational amplifier OPA2 becomes an output terminal of the output vout as a detection signal.

[0036] Therefore, regarding a pair of inputs of the operational amplifier OPA2, the difference between the pair of inputs is amplified and outputted according to the resistance value of the resistor Ri, with reference to the reference voltage vref.

[0037] Figure 2 Yes means Figure 1 A diagram showing an example of a set resistance value of each resistor in a circuit.

[0038] In this example, the settings are:

[0039] Resistance Ra = n*r1

[0040] Resistance Rb = r1

[0041] Resistance Rc = r1

[0042] Resistance Rd = n*R1

[0043] Resistance Re = (n + 1) R1

[0044] Resistance Rf = R2

[0045] Resistance Rg = R2

[0046] Resistance Rh = m*R2

[0047] Resistance Ri = R3

[0048] Resistance Rj=m*R2.

[0049] In this example, the coefficient "n" is the attenuation index in the attenuator att. When measuring large currents, a large value for n is used. In addition, the coefficient "m" defines the ratio of (vref-vout) to (vout-vcm) when iin=0A.

[0050] In this configuration, when the driving current iin=0A, vbp=vbn=vcm, and if the current flowing through the feedback resistor Ri (=R3) of the operational amplifier OPA2 is set to 0 to minimize the error, the potential difference between the positive and negative input terminals of the operational amplifier OPA2 becomes 0V.

[0051] therefore,

[0052] vc=vout.

[0053] Ideally, the driving current iin operates with positive and negative symmetry, and when vout changes from 0V to vref, when iin=0A, vout=vref / 2 is set.

[0054] In this case, when iin=0A, vc=vref / 2.

[0055] In addition, the negative input voltage of the operational amplifier OPA2 is

[0056] vc=vcm*m*R2 / [(1+m)R2]+vref*R2 / [(m+1)R2]

[0057] =vcm*m / (m+1)+vref / (m+1).

[0058] therefore,

[0059] vcm=vc*(m+1) / m-vref / m,

[0060] Substituting vc=vref / 2, we get

[0061] vcm=vref*(m+1) / (2m)-vref / m

[0062] =vref*(m-1) / (2m).

[0063] The gain of the current iin to the output voltage vout in this current sensing amplifier is defined by the resistor network in the pre-amplifier and the post-amplifier. Therefore, by adjusting the resistance values ​​of these resistors, the gain of the current sensing amplifier can be trimmed.

[0064] Here, if the current iin flowing through the driving transistor LS is 0 A, the difference between the pair of outputs of the preamplifier pre-amp will definitely be 0 V. However, if the preamplifier pre-amp has an offset voltage (vos), even if the current iin is 0, vbp-vbn will not be 0 due to the offset voltage vos.

[0065] If the settings are as shown in the figure, then

[0066] vbp-vbn=(1+(n+1)*R1 / r1)vos.

[0067] Thus, if the offset voltage vos is not 0, the offset voltage may cause temperature drift.

[0068] That is to say, the reason for the temperature drift is that the feedback resistor Re.Re (that is, R1) of the operational amplifier OPA1 and the resistors Ra, Rb, Rc (that is, r1) of the attenuator att have nothing to do with the temperature coefficient. The reason is that 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 the resistor R1 to the resistor r1 caused by the temperature will not affect vbp-vbn. Therefore, in this embodiment, the offset voltage vos of the preamplifier pre-amp is adjusted to 0V.

[0069] The current sense amplifier of the present embodiment includes a switch sw, and when the switch sw is turned on, vbn=vcm. Also, vcm=(vbn+vbp) / 2.

[0070] Therefore, when the switch sw is turned on and there is an offset voltage vos, both vbp and vbn are offset corresponding to half of vos.

[0071] In this embodiment, by turning on / off the switch sw, a voltage offset corresponding to the magnitude of the offset voltage vos can be detected. Therefore, while confirming the output offset caused by turning on / off the switch sw, the offset voltage vos can be made close to 0 by the offset adjustment function of the preamplifier pre-amp. Specifically, the adjustment is performed by trimming the input offset adjustment circuit provided in the input stage of the operational amplifier OPA1. This trimming can also be achieved by using resistors, currents, and transistors of different sizes.

[0072] Figure 3 This is a diagram showing the change in output vout caused by turning on switch sw. When the setting value of the trimming circuit for offset adjustment of OPA1 is changed relative to the preamplifier pre-amp with offset voltage, the situation at each setting value is represented by a different line. In this example, at the second setting from the top, the change in output vout caused by the on / off of switch sw is small. Therefore, it can be seen that it is sufficient to use this setting for trimming.

[0073] In addition, various well-known trimming methods may be used, such as changing the bias voltages of the positive and negative input terminals of the pre-amplifier pre-amplifier.

[0074] Figure 4 is a graph showing the temperature characteristics of vout when using the above-mentioned trimming setting values. It can be seen that, Figure 2 When the second setting from the top is set as the best, the change due to temperature is small, and the temperature dependence can be reduced by this setting.

[0075] 「Other configuration examples」

[0076] Figure 5 This is an example in which a pre-amplifier pre-amp is formed by two single-ended amplifiers, namely, a first operational amplifier OPA1-1 and a first operational amplifier OPA1-2.

[0077] In this example, the output of the first operational amplifier OPA1 - 1 is connected to the negative input terminal via the feedback resistor Rk.

[0078] On the other hand, the positive input terminal of the first operational amplifier OPA1-1 is directly output via the resistor Rw. Furthermore, the output of the first operational amplifier OPA1-1 and the output side of the resistor Rw form a pair of outputs, which are connected by split resistors Rt and Ru. Furthermore, the connection point of the split 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 between the resistor Rw and the split resistor Ru. In addition, the common mode voltage vcm is obtained by dividing the voltage of the resistors Rm and Rn connected between the reference voltage csaref and the ground electrode agnd. Here, the operational amplifier OPA1-2 is an inverting amplifier of the output of the operational amplifier opa1-1. With the common mode voltage vcm as a reference, when vbn is negative, the output to vbp is positive, and when vbn is positive, the action is the opposite. The amplitude is determined by the ratio of the resistors Rt and Ru. In this example, the resistance values ​​of the resistors Rt and Ru are both R7, and thus they operate as an inverting amplifier with a gain of 1. Furthermore, the comparison result of the operational amplifier OPA1-2 is fed back to the positive input terminal of the operational amplifier OPA1-1 via Rw.

[0079] In addition, in the negative feedback path of the post-amplifier, resistors R4, 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, thereby reducing the gain of the second operational amplifier OPA2.

[0080] In this configuration, OPA1-2 always maintains the common mode voltage at vcm, which may be useful for high-speed stabilization.

[0081] And, in this example, it is set as:

[0082] Resistance Ra = r1

[0083] Resistance Rb = r2

[0084] Resistance Rc = r2

[0085] r1:r2=4:1

[0086] Resistance Rk = 4*R1

[0087] Resistance Rw = 5*R1

[0088] Resistance Rf = R2

[0089] Resistance Rg = R2

[0090] Resistance Rh = 2*R2

[0091] Resistance Rj = 2*R2

[0092] Resistance Ri = R3

[0093] Resistance Rt = R7

[0094] Resistance Ru = R7

[0095] Resistance Rm = 3*R6

[0096] Resistance Ru = R6.

[0097] <When switch SW is off>

[0098] If iin=0, sw=disconnected, vcm=vcsaref / 4, r1=4*r2, the voltage of a pair of outputs of attenuator att is va, va+vos, the output voltage of the first operational amplifier OPA1-1 is vbn, the output side voltage of resistor Rw (the output voltage of OPA1-2) is vbp, and the output of the second operational amplifier OPA2 is sout, then the voltages at each point are as follows.

[0099] vcm=(vbp+vbn) / 2

[0100] va+vos=vbp*r2 / (r2+5*R1)

[0101] va=vbn*(r1 / / r2) / (r1 / / r2+4*R1)=vbn*r2 / (r2+5*R1) (In addition, the symbol [ / / " indicates parallel connection)

[0102] vos=(vbp-vbn)*r2 / (r2+5*R1)

[0103] vbp-vbn=vos*(r2+5*R1) / r2=vos*(1+5*R1 / r2)

[0104] vbp+vbn=2*vcm

[0105] vbp=vcm+vos*(r2+5*R1) / (2*r2)

[0106] sout(disconnected)=csaref / 3+(2 / 3)*[vcm+vos*(r2+5*R1) / (2*r2)]+(vbp-vbn)*R3 / R2

[0107] <When switch sw is turned on>

[0108] vbn=vcm

[0109] va=vcm*r2 / (r2+5*R1)

[0110] va+vos=vbp*r2 / (r2+5*R1)

[0111] vcm*r2 / (r2+5*R1)+vos=vbp*r2 / (r2+5*R1)

[0112] vbp=vcm+vos*(r2+5*R1) / r2

[0113] vbp-vbn=vos*(r2+5*R1) / r2

[0114] vbp+vbn=2*vcm+vos*(r2+5*R1) / r2

[0115] vbp=vcm+vos*(r2+5*R1) / r2

[0116] sout(on)=csaref / 3+(2 / 3)*[vcm+vos*(r2+5*R1) / r2]

[0117] +(vbp-vbn)*R3 / R2

[0118] <Difference due to on / off of switch sw>

[0119] Δsout=sout(on)-sout(off)=(2 / 3)*vos*(r2+5*R1) / (2*R2)=vos*(r2+5*R1) / (3*R2)

[0120] In this way, the difference Δsout between when the switch sw is turned on and when it is turned off is proportional to vos.

[0121] <Specific numerical example>

[0122] An example of specific setting values ​​in an actual application system is shown below.

[0123] The on-resistance Rs of the driving transistor LS may be 8 mΩ, and the ratio of the output voltage sout and the current iin of the current sensing amplifier may be sout / iin=1.5 V / 30 A.

[0124] In this case,

[0125] Total current to voltage gain Total_gain = 1.5V / (8mohm*30A) = 6.25

[0126] The current ir2 of resistor Rb = 169uA

[0127] The amplification ratio of the post-amp is R3 / R2=4.5

[0128] The output difference of the preamplifier pre-amp is Vbp-vbn=660mV

[0129] The resistance reference value of the feedback resistor of the preamplifier is R1 = 390 ohm

[0130] The resistance value of the attenuator ATT, Rb, Rc is r2 = 700 ohm

[0131] <Repair>

[0132] Figure 5 In the configuration, a variable power supply is provided near the positive and negative input terminals of the operational amplifier OPA1-1. This configuration adjusts the bias voltage by resistor trimming. The offset voltage vos of the preamplifier pre-amp can be adjusted by this trimming.

[0133] In addition, the resistor Rn used to determine the common mode voltage Vcm of the operational amplifier OPA1-2 and the resistor Rj of the reference voltage vref connection path can be set as a variable resistor. In this way, the offset of the output sout caused by the offset of the operational amplifier OPA1-2 or the operational amplifier OPA2, etc., which is generated other than the operational amplifier OPA1-1, and the error of the resistance ratio of the resistors Rf, Rh, Rg, and Rj can be adjusted.

[0134] In addition, by trimming the resistors Ri, R4, and R5 of the post-amplifier, the trimmed gain and the like can be changed.

[0135] 「Other components」

[0136] The pre-amplifier pre-amp output section can be implemented by an NMOS source follower, so that the in-phase voltage output by the pre-amplifier pre-amp is close to the GND level, so that a relatively large current flows through the module of the attenuator att.

[0137] The output of the post-amplifier post-amp is adjusted at the middle of the reference voltage vref, and therefore, each input terminal of the operational amplifier inside the post-amplifier post-amp can be connected to the reference voltage vref via a resistor.

[0138] Since the common mode voltage vcm outputted by the pre-amplifier pre-amp is made proportional to the reference voltage vref, the output voltage of the post-amplifier post-amp is made proportional to the reference voltage.

[0139] When the switch sw is turned on, the post-amplifier post-amp output voltage is offset in proportion to the offset voltage of the input stage of the pre-amplifier pre-amp, so sensitive nodes are not touched and can be measured for trimming.

[0140] [Explanation of Symbols]

[0141] LS: driver transistor

[0142] sw: switch

[0143] OPA: Operational Amplifier

[0144] R,r: resistance.

Claims

1. A current sensing amplifier for measuring a voltage drop in a driving transistor and a current flowing in the driving transistor, comprising: A preamplifier for inputting the voltage across the driving transistor and obtaining a positive output and a negative output corresponding to the voltage difference between the input voltage across the transistor; and A switch connecting an input terminal of a common mode voltage serving as an operating reference of the preamplifier and the negative output; and A change in the positive output caused by turning on / off the switch can be detected.

2. The current sensing amplifier according to claim 1 further includes a post-amplifier, which is input with the positive output and the negative output of the pre-amplifier, obtains an output corresponding to the difference in input voltage, and is capable of detecting the change of the positive output caused by turning on / off the switch as the change of the output of the post-amplifier. 3 . The current sensing amplifier according to claim 1 , wherein an attenuator is provided at a front stage of the preamplifier, and the attenuator uses a transistor having the same characteristics as the driving transistor.

4. The current sensing amplifier according to claim 2, wherein the post-amplifier is supplied with a reference voltage which serves as a reference for its operation, and a relationship between the output of the post-amplifier and the reference voltage when the current flowing through the driving transistor is zero is set by inserting a combination of resistors between an input terminal of the reference voltage and an input terminal of the common mode voltage. 5 . The current sensing amplifier according to claim 4 , wherein an output of the post-amplifier is set to ½ of the reference voltage.

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