Current sense amplifier

Abstract

The present invention is to improve temperature dependence in a current sense amplifier. The current sense amplifier of the present invention measures a voltage drop in a drive transistor (LS) to measure a current flowing in the drive transistor (LS). The current sense amplifier includes a pre-amp to which a voltage across the drive transistor (LS) is inputted, to obtain a positive output and a negative output corresponding to a voltage difference of the inputted voltage across, and a switch (sw) connecting an input terminal of a common mode voltage (vcm) which is an operation reference of the pre-amp to the negative output. A change in the positive output caused by turning on / off the switch (sw) can be detected.

Description

Technical Field

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

[0002] In drive circuits that drive loads such as motors, drive transistors are used to control the drive current. Furthermore, to control the drive current, it is necessary to detect the current flowing through the drive transistor; this is done using a current sensing amplifier. To achieve excellent motor control, the current sensing amplifier must meet the requirements of high speed, high accuracy, and low temperature drift.

[0003] Furthermore, it is desirable for the current sensing amplifier to operate with low power consumption. Therefore, connecting the current sensing resistor in series with the drive transistor is not suitable. Ideally, the current should be detected by the voltage drop generated by the on-resistance (Rdson) of the power MOSFET (metal-oxide-semiconductor field-effect transistor) used as the drive transistor.

[0004] However, the on-resistance Rdson changes nonlinearly depending on the current flow and temperature, making it difficult to use the on-resistance to detect current with high precision.

[0005] In particular, offset trimming in current sensing amplifiers is crucial for temperature-dependent compensation. This is because inaccurate offset trimming can cause temperature drift at 0A current, which is difficult for user applications to correct later.

[0006] [Existing Technical Documents]

[0007] [Patent Literature]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 7-113826 Summary of the Invention

[0009] [The problem the invention aims to solve]

[0010] When offset correction is performed correctly and the input offset is zero, the input bias current flows steadily. Therefore, the output current from the input stage will not flow into the current-to-voltage converter in the subsequent circuitry. On the other hand, when offset exists, the residual error current after adjusting the center voltage to the target value may cause temperature drift.

[0011] [Technical means to solve the problem]

[0012] The current sensing amplifier of the present invention measures the voltage drop in a driving transistor and measures the current flowing in the driving transistor, comprising: a preamplifier for receiving the voltage across the driving transistor as input, and obtaining a positive output and a negative output corresponding to the voltage difference between the input voltage across the transistor; a switch connected to the input terminal of a common mode voltage serving as the operating reference of the preamplifier and the negative output; and capable of detecting changes in the positive output and the negative output caused by turning the switch on / off.

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

[0014] [Invention Effects]

[0015] The preamplifier offset can be detected by the output change when the switch is turned on / off. Therefore, the offset voltage can be reduced by adjusting the preamplifier. This reduces the temperature dependence of the current sensing amplifier. Attached Figure Description

[0016] Figure 1 This is a circuit diagram showing the configuration of the current sensing amplifier in the implementation method.

[0017] Figure 2 It means Figure 1 A diagram showing examples of the set resistance values ​​for each resistor in a circuit.

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

[0019] Figure 4 It is a graph showing the temperature characteristics of vout under various trim settings.

[0020] Figure 5 This is an example of a preamplifier consisting of two single-ended amplifiers, namely the first operational amplifier OPA1-1 and the first operational amplifier OPA1-2. Detailed Implementation

[0021] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. Furthermore, the following embodiments are not intended to limit the present invention, and configurations that selectively combine multiple examples are also included in the present invention.

[0022] Overall composition

[0023] Figure 1This is a circuit diagram showing the configuration of the current sensing amplifier in the implementation method.

[0024] The driving transistor LS is a transistor that supplies drive current to the load. In this example, the driving transistor LS is composed of 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 in the driving transistor LS will be denoted as "iin", and the on-resistance will be denoted as "Rs".

[0025] The voltage sensing amplifier in this embodiment detects the drive current flowing in the drive transistor LS based on the voltage drop (voltage across the terminals) 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 ground terminal (gnd) of the driving transistor LS are input to the attenuator att. The attenuator att consists of three resistors Ra, Rb, and Rc. One end of resistor Ra is connected to the sensing terminal (sense), and one end of resistor Rc is connected to the ground terminal (gnd). The other end of resistor Ra is connected to the ground terminal (gnd) via resistor Rb. The other ends of resistors Ra and Rc form a pair of output terminals of the attenuator att. Resistors Ra, Rb, and Rc are transistors with approximately the same temperature characteristics as the driving transistor LS, and the same control signal as the gate of the driving transistor LS is supplied to the gate of each transistor.

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

[0029] A pair of outputs from the attenuator att are input to the preamplifier pre-amp. The output from resistor Ra is connected to the negative input of operational amplifier OPA1, and the output from resistor Rc is connected to the positive input of operational amplifier OPA1. The positive output of operational amplifier OPA1 (negative output relative to current iin) is connected to the negative input via feedback resistor Rd. Conversely, the negative output of operational amplifier OPA1 (positive output relative to current iin) is connected to the positive input via feedback resistor Re. A common-mode voltage vcm is supplied to the common-mode voltage input of operational amplifier OPA1.

[0030] Operational amplifier OPA1 operates on a pair of inputs (input voltages), using the common-mode voltage Vcm as the operating reference voltage (the average voltage of the positive and negative outputs). The voltages corresponding to the resistance values ​​of feedback resistors Rd and Re are amplified and output. The pair of output voltages are vbn and vbp, with the common-mode voltage Vcm = (vbn + vbp) / 2.

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

[0032] The preamplifier obtains a voltage difference between its negative and positive output terminals corresponding to the voltage drop of the driving transistor LS.

[0033] The negative and positive outputs of the preamplifier are input to the postamplifier. The negative output of the preamplifier is connected to the negative input of the second operational amplifier OPA2 via resistor Rf. Furthermore, the output of the second operational amplifier OPA2 is connected to its negative input via feedback resistor Ri. Finally, a reference voltage vref is supplied to the negative input of the second operational amplifier OPA2 (the junction of resistors Rf and Ri) via resistor Rh.

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

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

[0036] Therefore, for a pair of inputs of operational amplifier OPA2, with reference voltage vref as the reference, the difference between the pair of inputs is amplified and output as the resistance value of resistor Ri.

[0037] Figure 2 It means Figure 1 A diagram showing examples of the set resistance values ​​for each resistor in a circuit.

[0038] In this example, it is set as follows:

[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. A large value is used for n when measuring high currents. Additionally, the coefficient "m" defines the ratio of (vref - vout) to (vout - vcm) when iin = 0 A.

[0050] In this configuration, when the drive 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 in order to minimize the error, then 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 drive current iin operates symmetrically in both positive and negative directions. When vout changes from 0V to vref, vout is set to vref / 2 when iin = 0A.

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

[0055] In addition, the negative input voltage of 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 in this current-sensing amplifier with respect to the output voltage vout is defined by the resistor networks in the pre-amp and post-amp. Therefore, the gain of the current-sensing amplifier can be adjusted by changing the resistance values ​​of these resistors.

[0064] Here, if the current iin flowing through the driving transistor LS is 0A, then the difference between the two outputs of the preamp will definitely be 0V. However, if there is an offset voltage (vos) in the preamp, then even if the current iin is 0, vbp-vbn will correspond to the offset voltage vos and will not be 0.

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

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

[0067] Therefore, if the offset voltage vos is not zero, temperature drift may be caused by the offset voltage.

[0068] In other words, the temperature drift is due to the fact that the feedback resistor Re.Re (R1) of operational amplifier OPA1 and the resistors Ra,Rb,Rc (r1) of attenuator att have no temperature coefficient. This is because the resistor R1 of operational amplifier OPA1 is a standard resistor, and the resistor r1 of attenuator att is the on-resistance of a transistor (FET). However, when the offset voltage vos = 0, the temperature-induced change in the ratio of resistor R1 to resistor r1 does not affect vbp-vbn. Therefore, in this embodiment, the offset voltage vos of the preamplifier is adjusted to 0V.

[0069] In this embodiment of the current sensing amplifier, there is a switch sw. When the switch sw is turned on, vbn = vcm. In addition, vcm = (vbn + vbp) / 2.

[0070] Therefore, when 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 switching the switch sw on and off, the voltage offset corresponding to the magnitude of the offset voltage vos can be detected. Therefore, while confirming the output offset caused by switching the switch sw on and off, the offset voltage vos can be brought close to 0 by the offset adjustment function of the preamplifier. Specifically, this adjustment is performed by modifying the input offset adjustment circuit located in the input section of the operational amplifier OPA1. This modification can also be achieved by adjusting the resistor, current, or using transistors of different sizes.

[0072] Figure 3 This graph shows the change in output vout caused by turning on switch sw. Different lines represent the different cases when changing the setting of the offset adjustment circuit for OPA1 relative to the pre-amp with offset voltage. In this example, the change in output vout due to turning on / off switch sw is smaller in the second setting from the top. Therefore, it can be seen that this setting is sufficient for adjustment.

[0073] In addition, various well-known modification methods can be used, such as changing the bias voltages at the positive and negative input terminals of the preamplifier.

[0074] Figure 4 This is a graph showing the temperature characteristics of vout when using the aforementioned trimming settings. It can be seen that... Figure 2 The middle setting is the second best setting from the top, where temperature-related changes are minimal. This setting can be used to reduce temperature dependence.

[0075] Other examples of composition

[0076] Figure 5 This is an example of a preamplifier consisting of two single-ended amplifiers, namely the first operational amplifier OPA1-1 and the 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 of the first operational amplifier OPA1-1 is directly output via resistor Rw. Furthermore, the output of the first operational amplifier OPA1-1 and the output side of resistor Rw form a pair of outputs, connected by a dividing resistor Rt,Ru. The connection point of this dividing resistor Rt,Ru is input to the negative input of the first operational amplifier OPA1-2. A common-mode voltage vcm is input to the positive input 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 the dividing resistor Ru. Moreover, the common-mode voltage vcm is obtained by voltage division of resistors Rm,Rn connected between the reference voltage csaref and ground agnd. Here, operational amplifier OPA1-2 is an inverting amplifier of the output of operational amplifier OPA1-1. With the common-mode voltage vcm as a reference, when vbn is negative, the output is positive for vbp; when vbn is positive, the operation is reversed. Its amplitude is determined by the ratio of resistor Rt to Ru. In this example, the resistance values ​​of resistors Rt and Ru are both R7, therefore, it operates as an inverting amplifier with a gain of 1. Furthermore, the comparison result of operational amplifier OPA1-2 is fed back to the positive input of operational amplifier OPA1-1 via Rw.

[0079] Additionally, in the negative feedback path of the post-amplifier, resistors R4 and R5, along with the feedback resistor Ri, are connected in series with the switch LOWGAIN_SW. 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 stability.

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

[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] The resistance Ru = R6.

[0097] <When switch sw is open>

[0098] If we set iin = 0, sw = off, vcm = vcsaref / 4, r1 = 4*r2, the voltage of the pair of outputs of attenuator att is va, va + vos, the output voltage of the first operational amplifier OPA1-1 is vbn, the output voltage of resistor Rw (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 notation [ / / ] 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 (disconnect) = 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 (connected) = csaref / 3 + (2 / 3)*[vcm + vos*(r2 + 5*R1) / r2]

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

[0118] <Difference caused by the on / off state 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 switch sw is on and when it is off is proportional to vos.

[0121] <Specific numerical examples>

[0122] The following is an example of a specific setting in a real-world application system.

[0123] The following values ​​can be used: the on-resistance of the driving transistor LS is Rs = 8mΩ, and the ratio of the output voltage sou to the current iin of the current sensing amplifier is sou / iin = approximately 1.5V / 30A.

[0124] In this case,

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

[0126] The current ir2 of resistor Rb is 169uA.

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

[0128] The output difference of the preamplifier, Vbp - vbn, is 660mV.

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

[0130] The resistance values ​​of resistors Rb and Rc in the attenuator att are r2 = 700 ohms.

[0131] <Repair>

[0132] Figure 5 In this configuration, a variable power supply is placed near the positive and negative input terminals of the operational amplifier OPA1-1. This configuration adjusts the bias voltage through resistor adjustment. This adjustment can be used to adjust the offset voltage vos of the preamplifier.

[0133] Additionally, it can be configured as a variable resistor capable of adjusting the resistor Rn used to determine the common mode voltage Vcm of operational amplifier OPA1-2 and the resistor Rj for the reference voltage vref connection path. This allows adjustment of the offset in output sout caused by deviations other than those of operational amplifier OPA1-1, operational amplifier OPA1-2, or operational amplifier OPA2, as well as errors in the resistance ratios of resistors Rf, Rh, Rg, and Rj.

[0134] Additionally, adjusting the resistors Ri, R4, and R5 in the post-amplifier can change the adjusted gain, etc.

[0135] "Other components"

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

[0137] The output of the post-amp is adjusted at the midpoint of the reference voltage vref. Therefore, the input terminals of the operational amplifier inside the post-amp can be connected to the reference voltage vref via resistors.

[0138] Because the common-mode voltage vcm of the preamp output is proportional to the reference voltage vref, the output voltage of the postamp is also proportional to the reference voltage.

[0139] When switch SW is turned on, the output voltage of the post-amp is offset proportionally to the offset voltage of the input segment of the pre-amp. Therefore, it does not touch the sensitive node and can be measured for adjustment.

[0140] [Explanation of Symbols]

[0141] LS: Drive transistor

[0142] sw: switch

[0143] OPA: Operational Amplifier

[0144] R,r: Resistance.

Claims

1. A current sensing amplifier that measures a voltage drop in a driving transistor to measure a current flowing in the driving transistor, the current sensing amplifier comprising: A preamplifier is provided, wherein the voltage across the driving transistor is input to the preamplifier, and the preamplifier obtains a positive output and a negative output corresponding to the voltage difference between the voltages across the driving transistor. A post-amplifier, wherein the positive and negative outputs of the preamplifier are input to the post-amplifier, and the post-amplifier obtains an output corresponding to the difference between the positive and negative outputs of the preamplifier; and A switch that connects the input of the common-mode voltage to the negative output, the common-mode voltage being used as the operating reference for the preamplifier; in The change in the positive output caused by turning the switch on / off can be detected as a change in the output of the post-amplifier.

2. The current sensing amplifier according to claim 1, wherein an attenuator is provided at the front end of the preamplifier, the attenuator using a transistor with the same characteristics as the driving transistor.

3. The current sensing amplifier of claim 1, wherein the post-amplifier is supplied with a reference voltage having served as an operating reference for the post-amplifier, and the relationship between the output of the post-amplifier and the reference voltage when the current flowing in the driving transistor is 0 is set by a combination of resistors placed between the input terminal of the reference voltage and the input terminal of the common mode voltage.

4. The current sensing amplifier of claim 3, wherein when the current flowing through the driving transistor is 0, the output of the post-amplifier is set to 1 / 2 of the reference voltage.

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