Temperature-compensated magnetic tunnel junction-based sensing circuit for measuring external magnetic fields

Abstract

Compensates for temperature-dependent output fluctuations in magnetic detection. [Solution] The present invention provides a magnetic tunnel junction (MTJ) sensing circuit for measuring an external magnetic field, comprising a plurality of MTJ sensor elements connected in a bridge configuration, the MTJ sensing circuit having an input for receiving a bias voltage and for generating an output voltage proportional to the external magnetic field by multiplying the bias voltage and a gain sensitivity of the MTJ sensing circuit, wherein the gain sensitivity and the output voltage vary with temperature. The MTJ sensing circuit further comprises a temperature compensation circuit configured to provide a modulated bias voltage that varies as a function of temperature over a temperature range such that the output voltage is substantially constant as a function of temperature. Also disclosed is a method for compensating the output voltage for temperature.

Description

[Technical Field]

[0001] The present invention relates generally to magnetic sensors employing magnetic tunnel junctions, and more particularly to magnetic sensor output compensation based on the temperature of the magnetic sensor. [Background technology]

[0002] FIG. 1 shows a schematic cross-sectional view of a magnetic tunnel junction (MTJ) sensing element 10. The term MTJ is used to refer to a tunneling magnetoresistance (TMR) sensing element. The MTJ sensing element 10 includes a tunnel barrier layer 22 sandwiched between a sense ferromagnetic layer 21 having a sense magnetization 210 and a pinned ferromagnetic layer 23 having a pinned magnetization 230. The sense magnetization 210 is configured such that its orientation can change in an external magnetic field 42 while the orientation of the pinned magnetization 230 remains unchanged. In this configuration, an external magnetic field H can be used to switch the sense magnetization 210 from an orientation parallel to (same direction and parallel with) the pinned magnetization 230 to an orientation antiparallel to (opposite direction and parallel with) the pinned magnetization 230, or vice versa.

[0003] FIG. 2 shows a response curve of resistance R versus an external magnetic field H. The magnetoresistance response curve shows the change in resistance as a function of the angle between the orientations of the sense magnetization 210 and the pinned magnetization 230. When the sense magnetization 210 is parallel to the pinned magnetization 230, the resistance of the magnetic random access memory (MRAM) cell 10 is low (logic state “0”). When the sense magnetization 210 is antiparallel (opposite and parallel) to the pinned magnetization 230, the resistance of the MRAM cell 10 is high (logic state “1”). When the sense magnetization 210 and the pinned magnetization 230 are at a perpendicular angle, the resistance is intermediate between the high and low resistances. Around this value, the MTJ sensing element 10 has a linear response. The slope of the linear portion LP of the response curve is proportional to the sensitivity of the MTJ sensing element 10.

[0004] Compared with Hall effect elements and giant magnetoresistance (GMR) sensing elements, MTJ sensor elements have better temperature stability, higher sensitivity, lower power consumption, better linearity, and do not require additional flux concentrator structures. Compared with anisotropic magnetoresistance (AMR) elements, MTJ sensor elements have a wider linear range and do not require set or reset coils.

[0005] In circuit applications that modify the response signal of the MTJ sensing element 10, a resistive bridge is often employed to facilitate amplification of its output voltage. This also allows for common-mode signal reduction (signals flowing in the same direction), noise cancellation, reduced temperature drift, and minimization of other probe imperfections. MTJ sensing elements 10 may be connected in parallel and / or series to form a Wheatstone bridge or any other type of circuit bridge.

[0006] 3 shows a half-bridge MTJ sensing circuit 100. One of the two MTJ sensing elements 10 may have its pinned magnetization 230 rotated orthogonally to the pinned magnetization 23 of the other MTJ sensing element 10. The sense axis 250 of the MTJ sensing circuit 100 coincides with the direction of the pinned magnetization 230 of the top MTJ sensing element 10.

[0007] The MTJ sensing circuit 100 may be arranged as a full (Wheatstone) bridge circuit configuration consisting of two MTJ sensing elements 10 connected in series and two MTJ sensing elements 10 connected in series, as shown in FIG.

[0008] The MTJ sensing circuit 100 has three external contact pads, namely, a bias voltage V bias and an input section 101 to which an output voltage V outand ground. For a given value of the external magnetic field H, along the positive sense axis 250, the resistance increases for one (or two) MTJ sensing elements 10 and decreases for the other (of the two). When the external magnetic field H is applied in the opposite direction (along the negative sense axis 250), the resistance decreases for one (or two) and increases for the other (of the two).

[0009] The output voltage V of the MTJ detection circuit 100 out can be measured using many common methods. For example, V out A voltmeter is connected between V and ground. out The potential difference between this and ground is the output voltage.

[0010] The sensitivity of the magnetic sensor is mV / mT / V bias This is expressed as the ratio of the output voltage V from the MTJ sensing circuit 100 to the change in the external magnetic field H. out The sensitivity of the MTJ sensing element 10 and the MTJ sensing circuit 100 is ratiometric and is defined as a function of the bias voltage V bias The output voltage V from the MTJ sensing circuit 100 can be expressed as:

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[0011] Operating the MTJ sensing circuit 100 over a wide temperature range can result in temperature-induced sensitivity variations, which can hinder the accuracy of the MTJ sensing circuit. The operating temperature of the MTJ sensing circuit 100 can vary significantly, from -40°C to +155°C. Temperature variations also change the sensitivity of the MTJ sensing circuit 100. The sensitivity variation can be proportional to temperature with a linear slope or a high-order curve. The temperature coefficient of sensitivity can be as high as 500 ppm / °C (ppm / °C).

[0012] 5 shows the response curves measured in the linear portion of the half-bridge MTJ sensing circuit 100. In particular, FIG. 4 shows the response curves measured at three different temperatures: −30° C., 30° C., and 125° C.

[0013] Operating a sensor over a wide temperature range can result in temperature-induced sensitivity variations, reducing the sensor's accuracy. In some cases, the output drift can vary by as much as 5%. Therefore, temperature compensation of the magnetic sensor's output is necessary.

[0014] Patent Document 1 (US2019154735) discloses a current sensor that outputs an output voltage according to the magnitude of the current being measured. The current sensor includes a conductor through which a current flows, a magnetic sensor, and a compensator. The magnetic sensor detects the strength of a magnetic field generated by the current and outputs an output voltage from the current sensor that corresponds to the strength of the magnetic field.

[0015] Patent Document 2 (US2019339337) discloses a calibration device for calibrating a magnetic sensor configured to generate an output signal indicative of a magnetic field strength when a bias signal is applied.

[0016] Patent document 3 (EP3457154) discloses an electric field sensor device comprising a reference electric field sensor biased with a reference current and providing a reference sensor signal in response to an electric field, and a calibrated electric field sensor biased with an individually adjustable current and providing a calibrated sensor signal in response to an electric field. [Prior art documents] [Patent documents]

[0017] [Patent Document 1] US Patent Application Publication No. 2019 / 154735 [Patent Document 2] US Patent Application Publication No. 2019 / 339337 [Patent Document 3] European Patent Application Publication No. 3457154 Summary of the Invention

[0018] The present disclosure provides an MTJ sensing circuit for measuring an external magnetic field, the MTJ sensing circuit comprising a plurality of MTJ sensor elements connected in a bridge configuration. The MTJ sensing circuit has an input for receiving a bias voltage for the MTJ sensing circuit and generating an output voltage proportional to the external magnetic field by multiplying the bias voltage by a gain sensitivity of the MTJ sensing circuit, where the gain sensitivity and the output voltage vary with temperature. The MTJ sensing circuit further comprises a temperature compensation circuit configured to provide a modulated bias voltage that varies as a function of temperature over a temperature range such that the output voltage is substantially constant as a function of temperature.

[0019] The present disclosure further relates to a method for compensating an output voltage for temperature changes.

[0020] The invention will be better understood with the help of the description of an embodiment given by way of example and illustrated by the figures. [Brief explanation of the drawings]

[0021] [Figure 1] FIG. 1 shows a schematic cross-sectional view of an MTJ sensing element. [Figure 2] FIG. 2 shows the response curve of the resistance versus external magnetic field of the MTJ sensing element. [Figure 3] FIG. 3 shows a half-bridge MTJ sensing circuit. [Figure 4] Figure 4 shows a full-bridge MTJ sensing circuit. [Figure 5] Figure 5 shows the response curve of the half-bridge MTJ sensing circuit measured in the linear section. [Figure 6] FIG. 6 illustrates a schematic representation of the method steps for temperature compensated measurement of an external magnetic field according to one embodiment. [Figure 7] FIG. 7 reports the gain sensitivity in an MTJ sensing circuit that has been corrected for temperature changes according to the methods disclosed herein. [Figure 8] FIG. 8 shows a correction circuit according to one embodiment. [Figure 9] FIG. 9 illustrates a temperature sensor circuit according to one embodiment. [Figure 10] FIG. 10 shows a temperature sensor circuit according to another embodiment. [Figure 11] FIG. 11 shows a temperature sensor circuit according to yet another embodiment. [Figure 12] FIG. 12 shows a temperature sensor circuit according to yet another embodiment. [Figure 13] FIG. 13 shows the response curve of an MTJ sensing circuit where the bias voltage is not modulated by temperature. [Figure 14] FIG. 14 shows the response curve of an MTJ sensing circuit in which the bias voltage is modulated by temperature. DETAILED DESCRIPTION OF THE INVENTION

[0022] The temperature change may be proportional to the temperature, with a linear slope or a higher order curve.

[0023] The gain sensitivity G(t) varies as a function of temperature T as follows:

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[0024] In one embodiment, a method for measuring a temperature-compensated external magnetic field H using the MTJ sensing circuit 100 includes: bias The bias voltage V is set so that the value obtained by multiplying by the gain sensitivity G(T) is constant. bias with a temperature T.

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[0025] This method involves applying a modulated bias voltage V bias (T) to provide an output voltage V outThe method further comprises the step of measuring:

[0026] Modulation bias voltage V bias By providing a temperature T, the MTJ sensing circuit 100 provides an output voltage V that varies with an external magnetic field H but remains approximately constant as the temperature T changes. out can be output.

[0027] The method further comprises measuring the temperature T of the MTJ sensing circuit 100 and providing a compensated temperature signal 30 (see FIG. 10) proportional to the temperature T of the MTJ sensing circuit 100.

[0028] This method uses a compensated temperature signal to generate a bias voltage V bias and generate the bias voltage V bias and modulates the bias voltage V bias (T) bias (T) is provided by using a compensated temperature signal in combination with a temperature function f(T), such as a linear slope or a higher order curve.

[0029] FIG. 6 illustrates the method steps, particularly the step of measuring the temperature T of the MTJ sensing circuit 100 (M1) and the step of measuring the bias voltage V bias and a step (M2) of generating a modulated bias voltage V bias (T) to the input 101 of the MTJ sensing circuit 100; and bias Schematically shows step (M4) of inputting (T).

[0030] An analog circuit is used to measure the temperature of the MTJ sensing circuit 100 and adjust the bias voltage V bias generating a modulated bias voltage V bias The step of providing (T) can be performed.

[0031] 7 reports the gain sensitivity G(t) for the MTJ sensing circuit 100, which has been corrected for temperature changes according to the methods disclosed herein. The corrected gain sensitivity G(t) is substantially independent of temperature changes (compensating for the loss in sensitivity G(T)).

[0032] FIG. 8 illustrates the output voltage V of the MTJ sensing circuit 100 according to one embodiment. out 3 shows a correction circuit 300 that compensates for temperature changes. The correction circuit 300 includes a temperature sensor circuit 301 (reference generator) configured to generate a voltage reference V proportional to temperature T, which has been corrected for temperature changes according to the method disclosed herein. The correction circuit 300 also includes a modulated bias voltage V bias The correction circuit 300 further comprises a bias voltage control circuit 302 (MTJ sensing circuit regulator) configured to provide an output voltage V out The correction circuit 300 may be designed as a CMOS on-chip device.

[0033] In general, the MTJ sensing circuit 100 has a sensitivity G(T) that follows a monotonically decreasing function as the temperature T increases. More specifically, the sensitivity G(T) varies more strongly at temperatures below 25° C. and less strongly at temperatures above 25° C. Thus, the temperature function f(T) of the sensitivity G(T) is not linear with temperature.

[0034] In one embodiment, a constant (non-modulated) bias voltage V bias is input to the MTJ sensing circuit 100. The sensitivity G(T) of the MTJ sensing circuit 100 is then measured at regular intervals over a range of temperatures T. The sensitivity G(T) is expressed in mV / V / mT, i.e., the ratio of bias voltage V to a given magnetic field mT. bias Output V out The measured sensitivity G(T) is then tabulated and normalized to 25°C to determine the normalization factor for each interval. The normalized factor is then inverted and the bias voltage V biasThis results in a flat sensitivity G(T) with respect to temperature T.

[0035] In another embodiment, the output voltage V out is the output voltage V out increases with increasing temperature to obtain a flat sensitivity G(T) over temperature, in other words, to compensate for the loss of sensitivity G(T). The linear temperature function f(T) may be based on a first-order curve fit.

[0036] In yet another embodiment, the bias voltage V bias varies with a positive temperature slope complementary to the loss in sensitivity G(T) with temperature. In this configuration, the MTJ sensing circuit 100 operates with a bias voltage V bias The output voltage V out It is used as a scaling multiplier.

[0037] This can be achieved by applying an adjustable temperature independent voltage to the MTJ sensing circuit 100 (biasing the MTJ sensing circuit 100) mixed with a temperature dependent voltage proportional to absolute temperature (PTAT).

[0038] More specifically, the temperature sensor circuit 301 (reference generator) is combined with the bias voltage control circuit 302 (MTJ sensing circuit regulator) into a single stage circuit (see FIG. 9) using a single operational amplifier (op-amp). The input to this stage is a temperature independent current (I polyDown ) and PTAT current (I ptatRef ) combination. This stage has an array of selectable current mirrors (current DACs) that act as DACs and are injected into the op-amp circuit. Two current sources (I polyoffset , I ptat ) to convert the currents into summed voltages to obtain the desired temperature-dependent output reference (V bias ) is generated. The 5-bit code is the bias voltage V bias and another 5-bit code adjusts the bias voltage V biasThe PTAT component of the voltage is adjusted. PTAT cannot be used alone, so both PTAT and a fixed voltage are required. In fact, for PTAT voltage, the stronger the slope, the greater the voltage at 25°C. The voltage and slope are dependent functions.

[0039] In yet another embodiment, the linear temperature function f(T) has two different slopes: a first slope for temperatures T below 25° C. and a second slope for temperatures T equal to or greater than 25° C. The two-slope linear temperature function f(T) allows for better and more effective temperature compensation.

[0040] This embodiment can be implemented similarly to the third variant, but for two different slopes of the temperature function f(T). ptatX This shows how to determine the IC temperature T using the PTAT current Iptat into the resistor R ptatX The voltage across √{square root over (√Hz)} is directly sized with respect to temperature T (in Kelvin). Placing a comparator 311 on this temperature dependent voltage allows the selection of two different sets of 5-bit codes for the current DAC that change the compensation slope when the temperature transitions to 25°C.

[0041] A double slope approximation to the ideal correction curve can be implemented. Adding a filter to the output of the bias voltage control circuit 302 (regulator output) helps reduce glitches during transitions. The comparator 311 has hysteresis to avoid chattering near the transitions. The circuit of Figure 11 can still be used, but now the two sets of 5-bit codes are changed when the temperature exceeds 25°C. The number of bits does not necessarily have to be 5 (it could be 6, for example), this is specific to the required specifications for the implementation. The comparator output V comp A bidirectional digital multiplexer (not shown), triggered by the temperature (labeled "Temperature above 25C" in Figure 12), selects between two different sets of digital numbers, either stored in memory or hardwired.

[0042] In yet another embodiment, the loss of sensitivity G(T) with temperature is characterized for several temperature subranges. For each temperature subrange, the reference voltage V ref is determined and input to the bias amplifier of the MTJ sensing circuit 100. Here, the reference voltage V ref varies for each temperature subrange according to a digital look-up table (LUT). For example, in the temperature range from -40°C to 125°C, the reference voltage V ref and bias voltage V bias The temperature sub-range need not be 10° C., but may have any suitable value, such as 20° C., 30° C., etc., depending on the degree of accuracy desired for temperature compensation of the MTJ sensing circuit 100.

[0043] To improve the compensation scheme by accounting for nonlinearities in the sensitivity versus temperature curve, a piecewise linear approximation of the ideal inverse of the temperature sensitivity profile can be implemented. In the illustrated example of Figure 11, the temperature range from -40°C to 125°C is divided into 16 temperature subranges of 10°C. Multiple temperature-dependent voltages V_ Ti (V-40℃, V-30℃, ...V110℃, V120℃) are multiple sub-range resistance R ptat_i (One sub-range resistor R for each temperature-dependent voltage ptat_i ) into the resistor string consisting of PTAT current I pat The temperature dependent voltage V_Ti is generated by forcing a plurality of fixed voltage references V at a plurality of comparators 312. Ref_Ti (Vref-30℃, Vref-20℃, ... Vref110℃, Vref120℃) are compared in groups, and multiple hysteresis comparator outputs T aboveM3_Ti (T aboveM30℃ ,T aboveM20℃ ,…T aboveM110℃ ,T aboveM120℃ ) result.

[0044] Multiple hysteresis comparator outputs T aboveM3_Tiare connected to an array-based memory cell 315 (select 0, select 1, ... select 14, select 15) that selects a priority from 16 encoded memory locations based on the highest logic HI input. The array-based memory cell 315 has 16 registers, each of which is responsible for the desired analog bias voltage V for the MTJ sensing circuit 100. bias is represented digitally.

[0045] FIG. 12 shows a voltage reference V for the temperature sensor circuit 301 that changes 16 digital codes every 10°C. Ref_Ti The exact number of registers and temperature subranges is adjustable and depends on the desired resolution of the temperature compensation.

[0046] In an alternative configuration, a pair of comparators 312 provides a display area over a 10° C. temperature span. The pair of comparators 312 may be arranged so that the display area span is moved to different temperature locations based on the measured temperature. In such a configuration, only two comparators 312 are required for any number of temperature subranges required.

[0047] FIG. 13 shows the response curves of the MTJ sensing circuit 100 measured in the linear portion at room temperature RT, 125° C., and −20° C., where the bias voltage V bias is not modulated by temperature.

[0048] FIG. 14 shows the response curves of the MTJ sensing circuit 100 measured in the linear portion at room temperature RT, 125° C., 55° C., 85° C., −10° C., and −20° C., where the bias voltage V bias is modulated by temperature in accordance with the methods disclosed herein. Figure 8 shows the significant improvement in temperature compensation behavior of sensitivity observed using the MTJ sensing circuit 100 and the methods disclosed herein.

[0049] It is understood that the invention is not limited to the exemplary embodiments described above, but that other embodiments are possible within the scope of the claims.

[0050] For example, the method of the present invention can be used to perform similar temperature corrections for other types of sensors that have predictable temperature coefficients. [Explanation of symbols]

[0051] 10 MTJ sensing element 100 MTJ detection circuit 101 Input section 102 Output 2. Magnetic tunnel junction 21 Sense ferromagnetic layer 210 Sense Magnetization 22 Tunnel Barrier Layer 23 Pinned ferromagnetic layer 230 Pinned Magnetization 250 detection axis 30 Correction temperature signal 300 Correction Circuit 301 Temperature Sensor 302 Bias voltage control circuit 303 Analog Front-End Amplifier 311 Comparator 312 Comparators 315 Array-based memory cells H external magnetic field LP linear part T temperature V bias bias voltage V bias (T) Modulation bias voltage V out Output Voltage V Ref Voltage Reference

Claims

1. 1. An MTJ sensing circuit for measuring an external magnetic field, comprising: the MTJ sensing circuit comprises a plurality of MTJ sensor elements connected in a bridge configuration; the MTJ sensing circuit has an input for receiving a bias voltage and multiplying the bias voltage by a gain sensitivity of the MTJ sensing circuit to generate an output voltage proportional to an external magnetic field, wherein the gain sensitivity and the output voltage vary with temperature; the MTJ sensing circuit further comprising a temperature compensation circuit configured to provide a modulated bias voltage that varies as a function of temperature over a temperature range such that the output voltage is substantially constant as a function of temperature; The MTJ sensing circuit.

2. 2. The MTJ sensing circuit of claim 1, wherein the temperature compensation circuit is configured to provide the modulated bias voltage with a slope of a linear function of temperature across the temperature range, the slope of temperature being complementary to a variation of the gain sensitivity with temperature.

3. The MTJ sensing circuit of claim 2 , wherein the temperature compensation circuit comprises a temperature sensor circuit configured to measure the gain sensitivity over the temperature range.

4. 3. The MTJ sensing circuit of claim 2, wherein the temperature compensation circuit is configured to combine an adjustable temperature independent current and a temperature dependent proportional to absolute temperature (PTAT) current.

5. 5. The MTJ sensing circuit of claim 4, wherein the temperature compensation circuit comprises an operational amplifier to which the temperature independent current and the PTAT current are input, the operational amplifier converting the temperature independent current and the PTAT current into a voltage that generates the modulated bias voltage.

6. 2. The MTJ sensing circuit of claim 1, wherein the temperature compensation circuit is configured to provide the modulated bias voltage using a linear temperature function having a first slope and a second slope different from the first slope, the transition from the first slope to the second slope occurring at a predetermined transition temperature.

7. 7. The MTJ sensing circuit of claim 6, wherein the temperature compensation circuit comprises a PTAT current through a resistor to provide a temperature dependent voltage, the temperature dependent voltage being input to a comparator to change from the first slope to the second slope at the transition temperature.

8. 7. The MTJ sensing circuit of claim 6, wherein the transition temperature is about 25°C.

9. 2. The MTJ sensing circuit of claim 1, wherein the temperature compensation circuit is configured to generate a plurality of reference voltages, each reference voltage corresponding to a temperature sub-range of the temperature range, each reference voltage being determined for each temperature sub-range according to a digital look-up table, and providing a bias voltage.

10. 10. The MTJ sensing circuit of claim 9, wherein the temperature span of each temperature sub-range is approximately 10 degrees Celsius.

11. 2. The MTJ sensing circuit of claim 1, wherein the temperature range extends from -40°C to 125°C.

12. 1. A method for compensating an output voltage of an MTJ sensing circuit, comprising: the MTJ sensing circuit comprises a plurality of MTJ sensor elements connected in a bridge configuration; the MTJ sensing circuit has an input section for receiving a bias voltage and multiplying the bias voltage by a gain sensitivity of the MTJ sensing circuit to generate an output voltage proportional to an external magnetic field, wherein the gain sensitivity and the output voltage vary with temperature; the MTJ sensing circuit further comprising a temperature compensation circuit configured to provide a modulated bias voltage that varies as a function of temperature over a temperature range; The method comprises: measuring the temperature of the MTJ sensing circuit; using a temperature compensation circuit to modulate the bias voltage with temperature so that the modulated bias voltage multiplied by the gain sensitivity remains constant; The method comprising:

13. providing a compensated temperature signal proportional to the measured temperature; modulating the bias voltage using a compensated temperature signal in combination with a temperature function; The method of claim 12, comprising:

14. The method of claim 13 , wherein the temperature function comprises a linear temperature function over a temperature range with a slope.

15. 14. The method of claim 13, wherein the temperature function comprises a linear temperature function over a temperature range having a first slope and a second slope different from the first slope.

16. The temperature compensation circuit is configured to generate a plurality of reference voltages, each reference voltage corresponding to a temperature sub-range of a temperature range, and the method further comprises:

13. The method of claim 12, wherein adjusting the bias voltage with temperature comprises determining a voltage reference for each temperature subrange according to a digital lookup table and providing the bias voltage.

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