Sense amplifier circuit with temperature compensation

1. Field of the Invention

The present invention is related to a sense amplifier circuit with temperature compensation, and more particularly, to a sense amplifier circuit for use in MRAM and with temperature compensation.

2. Description of the Prior Art

Traditional memory normally stores data by charging/discharging capacitors, while magnetoresistive random-access memory (MRAM) stores data in magnetic domains. MRAM is characterized in high-speed data transmission, high cell density, light weight, low power consumption and high impact resistance, and thus particularly suitable for implementing on high-end portable electronic products.

FIG. 1 is a diagram illustrating a prior art MRAM array using spin-torque-transfer (STT) technique. The MRAM array includes a plurality of MRAM cells 10 , a plurality of word lines WL 1 -WLm, a plurality of sensing word lines SWL 1 -SWLm, a plurality of bit lines BL 1 -BLn, and a plurality of sense amplifiers SA 1 -SAn. Each MRAM cell 10 includes a select transistor 12 and a magnetic tunnel junction (MTJ) transistor 14 . Each select transistor 12 includes a first end coupled to a corresponding bit line, a second end coupled to a corresponding MTJ transistor 14 , and a control end coupled to a corresponding word line. Each MTJ transistor 14 includes a first end coupled to a corresponding bit line and a second end coupled to the second end of a corresponding select transistor 12 . Each MTJ transistor 14 includes a data ferromagnetic layer, a reference ferromagnetic layer, and a tunnel barrier layer. By applying an appropriate switching pulse via a corresponding bit line during the write operation, the magnetization direction of the data ferromagnetic layer may be varied so that the resistance of the MTJ transistor 14 may be switched between logic “0” state and logic “1” state, thereby storing bit information.

When the magnetization direction of the data ferromagnetic layer is aligned in an anti-parallel direction with respect to the magnetization direction of the reference ferromagnetic layer in the MTJ transistor 14 , the tunnel barrier layer has high impedance R H , thereby putting the MTJ transistor 14 in an OFF state for storing logic “1” data. When the magnetization direction of the data ferromagnetic layer is aligned in a parallel direction with respect to the magnetization direction of the reference ferromagnetic layer in the MTJ transistor 14 , the tunnel barrier layer has low impedance R L , thereby putting the MTJ transistor 14 in an ON state for storing logic “0” data. The impedance of the MTJ transistor 14 has a negative temperature coefficient, which means which means the high impedance R H and the low impedance R L of different states decrease as the temperature increases.

During the read operation, the select transistor 12 of a specific MRAM cell 10 is conducting so that the bit information stored in the corresponding MTJ transistor 14 may be transmitted to a corresponding bit line. The sense amplifiers SA 1 -SAn are respectively coupled to the bit lines BL 1 -BLn for receiving an activation signal SEN and respectively receiving a plurality of reference voltages REF 1 -REFn. Each sense amplifier is configured to amplify the current of a corresponding bit line in response to the activation signal SEN and determine the logic state of the MTJ transistor 14 of the corresponding MRAM cell 10 based on a corresponding reference voltage and the current flowing through a corresponding bit line, thereby outputting the corresponding sense amplifier signals SA_OUT 1 -SA-OUTn.

Each of the prior art sense amplifiers SA 1 -SAn includes four reference MTJ transistors 24 , wherein two reference MTJ transistors are set in the ON state and the other two reference MTJ transistors are set in the OFF state for providing reference impedance Rref whose value is equal to (R H +R L )/2. Due to process or material variations, different reference MTJ transistors may have different switching characteristics and thus different reference impedance Rref, which prevent the sense amplifiers from sensing the data stored in the MRAM cells accurately.

The present invention provides a sense amplifier circuit with temperature compensation. The sense amplifier circuit includes a sense amplifier, a switch and a temperature compensation circuit. The sense amplifier includes a first input end coupled to a target bit having high impedance in a first state and having low impedance in a second state, a second input end and an output end, wherein the high impedance has a first temperature coefficient and the low impedance has a second temperature coefficient. The switch is configured to operate according to a first control signal for providing reference impedance having a third temperature coefficient, and includes a first end coupled to the second input end of the sense amplifier, a second end and a control end for receiving the first control signal. The temperature compensation circuit is coupled to the control end of the first switch and configured to provide the first control signal having a fourth temperature coefficient. The first temperature coefficient, the second temperature coefficient and the third temperature coefficient have a same polarity. The fourth temperature coefficient and the first temperature coefficient have opposite polarities.

These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.

FIG. 1 is a diagram illustrating a prior art MRAM array using spin-torque-transfer technique.

FIG. 2 is diagram illustrating a sense amplifier circuit according to an embodiment of the present invention.

FIG. 3 is diagram illustrating the time-varying characteristic of the voltage established across the second transistor according to an embodiment of the present invention.

FIG. 4 is diagram illustrating the time-varying characteristic of the value of V T *Ln(n)*(1+R 1 /R 2 ) according to an embodiment of the present invention.

FIG. 5 is diagram illustrating the time-varying characteristic of the first control signal according to an embodiment of the present invention.

FIG. 6 is diagram illustrating the time-varying characteristic of the reference impedance provided by the switch, the high impedance of the target bit and the low impedance of the target bit according to an embodiment of the present invention.

FIG. 2 is diagram illustrating a sense amplifier circuit 100 according to an embodiment of the present invention. The sense amplifier circuit 100 includes a sense amplifier SA, switches SW 1 -SW 3 and a temperature compensation circuit 20 .

The sense amplifier SA includes a first input end coupled to a target bit TB, a second input end coupled to the switch SW 1 , and an output end for outputting a sense amplifier signal SA_OUT. The target bit may be an MTJ transistor of an MRAM cell which has high impedance R H in the OFF state and low impedance R L in the ON state. The impedance of the target bit TB has a negative temperature coefficient, which means the high impedance R H and the low impedance R L of different states decrease as the temperature increases.

The switch SW 1 includes a first end coupled to the second input end of the sense amplifier SA, a second end coupled to the switch SW 2 , and a control end for receiving a first control signal V BIAS1 The switch SW 1 is configured to provide reference impedance Rref having a negative temperature coefficient according to the first control signal V BIAS1 . The switch SW 2 includes a first end coupled to the second end of the switch SW 1 , a second end coupled to a ground voltage GND, and a control end for receiving a second control signal V BIAS2 . The switch SW 3 includes a first end coupled to the target bit TB, a second end coupled to the ground voltage GND, and a control end for receiving a third control signal V BIAS3 .

In an embodiment of the present invention, the switches SW 1 -SW 3 may be metal-oxide-semiconductor field-effect transistors (MOSFETs). Compared to the MTJ transistors adopted in the prior art, the characteristic of the switch SW 1 is relatively insensitive to process variations. The switch SW 1 can provide the reference impedance Rref having a negative temperature coefficient when operating according to the first control signal V BIAS1 which can reflect temperature variations.

In an embodiment of the present invention, the temperature compensation circuit 20 includes an operational amplifier OA, resistors R 1 -R 3 , a first transistor BJT 1 and N second transistors BJT 2 , wherein N is an integer larger than 1. The first end of the first transistor BJT 1 is coupled to the first input end (positive input end) of the operational amplifier OA, and the second end and the control end of the first transistor BJT 1 are coupled to the ground voltages GND. The first end of each second transistor BJT 2 is coupled to the second input end (negative input end) of the operational amplifier OA via the resistor R 3 , and the second end and the control end of each second transistor BJT 2 are coupled to the ground voltages GND, wherein VZ represents the voltage established on the second end of the second transistor BJT 2 . The resistor R 1 includes a first end coupled to the first input end of the operational amplifier OA and a second end coupled to the output end of the operational amplifier OA. The resistor R 2 includes a first end coupled to the second input end of the operational amplifier OA and a second end coupled to the output end of the operational amplifier OA. The operational amplifier OA is configured to provide the first control signal V BIAS1 according a voltage VX established on its first input end and a voltage VY established on its second input end.

In an embodiment of the present invention, the first transistor BJT 1 and the second transistor BJT 2 may be bipolar junction transistors (BJTs) each having a collector (first end), an emitter (second end) and a base (control end). Since the collector and the base of the first transistor BJT 1 are coupled to each other, the voltage established across the first transistor BJT 1 is equal to its base-emitter voltage V BE1 . Since the collector and the base of the second transistor BJT 2 are coupled to each other, the voltage established across the second transistor BJT 2 is equal to its base-emitter voltage V BE2 .

Due to the virtual short characteristic of the operational amplifier OA, its first input end and its second input end are kept at the same voltage level, which results in VX=VY=V BE1 . On the other hand, the voltage VZ established on the second end of the second transistor BJT 2 is equal to V BE2 . The voltage difference (VY−VZ) established across the resistor R 3 results in current Iptat having a positive temperature coefficient. The value of the current Iptat may be represented by the following equation (1), wherein V T represents thermal voltage and n represents the area ratio of the second transistors BJT 2 to the first transistor BJT 1 .

Iptat=( VY−VZ )/ R 3=( V BE1 −V BE2 )/ R 3= V T *Ln( n )  (1)

The value of the first control signal V BIAS1 outputted by the temperature compensation circuit 20 may be represented by the following equation (2). The variation of the first control signal V BIAS1 with time may be represented by the following equation (3)

V BIAS1 =V BE2 +V T *Ln( n )*(1+ R 1/ R 2)  (2)

∂ V BIAS1 /∂T=∂V BE2 /∂T +(∂ V T /∂ T )  (3)

FIG. 3 is diagram illustrating the time-varying characteristic of the voltage V BE2 established across the second transistor BJT 2 according to an embodiment of the present invention. FIG. 4 is diagram illustrating the time-varying characteristic of the value of V T *Ln(n)*(1+R 1 /R 2 ) in equation (2) according to an embodiment of the present invention. As well-known to those skilled in the art, the base-emitter voltage of a BJT has a negative temperature coefficient, and the thermal voltage V T has a positive temperature coefficient.

As depicted in FIG. 3 , FIG. 4 and equation (2), the value of the first control signal V BIAS1 is determined by the voltage V BE2 having a negative temperature coefficient and V T *Ln(n)*(1+R 1 /R 2 ) having a positive temperature coefficient. By adequately selecting the number N of the transistors BJT 2 (which adjusts the area ratio of the second transistors BJT 2 to the first transistor BJT 1 ) and the values of the resistors R 2 -R 3 , (∂V BIAS1 /∂ T ) in equation (3) may be set to 0 so that the first control signal V BIAS1 can have a positive temperature coefficient, as depicted in FIG. 5 .

FIG. 6 is diagram illustrating the time-varying characteristic of the reference impedance Rref provided by the switch SW 1 , the high impedance R H of the target bit TB and the low impedance R L of the target bit TB according to an embodiment of the present invention. By adjusting the value of R 2 /R 3 and the value of V BE2 , the value of the reference impedance Rref may be between the high impedance R H and the low impedance R L at any temperature.

In the present invention, the temperature compensation circuit 20 may adopt any type of bandgap reference circuit capable of providing the first control signal V BIAS1 having a positive temperature coefficient. However, the implementation of the temperature compensation circuit 20 does not limit the scope of the present invention.

In conclusion, the present invention adopts the switch SW 1 whose characteristic is relatively insensitive to process variations for providing the reference impedance Rref, and adopts the temperature compensation circuit 20 for providing the first control signal V BIAS1 having a positive temperature coefficient. Therefore, when the switch SW 1 operates according to the first control signal V BIAS1 , the influence of temperature variations on the reference impedance Rref may be compensated, thereby improving the accuracy of the present sense amplifier circuit.

Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.