Sense amplifier incorporating a symmetric midpoint reference

[0002] This invention relates to Magnetoresistive Random Access Memories (MRAMs) and other memories where the memory bit has at least two distinct resistance states, and more particularly to sense amplifier circuits for such memories.

[0003] Non-volatile memory devices, such as FLASH memories, are extremely important components in electronic systems. FLASH is a major non-volatile memory device in use today. Disadvantages of FLASH memory include high voltage requirements and slow program and erase times. Also, FLASH memory has a poor write endurance of 10⁴-10⁶cycles before memory failure. In addition, to maintain reasonable data retention, the scaling of the gate oxide is restricted by the tunneling barrier seen by the electrons. Hence, FLASH memory is limited in the dimensions to which it can be scaled.

[0004] To overcome these shortcomings, other types of nonvolatile memories are being evaluated. One such device is magnetoresistive RAM (hereinafter referred to as “MRAM”). To be commercially practical, however, MRAM must have comparable memory density to current memory technologies, be scalable for future generations, operate at low voltages, have low power consumption, and have competitive read/write speeds.

[0005] MRAM bit cells store data by varying the resistance of a magnetic tunnel junction (MTJ) between low (R) and high (R+dR) states. For most memories, the state of the memory cell is determined by comparing the memory cell to a reference, often a midpoint value. Accordingly, for MRAM, the reference is developed as a midpoint reference, an average of high and low states, to provide a mechanism to determine the stored value in a cell. In U.S. Pat. No. 6,236,611 entitled “Peak Program Current Reduction Apparatus and Method” by Naji a solution is offered wherein a combination of four reference bits, two high and two low, provide a midpoint reference. However, MTJ resistances are not linear. The series and parallel combination used by Naji does not produce a true midpoint reference. There is an asymmetry between the difference from the high value to the reference and the difference from the reference to the low value. Additionally, symmetry in bit line capacitance is desired and is more problematic with references using multiple resistances. In U.S. Pat. No. 6,269,040 by Reohr et al. entitled “Interconnection Network For Connecting Memory Cells to Sense Amplifiers”, a memory circuit is disclosed in which a midpoint reference is obtained by averaging memory reference cells, one high and one low. The midpoint reference shares signals between references from adjacent arrays via an interconnect network that is almost but not fully balanced. In the Reohr et al. memory, two sense amplifiers are required to perform the averaging. Thus, a need remains for a sensing circuit using a midpoint reference that requires a minimum of area, provides a true nearby midpoint reference, and maintains symmetry in the circuit path for balanced loading including parasitic capacitances and resistances.

[0006] The foregoing and further and more specific objects and advantages of the instant invention will become readily apparent to those skilled in the art from the following detailed description of the invention taken in conjunction with the following drawings:

[0007] FIG. 1 is a schematic diagram of a sense amplifier in accordance with the present invention; and

[0008] FIG. 2 is schematic diagram of another form of the sense amplifier in accordance with the present invention.

[0009] Skilled artisans appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale.

[0010] FIG. 1 illustrates a memory sense amplifier 10, a voltage bias circuit 11, and resistances R_B, R_H, R_Lassociated with a memory array. An operational amplifier 12 has a positive or a first input connected to a reference voltage, Vref, a negative or a second input, and an output connected to a control electrode or a gate of a P-channel transistor 14. The output of operational amplifier 12 provides the bias voltage of voltage bias circuit 11. A first current electrode or source of transistor 14 is connected to a first power supply terminal or supply voltage terminal for receiving a voltage V_DD. A second current electrode or drain of transistor 14 is coupled to the negative or second input of the operational amplifier 12 and coupled via one or more coupling devices (not shown) to a first terminal of a first reference high bit in the form of a resistance 18 that is a memory cell programmed to a high resistive state. Resistance 18 is therefore represented as having a resistance of value R_H. A second terminal of resistance 18 is coupled via one or more coupling devices (not shown) to a supply voltage terminal for receiving a voltage V_SS. A P-channel transistor 16 has a source connected to the V_DDsupply voltage terminal, a gate connected to the output of operational amplifier 12, and a drain is coupled to the negative or second input of the operational amplifier 12 and coupled via one or more coupling devices (not shown) to a first terminal of a first reference low bit or a resistance 20 that is a memory cell programmed to a low resistive state. Resistance 20 is therefore represented as having a resistance of value R_L. A second terminal of resistance 20 is coupled via one or more coupling devices (not shown) to the V_SSsupply voltage terminal.

[0011] Sense amplifier 10 functions to sense the memory state of the cell having a resistive value represented by R_B. Within sense amplifier 10, the output of operational amplifier 12 is connected to a gate of a P-channel transistor 22, to a gate of a P-channel transistor 24 and to a gate of a P-channel transistor 26. A source of each of transistors 22, 24 and 26 is connected to the V_DDsupply voltage terminal. A drain of transistor 22 is connected to a gate of a P-channel transistor 42 for providing a bias voltage, V_B. The drain of transistor 22 is coupled via one or more coupling devices (not shown) to a memory bit cell in the memory array of unknown memory state having a first terminal of a resistance 28 of value R_B. A second terminal of the memory bit cell 28 is coupled via one or more coupling devices (not shown) to the V_SSsupply voltage terminal. The drain of transistor 24 is coupled via one or more coupling devices (not shown) to a first terminal of a first reference memory cell 30 in the memory array that is in the high resistance state having the value R_H. A second terminal of a first reference memory cell 30 is coupled via one or more coupling devices (not shown) to the V_SSsupply voltage terminal. The drain of transistor 26 is coupled via one or more coupling devices (not shown) to a second reference memory cell 32 in the memory array that is in the low resistive state with the value R_L. A second terminal of the second reference memory cell 32 is coupled via one or more coupling devices (not shown) to the V_SSsupply voltage terminal. A P-channel transistor 34 has a source connected to the V_DDpower supply terminal, a gate connected to the drain of transistor 24, and a drain connected to a gate of an N-channel transistor 38 at a node 39. A P-channel transistor 36 has a source connected to the V_DDpower supply terminal, a gate connected to the drain of transistor 26, and a drain connected to the gate of transistor 38 at node 39 that functions as an output of sense amplifier 10. Transistor 34 conducts a current i_Hand transistor 36 conducts a current i_L. Transistor 38 has a drain connected to its gate thereof at node 39, and a source connected to the V_SSpower supply terminal. An N-channel transistor 40 has a gate connected to node 39, a source connected to the V_SSpower supply terminal, and a drain. A P-channel transistor 42 has a source connected to the V_DDpower supply terminal, a gate connected to the drain of transistor 22 for receiving the V_Bbias voltage, and a drain connected to the drain of transistor 40 at a node 41 that functions as an output of sense amplifier 10. Transistor 42 conducts a current equal to i_B. A P-channel transistor 43 has a source connected to the V_DDpower supply terminal, a gate connected to the drain of transistor 22 for receiving the V_Bbias voltage, and a drain connected to the drain of transistor 40 at node 41. Transistor 43 also conducts a current equal to i_B. An equalization switch 44 has a first terminal connected to node 39 and a second terminal connected to node 41. A control terminal of switch 44 is connected to an Equalization control signal, labeled EQ. A drain of an N-channel transistor 45 is connected to node 41 and to a gate thereof. A source of transistor 45 is connected to the V_SSpower supply terminal. A first terminal of a current source 47 for providing a current, iBias, is connected to the VDD power supply terminal. A second terminal of current source 47 is connected to the gate and drain of transistor 45. It should be well understood that current source 47 is optional. If current source 47 is not present, the current conducted by transistor 45 is [2i_B−(i_H+i_L)]. Sense amplifier 10 is a three-input sense amplifier wherein the three inputs are formed by the gate of transistor 34, the gate of transistor 36 and the common gates of transistors 42 and 43.

[0012] In operation, sense amplifier 10 is a three-input sense amplifier that detects a state of memory cell 28 by using current sources, summing and current doubling circuits in conjunction with a current differential amplifier. The three inputs are represented by V_H, V_Land V_B. The memory cell provides a memory cell current representing a resistance value, R_B, of the memory cell. Voltage bias circuit 11 functions to provide a bias voltage to transistors 22, 24 and 26 that provide a constant current source to each of resistances R_B, R_Hand R_Lfrom which voltages V_B, V_Hand V_Lare respectively developed. In other words, transistor 22 is a first constant current source, transistor 24 is a second constant current source and transistor 26 is a third constant current source. The bias voltage V_Bdrives P-channel current source transistors 42 and 43, and V_Hand V_Lrespectively drive P-channel current source transistors 34 and 36 in sense amplifier 10.

[0013] Transistors 34 and 36 form a summing circuit having a first input coupled to V_H, a second input coupled to V_Land an output formed by node 39.

[0014] Transistors 34 and 36 generate i_Hand i_L, respectively, that are reference currents corresponding to the reference bits R_Hand R_L. Therefore, transistor 34 is a first voltage to current converter and transistor 36 is a second voltage to current converter. Those currents are summed onto node 39 that functions as a summing node. Transistor 38 functions to mirror the summed reference current to transistor 40. That current is equal to (i_H+i_L). Transistors 42 and 43 each function as a voltage to current converter and develop the bit current i_Bthat is summed together at the output node 41 to form a current 2i_B.

[0015] Therefore, transistors 42 and 43 function as a current doubler. The current doubler is coupled between the output of the first constant current source, transistor 22, and the second input (node 41) of the current differential amplifier formed by transistors 38, 40 and 45. When the currents merge into output node 41, a difference current is conducted by transistor 45. The difference current is equal to [2i_B−(i_H+i_L)]. If 2i_Bis greater than (i_H+i_L), then the difference current will cause the capacitances at node 41 to charge so that the voltage at node 41 will be greater than the voltage at node 39.

[0016] Similarly, if 2i_Bis less than (i_H+i_L), then the difference current will cause the capacitances at node 41 to discharge so that the voltage at node 41 will be less than the voltage at node 39. Transistors 38, 40 and 45 in combination with current source 47 function as a current differential amplifier 15. It should be well understood that other circuit implementations may be used to implement a current differential amplifier. In the illustrated form, the physical sizing of transistors 38, 40 and 45, as accomplished by the transistor gate width and length dimensions, is important to the operation of the current differential amplifier in the proper region of current/voltage relationships of the low bit and high bit references. In particular, transistor 45 is sized significantly smaller than either transistor 38 or transistor 40 to function as a weak load device. Transistors 38 and 40 are matched to be substantially the same size and have the same current drive strength. A midpoint reference is obtained by summing the high and low reference currents. The midpoint reference current is compared with twice the bit current, 2i_B, by the current differential amplifier.

[0017] An effective midpoint is created by summing currents (i_H+i_L) at node 39. The midpoint is the average of the two currents. It represents the midpoint of the currents developed from the high and the low bit reference cells.

[0018] Voltage bias circuit 11 biases the current sources 22, 24 and 26 to obtain desired values for voltages V_B, V_Hand V_L. The desired values, by way of example only, may be approximately 300 mV relative to V_SS. Other voltages may be developed and used. It should be well understood that the desired values for these voltages is low enough to not damage the resistive memory cells, but high enough to obtain a significant differential between the voltages in order to obtain a suitable current differential in sense amplifier 10.

[0019] Symmetry of the reference voltages, V_Hand V_L, that are compared with the bit voltage, V_B, is desired. The voltages V_H, V_Land V_Bare converted to currents. In order to optimize a differential between the reference currents and either bit state that the bit current may assume, the optimal reference current value approximates an average of the reference currents, i_Hand i_L. The conversion to current introduces a square term as I∝(V_G−V_THP)². However, because the magnitude of the gate voltages, V_H, V_Land V_B, substantially exceeds the difference between these voltages, the reference current approximates a midpoint reasonably well.

[0020] Prior to a sensing operation, the EQ equalization signal is asserted for a predetermined equalization time period to make switch 44 conductive. Nodes 39 and 41 are equalized in potential prior to the current summing and sensing operation to improve the read speed.

[0021] In an alternative form, a sense amplifier 13 illustrated in FIG. 2 illustrates two alternatives to the circuitry of sense amplifier 10 of FIG. 1. Firstly, sense amplifier 13 functions without using voltage bias 11 of FIG. 1. In the alternative form of FIG. 2, similar elements in common with sense amplifier 10 are numbered the same for ease of comparison. In sense amplifier 13, each of P-channel transistors 22, 24 and 26 has its gate connected to the drain of transistor 24. It should be well understood that the implementation of transistors 22 and 26 would include additional dummy (i.e. not functionally used) gates to make the loading on the drains of each of transistors 22, 24 and 26 to be substantially the same. In addition, the sources of transistors 22, 24 and 26 may be connected to a V_BIASvoltage that is regulated (bias circuit not shown) to provide optimal operating points. In yet another alternative (not shown), the gates of each of transistors 22, 24 and 26 are respectively connected to their drains. In the later form, the transistors 22, 24 and 26 function as current mirror devices with respect to transistors 42 and 43, 34 and 36, respectively.

[0022] Secondly, sense amplifier 13 uses a different current differential amplifier than sense amplifier 10. An N-channel transistor 52 has a drain connected to node 39, a gate connected to node 41, and a source connected to the VSS supply voltage terminal. An N-channel transistor 54 has a drain connected to node 41, a gate connected to node 39, and a source connected to the VSS supply voltage terminal. An N-channel transistor 56 has a drain connected to a gate thereof and to node 39. Transistor 56 also has a source connected to the VSS supply voltage terminal. An N-channel transistor 58 has a drain connected to a gate thereof and to node 41. Transistor 58 also has a source connected to the VSS supply voltage terminal.

[0023] Transistors 52 and 54 function as a cross-coupled pair of transistors to differentiate which of two currents, 2iB and (iH+iL) has a greater magnitude. Depending upon which current has the greater magnitude will determine which logic value voltage will be present on the true and complement output terminals. The current differential amplifier implementation in FIG. 2 may be more balanced and hence parasitics may tend to be cancelled more favorably than in FIG. 1.

[0024] By now it should be apparent that there has been provided a three input sense amplifier for use in a memory such as an MRAM. By generating an effective midpoint reference to be used by the sense amplifier, the use of an externally supplied midpoint reference is avoided. External midpoint references are difficult to control and such references consume additional area on an integrated circuit than the circuitry used within the sense amplifier that permits the sense amplifier to generate an accurate midpoint reference. It is additionally preferable that sense amplifier 10 employs symmetry to minimize noise and optimize memory sense amplifier speed.

[0025] Various changes and modifications to the embodiments herein chosen for purposes of illustration will readily occur to those skilled in the art. For example, each of transistors 42 and 43 may be implemented as a single P-channel transistor having its physical dimensions sized to be twice the size of transistor 42 or transistor 43 in order to source a current of 2i_B. The sense amplifier and gain stage improvements disclosed herein are applicable to other memory types whose state is manifested as a change in the resistance value of the bit. Variations in the types of conductivities of transistors, the types of transistors (e.g. MOS, GaAs, bipolar, etc.), the sizing of the transistors, etc. may be readily made. Although specific logic circuits have been shown, numerous logic circuit implementations may be used to implement the functions discussed herein. To the extent that such modifications and variations do not depart from the spirit of the invention, they are intended to be included within the scope thereof that is assessed only by a fair interpretation of the following claims.

[0026] Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature or element of any or all the claims. As used herein, the terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. The terms a or an, as used herein, are defined as one or more than one. The term plurality, as used herein, is defined as two or more than two. The term another, as used herein, is defined as at least a second or more. The terms including and/or having, as used herein, are defined as comprising (i.e., open language). The term coupled, as used herein, is defined as connected, although not necessarily directly, and not necessarily mechanically.