A circuit for controlling a reference node in a sense amplifier

Abstract: A circuit (165) is proposed for controlling a reference node (Nr) in a sense amplifier (145) switchable between an operative condition and a stand-by condition, the reference node providing a reference voltage in the operative condition. The circuit includes means (Cp) for bringing the reference node to a starting voltage upon entry into the stand-by condition, first means (Rda1,Rda2) for keeping the reference node at a pre-charging voltage in the stand-by condition, second means (Rdb1,Rdb2) for providing a comparison voltage closer to the pre-charging voltage than the starting voltage, pulling means (Mu1) for pulling the reference node towards a power supply voltage, and control means (210) for activating the pulling means upon entry into the stand-by condition and for disabling the pulling means when the voltage at the reference node reaches the comparison voltage.

Description: The present invention relates to a circuit for controlling a reference node in a sense amplifier. Sense amplifiers are commonly used in several applications, for example for reading cells of a non-volatile memory device. A sense amplifier consists of a circuit that detects a low-level signal, which is compared with a pre-set reference voltage; the sense amplifier outputs a signal having two different values according to the result of the comparison. Typically, the sense amplifier enters a stand-by condition after a pre-set period has lapsed from a last reading operation. Particularly, most of the elements of the sense amplifier are switched off (bringing a reference node providing the reference voltage to a power supply reference value, or ground), in order to reduce the power consumption of the device embedding the sense amplifier, as well as the stress supported by reference cells commonly used for generating the reference voltage. When the sense amplifier is used in high performance devices (such as non-volatile memories with an access time lower than a few tens of ns), it is of the outmost importance to bring the voltage at the reference node back to its working value as quickly as possible upon exit from the stand-by condition. In this case, the reference node is pre-charged in the stand-by condition, in order to improve a dynamic response of the sense amplifier. Particularly, the reference node is connected to the central tap of a voltage divider providing a pre-charging voltage having a value close to the reference voltage. However, the reference node is generally associated with a stray capacitor having a not negligible capacitance; therefore, its pre-charging under the action of the voltage divider is very slow, due to the fact that the voltage divider must ensure a low power consumption in the stand-by condition. This problem is particular acute in a flash E 2 PROM, wherein a single reference node is used for all the sense amplifiers of the device (and it is then associated with a stray capacitor having a relatively high capacitance defined by all the memory cells). A solution known in the art for speeding up the pre-charging process is that of connecting the reference node to a capacitor charged at a power supply voltage upon entry into the stand-by condition. In this way, if the capacitor is suitably dimensioned, the reference node is instantaneously brought to the pre-charging voltage by charge sharing between this capacitor and the stray capacitor associated with the reference node. An example of a sense amplifier using this solution is disclosed in document US6052307. A drawback of the pre-charging circuit described above is that its correct operation is totally dependent on the stray capacitance associated with the reference node. However, it is very difficult (if not impossible) to estimate this value with precision. As a consequence, when the pre-charging circuit is not dimensioned correctly, the reference node is brought to a voltage that is quite different from the desired value. Therefore, the reference node will reach the pre-charging voltage very slowly only under the action of the voltage divider. Moreover, if the sense amplifier exits from the stand-by condition before completion of the aforementioned transient process, the voltage at the reference node will be at a value different from the expected one; in addition, the voltage at the reference node will vary widely according to the length of the stand-by condition. This causes spikes and surges, which increase the time needed for the voltage at the reference node to reach its correct value upon exit from the stand-by condition. It is an object of the present invention to overcome the above-mentioned drawbacks. In order to achieve this object, a circuit as set out in the first claim is proposed. Briefly, the present invention provides a circuit for controlling a reference node in a sense amplifier switchable between an operative condition and a stand-by condition, the reference node providing a reference voltage in the operative condition, wherein the circuit includes means for bringing the reference node to a starting voltage upon entry into the stand-by condition, first means for keeping the reference node at a pre-charging voltage in the stand-by condition, second means for providing a comparison voltage closer to the pre-charging voltage than the starting voltage, pulling means for pulling the reference node towards a power supply voltage, and control means for activating the pulling means upon entry into the stand-by condition and for disabling the pulling means when the voltage at the reference node reaches the comparison voltage. Moreover, the present invention provides a sense amplifier including this circuit and a non-volatile memory device including the sense amplifier; the invention also provides a corresponding method of controlling a reference node in a sense amplifier. Further features and the advantages of the solution according to the present invention will be made clear by the following description of a preferred embodiment thereof, given purely by way of a non-restrictive indication, with reference to the attached figures, in which: Fig.1 is a schematic block diagram of a reading section of a non-volatile memory device in which the circuit of the invention can be used; Fig.2a and Fig.2b show a circuit scheme of a pre-charging block and a circuit scheme of a discharging block, respectively, of the sense amplifier; and Fig.3 depicts a simplified timing diagram of the wave shapes relating to some electrical quantities of the sense amplifier. With reference in particular to Fig.1, a reading section of a memory device 100 (for example a flash E 2 PROM) is illustrated. The memory device 100 includes a matrix 115 of memory cells Cm consisting of floating gate MOS transistors. The memory device 100 has a so-called NOR architecture. The source terminals of all the memory cells Cm are connected to a common source line (typically taken at ground); the control gate terminals of the memory cells Cm of each row are connected to a corresponding word line. The drain terminals of the memory cells Cm of each column are connected to a corresponding bit line; the bit lines are grouped into a series of packets, each one for a bit of a word that is simultaneously processed by the memory device 100. A column decoder (COL_DEC) 120c is used to select the bit lines of a word, in response to a column address COL_ADR; a row decoder (ROW_DEC) 120r is likewise used to select a word line, in response to a row address ROW_ADR. The column decoder 120c is connected to a reading unit, which outputs a word read from the matrix 115 (defined by the values stored on the selected memory cells Cm). The reading unit includes a single reference cell Cr (consisting of floating gate MOS transistor) for all the memory cells Cm of the matrix 115. The reference cell Cr has a control gate terminal that is fed by a voltage +Vxr (for example 4.5V with respect to ground), which is higher than a power supply voltage +Vdd (for example 3.3V); a source terminal of the reference cell Cr is taken at ground, whereas a drain terminal of the reference cell Cr is connected to the first branch of a reference sense amplifier 140. The reference sense amplifier 140 includes a drain voltage regulator with a cascode configuration, which is formed by an n-channel MOS (or NMOS) transistor MCra having the source terminal connected to the drain terminal of the reference cell Cr; an inverter Ira has the input terminal and the output terminal connected to the source terminal and to the gate terminal, respectively, of the NMOS transistor MCra. A current mirror is formed by a p-channel MOS (or PMOS) transistor MSa (operating as a switch), which has the drain terminal connected to the drain terminal of the NMOS transistors MCra. The source terminal of the PMOS transistor MSa is connected to the drain terminal of a further PMOS transistor Mra; the source terminal of the PMOS transistor Mra is fed by the power supply voltage +Vdd. The gate terminal of the PMOS transistor Mra is short circuited to the drain terminal of the PMOS transistor MSa and to the gate terminal of a further PMOS transistor Mrb. The PMOS transistor Mrb has the source terminal that is fed by the power supply voltage +Vdd, and the drain terminal that is connected to the source terminal of a PMOS transistor MSb (operating as a switch). The gate terminal of the PMOS transistor MSb is short circuited to the gate terminal of the PMOS transistor MSa; the drain terminal of the PMOS transistor MSb is connected to a further voltage regulator, which is formed by an NMOS transistor MCrb and an inverter Irb. Multiple decoupling current mirrors are formed by an NMOS transistor MDa having the drain terminal connected to the voltage regulator MCrb,Irb and the source terminal taken at ground. The gate terminal of the NMOS transistor MDa is short circuited to its drain terminal and to the gate terminal of a series of NMOS transistors MDb, each one for a corresponding packet of bit lines. The NMOS transistor MDb has the source terminal taken at ground, and the drain terminal that is connected to a corresponding sense amplifier 145 (for reading the memory cells Cm connected to the bit lines of the packet). The sense amplifier 145 includes a voltage regulator Mcma,Ima connected between the NMOS transistor MDb and a first branch of a current mirror (referred to the power supply voltage +Vdd), which is formed by two PMOS transistors Mma and Mmb. The current mirror Mma,Mmb has a second branch connected to the column decoder 120c through a further voltage regulator MCmb,Imb. The current mirror Mma,Mmb has a structure with unbalanced loads, wherein the PMOS transistor Mma (on a reference side) is larger, and then with lower resistance, than the PMOS transistor Mmb (on a memory cell side). The drain terminal of the PMOS transistor Mma and the drain terminal of the PMOS transistor Mmb are connected to the non-inverting input terminal and to the inverting input terminal, respectively, of a comparator 147. The output terminal of the comparator 147 provides a bit of the word read from the matrix 115. A stand-by signal SB (generated by a state machine, not shown in the figure, controlling operation of the memory device 100) is supplied to a wave-shaping block (SHP) 150. The signal SB is used to switch off most of the circuits of the memory device 100 after a pre-set inactivity period (in order to reduce the stress supported by the reference cell Cr and the power consumption of the memory device 100). The wave-shaping block 150 outputs a delayed signal SBd and a pre-enabling signal PRE, together with their inverted signals SBd and PRE. The signal SBd is obtained from the signal SB delaying its leading edge (for example of a few ns), but without affecting its trailing edge. The signal PRE consists of a short pulse (for example of a few ns), which is triggered by the leading edge of the signal SBd. The signal SBd is supplied to the gate terminals of the PMOS transistors MSa and MSb. A pre-charging block (CHG) 165 controls a reference node Nr, defined by the drain terminal of the PMOS transistor MSa, in response to the signals SBd, PRE, SBd and PRE. A discharging block (DIS) 170 controls a further node Ns, defined by the source terminal of the PMOS transistor MCra, in response to the signals SBd, SBd and according to the voltage at the node Nr. During operation of the memory device 100, the signal SB and then also the signal SBd are at low voltage, so that the PMOS transistors MSa and MSb are both on. The current flowing through the reference cell Cr (always on) is transferred by the current mirrors MSa,Mra,Mrb,MSb and MDa,MDb to the reference side of each sense amplifier 145. When a word is to be read, the corresponding bit lines selected by the column address COL_ADR are biased to a reading voltage (for example 1V), while the other bit lines are left floating. The word line selected by the row address ROW_ADR is driven to the power supply voltage +Vdd, while the other word lines are kept at ground. The current flowing through the bit line of each selected memory cell Cm is compared with the current supplied by the reference cell Cr. In detail, the reference cell Cr is conductive (being not programmed and then featuring a low threshold voltage); therefore, the current flowing through the reference side of the sense amplifier 145 generates a voltage at the non-inverting input of the comparator 147, which is equal to the supply voltage +Vdd minus the voltage drop across the PMOS transistor Mma. When the selected memory cell Cm is not programmed as well, the same current flows through the corresponding bit line. However, the voltage drop across the PMOS transistor Mmb is higher than the one across the PMOS transistor Mma. As a consequence, the voltage at the inverting input of the comparator 147 is lower than the one at its non-inverting input terminal, and the comparator 147 outputs a high voltage (logic value 1). Conversely, when the selected memory cell Cm is programmed, it features a high threshold voltage and then it is not conductive. As a consequence, the inverting input terminal of the comparator 147 is driven to the supply voltage +Vdd (higher then the voltage at its non-inverting input terminal) and the comparator 147 outputs a low voltage (logic value 0). In both cases, the voltage at the inverting input terminal of the comparator 147 is different than the voltage at its non-inverting input terminal, so as to ensure a correct operation of the comparator 147. Similar considerations apply if the memory device has a different structure (for example with a NAND architecture), if the sense amplifiers have a different configuration (for example without any voltage regulator), if the NMOS and PMOS transistors are replaced with equivalent components, if two or more reference cells are employed, if the reference cell is directly coupled to the sense amplifiers (without any current mirror), and the like. Considering now Fig.2a, a stray capacitor Cr is associated with the node Nr (as shown in dashed lines in the figure between the node Nr and ground). The stray capacitor Cr has a relatively high capacitance, due to the fact that the reference node Nr is associated with all the cells of the memory device. The pre-charging block 165 includes a PMOS transistor Mr having the drain terminal connected to the node Nr and the gate terminal controlled by the signal SBd . The source terminal of the PMOS transistor Mr is connected to a central tap (node Nda) of a voltage divider formed by two resistors Rda1 and Rda2 in series. Particularly, the resistor Rda1 is connected between the node Nda and the drain terminal of a PMOS transistor Mda1, whose source terminal is fed by the power supply voltage +Vdd. The resistor Rda2 is connected between ground and the drain terminal of a PMOS transistor Mda2, whose source terminal is connected to the node Nda. The gate terminals of both PMOS transistors Mda1 and Mda2 are controlled by the signal SBd. A capacitor Cp is connected between the node Nda and ground. The node Nda is further connected to the drain terminal of a PMOS transistor Mp, whose source terminal is fed by the power supply voltage +Vdd. The gate terminal of the PMOS transistor Mp is controlled by the signal SBd. An additional voltage divider having a central tap defining a node Ndb is formed by two resistors Rdb1 and Rdb2 in series. The resistor Rdb1 is connected between the node Ndb and the drain terminal of a PMOS transistor Mdb1, whose source terminal is fed by the power supply voltage +Vdd. The resistor Rdb2 is connected between ground and the drain terminal of a PMOS transistor Mdb2, whose source terminal is connected to the node Ndb. The gate terminals of both PMOS transistors Mdb1 and Mdb2 are controlled by the signal PRE. The node Ndb is further connected to the drain terminal of a PMOS transistor Mdb3, whose source terminal is fed by the power supply voltage +Vdd. The gate terminal of the PMOS transistor Mdb3 is controlled by the signal PRE. The node Nr and the node Ndb are connected to the inverting input terminal and to the non-inverting input terminal, respectively, of a comparator 210. A supply terminal of the comparator 210 is connected to the drain terminal of a PMOS transistor Mpc, whose source terminal is fed by the power supply voltage +Vdd; the gate terminal of the PMOS transistor Mpc is controlled by the signal PRE. The output terminal of the comparator 210 (node Nc) controls a pull-up NMOS transistor Mu1; the source terminal and the drain terminal of the NMOS transistor Mu1 are connected to the node Nda and to the drain terminal of a PMOS transistor Mu2, respectively. The source terminal of the PMOS transistor Mu2 is fed by the power supply voltage +Vdd; the gate terminal of the PMOS transistor Mu2 is controlled by the signal PRE. Moving now Fig.2b, the discharging block 170 includes a voltage divider (having a central tap defining a node Ndc), which is formed by two resistors Rdc1 and Rdc2 in series. Particularly, the resistor Rdc1 is connected between the node Ndc and the drain terminal of a PMOS transistor Mdc1, whose source terminal is fed by the power supply voltage +Vdd. The resistor Rdc2 is connected between the node Ndc and ground. An NMOS transistor Mdc2 has the source terminal and the drain terminal connected to ground and to the node Ndc, respectively. The gate terminals of both the PMOS transistor Mdc1 and the NMOS transistor Mdc2 are controlled by the signal SBd. The node Nr and the node Ndc are connected to the inverting input terminal and to the non-inverting input terminal, respectively, of a comparator 215. A supply terminal of the comparator 215 is connected to the drain terminal of a PMOS transistor Mdc, whose source terminal is fed by the power supply voltage +Vdd; the gate terminal of the PMOS transistor Mdc is controlled by the signal SBd. The output terminal of the comparator 215 (node Nres) controls an NMOS transistor Md1. The source terminal of the NMOS transistor Md1 is connected to the drain terminal of an NMOS transistor Md2, whose source terminal is connected to ground; the gate terminal of the NMOS transistor Md2 is controlled by the signal SBd. Two inverters Id1 and Id2 are connected in a latch configuration between the NMOS transistor Md1 and a further NMOS transistor Md3. Particularly, the input terminal of the inverter Id1 and the output terminal of the inverter Id2 are connected to the drain terminal of the NMOS transistor Md1; the input terminal of the inverter Id2 and the output terminal of the inverter Idl are connected to the drain terminal of the NMOS transistor Md3. The source terminal of the NMOS transistor Md3 is connected to ground; the gate terminal of the NMOS transistor Md3 is controlled by the signal SBd. The drain terminal of the NMOS transistor Md1 (node Ndis) controls the gate terminal of a pull-down NMOS transistor Md4; the source terminal and the drain terminal of the NMOS transistor Md4 are connected to ground and to the node Ns, respectively. A stray capacitor Cs defined by the reference cell is associated with the node Ns (as shown in dashed lines in the figure between the source terminal of the transistor MCra and ground). Similar considerations apply if the pre-charging block and the discharging block have a different structure (for example replacing the pull-up transistor or the pull-down transistor with equivalent means), if the latch is formed by different elements, if the resistors have different resistance, if the voltage dividers are not resistive (for example, implemented with MOS transistors), and the like. As shown in the Fig.3 (to be considered together with Fig.2a and Fig.2b), in an operative condition of the memory device the signals SB, SBd, PRE are at low voltage, and then the inverted signals SBd, PRE are at high voltage. In this condition, the pre-charging block 165 is disabled and disconnected from the node Nr (transistors Mr, Mda1 and Mda2, Mdb1 and Mdb2, Mpc, and Mu2 off); the capacitor Cp is charged to the power supply voltage +Vdd (through the PMOS transistor Mp) and the node Ndb is kept at the power supply voltage +Vdd by the PMOS transistor Mdb3. When the memory device enters the stand-by condition, the signal SB is switched to high voltage. After a pre-set period (time t1), the signals SBd, PRE are brought to high voltage, and then the inverted signals SBd , PRE are brought to low voltage (in order to avoid glitches on the signals). In response thereto, the PMOS transistor Mp is switched off and the PMOS transistors Mr, Mda1, Mda2 are switched on, so that the node Nr instantaneously goes (by charge sharing between the capacitors Cp and Cr) to a starting voltage: Cp·Vdd + Cr·Vref Cp + Cr The capacitance of the capacitor Cp is set to a value so as to ensure that the starting voltage is lower (for example of 0.5V) than a pre-charging voltage Vpre of the node Nr in a steady condition, which is defined by the voltage divider Rda1,Rda2: V pre = Vdd·Rda 2 Rda 1 + Rda 2 The resistors Rda1, Rda2 have high resistance (for example of several hundreds of kΩ), in order to reduce the power consumption of the memory device in the stand-by condition; the voltage divider Rda1,Rda2 then features a very slow dynamic response, due to the fact that it drives a relatively high capacitive load (defined by the capacitors Cp and Cr). Moreover, the resistors Rda1, Rda2 are dimensioned so that the pre-charging voltage Vpre has a value close to the reference voltage Vref (typically higher then the reference voltage Vref, for example of 0.1V). At the same time, the PMOS transistors Mdb1, Mdb2 are switched on and the PMOS transistor Mdb3 is switched off, so that the node Ndb is brought from the power supply voltage +Vdd to the voltage: Vdd · Rdb 2 Rdb 1 + Rdb 2 The resistors Rdb1, Rdb2 have high resistance (for example of several hundreds of kΩ); however, the voltage divider Rdb1,Rdb2 features a very fast dynamic response, due to the fact that it drives a low capacitive load (defined by the non-inverting input terminal of the comparator 210). The resistors Rdb1, Rdb2 are dimensioned so that the voltage at the node Ndb is substantially the same as the pre-charging voltage Vpre; particularly, the voltage at the node Ndb has a value slightly lower than the pre-charging voltage Vpre (for example of a few tens of mV, such as 50mV), in order to compensate for an input offset voltage of the comparator 210. Moreover, the PMOS transistor Mpc is switched on, thereby supplying the comparator 210. The voltage at its inverting input terminal (node Nr) is lower than the voltage at its non-inverting input terminal (node Ndb), and the node Nc is then brought to high voltage. Therefore, both the transistors Mu1 and Mu2 are switched on, so that the node Nr is pulled up towards the power supply voltage +Vdd. As soon as the voltage at the node Nr reaches the voltage at the node Ndb (time t2), the node Nc is brought to low voltage and the NMOS transistor Mu1 is switched off. The node Nr is then maintained at the pre-charging voltage Vpre by the voltage divider Rda1 and Rda2. At a next time t3, the signal PRE is brought to low voltage and then the inverted signal PRE is brought to high voltage; the pulse defined by the signals PRE, PRE has a length that ensures completion of the above-described pre-charging process of the node Nr. As a consequence, the PMOS transistors Mpc, Mdb1, Mdb2 and Mu2 are switched off (thereby switching off the comparator 210 as well), and the PMOS transistor Mdb3 is switched on (bringing back the voltage at the node Ndb to the power supply voltage +Vdd). In the stand-by condition, the discharging block 170 is inactive (due to the action of the voltage regulator Mcra, Ira); the NMOS transistor Md3 is on, so that the latch Idl, Id2 is set. When the memory device exits from the stand-by condition (time t4), the pre-charging block is disabled and the node Nr is brought to the reference voltage Vref. Particularly, the node Nr is discharged through the NMOS transistors MCr, MDr2 and MDr1 (by charge sharing between the capacitors Cr and Cs) towards the voltage: V ref = Cr·Vpre + Cs·Vs Cr + Cs wherein Vs is the reading voltage (1V). At the same time, the PMOS transistor Mdc1 is switched on and the NMOS transistor Mdc2 is switched off, so that the node Ndc is brought (from ground) to the voltage: Vdd·Rdc 2 Rdc 1 + Rdc 2 The voltage divider Rdc1,Rdc2 features a very fast dynamic response, due to the fact that it drives a low capacitive load (defined by the non-inverting input terminal of the comparator 215); the resistors Rdc1, Rdc2 are dimensioned so that the voltage at the node Ndc is comprised between the pre-charging voltage Vpre and the reference voltage Vref. Moreover, the PMOS transistor Mdc is switched on, thereby supplying the comparator 215. The voltage at its inverting input terminal (node Nr) is higher than the voltage at its non-inverting input terminal (node Ndc), and the node Nres is then kept at low voltage. Therefore, the NMOS transistor Md1 remains off, while the NMOS transistor Md2 is on and the NMOS transistor Md3 is off. The latch Id1, Id2 remains in the set state (node Ndis at high voltage), so that the NMOS transistor Md4 is on and pulls down the node Nr towards ground. As soon as the voltage at the node Nr falls below the voltage at the node Ndc (time t5), the node Nres is brought to high voltage and the NMOS transistor Md1 is switched on. The latch Id1, Id2 is reset in response thereto (node Ndis at low voltage) and the NMOS transistor Md4 is switched off. The process of returning from the stand-by condition described above is then completed with the node Nr that reaches the reference voltage Vref. Similar considerations apply if the signals have different wave shapes, if the capacitor of the pre-charging block is dimensioned to provide a different starting voltage, if the voltages employed in the pre-charging and discharging blocks have different values, and so on. More generally, the present invention provides a circuit for controlling a reference node in a sense amplifier. The sense amplifier can be switched between an operative condition and a stand-by condition, with the reference node that provides a reference voltage in the operative condition. The circuit includes means for bringing the reference node to a starting voltage upon entry into the stand-by condition. First means are used for keeping the reference node at a pre-charging voltage in the stand-by condition. Second means provides a comparison voltage closer to the pre-charging voltage than the starting voltage. Pulling means are used for pulling the reference node towards a power supply voltage. Control means activates the pulling means upon entry into the stand-by condition and disables the pulling means when the voltage at the reference node reaches the comparison voltage. The solution of the invention strongly increases the speed of the pre-charging circuit, due to the pull-up action towards the power supply voltage exerted on the reference node. Moreover, the voltage at the reference node is controlled with a feedback scheme. In this way, the pre-charging process has a well-defined behaviour. As a consequence, the envisaged circuit substantially avoids any spike and surge at the reference node upon exit from the stand-by condition. The proposed solution improves the dynamic response of the sense amplifier; this is particularly advantageous when the sense amplifier is used in high performance devices (such as non-volatile memories with a very low access time), even if different applications of the circuit are not excluded. The preferred embodiment of the invention described above offers further advantages. For example, the reference node is brought to the starting voltage upon entry into the stand-by condition by means of a capacitor. This feature further improves the dynamic response of the sense amplifier. Moreover, the pre-charging voltage and the comparison voltage are generated by corresponding voltage dividers. The proposed structure is very simple, but at the same time effective. Preferably, the central tap of the second voltage divider is kept at the power supply voltage in the operative condition of the sense amplifier. This solution further improves the dynamic response of the pre-charging circuit upon entry into the stand-by condition. However, the solution according to the present invention leads itself to be implemented even without any capacitor, using different structures for generating the pre-charging voltage and the comparison voltage, or keeping the central tap of the second voltage divider at a different voltage in the operative condition of the sense amplifier. Advantageously, the pulling means (for example consisting of a pull-up transistor) is controlled by a comparator; the difference between the pre-charging voltage and the comparison voltage is set to an input offset voltage of the comparator. The proposed structure ensures that the voltage at the reference node has already reached its correct value when the pull-up transistor is switched off. Moreover, the second voltage divider and the comparator are disabled after a pre-set period from the entry into the stand-by condition. This feature strongly reduces the power consumption of the sense amplifier in the stand-by condition, once the voltage at the reference node has reached a steady value. Alternatively, the pull-up transistor is controlled by different elements, the comparison voltage has a different value, or all the components of the pre-charging block are always active in the stand-by condition. In a preferred embodiment of the invention, the reference node is pulled towards a further power supply voltage (for example ground) upon exit from the stand-by condition, until the voltage at the reference node reaches a further comparison voltage. The proposed feature ensures that the reference node is brought to the reference voltage very fast upon exit from the stand-by condition. Advantageously, the means employed for pulling the reference node towards ground (such as a pull-down transistor) is controlled by a latch, which is switched upon entry into the stand-by condition and when the voltage at the reference node reaches the further comparison voltage. In this way, a very compact structure keeps the pull-down transistor always in a correct condition, so as to discharge the reference node upon exit from the stand-by condition and not to interfere with operation of the sense amplifier otherwise. However, the solution of the invention is also suitable to be implemented with a different structure for controlling the pull-down transistor, generating the further reference voltage in a different manner, or even without any block for pulling the reference node towards ground (or any other power supply voltage) upon exit from the stand-by condition. Preferably, the proposed circuit is employed to control a sense amplifier of a non-volatile memory device. The solution of the invention is particularly advantageous in a flash E 2 PROM, due to the fact that the single reference node is associated with a stray capacitor (defined by the whole matrix of memory cells) having a relatively high capacitance. However, other applications of the proposed circuit are contemplated and within the scope of the invention. For example, the circuit is employed in an EPROM (wherein a column of reference cells is associated with each sense amplifier), or even in a different device (such as a sensor).

Claims: A circuit (165) for controlling a reference node (Nr) in a sense amplifier (145) switchable between an operative condition and a stand-by condition, the reference node providing a reference voltage in the operative condition, wherein the circuit includes means (Cp) for bringing the reference node to a starting voltage upon entry into the stand-by condition, and first means (Rda1,Rda2) for keeping the reference node at a pre-charging voltage in the stand-by condition, characterised in that the circuit further includes second means (Rdb1,Rdb2) for providing a comparison voltage closer to the pre-charging voltage than the starting voltage, pulling means (Mu1) for pulling the reference node towards a power supply voltage, and control means (210) for activating the pulling means upon entry into the stand-by condition and for disabling the pulling means when the voltage at the reference node reaches the comparison voltage. The circuit (165) according to claim 1, wherein the means for bringing the reference node to the starting voltage includes a capacitor (Cp). The circuit (165) according to claim 1 or 2, wherein the first means includes a first voltage divider (Rda1,Rda2) having a first central tap (Nda) for obtaining the pre-charging voltage from the power supply voltage, and wherein the second means includes a second voltage divider (Rdb1,Rdb2) having a second central tap (Ndb) for obtaining the comparison voltage from the supply voltage. The circuit (165) according to claim 3, further including means (Mdb3) for keeping the second central tap (Ndb) at the power supply voltage in the operative condition. The circuit (165) according to claim 4, wherein the control means includes a comparator (210) having a first input terminal connected to the first central tap (Nda), a second input terminal connected to the second central tap (Ndb), and an output terminal (Nc) for controlling the pulling means (Mu1), the difference between the pre-charging voltage and the comparison voltage being substantially equal to an input offset voltage of the comparator. The circuit (165) according to claim 5, further including means (Mpc,Mdb1,Mdb2) for disabling the second voltage divider (Rdb1,Rdb2) and the comparator (210) after a pre-set period of time from entry into the stand-by condition. The circuit (165,170) according to any claim from 1 to 6, further including third means (Mdc1,Mdc2) for providing a further comparison voltage comprised between the reference voltage and the pre-charging voltage, further pulling means (Md4) for pulling the reference node (Nr) towards a further power supply voltage, and further control means (215,Id1,Id2) for activating the further pulling means upon exit from the stand-by condition and for disabling the further pulling means when the voltage at the reference node reaches the further comparison voltage. The circuit (165,170) according to claim 7, wherein the further control means (215) includes a latch (Id1,Id2) switchable between a first and a second condition, the latch activating the further pulling means (Md4) in the first condition, means (Md3) for switching the latch to the first condition upon entry into the stand-by condition and means (Md2,215,Md1) for switching the latch to the second condition when the voltage at the reference node reaches the further comparison voltage. A sense amplifier (145) having a reference node (Nr) for providing a reference voltage, the circuit (165) according to any claim from 1 to 8 for controlling the reference node, an input node for receiving an input voltage, and means (147) for comparing the input voltage with the reference voltage. A non-volatile memory device (100) including a plurality of memory cells (Cm), at least one reference cell (Cr), at least one sense amplifier (145) according to claim 9 for comparing the input voltage provided by a selected memory cell with the reference voltage provided by the at least one reference cell. The non-volatile memory device (100) according to claim 10, wherein the memory device consists of a flash E 2 PROM. A method of controlling a reference node (Nr) in a sense amplifier (145) switchable between an operative condition and a stand-by condition, the reference node providing a reference voltage in the operative condition, wherein the method includes the steps of: bringing the reference node to a starting voltage upon entry into the stand-by condition, keeping the reference node at a pre-charging voltage in the stand-by condition, characterised by the steps of providing a comparison voltage closer to the pre-charging voltage than the starting voltage, pulling the reference node towards a power supply voltage by use of pulling means upon entry into the stand-by condition, and disabling the pulling means when the voltage at the reference node reaches the comparison voltage.

Inventors: DE AMBROGGI LUCA, GIOVE SALVATORE, NICOSIA SALVATORE, PIAZZA GIUSEPPE, PIPITONE FRANCESCO, PROMOD KUMAR, TOMAIUOLO FRANCESCO, DE AMBROGGI, LUCA, GIOVE, SALVATORE, NICOSIA, SALVATORE, PIAZZA, GIUSEPPE, PIPITONE, FRANCESCO, PROMOD, KUMAR, TOMAIUOLO, FRANCESCO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 2001
Main classification: G11C16
IPC codes: G11C7, G11C16