Supply voltages switch circuit for memory devices

Abstract: A circuit (1) for switching of power supply voltages in electrically programmable and erasable non-volatile semiconductor memory devices and of the type comprising a first switch-regulator block (23) connected to a main power supply line (2) with programming voltage (Vpp) to generate a first programming voltage signal (Vpcx) to be supplied to the memory device on a first programming line (6) comprises also a second switch-regulator block (28) powered by the first programming line (6) and designed to generate a second programming voltage (Vpcy) to be supplied to the memory device on a second programming line (10). There is also provided a detector block (27) connected to the line (6) of programming voltage (Vpcx) to detect reaching of a predetermined high level (RISE) on the rising front of the programming voltage signal (Vpcx) and reaching of a predetermined low level (FALL) on the falling front of said voltage signal (Vpcx) and emit at output a corresponding enablement signal (VPD-en) for a third regulator block (25) located downstream.

Description: Field of application The present invention relates to a circuit for the switching of programming voltages in electrically programmable and erasable non-volatile semiconductor memory devices. The present invention concerns in particular a circuit as indicated in the preamble of claim 1 and shown in the present fig. 1. Prior art As known, in many applications concerning electronic devices integrated on semiconductors there is a need to power the various circuit portions of the device with different voltage levels. Generally the electronic memory devices integrated on a semiconductor can be divided in two groups, i.e. with single power supply or double power supply. In the former case to the integrated device is supplied from the outside a single voltage value, for example a power supply voltage Vdd. In the second case there are supplied two voltage values, i.e. that of power supply Vdd and a higher programming voltage Vpp. For both types of device it is however necessary to generate and/or control inside the integrated memory device appropriate and different voltage values even higher than the power supply Vdd. In particular, in electrically programmable non-volatile memories such as EEPROM o flash EEPROM there are required different voltage values, for example: voltages Vpcx, Vpcy, to be applied respectively to the rows and columns of the cell matrix in the programming and decoding phases; a voltage Vpd to be applied to the column in the programming phase; voltages UPR-CG and UPR-PG for programming of the redundant circuitry UPROM; and BOOST voltages boosted starting from the power supply voltage Vdd. To generate all these different voltage levels the prior art proposes a solution illustrated in the annexed FIG. 1 in which is shown the diagrammatic view of a switching circuitry of known type. In this circuitry all the various voltage levels are virtually derived from the programming voltage Vpp which is present on a principal power supply line 2. A first switch-regulator block 3 is connected to the line 2 and to a reference power supply voltage Vdd to generate a first programming voltage Vpcx to be supplied to a memory device on a line 6. With the block 3 is associated a voltage-booster block 4 which is structurally independent of the block 3 which is in turn powered by the voltage Vpp and the voltage Vdd and connected at output to the line 6. A second switch-regulator block 5 is powered by the line 2 and by the reference power supply voltage Vdd. This block 5 is designed to generate a drain voltage Vpd to be supplied on a line 11 at the ends of the drain of the memory cells in the programming phase. The block 5 is interlocked with an activation signal VPD-cnt but is also connected downstream of a delay block 7 which emits at output an enablement signal VPD-en for the block 5. The delay block 7 receives at input an activation signal VPCX-cnt which is also applied to the block 3. Consequently the delay block 7 regulates the intervention of the block 5 only after a predetermined delay after the generation of the signal Vpcx by the block 3. A third switch-regulator block 8 is powered by the line 2 and by the reference power supply voltage Vdd. This block 8 is designed to generate a second programming voltage Vpcy to be supplied to the memory device on a line 10. The block 8 is interlocked with an activation signal VPCY-cnt. A fourth switch-regulator block 9 is powered by the line 2 and by the reference power supply voltage Vdd. This block 9 is designed to generate and regulate respective voltages UPR-CG and UPR-PG to be supplied in programming to redundancy registers UPROM through lines 12 and 13. The block 9 is interlocked with an activation signal VPP-UPR-cnt. From the above description it is apparent that the switching circuitry provided in accordance with the prior art arises from a design philosophy which calls for the employment of switch-regulators virtually independent from each other and that they were received in the switching circuitry according to the requirement for generating various voltage levels which appeared during the evolution of the memory devices. Although advantageous in some ways and virtually meeting the purpose, this proposed solution of the prior are displays a series of shortcomings which are set forth below. The number of switching-regulation circuits to be managed in an independent manner in the circuitry is considerable. This can involve imprecisions in the correct sequencing of the switching (snap-back phenomena). In particular, the presence of the delay block 7 makes imprecise and unsafe the switching interval of the second programming voltage Vpd. In addition, there are found high impedances on the lines of current use such as that emerging from the booster 4. This causes an imprecise correlation between the voltage level regulated upstream and the voltage level regulated downstream. Finally, the memory device configuration elements such as for example the redundancy registers can be exposed to spurious alterations due to undesired 'bump' phenomena. A voltage reference circuit is known from the US Patent No. 5,109,187 to Guliani. The described circuit generates a voltage reference which is independent from power supply. The technical problem underlying the present invention is to conceive a new circuit for the switching of power supply voltages in memory devices and having structural: and functional characteristics overcoming the shortcomings mentioned above with reference to the prior art. Summary of the invention The solution idea underlying the present invention is to provide a 'cascade' architecture of the switch-regulator blocks in such a manner that at least some of them,could take power from a switch-regulator block located upstream. In this manner there would be conferred greater protection on the memory device and any accidental events on the voltage values upstream would have no undesired effect on the downstream circuitry. On the basis of this solution idea the technical problem is solved by a circuit of the type indicated above and defined in the characterizing part of claim 1. The characteristics and advantages of the method in accordance with the present invention are set forth in the description of an embodiment thereof given below by way of non-limiting example with reference to the annexed drawings. In the drawings - Brief description of the drawings FIG. 1 shows a diagrammatic view of a switching circuit provided in accordance with the prior art, FIG. 2 shows a diagrammatic view of a switching circuit provided in accordance with the present invention, FIG. 3 shows the circuit details of the structure of the circuit of FIG. 2, and FIGS. 4 and 5 show diagrammatically respective additional details of the circuit in accordance with the present invention. Detailed description With particular reference to the example of FIG. 2, reference number 1 indicates as a whole and diagrammatically a circuit provided in accordance with the present invention to regulate the switching of power supply voltages in an electrically programmable and erasable non-volatile semiconductor memory device. The circuit in accordance with the present invention is designed specifically but not exclusively for a flash memory device with double power supply with 5-volt power supply voltage Vdd. The memory device is not shown in the drawings but is understood to be the type comprising a memory cell matrix organized in accordance with rows and columns and connected to a control, selection and decoding circuitry. The cell matrix has dimensions quantifiable in 16 megabytes and is divided in 16 segments of 1 megabyte each and with 16-bit memory words. In the following description structural details and parts having the same function as those described for FIG. 1 of the prior art are represented with the same reference numbers and signs. The circuit 1 in accordance with the present invention is powered by the programming voltage Vpp which is present on the main power supply line 2. A first switch-regulator block 23 is connected to the line 2 and receives voltage even from the power supply Vdd to generate a first programming voltage Vpcx to be supplied to the memory device on line 6. The block 23 receives at input an activation signal VPCX-cnt. The block 23 is closely connected to a booster block 24 which is in turn powered by the voltage Vpp. A second switch-regulator block 25 is powered by the line 2. This block 25 is designed to generate a drain voltage Vpd for the drain terminals of the memory cells in the programming phase. The block 25 is interlocked with an activation signal VPD-cnt. Advantageously, in accordance with the present invention the block 25 is also connected downstream of a detector block 27 which emits at output an activation signal VPD-en for the block 25. The detector block 27 is connected to the line 6 and detects the rising front and the falling edge of the signal Vpcx. Specifically the block 27 detects the reaching of a predetermined high level voltage RISE on the rising front of the signal Vpcx and the reaching of a predetermined low voltage level FALL on the falling front of the signal Vpcx. A third switch-regulator block 28 is powered by the line 6 and by the power supply voltage Vdd. This block 28 is designed to generate a second programming voltage Vpcy to be supplied to the memory device on a line 10. The block 28 is interlocked with an activation signal VPCY-cnt. A fourth switch-regulator block 29 is powered both by the line 6 and by the line 10. This block 29 is designed to generate and regulate respective voltages UPR-CG and UPR-PG to be applied in programming to redundancy registers UPROM through lines 12 and 13. The block 29 is interlocked with an activation signal VPP-UPR-cnt. Now with specific reference to the examples of FIGS. 3, 4 and 5 there is described in greater detail the internal structure of the circuit 1 in accordance with the present invention. The line 2 of the programming voltage Vpp is connected to the line 6 VPCX by the series of two power transistors P1 and P2 of the PMOS type. The first P1 of these has its body terminal connected to the line 2 while the second P2 has its body terminal connected to the line 6. The point of interconnection between the two transistors P1 and P2 is identified as node A. The control terminal of the first transistor P1 is connected to an output of a voltage booster block 30. The block 30 receives at input an enablement signal VPP-dis to force breaking of the connection between the lines 2 and 6 as explained below. Advantageously between the line 6 and the ground GND is present a natural charging capacitor C-vpcx due to the assembly of the devices connected to the line 6. This capacitor has a value of approximately 1nF and is advantageous as protection from overshoots since it contributes to providing a filtering structure against accidental variations in the voltage signal levels. In the circuit 1 are incorporated some booster blocks having the same structure and operation as the block 30. Therefore, there is described below only once the structure of the block 30 and the various booster blocks distributed in the circuit are identified by different reference numbers with the only purpose of facilitating their location in the circuit 1. But the description of their structure will be omitted to avoid uselessly lengthening the remainder of the description. The booster block 30 comprises a cell of transistors incorporating two complementary pairs of transistors with crossed connection. Each pair comprises a PMOS transistor connected to the power supply, in this case to the line 2 with voltage Vpp, and an NMOS transistor connected to a second reference voltage, for example a signal ground GND. The PMOS transistors of the first and second couples have their respective body terminals connected to the power supply, i.e. to the line 2. The interconnection point X between the PMOS and NMOS transistors of the first pair is connected to the control terminal of the PMOS transistor of the second pair. An analogous connection is provided between the second pair and the control terminal of the PMOS transistor of the first pair. In addition, the control terminals of the NMOS transistors are connected together with the interposition of an inverter. The enablement signal of the block 30 (in this case the signal VPP-dis) is applied to the input of the inverter. The output of the block 30 is taken in the interconnection node X. Even the second power transistor P2 has its control terminal connected to the output of a voltage booster block 31. The booster 31 is connected to a BSW body switch power supply line 15 and receives at input the signal VPP-dis. Between the interconnection node A and the control terminal of the transistor P2 is inserted a discharge transistor Ms of the NMOS type with grounded source terminal. The blocks 30 and 31 can be considered incorporated in the first switch-regulator block 23 which generates the first programming voltage Vpcx on the line 6. The activation signal VPCX-cnt indicated in FIG. 2 is to be understood as the result of a more complex articulation of signals which are discussed in greater detail with reference to FIG. 4. As seen above, the block 23 is powered also by the voltage Vdd. For this purpose there is provided a second power connection between a reference power supply voltage Vdd and the line 6. This connection comprises the series of two power transistors P3 and P4 of the PMOS type with the first P3 of which having its body terminal connected to the BWS line 15 while the second P4 has its body terminal connected to the line 6. The interconnection point between the two transistors P3 and P4 is identified as node B. To this node B leads a BOOST line 16 which is thus plugged in at the intermediate point of the switch block 23. The first transistor P3 of this second series connection has its control terminal driven by the output of booster block 32 receiving at input a BOOST-mode activation signal. The booster block 32 is powered by the BWS line 15. Even the second transistor P4 has its control terminal connected to the output of another booster block 33 which receives at input a BOOST-up enablement signal. The booster block 33 is powered by the line 2 with Vpp. Another booster block 34 is connected to the BOOST line 16 and receives at input a BOOST-mode activation signal. The block 34 displays a double output connection with a pair of transistors P5 and P6 of the PMOS type connected between the power supply Vdd and the BOOST line 16. Specifically the control terminals of the transistors P5 and P6 are respectively connected to the interconnection nodes of the first and second complementary pairs incorporated in the booster block 34. The body terminals of the transistors P5 and P6 as well as the interconnection node E between them are all connected to the BSW line 15. There is now discussed in detail the structure of the detector block 27 which emits at output an enablement signal VPD-en for the block 25 of FIG. 2. The detector block 27 is connected to the line 6 through a first circuit branch which calls for the series of a transistor P7 of the PMOS type in diode configuration connected to a transistor M7 of the NMOS type connected to ground GND. On the control terminal of the transistor M7 is applied the signal VPP-dis. Between the transistors P7 and M7 there is an interconnection node F from which is taken a FALL signal which indicates the reaching of a predetermined value on the falling front of the signal Vpcx. A second circuit branch of the detector block 27 comprises the series of a pair of PMOS transistors P8 and P9 the first of which is connected to the line 6 with an NMOS transistor M8 connected to ground. The transistors P8 and P9 are in diode configuration and the first P8 of these has its control terminal connected to the control terminal of the transistor P7 of the first circuit branch of the block 27. This control terminal and the control terminal of the transistor M8 receive a BSW signal. Between the transistors P9 and M8 there is an interconnection node H from which is taken a RISE signal which indicates the reaching of a predetermined value on the rising front of the signal Vpcx. There is now discussed the essential structure of the third switch-regulator block 28 designed to generate the second programming voltage Vpcy to be supplied to the memory device on the line 10. This block 28 is powered by the line 6 and by the power supply voltage Vdd. The series of two power transistors P10 an P11 of the PMOS type connects together the lines 6 and 10. The body terminal of the first P10 of these transistors is connected to the line 6 while the body terminal of the second P11 of these transistors is connected to the line 10. Advantageously, between the line 10 and the ground GND is present a natural charging capacitor C-vpcy due to the assembly of the devices connected to the line 10. This capacitor has a value of approximately 0.7nF and is advantageous as a second protection from overshoots since it contributes in turn to providing a filtering structure against accidental variations in the levels of the voltage signal. A voltage booster block 35 is provided for driving the control terminals of the transistors P10 and P11. The block 35 is powered by the line 6 and receives at output a signal emerging from a logic gate 38 of the NOR type with two inputs. The first input of the gate 38 receives the signal VPP-dis while on the second input of the gate 38 is applied a signal DMA. The DMA (Direct Memory Access) signal enables reading of the current of the memory cells to be able to go back to their threshold voltage. Another power transistor P12 of the PMOS type is inserted between the power supply Vdd and the line 10. Even this transistor has its control terminal connected to the output of a voltage booster block 36. The block 36 is powered by the line 2 with voltage Vpp and receives at input a signal emerging from a logic gate 37 of the NOR type with two inputs. The first input of this gate 37 receives the BOOST-up signal and on the second input is applied the DMA signal. The structure of the fourth switch-regulator block 29 is powered both by the line 6 and by the line 10. This block 29 is designed to generate and regulate respective voltages UPR-CG and UPR-PG to be applied in programming to configuration elements of the memory device such as for example the redundancy registers UPROM through lines 12 and 13. The block 29 comprises loads and overvoltage or electrostatic discharge protection elements which confer thereon a particular inertia towards spurious switching which might be forced by accidental events. The detailed description of the internal structure of the block 29 is omitted from the present description since it is the object of a specific and parallel patent application filed on the same date by this same applicant. There is now discussed FIG. 4 which illustrates a simple network 40 of logic gates which allow driving of the circuit 1 in accordance with the present invention with a series of activation and/or enablement signals. The network 40 comprises a first NOR gate 41 with two inputs and one output which receives the PRG signals and which starts the programming and DMA phase which starts the phase of measurement of the current of the memory cells. The output of the gate 41 is applied through an inverter 42 to an input of a second logic gate 43 of the NOR type with two inputs and one output. The other input of the gate 43 receives FALL signal emerging from the detector 27. The output of the NOR gate 43 produces the BOOST-up signal. The first input of the second NOR gate 43 is branched at the input of a retarder 44 which generates at output the signal VPP-dis. This signal commands disconnection of the programming voltage Vpp. The network 40 also comprises a logic gate 45 of the NAND type with two inputs and one output. This gate 45 receives on one input a VDD-mode signal and on the other input a DMAn signal which is the negation of the DMA signal taken through an inverter 46 from the second input of the first NOR gate 41. The output of the gate 45 represents a BOOST-mode signal applied to the booster blocks 33 and 34. Even the FIG. 5 illustrates a simple network 50 of logic gates which allow driving the circuit 1 in accordance with the present invention with a series of activation and/or enablement signals. The network 50 comprises an NAND logic gate 51 with two inputs and one output which receives on one input the PRG signal for starting programming and on the other input the RISE signal emerging from the detector 27. The output of the NAND gate 51 is applied to one input of a logic gate 52 of the NOR type with two inputs and one output. The other input of this gate receives a signal UPR-PROG which starts the programming phase of the UPROM registers. The output of the gate 52 is connected to the inverter of the voltage booster block 53 connected to the line 2 with voltage Vpp. The block 53 produces on the output nodes the PG signal and the corresponding negated PGn signal. There is discussed briefly below the operation of the circuit 1 in accordance with the present invention starting from an initial state in which is started the programming phase of the memory cells through the signal PGR = 1. The programming pulse PRG thus has high logical value and it is assumed that the other DMA signal on the gate 41 has a low logical value. Under these conditions the BOOST-mode signal is low if VDD-mode is high. Even the BOOST-up signal is low and the BSW line 15 is brought to the value of the power supply Vdd in the programming phase. Below the value Vdd the control returns to the BOOST line 16. The logical value FALL is set on approximately 6V and reaches the high value when even the signal VPP-dis is high. Under these conditions the transistor P1 is extinguished and the connection between the line 2 and the line 6 VPCX is broken. The line 6 falls to the value of approximately 6V before the FALL signal pushes the output of the gate 43 to produce a high BOOST-up signal which establishes a connection between the line 6 and the BOOST line 16 through the transistor P4. The FALL signal switches first but at the beginning is ignored. When the BOOST-up is high and FALL returns to the low value it is the BOOST line 16 which powers the line 6 VPCX. If that is not desired, it is necessary to set high the VDD-mode signal which deactivates BOOST-mode and allows taking voltage from the power supply Vdd. When the detector 27 detects reaching of a predetermined voltage value, for example 10V, on the rising front of the signal Vpcx, the value of RISE is brought high and the block 25 is enabled accurately only when the signal Vpcx is greater than 10V which allows prevention of snap-back phenomena during programming. After programming, the line 10 VPCY is activated with low VPP-dis signal. On the control terminal of the discharging transistor Ms is a high signal which allows discharging the line 6 VPCX. This signal is applied even to the control terminal of the power transistor P2 which however interprets this signal as relatively low. If the DMA signal is taken high the line 10 VPCY goes to the value of the line 6 VPCX. The circuit in accordance with the present invention solves the technical problem and achieves numerous advantages the first of which is certainly the fact that the 'cascade' architecture of the various power lines allows conferring greater protection against spurious power supply signals directed towards the memory matrix and other circuit parts susceptible to change. In particular, the fact that the drain voltage Vpd is generated following a detection on the rising front of the first programming voltage Vpcx ensures correct switching sequence. Furthermore the circuit in accordance with the present invention incorporates a smaller number of control structures for a given function with the circuits in accordance with the prior art. There is also obtained better entrainment of the BOOST line performed with PMOS transistors in the initial phase and reduced impedance of this line with better correlation of the voltage Vpcx regulated upstream with the other voltage Vpcy regulated downstream. Lastly, it can be stated that the circuit in accordance with the present invention achieves better protection of the programming circuits of the configuration elements thanks to the 'cascade' architecture of the power supply lines and to the inertia characteristics towards spurious switching signals.

Claims: Power supply circuit (1) for switching programming voltages to an electrically programmable and erasable non-volatile semiconductor memory device of the type comprising a first switch-regulator block (23) connected to a main power supply line (2) and generating a first programming voltage (Vpcx) on a first programming line (6) coupled to the memory device and at least one second switch-regulator block (28) generating a second programming voltage (Vpcy) on a second programming line (10) coupled to the memory device, characterized in that said second switch-regulator black (28) is connected to the first programming line (6) to receive the first programming voltage (Vpcx). Circuit in accordance with claim 1 and characterized in that it comprises a detector block (27) connected to the first line (6) of programming voltage (Vpcx) to detect reaching of a predetermined high level (RISE) on the rising front of the programming voltage signal (Vpcx) and reaching of a predetermined low level FALL on the falling front of this voltage signal (Vpcx) and emit at output a corresponding enablement signal (VPD-en) for a third regulator block (25) located downstream. Circuit in accordance with claim 1 and characterized in that it comprises another switch-regulator block (29) powered both by the first line (6) and by the second programming line (10) to generate and regulate respective voltage values (UPR-CG,UPR-PG) to be applied to configuration elements of the memory device. Circuit in accordance with claim 1 and characterized in that said first switch-regulator block (23) comprises a connection between the first programming line (6) and the main line (2) incorporating the series of two power transistors (P1,P2) of the PMOS type with at least one control terminal of said transistors (P1,P2) being connected to an output (X) of a voltage booster block (30) receiving at input an enablement signal (VPP-dis). Circuit in accordance with claim 4 and characterized in that said voltage booster block (30) comprises a cell of transistors incorporating two complementary pairs of transistors with crossed connection. Circuit in accordance with claim 4 and characterized in that between an interconnection node (A) of said transistors (P1,P2) and the control terminal of the second transistor (P2) is inserted a discharge transistor (Ms) of the NMOS type with grounded source terminal. Circuit in accordance with claim 1 and characterized in that it comprises a voltage booster block (24) designed to produce on a line 16 a boosted voltage (BOOST) with said line (16) of boosted voltage (BOOST) being plugged in at an intermediate point (B) of the first switch-regulator block (23). Circuit in accordance with claim 1 and characterized in that said second switch-regulator block (28) is powered by the first line (6) through the series of two power transistors (P10,P11) of the PMOS type with at least one control terminal of said transistors (P10,P11) being connected to an output of a voltage booster block (35) receiving at input an enablement signal through a logic gate (38). Circuit in accordance with claim 1 and characterized in that it comprises a capacitor (C-vpcx,C-vpcy) for protection from overshoots between each programming line (6,10) and a signal ground (GND). Circuit in accordance with claim 1 and characterized in that said detector block (27) comprises a first circuit branch with a complementary pair of transistors (P7,M7) and on which is taken the low level signal (FALL) and a second circuit branch comprising the series of a pair of PMOS transistors (P8,P9) and an NMOS transistor (M8) on which is taken the high level signal (RISE).

Inventors: PASCUCCI LUIGI, PASCUCCI, LUIGI
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1996
Main classification: G11C5
IPC codes: G11C16, G11C5, G05F1