Voltage level shifter device, particularly for a non-volatile memory

Abstract: In a first operation mode the level shifter (36) transmits as output a logic input signal (S i ) and in a second operation mode it shifts the high logic level of the input signal from a low (V dd ) to a high voltage (V pp ). The level shifter comprises a CMOS switch (42) and a pull-up transistor (43); the CMOS switch (42) comprises an NMOS transistor (45) and a PMOS transistor (44) which are connected in parallel between the input (37) and the output (39) of the shifter and have respective control terminals connected to a first supply line (4) at low voltage (V dd ) and, respectively, to a control line (46) connected to ground in the first operation mode and to the high voltage (V pp ) in the second operation mode; the pull-up transistor (43) is connected between the output (39) of the shifter and a second supply line (2) switchable between the low voltage and the high voltage and has a control terminal connected to the first supply line (4).

Description: The invention relates to a voltage level shifter device, particularly for a non-volatile memory. As is known, integrated electronic devices may comprise circuit parts which, although cooperating with each other, operate on different voltage levels and must therefore be supplied with power separately. Furthermore, some of these circuit parts may require different supply voltages according to the operation mode of the device of which they form part. For example, in non-volatile memory devices (EPROM, EEPROM, flash etc.), to which reference will be made in the rest of the description, the various circuit parts (such as those for decoding, for redundancy, for programming etc.) operate at different voltages according to the operation mode of the device (reading, programming, verifying etc.) and must therefore be connected by means of circuits known as "level shifters". A block diagram relating to the arrangement of a voltage shifter in a non-volatile memory 10 is shown in Fig. 1, in which the voltage shifter 1, connected to a first supply line 2 at voltage V pc , is inserted between a first circuit 3 connected to a second supply line 4 placed at the supply voltage V dd , and a second circuit 5 connected to the first supply line 2. The voltage V pc alternatively has, according to the operation mode present (programming, reading, verification, ...) the value of the supply voltage V dd or of a voltage V pp (typically, the programming voltage) greater than V dd . For this purpose, the supply line 2 is connected, by means of a controlled switch 7, to the supply line 4 or to a supply line 6 at V pp , where the voltage V pp may be tapped from a suitable pin or generated internally in the device with suitable charge pumps. In a first operation mode in which the voltage V pc is equal to the supply voltage V dd , the shifter 1 is not required to make any level shift but solely to reproduce the input signal correctly, with minimum delay and consumption; vice-versa, in a second operation mode, in which the voltage V pc is equal to the voltage V pp , the shifter 1 has to generate at the output a signal whose lowest logic state is close to the reference potential of the circuit (ground) and the highest logic state is close to the voltage V pp . Fig. 2 shows an embodiment of a level shifter 1' of known type, having an input 9, on which it receives an input signal S i from the circuit 3 operating at the voltage V dd , and an output 12, on which it supplies a signal S o to the circuit 5 connected to the supply line 2. The level shifter 1' comprises an NMOS pass transistor 15 having a first terminal connected to the input 9, a second terminal connected to an intermediate node 16 and a gate terminal connected to the supply line 4 at voltage V dd ; an inverter 17, having N input connected to the intermediate node 16, AN output connected to the output 12 and supplied at the voltage V pc ; and a pull-up transistor 19 having a gate terminal connected to the output 12, a source terminal connected to the supply line 2 and a drain terminal connected to the intermediate node 16. In the level shifter 1', in the first operation mode (reading, verification, ...), the voltage V pc on the supply line 2 is equal to V dd . If the logic state of the signal S i present at the input 9 is low, the NMOS transistor 15 is on and, transmits the low logic state onto the node 16. Consequently, the inverter 17 outputs the signal S o of high logic state, equal to V pp . This signal, also present on the gate terminal of the pull-up transistor 19, keeps the latter off. When, however, the logic state of the signal S i is high, the NMOS transistor 15 operates like a diode as it has two terminals placed at the same voltage, it thus determines a voltage drop between the input 9 and the intermediate node 16 which is equal to its threshold voltage V th . If the supply voltage V dd is of high value, this voltage drop does not involve problems, and the inverter 17 carries the output signal S o at low logic level. In this situation, the pull-up transistor 19 is on, stabilizing the voltage on the intermediate node 16 on the value V pp . Vice-versa, if the supply voltage V dd is low (2.7 V for example) this voltage drop may cause problems of functionality and of speed. In the second operation mode (voltage V pp on the supply line 2), the operation of the shifter is similar to that described above. In this mode also there may be problems of functionality and speed, and even in a situation of standard level of supply voltage, whenever the difference between the supply voltage V dd and the threshold voltage V th is high, such as not to permit the triggering of the inverter 17. Another known embodiment of the level shifter is shown in Fig. 3, in which the shifter, denoted by 1", comprises two PMOS transistors 26, 27, two NMOS transistors 28, 29 and an inverter 30; the PMOS transistor 26 has a source terminal connected to the.supply line 2 at voltage V pc , a gate terminal connected to the output 12 and a drain terminal connected to a node 32; the NMOS transistor 28 has a drain terminal connected to the node 32, a gate terminal connected to the input 9 and a source terminal connected to ground; the PMOS transistor 27 has a source terminal connected to the supply line 2, a gate terminal connected to the node 32 and a drain terminal connected to the output 12; the NMOS transistor 29 has a source terminal connected to ground, a drain terminal connected to the output 12, and a gate terminal connected to the output of the inverter 30; the inverter 30 is connected to the supply line 4 at voltage V dd and has the input connected to the input 9. Both PMOS transistors 26 and 27 have the substrate connected to the source terminal. In both modes of operation, when the level of the signal S i is high, the NMOS transistor 28 is on and on the node 32 the signal is low. Consequently, the PMOS transistor 27 is on and the signal S o is high and of value equal to V dd in the first operation mode and V pp in the second operation mode. Vice-versa, if the signal S i is low, the NMOS transistor 28 is off, whilst the NMOS transistor 29 is on and the output signal S o is low. In practice, the level shifter 1" functions correctly in both modes of operation, but it has low switching speed and large size, because of the large number of transistors and the complexity of the connections. This size is particularly disadvantageous when the levels of large data or address buses have to be "shifted", for which reason the solution of Fig. 3 is difficult to apply in this case. The object of the invention is to provide a voltage level shifter which does not have the disadvantages encountered in the known solutions. The invention provides a voltage level shifter device, particularly for non-volatile memories, as defined in Claim 1. For an understanding of the invention, two preferred embodiments will now be described, purely by way of non-exhaustive example, with reference to the accompanying drawings in which: Fig. 1 shows a block diagram of a non-volatile memory comprising a level shifter; Fig. 2 shows a circuit diagram of a first voltage level shifter of known type; Fig. 3 shows a diagram of a second voltage level shifter of known type; Fig. 4 shows a simplified circuit diagram of a first embodiment of this level shifter; Fig. 5 shows a diagram of a second embodiment of this level shifter. The level shifter 36 of Fig. 4 (belonging to the memory 10) has an input 37 at which is present the signal S i supplied by the circuit 3 operating at the voltage V dd and an output 39 supplying the signal S o to the circuit 5 operating at the voltage V pc . The shifter 36 comprises a CMOS switch 42 (of the type commonly known as "transmission gate") and a PMOS-type pull-up transistor 43. The CMOS switch 42 is formed by a PMOS transistor 44 and an NMOS transistor 45 having first terminals coupled and connected to the input 37, second terminals coupled and connected to the output 39 and gate terminals connected to a control line 46 and, respectively, to the supply line 4 at voltage V dd . The substrate of the PMOS transistor 44 is also connected to the supply line 2 at V pc . Present on the control line 46 is a control logic signal C, whose value determines the state (on or off) of the PMOS transistor 44. A switch 48, controlled by a signal T, alternatively connects the control line 46 to ground or to a second supply line 2 at voltage V pc , as described in detail below. The pull-up transistor 43 has a source terminal connected to the supply line 2, a drain terminal connected to the output 39 and a gate terminal connected to the supply line 4. Furthermore, the pull-up transistor 43 has substrate connected to the supply line 2, so as to prevent the presence of undesirable direct polarizations between the various regions of the transistor 43. The pull-up transistor 43 is designed in such a way that its internal resistance is much greater than that of the NMOS transistor 45, for the reasons explained below. The level shifter 36 operates as follows. The signal T (the programming command signal for example), in the case of the first operation mode at low voltage, has a first logic level such as to determine the connection of the control line 46 to ground and, in the case of the second operation mode at high voltage, has a second logic level such as to determine the connection of the control line 46 to the supply line 2. In practice, when the switch 48 determines the connection between the control line 46 and the supply line 2, the latter has the voltage V pp and thus the control line 46 is no longer supplied at the voltage V dd . In the first operation mode, as indicated above, the control line 46 is connected to ground, for which reason the signal C has low value, whereas the supply line 2 has the voltage V dd . In these conditions, the pull-up transistor 43 is off, in that the voltage V dd is present at both the gate terminal and the source terminal. On the other hand the state of the CMOS switch 42 depends on the level of the input signal S i at the node 37; in fact, if the signal S i is of low logic level, the PMOS transistor 44 is off, as its gate and source terminals are at the same voltage, whilst the NMOS transistor 45 is on and connects the input 37 to the output 39; consequently, the signal S o is low. Vice-versa, if the input signal S i is of high logic level, the NMOS transistor 45 is off as its gate and source terminals are at the same voltage, whilst the PMOS transistor 44 is on and connects the input 37 to the output 39; consequently, the signal S o is high (and of value V dd equal to that of the input signal S i ). In the second operation mode, the control line 46 is placed at the voltage V pp . Consequently the PMOS transistor 44 is off, regardless of the level of the input signal S i ; at the same time, the pull-up transistor 43 is on, having voltage V pp at the source terminal and voltage V dd at the gate terminal. If the signal S i is of low level, the NMOS transistor 45 is on and forms, together with the pull-up transistor 43, a voltage divider between the supply line 2 and the ground; given that the resistance of the pull-up transistor 43 is much greater than that of the NMOS transistor 45, the signal S o present at the output 39 is low. This condition does, however, comprise a static consumption due to the continuous passage of current from the supply line 2 towards ground, through the NMOS transistor 45 and the PMOS transistor 43. Vice-versa, if the input signal S i is of high level, the NMOS transistor 45 is off (having voltage V dd both at the source and at the gate terminal) and the pull-up transistor 43 ensures the presence of a high signal level, of value V pp , at the output 39. Fig. 5 shows a level shifter 36' similar to the level shifter 36 of Fig. 4. The level shifter 36' differs from the shifter 36 by the presence of an inverter 53 between the output node of the CMOS switch 42, denoted by 54, and the output 39, and by the fact that the pull-up transistor 43 has the drain terminal connected to the node 54 and the gate terminal connected to the output 39. The inverter 53 is connected to the supply line 2. In a way not shown, the control line 46 is connected to the switch 48, in a similar way to Fig. 4. The level shifter 36' operates as follows. In the first operation mode (control signal C of low logic level and voltage V dd on the supply line 2), if a signal S i of low logic level is present on the input node 37, the PMOS transistor 44 is off, whilst the NMOS transistor 45 is on; there is therefore a low logic state at the node 54 and a signal S o which is high and of value V dd at the output from the inverter 53. This signal S o also provides to keep the pull-up transistor 43 off. Vice-versa, if the signal S i is of high logic level on the input node 37, the NMOS transistor 45 is off, whilst the PMOS transistor 44 is on and there is a high logic level on the node 54; there is therefore a signal S o of low logic level at the output of the inverter 53. In this situation the pull-up transistor 43 is on, carrying the node 54 to the voltage level V pp . In the second operation mode, the control signal C is of high logic level and, consequently, the PMOS transistor 44 is off, regardless of the level of the input signal S i . If the signal S i is low, the NMOS transistor 45 is on and there is a low level on the node 54. At the output of the inverter 53 there is therefore a signal S o of high level and of value V pp (given that the inverter 53 is now supplied at that voltage) and the pull-up transistor 43 is off, having a high voltage at its gate terminal. If the signal S i is high and there is the switching from the first to the second operation mode, the PMOS transistor 44 is turned off, the NMOS transistor 45 remains off and the pull-up transistor 43 (which is on) carries the node 54 to the high state at V pp . The output 39 thus remains low. Vice-versa, if the input signal S i switches from the low level to the high level during the second operation mode, the NMOS transistor 45, initially on, transmits the level of the input signal S i to the node 54 until that signal reaches the value V dd - V th , where V th is the threshold voltage of the transistor 45, after which it turns off. The inverter 53 thus receives as input a signal whose maximum value is V dd - V th , which may not be sufficient to cause it to switch. If this value is sufficient, at the output of the inverter there is a signal S o of low logic value which turns the pull-up transistor 43 on; in this case the latter carries the voltage V pp on the node 54. If, vice-versa, the value is insufficient, the inverter 53 does not switch and does not permit the pull-up transistor 43 to turn on, so that, when the NMOS transistor 45 is off, the node 54 remains floating and there is no passage of signal towards the output 39. Consequently, the level shifter 36', although not having static consumption, is not always suitable for being used in applications in which the switching of the input signal S i may take place in conditions of high voltage (V pc = V pp ). In this case, therefore, it is preferable to use the shifter 36 of Fig. 4. On the other hand, this problem does not exist in the case of use of the shifter 36' in a non-volatile memory, in which the switching of the input signal takes place solely during the first operation mode (V pc = V dd ) and the input signal is constant during the second operation mode. The level shifters 36 (Fig. 4) and 36' (Fig. 5) offer advantages with respect to both the known solutions described above. In fact, the level shifters 36 and 36' are faster and have better functionality in conditions of low voltage compared with the level shifter 1' (Fig. 2), and are simpler and smaller in size compared with the level shifter 1" (Fig. 3). Furthermore, it will be clear that modifications and variants may be introduced to the level shifter described and illustrated here without thereby departing from the protective scope of the invention. In particular, in the embodiment of Fig. 4, the ground line may be replaced by a simple connection to the substrate, using the biasing of substrate already present for other components of the circuit (memory), given that it has solely the function of suitably biasing the bulk of the NMOS transistor 45 (in a manner not shown).

Claims: Voltage level shifter device, particularly for non-volatile memories, including: a first line (ground) set at a first reference potential; a second line (4) set at a second reference potential (V dd ) greater than the first; a third line (2) having a potential value (V pc ) switchable between said second reference potential (V dd ) in a first operation mode and a third reference potential (V pp ), greater than said second reference potential, in a second operation mode; an input (37) receiving an input signal (S i ) which switches between a first logic state corresponding to said first reference potential and a second logic state corresponding to said second reference potential (V dd ); an output (39) supplying an output signal (S o ) which switches between a first logic state corresponding to said first reference potential and a second logic state corresponding to said potential value (V pc ) ; a switch (42) defining a first and a second node (50, 54) connected respectively to said input (37) and to said output (39); and a pull-up transistor (43) interposed between said third line (2) and said second node (54) and having a control terminal receiving a control signal for turning said pull-up transistor (43) on in said second operation mode and in the presence of said second logic state of said input signal (S i ) ; characterized in that said switch (42) comprises a first and a second transistor (45, 44), complementary to each other, having respective first terminals coupled together and connected to said first node (50), respective second terminals coupled together and connected to said second node (54) and respective control terminals connected to said second line (4) and, respectively, to said first line (ground) in said first operation mode and to said third line (2) in said second operation mode. Device according to Claim 1, characterized in that said first transistor (45) is an NMOS transistor and that said second transistor (44) and said pull-up transistor (43) are PMOS transistors. Device according to Claim 1 or 2, characterized by a fourth line (46) connected to said control terminal of said second transistor (44) and a switching element (48) for connecting said fourth line (46) to said first line (ground) in a first switching position corresponding to said first operation mode and to said third line (2) in a second switching position corresponding to said second operation mode. Device according to one of the preceding Claims, characterized in that said control terminal of said pull-up transistor (43) is connected to said second line (4). Device according to one of Claims 1-3, characterized in that said control terminal of said pull-up transistor (43) is connected to said output (39) and that said device comprises an inverter (53) interposed between said second node (54) of said switch (42) and said output (39); said inverter (53) being connected to said third line (2). Device according to one of the preceding Claims, characterized in that said pull-up transistor (43) has a resistance much greater than said first transistor (45).

Inventors: BARCELLA ANTONIO, FONTANA MARCO, BARCELLA, ANTONIO, FONTANA, MARCO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1997
Main classification: G11C16
IPC codes: G11C8, G11C16