Bidirectional charge pump

Abstract: A charge pump comprises at least one charge pump stage (S1-Sn) comprising a first diode (D1-Dn) having an anode (A) and a cathode (K), and a capacitor (C1-Cn) having a first plate connected to the cathode (K) of the diode (D1-Dn) and a second plate connected to a clock signal (CK1,CK2) periodically varying between a reference voltage and a supply voltage (VDD), the anode (A) of said diode (D1-Dn) forming a first terminal (NEG) of the charge pump. The charge pump comprises a second diode (Dn+1) having an anode (A) connected to the cathode (K) of the first diode (D1-Dn) and a cathode (K) forming a second terminal (POS) of the charge pump, first switching means (SW1) for selectively coupling the first terminal (NEG) of the charge pump to the voltage supply (VDD) and second switching means (SW2) for selectively coupling the second terminal (POS) of the charge pump to the reference voltage. The first switching means (SW1) and the second switching means (SW2) are respectively closed and open in a first operating condition whereby the second terminal (POS) of the charge pump acquires a voltage of the same sign but higher in absolute value than said supply voltage (VDD). The first switching means (SW1) and the second switching means (SW2) are respectively open and close in a second operating condition whereby the first terminal (NEG) of the charge pump acquires a voltage of opposite sign with respect to said voltage supply (VDD).

Description: The present invention relates to a bidirectional charge pump, particularly for the integration in CMOS memory devices such as EPROMs, EEPROMs and Flash EPROMs. In several integrated circuits it is necessary to generate, internally to the chip, voltages which are higher than or opposite in sign of the supply voltage of the chip. This can be done by means of charge pumps. For example, in single-power-supply Flash EPROMs, it is necessary to generate a positive voltage higher than the supply voltage (VDD) for programming the memory cells, and a negative voltage for erasing the memory cells. Up to now, distinct charge pumps have been used to generate different voltages: positive charge pumps for generating voltages higher than the supply voltage, and negative charge pumps for generating negative voltages. On the other hand, it is known that the main problem of the integration of a charge pump is the Large chip area occupied by the capacitors of the charge pump. The necessity of providing more than one charge pump exacerbates this problem. In view of the state of the art described, it is an object of the present invention to provide a bidirectional charge pump, i.e. a charge pump that can be used to generate both positive voltages higher than the supply voltage, and negative voltages. According to the present invention, such object is attained by means of a charge pump comprising at least one charge pump stage comprising a first diode having an anode and a cathode, and a capacitor having a first plate connected to the cathode of the diode and a second plate connected to a clock signal periodically varying between a reference voltage and a supply voltage, the anode of said diode forming a first terminal of the charge pump, characterized by comprising a second diode having an anode connected to the cathode of the first diode and a cathode forming a second terminal of the charge pump, first switching means for selectively coupling the first terminal of the charge pump to the voltage supply, and second switching means for selectively coupling the second terminal of the charge pump to the reference voltage, the first switching means and the second switching means being respectively closed and open in a first operating condition whereby the second terminal of the charge pump acquires a voltage of the same sign but higher in absolute value than said supply voltage, the first switching means and the second switching means being respectively open and close in a second operating condition whereby the first terminal of the charge pump acquires a voltage of opposite sign with respect to said voltage supply. Thanks to the present invention, it is possible to use a single charge pump for generating both positive voltages higher than the voltage supply, and negative voltages. This allows for a great reduction in the chip area. These and other features of the present invention will be made more evident by the following detailed description of a particular embodiment, illustrated as a non-limiting example in the annexed drawings, wherein: Figure 1 is a schematic circuit diagram of a charge pump according to the present invention; Figures 2A and 28 are timing diagrams of clock signals driving the charge pump of Fig. 1; Figure 3 is a cross-sectional view of a diode of the charge pump of Fig. 1; Figure 4 is a circuit diagram showing a practical embodiment of the charge pump of Fig. 1; Figures 5A and 5B show in cross-sectional view a biasing scheme of the diodes of the charge pump, respectively when the charge pump is used to generate positive voltages and negative voltages; and Figure 6 is a circuit arrangement for achieving the biasing scheme of Figs. 5A and 5B. With reference to Fig. 1, there is shown a charge pump according to the present invention. The charge pump comprises a plurality of serially-connected charge pump stages S1, S2, S3,...Sn; each stage comprises a diode D1-Dn and a capacitor C1-Cn. The diode D1 of the first stage has an anode connected to a first output terminal NEG of the charge pump; terminal NEG can also be selectively coupled, by means of a switch SW1, to a voltage supply VDD (conventionally, supposing that the charge pump is integrated in a CMOS memory device, VDD is a 5V or a 3V voltage supply). The diode Dn of the last stage Sn has a cathode connected to an anode of a diode Dn+1, which in turn has a cathode connected to a second output terminal POS of the charge pump; terminal POS can also be selectively coupled, by means of a switch SW2, to a reference voltage supply (ground voltage). Capacitors C1, C3, ... are driven by a first clock signal CK1; capacitors C2,...,Cn are driven by a second clock signal CK2. Clock signals CK1 and CK2 are generated by a clock generator 1. Figs. 2A and 2B show the timing of clock signals CK1 and CK2, respectively; signals CK1 and CK2 are square-wave digital signals switching between ground and the voltage supply VDD; signals CK1 and CK2 are substantially in phase opposition to each other. When switch SW1 is closed and switch SW2 is open, a positive charge is transferred from each capacitor of the charge pump to the righthand adjacent one. Thus, positive charge is transferred from the voltage supply VDD to terminal POS. Consequently, output terminal POS takes (after a transient) a positive voltage higher than VDD; the exact value of the voltage at POS depends on the number of stages of the charge pump: ideally, such a voltage is equal to n times the value of VDD, where n is the number of stages of the charge pump; in the practice, the voltage at terminal POS is always lower than said ideal value, for example because the turn-on voltage of the diodes D1-Dn+1 is to be taken into account. When switch SW1 is open and switch SW2 is closed, a positive charge is again transferred from each capacitor of the charge pump to the righthand adjacent one. Thus, a positive charge is transferred from the output terminal NEG to ground, and terminal NEG takes a negative voltage, whose value again depends on the number of stages of the charge pump. Thus, the charge pump of Fig. 1 can work both as a positive charge pump and as a negative charge pump. Fig. 3 shows in cross-section the preferred structure of a diode D1-Dn+1 of the charge pump; such a structure is particularly useful for the integration in CMOS memory devices such as, for example, EPROMs, EEPROMs and Flash EEPROMs. A P type silicon substrate 2 represents the substrate wherein the CMOS memory device is formed. Inside substrate 2, an N type well 3 is formed, and inside N type well 3 a P type well 4 is formed. Inside P type well 4, a P+ contact region 5 is formed for biasing the P type well, and an N+ region 6 is also formed. N+ region 6 forms a cathode K of the diode Di, while the P type well 4 and the P+ contact region 5 form an anode A of the diode Di. Inside N type well 3 an N+ contact region 7 is formed for biasing the N type well 3; it is possible to see that the structure has inherently associated a parasitic diode Dp having anode coincident with that of diode Di, and cathode K' formed by the N type well 3 and the N+ contact region 7. Diode Dp must be kept in reverse-biasing condition in order to achieve correct working of the structure. In the following, it will be described how it is possible to guarantee the correct biasing of the parasitic diode Dp in each stage of the charge pump, both in positive-voltage and in negative-voltage generation modes of the charge pump. Fig. 4 shows a practical embodiment of switches SW1 and SW2. Switch SW2 is formed by means of an N-channel MOSFET M1 with drain connected to terminal POS, source connected to ground and gate driven by a control signal NEGGEN which is a logic "0" when the charge pump works as a positive charge pump, while NEGGEN is a logic "1" when the charge pump works as a negative charge pump. Switch SW1 is instead formed by means of a P-channel MOSFET M2 with drain connected to terminal NEG, gate connected to signal NEGGEN and source connected to a signal REL which when the charge pump works as a positive charge pump is connected to the voltage supply VDD, while when the charge pump works as a negative charge pump it is switched to ground to reduce the electrical stress of the n-well drain junction of MOSFET M2. Fig. 5A shows the biasing scheme of the N type well 3 of the diodes D1-Dn+1 of the charge pump of Fig. 1 when the latter works as a positive charge pump. In this figure, two adjacent diodes Dk, Dk+1 of the charge pump are shown in cross-section. The P type substrate 2 is conventionally kept grounded. In each diode, the N+ contact region 7 is electrically connected to the P+ contact region 5 (which is in turn connected to the N+ region 6 of the preceding diode); in this way, the N type well 3 of each diode is biased at the same potential of the anode A of the diode. Thanks to this arrangement, the junction between the N type well 3 and the P type well 4 can never be forward biased, and it is assured that the parasitic diode Dp associated to each diode of the charge pump is interdicted. Similarly to Fig. 5A, Fig. 5B shows the biasing scheme of the N type well of diodes D1-Dn+1 when the charge pump works as a negative charge pump. Again, the P type substrate 2 is kept grounded. In this case however, the N type well 3 of each diode is biased at the voltage supply VDD or at ground. More precisely, the N type well of all the diodes D1-Dn+1 of the charge pump are initially biased at VDD; then, when the voltage at terminal NEG takes a negative value with a rather high absolute value, the N type well of the diodes belonging to the charge pump stages near terminal NEG is switched to ground (switch SW3 in Fig. 5B), so to reduce the voltage difference between the N type well 3 and the P type well 4 of said diodes. In this way, reliability of the structure is increased. The N type well of the diodes belonging to charge pump stages sufficiently far away from terminal NEG is insted kept at VDD. Fig. 6 shows a circuit arrangement suitable for achieving the biasing schemes of Figs. 5A and 5B. Cathode K of diode Dk is electrically connected to a series of two P-channel MOSFETs M3 and M4. A gate electrode of MOSFET M3 is controlled by an output of an inverter I1 which is in turn supplied by an output of an inverter I2. Inverter I2 is supplied by an output 10 of a two-input AND gate 8 supplied with signals NEGGEN and REL. The output node 9 of inverter I2 is also electrically coupled to ground by means of a capacitor C. Output 10 of AND gate 8 is also connected to a source electrode of MOSFET M4; the gate of MOSFET M4 is connected to the cathode K of diode Dk. An intermediate node 11 between MOSFETs M3 and M4 is connected to the N+ contact region 7 of diode Dk+1. Node 11 is also connected to a drain electrode of an N-channel MOSFET M5 with gate connected to drain; the drain electrode of MOSFET M5 is connected to a drain of a P-channel MOSFET M6 having source connected to VDD; the gate of MOSFET M6 is driven by a logic complement of signal REL. The operation of the circuit of Fig. 6 wiLl be now explained. When the charge pump works as a positive charge pump to generate a positive voltage on terminal POS, signal NEGGEN is a logic "0", the output 10 of AND 8 is a logic "0" and MOSFETs M3 and M4 are respectively on and off. The N-type well 3 of diode Dk+1 is thus electrically connected, through M3, to the N+ contact region 6 of diode Dk, and since the latter is connected to the P+ contact region 5 of diode Dk+1, it follows that the N-type well 3 of diode Dk+1 is short-circuited to the P-type well 4 of the same diode, as discussed in connection with Fig. 5A. When the charge pump works as a negative charge pump to generate a negative voltage on terminal NEG, signal NEGGEN is a logic "1"; however, in this case the logic value of output signal 10 of AND gate 8 depends on the logic value of signal REL. Initially signal REL is a logic "1", signal 10 is a logic "1", MOSFET M3 is off and MOSFET M4 is on. The N-type well 3 of diode Dk+1 is biased at VDD through MOSFET M4 (N+ contact region 7 is connected, through MOSFET M4, to node 10, which in turn is at VDD). Then, after a prescribed time interval, signal REL switches to "0": node 10 switches to "0", but MOSFET M3 cannot turn on because in the meantime the cathode K of diode Dk has achieved a negative potential; for the same reason, MOSFET M4 remains on and biases the N type well 3 of diode Dk+1 to the potential of node 10, i.e. to ground. Switching of signal REL to "0" also causes MOSFET M6 to turn off, so to prevent the creation of a conductive path between VDD and ground through MOSFETs M6, M5, M4 and the pull-down MOSFET of AND gate 8. Inverters I1 and I2 and capacitor C are necessary to prevent turning on of MOSFET M3 during the switching of signal REL from "1" to "0". Inverters I1 and I2 and capacitor C introduce a delay in the switching of the gate voltage of M3 in consequence of switching of signal REL. Without such a delay, for a finite time interval during switching of signal REL the gate voltage of M3 would be 0V, the source voltage would be VDD and the drain voltage would be negative (because the drain of M3 is connected to the cathode of diode Dk): in these conditions, MOSFET M3 would be on, and a load would be connected to the cathode of diode Dk, thus reducing the efficiency of the charge pump. Thanks to the delay introduced by inverters I1 and I2 and capacitor C, the gate voltage of MOSFET M3 goes to ground when node 11 (i.e., the N-type well 3 of diode Dk+1) is already at ground. MOSFETs M5 and M6 allow to bias the N-type well of the diodes of the charge pump when the latter is not working. It should be noted that white AND gate 8, inverters I1 and I2, capacitor C, and MOSFET M6 can be unique for all the charge pump stages proximate to the output terminal NEG of the charge pump (for which it is desired that the N-type well 3 of the respective diode is switched from VDD to ground when the anode of the diode attains a rather high negative voltage), distinct MOSFETs M3, M4 and M5 are to be individually provided for each of said charge pump stages. For the remaining stages sufficiently far away from the output terminal NEG, for which it is not necessary to switch the N-type well 3 of the respective diode from VDD to ground, it is only necessary to provide a common resistive load in place of MOSFET M6, and individual MOSFETs M3, M4 and M5 for each stage, wherein MOSFET M3 and the source electrode of MOSFET M4 are directly controlled by signal NEGGEN. Advantageously, the capacitors C1-Cn of the charge pump can have the structure described in the co-pending European Patent Application No. 96830420.4 filed on 30 July 1996 in the name of the same Applicant, wherein integrated structure capacitors are described which can be used both for positive and for negative voltages at each of their plates.

Claims: Charge pump comprising at Least one charge pump stage (S1-Sn) comprising a first diode (D1-Dn) having an anode (A) and a cathode (K), and a capacitor (C1-Cn) having a first plate connected to the cathode (K) of the diode (D1-Dn) and a second plate connected to a clock signal (CK1,CK2) periodically varying between a reference voltage and a supply voltage (VDD), the anode (A) of said diode (D1-Dn) forming a first terminal (NEG) of the charge pump, characterized by comprising a second diode (Dn+1) having an anode (A) connected to the cathode (K) of the first diode (D1-Dn) and a cathode (K) forming a second terminal (POS) of the charge pump, first switching means (SW1) for selectively coupling the first terminal (NEG) of the charge pump to the voltage supply (VDD), and second switching means (SW2) for selectively coupling the second terminal (POS) of the charge pump to the reference voltage, the first switching means (SW1) and the second switching means (SW2) being respectively closed and open in a first operating condition whereby the second terminal (POS) of the charge pump acquires a voltage of the same sign but higher in absolute value than said supply voltage (VDD), the first switching means (SW1) and the second switching means (SW2) being respectively open and closed in a second operating condition whereby the first terminal (NEG) of the charge pump acquires a voltage of opposite sign with respect to said voltage supply (VDD). Charge pump according to claim 1, characterized in that said at Least one charge pump stage (S1-Sn) comprises a plurality of identical serially-connected charge pump stages (S1-Sn), the second terminal of the capacitor of a stage being connected to a respective clock signal (CK1,CK2) which is substantially in phase opposition with respect to the clock signals (CK1,CK2) of the adjacent charge pump stages. Charge pump according to claim 2, characterized in that said first (D1-Dn) and second (Dn+1) diodes are PN junction diodes. Charge pump according to claim 3, characterized in that each of said first (D1-Dn) and second (Dn+1) diodes comprises a first well region (3) of a first conductivity type formed in a semiconductor substrate (2) of a second conductivity type, a second well region (4) of the second conductivity type formed inside the first well region (3) forming a first electrode of the diode, and a doped region (6) of the first conductivity type formed inside the second well region (4) forming a second electrode of the diode. Charge pump according to claim 4, characterized by comprising first biasing means (8,I1,I2,C,M3,M5,M6) which in said first operating condition of the charge pump bias the first well region (3) of each diode (D1-Dn+1) at a same voltage as the second well region (4) of the diode. Charge pump according to claim 5, characterized by comprising second biasing means (8,I1,I2,C,M4-M6) which, in said second operating condition and for at least an initial portion of a working time of the charge pump in said second operating condition, bias the first_well region (3) of each diode (D1-Dn+1) at the supply voltage (VDD). Charge pump according to claim 6, characterized in that said second biasing means (8,I1,I2,C,M4-M6) comprise means (8) for switching a bias voltage of the first well region (3) of at least the charge pump stages (S1-Sn) proximate to the first terminal (NEG) to a ground voltage when a voltage of the second terminal (NEG) reaches a prescribed value. Charge pump according to claim 7, characterized in that said second switching means (SW1) comprise an N-channel MOSFET (M1) with a drain connected to the second terminal (POS) of the charge pump and a source connected to ground, and said first switching means (SW2) comprise a P-channel MOSFET (M2) with drain connected to the first terminal (NEG) of the charge pump and source connected to switchable supply line (REL) which in said second operating condition is kept at the voltage supply (VDD) during said initial portion of working time, while after said initial portion of working time is switched to ground. Charge pump according to claim 8, characterized in that said first conductivity type is the N type and said second conductivity type is the P type.

Inventors: CAMPARDO GIOVANNI, GHILARDELLI ANDREA, MULATTI JACOPO, CAMPARDO, GIOVANNI, GHILARDELLI, ANDREA, MULATTI, JACOPO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1996
Main classification: G11C5
IPC codes: G11C5