Abstract:
A charge pump comprises at least one charge pump stage including 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 that periodically varies between a reference voltage and a supply voltage, the anode of said diode forming a first terminal of the charge pump. The charge pump further comprises 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 are respectively closed and open in a first operating condition whereby the second terminal of the charge pump acquires a voltage of the same polarity but higher in absolute value than said supply voltage. The first switching means and the second switching means are respectively open and closed in a second operating condition whereby the first terminal of the charge pump acquires a voltage of opposite polarity with respect to said voltage supply.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a bidirectional charge pump, and more particularly to a bidirectional charge pump used in CMOS memory devices such as EPROMs, EEPROMs and Flash EPROMs. 
     2. Discussion of the Related Art 
     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 a single-power-supply Flash EPROM, it is necessary to generate a positive voltage higher than the supply voltage (VDD) for programming memory cells of the EPROM, 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. 
     SUMMARY OF THE INVENTION 
     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. 
     In one embodiment of the present invention a charge pump is provided. The charge pump includes at least one charge pump stage including a first diode, and a capacitor, an anode of the diode forming a first terminal of the charge pump, 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 a supply voltage, and a second switching means for selectively coupling the second terminal of the charge pump to a reference voltage, the first switching means and the second switching means being respectively closed and opened in a first operating condition whereby the second terminal of the charge pump acquires a voltage of the same polarity but higher in absolute value than the supply voltage, the first switching means and the second switching means being respectively opened and closed in a second operating condition whereby the first terminal of the charge pump acquires a voltage of opposite polarity with respect to the supply voltage. 
     In another embodiment, a charge pump is provided for coupling to first and second terminals of a voltage supply to receive a supply voltage having a magnitude and a polarity with respect to a reference voltage and for generating a boosted voltage having a magnitude and a polarity. The charge pump includes a first input coupled to the first terminal of the voltage supply to receive the supply voltage, a second input coupled to the second terminal of the voltage supply to receive the reference voltage, a charge pump circuit having a first voltage node and a second voltage node, the charge pump circuit being configured to transfer electrical charge from the first node to the second node. The charge pump further comprises a first switch that selectively couples the first node of the charge pump circuit to the first input, and a second switch that selectively couples the second node of the charge pump circuit to the second input. 
     In another embodiment, a charge pump is provided for coupling to a voltage supply to receive a supply voltage having a magnitude and a polarity with respect to a reference voltage for generating a boosted voltage having a magnitude and a polarity. The charge pump includes a charge pump circuit having a first voltage node and a second voltage node, the charge pump circuit being configured to transfer electrical charge from the first node to the second node, means for coupling the charge pump circuit to the first input for a first mode of operation of the charge pump for producing a voltage at the first node having a polarity opposite the polarity of the supply voltage and having a magnitude greater than the magnitude of the supply voltage, and means for coupling the charge pump circuit to the voltage supply for a second mode of operation of the charge pump circuit for producing a voltage at the second node having a polarity equal to the polarity of the supply voltage and having a magnitude greater than the magnitude of the supply voltage. 
     In another embodiment of the present invention, a method is provided for generating a boosted voltage having a magnitude and a polarity with respect to a reference voltage using a supply voltage having a magnitude and a polarity with respect to the reference voltage. The method includes steps of providing a charge pump circuit having a first voltage node and a second voltage node, the charge pump circuit being configured to transfer electrical charge from the first node to he second node, applying the supply voltage to the first node of the charge pump circuit to produce a boosted voltage at the second node, the boosted voltage having a polarity equal to the polarity of the supply voltage and having a magnitude greater than the magnitude of the supply voltage, and applying the reference voltage to the second node of the charge pump circuit to produce a boosted voltage at the first node, the boosted voltage having a polarity opposite the polarity of the supply voltage and having a magnitude greater than the magnitude of the supply voltage. 
     In another embodiment of the present invention a memory device is provided. The memory device includes a first input to receive a supply voltage having a magnitude and a polarity with respect to a reference voltage, a second input to receive the reference voltage, a charge pump coupled to the first and second inputs to respectively receive the supply voltage and the reference voltage, the charge pump including a charge pump circuit having a first voltage node and a second voltage node, the charge pump circuit being configured to transfer electrical charge from the first node to the second node, a first switch that selectively couples the first node of the charge pump circuit to the first input of the memory device, and a second switch that selectively couples the second node of the charge pump circuit to the second input of the memory device, and a first clock input to receive a first clock signal. The memory device further includes a clock generator coupled to the first clock input of the charge pump circuit to provide the first clock signal. 
     Thanks to embodiments of 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 of memory devices using charge pumps. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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 
     FIG. 1 is a schematic circuit diagram of a charge pump according to one embodiment of the present invention; 
     FIGS. 2A and 2B are timing diagrams of clock signals driving the charge pump of FIG. 1; 
     FIG. 3 is a cross-sectional view of a diode of the charge pump of FIG. 1; 
     FIG. 4 s a circuit diagram showing a practical embodiment of the charge pump of FIG. 1; 
     FIGS. 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 
     FIG. 6 is a circuit arrangement for achieving the biasing scheme of FIGS. 5A and 5B. 
    
    
     DETAILED DESCRIPTION 
     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 S 1 , S 2 , S 3 , . . . Sn; each stage comprises a diode D 1 -Dn and a capacitor C 1 -Cn. The diode D 1  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 SW 1 , 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 SW 2 , to a reference voltage supply (ground voltage). Capacitors C 1 , C 3 , . . . are driven by a first clock signal CK 1 ; capacitors C 2 , . . . , Cn are driven by a second clock signal CK 2 . Clock signals CK 1  and CK 2  are generated by a clock generator  1 . 
     FIGS. 2A and 2B show the timing of clock signals CK 1  and CK 2 , respectively; signals CK 1  and CK 2  are square-wave digital signals switching between ground and the voltage supply VDD; signals CK 1  and CK 2  are substantially in phase opposition to each other. 
     When switch SW 1  is closed and switch SW 2  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 D 1 -Dn+1 is to be taken into account. 
     When switch SW 1  is open and switch SW 2  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 D 1 -Dn+1 of the charge pump; such a structure is particularly useful for 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 SW 1  and SW 2 . Switch SW 2  is formed by means of an N-channel MOSFET M 1  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 SW 1  is instead formed by means of a P-channel MOSFET M 2  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 M 2 . 
     FIG. 5A shows the biasing scheme of the N type well  3  of the diodes D 1 -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 D 1 -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 D 1 -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 SW 3  in FIG.  5 B), 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 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 M 3  and M 4 . A gate electrode of MOSFET M 3  is controlled by an output of an inverter I 1  which is in turn supplied by an output of an inverter I 2 . Inverter I 2  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 I 2  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 M 4 ; the gate of MOSFET M 4  is connected to the cathode K of diode Dk. An intermediate node  11  between MOSFETs M 3  and M 4  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 M 5  with gate connected to drain; the drain electrode of MOSFET M 5  is connected to a drain of a P-channel MOSFET M 6  having source connected to VDD; the gate of MOSFET M 6  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 M 3  and M 4  are respectively on and off. The N-type well  3  of diode Dk+1 is thus electrically connected, through M 3 , 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.  5 A. 
     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 M 3  is off and MOSFET M 4  is on. The N-type well  3  of diode Dk+1 is biased at VDD through MOSFET M 4  (N+ contact region  7  is connected, through MOSFET M 4 , 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 M 3  cannot turn on because in the meantime the cathode K of diode Dk has achieved a negative potential; for the same reason, MOSFET M 4  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 M 6  to turn off, so to prevent the creation of a conductive path between VDD and ground through MOSFETs M 6 , M 5 , M 4  and the pull-down MOSFET of AND gate  8 . 
     Inverters I 1  and I 2  and capacitor C are necessary to prevent turning on of MOSFET M 3  during the switching of signal REL from “1” to “0”. Inverters I 1  and I 2  and capacitor C introduce a delay in the switching of the gate voltage of M 3  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 M 3  would be 0V, the source voltage would be VDD and the drain voltage would be negative (because the drain of M 3  is connected to the cathode of diode Dk): in these conditions, MOSFET M 3  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 I 1  and I 2  and capacitor C, the gate voltage of MOSFET M 3  goes to ground when node  11  (i.e., the N-type well  3  of diode Dk+1) is already at ground. MOSFETs M 5  and M 6  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 while AND gate  8 , inverters I 1  and I 2 , capacitor C, and MOSFET M 6  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 M 3 , M 4  and M 5  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 M 6 , and individual MOSFETs M 3 , M 4  and M 5  for each stage, wherein MOSFET M 3  and the source electrode of MOSFET M 4  are directly controlled by signal NEGGEN. 
     Advantageously, the capacitors C 1 -Cn of the charge pump can have the structure described in the co-pending European Patent Application No. 96830420.4 filed on Jul. 30, 1996 in the name of the same Applicant, incorporated herein by reference, wherein integrated structure capacitors are described which can be used both for positive and for negative voltages at each of their plates. 
     Having thus described at least one illustrative embodiment of the present invention, various alterations, modifications and improvements will readily occur to those skilled in the art. Such alterations, modifications and improvements are intended to be within the scope and spirit of the invention. Accordingly, the foregoing description is by way of example only. It is not intended as limiting. The invention&#39;s limit is defined only in the claims and the equivalent thereto.