Patent Abstract:
A circuit for generating a negative voltage on the basis of a positive voltage, including: at least one first transistor between a first terminal for applying a potential greater than a reference potential and a first node; a first capacitive element between the first node and a second node, a control terminal of said first transistor being linked to the second node; a first switch between the first node and a second terminal for applying the reference potential; a second switch between the second node and a third terminal for providing said negative voltage; a third switch between the second node and the second terminal; and a second capacitive element between the third terminal and the second terminal.

Full Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the national phase of International Application No. PCT/FR2014/052334, filed on Sep. 19, 2014, which claims the priority benefit of French Application No. 13/59117, filed on Sep. 23, 2013, which applications are hereby incorporated by reference to the maximum extent allowable by law. 
     BACKGROUND 
     The present invention generally relates to electronic circuits and, more particularly, to a circuit for generating a negative voltage from a positive power supply voltage. 
     DISCUSSION OF THE RELATED ART 
     Many charge pump circuits including circuits intended to generate a negative voltage from a positive power supply voltage are known. In particular, it has already been provided to use a field-effect transistor to supply, from a positive voltage, a switched-capacitance charge pump circuit. An example of such a circuit is described in article “Integrated Anti-Short-Circuit Safety Circuit in CMOS SOI for Normally-On JFET” of Khalil El Falahi et al. (CIPS 2012, Mar. 6-8, 2012, Nuremberg, Germany). A JFET transistor, used to recover the power from a positive power supply bus, has its gate permanently directly connected to ground. This circuit requires using a precharge circuit upstream of the capacitive charge pump circuit. 
     SUMMARY 
     An embodiment of the present disclosure aims at providing a circuit for generating a negative voltage from a positive voltage which overcomes all or part of usual solutions. 
     Another embodiment of the present disclosure aims at providing a circuit compatible with various applications capable of using a negative voltage. 
     Another embodiment of the present disclosure aims at a particularly simple solution. 
     Thus, an embodiment of the present disclosure aims at a circuit for generating a negative voltage from a positive voltage, comprising: 
     at least a first transistor between a first terminal of application of a voltage higher than a reference potential and a first node; 
     a first capacitive element between the first node and a second node, a control terminal of said first transistor being connected to the second node; 
     a first switch between the first node and a second terminal of application of the reference potential; 
     a second switch between the second node and a third terminal for providing said negative voltage; 
     a third switch between the second node and the second terminal; and 
     a second capacitive element between the third terminal and the second terminal. 
     According to an embodiment, the circuit comprises a first resistive element between the first terminal and the first transistor. 
     According to an embodiment, the circuit further comprises: 
     a fourth switch between the control terminal of the first transistor and the second node; and 
     a fifth switch between the control terminal of the first transistor and a fourth terminal of application of a potential higher than the reference potential. 
     According to an embodiment, the circuit further comprises: 
     at least a second transistor between said first terminal and the control terminal of the first transistor, the control terminal of the second transistor being connected to the second node; and 
     a second resistive element, interposed between the control terminal of the first transistor and the second node. 
     According to an embodiment, the circuit further comprises a third resistive element between the second transistor and the first terminal. 
     According to an embodiment, said transistor(s) are N-channel transistors. 
     According to an embodiment, all switches are N-channel MOS transistors. 
     The present invention also provides a method for controlling a circuit such as hereabove, wherein: 
     in a first phase, the first and second switches are off while the third switch is on; and 
     in a second phase, the first and second switches are on while the third switch is off. 
     According to an embodiment, the first and second phases are repeated. 
     According to an embodiment, intervals having durations shorter than those of the first and second phases, and where all switches are off, are interposed between the successive phases. 
     According to an embodiment: 
     during the first phase(s), the fourth switch is off and the fifth switch is on; and 
     during the second phase(s), the fourth switch is on and the fifth switch is off. 
     The invention also provides an electronic circuit comprising at least one circuit for generating a negative voltage from a positive voltage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, among which: 
         FIG. 1  very schematically shows in the form of blocks a first example of application of a circuit providing a negative voltage; 
         FIG. 2  schematically shows in the form of blocks a second example of application of a circuit providing a negative voltage; 
         FIG. 3  shows an embodiment of a circuit for generating a negative voltage from a positive voltage; 
         FIG. 4  illustrates, in the form of timing diagrams, the operation of the circuit of  FIG. 3 ; 
         FIG. 5  very schematically illustrates in the form of blocks an example of a circuit for controlling the circuit of  FIG. 3 ; 
         FIG. 6  illustrates a variation of the circuit of  FIG. 3 ; and 
         FIG. 7  shows another variation of the circuit of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
     The same elements have been designated with the same reference numerals in the different drawings. For clarity, only those elements which are useful to the understanding of the embodiments which will be described have been shown and will be detailed. In particular, the destination of the described charge pump circuit has not been detailed, the described embodiments being compatible with usual applications using a charge pump circuit for providing a negative voltage from a positive voltage. Further, when reference is made to the positive or negative character of the voltage, it is referred to a same intermediate potential between the positive voltage and the negative voltage. For simplification, it is considered that this reference potential is the ground (zero potential) of the electronic circuit, which will generally be true in practice, so that the positive and negative voltages correspond to the potentials of the corresponding terminals. However, all that will be described hereafter applies to positive and negative voltages defined by potentials, respectively upper and lower, to a reference potential which is not necessarily the ground (for example, potentials both negative with respect to ground, the ground then forming the upper potential of the positive voltage and the reference potential being the least negative potential). 
       FIG. 1  schematically shows, in the form of blocks, an example of application of a charge pump circuit  1  (NEG POW) for generating a negative voltage V− from a positive voltage V+. According to this embodiment, negative voltage V− is used to power a control circuit  2  (DRIVER) of a transistor  3  (typically, a MOS transistor) in series with a load  4  (Q) between terminals  12  of application of a positive potential V+ and  14 , for example, a ground potential, or a potential corresponding to the high point of the charge. In such an application, the voltages involved at the level of load  4  and the switching thresholds of transistor  3  result in the need for a negative potential V− in order to properly control transistor  3 . 
       FIG. 2  schematically shows in the form of blocks another example of application of a circuit for generating a negative voltage V−. Voltage V− is, here again, delivered to a circuit  2  for controlling a power transistor  3 , series-connected with a load  4  (Q) powered with a positive voltage. Transistor  3  is here connected on the ground side. Here again, according to the involved voltages and to the switching thresholds of transistor  3 , a negative potential may be needed in order to control it properly. 
     For example, the negative voltage may be used to lock a switch (transistor  3 ) having a normally-on state. Another example is the control of a power transistor having a threshold voltage close to zero and which requires a biasing of its control terminal with a negative voltage to draw away from its threshold voltage and avoid for parasitic voltages to modify the off or on state. Another example is the control of an IGBT transistor which sometimes uses a negative voltage to perform an efficient locking. 
       FIG. 3  shows an example of an electric diagram of an embodiment of a circuit  1  for generating a negative voltage, based on a capacitive charge pump. 
     A transistor M 1 , typically a normally-on MOS transistor, is connected, directly or via a resistive element R (illustrated in dotted lines), to a terminal  12  of application of a potential V+ positive with respect to ground (terminal  14 ). The other power terminal of transistor M 1  is connected to a node  16  via a first capacitive element C 1  and, to terminal  14 , by a first switch K 1 . The control terminal (the gate) of transistor M 1  is connected (directly connected) to node  16 . Node  16  is connected, by a second switch K 2 , to a terminal  18  for providing negative output voltage V− and, by a third switch K 3 , to terminal  14  of application of the reference potential. Terminal  18  is further connected to terminal  14  by a second capacitive element C 2 . Switches K 1 , K 2  are controlled in all or nothing by a signal CT 1 . Switch K 3  is controlled in all or nothing by a signal CT 3 . These switches are, preferably, N-channel MOS transistors. 
       FIG. 4  illustrates, in the form of timing diagrams, the operation of circuit  1  of  FIG. 3 . 
     These timing diagrams respectively show examples of shapes of signal CT 2 , conditioning the off or on state of switch K 3 , of voltage V 15  of node  15  between transistor M 1  and capacitor C 1 , of signal CT 1 , conditioning the off or on state of switches K 1  and K 2 , of voltage V 16  of node  16 , and of output voltage V−. 
     Taking the preferred example of switches K 1 , K 2 , K 3  formed of N-channel MOS transistors and, to within the threshold voltages, these transistors are turned on when their gates are connected to a positive potential (high states of signals CT 1  and CT 2 ) and are turned off when their gates are grounded (low states of signals CT 1  and CT 2 ). 
     For simplification, the on-state voltage drops in switches K 1  to K 3  are neglected (the on-state drain-source resistances RdsON are considered as negligible). 
     An initially discharged state of capacitors C 1  and C 2  is assumed and all switches K 1  to K 3  are off. Voltage V− is then zero. 
     A charge pump cycle starts at a time t 0  at which switch K 3  is turned on (signal CT 2  in the high state), switches K 1  and K 2  being off (signal CT 1  in the low state). Node  15  starts by being grounded. Transistor M 1  being normally on, the potential of node  15  increases until a time t 1  when voltage V 15  reaches threshold voltage VT of transistor M 1 . This amounts to charging capacitor C 1  up to the locking voltage (threshold voltage VT) of transistor M 1 . 
     Then, the states of the switches are inverted to transfer the charges from capacitor C 1  to capacitor C 2 . In practice, to avoid a simultaneous conduction of the switches, it is started, at a time t 2 , subsequent to time t 1 , by turning off switch K 3  (signal CT 2  in the low state) and then, at a time t 3 , subsequent to time t 2 , switches K 1  and K 2  are turned on (signal CT 1  in the high state). 
     The fact of taking node  15  to ground, by the turning-on of switch K 1 , causes the discharge of capacitor C 2  and decreases the potential of node  16 , and thus of terminal  18  (switch K 2  being on), generating negative voltage V−. 
     At a time t 4 , subsequent to time t 3 , a reverse switching phase is started, that is, switches K 1  and K 2  are turned off (signal CT 1  in the low state), after which, at a subsequent time, corresponding to time t 0  of beginning of the next cycle, switch K 3  is turned on. 
     In the assembly of  FIG. 3 , the minimum value (the most negative value) that voltage V− can take is −VT. 
     According to the power sampled from terminal  18 , value −VT is reached in one or a plurality of cycles. In the example of  FIG. 4 , two cycles are assumed to be necessary. 
     The duration of phase(s) T 1 , between time t 0  and t 2 , is selected to be longer than the duration necessary for the charge of capacitor C 1  at level VT. This duration is a function, in particular, of the capacitance of capacitor C 1  and of the on-state drain-source resistance of transistor M 1 . 
     The duration of phase(s) T 2 , between times t 3  and t 4 , is selected to be longer than the time of recharge of capacitor C 1  through transistor M 1 . 
     Durations T 1  and T 2  are not necessarily identical. For example, a shorter duration T 2 , particularly, at the starting, enables to limit current inrushes. 
     Intervals Ta between times t 2  and t 3 , and Tb between times t 2  and t 3 , are selected to guarantee an absence of simultaneous conduction of switches K 1  to K 3 . 
     The biasing of transistor M 1  enables to make it normally on, which avoids a starting circuit. 
     Optional resistive element R is used to limit current inrushes. 
     An advantage of the circuit described in relation with  FIGS. 3 and 4  is that it is compatible with an embodiment only using N-channel MOS transistors. 
     The fact of making transistor M 1  for supplying the switched-capacitance circuit switchable spares a start circuit. Further, advantage is taken of one of the switches used to switch the capacitive elements to switch the power supply transistor. 
       FIG. 5  shows, in simplified fashion and in the form of blocks, an example of a circuit for generating control signals CT 1  and CT 2 . 
     In this example, an oscillator  22  (OSC) controlled (activated) by a signal ACT delivered by a comparator  24  (COMP) between output voltage level V− and a threshold TH is used. For the circuit of  FIG. 3 , threshold TH corresponds to a level higher than level −VT to stop the oscillator and thus decrease the power consumption. As a specific embodiment, a ring oscillator having a period conditioning durations T 1  and T 2  may be used, signals CT 1  and CT 2  being sampled at the output of two different inverters of the oscillator to define intervals Ta and Tb (then identical). Oscillator  22  and comparator  24  are powered, for example, with a positive voltage Vdd, which is not necessarily identical to voltage V+. 
     According to an alternative embodiment, a single-pulse generator, triggered when voltage V− has not reached a set point TH, is used. 
     According to another alternative embodiment, an analog regulation which monitors the voltage across capacitive element C 1  and its discharge into capacitor element C 2  is provided. 
     More generally, any circuit capable of generating control signals to respect the above-described switching phases may be used. 
       FIG. 6  shows another embodiment intended to provide an output voltage V−, lower than −VT (higher, in absolute value, than the absolute value of the threshold voltage of transistor M 1 ). 
     As compared with the circuit of  FIG. 3 , the gate of transistor M 1  is further connected, via a switch K 4  controlled by signal CT 1 , to node  16  and, via a switch K 5  controlled by signal CT 2 , to a bias potential Vg higher than the reference potential (and lower than potential V+). Switches K 4  and K 5  preferably are NMOS transistors. During phase T 1  ( FIG. 4 ), switch K 4  is off and switch K 5  is on. Potential Vg, applied to the gate of transistor M 1 , results in the charging of capacitive element C 1  to a voltage VT+Vg. During the following phase T 2 , the inversion of the voltage generated by the capacitive switching results in that voltage V− can reach −Vg-VT. The generation of potential Vg from voltage V+ is not a problem (for example, a resistive bridge, preferably switchable to avoid a permanent power consumption, or a voltage regulator). 
       FIG. 7  shows another embodiment enabling to reach a voltage more negative than −VT. 
     As compared with the circuit of  FIG. 3 , a second MOS transistor M 2  connects, optionally in series with a resistive element R 1 , terminal  12  to the gate of transistor M 1 , now connected to node  16  by a resistive element R 2  (or a capacitive element to decrease the dc power consumption (dc)). The gate of transistor M 2  is connected to node  16 . 
     Thus, during phase T 1  ( FIG. 4 ), element C 1  charges to a value corresponding to the sum of the two threshold voltages of transistors M 1  and M 2  and the generated negative voltage has this value in absolute value. 
     The embodiment of  FIG. 7  may be extended to even lower negative voltages by adding other transistors on the basis of the same assembly (between the gate of transistor M 2  and node  16 ). 
     Various embodiments have been described. Various alterations and modifications will occur to those skilled in the art. In particular, time intervals Ta and Tb between periods T 1  and T 2  of the charge pump circuit may be adapted to the necessary switching times of the different transistors. 
     Further, although reference has been made to MOS transistors on the application circuit side, the generated negative voltage may be used to control any type of transistor (IGBT, JFET, etc.) and, more generally, to power any type of circuit requiring a negative voltage. 
     Further, the sizing of capacitive elements C 1  and C 2 , possibly made in the form of a plurality of capacitors in parallel, depends on the application and particularly on the expected power consumption of the element(s) connected to terminal  18 . 
     Finally, the practical implementation of the described embodiments is within the abilities of those skilled in the art based on the functional indications given hereabove and using, for the rest, usual electronic circuit sizing techniques.

Technology Classification (CPC): 7