Abstract:
Presented is a voltage regulating circuit for a capacitive load, which is connected between first and second terminals of a supply voltage generator. The regulating circuit has an input terminal and an output terminal, and includes an operational amplifier having an inverting input terminal connected to the input terminal of the regulating circuit and a non-inverting input terminal connected to an intermediate node of a voltage divider. The voltage divider is connected between an output node, which is connected to the output terminal of the regulating circuit, and the second terminal of the supply voltage generator. The operational amplifier has an output terminal connected, for driving a first field-effect transistor, between the output node and the first terminal of the supply voltage generator. The output terminal of the operational amplifier is also connected to the output node through a compensation network. The voltage regulating circuit also includes a second field-effect transistor connected between the output node and the second terminal of the supply voltage generator, which has its gate terminal connected to a constant voltage generating circuit means.

Description:
TECHNICAL FIELD 
     This invention involves semiconductor storage devices, and relates in particular to a voltage regulating circuit for an essentially capacitive load. A circuit such as this is used to output a precisely controlled voltage and exhibit fast re-establishment capability, even when a previously discharged capacitor C s  is connected to its output. The fast re-establishment ensures that the circuit can restore the output voltage promptly to its regulator-set value. 
     BACKGROUND OF THE INVENTION 
     A typical example of circuits in the field of the invention is that of a voltage regulator for reading word lines from multi-level non-volatile memories, where a precisely regulated voltage is vital to optimal reading conditions. 
     FIG. 1 of the drawings shows a word-line read circuit  10  in a storage device. Upon connection of a capacitor C s    12 , to an output OUT of a regulator  20 , a regulator output voltage V reg , which has a normal rating value of V R , falls by reason of the charge sharing effect that occurs between the total capacitive load C r    14 , connected to the regulator output and the capacitor C s    12 . In FIG. 1, the circuit connection is represented by a switch SW  16 , which is closed when C r    14  is to be connected to the regulator output OUT. 
     This fall in the regulator output voltage V reg  occurs very rapidly and may be excessive in the sense that it may bring the value of the voltage V reg  outside its set range. The return to the voltage V reg  should be sufficiently fast, i.e., the regulator output voltage must be quickly brought back into its set range. 
     Typical values for a storage device parameters may be: 
     V R =6V 
     C r =100 pF 
     C s =3 pF 
     ΔV max =50 mV, 
     where, ΔV max  is the maximum admitted deviation of V reg  from its rating value V R . In other words, the voltage V reg  is judged to have been re-established, following connection to the capacitor C s , once the voltage is brought back to within 50 mV of the rating value of V reg , and subsequently held within 50 mV of that value. 
     The appearance of a high capacitive load value delays the regulator  20  operation in that it slows down the re-establishment of the output voltage V reg  on the occurrence of charge sharing due to the previously discharged capacitor C s    12  having been connected to the voltage regulator output OUT. The amount of charge drawn by the capacitor C s    12  upon connection is:                Q   s     =       (       V   reg     -     Δ                   V   max         )     *     C   s                   =     5.95   ×   3                 pC                 =     17.85                   pC   .                                    
     Suppose that the re-establishment time is not to exceed 20 ns, then the current that the regulator  20  is to deliver for peak efficiency would be (17.85 pC)/(20 ns) 892.5 μA, assuming for simplicity that the process of re-establishing the output voltage is taking place at a constant current. Actually, this is not exactly the case, and the overall capacitive load would be charged with a decreasing current over time, so that the peak current supplied by the regulator  20  is bound to exceed the above value. 
     A prior solution provided a regulator for storage devices which was basically in the form of an operational amplifier  40  connected in a negative feedback loop. 
     This loop comprised, as shown in FIG. 2, a first stage consisting of a differential amplifier  42 , and a second stage consisting of a pull-up element  44  formed of a PMOS transistor and a pull-down element or resistor divider  46  formed of two resistors R 1    48 , and R 2    52 . The combined stages form the operational amplifier  40 . The inverting terminal of the differential amplifier  42  is applied a precise constant voltage, designated V BG  in FIG. 2. A junction node  50  between the resistors R 1    48  and R 2    52  is connected to a non-inverting input of the differential amplifier  42 , thereby closing the negative feedback loop. In order to provide the loop with adequate stability, a compensation network  54 , represented by a block COMP in FIG. 2, may consist of a capacitor connected between the gate and the drain of the pull-up PMOS transistor  44  in the second stage. Other compensation networks may be used, however, such as that discussed by D. B. Ribner and M. A. Copeland in an article “Design Techniques for Cascoded CMOS Op Amps with Improved PSRR and Common-mode Input Range”, IEEE Journal of Solid-State Circuits, vol. SC-19, No. 6, December 1984, pages 919-925. 
     If the loop gain of the feedback loop is sufficiently high, barring such inaccuracies as offset voltages, then the regulator output voltage V R  in the steady-state condition is given as V R =V BG *(1+R 1 /R 2 ). In an integrated circuit, the resistance ratio between two resistors can be provided with great precision, but for less-than-ideal effects, and the accuracy in value of V R  will depend essentially on the accuracy achieved for the voltage V BG . The latter accuracy can be obtained by means of a band-gap type of voltage reference generator, which is known to generate a fairly precise and stable voltage even with such varying factors as the supply voltage and temperature. 
     Upon connection of the capacitor C s    12  to the regulator  40  output, the charge originally stored into the capacitor C r    14  becomes shared with the capacitor C s . The regulator output voltage at the end of the charge sharing process is, assuming inaction of the control loop at this stage: 
     
       
         V reg =C s V R /(C s +C r )  (1) 
       
     
     Therefore, the theoretical voltage drop at the regulator output can be written as: 
     
       
         ΔV reg =V r /(1+C r /C s ){tilde over (=)}V R C s /C r   (2) 
       
     
     Substituting the values given above, we get ΔV r =180 mV, which exceeds the maximum error value admitted on line V reg (ΔV max =50 mV). Thus, the regulator  40  is to supply the required electric charge for re-establishing the voltage to its desired value. 
     With very high total capacitive loads (e.g., 100 pF) on the regulator  40  output, the voltage V reg  may not be re-established as quickly as desired, because the product of band by gain is limited in the amplifying structure. 
     Prior approaches to solving this problem presupposed that the capacitance of C s    12 , and the time when its connection to the regulator output node OUT is required, were known beforehand. In addition, such approaches involved of necessity the generation of appropriate clock drive signals. 
     However, such prior solutions cannot be used where the capacitance of C s    12  or the time when C s  is connected to the regulator output node OUT is not exactly known beforehand (as is the case when the problem is unrelated to the drive of word lines in a non-volatile memory). 
     Until now, no circuit solution was available that provides for fast re-establishment of the voltage V reg  upon a previously discharged capacitor being connected to the output terminal of the regulator, through the use of a circuit that is easy to implement, and not the prior capacitive compensation or capacitive boost techniques. 
     SUMMARY OF THE INVENTION 
     Embodiments of the invention include a voltage regulating circuit for a capacitive load, which is connected between a supply and a ground terminal of a supply voltage generator. The regulating circuit has an input terminal and an output terminal, and includes an operational amplifier having an inverting input terminal connected to the input terminal of the regulating circuit and a non-inverting input terminal connected to an intermediate node of a voltage divider. The voltage divider is connected between an output node, which is connected to the output terminal of the regulating circuit, and the second terminal of the supply voltage generator. The operational amplifier has an output terminal connected, for driving a first field-effect transistor, between the output node and supply terminal of the supply voltage generator. The output terminal of the operational amplifier is also connected to the output node through a compensation network. The voltage regulating advantageously includes a second field-effect transistor connected between the output node and the ground terminal of the supply voltage generator, which has its gate terminal connected to a constant voltage generating circuit. 
     In another embodiment, a third field effect transistor is coupled between the output node and the supply node of the supply voltage generator, which is driven by another constant voltage generating circuit. 
     The features and advantages of a voltage regulating circuit according to the invention will become apparent from the following description of an embodiment thereof, given by way of example and not of limitation with reference to the accompanying drawings. 
     It is generally noted that the description of the embodiments explained below includes language of especially preferred embodiments, such as transistors built to match other transistors and currents equaling one another. Strictly speaking, these features are not necessary to practice the invention, but are anyway disclosed to enable the reader to more fully understand the usefulness of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a regulator for regulating the read voltage in multi-level non-volatile memories according to the prior art. 
     FIG. 2 shows a voltage regulating circuit for a capacitive load, according to the prior art. 
     FIGS. 3 and 4 show two embodiments of a voltage regulating circuit for a capacitive load, according to this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A basic task of the feedback loop of the circuit shown in FIG. 2 is to prevent the occurrence of ringing, as apt to result in overshooting of the voltage V reg , during the transient associated with a capacitor C s    12  being connected to the output terminal of the regulator. The output node OUT of the regulator  40  has an instantaneous voltage V reg , and a desired regulated voltage of V R . In ideal conditions, V reg , will always equal V R , but due to the conditions mentioned above, they may differ. If the voltage V reg  rises above its rating value V R , its fall toward V R  must go through resistors R 1    48  and R 2    52 . This fall will be quite slow, due to the high capacitance of C r    14  unless sufficiently low resistances are selected for R 1    48  and R 2    52 . However, low resistances of R 1    48 , R 2    52  result in high DC power consumption of the regulator, which may be unacceptable in some cases. For example, a high power consumption may be unacceptable where a voltage regulator is connected in an integrated circuit which is supplied a lower single external supply voltage V DD  than the regulator own supply voltage; it being possible to drive the latter from V DD  using a voltage boosting circuit based on the charge pump technique that usually exhibits limited capacity for current output. 
     In the past, the need to prevent this behavior had prompted previous designers to design an amplifier with a very large phase margin, thus reducing the band and with it the rate of operation of the amplifier. In fact, lacking such a large phase margin, the risk of ringing and overshooting of the output voltage may be incurred as the closed loop system responds to the fall in voltage caused by connecting C s    12 . 
     To obviate such problems, an embodiment of the invention provides for a circuit structure  100  coupled to the regulator  40  of FIG.  2 . In this circuit  100 , a pull down PMOS transistor  110  is used, as shown in FIG. 3. A source of the transistor  110  is coupled to an output node OUT of a voltage regulator  40 , and its drain is connected to ground. Its gate electrode is driven with a constant voltage V A  of a suitable value. The aspect ratio W/L of the transistor  110  and the value of the voltage V A  should be selected to keep the transistor  110  saturated and produce a small DC (or bias current) flow through the transistor  110 , so as to limit the power consumption of the structure at rest. It is for this reason that the value V GS −V THP , where V GS  is the transistor gate-source voltage and V THP  is the transistor threshold voltage of the PMOS transistor  110 , is kept suitably low. 
     As a preliminary approach, a current I D  flowing through a saturated PMOS transistor is known to depend quadratically on the voltage V GS −V THP  when the transistor is operated in a region of strong inversion, that is, when the difference V GS −V THP  is negative and sufficiently high in absolute value, and is tied exponentially to V GS  as the difference V GS −V THP  approaches zero. At all events, I D  increases as the voltage V SG =−V GS , that is the difference between the source voltage and the gate voltage, increases. When the voltage at the output node of the regulator exhibits overshooting, the current flowing through the transistor  110  can become considerably larger than the current which flows through the same transistor in the rest condition (i.e., when V reg =V R ); the voltage V SG  at the transistor MPD is, in fact, equal to V reg −V A , and its value increases for positive overshoots of V reg . 
     While the power consumption is relatively low in the rest condition, with positive overshoots raising the voltage V reg  to a value higher than V R , the output node OUT discharge current becomes large and the value of V reg  falls very fast. Accordingly, the operational amplifier of the regulating loop can be dimensioned to have a lower phase margin, and therefore a wider band, than if no transistor  110  were provided. Thus, by providing the transistor  110 , the operational amplifier can be dimensioned to accommodate overshoots in the regulating loop output voltage. On the occurrence of such overshooting, the voltage can be quickly brought back to within the admitted range of values. 
     FIG. 3 also shows a simple circuit for generating the voltage V A . It includes a PMOS transistor  112  and a current generator  114  generating a current I B . Conventionally, the current generator  114  can be simply formed of an NMOS transistor driven with a constant voltage of a suitable level; for example, it could be the output section of a current mirror, the input section whereof is supplied a constant current of known value. The two transistors  110 ,  112  match each other, i.e., are identical with each other (at least nominally) but for an appropriate scaling factor K of the channel width W. In the rest condition, both transistors  110 ,  112  have the same gate-source voltage V GS ; they have the same source voltage because their respective sources are short-circuited, and have the same gate voltage because no current passes through a resistor  114  having a resistance R b . Both transistors  110 ,  112  also have the same threshold voltage V THP  (but for some minor differences arising from the manufacturing process being less than ideal). Accordingly, the direct current flowing through the transistor  110  will be essentially equal to K·I B . By an appropriate choice of the values of I B  and K, the bias current to the transistor  110  can be held sufficiently low and the power consumption of the structure at rest be reduced. Mismatching of the two transistors  110 ,  112  due to practical effects might indeed cause the current to become different from K·I B , but such differences can be minimized by appropriate component designing. 
     The resistance R B  of the resistor  116  multiplied by a capacitance C B  of a capacitor  118  forms a low-pass filter. In DC, the voltage V A  is the same as the voltage V B , and any quick changes in the voltage V B  (as caused by quick changing of the voltage V reg , for example) do not propagate to the voltage V A  because of the filtering action applied by the R B C B  combination of the resistor  116  and the capacitor  118 . Of course, both components  116 ,  118  would have to be suitably dimensioned, this being a simple matter for circuit designers. For example, to adequately “filter out” voltage variations at a characteristic time of less than 10 ns, R B =5′kΩ and C B =1 pF could be chosen. Other filter structures of the low-pass type may be used to make the voltage V B  virtually constant. 
     When the voltage V reg  drops rapidly below the regulated value of V R , the transistor  110 , having the voltage V reg −V th +V ov  applied to its gate, will tend to turn off and promote re-establishment to the regulated voltage, where V th  is the threshold voltage of the transistor  110  and where V ov  is the overvoltage of transistor  110 . 
     An advantage of the circuit shown in FIG. 3 lies in its great simplicity: in fact, above the required components already present for the voltage regulator  40 , only two additional transistors  110  and  112  are required, plus the resistor  116  and the capacitor  118 . For proper operation, no switches are needed as would require associated drive signals. The current draw at rest of the additional structure, i.e., the current through the transistors  110 ,  112 , can be kept fairly low, and the discharge current from the output node OUT of the voltage regulator  40 , as the voltage V reg  at the output node OUT undergoes sharp rises due to overshooting, can be much larger than the current flowing through transistor  110  at rest. As said before, this enables the operational amplifier  42  in the regulating loop to be designed with a moderate phase margin, and hence, with a higher band (and higher rate), than without the additional structure. 
     A further advantage of a circuit according to embodiments of the invention is as explained herein below. In the rest condition, the current flowing through the transistor  44  is equal to the sum of the currents flowing through the resistive divider  46  and the transistors  110 ,  112 . By a suitably scaling factor K, the current through the transistor  112  can be made trivial, so that the combined currents become substantially equal to the sum of the currents through the resistor divider  46  and the transistor  110 . 
     Should the voltage V reg  from the output node OUT of the voltage regulator  40  fall in operation rapidly below the regulated value V R  (in consequence of a previously discharged capacitor being connected to the regulator output OUT, for example), then the transistor  110  would draw less current than at rest. This difference becomes greater as the voltage V reg  drops further. Its dependence on the value of the voltage drop is as previously explained; this drop may be great enough to cause the transistor  110  to be blocked. On this account, for a given current at rest, the pull-up transistor  44  is now able to deliver a larger current to the external capacitive load than would be possible if the transistor  110  were not there. This contributes to making the re-establishment of the output current faster, for a given current at rest and, therefore, a given power consumption. 
     Mathematically, the relationship that leads to a transistor being turned off can be described as follows: with V ov  being the overdrive voltage to the transistor  110  at rest, the voltage V A  will be V R −|V THP |−|V ov  |. Upon the voltage V reg  falling rapidly below the regulated value by an amount |V ov |, the transistor  110  tends to turn off, thereby promoting re-establishment to the regulated voltage. 
     It should be noted, however, that the transistor  112  serves no clamping function, since the output voltage of the voltage regulator  40  is set by the regulating loop. 
     This embodiment can be improved by adding a second circuit structure  200  between the output of the voltage regulator  40  and a positive supply V DD , as shown in FIG.  4 . The second circuit structure  200  is similar to the circuit structure  100  shown in FIG. 3, but it is made of NMOS transistors, as will be explained below. 
     The portion affected by the addition shown in FIG. 4 includes an NMOS transistor  212  having its gate shorted to its drain. A gate/drain node V B2  is coupled to the positive supply V DD  through a fixed current generator  214  that generates the same amount of current as the underlying generator in FIG.  4 . The two current generators  114 ,  214  are matched together. The node V B2  is connected to a node V A2  via a resistance  216 . A capacitor  218  is connected between the node V A2  and ground. The node V A2  is connected to the gate of an NMOS transistor  210  having a drain connected to the positive supply V DD  and a source connected to the regulator output node OUT. The transistor  210  has a W/L ratio which is K times larger than that of  212 , where K is also the scaling factor between the aspect ratio of transistors  110  and  112  of the circuit structure  100 . This means that the W/L of the transistor  110  is K times larger than the W/L of  112 , as previously explained. Preferably, a cut-off frequency introduced by a resistance R B2  of the resistor  216  multiplied by a capacitance C B2  of the capacitor  218  is the same as that introduced by the combination of the resistance  116  and the capacitor  118  of the circuit  100 . Both combinations are low-pass filters; however, no difference is made should their cut-off frequencies be different, provided that they are sufficiently low, that is low compared to the variation frequency of V reg ; the most straightforward course is at any rate that of making the two cut-off frequencies equal each other. 
     A regulating loop, which includes the differential amplifier  42 , a leg including the pull-up transistor  44  and the resistive divider  46 , the compensation network  54 , and the feedback line, sets the DC value of the output voltage V reg  at the node OUT. The designer should choose a desired value for V reg  by suitable selection of the value of V BG  (in this example, equal to the band-gap voltage) and the value of the R 1   48  /R 2   52  ratio in the resistive divider  46 , as previously explained. The values of V B  and V B2  will depend on the value of V reg  determined by the regulating loop as above. 
     Specifically, V B  is equal to V reg −|V THP |−V ov   P , and V B2  is equal to V reg +V THN +V ov   N , where the symbols have the same meaning as before. Thus, the values of V B  and V B2  will automatically match the value of V reg , which depends on the values of the fabrication process parameters, and “follow” the value of V reg  if the latter changes “slowly” due for example to temperature changes, aging of the components, etc. The values of V A  and V A2  are respectively identical in DC with those of V B  and V B2 . The values of V A  and V A2  will be substantially identical with those of V B  and V B2 , respectively, even at a low frequency, that is lower frequencies than the cutoff frequencies of the filter formed by resistor  116  with the capacitor  118  and the filter formed by the resistor  216  with the capacitor  218 . The DC current flowing through the transistor  110  will be dependent on the ratio K of the W/L values for the transistors  110  and  112 , and, in particular, will be equal to K*I B . Likewise, the current flowing through the pull-up transistor  44  will be dependent on the ratio K and the W/L values for the transistors  210  and  212 . The value of K is the same for either structures, so that the current delivered from the transistor  212  will flow through the transistor  110 , at least in theory. 
     In DC, adding the circuit structures  100  and  200  to the voltage regulator  40  bears essentially no influence on the voltage V reg . In fact, the low output impedance of the feedback loop sets the value of V reg ; this, in turn, sets the DC values of the voltages V A  and V A2  which, as mentioned before, will “follow” the DC value of V reg . 
     Any reference to DC values infers reference to possible “slow” variations of these values over time, for example as due to changes in temperature, aging of components, etc. The bias of the transistors  210  and  110  will “match” the value of V reg  to cause the current through them to be the desired current, namely K*I B , but without substantially affecting the value of V reg . 
     At higher frequencies than the cutoff frequency of the RC combinations, the nodes V A  and V A2  do not follow the variations of V reg . If V reg  varies upwards of the regulated value, the transistor  210  would tend to turn off, and the transistor  110  to conduct more. This causes a current draw to come in through the terminal V reg  and discharge the total capacitance linked to the node OUT (in FIG. 1, C r    14 +C s    12 ), so that the voltage V reg  falls and is quickly restored to the desired value. Upon this value being attained, the current flowing through the transistor  210  will be same as that through the transistor  110 , and accordingly, the incoming current through the terminal OUT be cancelled. Moreover, the current through the pull-up transistor  44  also equals that through the resistive divider  46 , and a balanced condition is therefore achieved. On the other hand, if V reg  varies downwards of the regulated value, the transistor  210  would tend to conduct more and the transistor  110  tends to turn off. This causes a current to be output through the output terminal OUT and charge the total capacitance linked to the node OUT (in FIG. 1, C r    14 +C s    12 ), so that the voltage V reg  quickly rises back to the desired value. 
     The operation of the complementary circuit structure  200  is similar to that of the circuit structure  100 , except, of course, that the voltage and current polarities are now changed. 
     By providing the additional circuit structures  100  and  200 , the voltage V reg  at the output node OUT can be quickly restored to its set value, even in the presence of fast “noise” at the output. The operation does not go through the regulating loop, and can therefore be very fast, provided that the components are suitably dimensioned. Conventional techniques are based instead on operation of the regulating loop, which has its rate inherently limited by the need for a stable frequency. This represents a major advantage of the additional combined circuit structures  100  and  200 . 
     Furthermore, these circuit structures  100  and  200  can accommodate any overshooting of the regulating loop response, so that the loop can be designed for a moderate phase margin, and exhibit a wider band and improved frequency response. 
     The bias of the nodes V A  and V A2  “follows” the V reg  at the output node OUT, and is therefore dependent on the latter. The impedance of the two transistors  110 ,  210  to the node OUT is high at rest. The circuit structures  100 ,  200  operate quickly in the presence of small voltage deviations at V reg  from the regulated value. This is because of the biasing for the transistors  210  and  110 , i.e., due to “self-matching” of the bias voltages of their respective gate electrodes. Additionally, to save on power consumption, I B  can be kept small. 
     It is understood that transistors arranged to operate basically as switches could be introduced for zeroing the power consumption in those situations where power consumption is desired to be substantially nil. For example, a switch could be connected between the drain of the transistor  210  and the positive supply, and a switch connected between the drain of the transistor  110  and ground. Likewise, switches may be connected in the legs that generate the voltages V B  and V B2 . Also, the capacitors  118 ,  218  could be connected to the supply V DD  rather than to ground. 
     Changes can be made to the invention in light of the above detailed description. In general, in the following claims, the terms used should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims, but should be construed to include all methods and devices that are in accordance with the claims. Accordingly, the invention is not limited by the disclosure, but instead its scope is to be determined by the following claims.