Abstract:
A voltage regulator including an output stage to generate an output voltage based upon a control voltage determined as a function of a difference between a reference voltage and a voltage representative of the output voltage. A sense resistor is coupled in series with the output stage and an auxiliary power stage is coupled in parallel with the output stage and cooperates therewith to supply a load as a function of a voltage drop across the sense resistor. A scaled replica stage of the output stage is controlled by the control voltage to generate a replica voltage of the output voltage. A bias network biases the scaled replica stage and output stage with identical currents to keep constant bias voltages. The output stage, the auxiliary power stage, the scaled replica stage, and the bias network each have high voltage transistors. The bias network is input with a square-wave control signal and an externally generated boosted voltage, to bias the scaled replica stage and the output stage in conduction states with the identical currents at the externally generated boosted voltage, when the square-wave control signal is active.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to voltage regulators and particularly to an integrated voltage regulator suitable for a FLASH memory device with a reduced consumption when the memory is in a stand-by state. 
       BACKGROUND OF THE INVENTION 
       [0002]    The ever increasing scaling down of the size of integrated devices is leading to the realization of more compact and faster devices with reduced power consumption figures. These devices work typically with a supply voltage lower than that (3.3V) traditionally required for CMOS technology devices. 
         [0003]    Modern 0.13 μm CMOS fabrication technologies for nonvolatile memory devices allow fast and high density chips capable of operating at a low supply voltage (1.8V), but the wide diffusion of 3V systems and interfaces is slowing down transition toward 1.8V powered devices. In order to power 1.8V devices from 3V supply lines, a voltage regulator is normally used. 
         [0004]    In  FIG. 1 , two known voltage regulators are shown. They typically include an amplification stage that compares a scaled replica of the output voltage with a reference voltage VBG. These amplifiers generate a control voltage VREF of a MOSFET that is biased in a more or less deep conduction state depending on whether the voltage on the supplied load LOAD diminishes or increases. 
         [0005]    The schemes of  FIG. 1  are characterized by a negative feedback loop that includes the output stage. This requires potentially difficult compensation for obtaining an adequate stability frequency margin, thus penalizing response speed. This impairs the stability of the output voltage in the presence of fast variations of the load current. 
         [0006]    Response speed of the feedback loop is penalized also by limitations of the consumption during stand-by and during normal functioning. These limitations make the regulators of  FIG. 1  unsuitable for a NOR FLASH memory device that typically has fast varying current absorption. Moreover, the response delay when resuming from the stand-by state may penalize the access speed to stored data. 
         [0007]    The circuits disclosed in U.S. Patent Publication No. US/2006/0186865 to Placa et al., European Patent Application Nos. EP 1667158 and EP 1653315 to Pisasale et al. and shown in  FIG. 2  address these problems. A common characteristic of these circuits is that the output stage is not included in the feedback loop, with consequential improvements in terms of frequency stability and speed of response as disclosed in the article by Gerritt W. den Besten, et al., titled “Embedded 5 V-to-3.3 V Voltage Regulator for Supplying Digital IC&#39;s in 3.3 V CMOS Technology”, IEEE Journal of Solid-State Circuits. This is possible because of the presence of a “replica” stage that ensures the stability of the output voltage independently from the load current. The matching between the MOS M 1 -M 2  of the “replica” stage and the MOS M 3 -M 4  of the output stage allows tracking of process and temperature fluctuations, while keeping the output voltage VOUT at the desired value KVBG=VBG*(1+R″/R 1 ). VBG is a stable reference voltage, typically generated by a band-gap generator, and it is practically insensitive to process spread, supply and temperature fluctuations. 
         [0008]    These known circuits may ensure a slow but accurate control of the DC output voltage; therefore, the operational amplifier, that generates the signal VREF, can be biased with currents of about a 1 mA, with a negligible increase of the overall stand-by power consumption of the memory and of the voltage regulator. 
         [0009]    The current absorbed by the memory under working conditions is supplied by the output stage b) or c). In both cases, these stages are locally fed-back, but without the above mentioned limitations of the converters of  FIG. 1 . 
         [0010]    This result is possible because of particular topological/circuit choices, and because these stages are active only when the device resumes from stand-by, thus they do not worsen significantly the power consumption figures. Therefore, it is easier to obtain a fast response, appropriately limited by the filter capacitance CF (the order of magnitude of which is about 1 nF). This determines an enhanced stability of the output voltage VOUT, with a reduced ripple. 
         [0011]    The output stages b) and c) of the circuits disclosed in U.S. Patent Publication No. US/2006/0186865 to Placa et al. and European Patent Application Nos. EP 1667158 and EP 1653315 to Pisasale et al., respectively, differ from each other because of their functioning modes: analog the first, and mixed analog-digital the second. 
         [0012]    In the first case ( FIG. 2   b ), the LV (Low Voltage) natural N-channel MOS of the output power stage is controlled by a continuous analog signal that is a function of the voltage drop on a sense resistor Rsense proportional to the output voltage variation due to the varying current absorption by the load. 
         [0013]    In the second case ( FIG. 2   c ), a dynamic modulation of the channel width of the output power stage, that is a LV natural N-channel transistor controlled with a constant gate voltage VREF, is actuated. Modulation is based on an analog-to-digital conversion of the results of comparisons of the output voltage with three voltage references. The so-called “bidirectional domino” enabling of the portions in which the output transistor is subdivided, enhances accuracy allowing a finer control of the output voltage. 
         [0014]    The use of the so-called Low Voltage (LV) transistors, characterized by gate oxide thickness of about 3.5-4.0 nm, may impose an upper limit of about 4.2V of the applied external supply voltage. In order to prevent failure in the event of excessively high externally applied voltages, protection circuits for absorbing the external voltage that exceeds the limit are needed. 
         [0015]    The protection circuits may include High Voltage (HV) transistors, with 16 nm gate oxides and, as a consequence, with a gain reduced by one fourth compared to the gain of the LV MOS. These transistors, connected in series to the circuit to be protected, deliver all the current absorbed by the load of the converter and thus may be desirably relatively large for avoiding relevant voltage drops that would negatively influence the regulation of the voltage VOUT, especially when the output voltage is at or close to the minimum value of the supply range. In the analyzed cases, the required overall channel width for the HV protection MOS was comparable with that of the LV MOS to be protected, with a consequential significant overall silicon area occupation. 
         [0016]    Another characteristic of the circuits of  FIG. 2  is that the transistors M 2 , M 4  and the (N-channel) output power transistors are Low-Voltage natural transistors, that is they have a low threshold voltage. These transistors are used whenever an extended functioning range of the external voltage up to 2.4V is required without requiring boosting of the gate voltages VB 1  and VB 2 . 
       SUMMARY OF THE INVENTION 
       [0017]    A voltage regulator that may be advantageously made entirely with High Voltage transistors with substantially identical power consumption to that of the above discussed regulators of the prior art has been found. 
         [0018]    As many of the known voltage regulators, the regulator of this invention also comprises a first stage that generates the desired voltage and a second stage that replicates this voltage on an output node of the regulator. 
         [0019]    A characteristic of the regulator is that all employed transistors are High Voltage transistors and that a bias network of the two stages supplies, under stand-by conditions, a current at a boosted voltage only in correspondence to the active state of an externally generated square wave signal. The duty cycle of the square wave signal is such that the bias network keeps constant the bias voltages of the two stages. In this way, the voltage on the output node of the regulator remains practically constant and power consumption during stand-by is substantially reduced. Preferably, the two stages are both source followers. 
         [0020]    The voltage regulator is suitable for being integrated in a memory device, particularly a FLASH NOR memory. In this case, the boosted voltage used by the bias network is supplied by the charge pump generator of the memory that generates the read voltage of the cells. 
         [0021]    The voltage regulator further comprises an auxiliary power stage controlled by the voltage drop on a sense resistance in series to the load of the regulator, that cooperates with the source follower stage for contributing with the latter in supplying the load. According to the preferred embodiment, the auxiliary power stage includes a plurality of identical modules connected in parallel. Each module may comprise a signal path suitable for generating a current representative of the voltage on the sense resistor, and a comparator having a resistive input network, the total resistance of which is established by at least a control voltage, connected to the path such that the representative current flows therethrough, the comparator generating a high logic signal when the voltage on the resistive network exceeds a pre-established threshold. Each module further comprises a control stage suitable for supplying a pre-established current when the logic signal is high and the regulator is not in a stand-by condition. The control voltage(s) of each comparator is chosen among the voltage of a common ground node and those of logic signals generated by other comparators. 
         [0022]    Substantially, the auxiliary power stage may be a current generator controlled by an externally generated voltage, that may be useful also in circuits different from the voltage regulator requiring controlled contributory approaches of current supply. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIGS. 1   a  and  1   b  depict known voltage regulators according to the prior art. 
           [0024]      FIGS. 2   a ,  2   b  and  2   c  depict voltage regulators of the above mentioned prior patent applications. 
           [0025]      FIG. 3  depicts a basic architecture of a voltage regulator according to this invention. 
           [0026]      FIG. 4  shows an embodiment of the bias network of the voltage regulator, and of the related control signals, according to the present invention. 
           [0027]      FIG. 5  depicts an architecture of the auxiliary power stage employed in the voltage regulator according to this invention. 
           [0028]      FIG. 6  is a time diagram of the load current and of the load voltage, according to the present invention. 
           [0029]      FIG. 7  shows how to switch the load switches depicted in  FIG. 5  for obtaining a quasi-linear control characteristic. 
           [0030]      FIG. 8  illustrates in detail a module of a multi-module auxiliary power stage according to the present invention. 
           [0031]      FIG. 9  depicts an auxiliary power stage having a modular architecture in accordance with the present invention. 
           [0032]      FIGS. 10 and 11  are time diagrams of simulations of the regulator of  FIG. 5  for linear and quasi-linear control characteristics. 
           [0033]      FIG. 12  shows sample waveforms of the output voltage of the regulator of  FIG. 5  during a read operation of a datum from a memory array. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0034]    An embodiment of the voltage regulator of this invention is depicted in  FIG. 3 . The voltage regulator is particularly suitable for use in NOR FLASH memory devices and according to an embodiment is made with only HV enhancement transistors. As will become clearer hereinafter, the voltage regulator does not increase the area requirements and does not negatively affect performances. The pre-charge circuits commonly employed for starting up the regulator have been omitted for better focusing on the aspects of the embodiment. 
         [0035]    The meaning of the main signals of  FIG. 3  is defined in the following Table: 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 VBG 
                 Reference voltage 
               
               
                   
                   
               
             
             
               
                   
                 TDB 
                 Square-wave signal for 
               
               
                   
                   
                 keeping constant the bias 
               
               
                   
                   
                 of the source follower 
               
               
                   
                   
                 stage 
               
               
                   
                 SBY_N 
                 Stand-by signal 
               
               
                   
                 VXR 
                 Boosted read voltage 
               
               
                   
                   
               
             
          
         
       
     
         [0036]    At least for the portion dedicated to regulate the output voltage, the circuit has a circuit block that “replicates” the desired output voltage. The voltage replicating circuit has a double source follower (M 1 , M 2 ) configuration, as the corresponding circuit block (M 3 , M 4 ). This minimizes disturbances induced by output fluctuations on the node VREF, because the capacitive coupling is obtained by the two overlap capacitances connected in series with the transistors M 3  and M 4 . Disturbances are reduced also because of the double filtering action of the capacitors C 1  and C 3 , as in the known circuit disclosed in European Patent Application No. EP 1667158 to Pisasale et al. 
         [0037]    Similarly to the prior art circuits of  FIGS. 2   a ),  2   b ) and  2   c ), a control circuit generates the voltage VREF and a sense resistor Rs is connected to the drain of the NMOS M 4 . A mixed analog-digital power block POWER MODULE may be used as in the prior art circuit of  FIG. 2   c ), or it may have a particularly efficient structure as will be disclosed hereinafter. 
         [0038]    The block TIME DIVISION BIASER generates the bias currents Ib 1  and Ib 2  of the source followers M 1  and M 3 . The fact that there are no natural transistors implies that the gate voltages of the NMOS M 2  and M 4  should be boosted. 
         [0039]    FLASH memory devices normally include charge pump generators, for example, the charge pump generator that supplies the read voltage VXR. This voltage, typically of about 4.5V, is always available, both in stand-by conditions as well typically as in working conditions. Generating boosted voltages requires an increased power consumption ideally multiplied by N+1, being N the number of stages connected in series in the charge pump generator. 
         [0040]    Even if it would appear impossible to use only HV transistors without increasing significantly power consumption figures, investigations carried out by the applicant showed that this outstanding result can be attained because the output voltage replicating network and its corresponding output network in a double follower configuration can be biased through a time division biasing network TIME DIVISION BIASER, as the one depicted in  FIG. 4 . 
         [0041]    The MOS Mal that has the reference voltage VBG applied to its gate, generates the current Iq/(N+1). This current is multiplied by the MOS Ma 3  and Ma 4  by a multiplication factor (N+1) and is supplied to the source followers M 1  and M 3  through the switches Ma 5  and Ma 6 , controlled by the signal P_N. The N-channel MOS M 5  and M 6  ( FIG. 3 ) are controlled by the signal EN. 
         [0042]    The signals P_N and EN are logically opposite to each other, but have a different high logic level: the signal P_N is obtained by level shifting from VOUT to the voltage VXR, wherein VOUT is the regulated output voltage, whilst the high logic level of EN is that of the external supply voltage Vcc. 
         [0043]    The signal P_N depends on the signal TDB, that has a duty cycle of: 
         [0000]        d=Tr/T=[k ( N+ 1)]−1  k&gt; 1  (1) 
         [0044]    The transistors Ma 3  and Ma 4  supply a mean bias current d*Iq. The average current consumption of the charge pump VXR is: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         Ip 
                         = 
                           
                          
                         
                           
                             I 
                              
                             
                               ( 
                               
                                 Ma 
                                  
                                 
                                     
                                 
                                  
                                 1 
                               
                               ) 
                             
                           
                           + 
                           
                             avg 
                              
                             
                               [ 
                               
                                 I 
                                  
                                 
                                   ( 
                                   
                                     Ma 
                                      
                                     
                                         
                                     
                                      
                                     3 
                                   
                                   ) 
                                 
                               
                               ] 
                             
                           
                           + 
                           
                             avg 
                              
                             
                               [ 
                               
                                 I 
                                  
                                 
                                   ( 
                                   
                                     Ma 
                                      
                                     
                                         
                                     
                                      
                                     4 
                                   
                                   ) 
                                 
                               
                               ] 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             Iq 
                             / 
                             
                               ( 
                               
                                 N 
                                 + 
                                 1 
                               
                               ) 
                             
                           
                           + 
                           
                             2 
                              
                             
                                 
                             
                              
                             d 
                              
                             
                                 
                             
                              
                             Iq 
                           
                         
                       
                     
                   
                   
                     
                       
                         
                           = 
                             
                            
                           
                             
                               [ 
                               
                                 Iq 
                                 / 
                                 
                                   ( 
                                   
                                     N 
                                     + 
                                     1 
                                   
                                   ) 
                                 
                               
                               ] 
                             
                              
                             
                               ( 
                               
                                 1 
                                 + 
                                 
                                   2 
                                   / 
                                   k 
                                 
                               
                               ) 
                             
                           
                         
                         ; 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0000]    avg[.] being the function that calculates the mean value of its argument, and the increment of the current absorbed by the Vcc is: 
         [0000]      ( N+ 1) Ip= (1+2 /k ) Iq.   (3) 
         [0045]    For example, for k=2 the current absorbed by the supply is 2Iq instead of (2N+3)Iq, N being an integer number chosen in the set {2, 3, 4, 5}. Thanks to the time division biasing, stand-by power consumptions are reduced. 
         [0046]    When TDB=low the signal EN is grounded (GND), the NMOS switches M 5  and M 6  ( FIG. 3 ) are off and the voltage levels VB 1  and VB 2  on the gates of M 2  and M 4 , respectively, are held by the capacitors C 2  and C 3 . 
         [0047]    In working conditions, being SBY_N=high, the transistors Ma 5 , Ma 6 , M 5  and M 6  are always on, both because there are not stringent consumption constraints and the DC bias makes the circuit more robust against disturbances due to eventual (capacitive) coupling with other signals. 
         [0048]    According to a non-essential, though preferred, embodiment the output power block POWER MODULE may have a particularly efficient modular architecture.  FIG. 5  shows the basic architecture of the block in the form of an array of n current generators I i  each connected in series with a switch Swi having a threshold voltage Vti&gt;Vti−1. 
         [0049]    According to such a preferred embodiment of the voltage regulator of this invention, the MOS M 4  has a double function. It controls the DC component of the output voltage VOUT and it amplifies dynamically eventual voltage reductions of the output voltage VOUT on the output filter capacitor CF due to current absorption surges by the load that increase the drain current, incrementing of the voltage drop on the sense resistor Rs. 
         [0050]    Consequently, all switches the threshold voltage of which is Vt&lt;(Vcc−VSENSE) are on, thus allowing the respective generators of the array to supply current for restoring the charge extracted from the capacitor CF and restoring the output voltage VOUT to the correct level. 
         [0051]    Each current generator contributed to the current delivered to the load in on-off mode. Assuming that the load absorbs a constant current I Load  such that: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       I 
                       a 
                     
                     = 
                     
                       
                         
                           
                             ∑ 
                             
                               k 
                               = 
                               1 
                             
                             
                               i 
                               - 
                               1 
                             
                           
                            
                           
                               
                           
                            
                           
                             I 
                             k 
                           
                         
                         ≤ 
                         
                           I 
                           load 
                         
                         ≤ 
                         
                           
                             ∑ 
                             
                               k 
                               = 
                               1 
                             
                             i 
                           
                            
                           
                               
                           
                            
                           
                             I 
                             k 
                           
                         
                       
                       = 
                       
                         
                           
                             I 
                             b 
                           
                            
                           
                               
                           
                            
                           i 
                         
                         = 
                         1 
                       
                     
                   
                   , 
                   2 
                   , 
                   … 
                    
                   
                       
                   
                   , 
                   n 
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0052]    in this condition, the switches Swk (k=1, 2, . . . , i−1) are always on. 
         [0053]    Let us suppose that the drain current of M 4  is negligible with respect to I Load  In a time interval in which the i-th switch Swi is open, the currents will be 
         [0000]      I a &lt;I Load    
         [0000]    and the capacitor CF discharges itself with a constant current ΔIcd=I Load −I a . 
         [0054]    The output voltage VOUT decreases, the drain current of M 4  increases as far as the voltage drop on the sense resistor Rs surpasses the threshold voltage of the i-th switch Swi. When this switch is turned on, the currents will be 
         [0000]      I b &gt;I Load    
         [0000]    and the capacitor charges itself with a constant current 
         [0000]      Δ Icc=I   b   −I   Load . 
         [0055]    The output voltage Vout increases according to a slope waveform, the drain current of M 4  and also the voltage drop on the resistor Rs diminish. When the voltage drop on the sense resistor becomes smaller than the threshold voltage of the switch Swi, this switch turns off and a next cycle starts. 
         [0056]    The waveforms of the output voltage VOUT and of the current supplied by current generators are depicted in  FIG. 6 . 
         [0057]    The amplitude of the ripple Vri is proportional to 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           τ 
                           i 
                         
                         
                           C 
                           F 
                         
                       
                        
                       
                         I 
                         i 
                       
                        
                       
                           
                       
                        
                       i 
                     
                     = 
                     1 
                   
                   , 
                   2 
                   , 
                   … 
                    
                   
                       
                   
                   , 
                   n 
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
         [0000]    wherein τ i  is the turn on/turn off delay of the switch Swi. 
         [0058]    In order to minimize the ripple, in view of equation (5) it is desirable to opt for a large number n of current generators. Being I TOT  the current supplied with all switches on, the current I i  of the i-th generator will be 
         [0000]    
       
         
           
             
               
                 
                   
                     I 
                     i 
                   
                   = 
                   
                     
                       I 
                       TOT 
                     
                     n 
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
         [0059]    In this case, the current supplied by the POWER MODULE block varies according to a linear discrete (uniform stepwise) control law: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       I 
                       g 
                     
                     = 
                     
                       
                         
                           
                             I 
                             TOT 
                           
                           n 
                         
                          
                         i 
                          
                         
                             
                         
                          
                         i 
                       
                       = 
                       1 
                     
                   
                   , 
                   2 
                   , 
                   … 
                    
                   
                       
                   
                   , 
                   n 
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
         [0000]    and the ripple amplitude remains constant while the supplied current varies. 
         [0060]    Preferably, the control input node of each current generator ( FIG. 9 ) has a capacitance Ci that charges the node VSENSE. If n increases, also the overall capacitance increases, the response speed of the feedback loop slows down and the delay τ i  increases. Therefore, for a certain design I TOT , the number of generators n should be chosen to minimize the product τ i *I i . 
         [0061]    Depending on the current absorption characteristics of the load, different and more specifically suitable control laws or algorithms may be implemented. 
         [0062]    As already said, NOR FLASH memory devices are characterized by short pulse current absorptions, with peak values often larger than 100 mA, but with a mean value of just about 10 mA. The current peaks are due to switchings of digital circuits and of charge pumps. 
         [0063]    By contrast, analog circuits, relatively more sensitive to supply ripples, typically absorb moderately varying currents about a certain mean value. The regulated voltage converter should have a limited ripple when supplying relatively small currents, and at the same time it should possess an appropriate DC “driving capability” for ensuring a fast recovering of the charge lost by the filter capacitor CF upon absorption of a current pulse by the load. 
         [0064]    It has been found that in these applications the converter may be designed for supplying a DC current I TOT  of about 3-4 times the value of the typical mean power consumption of the memory, preferably implementing an exponential type control law: 
         [0000]      I i =Io2 i  i=0, 1, . . . , p  (8) 
         [0065]    In this way, a non-uniform stepwise function is obtained that allows lower steps at low current absorption and higher steps at large current absorption. Overall, the ripple will be reduced for supplied currents in the neighborhood of the mean absorption of the memory. 
         [0066]    Being 
         [0000]        p+ 1 &lt;n    
         [0000]    there are the advantages due to a reduced number of current generators (and related circuitry) and a shorter delay time τ i . 
         [0067]    In the hypothesis that the currents supplied by the current generators be determined according to the exponential progression defined by equation (8), a further reduction of the ripple may be obtained by acting on the thresholds of the connecting switches by introducing feedback loops. 
         [0068]    Let us consider two generic switches Swk and Swj, being k&lt;j≦p, with thresholds Vtk and Vtj, respectively, and let us suppose that Vtk&lt;Vtj. The voltage drop on the nodes of the resistor Rs ranges between Vtk and Vtj, thus Swk is on and Swj is off. When the voltage on the nodes of the sense resistor is larger than Vtj, the switch Swj turns on and through the feedback loop the threshold of Swk is incremented to the value Vtk′&gt;Vtj, thus opening it. The switch Swk will turn on when the voltage on the nodes of Rs will surpass the new threshold voltage Vtk′. 
         [0069]    For sake of illustration, for p=5, all possible states of the switches (“o” on, “x” off) are listed in the table of  FIG. 7  and the “direction” of the feedback from certain switches towards one or two switches of a lower threshold is also indicated (in general there may be more than two switches, that is the feedback may be carried out by the i-th generator towards the generator i−k, k≧3). In the last two columns of the table are indicated the overall current Ig and the variation steps ΔI of the supplied current, referred to the basic current Io. The amplitude of the initial steps is limited and, in particular, it is constant up to about one third of the maximum current. This characteristic is analogous to that obtainable with a linear control law (ΔI=4), but by using only 6 current generators instead of I TOT /ΔI=63/4≈16. 
         [0070]    The feedbacks allow establishing the control law by surreptitiously introducing steps of limited amplitude for obtaining a particular control law, hereinafter referred as “Q-lin” (Quasi-linear). Having so minimized both the number of current generators as well as the amplitude of current steps, the product τ i *I i  in the ripple equation (5) is minimized. 
         [0071]    The bonus of the further decrease of the ripple resulting from the introduction of feedbacks could be spent for reducing the value of the filter capacitance CF, that is of the most cumbersome component (over 50% of the overall area), if ripple specifications were already met without introducing feedbacks. 
         [0072]    A circuit embodiment of the power module implementing the just illustrated Q-lin control law is described hereinafter. A circuit diagram of a module of a current generator of this invention is shown in  FIG. 8 . 
         [0073]    The PMOS Mb 2 , that functions as a current generator, and the PMOS Mb 1 , that functions as a switch controlled by the inverted replica of the signal FB, are observable within the dash line perimeter of the block POWER DRIVER. How the gate signal Vgx it is generated will now be described. 
         [0074]    The multiplicity of the i-th power driver is indicated with m i =2i, being i=0, 1, . . . , p. The signal VSENSE_VT on the gate of the PMOS Mci, i=0, 1, . . . , p is obtained by the signal VSENSE through a level shift equal to about a threshold voltage of a PMOS. Therefore, the Mci overdrive equals the voltage drop Vs on the nodes of the sense resistor Rs. 
         [0075]    Suppose that the signals A and B and the enabling signal PWR_N are grounded. The signal FB assumes a high logic value when the signal Vc overcomes the switching threshold Vx of the inverter IV 1 . This happens because the drain current of the PMOS Mci overcomes the value: 
         [0000]        Idx=Vx/Rc   (9) 
         [0076]    that is when: 
         [0000]    
       
         
           
             
               
                 
                   
                     Vs 
                     &gt; 
                     Vt 
                   
                   = 
                   
                     
                       Vx 
                       
                         
                           Kp 
                            
                           
                             ( 
                             
                               W 
                               L 
                             
                             ) 
                           
                         
                          
                         Rc 
                       
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
         [0077]    The current generator is active when the voltage Vs on the nodes of the sense resistor Rs overcomes the threshold voltage Vt. From equation (10), it is observed that the threshold Vt increases if the channel width W of Mci decreases or if the resistance Rc decreases. 
         [0078]    According to a preferred embodiment, a “natural” threshold Vt (with A and B grounded) is fixed by acting on the width parameter W, and the resistance connected to the node Vc has been exploited for modifying the threshold of the generator in presence of at least one of the feedback inputs A, B at high logic level (of course the number of feedback inputs may be more than two). It is worth highlighting that the resistor Rc can be substituted by a current generator of value Vx/Rc for the same function. 
         [0079]    A more detailed circuit architecture of the power module is depicted in  FIG. 9 , that is substantially a current generator, according to the preferred embodiment. The power drivers have multiplicity from 1 up to 32 (p=5) and corresponding feedback paths as in  FIG. 7 . The enabling signal PWR_N is a high voltage (Vcc) replica of the standby signal (SBY_N) generated by the memory device. 
         [0080]    The level shifter by which the voltage VSENSE_VT is generated is realized with the diode-connected PMOS Md 6 , biased with a current Ib in the order of hundreds of nA, that is negligible in respect to the overall stand-by current consumption of about 10 μA of the FLASH memory and of the voltage regulator. 
         [0081]    The voltage Vgx is generated by subtracting a constant voltage, for example the voltage drop on three diodes, from the externally generated supply voltage Vcc. This voltage Vgx is used to impose, between the source and gate of the PMOS Mb 2 , a voltage that is substantially independent from the supply voltage Vcc between the source and gate of the PMOS Mb 2 , thus the “driving capability” of the converter does not depend on Vcc. 
         [0082]      FIG. 10  shows results of two SPICE™ simulations with load current having a saw-tooth waveform with relatively gradual slope (from 0 to 50 mA in 8 μs, in steady state conditions), Vcc=2.7V, CF=2 nF. 
         [0083]    The two simulations relate to the same circuit diagram, but for curve  1  the feedback inputs are grounded (exponential control law). In order to make easier the comparison, the output voltage VOUT(1) is increased by 200 mV. The curve  2  refers to the converter the power module of which uses the Q-lin control law. 
         [0084]    It is pointed out that the ripple is extremely reduced (±10 mV) for load currents up to about one third of the maximum value and, as desirable, is sensibly smaller than the case in which the converter works with an exponential control law ( 8 ).  FIG. 11  is a magnified view of a portion of the graph of  FIG. 10 . 
         [0085]      FIG. 12  depicts the waveform of the output voltage for a typical current absorption of a FLASH memory during a read operation, with a supply voltage at 2.7V and 5.5V. A sample load is realized with voltage controlled current sources (VCCS). The control signal PULSE 1  drives a generator having a transconductance g1=100 mS, the control signal PULSE 2  drives a generator having a transconductance g2=25 mS. 
         [0086]    Even in this case fluctuations of the output voltage VOUT are smaller than 100 mV, independently from the external supply voltage. 
         [0087]    The advantages of the voltage regulator are use of HV transistors only to ensure operability over a wide range of external supply voltage 3V/5V and that the occupied silicon area is comparable to that of prior art solutions that use also LV transistors because there are no protection circuits and the output power PMOS work with a larger overdrive than the output LV NMOS stages. Furthermore, the converter is suitable for processes that do not contemplate a dedicated masking step for natural transistors. This simplification of the fabrication process allows a larger fabrication throughput. The time division biasing reduces power consumption in a stand-by state (about 2 μA) within 10% of the typical power consumption of a FLASH memory, that substantially equals the power consumption of prior art regulators that use LV transistors with low threshold voltage. 
         [0088]    Moreover, circuit implementation of the current generator is simplified for maximizing the response speed, though only HV transistors are used and the control loop of the current generators may be modified by introducing dedicated feedback loops thus introducing additional regulation steps of the output current without increasing the number of current generators. The high response speed and adoption of a Q-lin control characteristic allow an effective limitation of the ripple on the regulated voltage, or alternatively a decrement of the value of the filter capacitance CF and of the relative silicon area requirement. Additionally, current consumption under working conditions is smaller than 1 mA (about 10% of the typical current consumption of a memory) and there is negligible delay when resuming from a stand-by condition and no reduction of the speed for accessing data stored in the memory.