Abstract:
A semiconductor memory device having a memory array is provided. A read unit reads information stored in a memory cell. A step-up unit steps up an externally supplied voltage, and supplies the stepped-up voltage to the memory cell. A start control unit has the step-up unit start the stepping up after a read cycle begins. A detection unit detects that the stepped-up voltage has reached a predetermined level, and has the read unit start the reading upon the detection. A stop control unit has the step-up unit stop the stepping up when a time period required for the reading has elapsed since the detection. With this construction, the time taken for stepping up the voltage supplied to the memory cell is minimized in accordance with the time taken for the reading. Hence the current consumption is reduced when compared with the case where the step-up time is set unnecessarily long.

Description:
[0001]    This application is based on an application No. 2002-25146 filed in Japan, the contents of which are hereby incorporated by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a semiconductor memory device, and in particular relates to techniques for reducing the current consumption of a semiconductor memory device that has a read circuit including a step-up power supply.  
           [0004]    2. Prior Art  
           [0005]    The following method is typically used to read information from a semiconductor memory device equipped in a microcomputer. One of a plurality of memory cells arranged in the form of a matrix is selected by a word line and a bit line, and stored information is sensed by a sense amplifier from a cell current of the selected memory cell. To supply power to the internal circuitry of such a semiconductor memory device, a step-up power supply that steps up an externally-supplied voltage may be equipped in the semiconductor memory device (e.g. Unexamined Japanese Patent Application Publication No. H10-302492).  
           [0006]    [0006]FIG. 1 shows a construction of a conventional semiconductor memory device that includes a step-up power supply.  
           [0007]    In the drawing, a microcomputer  130  is roughly made up of a CPU  121  which controls the overall microcomputer  130 , and a semiconductor memory device  120 .  
           [0008]    The semiconductor memory device  120  includes a step-up power supply  113 , a memory array  7 , word lines  111 , a row decoder  6 , bit lines  112 , a column decoder  8 , a sense amplifier  9 , a data latch  110 , and a pulse generator  5 .  
           [0009]    The memory array  7  is made up of a plurality of memory cells which are arranged in the form of a matrix. Each memory cell stores one-bit information.  
           [0010]    The word lines  111  are arranged in a Y direction of the memory array  7 .  
           [0011]    The row decoder  6  selects one of the word lines  111  using voltage Vg supplied from the step-up power supply  113  and address information AddY given from the CPU  121 .  
           [0012]    The bit lines  112  are arranged in an X direction of the memory array  7 .  
           [0013]    The column decoder  8  selects one of the bit lines  112  using address information AddX given from the CPU  121 .  
           [0014]    The sense amplifier  9  reads one-bit information from a memory cell selected by the row decoder  6  and the column decoder  8 .  
           [0015]    The data latch  110  latches the data read by the sense amplifier  9 .  
           [0016]    The pulse generator  5  controls the sense amplifier  9  using signals (SLOW and NDS) from the CPU  121 .  
           [0017]    The step-up power supply  113  includes a reference voltage generator  1 , a step-up pump  2 , a Vp detector  3 , and a differential amplifier  4 .  
           [0018]    The reference voltage generator  1  generates reference voltage VREF.  
           [0019]    The step-up pump  2  generates voltage Vp higher than power supply voltage VDD which is supplied from outside.  
           [0020]    The Vp detector  3  controls the step-up pump  2  in accordance with comparison between reference voltage VREF generated by the reference voltage generator  1  and voltage Vp generated by the step-up pump  2 .  
           [0021]    The differential amplifier  4  generates voltage Vg that is double the level of reference voltage VREF, using voltage Vp.  
           [0022]    In such a construction, a read is performed as follows. When address information AddX and AddY are input respectively to the column decoder  8  and the row decoder  6 , the row decoder  6  selects one word line  111  according to address information AddY, whereas the column decoder  8  selects one bit line  112  according to address information AddX. The sense amplifier  9  senses the storage contents of a memory cell selected by the word line  111  and the bit line  112 , and outputs signal DOUT which is high or low depending on the storage contents. The data latch  110  latches DOUT and outputs the data.  
           [0023]    When such a read is performed with a long cycle of several microseconds, usually the sense amplifier  9  is activated only while information is being read from the memory array  7 , to reduce current consumption. On the other hand, the step-up power supply  113  constantly generates voltage Vg to guarantee normal read operations.  
           [0024]    [0024]FIG. 2 shows an operation sequence of the semiconductor memory device  120 , when a read is performed with a long cycle.  
           [0025]    At time T2, read control signal NDS output from the CPU 121  makes a high to low transition. This causes output SAAV of the pulse generator  5  to transition from low to high. As a result, the sense amplifier  9  which receives SAAV is activated The sense amplifier  9  reads information stored in a selected memory cell, and outputs it as DOUT. At time T3, the pulse generator  5  changes SAAV from high to low. At this point, the data latch  110  latches DOUT, and keeps outputting the data until latching in the next cycle. The sense amplifier  9  is deactivated when SAAV becomes low at time T3.  
           [0026]    In the meantime, voltage Vg is steadily generated by the reference voltage generator land the differential amplifier  4 . On the other hand, voltage Vp generated by the step-up pump  2  has a waveform with some width, since the step-up pump  2  is activated/deactivated by the Vp detector  3  depending on detection of voltage Vp.  
           [0027]    When a read is performed with a long cycle, the sense amplifier  9  is activated only while information is actually being read from the memory array  7 , and deactivated once the information has been read and latched. This contributes to a lower current consumption, when compared with the case where the sense amplifier  9  is active during the whole cycle.  
           [0028]    Meanwhile, the step-up power supply  113  is active and generates voltage Vg during the whole cycle, as explained earlier. This causes unnecessary current consumption. Given that the step-up power supply  113  consumes a large amount of current, such unnecessary current consumption need be addressed.  
         SUMMARY OF THE INVENTION  
         [0029]    The present invention was conceived in view of the problem described above, and has an object of providing a semiconductor memory device in which the current consumption of a read circuit including a step-up power supply is reduced when compared with conventional semiconductor memory devices.  
           [0030]    The stated object can be achieved by a semiconductor memory device having a memory array, including: a read unit operable to read information stored in a memory cell in the memory array; a step-up unit operable to step up a voltage supplied from outside the semiconductor memory device, and supply the stepped-up voltage to the memory cell; a start control unit operable to have the step-up unit start the stepping up after a read cycle begins; a detection unit operable to detect that the stepped-up voltage has reached a predetermined level required for the read unit to read the information from the memory cell, and have the read unit start the reading upon the detection; and a stop control unit operable to have the step-up unit stop the stepping up, when a predetermined time period required for the read unit to complete the reading has elapsed since the detection.  
           [0031]    It should be noted here that the step-up unit does not step up the voltage to the predetermined level instantly. Rather, after the start control unit has the step-up unit start the step up, the step-up unit steps up the voltage with time, as a result of which the voltage reaches the predetermined level. The predetermined level here is higher than a power supply voltage supplied from outside, and is a multiple of the reference voltage as one example. The detection by the detection unit is carried out, for example, by comparing a submultiple of the stepped-up voltage (i.e. a voltage obtained by stepping down the stepped-up voltage by means of resistance voltage division) with a constant voltage lower than the power supply voltage.  
           [0032]    According to the above construction, the semiconductor memory device starts stepping up the voltage to be supplied to the memory cell, after the read cycle begins. Once the voltage has reached the predetermined level, the actual read operation is launched. The semiconductor memory device stops stepping up the voltage when the predetermined time period has elapsed since the read operation is launched. The predetermined time period here is a time period necessary for the read unit to perform the actual read operation. Thus, the time during which the voltage to be supplied to the memory cell is stepped up is minimized in association with the time taken for the read operation. This contributes to a lower current consumption, when compared with the case where the time during which the voltage is stepped up is unnecessarily long. Especially if the read cycle is relatively long such as several microseconds or more, the time during which the read operation is not performed is longer than the time during which the read operation is performed, within the read cycle. In such a case, if the step up is constantly performed throughout the read cycle, a current is unnecessarily consumed in a long period during which the read operation is not performed. According to the present invention, however, the step up is started little before the actual read operation is launched, and stopped at the same time as the completion of the read operation. This eliminates unnecessarily current consumption in the time during which the read operation is not performed.  
           [0033]    Here, the read unit may include: a sense amplifier operable to amplify a cell current of the memory cell and output the amplified cell current, wherein the step-up unit includes: a reference voltage generator operable to generate a constant reference voltage which is no higher than a power supply voltage supplied from outside the semiconductor memory device; a step-up pump operable to generate a first voltage which is higher than the power supply voltage; a differential amplifier operable to generate a second voltage using the first voltage and supply the second voltage to the memory cell, the second voltage increasing with time to the predetermined level that is a predetermined multiple of the reference voltage and is higher than the power supply voltage; and a first voltage detector operable to have the first voltage stay within a fixed range, by comparing the first voltage with the second voltage and activating/deactivating the step-up pump depending on a result of the comparison, wherein the detection unit includes: a second voltage detector operable to (a) detect that the second voltage has reached the predetermined level, by comparing a comparative voltage obtained by stepping down the second voltage with the reference voltage, and (b) activate the sense amplifier upon the detection, the start control unit activates the differential amplifier and the first voltage detector to have the step-up unit start the stepping up, and further activates the second voltage detector, after the read cycle begins, and the stop control unit deactivates the step-up pump, the differential amplifier, and the first voltage detector to have the step-up unit stop the stepping up, and further deactivates the second voltage detector, when the predetermined time period has elapsed since the activation of the sense amplifier.  
           [0034]    This construction corresponds to the semiconductor memory device  200  shown in FIG. 3. In detail, the step-up unit corresponds to the step-up power supply  201 , the reference voltage generator corresponds to the reference voltage generator  1 , the step-up pump corresponds to the step-up pump  2 , the differential amplifier corresponds to the differential amplifier  100 , the first voltage detector corresponds to the Vp detector  3 , the second voltage detector corresponds to the Vg detector  11 , and the start control unit and the stop control unit correspond to the control circuit  105 .  
           [0035]    Here, the semiconductor memory device may further include: a sense amplifier deactivator operable to deactivate the sense amplifier, when the predetermined time period has elapsed since the activation of the sense amplifier; and a sense amplifier output latch operable to hold the output of the sense amplifier for a fixed time period after the deactivation of the sense amplifier.  
           [0036]    In a semiconductor memory device, a step-up pump has the largest power consumption, and a sense amplifier has the second largest power consumption. Accordingly, by deactivating the sense amplifier along with the step-up unit including the step-up pump when the read operation ends, power consumption is further reduced. In this case, to obtain the output of the sense amplifier even after the sense amplifier is deactivated, the sense amplifier output latch is equipped in the semiconductor memory device.  
           [0037]    Here, the second voltage detector may output a predetermined signal to activate the sense amplifier, wherein the semiconductor memory device further include: a sense amplifier activation signal latch operable to hold the predetermined signal output from the second voltage detector for a set time period, and output the held signal to the sense amplifier.  
           [0038]    The second voltage detector outputs the predetermined signal upon detecting that the second voltage generated by the differential amplifier has reached the targeted level. Upon receiving this signal, the sense amplifier is activated. Here, the sense amplifier activation signal latch for latching the signal output from the second voltage detector is equipped in the semiconductor memory device. By activating the sense amplifier according to the output of this latch, even if noise acts upon the output of the differential amplifier and as a result the output of the second voltage detector oscillates, the sense amplifier can operate stably without being affected by such an oscillation. In this way, a malfunction caused by the occurrence of noise affecting the stepped-up voltage is prevented, with it being possible to realize stable read operations.  
           [0039]    Here, the differential amplifier may include: a first n-channel transistor operable to make the comparative voltage which is obtained by dividing the second voltage using a resistance voltage divider, equal to the second voltage upon the deactivation of the differential amplifier; a second n-channel transistor operable to interrupt a flow of current through a differential circuit portion of the differential amplifier, upon the deactivation of the differential amplifier; and a p-channel transistor having a gate connected to a node on which the second voltage is provided, and operable to short-circuit the node and the step-up pump upon the deactivation of the differential amplifier.  
           [0040]    This construction corresponds to the stop circuit  300  shown in FIG. 6. In detail, the first n-channel transistor corresponds to the n-channel transistor  25 , the second n-channel transistor corresponds to the n-channel transistor  23 , and the p-channel transistor corresponds to the p-channel transistor  22 .  
           [0041]    According to this construction, when the differential amplifier is deactivated, an intermediate node potential is set in anticipation of a change which may occur next time the differential amplifier is activated. This keeps the second voltage generated by the differential amplifier from unwantedly exceeding the targeted level upon the activation of the differential amplifier, with it being possible to ensure the stable operation of the differential amplifier.  
           [0042]    Here, the differential amplifier may include: a plurality of charge supply p-channel transistors each operable to supply a charge generated by the step-up pump to the differential amplifier; and a conduction control p-channel transistor connected in series to at least one charge supply p-channel transistor out of the plurality of charge supply p-channel transistors, and operable to bring the charge supply p-channel transistor into or out of conduction depending on a read mode.  
           [0043]    This construction corresponds to the switch circuit  301  shown in FIG. 6. In detail, the plurality of charge supply p-channel transistors correspond to the p-channel transistors  16  and  17 , and the conduction control p-channel transistor corresponds to the p-channel transistor  24 .  
           [0044]    According to this construction, when the read mode is a high-speed mode with a read cycle of several tens of nanoseconds, all of the charge supply p-channel transistors are used to enhance the charge supply capability and suppress a voltage drop by a load circuit. When the read mode is a low-speed mode with a read cycle of several tens of microseconds, on the other hand, only part of the charge supply p-channel transistors is used to lower the charge supply capability, thereby preventing an overshoot of unwantedly exceeding the targeted level. In this way, the voltage can be generated stably in both the high-speed mode and the low-speed mode.  
           [0045]    Here, the second voltage detector may include: a differential circuit portion including a first p-channel transistor and a second p-channel transistor which are current-mirror connected; a third p-channel transistor connected in parallel with the first p-channel transistor which has a gate and a drain connected, and operable to short-circuit the drain of the first p-channel transistor and an output of the power supply voltage upon the deactivation of the second voltage detector; a first n-channel transistor connected with the second p-channel transistor, and operable to ground a drain of the second p-channel transistor upon the deactivation of the second voltage detector; a second n-channel transistor operable to interrupt a flow of current through the differential circuit portion upon the deactivation of the second voltage detector; a fourth p-channel transistor connected with a node on which the second voltage is provided, and operable to change the comparative voltage which is obtained by dividing the second voltage using a resistance voltage divider, to a ground voltage upon the deactivation of the second voltage detector; a third n-channel transistor and a fifth p-channel transistor connected in series with each other, and operable to convert an output of the differential circuit portion to a logic signal; and a cutoff unit operable to bring the third n-channel transistor out of conduction upon the deactivation of the second voltage detector.  
           [0046]    This construction corresponds to the stop circuit  401  shown in FIG. 8. In detail, the first p-channel transistor and the second p-channel transistor correspond to the two p-channel transistors  30 , the third p-channel transistor corresponds to the p-channel transistor  37 , the first n-channel transistor corresponds to the n-channel transistor  36 , the second n-channel transistor corresponds to the n-channel transistor  33 , the fourth p-channel transistor corresponds to the p-channel transistor  39 , the fifth p-channel transistor corresponds to the p-channel transistor  38 , and the third n-channel transistor corresponds to the n-channel transistor  40 .  
           [0047]    According to this construction, when the second voltage detector is deactivated, an intermediate node potential is set in anticipation of a change which may occur next time the second voltage detector is activated. This keeps the second voltage detector from wrongly outputting a detection signal upon the activation. Hence the stable operation of the second voltage detector is ensured.  
           [0048]    Here, the second voltage detector may include: a first n-channel transistor having a gate which receives the comparative voltage obtained by dividing the second voltage using a resistance voltage divider; a second n-channel transistor having a gate which receives the reference voltage; and a third n-channel transistor connected in parallel with the first n-channel transistor so that a total capability of the first n-channel transistor connected with the third n-channel transistor is greater than a capability of the second n-channel transistor.  
           [0049]    According to this construction, even when the second voltage generated by the differential amplifier is stabilized at a level slightly lower than the targeted level, the second voltage detector is kept from a malfunction of not outputting a detection signal permanently. Hence the normal operation of the second voltage detector is ensured.  
           [0050]    Here, the semiconductor memory device may further include: a transistor diode-connected with a node on which the first voltage is provided, and operable to supply a charge from an external power supply to the node if the node falls below the power supply voltage after the deactivation of the step-up pump.  
           [0051]    According to this construction, the first voltage at the node corresponding to the output of the step-up pump decreases in a high impedance state due to a leakage of current, but does not fall below VDD−Vt. As a result, the step up in the next read cycle can be started not from a ground level but from VDD−Vt, which contributes to a lower current consumption when compared with the case where the step up is started from a ground level.  
           [0052]    Here, the semiconductor memory device may further include: a transistor diode-connected with a node on which the second voltage is provided, and operable to supply a charge from an external power supply to the node if the node falls below the power supply voltage after the deactivation of the differential amplifier.  
           [0053]    According to this construction, the second voltage at the node corresponding to the output of the differential amplifier decreases in a high impedance state due to a leakage of current, but does not fall below VDD−Vt. As a result, the step up in the next read cycle can be started not from a ground level but from VDD−Vt, which contributes to a lower current consumption when compared with the case where the step up is started from a ground level. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0054]    These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings which illustrate a specific embodiment of the invention.  
         [0055]    In the drawings:  
         [0056]    [0056]FIG. 1 shows a construction of a conventional semiconductor memory device including a step-up power supply;  
         [0057]    [0057]FIG. 2 shows an operation sequence of the semiconductor memory device shown in FIG. 1, when a read is performed with a long cycle;  
         [0058]    [0058]FIG. 3 shows a construction of a semiconductor memory device to which an embodiment of the invention relates;  
         [0059]    [0059]FIG. 4 is a time chart showing an operation of the semiconductor memory device shown in FIG. 3;  
         [0060]    [0060]FIG. 5 shows a relationship between Vp, Vg, VDD, and VREF;  
         [0061]    [0061]FIG. 6 shows a construction of a differential amplifier shown in FIG. 3;  
         [0062]    [0062]FIG. 7 is a time chart showing an operation of the semiconductor memory device shown in FIG. 3, when a read is performed with a short cycle; and  
         [0063]    [0063]FIG. 8 shows a construction of a Vg detector shown in FIG. 3. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0064]    The following is a description of an embodiment of the present invention with reference to drawings.  
         [0065]    (Semiconductor Memory Device  200 )  
         [0066]    [0066]FIG. 3 shows a construction of a semiconductor memory device to which the embodiment relates. It should be noted that construction elements which are the same as those in the conventional semiconductor memory device  120  shown in FIG. 1 are given the same reference numerals.  
         [0067]    In the drawing, a semiconductor memory device  200  includes a step-up power supply  201 , the memory array  7 , the word lines  111 , the row decoder  6 , the bit lines  112 , the column decoder  8 , the sense amplifier  9 , the data latch  110 , a Vg detector  11 , an R latch  107 , a pulse generator  12 , a control circuit  105 , and n-channel transistors  103  and  104 .  
         [0068]    The memory array  7  is made up of a plurality of memory cells that are arranged in the form of a matrix. Each memory cell stores one-bit information.  
         [0069]    The word lines  111  are arranged in the Y direction of the memory array  7 .  
         [0070]    The row decoder  6  selects one of the word lines  111 , using voltage Vg supplied from the step-up power supply  201  and address information AddY given from a CPU.  
         [0071]    The bit lines  112  are arranged in the X direction of the memory array  7 .  
         [0072]    The column decoder  8  selects one of the bit lines  112 , using address information AddX given from the CPU.  
         [0073]    The sense amplifier  9  reads one-bit information from a memory cell selected by the row decoder  6  and the column decoder  8 .  
         [0074]    The data latch  110  latches the data read by the sense amplifier  9 .  
         [0075]    The Vg detector  11  detects that voltage Vg is approximately double the level of reference voltage VREF.  
         [0076]    The R latch  107  stores a first leading edge of signal SAS output from the Vg detector  11  after signal NDS becomes low, and outputs signal SASS.  
         [0077]    The pulse generator  12  generates signal SAAV for a predetermined time period, upon receiving SASS from the R latch  107  after NDS becomes low.  
         [0078]    The control circuit  105  controls the reference voltage generator  1 , the Vp detector  3 , and a differential amplifier  100  in the step-up power supply  201 , the Vg detector  11 , and the pulse generator  12 .  
         [0079]    The step-up power supply  201  includes the reference voltage generator  1 , the step-up pump  2 , the differential amplifier  100 , the Vp detector  3 , a level shifter  106 , and p-channel transistors  101  and  102 .  
         [0080]    The reference voltage generator  1  generates reference voltage VREF.  
         [0081]    The step-up pump  2  generates voltage Vp.  
         [0082]    The differential amplifier  100  generates voltage Vg which is double the level of reference voltage VREF, using voltage Vp generated by the step-up pump  2 .  
         [0083]    The Vp detector  3  controls the step-up pump  2  based on comparison between voltage Vg and voltage Vp.  
         [0084]    The p-channel transistors  101  and  102  supply power supply voltage VDD to the step-up pump  2  and the differential amplifier  100 .  
         [0085]    [0085]FIG. 4 is a time chart showing an operation of the semiconductor memory device  200  having the above construction.  
         [0086]    Note here that FIG. 4 shows an operation of the semiconductor memory device  200  when a microcomputer is in a low-speed mode. Accordingly, signal SLOW is constantly high. In the low-speed mode, the semiconductor memory device  200  is controlled by external signal NDS, in the following way. When NDS is high, the semiconductor memory device  200  is on standby where a read is not performed. When NDS is low, a read is performed An interval from T1 at which NDS makes a low to high transition to next T1 is set as one read cycle.  
         [0087]    At time T1, NDS transitions from low to high. In response, the control circuit  105  changes signal RFACT from low to high and outputs it. As a result, the reference voltage generator  1  is activated. The reference voltage generator  1  generates reference voltage VREF that is stabilized by time T2.  
         [0088]    At time T2, NDS transitions from high to low. In response, the control circuit  105  changes signal DS from low to high and outputs it. As a result, the Vp detector  3 , the differential amplifier  100 , and the Vg detector  11  are activated. Also, the pulse generator  12  enters a state of waiting for signal SASS from the R latch  107 .  
         [0089]    The Vp detector  3  is active while DS is high. The Vp detector  3  activates or deactivates the step-up pump  2  according to a detection result of voltage Vp, to keep voltage Vp at about Vg+Vt (Vt is determined based on transistor characteristics, and is about 1V for example). In more detail, when Vp&lt;Vg+Vt, the Vp detector  3  outputs a signal to the step-up pump  2  to activate the step-up pump  2 . The step-up pump  2  responsively operates to generate voltage Vp which is higher than power supply voltage VDD, until Vp&gt;Vg+Vt (see a voltage relationship diagram of FIG. 5).  
         [0090]    The differential amplifier  100  is active while DS is high. The differential amplifier  100  generates voltage Vg which is double the level of reference voltage VREF, using voltage Vp.  
         [0091]    The Vg detector  11  is active while DS is high. At time T3, the Vg detector  11  detects that voltage Vg has reached the targeted level of VREF×2. Upon detecting this, the Vg detector  11  changes signal SAS from low to high and outputs it.  
         [0092]    When SAS becomes high, the R latch  107  stores it. The R latch  107  then changes signal SASS from low to high and outputs it.  
         [0093]    When SASS becomes high, the pulse generator  12  changes signal SAAV from low to high and outputs it. When the predetermined time period has passed after the low to high transition of SAAV, the pulse generator  12  changes SAAV from high to low. The predetermined time period here is a period necessary for the sense amplifier  9  to draw out a voltage from a selected memory cell of the memory array  7 .  
         [0094]    When voltage Vg has reached the targeted level at time T3, the row decoder  6  and the column decoder  8  select a word line  111  and a bit line  112  according to address information AddY and AddX, respectively.  
         [0095]    When SAAV becomes high, the sense amplifier  9  is activated. The sense amplifier  9  amplifies a voltage of a memory cell at an intersection of the selected word line  111  and bit line  112 , and outputs it as DOUT.  
         [0096]    When SAAV becomes low at time T4, the data latch  110  latches DOUT output from the sense amplifier  9 , and keeps outputting the latched data. Latching here means to hold a specific value. From time T4 of the current cycle to time T4 of the next cycle, the data latch  110  holds DOUT output from the sense amplifier  9  and keeps outputting it.  
         [0097]    When SAAV becomes low at time T4, the sense amplifier  9  is deactivated.  
         [0098]    Also, when SAAV becomes low at time T4, the control circuit  105  changes DS from high to low. As a result, the Vp detector  3 , the differential amplifier  100 , and the Vg detector  11  are deactivated.  
         [0099]    Thus, the semiconductor memory device  200  repeats the above read cycle of T1 to T4 in sync with NDS. In this repetition, all circuits in the semiconductor memory device  200  are inactive from time T4 of the current cycle to time Tl of the next cycle. Hence no current is consumed during this period. Also, all circuits that supply/extract a charge to/from voltages Vp and Vg are inactive from time T4 of the current cycle to time T2 of the next cycle. Accordingly, voltages Vp and Vg are in a high impedance (Hi-z) state at the levels of time T4, during this period.  
         [0100]    In the Hi-z state, a slight discharge occurs due to a junction leak (a leak of current from a junction). This causes voltages Vp and Vg to decrease. However, by stepping up voltages Vp and Vg by the amounts of discharge from time T2 of the next cycle onward, voltages Vp and Vg are brought back to their targeted levels. A time period required for such step up is shorter than a time period required for stepping up voltages Vp and Vg from power supply voltage VDD, with it being possible to significantly reduce the current consumption of the step-up power supply  201 .  
         [0101]    (Transistors  103  and  104 )  
         [0102]    The transistors  103  and  104  have the following function.  
         [0103]    Suppose voltages Vp and Vg drop to a ground level due to a leak between time T4 and time T2 of the next cycle. In such a case, a large amount of current consumption is needed for the step-up pump  2  and the differential amplifier  100  to raise voltages Vp and Vg from the ground level to the targeted levels from time T2 onward. The transistors  103  and  104  are provided to avoid such current consumption. The transistor  103  is diode-connected between power supply voltage VDD and voltage Vp. Likewise, the transistor  104  is diode-connected between power supply voltage VDD and voltage Vg. This being so, the transistors  103  and  104  serve to keep voltages Vp and Vg from dropping below VDD−Vt due to a current leakage.  
         [0104]    (R Latch  107 )  
         [0105]    The R latch  107  has the following function.  
         [0106]    Suppose SAS, which has become high at time T3, prematurely returns to low due to the occurrence of noise that affects voltage Vg or the like, as indicated by a dotted part of the waveform of SAS in FIG. 4. In such a case, the pulse generator  12  which operates in response to SAS changes SAAV to low before time T4, which causes the sense amplifier  9  to fail to completely read information from a selected memory cell. To prevent this, the R latch  107  is provided between the Vg detector  11  and the pulse generator  12 .  
         [0107]    According to this embodiment, upon detecting that voltage Vg has reached the targeted level, the semiconductor memory device  200  performs a read operation for the predetermined time period. All circuits in the semiconductor memory device  200  are deactivated except while the read operation is being performed. This contributes to a lower current consumption. Also, voltages Vp and Vg are kept from significant decreases until the next read cycle, with it being possible to reduce the amount of current required for stepping up voltages Vp and Vg. Furthermore, the R latch  107  is provided to ensure time necessary for the read operation of the sense amplifier  9 .  
         [0108]    (Differential Amplifier  100 )  
         [0109]    A circuit construction of the differential amplifier  100  for keeping voltage Vg from exceeding the targeted level (VREF×2) is described below.  
         [0110]    [0110]FIG. 6 shows a construction of the differential amplifier  100 .  
         [0111]    In the drawing, the differential amplifier  100  includes p-channel transistors  13 ,  16 ,  17 ,  22 , and  24 , n-channel transistors  15 ,  23 , and  25 , resistors  18 , and level shifters  20 . The two parts enclosed by dotted lines are a stop circuit  300  and a switch circuit  301 . Note that the p-channel transistors  13 ,  16 , and  17 , the n-channel transistors  15  and  23 , and the resistors  18  correspond to a construction of a conventional differential amplifier.  
         [0112]    (Stop Circuit  300 )  
         [0113]    The following explains how to prevent voltage Vg from exceeding the targeted level when voltage Vp is excessively supplied to the differential amplifier  100  upon the activation of the step-up power supply  201  at time T2.  
         [0114]    Before time T2, DS is low. Accordingly, the p-channel transistor  22  has a drain and a source conducting, whereas the n-channel transistors  23  and  25  have a drain and a source out of conduction. In this state, the potentials at nodes N1 and Vghf1 are Vp and Vg respectively.  
         [0115]    At time T2, DS becomes high. As a result, the p-channel transistors  22  ceases conduction, whilst the n-channel transistors  23  and  25  come into conduction. This being so, the voltage at node Vghf1 drops from Vg to Vg/2. Meanwhile, the voltage at node N1, which is Vp at time T2, does not decrease to such a level that brings the p-channel transistors  16  and  17  into conduction, until node Vghf1 falls below VREF. Thus, the stop circuit  300  enables proper comparison to be conducted as soon as the differential amplifier  100  is activated at time T2. Hence voltage Vg will not exceed the targeted level upon the activation.  
         [0116]    (Switch Circuit  301 )  
         [0117]    The following explains how to prevent an overshoot when a charge is supplied from voltage Vp to voltage Vg.  
         [0118]    The differential amplifier  100  needs to be capable of a high-speed mode too, where a read is performed with a short cycle of several tens of nanoseconds (see FIG. 7). In the high-speed mode, a current load on voltage Vg occurs with a trailing edge of NDS, as a result of which voltage Vg drops. Such a drop in voltage Vg need be eliminated by the time the next cycle starts after several tens of nanoseconds. Accordingly, the p-channel transistors  16  and  17  are both used to supply a charge from voltage Vp to voltage Vg.  
         [0119]    In the low-speed mode where a read is performed with a long cycle of several microseconds or more, on the other hand, if the p-channel transistors  16  and  17  which supply a charge from voltage Vp to voltage Vg are both used, voltage Vg increases so fast due to an excessive charge supply that the differential amplifier  100  cannot respond promptly. When this happens, voltage Vg exceeds the targeted level and results in an overshoot. If a read is performed with such a high voltage, a malfunction occurs. To avoid such an overshoot, the p-channel transistor  24  is brought out of conduction so that the p-channel transistor  16  is not used in the low-speed mode. Thus, the switch circuit  301  is constructed such that only the p-channel transistor  17  is used to supply a charge from voltage Vp to voltage Vg in the low-speed mode.  
         [0120]    (Vg Detector  11 )  
         [0121]    The following describes a circuit construction of the Vg detector  11  for preventing output of a wrong detection result immediately after activation and for reliably detecting voltage Vg at the targeted level.  
         [0122]    [0122]FIG. 8 shows a construction of the Vg detector  11 .  
         [0123]    In the drawing, the Vg detector  11  includes p-channel transistors  30 ,  37 ,  38 , and  39 , n-channel transistors  31 ,  32 ,  33 ,  34 ,  36 , and  40 , resistors  41 , a level shifter  47 , inverters  43 ,  45 , and  48 , and an NAND circuit  44 . The two parts enclosed by dotted lines are an offset circuit  400  and a stop circuit  401 . Note that the p-channel transistors  30  and  38 , the n-channel transistors  31 ,  32 ,  33 , and  40 , and the resistors  41  correspond to a construction of a conventional differential amplifier.  
         [0124]    (Stop Circuit  401 )  
         [0125]    The following explains how to prevent output of a wrong detection result at the time of activation at time T2.  
         [0126]    Before time T2, DS is low. Accordingly, the p-channel transistors  37  and the n-channel transistor  36  are conducting between a drain and a source, whilst the p-channel transistor  39  and the n-channel transistors  33  and  40  are out of conduction between a drain and a source. In this state, the potentials at nodes N2 and Vghf2 are both 0V.  
         [0127]    Since node N2 is 0V, the p-channel transistor  38  is conducting between a drain and a source. This means node N 4  is high, and SAS output from the inverter  48  is low.  
         [0128]    Also, since node Vghf2 is 0V, the n-channel transistors  32  and  34  are out of conduction between a drain and a source.  
         [0129]    At time T2, DS makes a low to high transition. As a result, the p-channel transistor  37  and the n-channel transistor  36  are brought out of conduction, whereas the p-channel transistor  39  and the n-channel transistors  33  and  40  are brought into conduction.  
         [0130]    The states of the nodes immediately after time T2 are the same as those when voltage Vg is below the targeted level of VREF×2. Therefore, SAS remains low immediately after time T2. In this way, a malfunction of switching SAS from low to high when voltage Vg has not reached the targeted level can be avoided.  
         [0131]    (Offset Circuit  400 )  
         [0132]    The following explains how to reliably detect voltage Vg at the targeted level.  
         [0133]    The differential amplifier  100  and the Vg detector  11  both use voltage VREF as a reference voltage.  
         [0134]    The differential amplifier  100  basically tries to bring voltage Vg to the targeted level of VREF×2. In actuality, however, voltage Vg stabilizes at a level 0.01V lower than the targeted level, due to variations in characteristic of circuit elements and changes in current load. This being so, if the detection level of the Vg detector  11  is set at Vg=VREF×2, SAS output from the Vg detector  11  never transitions from low to high. If SAS does not become high, the read cycle ends without the sense amplifier  9  being activated, which causes a malfunction. To avoid this, the n-channel transistor  34  is parallel-connected to the n-channel transistor  32  which has the same capability as the n-channel transistor  31 . In so doing, a differential circuit portion of the Vg detector  11  becomes slightly off-balance, so that the detection level of the Vg detector  11  is slightly decreased to Vg=VREF×2−0.05V which is 0.05V lower than Vg=VREF×2. The amount of decrease is 0.05V in this example, but may be adjusted by varying the capability of the n-channel transistor  34 .  
         [0135]    It should be noted that the embodied construction may be modified in such a manner that the polarity of each voltage is reversed. In this case, each n-channel transistor in the embodiment is replaced with a p-channel transistor, while each p-channel transistor in the embodiment is replaced with an n-channel transistor.  
         [0136]    Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art.  
         [0137]    Therefore, unless such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.