Patent Document

CROSS REFERENCES TO RELATED APPLICATIONS 
   Not Applicable. 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not Applicable. 
   BACKGROUND OF THE INVENTION 
   The present embodiments relate to electronic devices and circuits and are more particularly directed to a static random access memory (“SRAM”) with a temperature-dependent voltage control in its sleep mode. 
   Electronic circuits have become prevalent in numerous applications, including uses for devices in personal, business, and other environments. Demands of the marketplace affect many aspects of the design of these circuits, including factors such as device power consumption and cost of operation. Various of these electronic circuits include some aspect of digital signal processing and, quite often, these circuits include storage devices or memories. One type of popular memory is the SRAM. An SRAM has a static nature, that is, it does not require the data in each of its memory cells to be refreshed, which is required by way of contrast in a dynamic RAM (“DRAM”). Typically, the SRAM is also considered faster and more reliable than DRAM, and indeed it has found favor in numerous uses, including uses in cache memory. 
   With the prevalence of SRAM memory in numerous devices and applications, the SRAM also is subject to the above-introduced factors of power consumption and cost of operation. In this regard, the prior art now includes a so-called “sleep mode” of operation of the SRAM, which also is sometimes referred to with other terms such as “standby mode” or possibly still others. This mode is characterized as a period of time in which the data cells in the SRAM will not be accessed (i.e., either read or written), but the data state in each cell must be maintained because it is anticipated that eventually the operation of the SRAM will discontinue from the sleep mode, at which time the data previously stored therein will be needed; hence, there is a need to maintain the validity of that data during the sleep mode. With these considerations, the sleep mode typically is further characterized in that the voltage provided to the SRAM array of memory cells is reduced during the sleep mode as compared to the voltage supplied during all access operations. The reduction is typically to a level on the order of one-half of the voltage provided during data access operations of the SRAM, where the voltage applied to the SRAM array (often referred to as VDDA) during the data access operations is often set to a value referred to as V DD ; thus, during the sleep mode, the voltage V DDA  provided to the SRAM array is on the order of 0.5 V DD . In the prior art, the specific amount of reduction of V DDA , such as to a value of 0.5 V DD , is typically determined by the designer to accommodate worst case scenarios. For example, with the anticipated range of environmental and circuit conditions such as temperature, silicon variations, and the like, the reduced value of V DDA  is fixed at a level to ensure that the data state is maintained in each SRAM array cell during the sleep mode, while also accommodating any change in the environmental and circuit conditions. 
   While the above-described approaches have proven workable in various implementations, the present inventors have observed that the prior art may be improved. Specifically, in connection with the present preferred embodiments, it has been observed that as the temperature to which an SRAM is exposed decreases, the amount of voltage required to maintain the data state in the SRAM cells increases. Thus, with the fixed-voltage levels in the standby mode of the prior art, the supply voltage for the standby mode is presumably established in contemplation of a worst-case scenario, that is, in respect to the voltage required at the lowest temperature anticipated to be experienced by the SRAM data cells (with whatever additional tolerance). However, the present inventors have recognized that when the SRAM is in standby mode and experiences higher temperatures, then the preestablished fixed voltage supply to the SRAM will necessarily cause a certain amount of current leakage across the SRAM array. Of course, current leakage is undesirable for various reasons, including increased power consumption and operational cost. Thus, the preferred embodiments as set forth below seek to improve upon the prior art as well as these associated drawbacks. 
   BRIEF SUMMARY OF THE INVENTION 
   In one preferred embodiment, there is an electronic device, comprising a plurality of data storage cells, collectively operable in a data access mode and separately in a sleep mode. The sleep mode comprises a period of time during which the plurality of data cells are not accessed and during which a data state stored in each cell in the plurality of data cells is to be maintained at a valid state. The electronic device further comprises circuitry for providing at least one temperature-dependent voltage to at least one storage device in each cell in the plurality of data storage cells during the sleep mode. 
   In another preferred embodiment, there is a method supplying voltage to an electronic device, the electronic device comprising a plurality of data storage cells, collectively operable in a data access mode and separately in a sleep mode. The method comprises a set of steps comprising coupling a first voltage to at least one storage device in each cell in the plurality of data storage cells during the sleep mode and at a first temperature, and coupling a second voltage to the at least one storage device in each cell in the plurality of data storage cells during the sleep mode and at a second temperature, wherein the second temperature differs from the first temperature. 
   Other aspects are also disclosed and claimed. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       FIG. 1  illustrates a block diagram of a static random access memory (“SRAM”) system designated generally at  10  and according to the preferred embodiments. 
       FIG. 2  illustrates a schematic for providing a temperature-dependent voltage to the SRAM array of  FIG. 1  during the sleep mode. 
       FIG. 3   a  illustrates a plot demonstrating one preferred embodiment temperature-dependent voltage provided to the SRAM array of  FIG. 1  during the sleep mode. 
       FIG. 3   b  illustrates an alternative plot demonstrating temperature-dependent voltage that may be provided to the SRAM array of  FIG. 1  during the sleep mode. 
       FIG. 3   c  illustrates another alternative plot demonstrating temperature-dependent voltage that may be provided to the SRAM array of  FIG. 1  during the sleep mode. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  illustrates a block diagram of a memory system designated generally at  10  and according to the preferred embodiments. In one preferred embodiment, system  10  is constructed using a single integrated circuit and, indeed, additional circuitry is likely included within such an integrated circuit. However, to simplify the present illustration and discussion, such additional circuitry is neither shown nor described. Moreover, system  10  may be implemented in connection with numerous digital data systems, as ascertainable by one skilled in the art. 
   Looking to the blocks in system  10 , system  10  includes various items which in general are also known in the prior art, but additional control and operation as detailed later distinguishes the overall system. Looking by way of introduction to some of the blocks that are comparable to the prior art, they include an SRAM array  12 . SRAM array  12  is intended to demonstrate a collection of SRAM memory cells that are typically aligned in an array fashion, that is, to include either or both a physical and logical orientation wherein the memory cells are addressed according to a number R of rows and a number C of columns. The values of R and C may vary widely based on implementations. Each of the R×C memory cells may be constructed according to various techniques known to or ascertainable by one skilled in the art. An ADDRESS signal is coupled to a row access circuit  14  and a column access circuit  16 . Collectively, row access circuit  14  and column access circuit  16  facilitate the reading and writing of cells in SRAM array  12 , where the particular cells accessed in a given cycle are determined by the ADDRESS. The manner in which a row or rows, and some or all of the columns in a row or rows, are accessed in a given cycle also may vary. In any event, addressed cells are either written or read as indicated at the DATA input/output  10   I/O  of  FIG. 1 , where typically such I/O is provided in connection with the column access of the array. 
   Turning now to an inventive aspect of  FIG. 1  and which also combines with system  10  so as to provide an overall novel and improved system, system  10  includes an array voltage supply block  18 . In the preferred embodiment, array voltage supply block  18  provides one or more array voltages to the data storage cells of SRAM array  12 , where by way of a preferred example, three such system voltages are shown in  FIG. 1 . These three voltages include the array high supply voltage V DDA , the array low supply voltage V SSA , and an array back bias voltage V BBA . As known in the art, the array high and low supply voltages, V DDA  and V SSA , respectively, may be connected to various devices, such as selected source, drain, and/or gate nodes of various transistors within SRAM array  12 , as may the low supply voltage, V SSA . Further, the array back bias voltage V BBA , may connect to the backgates or other appropriate threshold voltage connections of at least some of the transistors in SRAM array  12 . Further in this regard, while a single back bias voltage, V BBA , is shown in  FIG. 1 , in one preferred embodiment two separate back bias voltages are provided, one for p-channel transistors and one for n-channel transistors, where as known in the art a different back bias voltage is generally applied to p-channel transistors compared to the back bias voltage applied to n-channel transistors, and a different amount of voltage adjustment is required to cause an equal change in the threshold voltage of these different conductivity type devices. In an alternative embodiment, V BBA  may be adjusted with respect to p-channel transistors while an increase in V SSA  relative to substrate voltage may be used to increase a back bias on n-channel transistors so as to increase the threshold voltage of those transistors. Lastly, and for reasons evident below, array voltage supply block  18  also includes an input  18   S  for receiving a SLEEP signal. 
   The particular operation of array voltage supply block  18  is now explored and provides an inventive aspect as will be appreciate by one skilled in the art. Specifically, during data access operation of system  10 , array voltage supply block  18  may provide the array voltages, V DDA , V SSA , and V BBA , according to techniques and methodologies as known in the art. However, the preferred embodiments contemplate that the SLEEP signal is asserted when it is desired for system  10  to enter into a sleep mode of operation, that is, a period of time where it is known that the data cells in SRAM array  12  will not be accessed (i.e., either read or written), but where it is required that each cell maintain its valid data state. The assertion of the SLEEP signal in this manner may be performed according to the art. In response to asserted SLEEP signal, array voltage supply  18  adjusts one or more of the array voltages, V DDA , V SSA , and V BBA , so as to reduce current consumption of the cells in SRAM array  12  during the corresponding period of standby operation. For example, V DDA  may be decreased and/or V SSA  may be increased to the cells. As another example, V BBA  may be adjusted so as to increase the threshold voltage of the transistor(s) in each SRAM cell, where, for example, increasing V BBA  of a p-channel transistor will increase its threshold voltage. Importantly, however, also in the preferred embodiment, any one or more of V DDA , V SSA , and V BBA  are further adjusted according to the temperature experienced by system  10 . In other words, and as further appreciated below from one preferred embodiment approach, array voltage supply  18  operates in a fashion that is deliberately temperature-dependent such that any of V DDA , V SSA , or V BBA  may be altered in response to temperature. For sleep mode, the applied voltages must be set at values that will allow retention of the data while preferably reducing IDDQ. Since transistor characteristics change with temperature, the voltages required for data retention may change with temperature, and IDDQ may change with temperature. A larger voltage across the cell (V DDA -V SSA ) may be required for data retention at lower temperature than at higher temperature. Also, for a given set of voltages, IDDQ may be higher at higher temperature than at lower temperature. Thus, these voltage alterations may be constructed so that when temperature decreases, the net voltage, V DDA -V SSA , applied to SRAM array  12  is increased, where in contrast when temperature increases, that net voltage, V DDA -V SSA , applied to SRAM array  12  is decreased. Additionally, as temperature increases, the threshold voltage of data cell transistors may decrease for a given back bias. Thus the applied back bias voltage may be adjusted to increase the threshold voltage at higher temperature and to decrease the threshold voltage at lower temperature. Note that these net voltage adjustments are such that when temperature decreases, the voltage across the cell is increased so as to maintain the valid data state in cells of SRAM array  12 ; however, as a benefit, when temperature increases, the net voltage is decreased and/or the threshold voltage is increased with a corresponding reduction in leakage current as compared to that which would occur if the voltage remained the same as it was at lower temperatures. Therefore, during the sleep mode of operation, array voltage supply block  18  provides a first set of voltage levels to SRAM array  12  for a first temperature, and array voltage supply block  18  provides a second set of voltage levels to SRAM array  12  for a second temperature, where the first set of voltages has at least one voltage that differs from the comparable voltage in the second set of voltages, and where the first temperature differs from the second temperature. 
     FIG. 2  illustrates a schematic of one preferred embodiment for implementing a portion of voltage supply block  18  for sake of generating the array high supply voltage V DDA  and which is designated as system  18 ′. By way of introduction, system  18 ′ is such that the voltage V DDA  it outputs decreases with an increase in temperature and such that this voltage increases with a decrease in temperature, consistent with the above teachings and for use when the SLEEP signal is asserted to supply  18  in  FIG. 1 . Looking to the connectivity of the schematic, it includes a node  30  coupled to receive a supply voltage V CC  that preferably does not vary significantly with temperature, as may be produced by various voltage supply circuits known in the art. Looking to the left of the schematic, in general a subsystem SB 1  is generally segregated for sake of illustration and later functional description, and is as follows. Node  30  is connected to the source and backgate of a p-channel transistor  32 , which has its gate connected to a node  34  and its drain connected to a node  36 . Node  34  in the illustrated embodiment is connected to ground. Node  36  is further connected to the base and collector of a bi-polar junction transistor (“BJT”)  38 , which has its emitter connected to a base and collector of a BJT  40 . The emitter of BJT  40  is connected to node  34 . Returning to node  30 , it is also connected to the source and backgate of a p-channel transistor  42 , which has its gate connected to its drain and also to the gate of a p-channel transistor  44 . The drain of p-channel transistor  42  is also connected to the collector of a BJT  46 , which has its base connected to node  36  and its emitter connected to a first terminal (or node)  48  of a resistor  50 , and where a second terminal  52  of resistor  50  is connected to node  34 . 
   Continuing with  FIG. 2 , system  18 ′ includes another subsystem SB 2 , also segregated for sake of illustration and later functional description. In subsystem SB 2 , node  30  is connected to the backgate of p-channel transistor  44  and also to the backgate of a p-channel transistor  54 . The drain of p-channel transistor  44  is connected to a node  56 , which as detailed later provides the temperature-dependent voltage, V DDA . The drain of p-channel transistor  54  is connected to a node  58  and the gate of that p-channel transistor  54  is connected to a node  60 , which is connected to the drain of a p-channel transistor  62  and the drain of an n-channel transistor  64 . P-channel transistor  62  has its source and backgate connected to node  30 , and its gate connected to a node  66 . The gate of n-channel transistor  64  is connected to node  30  and its source and backgate are connected to node  34 . Node  66  is also connected to the drain and gate of a p-channel transistor  68 , having its source and backgate connected to node  30 . Node  66  is also connected to the gate and drain of a p-channel transistor  70 , having its source and backgate connected to node  30 . Node  66  is also connected to the gate of a p-channel transistor  72 , having its source and backgate connected to node  30  and its drain connected to node  56 . Node  66  is also connected to the collector of a BJT  74 , having its base connected to node  58  and its emitter connected to a terminal  76  of a resistor  78 . Another terminal  80  of resistor  78  is connected to node  34 . Returning to p-channel transistor  70 , its drain is connected to the base and collector of a BJT  82 , which has its emitter connected to node  34 . Finally, node  56  is also connected to a terminal  84  of a resistor  86 , which has another terminal  88  connected to node  34 . 
   The operation of system  18 ′ is now discussed, and may be appreciated by first turning to sub-system SB 2 . In general, sub-system SB 2  operates as a bandgap subcircuit to provide a current that has a known dependency with temperature. Specifically, in the preferred embodiment, BJT  74  is on the order of four times the physical size of BJT  82 . However, due to the current mirror produced by p-channel transistors  68  and  70 , each of BJTs  74  and  82  has a same amount of current passing through the collector-emitter paths of those devices. The result of this equivalent current flow is that the base-to-emitter voltage (“VBE”) is reduced through BJT  74  relative to BJT  82 , since the current through BJT  74  per unit area is one-fourth that through BJT  82 . Thus, there is a ΔV BE  between ground at node  34  and terminal  76  of resistor  78 ; this ΔV BE  may be in a slight sense temperature dependent, but may be assumed to be temperature independent at least to a first order of approximation. Given the preceding, the amount of resistance provided by resistor  78  defines the amount of current through subsystem SB 2 , that is, the mirrored current through p-channel transistors  68  and  70 . The current through resistor  78  is also mirrored through resistor  86 ; also, note that resistors  78  and  86  are preferably matched in terms of fabrication, but in terms of resistance one may be a multiple of the other. Likewise, therefore, the current mirrored through resistor  78  and resistor  86  may be the same or one may be a multiple of the other. In any event, therefore, subsystem SB 2  primarily contributes a temperature-dependent current through resistor  86 , which is inversely proportional to its resistance, thereby providing V DDA  in this respect as a temperature-independent voltage. Thus, subsystem SB 2  may be provided so as to provide a substantially temperature-independent V DDA  over a certain range of temperatures, such as for higher operating temperatures of memory system  10 . 
   Continuing with the operation of system  18 ′, attention is now directed to sub-system SB 1 . In general and as further detailed below, sub-system SB 1  operates to contribute to system  18 ′ a characteristic of a voltage that is temperature-dependent so that this element may be used to include temperature dependence into V DDA  below a certain desirable temperature threshold. With this inclusion, therefore, V DDA  may be increased either linearly or otherwise below the desired temperature threshold during the standby mode of system  10 , thereby providing sufficient voltage to maintain data in SRAM array  12  during that mode, whereas when the temperature increases above the desirable temperature threshold during the standby mode, the effect of sub-system SB 1  is minimized or avoided, thereby leaving the reduced and relatively fixed or temperature-independent characteristic of V DDA  as provided by sub-system SB 2  so as to also maintain state in SRAM array  12  while also decreasing leakage current as compared to that which would occur if the voltage were left at a higher range as is provided with the inclusion of the characteristic of sub-system SB 1  at lower temperatures. The details of such operation of subsystem SB 1  are further appreciated below. 
   Looking now in greater detail to the operation of subsystem SB 1 , a current is generated through p-channel transistor  32 , and this current is mirrored through BJTs  38  and  46 , thereby providing a same voltage at the emitter of each of BJTs  38  and  46 . Thus, the voltage across resistor  50  is the same as a V BE  across BJT  40 . Note, however, that the temperature coefficient of the V BE  across BJT  40  will vary considerably with temperature. For example, the V BE  may be 0.9 volts at −40 degrees Celsius and 0.4 volts at 150 degrees Celsius. Further, the resistance of resistor  50  increases with temperature. Thus, at lower temperatures, these factors combine to produce more current in the current mirror output that includes p-channel transistors  42  and  44 . This current may be scaled and is added to the current from p-channel transistor  72  (in subsystem SB 2 ), and this combined current passes through resistor  86 , thereby contributing to the voltage, V DDA , across that resistor  86 . Thus, the temperature dependence of subsystem SB 1  combines with the relative temperature independence of subsystem SB 2 . Accordingly, a characteristic of V DDA  may have the shape as shown as a simplified example in  FIG. 3   a . Particularly,  FIG. 3   a  plots V DDA  with respect to temperature, as produced by system  18 ′. As seen, below a temperature threshold THR 1 , V DDA  is fairly linear along a first line L 1 , and that line indicates that V DDA  increases with a decrease in temperature, as achieved through the added functionality provided by subsystem SB 1 . Above voltage threshold THR 1 , V DDA  is also linear, but along a second and different line L 2 . Also, while not shown, the transition between lines L 1  and L 2  may include a curved transition. In any event, V DDA  along line L 2  is fairly temperature-independent, as provided by subsystem SB 2 , in a range of temperature above THR 1  where the effect of subsystem SB 1  is substantially reduced or eliminated. In other words,  FIG. 3   a  illustrates that for system  18 ′, V DDA  is generated by the adding of a constant current and a temperature-dependent current through resistor  86 . In the low temperature region below threshold THR 1 , the temperature-dependent current is larger and predominantly determines the voltage, thereby increasing V DDA  as temperature decreases below threshold THR 1 . In the high temperature region, the constant current is larger and predominantly determines the voltage, thereby maintaining V DDA  as a relative constant above threshold THR 1 . At the transition region between lines L 1  and L 2 , the two currents are of comparable magnitude. In all events, it may be seen that lines L 1  and L 2  provide a range for V DDA , whereby generally at lower temperatures the provided voltage is greater than that at higher temperatures, thereby providing the operation and benefits described above. 
   In addition to the preceding, note further that in the preferred embodiments V DDA  may be supplied relative to V SSA . Thus the voltage across the cell may be increased at lower temperature by increasing V DDA  as shown in  FIG. 3   a , or by lowering V SSA  while holding V DDA  constant. Any combination of raising or lowering V DDA  and raising or lowering V SSA  may be used to obtain the desired voltage across the cell as a function of temperature. This may be done in conjunction with adjustment of back bias voltages or with constant back bias voltages to adjust the threshold voltages of the n-channel and p-channel transistors. For example, for a given V BBA  applied as back bias to the p-channel transistors and a given substrate voltage applied to the n-channel transistors, raising V DDA  and V SSA  together will lower the magnitude of the p-channel transistor threshold voltage and increase the magnitude of the n-channel transistor threshold voltage. Still further, note that the preceding techniques, with respect to V DDA , V SSA , and/or V BBA , may be applied based on other plots of voltage change with temperature. For example,  FIGS. 3   b  and  3   c  illustrate alternative plots demonstrating temperature-dependent voltage that may be provided to the SRAM array of  FIG. 1  during the sleep mode. Briefly, therefore,  FIG. 3   b  illustrates a first range of temperature below a threshold THR 2  wherein V DDA  is constant, and it further illustrates a second range of temperature above threshold THR 2  wherein V DDA  decreases linearly with temperature. As another alternative,  FIG. 3   c  illustrates different voltage characteristics across three temperature ranges. In a first range of temperature below a threshold THR 3 , V DDA  is constant, and in a second range of temperature above a threshold THR 4 , V DDA  is also constant. However, between temperatures THR 3  and THR 4  is a third range of temperature wherein V DDA  decreases linearly with temperature. 
   From the above, it may be appreciated that the above embodiments provide an SRAM with a temperature-dependent voltage control in its sleep mode. The SRAM provides particular benefits over the prior art. For example, current consumption during the sleep mode is reduced as compared to the prior art. This reduction leads to corresponding benefits, such as reduced power consumption and reduced cost of operation. These benefits may be particularly advantageous in battery-operated applications, which are quite common in contemporary applications. As another benefit, the present inventive teachings may be applied to numerous forms of SRAMs and in the numerous devices into which such SRAMs are included. As yet another benefit, while  FIG. 2  illustrates one approach to providing a temperature-dependent form of V DDA , one skilled in the art may ascertain other approaches as well as circuits for providing a temperature-dependent form of V SSA  and/or V BBA . Still further, rather than using current sources to provide a temperature-dependent form for any one or more of V DDA , V SSA , and V BBA , such a voltage or voltages may be provided by a number of voltage sources, where different ones of the voltage sources have different temperature dependencies. Thus, these benefits further demonstrates the flexibility of the preferred embodiments, and further demonstrate that while the present embodiments have been described in detail, various substitutions, modifications or alterations could be made to the descriptions set forth above without departing from the inventive scope which is defined by the following claims.

Technology Category: 3