Abstract:
A method of maintaining a power supply voltage during a brownout is disclosed. The method includes the step of storing a charge in a charge reservoir ( 608 ) and storing a charge on a power supply capacitor ( 832 ). A charge from the charge reservoir is applied to the power supply capacitor in response to a power supply fail signal (BROWNOUT).

Description:
CLAIM TO PRIORITY OF NONPROVISIONAL APPLICATION 
       [0001]    This application claims the benefit under 35 U.S.C. §119(e) of Provisional Appl. No. 61/547,496 (TI-69575PS): filed Oct. 14, 2011, which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Nonvolatile memory circuits such as electrically erasable programmable read only memories (EEPROM) and Flash EEPROMs have been widely used for several decades in various circuit applications including computer memory, automotive applications, and video games. Many new applications, however, require the access lime and packing density of previous generation nonvolatile memories in addition to low power consumption for battery powered circuits. One nonvolatile memory technology that is particularly attractive for these low power applications is the ferroelectric memory cell. A major advantage of these ferroelectric memory cells is that they require approximately three orders of magnitude less energy for write operations than previous generation floating gate memories. Furthermore, they do not require high voltage power supplies for programming and erasing charge stored on a floating gate. Thus, circuit complexity is reduced and reliability increased. 
         [0003]    The term ferroelectric is something of a misnomer, since present ferroelectric capacitors contain no ferrous material. Typical ferroelectric capacitors include a dielectric of ferroelectric material formed between two closely-spaced conducting plates. One well-established family of ferroelectric materials known as perovskites has a general formula ABO 3 . This family includes Lead Zirconate Titanate (PZT) having a formula Pb(Zr x Ti 1-x ) O 3 . This material is a dielectric with a desirable characteristic that a suitable electric field will displace a central atom of the lattice. This displaced central atom, either Titanium or Zirconium, remains displaced after the electric field is removed, thereby storing a net charge. Another family of ferroelectric materials is Strontium Bismuth Titanate (SBT) having a formula SbBi 2 Ta 2 O 9 . SBT has several advantages over PZT. Memories fabricated from either ferroelectric material have a destructive read operation. In other words, the act of reading a memory cell destroys the stored data so that it must be rewritten before the read operation is terminated. Thus, a power supply interruption or brownout during a read or write operation may produce a data loss in a ferroelectric memory. This is also true for other nonvolatile memories such as flash memories that experience a brownout during a write operation. 
         [0004]    A typical one-transistor, one-capacitor (1T1C) ferroelectric memory cell of the prior art is illustrated at  FIG. 1 . The ferroelectric memory cell is similar to a 1T1C dynamic random access memory (DRAM) cell except for ferroelectric capacitor  100 . The ferroelectric capacitor  100  is connected between plateline  110  and storage node  112 . Access transistor  102  has a current path connected between bitline  108  and storage node  112 . A control gate of access transistor  102  is connected to wordline  106  to control reading and writing of data to the ferroelectric memory cell. This data is stored as a polarized charge corresponding to cell voltage V CAP . Capacitance of bitline BL is represented by capacitor C BL ,  104 . 
         [0005]    Referring to  FIG. 2 , there is a hysteresis curve corresponding to the ferroelectric capacitor  100 . The hysteresis curve includes net charge Q or polarization along the vertical axis and applied voltage along the horizontal axis. By convention, the polarity of the ferroelectric capacitor voltage is defined as shown in  FIG. 1 . A stored “0”. Therefore, is characterized by a positive voltage at the plateline terminal with respect to the access transistor terminal. A stored “1” is characterized by a negative voltage at the plateline terminal with respect to the access transistor terminal. A “0” is stored in a write operation by applying a voltage Vmax across the ferroelectric capacitor. This stores a saturation charge Qs in the ferroelectric capacitor. The ferroelectric capacitor, however, includes a linear component in parallel with a switching component. When the electric field is removed, therefore, the linear component discharges and only the residual charge Qr remains in the switching component. The stored “0” is rewritten as a “1” by applying −Vmax to the ferroelectric capacitor. This charges the linear and switching components of the ferroelectric capacitor to a saturation charge of −Qs. The stored charge reverts to −Qr when the voltage across the ferroelectric capacitor is removed. Finally, coercive points V C  and −V C  are minimum voltages on the hysteresis curve that will degrade a stored data state. For example, application of V C  across a ferroelectric capacitor will degrade a stored “1” even though it is not sufficient to store a “0”. Thus, it is particularly important to avoid voltages near these coercive points unless the ferroelectric capacitor is being accessed. Moreover, power supply voltage across a ferroelectric capacitor must exceed these coercive voltages during a brownout to successfully write or rewrite a desired data state. Otherwise, the data state is lost. 
         [0006]    Referring to  FIG. 3 , there is illustrated a typical write sequence for a ferroelectric memory cell as in  FIG. 1 . Initially, the bitline (BL), wordline (WL), and plateline (PL) are all low. The upper row of hysteresis curves illustrates a write “1” and the lower row represents a write “0”. Either a “1” or “0” is initially stored in each exemplary memory cell. The write “1” is performed when the bitline BL and wordline WL arc high and the plateline PL is low. This places a negative voltage across the ferroelectric capacitor and charges it to −Qs. When plateline PL goes high, the voltage across the ferroelectric capacitor is 0V and the stored charge reverts to −Qr. At the end of the write cycle, both bitline BL and plateline PL go low and stored charge −Qr remains on the ferroelectric capacitor. Alternatively, the write “0” occurs when bitline BL remains low and plateline PL goes high. This places a positive voltage across the ferroelectric capacitor and charges it to Qs representing a stored “0”. When plateline PL goes low, the voltage across the ferroelectric capacitor is 0 V, and the stored charge reverts to Qr representing a stored “0”. 
         [0007]    A read operation is illustrated at  FIG. 4  for the ferroelectric memory cell at  FIG. 1 . The upper row of hysteresis curves illustrates a read “0”. The lower row of hysteresis curves illustrates a read “1”. Wordline WL and plateline PI, are initially low. Bitlines BL are precharged low. At time Δt 0  bitline precharge signal PRE goes low permitting the bitlines BL to float. At time Δt 1  both wordiine WL and plateline PL go high, thereby permitting each memory cell to share charge with a respective bitline. A stored “1” will share more charge with parasitic bitline capacitance C BL  and produce a greater bitline voltage than the stored “0” as shown. A reference voltage ( FIG. 6 ) is produced at each complementary bitline of an accessed bitline. This reference voltage is between the “1” and “0” voltages. Sense amplifiers are activated at the time boundary between Δt 1  and Δt 2 . When respective bitline voltages are fully amplified in lime Δt 2 , the read “0” curve cell charge has increased from Qr to Qs. By way of comparison, the read “1” data state has changed from a stored “1” to a stored “0”. Thus, the read “0” operation is nondestructive, but the read “1” operation is destructive. At time Δt 3 , plateline PL goes low and applies −Vmax to the read “1” cell, thereby storing −Qs. At the same time, zero voltage is applied to the read “0” cell and charge Qr is restored. At the end of time Δt 3 , signal PRE goes high and precharges both bitlines BL to zero volts or ground. Thus, zero volts is applied to the read “1” cell and −Qr is restored. 
         [0008]    Referring to  FIGS. 5A and 5B , there are timing diagrams illustrating two different types of sensing that may be used in ferroelectric memory circuits. A primary difference between these two schemes is that the step sensing scheme ( FIG. 5A ) uses a single pulse of plateline PL, while the pulse sensing scheme ( FIG. 5B ) uses a double pulse of plateline PL. For both types of sensing, bitline precharge signal PRE goes low at time t 0 , thereby permitting the bitlines BL to float. Next, wordiine WL goes high at time t 1  to turn on access transistors of a row of memory cells. Plateline PL goes high between limes t 1  and t 2 , permitting ferroelectric memory cells share charge with their respective bitlines BL and develop respective difference voltages. Here, V 1  represents a data “1” and V 0  represents a data “0”. The difference voltage available for sensing is the difference between one of V 1  and V 0  and a reference voltage ( FIG. 6 ) which lies approximately midway between voltages V 1  and V 0 . This difference voltage is amplified at time t 3  for the step sensing scheme ( FIG. 5A ) so that Hill bitline BL voltages are developed before the plateline PL goes low at time t 4 . The data “0” cell is fully restored between time t 3  and time t 4  while plateline PL is high and the data “0” bitline BL is low. At time t 4 , the plateline PL goes low while the data “1” bitline BL remains high. Thus, the data “1” cell is restored between time t 4  and time t 5 . Bitline precharge signal PRE goes high at time t 5 , thereby precharging the bitlines BL to ground or 0 V. The step sensing cycle is completed when wordline WL goes low at time t 6  to store respective data in the row of memory cells. 
         [0009]    Referring now to  FIG. 5B  the first pulse of plateline PL develops a difference voltage at time t 2 . Plateline PL then goes low at time t 3 , and the common mode difference voltage goes to near 0 V. Then the difference voltage is amplified at time t 4 , and full bitline BL voltages are developed while the plateline PL is low. Thus, the data “1” cell is restored between time t 4  and time t 5  while plateline PL is low and the data “1” bitline BL is high. At time t 5 , the plateline PL goes high while the data “0” bitline BL remains low. Thus, the data “0” cell is restored between time t 5  and time t 6 . The data “1” cell is again restored between time t 6  and time t 7  while plateline PL is low and the data “1” bitline BL is high. Bitline precharge signal PRE goes high at time t 7 , thereby precharging the bitlines BL to ground or 0 V. The pulse sensing cycle is completed when wordline WL goes low at time t 4 . 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    In a preferred embodiment of the present invention, a method of maintaining a power supply voltage is disclosed. The method includes storing a charge in a charge reservoir and storing a charge on a power supply capacitor. Upon receiving a power fail signal, a charge from the charge reservoir is applied to the power supply capacitor. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0011]      FIG. 1  is a circuit diagram of a ferroelectric memory cell of the prior art; 
           [0012]      FIG. 2  is a hysteresis curve of the ferroelectric capacitor  100  of  FIG. 1 : 
           [0013]      FIG. 3  is a timing diagram showing a write operation to the ferroelectric memory cell of  FIG. 1 ; 
           [0014]      FIG. 4  is a timing diagram showing a read operation from the ferroelectric memory cell of  FIG. 1 : 
           [0015]      FIG. 5A  is a timing diagram of a step sense read cycle; 
           [0016]      FIG. 5B  is a timing diagram of a pulse sense read cycle; 
           [0017]      FIG. 6A  is a schematic diagram of a charge pump of the present invention: 
           [0018]      FIG. 6B  is a waveform diagram showing operation of the charge pump of  FIG. 6A ; 
           [0019]      FIG. 7  is a control circuit for the charge pump of  FIG. 6A ; 
           [0020]      FIG. 8  is a schematic diagram of an embodiment of a charge pump circuit of the present invention; 
           [0021]      FIG. 9  is a schematic diagram of another embodiment of a charge pump circuit of the present invention: and 
           [0022]      FIG. 10  is a timing diagram showing operation of the charge pump circuit of  FIG. 9 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    Preferred embodiments of the present invention provide significant advantages in completing read and write operations in nonvolatile memories after a temporary power failure, often referred to as a brownout, in the following discussion, the same reference numerals are used in the drawing figures to indicate common circuit elements. 
         [0024]    Referring to  FIG. 8 , there is a schematic diagram of a power supply circuit of the present invention. Here, a memory circuit  830  is modeled as a power supply capacitor  832  in parallel with a current source  834 . The power supply capacitor  832  may include parasitic capacitance as well as thin oxide decoupling capacitors. The current source  834  represents an average current during a read or write operation of the memory circuit. A p-channel transistor  800  couples power supply terminal VDD to the power supply capacitor  832  via lead  620 . P-channel transistor  800  has a bulk terminal coupled to lead  620 , so the ramp rate of the power supply voltage VDD must be controlled during power up to avoid exceeding a diode drop across the transistor current path. P-channel transistor  800  is preferably surrounded by a P+ guard ring as is known in the art. The power supply voltage on lead  620  or VDD_MEM is dedicated to the memory circuit as well as other circuits in the following discussion. The power supply circuit includes plural pump circuits  818 - 822  coupled to lead  620 . Each pump circuit  818 - 822  includes a respective control circuit  812 - 816  as will be discussed in detail. The control circuits  812 - 816  are each coupled to receive a power fail signal BROWNOUT on lead  702  to indicate a temporary power failure at terminal VDD. The BROWNOUT signal is typically generated by a memory controller and powered by VDD_MEM or another power source which is not impacted by the VDD brownout. Alternatively, the BROWNOUT signal may be generated by the memory circuit by comparing power supply voltage VDD to a predetermined threshold voltage. The control circuits  812 - 816  are also coupled to receive a reset signal RST on lead  706 . The RST signal is preferably a pulse of short duration that is generated at the beginning of each memory access. Control circuit  812  is also coupled to receive wordline enable signal WLEN as will be discussed in detail. 
         [0025]    Turning now to  FIGS. 6A and 6B , pump circuits  818 - 822  will be described in detail. Each pump circuit  818 - 822  is substantially the same as pump circuit  600 , except that transistor and capacitor sizes may vary. Pump circuit  600  includes p-channel transistor  602  having a control gate coupled to receive pump enable signal PUMPEN. The PUMPEN signal is low during normal operation so that p-channel transistor  602  is on. This charges reservoir capacitor  608  to VDD_MEM. The PUMPEN signal is also applied to inverter  604  and produces a high output signal. The high output signal is applied to inverter  606  to produce a low output at lead  614  of pump capacitor  610 . Pump capacitor  610 , therefore, is initially charged to VDD_MEM.  FIG. 6B  illustrates operation of pump circuit  600  in the power supply circuit of  FIG. 8  when PUMPEN, indicated by the solid line, goes high at time t 0 . The high level of PUMPEN turns off p-channel transistor  602 , thereby isolating reservoir capacitor  608  from VDD_MEM at lead  620 . The high level of PUMPEN also produces a low level at the output of inverter  604  and a corresponding high level at the input of inverter  606 . Responsively, the reservoir capacitor  608  partially discharges through inverter  606  as indicated by dashed waveform  612 . The output of inverter  606  charges lead  614  as indicated by short dashed waveform  614 . The transition of waveform  614  pumps charge onto lead  620  and power supply capacitor  832  as indicated by dotted waveform  620 . At time t 1 , PUMPEN returns to a low level and turns on p-channel transistor  602 . This applies VDD_MEM to lead  612  and recharges reservoir capacitor  608 . The low level of PUMPEN also produces a low level at lead  614  to recharge pump capacitor  610  to VDD_MEM. Thus, pump circuit  600  returns to its initial state at time t 2 . In practice, reservoir capacitor  608  and pump capacitor  610  are designed for a predetermined charge transfer with minimum capacitor size. If there is a large difference in size between reservoir capacitor  608  and pump capacitor  610 , the smaller capacitor will limit the effectiveness of the larger capacitor. Both capacitors are designed to pump the predetermined charge onto power supply capacitor  832 . 
         [0026]    Referring now to  FIG. 7 , there is a schematic diagram of a control circuit  700  that is substantially the same as control circuits  812 - 816  of  FIG. 8 . The control circuit produces control signal PUMPER which is applied to a respective pump circuit as shown in  FIG. 8 . In operation, the high level of RST produces a low level output from inverter  704 , a high level output from NAND gate  710 , and a low level output from NOR gale  712  at the S input of SR Hip flop  714 . Together with the high level of RST at the R input, RST resets the SR flip flop at the beginning of each memory access cycle so that the Q output is low. The low Q output produces a low output (PUMPEN) from AND gate  718  at lead  716 . The short duration RST pulse then returns low and produces a high output from inverter  704  to enable NAND gate  710 . Power fail signal BROWNOUT remains low in normal operation, thereby producing a high level output from NAND gate  710  and a corresponding low level output from NOR gate  712 . In this normal mode of operation, PUMPEN remains low as long as power fail signal BROWNOUT remains low without regard to the level of BOOSTn on lead  708 . At any time during a memory access cycle, power fail signal BROWNOUT may go high indicating a power failure. This produces a low level output from NAND gate  710  and a high level output from NOR gate  712 . The high level at the S input sets SR flip flop  714  to produce a high level at the Q output. Subsequently, when BOOSTn goes high it produces a high level PUMPEN signal from AND gate  718  on lead  716 . This high level of PUMPEN enables a respective one of pump circuits  818 - 822  as previously described. 
         [0027]    Returning now to the power control circuit of  FIG. 8 , control circuit  812  is coupled to receive wordline enable signal WLEN, which corresponds to BOOSTn ( FIG. 7 ) in the previous discussion. WLEN is also applied to delay circuit  808 . Delay circuit  808  may include series connected inverters with a capacitor and preferably produces a rising edge delay with a negligible falling edge delay. The output of delay stage  808 , a delayed version of the rising edge of WLEN, is then applied to control circuit  814 . Responsively, control circuit  814  produces a respective PUMPEN signal to activate pump circuit  820 . The output of delay stage  808  is also applied to delay stage  810  to produce a further delayed version of WLEN which is applied to control circuit  816 . Responsively, control circuit  816  produces a respective PUMPEN signal to activate pump circuit  822 . In this manner, multiple pump circuits such as pump circuits  818 - 822  may be sequentially activated to maintain power supply voltage VDD_MEM until completion of the memory access cycle. This is highly advantageous for several reasons: First, sequential activation of pump circuits avoids large voltage spikes of VDD_MEM on lead  620 . Second, data will not be lost due to an incomplete read or write in a memory access cycle of a nonvolatile memory. Finally, power supply capacitor  832  may be smaller than a comparable decoupling capacitor that would maintain a sufficient voltage of VDD_MEM without the pump circuits. 
         [0028]    Turning now to  FIGS. 9 and 10 , there is another embodiment of the power supply circuit of the present invention. The power supply circuit of  FIG. 9  is substantially the same as the power supply circuit of  FIG. 8  except that each of control circuits  812 - 816  is activated by an enable signal for a respective memory circuit. For example, wordline enable signal WLEN activates control circuit  812  as previously discussed at time t 0  ( FIG. 10 ). A first plateline enable signal PLEN 1  activates control circuit  814  at time t 1 . A second plateline enable signal PLEN 1  activates a second plateline control circuit (not shown) at time t 2 . At time t 3 , sense amplifier enable signal SAEN activates control circuit  816 . Finally, at time t 4  the memory cycle ends and all enable signals return low. The power supply circuit of  FIG. 9  offers all the advantages of the power supply circuit of  FIG. 8 . Additionally, since each control circuit is activated by a respective memory circuit enable signal, both the reservoir capacitor  608  and the pump capacitor  610  of each respective pump circuit  600  may be designed to supply the charge required for that respective memory circuit. For example, the reservoir capacitor  608  and pump capacitor  610  of pump circuit  818  arc designed to supply the charge required by the wordline and other circuitry until the next pump circuit  820  is activated. The reservoir capacitor  608  and pump capacitor  610  of pump circuit  820  are designed to supply the charge required by the plateline and other circuitry until the next pump circuit is activated. Likewise, the reservoir capacitor  608  and pump capacitor  610  of pump circuit  822  are designed to supply the charge required by the sense amplifiers and other circuitry for the rest of the access. Thus, power fail signal BROWNOUT may occur any time in the memory cycle, and only the charge required for the remainder of the memory cycle is supplied to the VDD_MEM power supply capacitor  832 . 
         [0029]    Still further, while numerous examples have thus been provided, one skilled in the art should recognize that various modifications, substitutions, or alterations may be made to the described embodiments while still falling with the inventive scope as defined by the following claims. For example, any number of pump circuits may be used in the previously described embodiments of  FIGS. 8 and 9 . Moreover, a combination of the embodiments of  FIGS. 8 and 9  may be desirable. For example, it may be desirable to sequentially activate more than one pump circuit in response to sense amplifier enable signal SAEN. In this case sequential activation may be accomplished by delayed versions of SAEN. In another example in the circuit of  FIG. 7 , the leading edge of the BOOSTn signal could be a delayed version of the BROWNOUT signal. Other combinations will be readily apparent to one of ordinary skill in the art having access to the instant specification.