Abstract:
A sense amplifier circuit and a method for reading a memory cell. A circuit comprises a first bit line associated with a memory cell. A first input of a latch is coupled to the first bit line and a second input of the latch is coupled to a second node. There is a means for biasing the first input and the second input of the latch to a differential voltage between the first node coupled to the first bitline and the second node. There is also a means for switching the latch according to memory cell current. There is also a means for producing an output signal indicating the direction of switch. A method of reading a memory cell comprises precharging a first bit line which is associated with a memory cell. The memory cell current is driven according to the programmed state of the memory cell. Latch circuitry is biased based on a differential voltage between a first node coupled to the first bit line and a second node. The latch circuitry is then activated and the latch circuitry switches according to the memory cell current. An output signal indicating the direction of the latch circuitry&#39;s switch is then produced.

Description:
TECHNICAL FIELD 
   The present invention relates to sense amplifiers for reading non-volatile memory cells. 
   BACKGROUND INFORMATION 
   In memory integrated circuits, sense amplifiers detect and determine the data content of a selected memory cell. In electrically erasable programmable read only memories (“EEPROM”) and Flash memories, the sense amplifier serves two functions. First, the sense amplifier charges the bit line to a clamped value. Second, the sense amplifier senses the current flowing into the bitline due to the memory cell state. Both the reliability, in terms of endurance and retention, and the performance of the memory, in terms of access time and power consumption, are dependent on the design of the sense amplifier. 
   Usually, integrated sense amplifier structures are based on a differential amplifier comparing the current coming from the selected memory cell to the current of a reference cell. Reference cells can be implemented in a number of ways, including arrays of reference cells. A reference current may also be supplied by a “dummy” bit line equivalent to a standard bit line. When reference cells are employed, they are programmed once during the testing of the memory, increasing testing time. 
   In order to ensure good functionality of the sense, the ratio I cell /I ref , where I cell  is the memory cell current and I ref  is the reference current, must be maintained high enough to take account of process fluctuations in the memory and references cells as well as the impact of memory cycling. It has been shown that the speed, performance, and reliability of standard differential amplifier sense amplifiers are highly reduced for supply voltages less than 2 V. 
   In general, previous attempts to design sense amplifiers that do not employ reference cells are fully asynchronous and are not very suitable at a low supply voltage (i.e., V DD &lt;1.2 V). Therefore, it would be desirable to have an improved sense amplifier design. 
   SUMMARY OF THE INVENTION 
   In one embodiment, a method of reading a memory cell comprises precharging a first bit line coupled to the memory cell. The memory cell is driven according to a programmed state of the memory cell. Latch circuitry is biased based on a differential voltage between a first node coupled to the bit line and a second node. The latch circuitry is activated and switches according to the memory cell current. An output signal indicating a direction of the latch circuitry switch is produced. 
   In another embodiment, a circuit comprises a first bit line coupled to a memory cell. There is a means for biasing a first input and second input of a latch to a differential voltage between a first node coupled to the first bit line and a second node. There is also a means for switching the latch according to memory cell current and a means for producing an output signal indicating a direction of the switch. 
   In yet another embodiment, a circuit comprises a first bit line coupled to a memory cell. A first input of a latch is coupled to the first bit line and a second input of the latch is coupled to a second node. Latch biasing circuitry is configured to bias the first input and second input of the latch to a differential voltage between a first node coupled to the bit line and the second node, the latch configured to switch after activation, the switch made according to memory cell current. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an electrical schematic diagram of one embodiment of the invention. 
       FIG. 2  is an electrical schematic diagram of one embodiment of the invention. 
       FIG. 3  is an electrical schematic diagram of one embodiment of circuitry to provide a bias voltage to the circuit of  FIG. 2 . 
       FIG. 4  is a flow chart showing one embodiment of the operation of the invention. 
       FIG. 5  is a timing diagram of one embodiment of the invention. 
       FIG. 6  is a block diagram showing a detectable range of memory cell current. 
       FIG. 7  is a block diagram showing sequencing circuitry of one embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows one embodiment of the sense circuit  10  featuring a latch  12  and latch biasing circuitry  60 . The latch  12  has two NMOS transistors  14 ,  16 . The source terminal of each of the NMOS transistors  14 ,  16  is coupled to a ground potential, and the gate of each NMOS transistor  14 ,  16  is coupled to the drain terminal of the other NMOS transistor  14 ,  16  of the latch. 
   In the latch biasing circuitry  60 , a bit line  46  associated with a memory cell (not shown) has a capacitor  34 . The source terminal of PMOS transistor  38  is coupled to V DD    42  and its drain terminal coupled to the bit line  46  at node BL  30 . The gate of the PMOS transistor  38  is coupled to a ground potential. The source terminal of PMOS transistor  24  is coupled to the drain terminal of PMOS transistor  38 . The gate of PMOS transistor  24  is coupled to the gate of PMOS transistor  26  (discussed below). A resistor  52  is coupled to the drain of PMOS transistor  24 . Resistor  52  is coupled to switch  56  which is coupled to resistor  54 . 
   A dummy bit line  48  has a capacitor  36 . The source terminal of PMOS transistor  40  is coupled to V DD    42  and the drain terminal of the PMOS transistor  40  is coupled to the dummy bit line  48  at node CL  32 . The source terminal of PMOS transistor  26  is coupled to the drain terminal of PMOS transistor  40 . The drain terminal of PMOS transistor  26  is coupled to resistor  54  and the gate of PMOS transistor  26  is coupled to the gate of PMOS transistor  24 . 
   The latch  12  is coupled to the latch biasing circuitry  60 . The drain terminal of PMOS transistor  24  is coupled to the drain terminal of NMOS transistor  14  while the drain terminal of PMOS transistor  26  is coupled to the drain terminal of NMOS terminal  16 . The voltage terminal at the drain of NMOS transistor  14  is V 2   22 . The voltage at the drain terminal of NMOS transistor  16  is V 1   58 . 
   With regard to  FIG. 2 , the sense circuit  10  includes precharging circuitry. In one embodiment, PMOS transistors  62 ,  64  have source terminals coupled to supply voltage V DD  (in one embodiment, supply voltage V DD  is 1.2 V; other voltages may be used in other embodiments). The gates of these transistors  62 ,  64  are coupled to a precharge signal line. The drains of transistors  62 ,  64  are coupled to the bit line  46  and dummy bit line  48 , respectively. In this embodiment, the bit line decoder (not shown) couples the precharge circuitry to the bit line. Any bit line decoder known to those of skill in the art may be employed with the sense circuitry. 
   In  FIG. 3 , one embodiment of the circuitry  104  to generate bias voltage for the sense circuit is shown. The source terminal of a PMOS transistor  82  is coupled to V DD    42  and the drain terminal of the PMOS transistor  82  is coupled to the gate of the PMOS transistor  82  and the source terminal of another PMOS transistor  84 . The drain terminal of the PMOS transistor  84  is coupled to the gate of the PMOS transistor  84  and the drain terminal of NMOS transistor  86 , whose source terminal is coupled to a ground potential. The gate of NMOS transistor  86  is coupled to an inverter  80 . The inverter  80  input is the rdn signal, which is low during a read operation. During a read operation, the circuitry  104  produces an output signal, vbias, or bias voltage, which is supplied to the gates of PMOS transistors  24  and  26  of the sensing circuitry in  FIGS. 1 and 2 . 
   Returning to  FIG. 2 , the signals which control latches  74 ,  76 , and  78  are shown. Latch  78  is controlled by the signals latch and latchn. The latchn signal is an inversion of the latch signal. Latches  74  and  76  are controlled by the latchd and latchdn signals. The latchdn signal is an inversion of the latchd signal. The timing of these signals will be discussed in greater detail below in  FIG. 5 . 
   In one embodiment, shown in  FIG. 4 , the read operation of a memory cell begins with initializing the dummy bit and word lines (by discharging the lines), precharging a dummy word line and a dummy bit line, and precharging the word line and bit line associated with the memory cell to be read (block  90 ). The end of the precharge operation is detected by detection circuitry (an exemplary embodiment of which is discussed below in  FIG. 7 ) (block  92 ). At the end of the precharge operation, the memory cell is correctly biased for read. The memory cell drives a current according to its programmed state (i.e., the current will vary depend on whether the memory cell is programmed with a “1” or “0”) (block  94 ). The memory cell current creates a voltage variation on the bit line. This is a current to voltage conversion on the bit line. This voltage variation is amplified by the latch biasing circuitry and the inputs of the latch are biased with a differential voltage (discussed below) (block  96 ). The latch is then activated (block  98 ). The latch then switches according to the memory current (block  100 ). An output signal indicating the result of the read operation is then produced (block  102 ). 
   A timing diagram of one embodiment is shown in  FIG. 5 . The read operation begins with an address transition detection (“atd”) signal. (For purposes of simplicity, standard address transition detection circuitry is not shown in  FIGS. 2 and 3  but is well-known to those of skill in the art.) The atd signal pulse is obtained using standard address transition detection circuitry. The atd signal goes low after an internally controlled delay (the signal stays high as long as the input address bits are toggling). The pulse delay on the atd signal is used to discharge the dummy bit line and dummy word line during the initialization phase. Once this initialization phase has occurred, the precharge operation starts. After the address is verified, the prech signal goes high to begin the precharge operation. During the precharge operation, the bit line is precharged (when the signal prechBl goes low). Once the dummy bit line reaches the desired precharge voltage (this is detected by circuitry connected at the end of the dummy bit and word lines, which, for purposes of simplicity, is not shown in either  FIGS. 1  or  2 , but would be well-known of skill to the art, the precharging of the bit lines and word lines is stopped by the signals StopprechBl and EndprechWl going high. The prech signal then goes low to end the precharge operation. Delay signals d 1  and d 2  are added by delay circuitry to provide delays between the end of the precharge and latch biasing operations and the beginning of the latch activation operations (i.e., the latch signal going high and then the latchd signal going high). (In one embodiment, d 1  is less than 5 nanoseconds while d 2  is less than 2 ns. However, these delays may vary in other embodiments.) Delay d 1  is the biasing latch time and delay d 2  is a security delay (communication latch time) before latching data out. After the precharge operation ends, the latch and latchd signals go high during the latch activation period. While the latchd signal is high, the latch data is valid. The access time (the time required for a read operation) is derived as follows:
 
Access time=Initialization delay+Precharge delay+Latch biasing delay+Latching delay+Dataout delay
 
   Returning to  FIG. 2 , switch  78  is on during the precharge and biasing operations. A current can flow through the resistors R 1   54  and R 2   52  and the switch  78 . Resistors R 1   54  and R 2   52  are of equivalent value (R 1 =R 2 =R). At the end of the precharge operation, the nodes BL  30  and CL  32  are set to their precharge voltage. The precharge voltage on node BL  30  is equal to V DD −R P ·Ibias 2 , where Ibias 2  is the current flowing through PMOS transistor  24  and R P  is the equivalent resistance of PMOS transistor  38  biased in linear mode. On node CL  32 , the precharge voltage is equal to V DD −R P ·Ibias 1 , where Ibias 1  is the current flowing through PMOS transistor  26  and R P  is the equivalent resistance of PMOS transistor  40  biased in linear mode. In one embodiment, the precharge voltage is V DD −100 mV. Other precharge voltages may be used in other embodiments. Both of the voltages on nodes BL  30  and CL  32  may be made very close to V DD  through selection of structure size. 
   Since Ibias 1  is not equal to Ibias 2 , there is a current imbalance in the circuit. A current I Rinit  flows through R 1   54 , R 2   52 , and the switch  78 . Current I Rinit  fixes the DC biasing conditions of the latch following the precharge operation. In one embodiment, an initial voltage V Rinit =V 2 −V 1 =(R 1 +R 2 +R switch )I Rinit =(2R+R switch ) (again, this assumes that R 1 =R 2 =R). The current imbalance is obtained by selecting the size of certain elements of the circuitry. For example, the drive of PMOS transistor  24  can be tuned to be larger than the drive of PMOS transistor  26  by appropriately selecting the size of the transistors  24  and  26  (or transistors  38 ,  40 ). In one embodiment, given the voltages at nodes V 2   22  and V 1   58 , a positive differential DC voltage is obtained at the inputs of the latch. 
   As noted above, the memory cell current can change the voltage on the bit line  46 . The voltage variation at node BL  30  due to the memory cell current, Icell, can be explained as: 
             Δ   ⁢           ⁢     V   BL       =       -     (     Rp       Rp   ·     gm     p   ⁢           ⁢   24         +   1       )       ·   Icell           
where Rp is the equivalent resistance of transistor  38  biased in linear mode and gm p24  is the transconductance of transistor  24  biased in saturation mode. Since there is no memory cell on the dummy bit line  48 , node CL  32  is stable at its precharge value.
 
   The voltage variation on the bit line  46  generates an amplified variation at the inputs of the latch thanks to the biasing circuitry. The variation of differential voltage V R  due to the cell current can be expressed (by neglecting g ds ) as: 
             Δ   ⁢           ⁢     V   R       =         -       gm     p   ⁢           ⁢   24           (       gm     p   ⁢           ⁢   24       +     G   p       )     ·     (         G   ·     gm     N   ⁢           ⁢   14           gm     N   ⁢           ⁢   16         -     gm     N   ⁢           ⁢   14       +   G     )           ·   Icell     =     f   ·   Icell             
where Gp=1/Rp, gmN 16  and gmN 14  are the transconductances of transistors  16  and  14 , respectively, and
 
             G   =       1       2   ⁢   R     +     R   switch         =     1     2   ⁢   R           ,         
where R=R 1 =R 2  and R switch  is the equivalent resistance of the switch  78  (which can be made negligible compared to R). Based on the above equations, the following is obtained:
 
           G   ≥         gm     N   ⁢           ⁢   16       ·     gm     N   ⁢           ⁢   14             gm     N   ⁢           ⁢   16       +     gm     N   ⁢           ⁢   14                 
This expression is used to correctly size the resistance R.
 
   At the end of the precharge operation (after memory cell current is flowing), the inputs of the latch are biased to a differential voltage value. The value of this differential voltage V R  is:
 
 V   R   =V   Rinit   −ΔV   R   =V   Rinit   −f·Icell 
 
   When the inputs of the latch are correctly biased to the DC V R  value, the latch circuitry can be activated. To activate the latch, the switch  78  must be OFF. Once activated, the latch switches according to the initial DC input conditions given by V R . The latch switching operation is very fast due to positive feedback. If the NMOS transistors  14 ,  16  in the latch are perfectly identical (i.e., there is no mismatch is the latch circuitry), the theoretical condition to get a correct latch switching operation is |V R |≧0, where |V R | is the absolute value of the differential voltage V R . However, given a mismatch between transistors  14  and  16 , the practical condition for a latch switching operation is |V R |≧3·σ VTN , where σ VTN  is the standard deviation of the threshold voltage (“VTN”) of NMOS transistors  14  and  16 . This condition ensures the latch will switch correctly in the direction imposed by the biasing of the latches with V R  at the end of the precharge operation. If V R  is negative, i.e., V 2 &lt;V 1 , then V 2  will go low while V 1  will go high. If V R  is positive, i.e., V 2 &gt;V 1 , then V 2  will go high while V 1  goes low. 
   In order to be correctly sensed by the sense circuitry, the memory cell current should meet certain conditions. Given the practical condition for a latch switching operation (as discussed above) |V R |≧3·σ VTN , the following is obtained: 3·σ VTN &lt;V Rinit −f·Icell&lt;−3·σ VTN , resulting in the following conditions for the memory cell current: 
                 Condition   ⁢           ⁢   1   ⁢     :               Icell   &gt;         3   ⁢           ⁢     σ   VTN       +     V   Rinit       f       =     I     L   ⁢           ⁢   1                   Condition   ⁢           ⁢   2   ⁢     :               Icell   &lt;         3   ⁢           ⁢     σ   VTN       -     V   Rinit       f       =     I     L   ⁢           ⁢   2                   
If condition 1 is fulfilled, V 2  will go low when the latch is activated. If condition 2 is fulfilled, the latch will switch in the opposite direction and V 2  will go high. As shown in  FIG. 6 , in some embodiments, if the memory cell current is between I L1  and I L2 , the latch output is unknown due to mismatching devices.
 
   With regard to  FIG. 2 , identical structures are on the output nodes V 1   58  and V 2   22  to match or closely match the capacitances on these nodes  58 ,  22 . The voltage on node V 2   22  is transferred to the dout node  88  once the sensing operation has been performed (i.e., the latch switching operation has occurred). Switch  74  is activated by the latchd and latchdn signals. The signal passing through switch  74  is inverted  66  and before the output signal is transferred to the dout node  88 . As has been discussed above, the output switches  74 ,  76  are off during the sensing operation. Data transfer to the output node  88  occurs when the latchd and latchdn signals activate the output switches  74 ,  76  (for example, latchd is set to “1” while latchdn is set to “0”). 
   Sequencing circuitry is shown in  FIG. 7 . The atd signal controls the bit and word line discharge circuitry used for initialization (block  106 ). The precharge circuitry then precharges the dummy bit line and dummy word line as well as the bit line and word line (block  108 ). Once the dummy bit line and dummy word line are precharged, the signals EndprechWl and StopprechBl are set to high and the reset signal is set to low. The output of the register goes low and NAND cell  124  sets signal prechBl high to stop precharge of the bitline (block  110 ). The precharge operation is turned off when the EndprechWl signal goes high (block  112 ). A NAND cell  122  is used to activate the circuitry to generate delays. A first delay signal d 1  is asserted on a delay line (block  118 ) between the end of the precharging operation and the latch signal going high (block  114 ), at which time data is read. A second delay signal d 2  is asserted on another delay line (block  12 ) before the latch d signal goes high (block  116 ), at which time data is valid. In one embodiment, the register is a D flip flop with clear (the register is clear when the reset signal is low). The delay circuits are inverters with capacitors. 
   The sense circuit described above is able to operate at very low supply voltages (for instance, 1.2 V, though other voltages (greater and smaller than 1.2 V) may be used). The circuit also provides for perfect control of the latch DC biasing conditions before latch activation. The circuit also consumes little power (for example, 15 μA per sense in 0.13 μm technology has been achieved). The circuitry is self-synchronized and there is no need for an external clock. 
   The sense circuit described above may have different configurations in other embodiments. For instance, dummy bit lines and dummy word lines are not required. Instead of a dummy bit line, a capacitor nearly equal to the bit line capacitor can be used.