Abstract:
Some embodiments include a device having storage node and a latch circuit coupled to the storage node to latch data provided to the storage node during one of a first mode and a second mode of the device. The latch circuit includes a first transistor, a second transistor, and a third transistor coupled between a first voltage node and a second voltage node. The third transistor is configured to selectively turn on and off in the first and second modes. Other embodiments are described.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. application Ser. No. 11/470,536, filed on Sep. 6, 2006, which is incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to high-voltage data latches that are used for writing data into non-volatile memories and, more particularly, to apparatus and a method for limiting leakage current drawn from a high voltage supply or generator, such as an on-chip charge pump circuit that provide a high voltage to the high-voltage data latches during a high-voltage write mode of operation. 
       BACKGROUND ART 
       [0003]    A latch is used to store data bits to be written into pre-selected memory cells of a non-volatile memory. Except when data bits are being written into the memory cells, the latch is normally supplied with a low-voltage power supply, such as, for example, 3 volts or less. During a write mode of operation, the latch is supplied with a high voltage of 7-15 volts, as required for writing data into the non-volatile memory cells. A single non-volatile memory chip may contain a large number, for example, 512 or more, of high-voltage latch circuits. These latch circuits are typically called high-voltage latch circuits, although a high voltage supply is only required for write operations. An on-chip high-voltage supply or generator, such as a charge pump circuit, provides the high-voltage for writing the data bits into the non-volatile memory cells. The high-voltage generator typically has limited current capability and excessive leakage currents in some of the high-voltage latches may load down the generator so much as to cause the high-voltage level to be less than what is required for proper writing of data bits into the memory cells of the non-volatile memory. 
         [0004]      FIG. 1  illustrates a typical cross-coupled high-voltage latch circuit  10  that includes a first CMOS inverter circuit  12  and a second CMOS inverter circuit  14 . The first CMOS inverter circuit  12  includes a first pull-up PMOS transistor  16  that has a source connected to a HV node  18  and a drain connected to a latch input node A. The first CMOS inverter circuit  12  also includes a first pull-down NMOS transistor  20  that has a drain connected to the latch input node A and a source connected to ground. The gates of the first pull-up PMOS transistor  16  and the pull-down NMOS transistor  20  are connected together. Note that the HV node  18  is supplied with low voltage except when a write mode of operation occurs. 
         [0005]    The second CMOS inverter circuit  14  includes a second pull-up PMOS transistor  22  that has a source connected to the HV node  18  and a drain connected to a data storage output node B. The second CMOS inverter circuit  14  also includes a second pull-down NMOS transistor  24  that has a drain connected to the data storage output terminal B and a source connected to ground. The gates of the second pull-up PMOS transistor  22  and the second pull-down NMOS transistor  24  are connected together. 
         [0006]    To enable operation of the high-voltage latch circuit  10  with a normal low Vdd voltage being supplied at the HV node  18 , the second pull-down NMOS transistor  24  is a low-threshold voltage Vt, high-voltage NMOS transistor, which tends to have a high leakage current at high write voltages because of its susceptibility to punch through at high voltages. Thus, a leakage path is provided from the HV node  18  to ground through a leaky second pull-down NMOS transistor  24  with a low threshold voltage, Vt. 
         [0007]    A reset NMOS transistor  32  is connected between the latch input node A and ground. A HIGH RESET signal is applied to a RESET terminal  34  to turn on the reset NMOS transistor  32  and pull the latch input node A to ground. 
         [0008]    The latch input node A is connected through a load input NMOS transistor  26  to a DATA In terminal  28 . A LOAD signal is provided at a gate terminal  30  of the load input NMOS transistor  26  to load a data bit at the DATA IN terminal  28  into the latch input node A. 
         [0009]    An OUTPUT terminal  36  provides the signal from the latch input node A that is provided to write to the memory. 
         [0010]    When the non-volatile chip is not being used in a high-voltage write mode of operation, a Vdd logic-circuit power supply voltage of 3 volts, for example, is provided to the HV node  18  to power the two inverters  12 ,  14  forming the high-voltage latch  10 . When the non-volatile chip is actually being used in a high-voltage write mode of operation, a suitable high-voltage power supply of, for example, 7-15 volts is provided to the HV node  18  to power the two inverters  12 ,  14  forming the high-voltage latch. The high-voltage is supplied from a high-voltage generation circuit, such as, for example, a charge-pump circuit that is provided on the chip. 
         [0011]    In order to provide for proper switching operation of the latch with a low Vdd logic-circuit supply voltage, such as, for example, 3 volts or less, the NMOS transistor  24  is a high-voltage, low Vt threshold device. A low Vt threshold device is required because it is difficult to load a HIGH or “1” level to the latch because of the Vt voltage drop across the load input NMOS transistor  26  that makes it difficult to load a HIGH or “1” level into the latch input node A. 
         [0012]    When the chip is in a high-voltage write mode of operation with the HV terminal  18  at 7-15 volts and when the data storage output node B is at a HIGH, “1”, logic level, the high-voltage pull-up PMOS transistor  22  is turned on and the high-voltage pull-down, low-threshold voltage NMOS transistor  24  is turned off. This essentially places almost all of the 7-15 volts from the HV terminal  18  across the low-threshold NMOS transistor  24 . If the high-voltage pull-down NMOS transistor  24  is leaky because of the presence of a punch through path in it, a leakage path goes from the high voltage at the data storage output node B to ground through the leaky pull-down, low-threshold NMOS transistor  24 . 
         [0013]    A non-volatile memory chip has 512 or more high-voltage latches like the typical high-voltage latch circuit  10 , some or all of which may be leaky with a high voltage at their HV voltage supply terminals. Excessive leakage currents taken from the on-chip high voltage generation circuit, such as, for example, an on-chip charge pump, that supplies a nominal 15 volts, may cause the voltage at the HV terminal  18  to be pulled down to, say, 12 volts. The reduced high voltage at the HV terminal  18  may cause malfunctions in a memory write function. 
         [0014]      FIG. 2  is a timing diagram that illustrates operation of the typical high-voltage latch circuit  10  of  FIG. 1 , when the DATA IN signal at the DATA IN terminal  28  is LOW, or at 0 volts. A LOAD signal is initially at a LOW level at the gate terminal  30  of the NMOS load input transistor  26  to keep the NMOS load input NMOS transistor  26  off. Initially, the RESET signal at terminal  34  is HIGH, which turns on the reset NMOS transistor  32  to pull the latch input node A to ground. The HV_ENABLE signal is initially LOW, which provides a Vdd voltage at terminal  18 . When the LOAD signal is raised HIGH to Vdd, the NMOS load input NMOS transistor  26  is turned on to provide a LOW logic level DATA IN signal to the latch input node A and the voltage on the data storage output node B goes HIGH to Vdd. Subsequently, the HV_ENABLE control signal goes high to apply a high voltage HV from a high voltage generation circuit to the HV node  18 . The second pull-up PMOS transistor  22  is turned on so that the voltage at the data storage output node B is at essentially the same high voltage as at the HV node  18 . The HV voltage at the BV node  18  is initially at a Vdd level. However, after the HV_ENABLE control voltage goes HIGH to connect the high voltage generation circuit to the HV node  18 , the HV voltage at node  18  rises to a HV(Actual) level that is less than the full HV(Target) level because of the extra leakage current that the high voltage generation circuit must provide to the leaky pull-down NMOS transistor  24  for a number of such high-voltage latch circuits. The full HV(Target) level is, for example, 15 volts while the HV(Actual) level is, for example, 12 volts due to leakage in various high voltage latch circuits. The voltage at the latch input node A and the OUTPUT terminal  36  remains at a LOW state. The voltage at the data storage node B tracks the HV voltage and only rises to the HV(Actual) level. 
         [0015]      FIG. 3  is a timing diagram that illustrates operation of the typical high-voltage latch circuit  10  of  FIG. 1 , when the DATA IN signal at the DATA IN terminal  28  is HIGH. The LOAD signal is initially at a LOW level at the gate terminal  30  of the NMOS load input transistor  26  to keep the NMOS load input NMOS transistor  26  off. Initially, the RESET signal at terminal  34  is HIGH, which turns on the reset NMOS transistor  32  to pull the latch input node A to ground. The HV_ENABLE signal is initially LOW, which provides a Vdd voltage at terminal  18 . When the LOAD signal is raised to Vdd, the load input NMOS transistor  26  is turned on to provide a HIGH logic level DATA IN signal to the latch input node A and the voltage on the data storage output node B goes low to 0 volts when the pull-up PMOS transistor  22  is turned off and the pull-down NMOS transistor  24  is turned on. Subsequently, the HV_ENABLE control signal goes high to apply the high voltage HV from a high voltage generation circuit to the HV node  18 . The first pull-up PMOS transistor  16  is turned on so that the voltage at the data storage output node B is LOW. The HV voltage at the HV node  18  is initially at the Vdd voltage level. After the HV_ENABLE control voltage goes high to connect the high voltage generation circuit to the HV node  18 , the HV voltage at node  18  rises to the full HV(Target) level because there is no leakage current through the pull-down NMOS transistor  24 . The signal at latch input node A and the OUTPUT terminal  36  tracks the HV level at the HV terminal  18 . 
         [0016]    Various possible remedies for reducing the effect of leakage through the pull-down NMOS transistor  24 , when the voltage at the data storage output terminal B is at a high-voltage level, have some disadvantages. Changing the process parameters for fabrication of the pull-down NMOS transistor  24  may reduce leakage; but this can cause its threshold voltage Vt to increase and adversely affect low-voltage operation. 
         [0017]    To decrease leakage current, the resistance of the pull-down NMOS transistor  24  can be increased by increasing the gate length L of the pull-down NMOS transistor; but this takes more area on the chip and increases the size of the chip. The current output, or strength, of the HV generation circuit can be increased; but this may require a larger pump circuit, which takes more area on the chip and increases the size of the chip. Increasing the strength of the HV generation circuit may also require a higher clock frequency to provide a greater write current. 
       SUMMARY OF THE INVENTION 
       [0018]    The present invention provides a non-volatile memory having a plurality of high-voltage CMOS latches. Each high-voltage CMOS latch includes a HV terminal that is connected to a Vdd supply voltage during a standby mode of operation and during a load data mode of operation and that receives a HIGH-VOLTAGE supply voltage during a high-voltage write mode of operation. A first CMOS inverter and a second CMOS inverter, each having respective input and output terminals, are each connected between the HV terminal and a ground terminal. The input terminal of the second CMOS inverter and the output terminal of the first CMOS inverter are connected to a latch input node A. The input terminal of the first CMOS inverter and the output terminal of the second CMOS output terminal are connected to a latch output node B. 
         [0019]    The first CMOS inverter has a first PMOS pull-up transistor that is connected between the HV terminal and the latch input node A. The first CMOS inverter also has a first NMOS pull-down transistor connected between the latch input node A and the ground terminal. The second CMOS inverter has a second PMOS pull-up transistor connected between the HV terminal and the latch output node B. The second CMOS inverter has a pass-gate high-voltage NMOS transistor with a VT implant connected between the latch output node A and a second high-voltage, low-threshold NMOS pull-down transistor that is connected to the ground terminal. The pass-gate high-voltage NMOS transistor has a gate connected to a STANDBY terminal that receives a HIGH LOGIC signal with a value of at most Vdd to turn on the pass-gate high-voltage NMOS transistor when the high-voltage CMOS latch is in a data-loading mode of operation and during a high-voltage write mode of operation. The pass-gate high-voltage NMOS transistor limits the voltage across the second high-voltage, low-threshold NMOS pull-down transistor and reduces punch-through current and drain-to-substrate leakage of the second high-voltage, low-threshold NMOS pull-down transistor. 
         [0020]    Each high-voltage CMOS latch circuit has a DATA IN input terminal connected to the latch input node A through a NMOS load input NMOS transistor at a gate terminal of which is provided a DATA LOAD signal to turn on the NMOS load input NMOS transistor. 
         [0021]    Each high-voltage CMOS latch circuit has a reset NMOS transistor that is connected between the latch input node A and the ground terminal and that has a gate terminal at which is provided a HIGH RESET signal to turn on the reset NMOS transistor during the standby mode of operation and at which is provided a LOW RESET signal to turn off the reset NMOS transistor during a data-loading mode of operation and during a high-voltage write mode of operation. 
         [0022]    The present invention also provides a method of limiting leakage current in one or more high-voltage latches that are used for high-voltage writing of data into a non-volatile memory. The method includes the steps for each of the one or more latches of: connecting a cross-coupled CMOS latch between a HV terminal and a ground terminal by connecting a first CMOS inverter between a HV terminal and a ground terminal and by connecting a second CMOS inverter between the HV terminal and a ground terminal; connecting an input terminal of the second CMOS inverter and output terminal of the first CMOS to a latch input node A for the latch circuit; connecting the latch input node A through a NMOS load input NMOS transistor to a DATA In input terminal of the latch; providing a LOAD signal at a gate terminal of the NMOS load input NMOS transistor to turn on the NMOS load input NMOS transistor; connecting an input terminal of the first CMOS inverter and an output terminal of the second CMOS output terminal to a latch output node B for the latch circuit; connecting a high-voltage, pass-gate NMOS transistor between the latch output node A and one terminal is a low-threshold NMOS pull-down transistor that has another terminal connected to ground; and limiting the voltage across the second high-voltage, low-threshold NMOS pull-down transistor and the reducing punch through current and drain-to substrate leakage by turning on the pass-gate high-voltage NMOS transistor with a HIGH signal with a value of Vdd at most. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention: 
           [0024]      FIG. 1  is a circuit diagram of a prior art high voltage latch circuit having a high-voltage leakage path from an output terminal to ground. 
           [0025]      FIG. 2  is a timing diagram for the circuit of  FIG. 1  that is loaded with a LOW DATA IN signal. 
           [0026]      FIG. 3  is a timing diagram for the circuit of  FIG. 1  that is loaded with a HIGH DATA IN signal. 
           [0027]      FIG. 4  is a circuit diagram of a high voltage latch circuit according to the present invention. 
           [0028]      FIG. 5  is a timing diagram for the circuit of  FIG. 4  that is loaded with a LOW DATA IN signal. 
           [0029]      FIG. 6  is a timing diagram for the circuit of  FIG. 4  that is loaded with a HIGH DATA IN signal. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]      FIG. 4  illustrates an improved high-voltage latch circuit  100  according to the present invention. The latch circuit includes a first CMOS inverter circuit  102  and a second CMOS inverter circuit  104 . The first CMOS inverter circuit  102  includes a first pull-up PMOS transistor  106  that has a source connected to a HV node  108  and a drain connected to a latch input node A. The first CMOS inverter circuit  102  also includes a first pull-down NMOS transistor  110  that has a drain connected to the latch input node A and a source connected to ground. The gates of the first pull-up PMOS transistor  106  and the first pull-down NMOS transistor  110  are connected together. 
         [0031]    The second CMOS inverter circuit  104  includes a second pull-up PMOS transistor  112  that has a source connected to the HV node  108  and a drain connected to a data storage output node B. The second CMOS inverter circuit  104  includes a pass-gate high-voltage NMOS transistor  116  with a VT implant that is connected between the latch output node B and a drain of a second high-voltage, low-threshold NMOS pull-down transistor  118  that has a source terminal connected to the ground terminal. The pass-gate high-voltage NMOS transistor  116  has a gate connected to a STANDBY terminal  120  that receives a HIGH LOGIC signal with a value of at most Vdd to turn on the pass-gate high-voltage NMOS transistor when the high-voltage CMOS latch is in a data-loading mode of operation and during a high-voltage write mode of operation. 
         [0032]    The STANDBY signal is Low when the memory system is in a standby mode of operation. The STANDBY signal is HIGH when the latch is loading data and during a high-voltage write operation. The STANDBY signal is not a high-voltage signal and is limited to Vdd in its HIGH state. By allowing the STANDBY signal to only reach Vdd, the pass-gate high-voltage NMOS transistor  116  will shut off once the Vg (STANDBY voltage)−Vs (source voltage of transistor  116 )−Vt (threshold voltage of transistor  116 )=0. This limits the voltage across the second high-voltage, low-threshold NMOS pull-down transistor  118  to be Vdd−Vt. This reduces punch through current and drain-to-substrate leakage of the second high-voltage, low-threshold NMOS pull-down transistor  118 . 
         [0033]    A reset NMOS transistor  122  is connected between the latch input node A and the ground terminal. A gate terminal of the reset NMOS transistor  122  in connected to a RESET terminal  124 , at which is provided a HIGH RESET signal to turn on the reset NMOS transistor  122  during a standby mode of operation. A LOW RESET signal turns off the reset NMOS transistor  122  during a data-loading mode of operation and during a high-voltage write mode of operation. 
         [0034]    A load input NMOS transistor  126  is connected between the latch input node A and a DATA IN input terminal  128 . A DATA LOAD signal is provided at a DATA LOAD terminal that is connected to a gate terminal of the load input NMOS transistor  126 . A HIGH DATA LOAD signal turns on the load input NMOS transistor  126  to connect the DATA IN input terminal to the latch input node A. An output terminal  136  provides the signal from the latch input node A that is provided to write to the memory. 
         [0035]      FIG. 5  is a timing diagram that illustrates operation of the improved high-voltage latch circuit  100  for a LOW DATA IN signal at the DATA IN input terminal  128 . The high-voltage latch circuit  100  operates in three modes: a standby mode, a data-loading mode, and a high-voltage write mode. The standby mode of operation occurs when the latch circuit  100  is powered by a LOW Vdd voltage on the HV node  108 . The data-loading mode of operation occurs when the input data signal at the DATA IN terminal  128  is loaded into the latch circuit  100  that is still operated with the low Vdd voltage at terminal  108 . The high-voltage writer mode of operation occurs when the high voltage is applied to the HV node  108  to write data into the non-volatile memory cells. 
       Standby Mode 
       [0036]    During the standby mode of operation, the STANDBY signal is at a LOW level Vdd to cutoff the pass-gate high-voltage NMOS transistor  116 . The HV-ENABLE signal is at a 0 (LOW) level to provide a Vdd voltage at terminal  108 . Data at the latch input node A and the data storage output node B is either HIGH or LOW. Prior to the standby mode of operation ending, the STANDBY signal goes to a HIGH signal level to turn on the pass-gate high-voltage NMOS transistor  116 . The RESET signal at terminal  124  goes LOW to cut the reset NMOS transistor  122 . 
       Data Load Mode 
       [0037]    The data-loading mode of operation begins when the LOAD signal at a Vdd level is provided at the gate terminal  130  of the load input NMOS transistor  126 . In this mode of operation, a LOW input signal at the DATA IN terminal  128  is loaded into the latch input node A. This turns on the second pull-up PMOS transistor  112  and turns off the second NMOS pull-down transistor  118  and causes the data storage output node B to go to a HIGH Vdd level. 
       High-Voltage Write Mode 
       [0038]    During the high-voltage write mode of operation, the HIGH STANDBY voltage at input terminal  120  continues to turn on the pass-gate high-voltage NMOS transistor  116 . Note that the HIGH state of the STANDBY voltage is at most Vdd. The pass-gate high-voltage NMOS transistor  116  receives the STANDBY signal that has a value of at most Vdd to turn on the pass-gate high-voltage NMOS transistor when the high-voltage CMOS latch is in a data-loading mode of operation and during a high-voltage write mode of operation. The Vdd STANDBY voltage at the gate terminal of the pass-gate high-voltage NMOS transistor  116  limits the voltage across the second high-voltage, low-threshold NMOS pull-down transistor to Vdd−Vt and reduces punch-through current and drain-to-substrate leakage. 
         [0039]    During the high-voltage write mode of operation, a high voltage supply is applied to the HV node  108  and the voltage at the data storage output node B follows the voltage on the HV node  108 .  FIG. 5  shows these two voltages HV and B ramping up to the HV voltage target level, for example, 15 volts. The output voltage at terminal  136  stays at a LOW state. 
         [0040]      FIG. 6  is a timing diagram that illustrates operation of the improved high-voltage latch circuit  100  for a HIGH DATA IN signal at the DATA IN input terminal  128 . 
       Standby Mode 
       [0041]    During the standby mode of operation, the STANDBY signal is at a LOW level Vdd to cutoff the pass-gate high-voltage NMOS transistor  116 . The HV-ENABLE signal is at a 0 (Low) level to provide a Vdd voltage at terminal  108 . Data at the latch input node A and the data storage output node B is either HIGH or LOW. Prior to the standby mode of operation ending, the STANDBY signal goes to a HIGH signal level to turn on the pass-gate high-voltage NMOS transistor  116 . The RESET signal at terminal  124  goes LOW to cut the reset NMOS transistor  122 . 
       Data Load Mode 
       [0042]    The data-loading mode of operation begins when the LOAD signal at a Vdd level is provided at the gate terminal  128  of the load input NMOS transistor  126 . In this mode of operation, a HIGH input signal at the DATA IN terminal  130  is loaded into the latch input node A. This turns off the second pull-up PMOS transistor  112  and which turns on the second NMOS pull-down transistor  118  and causes the data storage output node B to go to a LOW level. 
       High-Voltage Write Mode 
       [0043]    During the high-voltage write mode of operation, the HIGH STANDBY voltage at input terminal  120  continues to turn on the pass-gate high-voltage NMOS transistor  116 . Note that the HIGH state of the STANDBY voltage is at most Vdd. The pass-gate high-voltage NMOS transistor  116  receives the STANDBY signal that has a value of at most Vdd to turn on the pass-gate high-voltage NMOS transistor when the high-voltage CMOS latch is in a data-loading mode of operation and during a high-voltage write mode of operation. The Vdd STANDBY voltage at the gate terminal of the pass-gate high-voltage NMOS transistor  116  limits the voltage across the second high-voltage, low-threshold NMOS pull-down transistor to Vdd−Vt and reduces punch-through current and drain-to-substrate leakage. 
         [0044]    During the high-voltage write mode of operation, a high voltage supply is applied to the HV node  108  and the voltage at the data storage output node A follows the voltage on the HV node  108 .  FIG. 6  shows these two voltages HV and A ramping up to the HV voltage target level, for example, 15 volts. Terminal  136  provides the node A voltage as an output signal to write the memory. 
         [0045]    The foregoing descriptions of specific embodiments of the present invention has been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the Claims appended hereto and their equivalent.