Patent Document

BACKGROUND  
         [0001]    1. Technical Field  
           [0002]    This disclosure relates to electronic circuits and more particularly, to an adjustment circuit for a MOSFET switch.  
           [0003]    2. Description of the Related Art  
           [0004]    Generally, a semiconductor memory device comprises memory blocks for storing a plurality of binary information and memory peripheral circuits for driving the memory blocks. The memory device further comprises at least one voltage down converter for converting a supply voltage from an external power supply circuit to a desired level and supplying the converting voltage to an internal circuit which includes the memory blocks and the memory peripheral circuits. This down converting is necessary to ensure the reliability of the transistors by providing a lower power of operation.  
           [0005]    Since components in the semiconductor device are micronized (generally about 0.5 microns or less), the reliability of the transistors would be greatly reduced if the full supply voltage were used to drive them. Reliability problems would arise in the form of break-down of insulating films of metal oxide semiconductor field effect transistors (MOSFET&#39;s).  
           [0006]    A simple and area efficient way to create current pulses in voltage down converters is to use a MOSFET switch that is controlled by a timer circuit. Referring to FIG. 1, a prior art scheme is shown for a voltage down converter. MOSFET switch  2  has its source and drain connected between supply voltage V sup  and bit line high voltage V blh . MOSFET switch  2  is controlled by a timer circuit  4 . Generally, MOSFET switch  2  is synchronized with sense amplifiers  6  through timer circuit  4 . Timer circuit  4  turns on MOSFET switch  2  for a preset amount of time. This allows a fixed amount of charge to flow from node V sup  to node V blh . The sense amplifiers  6  sense a voltage differential between two complementary bit lines  8  of the memory block. When a difference is sensed by a sense amplifier, one of the two bit lines is brought high and the other is brought low. Low is generally ground potential where as high is Vblh. Ideally the amount of charge that flows through MOSFET switch  2  is identical to the amount of charge consumed by sense amplifiers  6  during the activation period.  
           [0007]    The disadvantage of this scheme is the strong dependency of the transistor current on the parameters of the circuit, especially the threshold voltage of the MOSFET switch  2 . A variation of V T  has a major influence on the current that is produced by the MOSFET switch  2 .  
           [0008]    Threshold voltage of a transistor is a function of many parameters including manufacturing processes, doping levels for the sources and drains, etc. In order to reduce the effects of threshold voltage variations on MOSFET switches it is necessary to compensate for these parameters. Thus, a need exists for an improved circuit which can account for variations in the threshold voltage of a MOSFET switch.  
         SUMMARY OF THE INVENTION  
         [0009]    A voltage converter circuit for an electronic device includes a transistor switch for providing current pulses to a current input node. The transistor switch has a gate and a turn-on threshold voltage. An adjustment circuit provides a controlled voltage to the gate for turning on the transistor switch and the adjustment circuit includes means for compensating for variations in the turn-on threshold voltage of the transistor switch. A timer for enables the adjustment circuit for a preset period of time.  
           [0010]    In one embodiment, the transistor switch is a metal oxide semiconductor field effect transistor (MOSFET). In another embodiment, the adjustment circuit contains a transistor having a threshold voltage substantially equal to the threshold voltage of the transistor switch. The adjustment circuit may also include a voltage divider circuit. The voltage divider circuit has at least one resistor which can be dimensioned to adjust the gate voltage of the transistor switch. In another embodiment, the voltage divider circuit includes at least one transistor having a threshold voltage substantially the same as the transistor switch.  
           [0011]    An adjustment circuit for a semiconductor memory devices includes a first node, a second node for connecting to a gate of a MOSFET switch and a feedback circuit connected to the first and second nodes for monitoring voltage changes at the first node and adjusting voltage at a second node to compensate for variations in current flow through the MOSFET switch. In an alternate embodiment, the feedback circuit further includes a differential amplifier to supply a substantially constant voltage to the first node, a resistor connected between the first node and ground such that a constant current flows through a first transistor, the first transistor having a gate connected to the second node. The first transistor and the MOSFET switch can both be either PMOSFET&#39;s or NMOSFET&#39;s with a threshold voltage substantially equal to the threshold voltage of the MOSFET switch. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0012]    Various embodiments will be described in detail in the following description of preferred embodiments with reference to the following figures wherein:  
         [0013]    [0013]FIG. 1 shows a schematic diagram of a prior art MOSFET switch that is controlled by a timer circuit;  
         [0014]    [0014]FIG. 2 shows a schematic/block diagram of an adjustment circuit enabled by a timer circuit and connected to a MOSFET switch;  
         [0015]    [0015]FIG. 3 is a schematic diagram of an embodiment of an adjustment circuit connected to a MOSFET switch;  
         [0016]    [0016]FIG. 4 is a schematic diagram of an alternate embodiment of the embodiment shown in FIG. 3 showing a plurality of transistors in series added to an adjustment circuit; and  
         [0017]    [0017]FIG. 5 is a schematic diagram of another embodiment of an adjustment circuit showing a differential amplifier and a feedback loop used in conjunction with a MOSFET switch. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]    The present disclosure describes an adjustment circuit for minimizing effects due to the threshold voltage variations for a metal oxide semiconductor field effect transistor (MOSFET) switch. The MOSFET switch includes a timer for closing the switch for a preset period of time. This is useful for applications such as when sense amplifiers of a dynamic random access memory (DRAM) chip sense a differential voltage between a corresponding pair of bit lines. The switch connects between the supply voltage node and a bit line high voltage node. When the switch is activated current flows through the switch to sense amplifiers which are connected to complementary pairs of bit lines.  
         [0019]    Referring now in specific detail to the drawings in which like reference numerals identify similar or identical elements throughout the several views, and initially to FIG. 2, a block diagram of a circuit of the present invention is shown. A timer circuit  16  sends a signal from a timer output  26 . An adjustment circuit  14  is connected to timer output  26  at adjustment circuit input  28 . The signal from output  26  enables adjustment circuit  14 . Adjustment circuit  14  adjusts the voltage to an adjustment circuit output  24 . A MOSFET switch  12  has a gate  22  connected to adjustment circuit output  24 . When the adjusted voltage is applied to gate  22 , current flows through switch  12  into the V blh  node. V blh  supplies sense amplifiers  6  to drive one of the complementary pair of bit lines  8  high. Adjustment circuit  14  is designed to provide gate  22  with appropriate voltage to allow a predetermined amount of current flow through MOSFET switch  12  to provide enough current to sense amplifiers  6  during activation. Although shown as a PMOSFET, MOSFET switch may also be an NMOSFET.  
         [0020]    Referring to FIG. 3, an adjustment circuit  14  in accordance with one embodiment of the invention is schematically illustrated. Adjustment circuit  114  has an input  128  from a timer circuit  16 . Input  128  is connected to a gate  139  of a transistor  140 , for example a MOSFET. Transistor  140  acts as an activation switch-for the adjustment circuit  114 . When the appropriate signal is received from-timer circuit  16 , gate  128  is activated allowing current to flow through transistor  140 . Transistor  140  has its drain  141  connected to ground  143  and its source  145  connected to node  138 . A resistor R 2  is connected between node  138  and a node  124 . Node  124  is connected to a transistor  132 , preferable a PMOSFET. Transistor  132  has a source  144 , a gate  134  and a drain  136 . Both gate  134  and drain  136  are connected to node  124 . Resistor R 1  connects between source  144  of transistor  132  and a node  142 . Node  142  is at the supply voltage V sup  potential.  
         [0021]    Node  124  is connected to a gate  122  of a transistor switch  112 . Transistor switch  112  is preferably a MOSFET. Transistor switch has a source  111  and a drain  113 . Drain  113  is connected to V blh  node, and source is connected to node  118  which remains at the supply voltage V sup  potential. It is preferred to have switch  112  be of the same kind as transistor  134 , i.e. both of the transistors are PMOSFET&#39;s or both of the transistors are NMOSFET&#39;s. Preferably, transistor  134  and transistor switch  112  can share the same doped regions for their respective sources and drains. In this way the threshold voltage V T  across the transistor (the voltage difference between the source and drain of a transistor) is the same for both transistor  132  and transistor switch  112 .  
         [0022]    The resistors R 1  and R 2  and transistor  132  act as a voltage divider circuit. Timer circuit  16  sends an enable signal to input  128  which applies a voltage to gate  139 . This allows current to flow from node  138  to ground  143 . This current draw causes current flow through resistor R 1  creating a potential drop across the resistor R 1 . This voltage is applied to gate  134  of transistor  132  allowing current to flow through resistor R 2  as well. The voltage at node  124  can be calculated as follows:  
         V     node                 124       =           V   sup     -     V   T           R   1     +     R   2         ×     R   2                             
 
         [0023]    V node 124  is the applied voltage to gate  122 . The voltage that is applied to the gate is sufficient to allow current to flow from source  111  to drain  113 . It is important that the charging current that passes through switch  112  be sufficient to enable operation of the sense amplifiers during their activation period. As such, the voltage that is applied to the gate is sufficiently high to achieve this goal. Since transistor  132  and switch  112  share the same doped regions for their respective sources and drains, any threshold voltage V T  changes across the transistors are the same. If, for example, the threshold voltage V T  of transistor  134  and switch  112  is increased, than their current flow from source to drain decreases. Consequently, the voltage at node  124  changes according to the formula:  
           V     node                 124       +     delta                   V     node                 124                V   sup     -     (       V   T     +     delta                   V   T         )           R   1     +     R   2         ×     R   2                                          
 
         [0024]    where:  
         [0025]    delta V node 124  represents the change voltage differential from the voltage prior to the increased threshold voltage V T ; and  
         [0026]    delta V T  represents the increase in threshold voltage V T  across the transistors.  
         [0027]    The gate voltage of switch  112  is decreased and by this, the current flow from source  111  to drain  113  is increased. The combination of these two effects leads to reduced influence of the threshold voltage variations on the current flow through switch  112 . The value of the voltage V node 124  can be controlled by dimensioning the resistors R 1  and R 2 .  
         [0028]    Referring to FIG. 4, another embodiment of the adjustment circuit includes the addition of transistors to further reduce the influence of threshold voltage changes on switch  112 . A plurality of transistors designated as FET 1  through FETn are connected serially between resistor R 1  and node  224 . Each transistor is preferably the same type as a transistor switch  212 , i.e. all transistors are PMOSFET&#39;s, or all NMOSFET&#39;s. Each of the plurality of transistors FET 1  through FETn has its drain connected to its gate. It is preferred to have all the sources and drains of the transistors FET 1  through FETn share the same doped regions for their respective sources and drains so that any threshold voltage V T  changes across the transistors are the same for each transistor. The compensation effect on switch  212  is increased. This means that the voltage at node  224  is more reliably achieved making the current flow rate through switch  224  more efficient and repeatable. If n is used to denote the number of transistors used, the voltage at node  224  can be calculated according to the following formula:  
           V     node                 224       +     delta                   V     node                 224           =           V   sup     -     (       V   T     +     (     n   ×   delta                   V   T       )       )           R   1     +     R   2         ×     R   2                             
 
         [0029]    where:  
         [0030]    delta V node 224  represents the change voltage differential from the voltage prior to the increased threshold voltage V T ;  
         [0031]    n is the number of transistors introduced in series; and  
         [0032]    delta V T  represents the increase in threshold voltage V T  across the transistors.  
         [0033]    Referring to FIG. 5, another embodiment of an adjustment circuit  314  includes a two stage differential amplifier  310 . An input  346  to differential amplifier  310  is at reference voltage V REF . Differential amplifier  310  maintains voltage V REF ′ at a node  326 . A feedback circuit  354  includes a transistor  342  of differential amplifier  310 , a transistor  332 , a transistor  330  and node  352 , node  326  and node  324 . Since the voltage at node  326  is held constant by differential amplifier  310 , the current through resistor R 3  is constant. I 2 =V REF ′/R 3 .  
         [0034]    Transistor  332  is used to track changes in threshold voltage V T  and adjusts the voltage in node  324  accordingly. Transistor  332  has a source  334 , gate  336  and a drain  328 . Drain  328  is connected to node  326  and gate  336  is connected to node  324 . Transistor  330  connects node  352  to node  324  to complete feedback circuit  354 . Transistor  330  has its gate  338  connected to node  352  and its drain  340  connected to node  324 . If, for example, the threshold voltage of transistor  332  increases, the voltage at node  324  is lowered in order to maintain the constant voltage V REF ′ at node  326  (I 2 =V REF ′/R 3 ). Feedback loop  354  adjusts the voltage of gate  336  to maintain I 2  constant. This means that the voltage at node  324  is compensated for variations in the threshold voltage in transistor  332 , maintaining a predetermined voltage at node  324 . Resistor R 4  keeps the voltage at node  324  regulated by allowing current form transistor  330  to flow to ground.  
         [0035]    Transistors  332  and transistor switch  312  are of the same type, i.e. all PMOSFET&#39;s, or all NMOSFET&#39;s. So that tracking threshold voltage variations can be achieved in switch  312 . It is preferred that transistor  332  and transistor switch  312  are made to share the same doped region of the chip. It is desired to achieve similar current densities in transistor  332  and transistor switch  312 . The transition voltages would therefore be nearly the same for these transistors, hence threshold voltage variations can be more closely tracked.  
         [0036]    A transistor  328  receives an enable signal from timer circuit  16  begins the activation cycle in which current flows through transistor switch  312  from V sup . Node  324  is connected to a gate  309  of switch  312 . This allows current to flow from V sup  toward V blh . Since the voltage at node  324  is compensated, effects due variations in threshold voltages are minimized, thereby improving the performance and efficiency of switch  312 .  
         [0037]    While this invention has been described in terms of several illustrative embodiments, there are alterations, permutations, and equivalents which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the processes of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, and equivalents as fall within the true spirit and scope of the present invention.

Technology Category: g