Abstract:
An output buffer, in accordance with the present invention includes a first driver circuit for coupling a first voltage to an output when the first driver circuit is turned on, and a second driver circuit for coupling a second voltage to the output when the second driver circuit is turned on. An input connects to the first and second driver circuits for turning the first and second driver circuits on and off according to a first input signal. An adjustment circuit is coupled to the first and second driver circuits for adjusting the strength of the first and second driver circuits according to a data pattern, the data pattern including the first input signal and input signals of a plurality of output buffers. Also, included is a method for adjustment of driver strength of the output buffer.

Description:
BACKGROUND 
     1. Technical Field 
     This disclosure relates to semiconductor devices and more particularly, to an adjustable strength driver circuit for off chip drivers used in semiconductor memories and a method of adjustment. 
     2. Description of the Related Art 
     Semiconductor memories, such as dynamic random access memories (DRAMs), include an off chip driver (OCD) or an output buffer which provide a signal to be sent off the semiconductor chip during operation. A memory chip often includes an array of output buffers to permit simultaneous output of multiple databits. When several of the drivers of the array of buffers are in operation, the rise and/or fall time of the outgoing signals are slowed down. This is true for signals driven off the memory chip. The slow down is caused primarily by power and ground supply noise, and the resulting decrease in a gate to drain voltage (V gs ) and drain to source voltage (V ds ) of driver transistors of the buffer array. The load at the output is driven by less overdrive voltage V gs  and less voltage difference V ds  between source and drain of the driving transistor. 
     In DRAMs, there are typically between 4 to 32 output buffers, arranged in an array, and having a dedicated power supply. Package parasitics include inductances of a lead frame and a bond wire and capacitive loads including internal and external output loads as indicated in FIG.  1 . When, as a worst case, a majority of output buffers have to drive data of a same polarity (either “0” or “1”, single-sided), this causes a decrease of V gs  and V ds . over the driving transistor. This slows down the rising or falling edge of the output signal and results in a lower speed performance of the DRAM. Although the rising or falling edge delay may be compensated for by increasing the width of the driving transistor, this cannot be performed dynamically and may therefore violate maximum slew rates in best cases. 
     Referring to FIG. 1, a typical output buffer  10  is shown having parasitic load  12  and capacitive loads  13  and  15  associated with lead frame and bond wire. Output buffer  10  includes two driving transistors  14  and  16 . Transistor  14  is driven by an n-logic signal while transistor  16  is driven by a p-logic signal. Transistor  14  has a source coupled to a first supply voltage (i.e., VSSD) while transistor  16  has a source coupled to a second supply voltage (VDDQ). When n-logic drives transistor  14  (which is an NFET pull transistor) to drive node OUT low, node  2  will temporarily bounce up (dI/dt noise, i.e., &lt;U=L∃dI/dt where &lt;U; is voltage variation caused by parasitic inductances L and dI/dt is the derivative of the current with respect to time). This voltage change caused by the inductance is not negligible and may significantly contribute to the decrease in V gs  and V ds  for transistor  14 . When a majority of output buffers in an array drive the same data (1&#39;s or 0&#39;s), this effect is further enhanced and causes a slowing down of the signal&#39;s OUT falling edge because V gs  and V ds  decrease further. Output timings become data pattern dependent resulting in reduced timing margins especially when the DRAM chip is operated at high frequencies. 
     Therefore, a need exists for an output buffer which dynamically adjusts the drive strength according to a data pattern to be output from an array of output buffers in its vicinity. 
     SUMMARY OF THE INVENTION 
     An output buffer, in accordance with the present invention includes a first driver circuit for coupling a first voltage to an output when the first driver circuit is turned on, and a second driver circuit for coupling a second voltage to the output when the second driver circuit is turned on. An input connects to the first and second driver circuits for turning the first and second driver circuits on and off according to a first input signal. An adjustment circuit is coupled to the first and second driver circuits for adjusting the strength of the first and second driver circuits according to a data pattern, the data pattern including the first input signal and input signals of a plurality of output buffers. 
     In alternate embodiments, the data pattern preferably includes bits, and the adjustment circuit adjusts the strength of the first and second driver circuits according to a number of bits having a same value. The first and the second driver circuits preferably include field effect transistors. The adjustment circuit preferably adjusts the strength of the driver circuits according to a graduated strength scale, the number of graduations being the number of inputs of the data pattern plus one. The adjustment circuit may overdrive the driver circuits to adjust the driver circuit strength. The adjustment circuit may include logic circuitry to adjust the first and second driver circuits. The plurality of output buffers may be disposed in an array of output buffers and may include adjacent output buffers. 
     Another output buffer includes a first driver device for coupling a first voltage to an output when the first driver device is turned on, and a second driver device for coupling a second voltage to the output when the second driver device is turned on. An input is connected to the first and second driver devices for turning the first and second driver devices on and off according to a first input signal. A NOR gate is provided having an output coupled to a first driver circuit for turning the first driver circuit on and off, the first driver circuit for coupling the first voltage to the output when the first driver circuit is turned on. A NAND gate is also provided having an output coupled to a second driver circuit for turning the second driver circuit on and off, the second driver circuit for coupling the second voltage to the output when the second driver circuit is turned on. The NOR gate and NAND gate receive an input data pattern such that upon logically combining the plurality of inputs the first and second driver circuits are turned on and off in conjunction with the first and second driver devices to adjust a driving strength to the output according to the data pattern, the data pattern including the first input signal and input signals of a plurality of output buffers. 
     In other embodiments, the data pattern preferably includes bits, and the driving strength to the output may be adjusted when the data pattern includes all bits having a same value. The first and the second driver devices preferably include field effect transistors. The first and second driver circuits include field effect transistors of the same type as the first and the second driver devices, respectively. The driver devices may include gates for activation of the driver devices, the gates being overdriven to adjust driver circuit strength. The plurality of output buffers may be disposed in an array of output buffers and may include adjacent output buffers. The first and second driver circuits may each include a control circuit which outputs a pulse to activate at least one driver device to assist in adjusting the driver strength of one of the first driver device and the second driver device. 
     A method for adjusting driver strength for output buffers includes the steps of providing an output buffer including a first driver circuit for coupling a first voltage to an output when the first driver circuit is turned on, a second driver circuit for coupling a second voltage to the output when the second driver circuit is turned on, an input connected to the first and second driver circuits for turning the first and second driver circuits on and off according to a first input signal and an adjustment circuit coupled to the first and second driver devices for adjusting the strength of the first and second driver circuits according to a data pattern, the data pattern including the first input signal and input signals of a plurality of output buffers, inputting the data pattern into the adjustment circuit, adjusting the strength of the first and second driver circuits according a number of high and low bits in the data pattern and outputting the first input signal. 
     In alternate methods, the step of adjusting may include the step of adjusting the strength of the driver circuits when the data pattern includes bits all having the same value. The driver circuits may include transistors with gates for activating the transistors and may further include the step of overdriving the gates to adjust driver circuit strength. The step of adjusting the strength of the first and second driver circuits according a number of same bits in the data pattern may include the steps of summing one of the bits, comparing the sum of the bits to a graduated scale of driver strengths and adjusting the driver strength according to the scale. The step of adjusting the strength of the first and second driver circuits according a number same bits in the data pattern may include the steps of inputting the data pattern into a logic gate and adjusting the driver strength according to an output of the logic gate. The step of adjusting the driver strength according to the output of the logic gate may include adjusting the driver strength when the data pattern has all bits of the same value. The logic gate may include a NAND gate and/or a NOR gate. 
    
    
     These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings. 
     BRIEF DESCRIPTION OF DRAWINGS 
     This disclosure will present in detail the following description of preferred embodiments with reference to the following figures wherein: 
     FIG. 1 is a schematic diagram of an output buffer for a dynamic random access memory in accordance with the prior art; 
     FIG. 2 is a schematic diagram of an output buffer having an adjustment circuit for adjusting driver strength of the output buffer in accordance with the present invention; 
     FIG. 3 is a schematic diagram of an output buffer having an adjustment circuit including NAND and NOR gates for adjusting driver strength of the output buffer in accordance with the present invention; 
     FIG. 4 is a schematic diagram of a control circuit for outputting pulses for adjusting driver strength by driving gates of the drivers of the output buffer in accordance with the present invention; and 
     FIG. 5 is a schematic diagram of another control circuit for outputting pulses for adjusting driver strength by driving gates of the drivers of the output buffer in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     This disclosure relates to semiconductor devices and more particularly, to an adjustable strength driver circuit for off chip drivers used in semiconductor memories. In accordance with the present invention, a proper adjustment of the drive strength of an individual output buffer or off chip driver (OCD) of an array of OCD&#39;s is provided. Data to be driven from a dynamic random access memory (DRAM) is known in advance of being driven by the OCD and the array of OCD&#39;s. The drive strength can therefore be adjusted and optimized depending on the data pattern to be output from the array of OCD&#39;s. The present invention will now be described by way of example for a DRAM output buffer. Other devices are contemplated for use with the present invention where it is advantageous to adjust driver strength. 
     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, an output buffer  50  is provided. An adjustment circuit  54  includes a group of inputs (IN) which share a VSSQ/VDDQ pair. The number of inputs may be greater or less depending on the design. One input represents the output signal to be driven at output  52  while the other inputs represent the outputs of output buffers in the vicinity of buffer  50 . The data in this group of inputs can be evaluated in advance using a sum of all bits approach. The input bits have a voltage of either high (VDDQ) or low (0). Driver strength, i.e., the strength of the driving transistors, for a driver circuit  56  is adjusted according to TABLE 1 where the sums of the data states are equated to a driver strength which includes a graduated strength scale having as the number of graduations the sum of the inputs+1. Since circuit  54  has 4 inputs, TABLE 1 illustratively includes 5 graduations of strength for the driver transistor(s). In accordance with a data pattern on DQs, adjustment circuit  54  calculates the strength needed to properly drive output  52 . Drive strength is modified in one embodiment by selecting an appropriately sized transistor based on the strength needed, i.e., TABLE 1. 
     
       
         
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Option 
                 
                   
                     
                       
                         
                           ∑ 
                           
                             i 
                             = 
                             1 
                           
                           4 
                         
                          
                         
                           ( 
                           
                             
                               b 
                                
                               
                                   
                               
                                
                               D 
                                
                               
                                   
                               
                                
                               
                                 Q 
                                 j 
                               
                             
                             = 
                             0 
                           
                           ) 
                         
                       
                     
                             
                     
                         
                     
                   
                 
                 bDQ j   
                 Modify drive devices for DQ j   
               
               
                   
               
             
             
               
                 1 
                 4 
                 0 
                 activate additional strong driver 
               
               
                   
                   
                   
                 device 
               
               
                 2 
                 3 
                 0 
                 activate additional weak driver 
               
               
                   
                   
                   
                 device 
               
               
                   
                   
                 VDDQ 
                 use default drive strength 
               
               
                 3 
                 2 
                 0 
                 use default drive strength 
               
               
                   
                   
                 VDDQ 
                 use default drive strength 
               
               
                 4 
                 1 
                 VDDQ 
                 activate additional weak driver 
               
               
                   
                   
                   
                 device 
               
               
                   
                   
                 0 
                 use default drive strength 
               
               
                 5 
                 0 
                 VDDQ 
                 activate additional strong driver 
               
               
                   
                   
                   
                 device 
               
               
                   
               
             
          
         
       
     
     Σ(bDQ j =0) is the sum of input data bits being zero for i buffers in the group (in this case, 4 buffers are grouped). bDQ j  is the input of the buffer that has its drive strength adjusted. The additional driver devices may be represented connected as ansistors  106  and  110  of FIG.  3 . In one embodiment, additional strong and weak transistors are PFETs for options 1 and 2, and strong and weak transistors are NFETs for options 4 and 5. To provide all additional options in Table 1 additional drivers may be added. The results of the strength adjustment may be implemented by increasing the number of driver transistors, increasing their width or using the drivers to overdrive an appropriate gate to avoid the drop of V gs  below a lower threshold. In this way, driver strength in buffer  50  may be adjusted in accordance with the data pattern input to the output drivers in its vicinity. Additional drive transistors may be added by connecting sources and drains of the additional transistors across the same nodes as transistors  106  and  110  of FIG.  3 . Adjustment circuit  54  preferably controls which transistors are activated in accordance with the invention. 
     Referring to FIG. 3, an off chip driver circuit  100  is shown in accordance with the present invention. Circuit  100  includes a NOR gate  102  and a NAND gate  104 . NOR gate  102  and NAND gate  104  receive a same set of inputs from a data pattern to be driven by an array of output buffers (OCD&#39;s). A set of pins or DQs carry the output signal of the OCD. These DQs are grouped and supplied by a specific VDDQ/VSSQ pair. The data in the illustrative embodiment shown in FIG. 3 includes 4 DQs being shared by the VDDQ/VSSQ pair. However, the number of DQs may be greater or less as well. 
     NOR gate  102  has its output coupled to a circuit  122  which creates a low going pulse when the output of NOR  102  is rising. The output of circuit  122  is coupled to a gate of transistor  106 . PFET transistor has its source coupled to the source of a transistor  108 . Transistors  106  and  108  are preferably transistors of a same conductivity, and more preferably PFET transistors. The sources of transistors  106  and  108  are coupled to VDDQ, and the drains of transistors  106  and  108  are connected to node  3 . NAND gate  104  has its output coupled to a circuit  124  which creates a high going pulse when the output of NAND gate  104  is falling. The output of circuit  124  is coupled to a gate of transistor  110 . NFET transistor has its source coupled to the source of a transistor  112 . Transistors  110  and  112  are preferably transistors of a same conductivity, and more preferably NFET transistors. The sources of transistors  110  and  112  are coupled to VSSQ, and the drains of transistors  110  and  112  are connected to node  3 . The gates of transistors  108  and  112  include an input signal line (bDQ&lt;1&gt;) for activating transistors  108  and  112 . Transistor  106 ,  108 ,  110  and  112  are all driving transistors. 
     Inputs to NOR and NAND gates  102  and  104 , respectively, include data to be output by this and other OCD&#39;s (corresponding to DQ&lt;1:4&gt;) in the array in the vicinity of the OCD of circuit  100 . These inputs include bDQ&lt;1:4&gt;. NOR gate  102  and NAND gate  104  provide driver strength compensation for worst cases (all 1&#39;s or all 0&#39;s) in the vicinity of circuit  100 . Inputs bDQ are NORed and NANDed, and the outputs of NOR gate  102  and NAND gate  104  are used to activate additional driver transistors  106  and  110 . In this way, driver strength is increased for the worst case data pattern of all 1&#39;s or all 0&#39;s, in accordance with the present invention. The same scheme may be utilized to either evaluate a greater number of DQ data or to add more discrete levels to the strength adjustment. The timing of the activation of the additional drive transistors  106  and  110  may be selected to comply with design specifications, e.g., maximum and/or minimum currents, etc. 
     FIG. 3 shows a circuit capable of implementing options 1, 3 and 5 shown in Table 1 above. Option 3 uses default driver strength. Option 1 is implemented when bDQs are low. Output of NOR gate  102  goes high and generates a low pulse (of variable width) on a signal which is otherwise held high. This signal activates transistor  106  (which functions as a strong transistor in Table 1) and assists transistor  108  in driving output DQ(i) high. Output of NAND gate  104  stays high, and transistors  112  and  110  are not switched on. Option 5 is implemented when all bDQs are high. Output of NOR gate  102  stays low, and transistors  106  and  108  are not switched on. Output of NAND gate  104  goes low and generates a high pulse (of variable width). Transistors  110  (which functions as a strong transistor in Table 1) and  112  are switched on and drive the output DQ(i) low. In all other combinations, transistor  108  or transistor  112  is active. 
     Circuit  122  provides the pulse shown in FIG.  3 . Referring to FIG. 4, circuit  122  preferably includes an input (in) split into two legs. One leg includes a series of inverters  126  which are coupled to a NAND gate  128 . NAND gate  128  NANDs the input and the output of the inverters  126  to generate the low going pulse as described for FIG.  3 . Circuit  124  provides the pulse shown in FIG.  3 . Referring to FIG. 5, circuit  124  preferably includes an input (in) split into two legs. One leg includes a series of inverters  130  which are coupled to a NOR gate  132 . NOR gate  132  NORs the input and the output of the inverters  130  to generated the high going pulse as described for FIG.  3 . 
     The embodiments described above for the present invention include a device which can quickly adjust the drive strength of an OCD. Data are known in advance of driving the output, therefore, setting up the drivers can be performed efficiently. Only the OCDs which need additional drive strength receive it. In preferred embodiments, drive strength may be optimized with respect to package parasitics. In combination with trim fuses or other devices, the present invention can be used to optimize a DRAM for its specific application environment. 
     Having described preferred embodiments for a novel adjustable strength driver circuit device for semiconductor memories (which are intended to be illustrative and not limiting), it is noted that modifications and variations can be made by persons skilled in the art in light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments of the invention disclosed which are within the scope and spirit of the invention as outlined by the appended claims. Having thus described the invention with the details and particularity required by the patent laws, what is claimed and desired protected by Letters Patent is set forth in the appended claims.