Abstract:
Circuits, methods, and apparatus that provide output drivers that consume relatively little integrated circuit area and provide fast output switching. An exemplary embodiment provides an output driver including pull-up and pull-down devices, each device driven by a pre-driver stage. The pre-driver for the pull-down device is supplied from an auxiliary power supply, which has a higher voltage than the supply seen by the pull-up device. The pre-driver for the pull-down is biased by a voltage that tracks the higher of the auxiliary and output supplies. In some embodiments, the output driver may be part of an input/output cell. In that case, the well for the pull-up device is biased by a voltage that tracks the highest of the output supply and input received voltage, while the pull-up predriver circuit bias is the higher between the auxiliary and output supplies and the input received voltage.

Description:
BACKGROUND 
   The present invention relates to input/output (IO) cells in general and to high speed IO cells using an auxiliary power supply in particular. 
   A goal in integrated circuit manufacturing is to increase circuit density and functionality. Accordingly, there has been a great deal of effort put into reducing the size of individual transistors so that more transistors, and thus more functionality, can be placed on each device. 
   There is a downside to these higher densities and smaller devices. For example, smaller devices can only standoff or support a limited voltage before breakdown occurs. Higher densities can result in an increase in power supply dissipation per unit area of an integrated circuit, which can limit operability and lifetime, as measure in mean time before failure, of the circuit. To mitigate both these consequences, the power supply voltages applied to integrated circuits has been progressively lowered over the years, from 5 volts to 3.3, then to 2.5 and recently 1.8 volts and even lower. 
   This reduction in power supply voltage has taken its toll on some of the individual circuits that are used in the design and manufacture of integrated circuits. One type of circuit that has been particularly effected are output drivers. The reduction in supply voltage has meant a corresponding decrease in their drive capability. 
   To compensate for this, the size of output devices has often been increased. This has the undesirable results of consuming more die area for the output drivers, and also necessitate the increase in size of predriver circuits that drive the output drivers themselves. The increase in size of these circuits increases their power supply currents, thus offsetting some of the gains achieved by having the smaller devices and lower power supplies in the first place. Alternately, a slower output driver may be used, but these outputs are more susceptible to noise and jitter. 
   Accordingly, what is needed are circuits, methods, and apparatus that provide fast output drivers using lower power supplies but do not require large integrated circuit areas for their implementation. 
   SUMMARY 
   Accordingly, embodiments of the present invention provide circuits, methods, and apparatus that provide output drivers that consume relatively little integrated circuit area and provide fast output switching. An exemplary embodiment provides an output driver including pull-up and pull-down devices, each device driven by a pre-driver stage. The pre-driver for the pull-down device is supplied from an auxiliary power supply that has a higher voltage than the supply seen by the pull-up device. The pre-driver for the pull-down is biased by a voltage that tracks the higher of the auxiliary and output supplies. In some embodiments, the output driver may be part of an input/output cell. In that case, the well for the pull-up device is biased by a voltage that tracks the highest of the output supply and input received voltage, while the pull-up predriver circuit bias is changed to the higher between the auxiliary and output supplies and the input received voltage. Various embodiments of the present invention may incorporate one or more of these and the other features discussed herein. 
   A better understanding of the nature and advantages of the present invention may be gained with reference to the following detailed description and the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a simplified block diagram of a programmable logic device that can implement embodiments of the present invention; 
       FIG. 2  is a block diagram of an electronic system that may incorporate embodiments of the present invention; 
       FIG. 3  is a schematic of an input/output (I/O) cell consistent with an embodiment of the present invention; 
       FIG. 4  is a plot illustrating drain current as a function of drain-to-source voltage for different drain-to-gate voltages for an NMOS transistor; 
       FIG. 5  is a schematic of a pre-driver that may be used as the pre-drivers in  FIG. 3 , or as a pre-driver in other embodiments of the present invention; 
       FIG. 6  is a schematic of a level shift circuit that may be used as the level shift circuit in  FIG. 5 , or as a level shift circuit in other embodiments of the present invention; 
       FIG. 7  is a schematic of an output well-bias circuit that may be used as the output well-bias circuit  360  in  FIG. 3 , or as an output well-bias circuit in other embodiments of the present invention; and 
       FIG. 8  is a schematic of a pre-driver well-bias circuit that may be used as the pre-driver well-bias circuit  350  in  FIG. 3  or as a pre-driver well-bias circuit in other embodiments of the present invention. 
   

   DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     FIG. 1  is a simplified partial block diagram of an exemplary high-density programmable logic device  100  wherein techniques according to the present invention can be utilized. PLD  100  includes a two-dimensional array of programmable logic array blocks (or LABs)  102  that are interconnected by a network of column and row interconnects of varying length and speed. LABs  102  include multiple (e.g., 10) logic elements (or LEs), an LE being a small unit of logic that provides for efficient implementation of user defined logic functions. 
   PLD  100  also includes a distributed memory structure including RAM blocks of varying sizes provided throughout the array. The RAM blocks include, for example, 512 bit blocks  104 , 4K blocks  106  and a M-Block  108  providing 512K bits of RAM. These memory blocks may also include shift registers and FIFO buffers. PLD  100  further includes digital signal processing (DSP) blocks  110  that can implement, for example, multipliers with add or subtract features. I/O elements (IOEs)  112  located, in this example, around the periphery of the device support numerous single-ended and differential I/O standards. It is to be understood that PLD  100  is described herein for illustrative purposes only and that the present invention can be implemented in many different types of PLDs, FPGAs, and the like. 
   While PLDs of the type shown in  FIG. 1  provide many of the resources required to implement system level solutions, the present invention can also benefit systems wherein a PLD is one of several components.  FIG. 2  shows a block diagram of an exemplary digital system  200 , within which the present invention may be embodied. System  200  can be a programmed digital computer system, digital signal processing system, specialized digital switching network, or other processing system. Moreover, such systems may be designed for a wide variety of applications such as telecommunications systems, automotive systems, control systems, consumer electronics, personal computers, Internet communications and networking, and others. Further, system  200  may be provided on a single board, on multiple boards, or within multiple enclosures. 
   System  200  includes a processing unit  202 , a memory unit  204  and an I/O unit  206  interconnected together by one or more buses. According to this exemplary embodiment, a programmable logic device (PLD)  208  is embedded in processing unit  202 . PLD  208  may serve many different purposes within the system in  FIG. 2 . PLD  208  can, for example, be a logical building block of processing unit  202 , supporting its internal and external operations. PLD  208  is programmed to implement the logical functions necessary to carry on its particular role in system operation. PLD  208  may be specially coupled to memory  204  through connection  210  and to I/O unit  206  through connection  212 . 
   Processing unit  202  may direct data to an appropriate system component for processing or storage, execute a program stored in memory  204  or receive and transmit data via I/O unit  206 , or other similar function. Processing unit  202  can be a central processing unit (CPU), microprocessor, floating point coprocessor, graphics coprocessor, hardware controller, microcontroller, programmable logic device programmed for use as a controller, network controller, and the like. Furthermore, in many embodiments, there is often no need for a CPU. 
   For example, instead of a CPU, one or more PLD  208  can control the logical operations of the system. In an embodiment, PLD  208  acts as a reconfigurable processor, which can be reprogrammed as needed to handle a particular computing task. Alternately, programmable logic device  208  may itself include an embedded microprocessor. Memory unit  204  may be a random access memory (RAM), read only memory (ROM), fixed or flexible disk media, PC Card flash disk memory, tape, or any other storage means, or any combination of these storage means. 
     FIG. 3  is a schematic of an input/output cell that is consistent with an embodiment of the present invention. This I/O cell may be used as the I/O cell  112  in  FIG. 1 , or as an I/O cell in other integrated circuits. This figure, as with the other included figures, is shown for illustrative purposes only and does not limit either the possible embodiments of the present invention or the claims. 
   Include are output driver devices M 1   310  and M 2   320 , pre-drivers  330  and  340 , PD well bias circuit  350 , output well bias circuit  360 , and optional input receiver or buffer  370 . When the optional input receiver  370  is removed, this circuit is an output cell. When the optional input receiver  370  is included, the circuit forms an IO cell. 
   When configured as an output cell, the output may be active or tristated. For example, when configured as an output, the output pin VOUT on line  315  may be connected to a tristate bus, and the output cell may need to be tri-stated on occasion. When configured as an IO cell, the circuit may act as an input or an output. When the circuit is receiving signals as a input, the output portion of the circuit is tristated. Again, when the circuit is an output, the output may be active or tristated. 
   When the circuit is acting as an active output, whether as an output cell or an IO cell, an input signal VINA is received on line  342  from the integrated circuit internal core. Also, a signal VINB, which may be the same signal as VINA, is received on line  332  from the core. VINA (and VINB) is typically a logic signal that swings between the power supply voltages in the core of the integrated circuit. Often, these voltages are kept low to reduce the power dissipation for the integrated circuit, for example the core voltages may be 1.2 volts and ground. Pre-driver  330  translates its input signal to a logic signal having a high-voltage approximately equal to the voltage applied at VPD on line  335 , and a low voltage approximately equal to ground. Pre-driver  340  translates its input signal to logic signal having a high-voltage approximately equal to the higher of the voltage applied at VPD on line  335  and VCCN on line  325 , and a low voltage approximately equal to ground. 
   The input signal VINA (and VINB) is buffered by the pre-drivers  340  and  330 , the outputs of which drive pull-up device M 1   310  and pull-down device M 2   320 . The drains of M 1   310  and M 2   320  connect to line  315 . Typically, line  315  connects to a pad, which in turn connects to a pin of the integrated circuit device package. The source of M 1   310  connects to a supply VCCN, which is 1.8 volts in a specific embodiment of the present invention. The source of M 2   320  connects to ground. In this mode, the output well bias circuit  360  connects the well of the pull up device M 1   310  to VCCN. 
   To improve the switching performance of the circuit when it is an active output, the pre-driver  330  supply voltage is supplied by a separate supply, VPD on line  335 . By using the higher supply VPD on line  335  to bias the pre-driver  330 , a larger drive is seen at the gate of M 2   320 . This larger gate translates to a larger VGS (specifically, the maximum value of VGS is equal to VPD), and hence a larger pull-down current for device M 2   320  when it is on. The pre-driver  340  is biased by the higher of the supply voltages, VCCN and VPD. A higher voltage here does not translate to a performance improvement to the same degree. Since the source of M 1  is tied to VCCN on line  325 , the maximum VGS for M 1   310  is still VCCN, the larger swing simply shuts off M 1   310 . 
   When the output cell is tristated, either because the cell is configured as an IO cell and it is acting as an input, or it is configured as an output but tristated, VINA on line  342  and VINB on line  332  are separate signals. In this case, the input VINA on line  342  is high, such that the gate of the pull-up driver M 1   310  is high, and M 1   310  is off, and the input VINB on line  332  is low, such that gate of the pull-down driver M 2   320  is low and M 2   320  is off. 
   When the cell is acting as an input, an input signal is received on line  315  and buffered by input buffer  370 . Input buffer  370  is typically supplied by the core voltage power supplies. In this configuration, conventional cells are susceptible to problems is the parasitic drain-to-bulk diode of the pull-up device M 1   310  turns on. Specifically, if the input voltage on line  315  exceeds the voltage VCCN on line  325  by a diode drop (approximately 0.7 volts) and the well of the pull-up device M 1   310  is tied to VCCN on line  325 , the drain-to-bulk diode of M 1   310  conducts current from the input line  315  to VCCN on line  325 . To avoid this, the output well bias circuit biases the well of M 1   310  to the higher voltage between VCCN on line  325  and the received signal on line  315 . 
   It will be appreciated by one skilled in the art that although this figure illustrates circuitry operating between positive power supplies and ground, other power supply schemes are possible consistent with embodiments of the present invention. For example, these circuits may operate between ground and one or more negative supplies. Alternately, the circuit may operate between on or more positive power supplies and one or more negative power supplies, and may also possibly include ground. 
   Also, this and the following figures show circuitry as being made of CMOS devices. In other embodiments, other processes, such as bipolar, BiCMOS, HFETs, HBTs, and other processes and technologies may be used consistent with the present invention. 
     FIG. 4  is a plot illustrating drain current as a function of drain-to-source voltage for different drain-to-gate voltages for an NMOS transistor, such as the pull-down transistor M 2   320  in  FIG. 3 . Drain current Id is plotted along a Y-axis  410  as a function of VDS along an X-axis  420 . Trace  450  illustrates how zero, or near zero drain current flows when VGS is approximately equal to the threshold voltage Vth for the device. As VGS is increased, for example to VCCN, trace  440  indicates that a higher current flows at a corresponding VDS. As VGS is further increased, for example to VPD, the drain current increases further, as indicated by trace  430 . 
   In this way, the higher VGS provided to the pull-down driver M 2   320  in  FIG. 3  results in a larger pull-down current in that device. This advantage may be used in one of two ways. The device size may be kept large, in which case the switching speed is improved. Alternately, device size may be reduced while maintaining similar switching characteristics. Often, a compromise between these two may be desirable, where the device sizes reduced and switching performance is improved, but both to a lesser degree. 
     FIG. 5  is a schematic of a pre-driver that may be used as the pre-drivers  330  and  340  in  FIG. 3 , or as a pre-driver in other embodiments of the present invention. Include are level shift circuit  510  and inverters  520 ,  530 , and  540 . 
   An input signal is received on line  505  by the level shift circuit  510 . Typically, this signal operates in a voltage range provided to the core of the integrated circuit. For example, the input may be a digital signal operating between 1.2 volts and ground. The level shift circuit  510  translates this signal to a signal operating between VCC on line  555  and ground on line  515 . This signal is then buffered by inverters  520 ,  530 , and  540 . Inverter  540  provides an output voltage VOUT on line  545 . 
   The buffers  520 ,  530 , and  540  typically increase progressively in size, with inverter  520  being the smallest and inverter  540  being the largest. A typical ratio of device widths is on the order of 3:1 to 5:1. In a specific embodiment of the present invention, this ratio is approximately 4:1. In this way, a signal with a comparatively low drive is boosted in power, until it is capable of driving a large output device. Each of the inverters  520 ,  530 , and  540  typically are formed of a series of a p-channel and n-channel coupled between VCC on line  555  and ground on line  515 . The gates of the p-channel and n-channel devices both connect to the input, while the drains of the devices tie together to form the output. 
   Typically the p-channel device in an inverter has a longer width than the n-channel. This is to compensate for differences in the mobility of the majority carriers between the p-channel and n-channel devices. By adjusting the widths of the devices to compensate for differences in mobility, the threshold voltage of the inverter remains near one-half of VCC. In a specific embodiment however, this is not desirable, since the output of the level shift circuit  510  is asymmetrical. In this specific embodiments, the lengths of the p and n-channel inverter  520  is close to a 1:2 ratio. This moves the threshold for the inverter closer to ground to compensate for the asymmetry in the voltage swing at the output of the level shift  510 . In this embodiment the lengths of devices in inverter  530  are very close to 1:1, while the ratio of devices in inverter  540  have approximately a 3:1 ratio. 
   Again, VCC for the pre-driver  330  is equal to VPD. This voltage is typically higher than the core power supply, or the output supply VCCN. In this way, a greater swing is provided to the n-channel pull down driver M 2   320  in  FIG. 3 . 
     FIG. 6  is a schematic of a level shift circuit that may be used as the level shift circuit  510  in  FIG. 5 , or as a level shift circuit in other embodiments of the present invention. Included are p-channel transistors M 1610  and M 2   620 , n-channel devices M 3   630  and M 4   640 , and inverter I 1   650 . 
   An input signal VIN is received on line  605 . Again, this signal is typically provided by a core logic element LE in logic array block LAB  102 . This input signal typically transitions between a high voltage and a low voltage, for example, 1.2 volts and ground. The level shift circuit provides an output VOUT on line  625 . The output VOUT on line  625  swings between a voltage VCC applied on line  615  and ground. 
   The input signal VIN on line  605  drives the gates of n-channel device M 3   630  and inverter I 1   650 . Inverter I 1   650  inverts the signal VIN on line  605  and drives the gate of M 4   640 . When the input VIN on line  605  is high, device M 3   630  is on, while device M 4   640  is off. Device M 3   630  conducts current, thus pulling down the gate of device M 2   620 . The device M 2   620  in turn pulls VOUT on line  625  to the voltage VCC applied on line  615 . When the input voltage VIN on line  605  his low, device M 3   630  is off, while device M 4   640  is on. Device M 4   440  pulls VOUT on line  645  to ground. The devices M 1610  and M 2   620  are crossed coupled to provide positive feedback for faster switching. 
   The inverter I 1   650  may be powered by the core voltage or by the voltage VCC applied on line  615 . In a specific embodiment, I 1   650  is powered by VCC on line  615 . This means that device M 3   630  and M 4   440  receive different input voltages. For this reason, M 4   440  receives a stronger drive, thus resulting in a lower average voltage for VOUT on line  625 . For this reason, the inverters driven by the output VOUT on line  645  have their input thresholds set low to compensate. 
     FIG. 7  is a schematic of an output well-bias circuit that may be used as the output well-bias circuit  360  in  FIG. 3 , or as an output well-bias circuit in other embodiments of the present invention. Included are p-channel devices M 1   710 , M 2   720 , M 3   730 , and M 4   740 . 
   Again, this is circuit tracks the higher of voltage between VCCN on line  705  and VPIN on line  715 . If the two voltages are equal, or within a threshold voltage of each other, the bias voltage VBIAS on line  735  is set by the diodes M 1   710  or M 4   740 . If the difference between the voltage VCCN on line  705  and VPIN on line  715  is greater than a threshold voltage, one of the devices M 2   720  or M 3   730  shorts VBIAS on line  735  to it the respective voltage line. 
   Specifically, when VCCN on line  705  is higher than VIN on line  715  by more than a threshold voltage, device  720  conducts and shorts VCCN on line  705  to VBIAS on line  735 . Conversely, if VPIN on line  715  is higher than VCCN on line  705  by more than a threshold voltage, device M 3   730  conducts, thus shorting VPIN on line  715  to VBIAS  735 . In this way, the well of pull-up device M 1   310  is sufficiently high such that an excess received voltage at the pad VOUT on line  315  does not cause conduction in its drain-to bulk diode. 
   A table listing exemplary voltages for VCCN on line  705  and VPIN on line  715 , and the resulting voltage for VBIAS on line  735  follows: 
   
     
       
             
             
             
           
         
             
                 
             
             
               VCCN 
               VPIN 
               VBIAS 
             
             
                 
             
           
           
             
               1.5 
               2.5 
               2.5 
             
             
               1.8 
               2.5 
               2.5 
             
             
               1.8 
               1.8 
               1.8-V TP   
             
             
               2.5 
               1.8 
               2.5 
             
             
               2.5 
               1.5 
               2.5 
             
             
                 
             
           
        
       
     
   
   Where V TP  is the threshold voltage for a p-channel device. 
     FIG. 8  is a schematic of a pre-driver well-bias circuit that may be used as the pre-driver well-bias circuit  350  in  FIG. 3  or as a pre-driver well-bias circuit in other embodiments of the present invention. Included are P-channel devices M 1   810 , M 2   820 , M  830 , M 4   840 , M 5   850 , M 6   860 , and M 7   870 . 
   Devices M 1   810 , M 2   820 , M 3   830 , and M 4   840  operate similarly as described regarding the output well-bias circuit in  FIG. 7 . Additionally, VPD on line  825  is received by this circuit. In some embodiments of the present invention, this circuit is used to bias both in the pre-driver  340  and the well of the output pull up device M 1   310 . 
   When VPD on line  825  is higher than the voltages VCCN on line  805  or VPIN on line  815 , which is often the case, device M 6   860  conducts, thus shorting VPD on line  825  to VBIAS on line  835 . If the voltage VPD on line  825  is lower than other the voltages VCCN on line  805  or VPIN on line  815 , then the voltage at the intermediate node  860  in shorted to VBIAS on line  835 . In this case, the intermediate voltage on line  860  is set as before by VCCN on line  805  and VPIN on line  815 . If two or more of the voltages VPD on line  825 , VCCN on line  805  and VPIN on line  815  are within a threshold voltage of each other (and higher than the remaining voltage, if applicable), then VBIAS is approximately a threshold voltage below the highest voltage level. 
   A table listing exemplary voltages for VPD on line  825 , VCCN on line  805  and VPIN on line  815 , and the resulting voltage VBIAS on line  835  follows: 
   
     
       
             
             
             
             
             
           
         
             
                 
                 
             
             
                 
               VPD 
               VCCN 
               VPIN 
               VBIAS 
             
             
                 
                 
             
           
           
             
                 
               1.5 
               1.5 
               2.5 
               2.5 
             
             
                 
               1.8 
               1.5 
               2.5 
               2.5 
             
             
                 
               1.8 
               1.5 
               1.8 
               1.8-V TP   
             
             
                 
               2.5 
               1.5 
               1.8 
               2.5 
             
             
                 
               2.5 
               1.5 
               1.5 
               2.5 
             
             
                 
                 
             
           
        
       
     
   
   The above description of exemplary embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form described, and many modifications and variations are possible in light of the teaching above. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated.