Abstract:
The present invention significantly reduces the chip size of a metal-oxide-semiconductor (MOS) field-effect transistor, which serves as a driver for output impedance drivers, such as, but not limited to, double data rate (DDR2) synchronous dynamic random access memory (SDRAM). In an embodiment of the invention, a voltage drop across the driver is a decreased ratio of the supply voltage, e.g., three-tenths of the supply voltage, lower than half of the supply voltage. A smaller voltage drop allows a lower current and a higher impedance to be used in the driver. By having a higher impedance across the driver, the physical area needed for the DDR2 driver is reduced because a transistor with a smaller physical width can be used. A DDR2 driver operating at the decreased ratio is the functional equivalent of the driver operating with the supply voltage or half of the supply voltage, with the added advantage of the reduced area.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention relates to electronic drivers and more specifically, to an improved impedance control technique for significantly reducing chip size of a metal-oxide-semiconductor (MOS) field-effect transistor, which serves as a driver for double data rate synchronous dynamic random access memory (SDRAM). 
     2. Description of Related Art 
     Stub series terminated logic (SSTL) is an electrical interface for driving transmission lines commonly used with double data rate (DDR) dynamic random access memory (DRAM) integrated circuits and memory modules. SSTL comes in three different standards as defined by the JEDEC Solid State Technology Association: SSTL — 2, SSTL — 3, and SSTL — 18. SSTL assumes that transmission line termination is required. As such, SSTL has specifications for output impedance and different methods of termination. Impedance is a measure of opposition to current, extending the direct current (DC) concept of resistance to alternating current (AC) circuits. Termination is important for high-speed signaling because a properly terminated transmission line reduces reflections, reduces electromagnetic interference (EMI) emissions, improves settling time, and improves timing margins. 
     SSTL — 2 and SSTL — 3 are used with DDR memory and define two classes of drivers that are targeted at different termination schemes. SSTL — 18 is used with DDR2 memory, which is the next generation of DDR. DDR and DDR2 are both types of SDRAM. SDRAM has a synchronous interface, meaning that it waits for a clock signal before responding to control inputs and is therefore synchronized with a computer&#39;s system bus. The clock is used to drive an internal finite state machine that pipelines incoming instructions. This allows the chip to have a more complex pattern of operation than an asynchronous DRAM, which does not have a synchronized interface. DDR2 supersedes the original DDR SDRAM specification and the two are not compatible. In addition to double pumping the data bus as in DDR SDRAM (transferring data on the rising and falling edges of the bus clock signal), DDR2 allows higher bus speed and requires lower power by running the internal clock at one quarter the speed of the data bus. The two factors combine to require a total of four (4) data transfers per internal clock cycle. Thus, without speeding up the memory cells themselves, DDR2 can effectively operate at twice the bus speed of DDR. 
     With data being transferred 64 bits at a time, DDR2 SDRAM gives a transfer rate of (memory clock rate)×2 (for bus clock multiplier)×2 (for dual rate)×64 (number of bits transferred)/8 (number of bits/byte). Thus, with a memory clock frequency of 100 MHz, DDR2 SDRAM gives a maximum transfer rate of 3200 MB/s. Since the DDR2 clock runs at half the DDR clock rate, DDR2 memory operating at the same external data bus clock rate as DDR will provide the same bandwidth, but with higher latency, resulting in inferior performance. Alternatively, DDR2 memory operating at twice the external data bus clock rate as DDR may provide twice the bandwidth with the same latency. The best-rated DDR2 memory modules are at least twice as fast as the best-rated DDR memory modules. 
     The SSTL — 18 standard has been developed particularly with the objective of providing a relatively simple upgrade path from metal oxide semiconductor (MOS) push-pull interface designs. The standard is particularly intended to improve operation in situations where busses must be isolated from relatively large stubs. External resistors provide this isolation and also reduce the on-chip power dissipation of the drivers. Busses may be terminated by resistors to an external termination voltage. To achieve better signal integrity, the output impedance of a DDR2 driver should be controlled, i.e., calibrated to a desired impedance as established by an external resistor. 
       FIG. 1  illustrates a conventional output impedance control circuit  100  that compares the impedance of (or the voltage across) a driver  135  (e.g., a SSTL — 18 output driver PMOS transistor and in series on-chip resistor, an output driver, a calibrated resistance driver, etc.) with the impedance of (or the voltage across) an external precision resistor  150  (e.g., a fixed resistor, a 38 ohm external precision resistor, etc.). The implementation of the conventional output impedance control circuit  100  is readily apparent to one of ordinary skill in the art. SSTL — 18 requires a supply voltage to the output buffers of a DDR2 memory chip, V DDQ , to be approximately 1.8 volts, and a tracking termination voltage, V TT , to be approximately 0.9 volts (i.e., ½ of V DDQ ). The driver  135  comprises “N” number of fingers  140 A-N (e.g., PMOS transistor fingers, a transistor and in series on-chip resistor fingers, etc.) comprising a transistor and an in series on-chip resistor. The conventional output impedance control circuit  100  further includes a comparator  130  and a counter  120 . Although the fingers  140  are illustrated as being a PMOS transistor, calibration can just as easily be performed using an NMOS transistor. 
     In operation, the impedance of the external precision resistor  150  is compared to the impedance of one or more of the fingers  140 A-N of the driver  135 . Initially, the comparator  130  compares the voltage across a first finger  140 A with the voltage across the external precision resistor  150 . If the voltage across the external precision resistor  150 , which is input into the positive input of the comparator, is less than V TT  (i.e., half of the supply voltage, typically 0.9 volts), which is the voltage into the negative input of the comparator, the output voltage of the comparator  130  is high, and the output is then fed into the counter  120 . The high input of the counter  120  increments the value stored in the counter  120  by one, which enables one more finger  140 B of the driver  135 . The counter  120  registers the number of PMOS transistor fingers enabled in the driver  135 . Enabling the finger  140 B decreases the impedance of the driver  135  because the enabled fingers  140 A-B are impedances connected in parallel, which reduces the equivalent impedance. The decreased impedance across the driver  135  results in a decreased voltage across it. Because the driver  135  and the external precision resistor  150  form a voltage divider, the decreased voltage across the driver  135  increases the voltage across the external precision resistor  150  towards V TT . 
     In one example, the supply voltage is 1.8 V, the counter  120  has four bits that enable 16 of the fingers  140 , with a minimum of five of the fingers  140  enabled, where each finger  140  is set at 500 ohms, and the external precision resistor  150  is set at 38 ohms. In this example, when the counter  120  enables five of the fingers  140 , the equivalent resistance is 100 ohms and the voltage across the external precision resistor  150  is 0.5 V. When ten of the fingers  140  are enabled, the equivalent resistance drops to 50 ohms and the voltage across the external precision resistor  150  increases to 0.78 V. In this example, the voltage across the external precision resistor  150  reaches half of the supply voltage when 14 of the fingers  140  are enabled, where the equivalent resistance is 36 ohms. As shown in this example, as more fingers  140  are enabled, the equivalent resistance of the fingers  140  decreases and the voltage across the external precision resistor  150  increases towards half of the supply voltage. The rate of decrease of the equivalent resistance of the fingers  140  is progressively less as more fingers are enabled (e.g., 150 ohms for five fingers, 50 ohms for 10 fingers, and 33 ohms for 15 fingers). 
     The resistance of each of the fingers  140  (e.g., 500 ohms in the example above) has a minimum, maximum, and typical value to allow the voltage across the external precision resistor  150  to reach half of the supply voltage. In this example, the minimum resistance is 190 ohms and the maximum resistance is 760 ohms. The typical resistance is selected far enough away from the minimum and maximum resistance values (e.g., 380 ohms), because the transistors of the driver  135  vary with process (e.g., channel length, threshold voltages, etc.), voltage (e.g., 1.7 V to 1.9 V), and temperature (e.g., −40° C. to +125° C.). If each of the fingers  140  is set to 190 ohms, 380 ohms, or 760 ohms, respectively, the voltage across the external precision resistor  150  reaches half of the supply voltage when five, ten, or  20  fingers, respectively, are enabled. 
     The comparator  130  compares the voltage V TT  (i.e., ½ of V DDQ ) in the positive input of the compactor  130 , with the voltage across the external precision resistor  150  at the negative input of the comparator  130 . The fingers  140 A-N are interactively enabled, lowering the impedance of the driver  135  towards the impedance of the external precision resistor  150 . Each time the impedance of the driver  135  is greater than the impedance of the external precision resistor  150 , one more finger of the driver  135  is enabled, thereby decreasing the impedance of the driver  135 . This iterative calibration process continues until the difference between the input terminals of the comparator  130  is approximately zero. At this point, the voltage drop across the external precision resistor  150  equals the voltage drop across the driver  135 , both voltage drops being V TT . The current flows directly through the driver  135  and the external precision resistor  150 . Accordingly, the impedance of the driver  135  matches that of the external precision resistor  150 . The counter  120  stops when matching impedances of the driver  135  and external precision resistor  150  are found, such that the optimum number of enabled fingers  140 A-N of the driver  135  is registered in the counter  120 . 
     In one embodiment, the impedance of the driver  135  is approximately 38 ohms, 18 ohms from the transistor and 20 ohms from the in series on chip resistor, matching 38 ohms of the external precision resistor  150 . The voltage across the driver  135  and the external precision resistor  150  are both V TT . In another embodiment, the impedance of the PMOS transistor is another value less than 21 ohms. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes these and other disadvantages of the prior art by employing a ratio of the voltage drop less than half of the supply voltage across an output impedance driver, which in turn significantly reduces the chip size of a metal-oxide-semiconductor (MOS) field-effect transistor, which serves as a driver for output impedance drivers, such as, but not limited to, double data rate (DDR2) synchronous dynamic random access memory (SDRAM). In an embodiment of the invention, a voltage drop across the driver is a decreased ratio of the supply voltage, e.g., three-tenths of the supply voltage, lower than half of the supply voltage. A smaller voltage drop allows a lower current and a higher impedance to be used in the driver. By having a higher impedance across the driver, the physical area needed for the DDR2 driver is reduced because a transistor with a smaller physical width can be used. 
     In an embodiment of the invention, a device comprises: a reference voltage set to be a supply voltage or half of the supply voltage; one or more external resistors connected in series with the reference voltage, the one or more external resistors having one or more impedance values; and a driver connected in series with the external resistors, the driver having an equivalent impedance value and a voltage drop across the driver, wherein a first value for the one or more impedance values and a second value for the equivalent impedance value are calculated with the voltage drop being a voltage drop of half of the supply voltage, wherein the one or more impedance values are set larger than the first value and the equivalent impedance value is set larger than the second value with the voltage drop being less than half of the supply voltage. The device may further comprise a minimum current corresponding to the voltage drop of half of the supply voltage, wherein a current through the driver is less than the minimum current. The one or more impedance values are set four times larger than the first value and the equivalent impedance value is set four times larger than the second value. The driver may comprise two impedances such as an in series on-chip impedance and a transistor impedance. The transistor impedance is calibrated iteratively based on the in series on-chip impedance and the one or more external resistors. The reference voltage is the supply voltage, the one or more external resistors comprise two resistors. The two resistors connect at a termination node, wherein the termination node is tied to a comparator for calibrating the equivalent impedance value. The voltage drop may be determined by a JEDEC SSTL — 18 standard. 
     In another embodiment of the invention, a method comprises: setting a reference voltage to be a supply voltage or half of the supply voltage; using one or more external resistors connected in series with the reference voltage, the one or more external resistors having one or more impedance values; using a driver connected in series with the external resistors, the driver having an equivalent impedance value and a voltage drop across the driver; calculating a first value for the one or more impedance values and a second value for the equivalent impedance value with the voltage drop being a voltage drop of half of the supply voltage; and setting the one or more impedance values larger than the first value and the equivalent impedance value larger than the second value with the voltage drop being less than half of the supply voltage. The method may further comprise a minimum current corresponding to the voltage drop of half of the supply voltage, wherein a current through the driver is less than the minimum current. The impedance values are set four times larger than the first value and the equivalent impedance value is set four times larger than the second value. The driver may comprise two impedances such as an in series on-chip impedance and a transistor impedance. The transistor impedance is calibrated iteratively based on the in series on-chip impedance and the one or more external resistors. The reference voltage is the supply voltage, the one or more external resistors comprise two resistors. The two resistors connect at a termination node, wherein the termination node is tied to a comparator for calibrating the equivalent impedance value. The voltage drop may be determined by a JEDEC SSTL — 18 standard. 
     The present invention provides several advantages. First, the output driver impedance is calibrated under a voltage condition that is closer to the actual voltage across the driver at the time of functional switching. Second, a more optimal PMOS (or NMOS) transistor size can be selected. When the voltage drop across the PMOS (or NMOS) transistor is overestimated, a transistor with more fingers needs to be selected, at the cost of a larger size, in order to provide a range of impedances for the transistor which cover the impedance at the calibration point. When the impedance at the functional switching voltage is implemented, the size of the transistor can be made smaller to achieve the same desired output impedance during switching. In the present invention, the impedance is actually optimized at the operational point, enabling the SSTL — 18 DDR2 specification to be met with a smaller PMOS (or NMOS) transistor. In principle it has been found that due to this type of calibration, the output driver can actually reduce the size of the driver. Post layout area savings for the driver have been recorded at least between 20% to 30%, although larger and small area savings are possible as well. 
     The foregoing, and other features and advantages of the invention, will be apparent from the following, more particular description of the preferred embodiments of the invention, the accompanying drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the background of the present invention, reference is now made to the ensuing descriptions taken in connection with the accompanying drawings briefly described as follows: 
         FIG. 1  illustrates a conventional output impedance control circuit that compares a driver with an external precision resistor; 
         FIG. 2  illustrates a conventional DDR2 driver circuit; 
         FIG. 3  illustrates a simplified equivalent circuit of the conventional DDR2 driver circuit illustrated in  FIG. 2 ; and 
         FIG. 4  illustrates a DDR2 driver circuit according to an exemplary embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying  FIGS. 2-4 , wherein like reference numerals refer to like elements. Although the invention is described in the context of a DDR2 driver application, one of ordinary skill in the art readily appreciates that the present invention can be implemented in other high-speed devices such as, but not limited to peripheral component interconnect express (PCI express or PCIe) expansion cards and DDR3 memory modules. 
     In general, the present invention provides an output impedance control circuit for an SSTL — 18 electrical interface employing a lower reference voltage across the driver. However, if a lower reference voltage is used (e.g., 0.3 V DDQ ), i.e., a voltage lower than half of V DDQ , the impedance of the transistor cannot be compared directly to the desired impedance of an external resistor. When the voltage across the external precision resistor reaches half of the supply voltage due to the iteratively smaller voltage drop across the driver, the voltage across the driver is still proportional to the supply voltage, but at a lower ratio. 
     In an exemplary embodiment of the invention, the impedance calibration of a DDR2 driver may be performed with external resistors of approximately 250 ohms and 100 ohms with the driver impedance of approximately 150 ohms (approximately 70 ohms plus 80 ohms). The impedance of the driver is proportional to the supply voltage (e.g., 1.8 volts), such that a voltage drop (e.g., 0.9 volts), compared to the actual voltage drop (e.g., 0.6 volts), leads to an overestimated minimum current (e.g., 13.4 mA). By lowering (e.g., relaxing) the minimum current below the overestimated minimum current (e.g., less than 13.4 mA), the impedance of the DDR2 driver can be calibrated with less fingers, allowing a smaller transistor to be selected. 
       FIG. 2  illustrates a conventional DDR2 driver circuit  200  (e.g., an output buffer, a typical DDR2 driver circuit, etc.) under the SSTL — 18 standard. See JEDEC Solid State Technology Association, “Stub Series Terminated Logic for 1.8 V (SSTL — 18),” JEDEC, p. 7 (2003), the disclosure of which is incorporated by reference herein in its entirety. The DDR2 driver circuit  200  comprises an external termination resistor  210  (e.g., an equivalent termination resistance, a termination resistor, etc.) with a resistance of 25 ohms, an in series on-chip resistor  220  (e.g., a stub series resistor, an off-chip resistor, etc.) with a resistance of 20 ohms. A NMOS transistor  230  and a PMOS transistor  240  are both shown, as either one of which can be calibrated, although not both at the same time.  FIG. 2  illustrates only one of the multiple DDR2 fingers for simplicity. Additionally, a counter, such as counter  120 , coupled to the gates of the NMOS transistor  230  and the PMOS transistor  240 , is also not shown for simplicity. In one embodiment, the external termination resistor  210  and the in series on-chip resistor  220  are set such that the transistor impedance is calibrated to 18±3 ohms. 
     The specifications of the SSTL — 18 standard set the external termination resistor  210  to be 25 ohms to achieve the minimum current. The external termination resistor  210  terminates to V TT , which is half the supply voltage, V DDQ . The SSTL — 18 standard sets V DDQ  at 1.8 volts and thus, V TT  is 0.9 volts. V SS  is connected to ground. A further simplified and equivalent DDR2 driver application substitutes either one of the PMOS transistor  240  or the NMOS transistor  230 , although not both at the same time, with a single impedance, for calibration calculation purposes, with a transistor impedance  330  representing the NMOS transistor  230  as illustrated in  FIG. 3 . 
       FIG. 3  illustrates a DDR2 driver circuit  300  (e.g., an equivalent impedance circuit, an equivalent DDR2 driver circuit, a DDR2 driver application, an output buffer, etc.) simplified with the equivalent impedance of the DDR2 driver circuit  200  depicted in  FIG. 2 . The purpose of the DDR2 driver circuit  300  is to present a simplified, equivalent circuit of the DDR2 driver circuit  200  with the transistor impedance  330  of  FIG. 3  representing the NMOS transistor  230  of  FIG. 2 , which makes it easier to observe the voltage divider apparent between an external termination resistor  310  and an equivalent impedance value of driver  335  (e.g., a memory driver). 
     The DDR2 driver circuit  300  comprises the external termination resistor  310  (e.g., an equivalent termination resistance, a termination resistor, etc.) with a resistance of 25 ohms, an in series on-chip resistor  320  (e.g., a stub series resistor, an off-chip resistor, etc.) with a resistance of 20 ohms and a transistor impedance  330  with an impedance of 18 ohms, collectively the equivalent impedance value of driver  335  is 38 ohms. The in series on-chip resistor  320  and transistor impedance  330  represent one or more fingers of the DDR2 circuit  300 . The counter and comparator have large impedances, which do not affect the DDR2 driver circuit  300  and as such are not shown. 
     According to SSTL — 18 standard specifications, the external termination resistor  310  is set to 25 ohms and is terminated to V TT , which is half the supply voltage V DDQ . SSTL — 18 standards set V DDQ  at 1.8 volts, and thus, V TT  is 0.9 volts. The in series on-chip resistor  320  has an impedance of 20 ohms. The SSTL — 18 standard requires a minimum 13.4 mA of current flowing through the external termination resistor  310  and the equivalent impedance value of driver  335  of the DDR2 driver circuit  300 . 
     The equivalent impedance value of the driver  335  is the sum of the impedances of the in series on-chip resistor  320  and the transistor impedance  330 , which is (20+18) ohms, or 38 ohms. The voltage divider between the equivalent impedance value of driver  335  and the external termination resistor  310  yields ½ V DDQ ×(38)/(38+25), which is approximately 0.3 V DDQ  across the driver. Therefore, a correct impedance calibration scheme for the output driver would yield 0.3 V DDQ  (rather than 0.5 V DDQ ) across the equivalent impedance value of driver  335 . 
       FIG. 4  illustrates a DDR2 driver circuit  400  for an output impedance driver according to an exemplary embodiment of the invention. The DDR2 driver circuit  400  comprises an external precision resistor  450  with a resistance of 250 ohms connected to a termination node, an external termination resistor  410  (e.g., an equivalent termination resistance, a termination resistor, etc.) with a resistance of 100 ohms connected to the termination node, and an equivalent impedance value of driver  435  (e.g., a memory driver with a minimum, maximum, and typical resistance value) calibrated to 152 ohms, rounded to 150 ohms, comprising an in series on-chip resistor  420  (e.g., a stub series resistor, an off-chip resistor, etc.) with a resistance of 80 ohms and a transistor impedance  430  with an impedance of 72 ohms. The in series on-chip resistor  420  and transistor impedance  430  represent the driver of the DDR2 circuit  400 . Although the transistor impedance  430  is illustrated as representing an NMOS transistor, it can instead represent a PMOS transistor. The counter and comparator have large impedances which do not affect the DDR2 driver circuit  400  and as such are not shown. 
     Based on the configuration shown in  FIG. 4 , the physical width (i.e. the cross-section) of components for calibrating the DDR2 driver circuit  400  can be selected with a smaller size than the DDR2 driver circuit  200  and the DDR2 driver circuit  300 . For a transistor, resistance is inversely proportional with the physical width (i.e., a large resistance requires a less transistor fingers and as such a small transistor can be selected). 
     The external precision resistor  450 , the external termination resistor  410 , and the equivalent impedance value of driver  435  can be multiplied by a common factor to increase resistance, and this increased resistance lowers the current used during calibration without affecting the voltage driving the driver  435 . Selecting the equivalent impedance value of driver  435  to be a factor of four larger reduces the current flow during calibration and allows a smaller driver to be selected. In one embodiment, the impedance is multiplied by a factor of four. In another embodiment, the impedance is multiplied by a factor other than four, including any real number greater than one (e.g., 1.7, pi, 5, 8.6, etc.). Post layout area savings for the driver  435  have been recorded at least between 20% to 30%, although larger and small area savings are possible as well. Accordingly, the 25 ohm external termination resistor  310  is increased to become the 100 ohm external termination resistor  410 . Additionally, the equivalent impedance value of driver  335  (comprising the 20 ohm in series on-chip resistor  320  and the 18 ohm transistor impedance  330 ) is increased from 38 ohms to 152 ohms (or 150 ohms for simplification), embodied in  FIG. 4  as the equivalent impedance value of driver  435  (comprising the 80 ohm in series on-chip resistor  420  and the 72 ohm transistor impedance  430 ). The DDR2 driver circuit  400  also embodies the external precision resistor  450  with a resistance of 250 ohms. 
     A voltage V DDQ  is applied to the external precision resistor  450 , and a voltage divider is formed with the external precision resistor  450  (250 ohms) and the external termination resistor  410  (100 ohms) on one hand, and the equivalent impedance value of driver  435  (150 ohms for simplification) on the other. This voltage divider yields a voltage of V DDQ ×(150)/(250+100+150) or 0.3 V DDQ  across the equivalent impedance value of driver  435 . The voltage 0.3 V DDQ  is the desired voltage across the equivalent impedance value of driver  435 . The desired voltage 0.3 V DDQ  is demonstrated in  FIG. 3  across the equivalent impedance value of driver  335  (comprising the in series on-chip resistor  320  and the transistor impedance  330 ). Therefore, the desired voltage 0.3 V DDQ  is achieved while the components calibrating the DDR2 driver circuit  400  are reduced in size. 
     This invention has been described herein using specific embodiments for the purposes of illustration only. It will be readily apparent to one of ordinary skill in the art, however, that the principles of the invention can be embodied in other ways. Therefore, the invention should not be regarded as being limited in scope to the specific embodiments disclosed herein, but instead as being fully commensurate in scope with the following claims.