Patent Publication Number: US-6218822-B1

Title: CMOS voltage reference with post-assembly curvature trim

Description:
The present invention is related to U.S. patent application Ser. No. 09/416,899, entitled “CMOS VOLTAGE REFERENCE WITH A NULLING AMPLIFIER” filed Oct. 13, 1999; U.S. patent application Ser. No. 09/416,896, entitled “SLOPE AND LEVEL TRIM DAC FOR VOLTAGE REFERENCE” filed Oct. 13, 1999; and U.S. patent application Ser. No. 09/416,898, entitled “LOW DROPOUT VOLTAGE REFERENCE” filed Oct. 13, 1999; all applications are commonly assigned to the assignee of the present invention, and the disclosures of which are herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the field of CMOS voltage references, and more particularly to an apparatus and method for providing post-assembly curvature trim. 
     2. Description of the Related Art 
     Using a CMOS process to make a voltage reference has cost advantages over a precision-trimmed bipolar process. Problems with the accuracy and stability of CMOS devices must be overcome, however, in order to make a CMOS reference competitive in performance with bipolar references. Specifically, the lack of high-value stable and trimmable resistors presents a problem for circuit designers. 
     In order to adjust for variances in each circuit, voltage references are “trimmed” after manufacture in order to bring the output values within a specified range. This is generally accomplished by using lasers to etch away certain thin-film resistors (thereby increasing the resistance by decreasing the cross-sectional area). With proper design, most devices can be brought within the specified range using this technique. However, once the device (i.e. silicon die) is placed into a package, the mechanical stresses caused by the packaging can once again cause the circuit parameters to vary. Therefore, a competitive CMOS voltage reference must be designed such that the circuit may be “trimmed” after the final assembly of the die into a package. 
     One aspect of a voltage reference design requiring special consideration is “curvature correction.” Curvature correction has been the subject of many papers and patents over the past two decades. One of the earliest circuits is disclosed in U.S. Pat. No. 4,250,445 entitled “BAND-GAP VOLTAGE REFERENCE WITH CURVATURE CORRECTION” by Brokaw. As shown in FIG. 1, the disclosed circuit uses a diffused resistor Rb to add a low-order (temperature squared or T 2 ) correction term to the reference output. The T 2  term is generated not by the resistor Rb itself (which is assumed to be linear) but by the combination of a positive temperature coefficient (TC) in the resistor Rb and the positive TC in the current forced through the resistor. The operation of the band-gap cell forces the current in all resistors to be “proportional to absolute temperature” (PTAT). As long as Rb has a TC more positive that the other resistors in the circuit, the voltage across the resistor will have a T 2  term. Thus, the Brokaw curvature correction technique is only a second order (T 2 ) correction, whereas real band-gap circuits have a significant amount of higher order curvature. Also, there is no mechanism to easily trim the curvature, once the device is packaged. 
     Efforts to improve on the Brokaw technique have generally used sophisticated circuits to generate higher order correction terms, often attempting to match the theoretical “TlnT” characteristic of an “ideal” band-gap reference. Two such techniques are disclosed in U.S. Pat. No. 5,352,973 entitled “TEMPERATURE COMPENSATION BANDGAP VOLTAGE REFERENCE AND METHOD” and U.S. Pat. No. 5,519,308 entitled “ZERO-CURVATURE BANDGAP REFERENCE CELL.” These circuits generally require components not available in a low-cost CMOS process, however. Also, as discussed below, real voltage references deviate significantly from the ideal “TlnT” characteristic. 
     Thus, it would be desirable to have an improved curvature trim technique, suitable for use with CMOS voltage references and providing post-assembly trim. 
     SUMMARY OF THE INVENTION 
     The present invention is an apparatus and method for performing curvature trim in a voltage reference circuit that allows a curvature error to be trimmed even after the circuit has been packaged. In one embodiment, the curvature trim is performed by connecting one or more non-linear resistors, such as diffused lightly-doped drain (LDD) resistors, to a band-gap reference. The curvature characteristics of the non-linear resistor is such that the negative curvature error associated with a band-gap reference is cancelled. In fact, this curvature correction is better than that of an ideal “TlnT” corrector. In another embodiment, a set of series non-linear resistors may be selected via selection bits stored in an EEPROM (or other similar non-volatile memory). Since various combinations of the resistors may be selected by programming the EEPROM, the curvature of a band-gap reference can be adjusted after final packaging. This curvature correction method achieves a reliable and accurate correction for the curvature variations that occur with process changes. 
     In yet another alternative embodiment, one or more resistors associated with the band-gap core may be formed using non-linear resistors, such as diffused LDD resistors. As in the other embodiments, the curvature of the diffused resistors compensates for the normal band-gap curvature, and combinations of resistors may be selected via a programmable non-volatile memory, even after the circuit has been packaged. 
     Once the voltage reference is packaged, the voltage reference may be calibrated by programming the non-volatile memory. Thus, the present invention provides an improved curvature trim technique, suitable for use with CMOS voltage references and providing post-assembly trim. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be readily understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which: 
     FIG. 1 is a schematic diagram of a prior art band-gap reference incorporating curvature correction; 
     FIG. 2 is a block diagram of a CMOS voltage reference incorporating the present invention; 
     FIG. 3 is a graph of output voltage vs. temperature for actual data from band-gap cores; 
     FIG. 4 is a graph of the data of FIG. 3, after the slope has been trimmed; 
     FIG. 5 is a graph of the data of FIG. 3, after both the slope and level have been trimmed; 
     FIG. 6 is a graph of the residual error vs. polynomial order, in other words, the root-mean-square deviation of the data points from a best fitting polynomial; 
     FIG. 7 is a graph of various curvature correction schemes and the average of the experimental data; 
     FIG. 8 is a graph of the resistance vs. temperature for an n-type lightly-doped drain (LDD) resistor according to the present invention; and 
     FIG. 9 is a graph of the data of FIG. 7, showing the differences between the experimental data and various correction curves. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is provided to enable any person skilled in the art to make and use the invention and sets forth the best modes contemplated by the inventor for carrying out the invention. Various modifications, however, will remain readily apparent to those skilled in the art, since the basic principles of the present invention have been defined herein specifically to provide an apparatus and method for providing post-assembly curvature trim. 
     A CMOS voltage reference  10  incorporating the present invention is shown in FIG.  2 . The voltage reference comprises a band-gap core  12 , connected to a primary amplifier  18 , an output FET M 11 , and a null amplifier  20 . The circuit further comprises a slope trim DAC  14  and a level trim DAC  16  for adjusting the slope and level of the output V REF . A level select R 4 A selects one of the available output voltage options, for example, the circuit can be designed to output three different V REF  values. Finally, the curvature trim DAC R 4 B is shown as a potentiometer to illustrate that it has a variable resistance, but as described below, it actually consists of a network of non-linear resistors that can be controlled by setting a non-volatile memory. In fact, the slope, level and curvature trims can be performed after final packaging via the non-volatile memory. The CMOS voltage reference of FIG. 2 provides a precision voltage reference that can be manufactured in a standard CMOS process and trimmed after final assembly. 
     According to the present invention, curvature trim may be performed by switching in segments of a non-linear resistor, such as an n-type lightly doped drain (LDD) diffused resistor, having a curvature characteristic that is opposite to the normal band-gap curvature. Specifically, in FIG. 2, R 4 B is a set of series LDD resistors, which may be selected via selection bits stored in an EEPROM (or other similar non-volatile memory, not shown). Polysilicon fuses may also be used to select the appropriate combination of resistors. Since various combinations of the resistors may be selected by programming the EEPROM, (or burning the fuses) the curvature of a band-gap reference can be adjusted after final packaging. This curvature correction method achieves a reliable and accurate correction for the curvature variations of a low-cost CMOS voltage reference. 
     In FIG. 3, plots of the actual output characteristics for several untrimmed voltage references (units) are shown. These measurements were taken on “core-cell” circuits similar to that of FIG. 2, but without the level and slope trim, or any curvature correction (R 4 A=R 4 B=0). As shown in FIG. 3, the major variation appears to be a “pivoting” of the curves about T=−273 (absolute zero). These variations come primarily from mismatches in the core-cell transistors Q 11 , Q 21  and offsets in the primary amplifier. Also, there is a downward or “negative” curvature in each of these units. This curvature comes partly from the inherent behavior of the bipolar transistors and partly from “aberrations”—or non-fundamental deviations in the behavior of real circuits, compared to the theoretical ideal. Some of these aberrations are predictable, at least experimentally, by making measurements on a particular process, but some are erratic, and cannot be corrected no matter how sophisticated the correction circuit. An example would be hysteresis, i.e. the variation in repeated measurements at the same temperature due to prior changes in temperature. 
     From the experimental data on the CMOS process used (a low-cost, 0.72 μm, CMOS process) it can be observed that corrections beyond the third-order are futile. The aberrations are such that no further reduction in error is achievable with higher order corrections. In fact, the ideal theoretical “T ln(T)” correction model is not as good as the present third-order curvature correction solution. The “skew” in the curvature is actually larger than theory predicts, and the present invention accommodates that extra skew. It can also be shown that the residual errors, after trimming out third-order curvature, are much smaller than other factors that limit the accuracy of commercial CMOS voltage references. 
     The following background discussion of band-gap theory is presented, in order to explain the operation of the present curvature correction technique. As shown in FIG. 2, a band-gap core comprises a pair of bipolar transistors Q 11 , Q 21  which generate a voltage proportional to absolute temperature (PTAT). A network of resistors connected to these transistors Q 11 , Q 21  are arranged to multiply the PTAT voltage and add it to the base-emitter of one of the transistors so that the total voltage is constant over temperature. A more thorough derivation is presented in Gray &amp; Meyer,  Analysis and Design of Analog Integrated Circuits,  “Band-Gap-Referenced Biasing Circuits,” section A4.3.2,3 rd  ed. (Wiley, 1993), the standard textbook in this field. In U.S. Pat. No. 5,519,308 (Gilbert), the textbook equations are presented in a more readable form, which is what is presented here. 
     The base-emitter voltage of a bipolar transistor can be written as 
     
       
           V   BE ( T ) =V   T   ln ( I   C   /A   J   J   S ( T ))  (1)  
       
     
     where 
     
       
           V   T   =kT/q  thermal voltage  (2)  
       
       
         
           
             and 
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     The temperature-dependent factors in this equation can be further expanded as 
     
       
           J   s   =qn   i   2   Tμ/Q   B   (3)  
       
     
     
       
           n   i   2 ( T )= c   1   T   3  exp(− V   G0   /V   T ) intrinsic carrier density   (4)  
       
     
     
       
         μ( T )= c   2   T   −n  base carrier mobility  (5)  
       
     
     where V G0  is the band-gap voltage, extrapolated to 0° K, and the other constants are independent of temperature and will cancel out in the final formula. 
     Combining these equations, and lumping all the constants into c 3 , gives an explicit function of T 
     
       
           V   BE ( T )= V   G0   +V   T   ln ( I   c   T   −γ   c   3 )  (6)  
       
     
     where 
     
       
         γ=4 −n  temperature exponent of current density  (7)  
       
     
     Equation 6 can be put in a more useful form by defining a reference temperature T R , typically 25° C., and a temperature factor H.                  V   BE          (   T   )       =       V   G0     -     H        (       V   G0     -     V   BER       )       +       HV   TR          ln        [       (       I   C       I   CR       )          H     -   γ         ]                   (   8   )               Where                   {           H   =     T   /     T   R               T                 in                 degrees                 Kelvin                 V   BER     =       V   BE          (     T   R     )                                 V   TR     =       kT   R     /   q                               I   CR     =       I   C          (     T   R     )                                     (   9   )                         
     Further simplification can be made if we assume I C  can be modeled as a simple exponential. Then 
     
       
           I   C   /I   CR   =H   α   (10)  
       
     
     
       
           V   BE ( T )= V   G0   −H ( V   G0   −V   BER )−(γ−α) V   TR   Hln ( H )  (11)  
       
     
     The band-gap circuit is designed to add to this V BE  a PTAT voltage 
     
       
           V   REF ( T ) =V   BE ( H ) +KH   (12)  
       
     
     Adjusting the constant K to give zero slope at H=1, gives the final result 
     
       
           K= ( V   G0   −V   BER )+(γ−α) V   TR   (13)  
       
     
     
       
           V   REF ( H )= V   G0 +(γ−α) V   TR   H (1− lnH )  (14)  
       
     
     Typical values are V G0 =1.205,γ=3.2, and α=1 (from Gray &amp; Meyer). With these values and V TR =25.7mV at 298° K, 
     
       
           V   REF ( T )=1.205+0.0565 H (1− lnH )=1.262 at  H= 1  (15)  
       
     
     From these equations, the following observations and conclusions can be made. The entire temperature dependence of equation 14 is contained in the simple function H(1−lnH), where H is the absolute temperature, normalized to the reference temperature 25° C. This function has a maximum value 1 at H=1, and drops 3% on either side over the range −44 to +102° C. This slight asymmetry compared to a parabola has been the focus of much effort over the last twenty years as designers try to improve on the simple parabolic correction developed by Brokaw, as shown in FIG.  1 . 
     The accuracy of the TlnT theory depends on assumptions that the constants V G0 , γ and α, do not vary with temperature. This is clearly not true for real transistors. The constant γ, for example, which represents the temperature exponent of collector current, includes the temperature exponent of mobility, which can vary from −2.42 in lightly-doped n-type silicon at high temperature, to +0.6 heavily-doped p-type silicon at low temperature. According to equation 16, a variation of 0.1 in this parameter causes a variation of 2.6 mV in the reference voltage. Given this variation, it is rather amazing that band-gaps can be trimmed to better than 1 mV. The temperature variation of γ is larger for the more heavily-doped materials, which leads to the expectation that real band-gap circuits will become ever more deviant from the ideal as the industry moves to CMOS processes designed for ever smaller digital circuits. 
     In order to further evaluate the real data shown in FIG. 3, the slope error is trimmed by adjusting the slope-trim DAC in FIG. 2 for each voltage reference. The slope and level trim may be performed as disclosed in related U.S. patent application Ser. No. 09/416,896, entitled “SLOPE AND LEVEL TRIM DAC FOR VOLTAGE REFERENCE” filed Oct. 13, 1999. The teachings of the present invention, however, may be applied to voltage reference circuits using any slope and level trim techniques known to those skilled in the art. As a result of the slope trim, a much tighter curve distribution is produced as shown in FIG.  4 . The spread at room temperature has now been reduced from 30 mV (3%) to 7 mV (0.6%). What remains is the variation in the so-called “magic voltage” (the voltage which makes the reference “flat” at room temperature). It is clear that this variation cannot be accounted for by variations in γ or α. These constants affect the curvature, which appears to be fairly uniform in FIG.  4 . 
     After trimming out the level variations of the curves in FIG. 4, a residual curvature remains as shown in FIG.  5 . From FIG. 5, the following observations can be made: 
     1. The curvature is nearly parabolic, with the major variation being the height of the parabola (1.5 mV to 1.9 mV for this process). 
     2. The parabola has a little “skew” to the left, suggesting that there might be some benefit to including a small amount of third-order correction. 
     3. There are some “dents” in the curves, representing aberrations on the order of 50 μV that may be impossible to correct. 
     Next, an optimum polynomial to fit each voltage reference was determined, and a residual error as a function of the polynomial order was computed. The results in FIG. 6 show that there is some benefit for most references in adding a small amount of third-order correction, but any higher-order corrections make no difference. The minimum error is 20 to 40 μV, and this minimum is achieved with a third-order correction. 
     The errors shown in FIG. 6 are much smaller than other factors that limit the precision of commercial voltage references. A 1.25 V reference that must meet a specification of 10 ppm/° C., for example, can have a 2 mV variation over a 150° C. range. So, as a practical matter, there is no need to waste trim bits on the third-order coefficient in a real curvature corrector. According to the present invention, the circuit trims the second-order coefficient, but just uses a fixed value for the third-order coefficient. 
     FIG. 7 shows the final result of various trim methods on the experimental data. The diamonds show the average of experimental data for seven voltage references. The R 2 (T) curve is a parabolic correction, and the R 3 (T) curve shows a correction using a diffused resistor made from the same implant as used for the “lightly-doped drain” (LDD) extensions of NMOS transistors in certain CMOS processes. Measurements on eight of these LDD resistors produce the following characteristics: 
     
       
           R ( T )= R   0 *(1+ C   T1   ΔT+C   T2   ΔT   2   +C   T3   ΔT   3 )  (16)  
       
       
         
           
             
               
                 
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     As shown in FIG. 8, an LDD-type resistor has a “positive” curvature characteristic. When this resistor is added to the circuit of FIG. 2 as R 4 B, the positive curvature generated by C T2  will cancel the negative curvature of the core cell. By trimming the value of this resistor R 4 B, the total second-order curvature can be adjusted to zero. For example, assuming 10 μA flows through resistors R 3 B and R 4 A, and R 4 B is trimmed to 2.43 K, the correction will be 
     
       
         Δ V= 24.3 mV *(1+ C   T1   ΔT+C   T2   ΔT   2   +C   T3   ΔT   3 )  (18)  
       
     
     Taking out an equal amount of resistance from R 4 A offsets the constant term. The difference between the slope term and the resistance removed from R 4 A is taken out by the slope-trim DAC. The curvature term generates a lift of 1.75 mV at −50 and +100° C., just what is needed to offset the curvature of FIG.  5 . In fact, the third-order term generates a small amount of “skew” which nearly matches the skew of FIG.  5 . FIG. 9 shows the averaged data of FIG. 7 after application of various curvature correction techniques. R 2 (T) is an ideal parabolic corrector, R 3 (T) is the third-order correction generated by the non-linear resistor network of the present invention. The T( 1 −lnT) curve is the theoretical ideal correction curve. Notice that the R 3 (T) provides a better correction than the “ideal” curve. 
     Since R 4 B must be trimmed to adjust C T2 , the skew correction is, in effect, determined by the ratio C T3 /C T2 . From FIG. 7, this correction appears to be just a little more than needed to fit the data, but it actually gives a better fit than the ideal TlnT corrector. In other words, the present invention provides a better curvature correction for real circuits, than a theoretical “ideal” corrector. If even better correction is ever needed, or if a given CMOS process does not provide LDD resistors with an appropriate amount of skew, the ratio C T3 /C T2  can be modified by making a network with small sections of other resistors. The n+ resistor in a standard CMOS process, for example, has a strong negative curvature and a different ratio C T3 /C T2 . 
     The curvature-trim DAC must adjust for both the variations seen in FIG. 5 (+/−10%), and the variations due to ratio error between the diffused resistor R 4 B and the other resistors R 3 A, R 3 B and R 4 A, which are polysilicon. This ratio error can be as large as +/−20% in a current CMOS process. A 4-bit curvature DAC covering a range of +/−30% should allow adjustment to within +/−2% (or +/−40 μV) which is near the minimum residual error shown in FIG.  5 . Trimming only the resistor ratio error, a procedure requiring no temperature cycling, will reduce the error to just +/−200 μV, which is good enough for most commercial references. 
     In the Brokaw solution, a diffused resistor Rb is used, but it is assumed that the resistor is linear with temperature and has a positive TC. The current through the cell also has a positive TC, and the current times the resistance gives a quadratic coefficient that is used to compensate for the curvature. The Brokaw circuit, however, does not use the curvature of the resistor itself to provide the curvature correction. In contrast, the present invention relies entirely on the quadratic and higher order terms in the resistor R 4 B itself, and assumes the current is constant, since V REF  is constant. Resistor R 4 B has a non-linear temperature characteristic chosen to match the second and third-order curvature of the band-gap core. The embodiments disclosed herein use LDD-type resistors, but any network of non-linear resistors having similar temperature curvature can be used without departing from the scope of the present invention. 
     In one implementation of the present invention, a voltage reference as shown in FIG. 2 can be constructed for producing three different voltage options: 2.048 V, 2.5 V and 4.096 V. The desired output voltage V REF  is selected by switching in one of three resistors used to form the level-select R 4 A. In order to provide the necessary curvature correction for each of these voltage options, R 4 A contains some additional segments of LDD resistance. R 4 B consists of a series of 16 LDD resistors controlled by a four-bit code. The curvature-trim DAC, together with the level-select bits, thus select an appropriate combination of resistors, in order to adjust the curvature. 
     For example, the following typical resistor values are used for each output voltage range: 
     
       
         V REF =2.048 V 0K+6.4K+[N×800]Ω 
       
     
      V REF =2.5 V 2.8K+6.4K+[N×800]Ω 
     
       
         V REF =4.096 V 13K+6.4K+[n×800]Ω 
       
     
     where n is selected to provide the appropriate trim for each different voltage range, and each voltage reference during calibration. The values of “n” are stored in an EEPROM after final packaging and calibration, thus providing a technique to trim the curvature of a voltage reference after the final packaging has occurred. 
     Note that in the embodiments discussed above, the resistors comprising R 4 B are diffused, and the other resistors are polysilicon. The poly resistors track across temperature, whereas the diffused R 4 B resistors have positive curvature that overcomes the normal negative effect of the band-gap curvature. This same technique may be advantageously applied to other resistors in the circuit as well. For example, sections of the core resistors R 2 A, R 2 B, and R 2 C could be LDD-type resistors. The positive curvatures could be adjusted to overcome the negative curvature of the band-gap. In this case, however, the switches for selecting the appropriate resistance values would probably need to be p-channel devices to overcome the higher gate voltages at this position in the circuit. The curvature correction could be performed entirely by R 2 A, for example, or a combination of the R 2  resistors and R 4 B. This embodiment has advantages for low-voltage references in which the “lift” in V REF  from R 4 B is undesirable. 
     Another solution to correct the negative band-gap curvature, while avoiding lift caused by R 4 B is to add negative curvature to R 1 V. The negative curvature in R 1 V actually adds positive curvature to the output, since the ratio of R 2 /R 1 V controls the output. Again, the disadvantage of this approach is that it requires a more complicated switching structure (complementary switches in this case). The key point is that the present invention provides trimmable curvature correction by using one or more non-linear resistors to generate curvature opposite to that of the normal band-gap curvature in the output. 
     Those skilled in the art will appreciate that various adaptations and modifications of the just-described preferred embodiments can be configured without departing from the scope and spirit of the invention. Therefore, it is to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described herein.