Patent Publication Number: US-7898320-B2

Title: Auto-nulled bandgap reference system and strobed bandgap reference circuit

Description:
RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. 121 to U.S. patent application Ser. No. 11/903,113, filed Sep. 20, 2007, entitled Auto-Nulled Bandgap Reference System and Strobed Bandgap Reference Circuit, which is herein incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to an auto-nulled bandgap reference system and also to a strobed bandgap reference circuit adapted for use with an auto-nulled or other bandgap reference. 
     BACKGROUND OF THE INVENTION 
     As the semiconductor industry continues to mature, cost pressures persist that drive companies to continually reduce manufacturing costs. A direct result of this pricing pressure is the movement to smaller geometry fabrication processes with reduced feature sets. A consequence of the reduced process feature set is the removal of dedicated (non-substrate) bipolar devices that would require extra processing steps, and therefore cost, to implement. Note bipolar devices typically exhibit substantially smaller (and more predictable over temperature) offset voltages and have less noise when compared with MOS (metal-oxide-semiconductor) devices. However since dedicated bipolar devices are not available in most reduced feature set processes of today, MOS devices must typically be used. The larger and less predictable device mismatch levels in MOS devices result in larger and less predictable circuit performances both for initial tolerances and drift over temperature. Additionally, increasing relative noise levels in circuits using MOS devices are exacerbated by reductions in process line width due to thinner gate oxides. The increased noise levels and larger voltage shifts over temperature resulting from MOS devices are un-desirable features in a voltage reference. 
     A standard substrate bandgap reference current uses a pair of diode connected substrate bipolar junction transistors with different current densities to develop a proportional-to absolute-temperature (PTAT) voltage (ΔV BE ) across associated resistors. Though there are several other contributors to the temperature variation in the reference e.g. transistor temperature coefficients, differential temperature coefficients in the resistors, V BE  curvature, and accuracy of the bandgap voltage, the dominant factor, and the one addressed by this invention, is the offset and drift of the amplifier when an MOS amplifier is used. Like all bipolar devices, the buried junctions of the substrate bipolar junction transistors have a relatively small response to package stress and typically match closely during fabrication. Layout techniques enhance this behavior and initial matching errors can be trimmed by adjusting the output voltage of the reference. The resistor-to-resistor temperature coefficient variation benefits from all the same layout techniques that improve resistor matching and can typically be reduced to the point where it is not an issue. V BE  curvature, a nonlinearity in the transistors that results in an undesired shift in the reference voltage over temperature, can be reduced to acceptable levels using one of many known correction techniques. The bandgap voltage is usually very stable on a given process and is not typically the limitation for a reference design. This leaves the non-idealities of the MOS amplifier, input referred offset, temperature drift, and noise, as the dominant error sources in the reference. The input referred offset and noise voltage of the MOS amplifier are gained up by an approximate factor of (1+R 2 /R 3 ) to the output of the reference. Though this gain can be minimized by increasing the PTAT voltage, practical limitations on the ratio of current densities in the substrate bipolar junction transistors place the gain factor (on a single bandgap) in the 8X-12X range. Note, this means that a 1 uV/° C. drift in the amplifier results in almost a 10 ppm/° C. drift in the reference. Thus, random drift offsets, and low frequency noise of the MOS amplifier are the main impediment for achieving a tight temperature coefficient specification for the reference. 
     BRIEF SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to provide an improved auto-nulled bandgap reference system. 
     It is a further object of this invention to provide such an improved auto-nulled bandgap reference system which removes or at least reduces both the offset and low frequency noise effects of the amplifier. 
     It is a further object of this invention to provide such an improved auto-nulled bandgap reference system which reduces the voltage shift of the reference over temperature. 
     It is a further object of this invention to provide such an improved auto-nulled bandgap reference system which re-locates or spreads the noise within the pass band of the intended application. 
     It is a further object of this invention to provide such an improved auto-nulled bandgap reference system which can apply the auto-nulling effect at a constant frequency, by frequency hopping or by random or spread spectrum frequency techniques. 
     It is also an object of this invention to provide an improved strobed bandgap reference circuit for use with an auto-nulled or other bandgap reference. 
     It is a further object of this invention to provide such an improved strobed bandgap reference circuit which powers up periodically to refresh the voltage reference and between refreshes, powers down the reference to save current. 
     It is a further object of this invention to provide such an improved strobed bandgap reference circuit which can reduce the average supply current by a factor or 1000 or more. 
     The invention results from the realization that a truly effective removal or reduction of amplifier offset and noise errors can be achieved with an auto-nulled bandgap reference system employing a substrate PTAT bandgap reference circuit with primary and auxiliary amplifiers and a switching circuit which in a first mode develops a voltage to null the offset and noise errors of the auxiliary amplifier and then in the second mode uses the nulled auxiliary amplifier to develop a voltage to null the offset and noise errors of the primary amplifier to maintain its output PTAT. 
     The invention further realizes that a significant reduction in average power required to operate auto-nulled and other bandgap references can be accomplished with a strobe circuit including an output storage device and a strobe control circuit for periodically powering up a bandgap reference circuit to charge the output storage device and powering down the bandgap reference circuit to conserve power. 
     The subject invention, however, in other embodiments, need not achieve all these objectives and the claims hereof should not be limited to structures or methods capable of achieving these objectives. 
     This invention features an auto-nulled bandgap reference system including a substrate bandgap core having a differential output, a primary amplifier for receiving at its input the differential output of the bandgap core, and a feedback circuit responsive to the primary amplifier to drive to zero the differential signal at its input. The primary amplifier has inherent offset and noise errors. A first storage device is connected to a nulling input of the primary amplifier. There is an auxiliary amplifier having inherent offset and noise errors and there is a second storage device connected to a nulling input of the auxiliary amplifier. A switching circuit selectively, in a first mode, shorts the inputs of the auxiliary amplifier and connects its output to the second storage device to develop a compensation voltage for nulling the noise and offset error of the auxiliary amplifier and in a second mode connects the input of the nulled auxiliary amplifier in parallel with the input to the primary amplifier and connects the output of the nulled auxiliary amplifier to the first storage device to develop a compensation voltage for nulling the noise and offset error of the primary amplifier. 
     In a preferred embodiment the nulling inputs may be differential inputs. The nulling inputs may further include a nulling control circuit for providing a switching signal to drive the switching circuit between the first and second modes. The switching signal may be a constant frequency. The switching signal may be “hopping” between different frequencies. The switching signal may be spread spectrum. The substrate bandgap core may include a pair of pn junctions. The substrate bandgap core may include multiple pairs of stacked pn junctions. The pn junctions may be implemented with diode connected bipolar junction transistors. It may further including a strobe circuit including an output storage device interconnected with the output of the primary amplifier, and a strobe control circuit for periodically powering up the auto-nulled bandgap reference system to charge the output storage device and powering down the auto-nulled bandgap reference system to conserve power. The output storage device may be interconnected through a switch to the output of the primary amplifier. It may further include a start up circuit responsive to a predetermined condition at the output storage device for enabling the strobe control circuit. The predetermined condition may be a voltage level. The start up circuit may include a comparator circuit. It may further include a temperature sensor circuit for varying the rate of periodicity of the strobe control circuit as a function of temperature. The amplifiers may be MOS devices. The output storage device may include a capacitor. 
     This invention also features a strobed bandgap reference circuit including a bandgap reference circuit and a strobe circuit including an output storage device interconnected with the output of the bandgap reference circuit. A strobe control circuit periodically powers up the bandgap reference circuit to charge the output storage device and powers down the bandgap reference circuit to conserve power. 
     In a preferred embodiment the output comparator may be interconnected through a switch to the output of the primary amplifier. The bandgap reference circuit may include a substrate bandgap reference circuit. The strobed bandgap reference circuit may further include a substrate bandgap core having a differential output, a primary amplifier for receiving at its input the differential output and a feedback circuit responsive to the primary amplifier to drive to zero the differential output at its input. The primary amplifier may have inherent offset and noise errors. A first storage device may be connected to a nulling input of the primary amplifier. There may be an auxiliary amplifier having inherent offset and noise errors. A second storage device may be connected to a nulling input of the auxiliary amplifier. A switching circuit may selectively, in a first mode, short the inputs of the auxiliary amplifier and connect its output to the second storage device to develop a compensation voltage for nulling the noise and offset error of the auxiliary amplifier and in a second mode connect the input of the nulled auxiliary amplifier in parallel with the input to the primary amplifier and connect the output of the nulled auxiliary amplifier to the first storage device to develop a compensation voltage for nulling the noise and offset error of the primary amplifier. 
     In a preferred embodiment the nulling inputs may be differential inputs. There may be a nulling control circuit for providing a switching signal to drive the switching circuit between the first and second modes. The switching signal may be a constant frequency. The switching signal may be frequency hopping. The switching signal may be spread spectrum. The substrate bandgap core may include a pair of pn junctions. The substrate bandgap core may include multiple pairs of stacked pn junctions. The pn junctions may be implemented with diode connected bipolar junction transistors. An output comparator may be interconnected through a switch to the output of the primary amplifier. There may be a start up circuit responsive to a predetermined condition at the output storage device for enabling the strobe control circuit. The predetermined condition may be a voltage level. The start up circuit may include a comparator circuit. There may be a temperature sensor circuit for varying the rate of periodicity of the strobe control circuit as a function of temperature. The amplifiers may be constructed from MOS devices. The output storage device may include a capacitor. 
     This invention also features an auto-nulled bandgap reference system including a substrate bandgap reference circuit including a primary amplifier having inherent offset and noise error. There is an auxiliary amplifier circuit having inherent offset and noise error. A switching circuit, in a first mode, develops a voltage to null the offset and noise errors of the auxiliary amplifier and in a second mode connects together in parallel the inputs of the primary and auxiliary amplifiers for developing with the nulled auxiliary amplifier a voltage to null the offset and noise errors of the primary amplifier. In a preferred embodiment the substrate bandgap reference circuit may include a substrate bandgap core having a differential output. The primary amplifier may receive at its output the differential output. A feedback circuit responsive to the primary amplifier output to drive to zero the differential output at its input. There may be a first storage device connected to a nulling input of the primary amplifier, and a second storage device connected to a nulling input of the auxiliary amplifier. The switching circuit in a first mode may short the input of the auxiliary amplifier and connects its output to the storage device to develop a compensation voltage for nulling the noise and offset error of the auxiliary amplifier and in a second mode may connect the input of the nulled auxiliary amplifier in parallel with the input of the primary amplifier and connecting the output of the nulled auxiliary amplifier to the first storage device to develop a compensation voltage for nulling to noise and offset error of the primary amplifier. The nulling inputs may be differential inputs. There may be a nulling control circuit for providing a switching signal to drive the switching circuit between the first and second modes. The switching signal may be a constant frequency. The switching signal may be frequency hopping. The switching signal may be spread spectrum. The substrate bandgap core may include a pair of pn junctions. The substrate bandgap core may include multiple pairs of stacked pn junctions. The pn junctions may be implemented with diode connected bipolar junction transistors. There may be a strobe circuit including an output storage device interconnected with the output of the primary amplifier, and a strobe control circuit for periodically powering up the auto-nulled bandgap reference system to charge the output storage device and powering down the auto-nulled bandgap reference system to conserve power. The output storage device may be interconnected through a switch to the output of the primary amplifier. There may be a start up circuit responsive to a predetermined condition at the output storage device for enabling the strobe control circuit. The predetermined condition may be a voltage level. The start up circuit may include a comparator circuit. There may be a temperature sensor circuit for varying the rate of periodicity of the strobe control circuit as a function of temperature. The amplifiers may be constructed from MOS devices. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Other objects, features and advantages will occur to those skilled in the art from the following description of a preferred embodiment and the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram of a prior art substrate bandgap reference circuit; 
         FIG. 2A  illustrates PTAT, CTAT, (complementary to absolute temperature) and PTAT plus CTAT (VBG) characteristics of the circuit of  FIG. 1 ; 
         FIG. 2B  illustrates the noise power spectral density; 
         FIG. 3  is a schematic diagram of an auto-nulling substrate bandgap reference system according to this invention; 
         FIG. 4  is a schematic diagram of a portion of the auto-nulling substrate bandgap reference system of  FIG. 3  instead using multiple diode connected bipolar junction transistors to implement a multiple (or “stacked”) bandgap reference. 
         FIG. 5  is a schematic diagram of another embodiment of the auto-nulling substrate bandgap reference system of this invention with the strobe circuit of this invention; 
         FIG. 6  is a schematic diagram of another embodiment of the strobe circuit of this invention; and 
         FIG. 7  illustrates the waveforms of the compensation voltage, start up signal and strobe signal generated by the strobe circuit and substrate bandgap reference system of this invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Aside from the preferred embodiment or embodiments disclosed below, this invention is capable of other embodiments and of being practiced or being carried out in various ways. Thus, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of components set forth in the following description or illustrated in the drawings. If only one embodiment is described herein, the claims hereof are not to be limited to that embodiment. Moreover, the claims hereof are not to be read restrictively unless there is clear and convincing evidence manifesting a certain exclusion, restriction, or disclaimer. 
     There is shown in  FIG. 1  a basic substrate bandgap reference circuit  10  contained on the single MOS chip  12  including an amplifier  14  and a substrate PTAT bandgap core  16  which has a differential output  18 ,  20  to the differential input  22 ,  24  of amplifier  14 . Amplifier  14  operates with a feedback circuit  15  which responds to the output of amplifier  14  to drive to zero the differential output  18 ,  20  at inputs  22  and  24 . The inherent offset and noise errors associated with such amplifiers is a problem. Bandgap core  16  includes a pair of bandgap reference diodes  26  and  28  which may be diodes but are typically diode connected substrate bipolar junction transistors. Bandgap core  16  also includes three resistors R 1    30 , R 2    32 , and R 3    34 . In this circuit a differential base emitter voltage (ΔV BE ) is developed by running different current densities into two bipolar devices  26  and  28 . The same ΔV BE  can be achieved by using differently sized bipolar devices or a combination of these effects. The resulting differential voltage is used to develop a proportional to absolute temperature (PTAT) current that in turn is used to generate larger PTAT voltages. The circuit is configured so that the base emitter voltage with the substrate bipolar known to be complementary to absolute temperature (CTAT) is added to the generated PTAT voltage so that the resulting reference output voltage shows little variation over temperature. This is standard architecture and well known to those skilled in the art. In MOS process the largest contribution offset and low frequency noise in such a reference circuit results from the MOS devices used in the amplifier that provide the large loop gain necessary in this circuit. In accordance with this invention those noise sources are removed or reduced. 
     The characteristic PTAT  40  and CTAT  42  voltages,  FIG. 2A , combine to form a virtually temperature insensitive voltage VBG. The nulling technique of this invention which removes offset in the amplifier has the same effect on low frequency noise as it has on offset voltages in that it removes or greatly reduces the noise that occurs at frequencies well below the frequency at which the auxiliary amplifier is switched in and out of the main loop. As low frequency (l/f) noise becomes more and more significant and as process geometries are reduced this becomes more and more important. This is shown in  FIG. 2B , where the thermal noise  46  and l/f noise  48  are shown in a plot of log of noise power versus log of the frequency. 
     The auto-nulled bandgap reference system  50  according to this invention is shown in  FIG. 3 , where similar parts have been given similar numbers accompanied by a lower case letter. Auto-nulled bandgap reference system  50  is located on a single chip  12   a  along with a basic substrate bandgap reference circuit  10   a , but there is added in this invention a second auxiliary amplifier  52  in addition to the first primary amplifier  14   a . Also included are storage devices such as capacitors  54  and  56  and a switching circuit  58  which includes switches  60 , and  62 , which are closed in a first mode φ 1  and two switches  64  and  66  which are closed during a second mode φ 2 . The switches are operated on a break before make basis so that at no time will switches  60  and  62  be on at the same time as switches  64  and  66 . Switches  60 ,  62 ,  64 , and  66  are operated by nulling control circuit  68  which provides at its output the φ 1  signal in the first mode, and φ 2  signal in the second mode. Nulling control circuit  68  may be commanded to alternate between the two modes either at a fixed frequency  70  or at a hopping frequency  72  or using a pseudo random or spread spectrum approach  74 . The nulling frequency therefore may be moved during operation depending on where noise will cause problems in the application. In some applications it may not be practical or possible to find a safe frequency range for the auto nulling frequency to be set in order to avoid a potential frequency bands of interest. In those cases the auto nulling can be spread spectrum to avoid placing too much unwanted frequency content in a given band. 
     In operation, in a first mode switches  60  and  62  are closed. Switch  62  shorts together the inputs to auxiliary amplifier  52 . With switch  60  also closed a secondary feedback loop  76  is created so that the output of auxiliary amplifier  52  is fed back on the nulling input  78  of auxiliary amplifier  52  to develop a voltage sufficient to cancel any offset voltage at or imbalance between its inputs. This voltage will be stored on storage device or capacitor  56 . Switches  60  and  62  are now open preserving that voltage at least for a finite period of time during which auxiliary amplifier  52  operates as a “perfect” amplifier with the offset and low frequency noise at its input reduced to zero, or nearly so. Then in the second mode switches  64  and  66  which have been opened during the first mode are closed. With the closing of switch  66  the inputs to auxiliary amplifier  52  are connected in parallel with the inputs of primary amplifier  14   a . And the output of auxiliary  52  is delivered to the nulling input  80  of primary amplifier  14   a . Auxiliary amplifier  52  now perfectly senses the offset and low frequency noise errors at the input of primary amplifier  14   a  and through feedback loop  82  and closed switch  64  develops a voltage at nulling input  80  which compensates for the offset and low noise error at the input of amplifier  14   a . Primary amplifier  14   a  now operates as a virtually perfect amplifier as did auxiliary amplifier  52 . Initially, the auxiliary amplifier  52  is nulling itself and the main amplifier  14   a  controls the loop with its offset set as a function of its inherent offset plus the effect of the voltage stored at its nulling input. Once this phase is complete, the switch connected to the nulling cap input of the auxiliary amplifier  52  is first opened, followed by the switch shorting its inputs together. Next, the switch connecting the inputs of the auxiliary amplifier  52  to the inputs of the main amplifier  14   a  is shorted followed by connecting the output of the auxiliary amplifier  52  to the nulling input of the main amplifier  14   a . When this clock phase is completed, the connection from the output of the auxiliary amplifier  52  to the nulling input of the main amplifier  14   a  is first disconnected followed by the connection of the auxiliary amplifier  52  inputs to the inputs of the main amplifier  14   a . To complete the cycle, the inputs of the auxiliary amplifier  52  are first shorted followed by the connection of the output of the auxiliary amplifier  52  to the auxiliary inputs of the main amplifier  14   a . And thus, the cycle continues. 
     While in  FIG. 3  the bandgap references are shown as the single pair of diodes or diode connected transistors  26   a  and  28   a , commonly referred to as a “single bandgap,” this is not a necessary limitation of the invention. For example as shown in  FIG. 4 , each leg may include two or more such diode devices, e.g.  26   a   1 ,  26   a   2  . . .  26   a n:  28   a   1 ,  28   a   2  . . .  28   a n, in order to implement a multiple or (“stacked”) bandgap reference. 
     In another embodiment,  FIG. 5 , primary amplifier  14   b  and auxiliary amplifier  52   b  each have differential nulling inputs  80   b ,  80   bb ,  78   b ,  78   bb , respectively, so there are two storage devices capacitors  54   b ,  54   bb  and capacitors  56   b  and  56   bb  and the switches are both double pole switches  64   b  and  60   b.    
     A strobe circuit  98  according to this invention is also shown in  FIG. 5 . Strobe circuit  98  includes output storage device  104 , refresh switch  102 , and strobe control circuit  100 . Strobe control circuit  100  provides a periodic strobe signal which closes switch  102  to periodically charge output storage device, capacitor  104 , and simultaneously enable the primary  14   b  and auxiliary  52   b  amplifiers which are otherwise disabled in order to conserve power. This is done at a predetermined rate in accordance with the particular application. Since the charge leakage from output storage capacitor  104  is effected by temperature, a temperature sensor circuit  108  may be used to change the rate at which strobe control circuit refreshes storage capacitor  104  in accordance with the temperature sensed. When strobe control circuit  100  powers up amplifiers  14   b  and  52   b , it also enables nulling control circuit  68  to begin its auto nulling operation. Also shown in  FIG. 5  is start up circuit  110  which includes a comparator  112  and a timer  114 . Comparator  112  senses the voltage on output terminal  114  and as it rises provides an output signal when it reaches a reference voltage V X . This operates timer  114  so that after a predetermined period of time when the voltage has stabilized a signal is sent on line  116  to enable strobe control circuit  100 . 
     Strobe control circuit  98   c  is shown more generally in  FIG. 6  as including only strobe control circuit  100   c  and output storage device capacitor  104   c  as switch  102   c  may not always be necessary. The strobe signal on line  122  could simply enable or disable an associated bandgap reference circuit  120  such as auto-nulled bandgap reference system  50 ,  FIG. 3 , or  50   b ,  FIG. 5 , so that it provides output or no output to capacitor  104   c  without the supervision of a switch such as switch  102   c . If there no switch it may simply have a high impedance output when disabled to allow the output capacitor  104   c  to float. 
     The average current is substantially reduced by the strobe circuit operation whereby the bandgap reference is powered up periodically to refresh the voltage on the output capacitor and after a short period of refresh the bandgap reference is then completely powered down to save current. When it is powered down and it presents a high impedance output (or if a switch is opened) the output capacitor is then allowed to float and this process repeats cyclically at a predetermined rate. 
     In operation, at startup referring to  FIG. 7 and 5  the compensation voltage  130 ,  FIG. 7 , rises until it reaches the reference voltage V X  of comparator  112 ,  FIG. 5 . This triggers an output from comparator  112  at  132  which operates timer  114  for a period of time until it reaches time  134 . At that point it provides an enable signal  136  to the strobe circuit which then begins refreshing output capacitor  104  and, alternately, powering up to refresh output capacitor  104  and powering down to conserve power. Strobe signals repeat periodically as shown at  138  in dependence upon how quickly the charge on output capacitor  104  depletes as indicated by the saw tooth shape  140  of the compensation voltage  130 . It is during each of these strobe periods  138  that the nulling control circuit provides the auto-nulling operation using switching circuits  58 ,  58   b . Note that if the strobing is used in conjunction with the auto nulling the refresh period should be over several cycles of the auto nulling period to get the benefits from auto nulling. 
     Although specific features of the invention are shown in some drawings and not in others, this is for convenience only as each feature may be combined with any or all of the other features in accordance with the invention. The words “including”, “comprising”, “having”, and “with” as used herein are to be interpreted broadly and comprehensively and are not limited to any physical interconnection. Moreover, any embodiments disclosed in the subject application are not to be taken as the only possible embodiments. 
     In addition, any amendment presented during the prosecution of the patent application for this patent is not a disclaimer of any claim element presented in the application as filed: those skilled in the art cannot reasonably be expected to draft a claim that would literally encompass all possible equivalents, many equivalents will be unforeseeable at the time of the amendment and are beyond a fair interpretation of what is to be surrendered (if anything), the rationale underlying the amendment may bear no more than a tangential relation to many equivalents, and/or there are many other reasons the applicant can not be expected to describe certain insubstantial substitutes for any claim element amended. 
     Other embodiments will occur to those skilled in the art and are within the following claims.