Patent Publication Number: US-6664847-B1

Title: CTAT generator using parasitic PNP device in deep sub-micron CMOS process

Description:
FIELD OF THE INVENTION 
     The present invention relates to an integrated circuit, and, more particularly, to a low voltage bandgap reference manufactured using a deep sub-micron CMOS process having a current complementary to absolute temperature sub-circuit coupled to provide a current substantially constant over temperature. 
     BACKGROUND OF THE INVENTION 
     Various systems, such as analog-to-digital converters (ADC), digital-to-analog converters (DAC), temperature sensors, measurement systems and voltage regulators use bandgap reference circuits to establish the accuracy of the system. Bandgap reference circuits provide local reference voltages of a known value that remains stable with both temperature and process variations. As such, the bandgap reference circuit provides a stable, precise, and continuous output reference voltage for use in various analog circuits. A known bandgap reference circuit derives its reference voltage by compensating the base-emitter voltage of a bipolar transistor V BE  for its temperature dependence (which is inversely proportional to temperature) using a proportional to absolute temperature (PTAT) voltage. With reference to FIG. 2, the difference between the base-emitter voltages, V BE1  and V BE2  or ΔV BE , of two transistors that are operated at a constant ratio between their emitter-current densities forms the PTAT voltage. 
     The emitter-current density is conventionally defined as the ratio of the collector current to the emitter size. Thus, the basic PTAT voltage ΔV BE  is given by: 
     
       
         Δ V   BE   =V   BE1   −V   BE2   (1) 
       
     
     
       
         Δ V   BE =( kT/q ) In ( J   1   /J   2 )  (2) 
       
     
     where k is the Boltzmann&#39;s constant, T is the absolute temperature in degree Kelvin, q is the electron charge, J 1  is the current density of a transistor T 1 , and J 2  is the current density of a transistor T 2 . As a result, when two silicon junctions are operated at different current densities, J 1  and J 2 , the differential voltage ΔV BE  is a predictable, accurate and linear function of temperature. Consequently, the output current I out2  is proportional to absolute temperature since I out2 =ΔV BE /R 2 . In some applications, however, to better control power consumption, a current substantially independent of temperature is desirable. 
     In an effort to provide a reference voltage and current that is constant and substantially independent of temperature, a current source that provides a current complementary to absolute temperature (CTAT) is necessary, wherein the PTAT current from the bandgap reference circuit shown in FIG.  2  and the CTAT current are combined. A temperature independent reference current is provided when the PTAT current, that increases with temperature, and the CTAT current, that decreases with temperature are summed together. If the two slopes of both currents, PTAT and CTAT, are equal in magnitude but opposite in sign, the sum will be independent of temperature. This constant current is applied to a resistor to create a constant voltage. 
     Conventionally, a CTAT current is provided using current that is proportional to the base-emitter voltage of a bipolar transistor V BE  for its temperature dependence which is inversely proportional to temperature. The current source shown in FIG. 1 follows this approach. In processes where the gain β of the bipolar device Q 1  is greater than 50, the base current of the bipolar device is ignored. Thus, the output current I out1  equals V BE /R 1 , where V BE  is the base emitter voltage of bipolar device Q 1 . Since the base emitter voltage V BE  includes a negative temperature coefficient, the output current I out1  represents a CTAT current. In a CMOS digital process such as Texas Instrument&#39;s ® 1833c05 process, however, the gain β of bipolar device Q 1 . is less than 10. As such, the base current I B  of the bipolar device Q 1 . cannot be ignored. Thereby, the total output current I out1  equals the sum [(V BE /R)+I B ]. Thus, the conventional CTAT current source will not provide a CTAT current in a CMOS digital process. 
     Another approach that provides a current that is temperature independent may include an external resistor to set a temperature independent bias current. Although the external resistor has an adjustable value, most preferred implementations require that all the components be included on the chip. 
     Another popular approach is to apply a temperature independent reference voltage V ref  to a resistor to generate a temperature independent current. Since the resistor&#39;s temperature coefficient cannot be compensated, the output current becomes temperature dependent. This design, however requires an additional buffer stage. 
     Thus, a need exists for a current source that provides a CTAT current void of bipolar transistor base current, regardless of whether it is implemented in a CMOS digital process or not. This current source must not be a complex circuit requiring an additional buffer stage. 
     SUMMARY OF THE INVENTION 
     To address the above-discussed deficiencies of current sources that provide CTAT current, the present invention teaches a current source that provides a current CTAT void of bipolar transistor base current, regardless of whether it is implemented in a CMOS digital process or not. This current source does not require an additional buffer stage. 
     A control circuit according to the present invention includes a bandgap reference for providing a PTAT current connected a first current mirror to generate a current proportional to the PTAT current. A novel complementary to absolute temperature (CTAT) current source in accordance with the present invention connects to the first current mirror such that the current proportional to the PTAT current and the CTAT current are summed together to provide the current that remains substantially constant over temperature. 
     This CTAT current source includes a first bias current source which connects to a first resistive circuit and a first subcircuit portion. The first subcircuit portion, including a first bipolar transistor, generates a current proportional to the base emitter voltage of the first bipolar transistor and the base current of the first bipolar transistor. A second bias current source connects to a second resistive circuit and a second subcircuit portion. The second subcircuit portion, including a second bipolar transistor, generates a current proportional to the base current of the second bipolar transistor. A second current mirror connects between the first subcircuit portion and the second subcircuit portion to subtract the base current from the first subcircuit portion. A third current mirror connects between the second current mirror and the first current mirror to provide the current that remains substantially constant over temperature. 
    
    
     These and other features and advantages of the present invention will be understood upon consideration of the following detailed description of the invention and the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawing in which like reference numbers indicate like features and wherein: 
     FIG. 1 illustrates a known CTAT current source; 
     FIG. 2 displays a known PTAT current generator; 
     FIG. 3 shows a control circuit in accordance with the present invention; 
     FIG. 4 illustrates the PTAT and CTAT currents with respect to temperature; and 
     FIG. 5 shows the current that remains substantially constant over temperature as provided from the circuit of FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set for the herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. 
     FIG. 3 illustrates the schematic of the control circuit in accordance with the present invention that produces a current substantially constant over temperature. The IPAT current source couples to a first current mirror including transistors M 14  and M 13  to generate a current I 8  that is proportional to the PTAT current. 
     FIG. 2 illustrates an embodiment of a known PTAT current source that may be incorporated into the control circuit of FIG.  3 . In this particular implementation, the base-emitter area of transistor Q 3  is made eight times as large as that of transistor Q 2 . Thus, currents, I 1  and I 2 , equations are as follows:                I   1     =                  I   S          exp        (       V   BE1       V   T       )                       I   2     =                8        I   S          exp        (       V   BE2       V   T       )                         I   2          R   2       =                    V   BE1     -     V   BE2       =       V   T        ln                 8                             
     The current mirror formed by transistors, M 3  and M 4 , set currents I 1  and I 2  equal to one another, such that the currents are equal as follows:          I   1     =       I   2     =           V   BE1     -     V   BE2         R   2       =         V   T        ln                 8       R   2                           
     The temperature coefficient of R 2  can be ignored. Thus, current I 2  is a current proportional to absolute temperature (PTAT). With reference to FIG. 3, current I 2  is fed into the first current mirror including transistors M 14  and M 13  to generate a current I 8  that is proportional to the PTAT current I 2 . 
     With further reference to FIG. 3, the value of resistors, R 3  and R 4 , and the size of transistors M 7  and M 8  are set equal such that currents, I 3  and I 4 , across the base and emitter of transistors Q 4  and Q 5  are equal, as follows: 
       I   R   =I   3   =I   4   =V   BE   /R   3   
     From the above equation, currents, I 3  and I 4 , are proportional to the base-emitter voltage V BE  for transistors, Q 4  and Q 5 , which includes a negative temperature coefficient. 
     In a CMOS digital process such as Texas Instrument&#39;s ® 1833c05 process, the gain α of each bipolar device, Q 4  and Q 5 , is less than 10. As such, the base current I B  of each bipolar device, Q 4  and Q 5 , cannot be ignored as compared to the collector current I C  for each bipolar device, Q 4  and Q 5 . Thereby, the total current across transistor M 7  equals the sum [2(V BE /R)+I B ]. This current is not exactly a CTAT current. Thus, the use of the extra transistors of M 7 -M 12  are necessary to extract a true CTAT current. 
     The current through transistor M 8  equals the base current I B  of transistor Q 5 . The base current I B  of transistor Q 4  equals the base current I B  of Q 5 . The current through transistor M 7  equals to (2I R +I B ). By using the current mirror including the transistor pair, M 9  and M 10 , the base current I B  is cancelled out from the current that flows through transistor M 7 . The third current mirror including transistor pair, M 11  and M 12 , is connected to the second current mirror including the transistor pair, M 9  and M 10 , such that current of only 2I R  flows to transistor M 12  to be added with the PTAT current I 8  to provide a current I constant  substantially constant over temperature, wherein: 
     
       
           I   constant   =kI   PTAT +(2 V   BE   /R ) 
       
     
     In spite of the temperature-dependent resistors, R 2 , R 3  and R 4 , the value of k can always be adjusted such that current I constant  remains substantially constant over temperature, as long as k is linear. 
     FIG. 4 shows the CTAT current from the control circuit of FIG. 3 along with the PTAT current from the known bandgap reference of FIG.  2 . As shown, the PTAT current increases with temperature and the CTAT current decreases with temperature. 
     FIG. 5 displays the current that remains substantially constant over temperature which is the sum of the CTAT current and PTAT current. There is minimal curvature of approximately −0.20 μamps which those skilled in the art can recognize may be eliminated using known curvature correction circuits. 
     Those of skill in the art will also recognize that the physical location of the elements illustrated in FIG. 3 can be moved or relocated while retaining the function described above. 
     The reader&#39;s attention is directed to all papers and documents which are filed concurrently with this specification and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference. 
     All the features disclosed in this specification (including any accompany claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. 
     The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.