Patent Publication Number: US-7589580-B2

Title: Reference current generating method and current reference circuit

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to semiconductor devices. More particularly, the invention relates to a method of generating a reference current and a current reference circuit implementing the method. 
   This application claims the priority of Korean Patent Application No. 10-2006-0047529, filed on May 26, 2006, the subject matter of which is hereby incorporated by reference. 
   2. Description of the Related Art 
   The performance characteristics of contemporary semiconductor devices are carefully defined in relation to a fairly narrow range of applied operating voltages and currents. The operating voltages and currents allow proper operation of the electrical circuits within semiconductor devices and must remain stable across a range of operating temperatures. One type of circuit commonly providing stable current over a range of operating conditions is referred to as a current reference circuit. 
   Most conventional current reference circuits generate a constant reference current irrespective of operating temperature change by compensating for a first current component proportional to absolute temperature and a second current component inversely proportional to absolute temperature. Examples of conventional reference circuits are disclosed, for example, in U.S. Pat. No. 5,990,727 and U.S. Pat. No. 6,693,332. 
     FIG. 1  is a diagram of an exemplary band gap reference circuit which is commonly used in conventional current reference circuits. Referring to  FIG. 1 , the band gap reference circuit is implemented with resistors R 1  and R 2  and diodes Q 1 -Q 3  and generates a reference voltage (REF) and a corresponding reference current. 
   The band gap reference circuit includes a PTAT generating unit  11  generating a PTAT current component (I_PTAT) and a CTAT generating unit  13  generating a CTAT current component (I_CTAT). The PTAT generating unit  11  includes PMOS transistors P 1 -P 3 , NMOS transistors N 1  and N 2 , a resistor R 1 , and bipolar transistors Q 1  and Q 2 . The CTAT generating unit  13  includes a resistor R 2  and a bipolar transistor Q 3 . 
   With this circuit configuration, a PTAT current component (I_PTAT), which is proportional to changes in temperature, flows to the PMOS transistor P 3  of the PTAT generating unit  13 , and a CTAT current component (I_CTAT), which is inversely proportional to the change in temperature, flows through the resistor R 2  of the CTAT generating unit  13 . Thus, the PTAT current component (I_PTAT) and the CTAT current component (I_CTAT) provide temperature compensation to generate the reference voltage (REF) and the corresponding reference current. 
   As described above, conventional current reference circuits such as the band gap reference circuit require separate circuits for generating the PTAT current component and the CTAT current component. For this reason, when conventional current reference circuits are implemented in contemporary semiconductor devices, they occupy a disproportionately large area within the device. In addition, the use of resistors within the conventional current reference circuits may lead to mismatches caused by variations in process used to fabricate the resistor, variations in the respective voltages applied to the resistors, as well as circuit local temperature variations. Such mismatches have the potential to interfere with proper operation of the conventional current reference circuit. 
   SUMMARY OF THE INVENTION 
   Embodiments of the invention provide a reference current generating method and a related circuit within a semiconductor integrated circuit occupying a relatively small chip area. The current reference circuit, according to embodiments of the invention, provide enhanced immunity to variations in operating conditions, yet do not suffer the potential resistor mismatch problems associated with similar conventional circuits. 
   In one embodiment, the invention provides a reference current generating method comprising; generating a first current using an NMOS transistor and generating a second current using a PMOS transistor, calculating a current difference between the first and second currents, generating a third current which has a similar current/temperature slope as the second current by multiplying the current difference by a proportional constant, and generating a reference current by subtracting the third current from the second current. 
   In another embodiment, the invention provides a current reference circuit comprising; a first current generating unit generating a first current using an NMOS transistor, a second current generating unit generating a second current using a PMOS transistor, a current difference generating unit generating a current difference between the first and second currents, a third current generating unit generating a third current which has a similar current/temperature slope as the second current by multiplying the current difference by a proportional constant, and a reference current generating unit generating a reference current by subtracting the third current from the second current. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram of an exemplary band gap reference circuit such as those commonly used in conventional current reference circuits; 
       FIGS. 2A through 2C  are graphs illustrating the operational concept of a reference current generating method and circuits according to an embodiment of the invention; 
       FIG. 3  is a flowchart summarizing a reference current generating method according to an embodiment of the invention; and 
       FIG. 4  is a circuit diagram of a current reference circuit according to an embodiment of the invention and may be used to implement the reference current generating method of  FIG. 3 . 
   

   DESCRIPTION OF EMBODIMENTS 
   Several embodiments of the invention will be described with reference to the attached drawings. Throughout the written description and drawings, like reference numerals denote like or similar elements. 
   Those of ordinary skill in the art will recognize that the illustrated embodiments are selected examples of the invention which may be otherwise embodied. Indeed, numerous circuit and method variations are contemplated within the scope of the invention, as defined by the following claims. 
     FIGS. 2A through 2C  are graphs illustrating the basic operation concept of a reference current generating method according to an embodiment of the invention.  FIG. 2A  is a graph of the current characteristics for various transistors with respect to temperature.  FIG. 2B  is a graph of current that varies with respect to a constant (h).  FIG. 2C  is a graph of reference current which is unaffected by a change in temperature. 
   Referring to  FIG. 2A , one current characteristic (Ip) for a PMOS transistor with respect to temperature, and another current characteristic (In) for an NMOS transistor with respect to temperature have different slopes due to different mobilities and threshold voltages associated with the different transistor types. For example, a PMOS transistor and an NMOS transistor designed to have the same current at one arbitrary temperature point (Te) will none the less have different current/temperature characteristics at temperatures below arbitrary temperature Te, e.g., temperatures down to temperature Ts. In other words, there is a difference (In−Ip) between the currents associated with the NMOS transistor and the PMOS transistor at temperatures below the arbitrary temperature Te. 
   Referring to  FIG. 2B , the current difference (In−Ip) is “0” at the arbitrary temperature Te and gradually increases as temperature falls further from the arbitrary temperature Te. Current curves (h*(In−Ip)) with different slopes can be obtained at the temperature Te with y=0 by multiplying the current difference (In−Ip) by a given proportional constant (h). 
   A reference current (Iref) may thus be defined by the equation:
 
 Iref=Ip−[h *( In−Ip].  
 
   This reference current is almost constant irrespective of a change in temperature and may be obtained by determining the proportional constant (h) that results in a current/temperature slope for the derived current curve (h*(In−Ip)) that is similar to the current/temperature slope of current curve (Ip) associated with the PMOS transistor, and then subtracting the product h*(In−Ip) from the current curve (Ip). See,  FIG. 2C . 
     FIG. 3  is a flowchart of a reference current generating method according to an embodiment of the invention. As will be apparent, the operation concepts illustrated in  FIGS. 2A through 2C  may be implemented by the exemplary reference current generating method illustrated in  FIG. 3 . 
   Referring to  FIG. 3 , a first current is generated using an NMOS transistor (S 1 ). That is, a first current (In) associated with the NMOS transistor ( FIG. 2A ) is generated. A second current is also generated using a PMOS transistor (S 2 ). That is, a second current (Ip) associated with the PMOS transistor ( FIG. 2A ) is generated. Then, a current difference (In−Ip) between the first current (In) and the second current (Ip) is calculated (S 3 ). 
   In one embodiment of the invention, the calculation of the current difference (In−Ip) includes obtaining a first mirror current that has the same level as the first current (In) by mirroring the first current (In), obtaining a second mirror current that has the same level as the first current (Ip) by mirroring the second current (Ip); and obtaining the current difference (In−Ip) by subtracting the second mirror current from the first mirror current. 
   Next, a third current having the same slope as the second current (Ip), i.e., a current (h*(In−Ip)) in  FIG. 2B  is obtained by multiplying the current difference (In−Ip) by a proportional constant (h) (S 4 ). In one embodiment of the invention, obtaining the third current (h*(In−Ip)) includes mirroring the current difference (In−Ip). 
   Next, a reference current (Iref=Ip−h*(In−Ip)) in  FIG. 2C  is generated by subtracting the third current (h*(In−Ip)) from the second current (Ip) (S 5 ). Finally, a final reference current is generated by mirroring the reference current (Iref=Ip−h*(In−Ip)) (S 6 ). 
   As described above, in the reference current generating method according to an embodiment of the invention, since the slope of the second current (Ip) and the slope of the third current (h*(In−Ip)) are the same, the reference current (Iref=Ip−h*(In−Ip)), which corresponds to the difference between the two currents, has an almost constant level irrespective of a change in temperature. 
     FIG. 4  is a circuit diagram of a current reference circuit according to an embodiment of the invention. This circuit is one example of a class of circuits capable of implementing the reference current generating method of  FIG. 3 . 
   Referring to  FIG. 4 , the current reference circuit includes a first current generating unit  41 , a second current generating unit  42 , a current difference generating unit  43 , a third current generating unit  44 , a reference current generating unit  45 , and a final reference current generating unit  46 . 
   The first current generating unit  41  generates a first current (In) using an NMOS transistor. The second current generating unit  42  generates a second current (Ip) using a PMOS transistor. Since final reference currents Iref 1  and Iref 2  are generated from the first current (In) and the second current (Ip), the first and second current generating units  41  and  42  are implemented so as for variations in the first current (In) and the second current (Ip) with respect to temperature to be small. 
   The current difference generating unit  43  is coupled to an output node O 1  of the first current generating unit  41  and an output node O 2  of the second current generating unit  42  and generates a current difference (In−Ip) between the first current (In) and the second current (Ip). In particular, the current difference generating unit  43  generates a first mirror current (In) which has the same level as the first current (In) by mirroring the first current (In), generates a second mirror (Ip) which has the same level as the second current (Ip) by mirroring the second current (Ip), and generates the current difference by subtracting the second mirror current (Ip) from the first mirror current (In). 
   The third current generating unit  44  is coupled to an output node O 3  of the third current generating unit  44  and generates a third current (h*(In−Ip)) which has the same slope as the second current (Ip) by multiplying the current difference (In−Ip) by a proportional constant (h). 
   The reference current generating unit  45  is coupled to an output node O 4  of the third current generating unit  45  and the output node O 2  of the second current generating unit  42  and generates a reference current (Iref=Ip−h*(In−Ip)) by subtracting the third current (h*(In−Ip)) from the second current (Ip). 
   The final reference current generating unit  46  is coupled to the reference current generating unit  45  and generate the final reference currents reference current (Iref 1  and Iref 2 ) by mirroring the reference current (Iref=Ip−h*(In−Ip)). 
   Since the above-described current reference circuit is designed such that the slope of the second current (Ip) is the same as the slope of the third current (h*(In−Ip)), the reference current (Iref=Ip−h*(In−Ip)), which corresponds to the difference between these two currents, has an almost constant level irrespective of a change in temperature. 
   The structure of each element of the above-described current reference circuit will now be described in some additional detail. The first current generating unit  41  includes a first PMOS transistor P 11 , a first NMOS transistor N 12 , and a second NMOS transistor N 13 . The first PMOS transistor P 11  has a source receiving an applied power voltage (VDD), and a source and a drain which are commonly coupled to the output node O 1  of the first current generating unit  41 . The first NMOS transistor N 12  has a drain and a gate which are commonly coupled to the output node O 1 . The second NMOS transistor N 13  has a drain coupled to a source of the first NMOS transistor N 12 , a gate coupled to the output node O 1 , and a source connected to ground (VSS). 
   The second current generating unit  42  includes a second PMOS transistor P 14 , a third PMOS transistor P 15 , and a third NMOS transistor N 16 . The second PMOS transistor P 14  has a source receiving applied power voltage (VDD), and a gate coupled to the output node O 2  of the second current generating unit  42 . The third PMOS transistor P 15  has a source coupled to a drain of the second PMOS transistor P 14 , and a gate and a drain which are commonly coupled to the output node O 2 . The third NMOS transistor N 16  has a drain and a gate which are commonly coupled to the output node O 2 , and a source connected to ground (VSS). 
   The current different generating unit  43  includes a fourth PMOS transistor P 21 , a fourth NMOS transistor N 21 , and a fifth NMOS transistor N 23 . The fourth PMOS transistor P 21  has a source receiving applied power voltage (VDD), a gate coupled to the output node O 1  of the first current generating unit  41 , and a drain coupled to the output node O 3  of the current different generating unit  43 . The fourth NMOS transistor N 21  has a drain coupled to the output node O 3 , a gate coupled to the output node of the second current generating unit  42 , and a source connected to ground (VSS). The fifth NMOS transistor N 23  has a drain and a gate which are commonly coupled to the output node O 3 , and a source connected to ground (VSS). 
   The first PMOS transistor P 11  of the first current generating unit  41  and the fourth PMOS transistor P 21  of the current difference generating unit generate a mirror current. The size of the first PMOS transistor P 11  is designed to be the same as the size of the fourth PMOS transistor P 21 . Accordingly, the level of the first mirror current (In) flowing through the fourth PMOS transistor P 21  is the same as the level of the first current (In) flowing through the first PMOS transistor P 11 . 
   In addition, the third NMOS transistor N 16  of the second current generating unit  42  and the fourth NMOS transistor N 21  of the current difference generating unit generates a mirror current. Here, the size of the third NMOS transistor N 16  is designed to be the same as the size of the fourth NMOS transistor N 21 . Accordingly, the level of the second mirror current (Ip) flowing through the fourth NMOS transistor N 21  is the same as the level of the second current (Ip) flowing through the third NMOS transistor N 16 . As a result, the level of a current which corresponds to the current difference (In−Ip) between the first mirror current (In) and the second mirror current (Ip) flows through the fourth NMOS transistor N 23 . 
   The third current generating unit  44  includes a sixth NOMS transistor N 33 . The sixth NMOS transistor N 33  has a drain coupled to an output node O 4  of the third current generating unit  44 , a gate coupled to the output node O 3  of the third current generating unit  44 , i.e., the gate of the fifth NMOS transistor N 23 , and a source connected to ground (VSS). 
   The fifth NMOS transistor N 23  and the sixth NMOS transistor N 33  generate a mirror current. Here, the size of the sixth NMOS transistor N 33  is designed to be h (proportional constant) times than the size of the fifth NMOS transistor N 23 . Accordingly, a third current (h*(In−Ip)) that is h times larger than the amount of the current (In−Ip) flowing through the fifth NMOS transistor N 23  flows through the sixth NMOS transistor N 33 . The proportional constant h is determined for the slope of the third current (h*(In−Ip)) to be equal to the slope of the second current (Ip). 
   The reference current generating unit  45  includes a fifth PMOS transistor P 31 , a sixth PMOS transistor P 32 , and a seventh NMOS transistor N 34 . The fifth PMOS transistor P 31  has a source receiving applied power voltage (VDD) and a gate coupled to the output node O 2  of the second current generating unit  42 . The sixth PMOS transistor P 32  has a source coupled to a drain of the fifth PMOS transistor. P 31 , a gate coupled to the gate of the fifth PMOS transistor P 31 , and a drain coupled to an output node of the reference current generating unit  45 . The output node of the reference current generating unit  45  is coupled to the output node O 4  of the third current generating unit  44 . The seventh NMOS transistor N 34  has a drain and a gate which are commonly coupled to the output node O 4  of the reference current generating unit  45 , and a source connected to ground (VSS). 
   The second and third PMOS transistors P 14  and P 15  of the second current generating unit  42  and the fifth and sixth PMOS transistors P 31  and P 32  of the reference current generating unit  45  generate a mirror current. Here, the size of the second and third PMOS transistors P 14  and P 15  are designed to be the same as the size of the fifth and sixth PMOS transistors P 31  and P 32 . Accordingly, the level of the mirror current (Ip) flowing through the fifth and sixth PMOS transistors P 31  and P 32  is the same as the level of the second current (Ip). As a result, a reference current (Iref=Ip−h*(In−Ip)) that corresponding to the current difference between the mirror current (Ip) and the third current (h*(In−Ip)) flows through the seventh NMOS transistor N 34 . 
   As described above, since the slope of the current (Ip) and the slope of the third current (h*(In−Ip)) are the same, the reference current (Iref=Ip−h*(In−Ip)) has an almost constant value irrespective of a change in temperature. 
   The final reference current generating unit  46  includes an eighth NMOS transistor N 42 , a seventh PMOS transistor P 41 , an eighth PMOS transistor P 51 , and a ninth NMOS transistor N 52 . The eighth NMOS transistor N 42  has a gate coupled to the output node O 4  of the reference current generating unit  45  and a source connected to ground (VSS). The seventh PMOS transistor P 41  has a source receiving applied power voltage (VDD), and a gate and a grain which are commonly coupled to a drain of the eighth NMOS transistor N 42 . The eighth PMOS transistor P 51  has a source receiving applied power voltage (VDD), a gate coupled to the gate of the seventh PMOS transistor P 41 , and a drain through which a first final reference current (Iref 1 ) flows. The ninth NMOS transistor N 52  has a gate coupled to the gate of the eighth NMOS transistor N 42 , a source connected to ground (VSS), and a grain through which a second final reference current (Iref 2 ) flows. 
   The eighth NMOS transistor N 42  of the final reference current generating unit  46  and the seventh NMOS transistor N 34  of the reference current generating unit  45  generate a mirror current. The ninth NMOS transistor N 52  of the final reference current generating unit  46  and the seventh NMOS transistor N 34  of the reference current generating unit  45  generate a mirror current. In addition, the seventh PMOS transistor P 41  and the eighth PMOS transistor P 51  generate a mirror current. 
   As described above, in a reference current generating method and a current reference circuit according embodiments of the invention, a difference in the mobility of carriers associated with PMOS and NMOS transistors, (i.e., the difference in temperature-dependent current characteristic between PMOS and NMOS transistors) are used for the purpose of temperature compensation. Accordingly, circuits generating the conventionally used PTAT and CTAT current components are not required. Thus, semiconductor devices incorporating the embodiments of the invention save increasingly scarce chip space. In addition, the reference current generating method and corresponding current reference circuit according to embodiments of the invention may be implemented using only CMOS components instead of resistors. Thus, the potential for resistive component mismatch is eliminated. 
   While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the scope of the present invention as defined by the following claims.