Abstract:
An accurate temperature monitoring system that uses a precision current control circuit to apply accurately ratioed currents to a semiconductor device, which may be a bipolar junction transistor (BJT), used for sensing temperature. A change in base-emitter voltage (ΔV BE ) proportional to the temperature of the BJT may be captured and provided to an ADC, which may generate a numeric value corresponding to that temperature. The precision current control circuit may be configured to generate a reference current, capture the base current of the BJT, generate a combined current equivalent to a sum total of the base current and a multiple of the reference current, and provide the combined current to the emitter of the BJT. In response to this combined current, the collector current of the BJT will be equivalent to the multiple of the reference current. The ratios of the various collector currents conducted by the BJT may thus be accurately controlled, leading to more accurate temperature measurements.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates generally to the field of integrated circuit design and, more particularly, to the design of temperature sensor and measurement devices.  
         [0003]     2. Description of the Related Art  
         [0004]     Many digital systems, especially those that include high-performance, high-speed circuits, are prone to operational variances due to temperature effects. Devices that monitor temperature and voltage are often included as part of such systems in order to maintain the integrity of the system components. Personal computers (PC), signal processors and high-speed graphics adapters, among others, typically benefit from such temperature monitoring circuits. For example, a central processor unit (CPU) that typically “runs hot” as its operating temperature reaches high levels may require a temperature sensor in the PC to insure that it doesn&#39;t malfunction or break due to thermal problems.  
         [0005]     Often, integrated circuit (IC) solutions designed to measure temperature in a system will monitor the voltage across one or more PN-junctions, for example a diode or multiple diodes at different current densities to extract a temperature value. This method generally involves amplifying (or gaining up) a small voltage generated on the diode(s), and then subtracting voltage from the amplified temperature-dependent voltage in order to center the amplified (gained) value for conversion by an analog-to-digital converter (ADC). In other words, temperature-to-digital conversion for IC-based temperature measuring solutions is often accomplished by measuring a difference in voltage across the terminals of typically identical diodes when different current densities are forced through the PN junctions of the diodes. The resulting change (ΔV BE ) in the base-emitter voltage (V BE ) between the diodes is generally proportional to temperature. (It should be noted that while V BE  generally refers to a voltage across the base-emitter junction of a diode-connected transistor and not a voltage across a simple PN-junction diode, for the sake of simplicity, V BE  is used herein to refer to the voltage developed across a PN-junction in general.) More specifically, a relationship between V BE  and temperature is defined by the equation  
               V   BE     =     η   ⁢     kT   q     ⁢   ln   ⁢       I   C       I   S                 (   1   )             
 
 where η is the ideality factor of the PN junction, k is Boltzman&#39;s constant, q is the charge of a single electron, T represents absolute temperature, I s  represents saturation current and I C  represents the collector current. A more efficient and precise method of obtaining ΔV BE  is to supply the PN junction of a single diode with two separate and different currents in a predetermined ratio. Consequently, ΔV BE  may be related to temperature by the equation  
               Δ   ⁢           ⁢     V   BE       =     η   ⁢     kT   q     ⁢     ln   ⁡     (   N   )                 (   2   )             
 
 where N is a constant representing a pre-selected ratio of the two separate collector currents that are supplied to the PN junction of the diode. 
 
         [0006]     In certain cases, for example when measuring the temperature of a semiconductor device such as a CPU, the PN-junction used in performing the temperature measurement may be comprised in a PNP device configured on the same substrate as the CPU. When using a small geometry process substrate transistor as the PNP device, the β (common-emitter current gain) of the transistor may be very low and may vary over process and temperature, as well as over collector current levels. Typical present day temperature measurement systems operate by applying controlled, ratioed currents to the emitter of a transistor used as the temperature measurement PNP device, and are therefore prone to temperature measurement errors due to the β variation in the transistor.  
         [0007]     Other corresponding issues related to the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.  
       SUMMARY OF THE INVENTION  
       [0008]     In one set of embodiments, an accurate temperature monitoring system may use a precision current control circuit to apply accurately ratioed currents to a semiconductor device, which may be a bipolar junction transistor (BJT), used for sensing temperature. A change in base-emitter voltage (ΔV BE ) proportional to the temperature of the BJT may be captured and provided to an ADC, which may generate a numeric value corresponding to that temperature. The precision current control circuit may be configured to generate a reference current, capture the base current of the BJT, generate a combined current equivalent to a sum total of the base current and a multiple of the reference current, and provide the combined current to the emitter of the BJT. In response to this combined current, the collector current of the BJT will be equivalent to the multiple of the reference current.  
         [0009]     In one embodiment, the current control circuit comprises a β compensation circuit configured to generate the emitter current provided to the BJT, and a β detection circuit configured to generate a reference current, which corresponds to the desired collector current to be developed in the BJT used for sensing temperature. The β compensation circuit may operate in at least two states, being configured to generate the emitter current corresponding to a low collector current (for the BJT) in the first state, and to generate the emitter current corresponding to a high collector current in the second state.  
         [0010]     In the first state, the base current from the BJT may be fed into the drain of a first NMOS device and mirrored in a second NMOS device. The mirror current, equivalent to the base current, may then be summed with the reference current, and the summed current applied to (drawn from) the drain of a first PMOS device. The summed current may be mirrored in a second PMOS device, which may have its drain coupled to the emitter of the BJT, thereby providing the summed current to the emitter of the BJT. As a result of the emitter current being equivalent to a sum total of the base current and the reference current, the collector current developed in the BJT will be equivalent to the reference current.  
         [0011]     In the second state, a specified number (N−1) of parallel-coupled identical NMOS devices may be switched to couple in parallel with the first NMOS device, thereby creating an NMOS array such that the base current from the BJT may be equally split between the respective drains of the individual NMOS devices comprised in the NMOS array. The total number of individual NMOS devices in the NMOS array may reflect the desired ratio between the low collector current and the high collector current. Thus, in the second state, the current mirrored in the second NMOS device may be equivalent to the base current divided by N. The mirror current, equivalent to the base current divided by N, may then be summed with the reference current, and the summed current again applied to (drawn from) the drain of a first PMOS device. In the second state, the summed current may be mirrored N to 1 in a PMOS array comprising N−1 parallel-coupled identical PMOS devices switched to couple in parallel to the second PMOS device, where the respective drains of the individual PMOS devices comprised in the PMOS array may be coupled together, and to the emitter of the BJT, thereby providing an N multiple of the summed current to the emitter of the BJT. As a result of the emitter current being equivalent to a sum total of the base current and N times the reference current, the collector current developed in the BJT will be equivalent to an N multiple of the reference current.  
         [0012]     The β detection circuit may be operated to optimally set the reference current (and thus the collector current for the BJT) such that the emitter current generated by the β compensation circuit through the PMOS array is maximized but not overdriven. Maximizing the emitter current, and hence the collector current, may provide the added benefit of reducing the effects of electromagnetic interference (EMI) on the temperature measurement, and maximizing the amount of filtering capacitance that may be coupled across the base emitter junction of the BJT. In addition, the larger the current in the PMOS array, the less current ratio error may be incurred as a result of PMOS device mismatch, which may also translate to a considerable reduction in temperature measurement errors. In one embodiment, the β detection circuit may be configured to set the reference current to a low initial value, and progressively increase the value of the reference current, until the summed current (equivalent to the sum total of the base current and the reference current, and applied to the drain of the first PMOS device comprised in the β compensation circuit) exceeds a previously determined optimal value. In one embodiment, the reference current is increased in steps, where each step corresponds to a specified unit value, for example 1 μA. Once the summed current exceeds the previously determined optimal value, the value of the reference current may be reduced by a unit step, and may be held at the thus obtained value for the duration of the temperature conversion.  
         [0013]     The ratios of the various collector currents conducted by the BJT may thus be accurately controlled, leading to more accurate temperature measurements. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The foregoing, as well as other objects, features, and advantages of this invention may be more completely understood by reference to the following detailed description when read together with the accompanying drawings in which:  
         [0015]      FIG. 1  illustrates a temperature measurement system that utilizes an ADC and a current control circuit in accordance with principles of the present invention;  
         [0016]      FIG. 2  illustrates one embodiment of a current control circuit;  
         [0017]      FIG. 3  illustrates an alternate embodiment of a current control circuit; and  
         [0018]      FIG. 4  illustrates one embodiment of a current control circuit comprising a beta compensation component combining with a beta detection component. 
     
    
       [0019]     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. Note, the headings are for organizational purposes only and are not meant to be used to limit or interpret the description or claims. Furthermore, note that the word “may” is used throughout this application in a permissive sense (i.e., having the potential to, being able to), not a mandatory sense (i.e., must).“The term “include”, and derivations thereof, mean “including, but not limited to”. The term “coupled” means “directly or indirectly connected”.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]     As used herein, the word “alternately” is meant to imply passing back and forth from (or between) one state, action, or place to another state, action, or place, respectively. For example, “alternately closing and opening a switch” would mean closing the switch, then opening the switch, then closing the switch, then opening the switch, and so on.  
         [0021]     A “PN-junction-voltage” (V BE ) refers to a voltage measured across a PN-junction, or a difference in voltage between a voltage measured at the anode of the PN-junction with respect to a common ground and a voltage measured at the cathode of the PN-junction with respect to the common ground. A “change in PN-junction-voltage” (ΔV BE ) refers to a change in PN-junction-voltage for a chosen PN-junction, either in time or in different circuit configurations. For example, if in one circuit configuration V BE =700 mV for a PN-junction, and in a different circuit configuration V BE =655 mV for the PN-junction, then AV BE =45 mV for the PN-junction when referencing to the two different circuit configurations. Similarly, for example, if at a time point tl V BE =650 mV for a PN-junction, and at a time point t 2  V BE =702 mV for the PN-junction, then ΔV BE =52 mV for the PN-junction when referencing time points t 1  and t 2 . “Storing” a V BE  or V BE  value in an integrator generally refers to developing a charge corresponding to the V BE  value within the integrator. “Adding” and/or “subtracting” a V BE  or V BE  value in the integrator generally refers to increasing and/or decreasing the developed charge within the integrator, correspondingly to the V BE  value. A PN-junction may be comprised in a base-emitter junction of a bipolar junction transistor (BJT). Various embodiments of the circuit are described as utilizing a BJT. However, in other embodiments, the operation performed by the BJT may be achieved through PN-junctions (or NP-junctions) present in devices other than a BJT, having characteristics similar to that of a BJT.  
         [0022]     Considering a high collector current and a low collector current as the two separate and different currents in a pre-selected ratio, equation 2 may be re-written as:  
                 Δ   ⁢           ⁢     V   BE       =     η   ⁢     kT   q     ⁢     ln   ⁡     (       I   CH       I   CL       )           ,           (   3   )             
 
 where I CH  represents the high collector current and I CL  represents the low collector current, where 
 
 I   CH   =N*I   CL .  (4) 
 
         [0023]     The relationship between the emitter current I E  and the collector current I C  is given by:  
               I   C     =         I   E     ⁡     (     β     1   +   β       )       .             (   5   )             
 
 Considering a gain of β for the transistor at the high current, with a Δβ change in P when switching from the high current to the low current, the following relationships can be obtained from equation 5:  
               I   CH     =     N   *       I   E     ⁡     (     β     1   +   β       )                 (   6   )                   I   CL     =       I   E     ⁡     (       β   +     Δ   ⁢           ⁢   β         1   +   β   +     Δ   ⁢           ⁢   β         )         ,           (   7   )             
 
 where I E  is the unit emitter current corresponding to I CL , multiplied by ‘N’ in equation 6 according to equation 4 in order for the relationship established in equation 5 to be satisfied. Equation 3 can then be re-written, substituting the corresponding expressions from equations 6 and 7 for I CH  and I CL , respectively.  
               Δ   ⁢           ⁢     V   BE       =       η   ⁢     kT   q     ⁢     ln   ⁡     (   N   )         +     η   ⁢     kT   q     ⁢       ln   ⁡     (       β   *     (     1   +   β   +   Δβ     )           (     1   +   β     )     *     (     β   +   Δβ     )         )       .                 (   8   )             
 
 In equation 8, ΔV BE  corresponds to the correct (expected) measured temperature plus a temperature error incurred as a result of Δβ. The first term on the right hand side of equation 8 represents the expected ΔV BE  without error, and the second term is the error term resulting from a change in the β of the transistor (Δβ). As indicated in equation 8, as Δβ goes to zero, the error term is eliminated. Note also, that even for a finite Δβ value, large values of β will result in a smaller error term. However, in the case of small geometry process substrate transistors having small values of β, a finite Δβ value will have a greater effect on the overall value of ΔV BE , and will thus lead to increased measurement errors. 
 
         [0024]      FIG. 1  illustrates a block diagram of one embodiment of a temperature sensor system implemented in accordance with the present invention. In the embodiment shown, current control circuit  102  is used to control emitter current I E  and collector current I C  in BJT  104 , according to base current I B  of BJT  104  that is received by current control circuit  102 . BJT  104  may be coupled to the inputs of ADC  110  as shown. For more detail on possible embodiments of ADC  110  and the coupling of BJT  104  to ADC  110 , please refer to U.S. Pat. No. 6,847,319 titled “Temperature-to-Digital Converter” invented by Troy L. Stockstad, which is hereby incorporated by reference in its entirety as though fully and completely set forth herein. In one set of embodiments, current control circuit  102  may be operated to alternately develop a high I C  current and a low I C  current—I CH  and I CL , respectively, in equation 3—in BJT  104 , with the resulting ΔV BE  developed across the base-emitter junction of BJT  104  used by ADC  110  to generate a numeric (digital) value corresponding to the temperature of BJT  104 . ADC  110  may provide an M-bit output, where M may be selected based on the desired resolution of the digital value. In one set of embodiments, the value of I CH  may be an N-multiple of the value of I CL , as also shown in equation 4.  
         [0025]      FIG. 2  shows one possible embodiment of current control circuit  102 , used for establishing I E  and I C  in BJT  104 , according to I B . PMOS transistor  200  may be configured to provide the emitter current I E  to BJT  104 . PMOS device  202  may be configured to mirror this emitter current. In other words, PMOS device  202  may be coupled to PMOS device  200  such that a current flowing into the source of PMOS device  200  is replicated by PMOS device  202 , where the current flowing into the source of PMOS device  202  is equivalent to the current flowing into the source of PMOS device  200 . Those skilled in the art will appreciate that the value of an equivalent mirror current may typically be within 1% of the value of the mirrored current, and that various techniques may be employed to minimize or eliminate mismatch errors between PMOS devices  200  and  202 . Such mismatch errors may be present due to fabrication process variations, for example, and may be remedied using well known methods in the art such as dynamic element matching (DEM), for example. The base of BJT  104  may be coupled to NMOS device  210 , effectively applying I B  to the drain and coupled gate of NMOS device  210 .  
         [0026]     In one embodiment, NMOS device  212  is coupled to NMOS device  210  to mirror the base current IB that is conducted by NMOS device  210 . That is, the current flowing into the drain of NMOS device  212  may be equivalent to the I B  current conducted by NMOS device  210 . NMOS device  212  may further be configured to couple to node  224 , which may couple the drain of PMOS device  202  to the source of PMOS device  206 , NMOS device  212  thereby pulling a current equivalent to I B  from node  224 . This results in a current equivalent to I E -I B  flowing out of the drain of PMOS device  206 , which by definition is equivalent to the collector current I C  conducted by BJT  104 . Current source  220  may be configured to provide a supply current I SUPPLY  to NMOS device  218 . The supply current may be switched between values representing I HIGH  or I LOW , that is, a desired high collector current I CH  and low collector current I CL , respectively, for BJT  104 . NMOS device  214  may be configured to mirror the current conducted by PMOS device  218 , with the drain of PMOS device  206  coupled to the drain of NMOS device  214  forming a high impedance node that may drive non-inverting input of amplifier  216 . In one embodiment, the output of amplifier  216  is configured to drive the gates of PMOS devices  202  and  200 , thereby adjusting the emitter current I E  flowing into BJT  104  such that the collector current I C  flowing out of BJT  104  is equivalent to the current (I HIGH  or I LOW ) flowing in NMOS device  218 . Current source  220  may be configured to provide accurately ratioed currents (that is, satisfying equation 4), resulting in accurately ratioed collector currents being developed in BJT  104 . By controlling the collector current developed in BJT  104 , β-errors may be greatly reduced and/or eliminated during temperature measurements.  
         [0027]      FIG. 3  shows an alternate embodiment of current control circuit  102 . In this embodiment, current I E  is provided to BJT  104  by PMOS device  300 , which is coupled to the emitter of BJT  104  as shown. The base current from BJT  104  may flow into NMOS device  306 , and may be mirrored by NMOS device  308 , such that the current flowing into the drain of NMOS device  308  is equivalent to I B  flowing into the drain of NMOS device  306 . Current source  312  may be configured to provide a current to NMOS device  314 , which may be mirrored by NMOS device  310 , such that the current flowing into the drain of NMOS device  310  is equivalent to the current I C  conducted by NMOS device  314 . I C  and I B  may be effectively summed together by coupling the drain of NMOS device  308  and the drain of NMOS device  310  together, the resulting current (I B +I C =I E ) being drawn from PMOS device  302 , and mirrored by PMOS device  300 . The collector current flowing out of BJT  104  may therefore be controlled and/or kept constant regardless of the β of BJT  104 .  
         [0028]     To provide ratioed collector currents (I CH  and I CL ), current source  312  may be configured to be capable of providing two ratioed currents, or PMOS device  300  and NMOS device  306  may each be replaced by corresponding parallel-coupled PMOS and NMOS device arrays, respectively. In one set of embodiments, parallel-coupling the NMOS devices comprises coupling the gates of the NMOS devices together, coupling the drains of the NMOS devices together, and coupling the sources of the NMOS devices together (in the embodiment shown in  FIG. 3 , the respective source of each of the parallel-coupled NMOS devices is coupled to ground). Similarly, parallel-coupling the PMOS devices may comprise coupling the gates of the PMOS devices together, coupling the drains of the PMOS devices together, and coupling the sources of the PMOS devices together (in the embodiment shown in  FIG. 3 , the respective source of each of the parallel-coupled PMOS devices is coupled to the supply voltage). These PMOS and NMOS device arrays may be configured to allow switching back and forth between a single device conducting current and N devices conducting current, with the current provided by current source  312  remaining constant. With N devices in the circuit for both the PMOS device array (replacing PMOS device  300 ) and the NMOS device array (replacing NMOS device  306 ), I B  may flow into the NMOS device array and may be mirrored N-to-1 in NMOS device  308 , such that the current flowing into the drain of NMOS device  308  is equivalent to I B  divided by N. The current supplied by NMOS device  308  and NMOS device  310  to PMOS device  302  may then become I B /N+I C . This current may be mirrored 1-to-N by the PMOS device array, where the current flowing into the emitter of BJT  104  would be I B +N*I C . Hence, the collector current I C  in BJT  104  may be N*I C , and I C  when the NMOS array and PMOS array are each switched to only a respective single device conducting current. This is illustrated in  FIG. 4  as part of β compensation circuit  416 .  
         [0029]      FIG. 4  shows yet another embodiment of current control circuit  102 , comprising a β compensation circuit  416 , and a β detection circuit  432  configured to generate reference current I REF , which corresponds to the desired I CL  in BJT  104 . Circuit  416  may operate in two states, with the first state having switches  407  and  408  open. While in the state where switches  407  and  408  are open, the base current, I B  flowing from the base of BJT  104  may be applied to DN pin  108  of circuit  416 . I B  may consequently be applied to the drain of NMOS device  410 , and may be mirrored, one for one, by NMOS device  414 , thereby establishing a current equivalent to I B  flowing into the drain of NMOS device  414 . This equivalent current may be summed with I REF  generated by circuit  432 , resulting in a combined current of (I REF +I B ). This combined current may be drawn from PMOS device  406 , and may be mirrored, one for one, by PMOS device  402 , thereby establishing a current equivalent to (I REF +I B ) flowing out of pin DP  106 , and into the emitter of BJT  104 . When the emitter current I E  of BJT  104  is equivalent to I REF +I B , by definition (I E =I C +I B ), I C  is equivalent to I REF .  
         [0030]     As shown in  FIG. 4 , circuits  416  and  432  may both comprise single NMOS and/or PMOS devices and parallel-coupled NMOS and/or PMOS arrays. The number of individual devices is indicated by the value of ‘m’ in each case. For example, PMOS device  402  represents a single PMOS device, while PMOS  404  may represent ‘N−1’ parallel-coupled PMOS devices, where ‘N’ represents the ratio between I CH  and I CL  as defined in equation 4. The respective gates of the individual PMOS devices comprised in PMOS array  404  may be coupled together and to the gate of single PMOS device  402 , while the respective drains of the individual PMOS devices comprised in PMOS array  404  may each be coupled to the drain of PMOS device  402  via switch  408 . It should be noted that switch  408  may be representative of ‘N−1’ switches, each switch configured to couple the drain of a respective PMOS device comprised in PMOS array  404 . NMOS  412  may similarly represent ‘N−1’ parallel-coupled NMOS devices coupled to NMOS device  410  in a manner similar to that described for PMOS array  404 . NMOS array  412  may be switchably coupled using switch  407 , which may represent ‘N−1’ number of switches, one switch for the respective drain of each NMOS device comprised in NMOS array  412 .  
         [0031]     While in the state where switches  407  and  408  are closed, the base current, I B  of BJT  104  may flow out of the base of BJT  104  and into DN pin  108  of circuit  416 . I B  may be distributed between the respective drains of NMOS device  410  and the ‘N−1’ NMOS devices comprised in NMOS array  412 . In one set of embodiments, in order to obtain substantially equivalent currents flowing into the drain of NMOS device  410  and the respective drains of the individual devices of NMOS array  412 , NMOS device  410  and the individual devices of NMOS array  412  may be designed to be identical. PMOS device  402  and the individual devices of PMOS array  404  may similarly be designed to be identical. I B  may be mirrored, one for N, by NMOS device  414 . That is, the mirror current flowing into the drain of NMOS device  414  may be equivalent to I B /N, which represents the value of an individual current flowing into the drain of NMOS device  410  or the respective drain of any of the NMOS devices comprised in NMOS array  412 . This mirror current, equivalent to I B /N, may be summed with I REF  generated by circuit  432 , resulting in a combined current of (I REF +I B /N). This combined current may be drawn from PMOS device  406 , and may be collectively mirrored, N for one, by PMOS device  402  and PMOS array  404 , thereby establishing a current equivalent to (N*I REF +I B ) flowing out of pin DP  106 , and into the emitter of BJT  104 . When the emitter current I E  of BJT  104  is equivalent to N*I REF +I B , by definition (I E =I C +I B ), I C  is equivalent to N*I REF .  
         [0032]     Circuit  416  may therefore be operated to accurately control the collector current in BJT  104 , since I C  will be equivalent to I REF  or a multiple of I REF . This multiple, or ratio, may be determined by the mirror ratio used when configuring NMOS array  412  and/or PMOS array  404 . The respective number of—preferably identical—devices used in configuring each array, NMOS array  412  and/or PMOS array  404 , may determine the actual collector current ratio. In the embodiment shown, the ratio of I CH  and I CL  is ‘N’. By using this technique, the impact that the β of BJT  104  has on temperature readings and accuracy may be greatly reduced and/or eliminated.  
         [0033]     The embodiment of current control circuit  102  shown in  FIG. 4  also features a β detection circuit  432 , which may be operated to optimally set the collector current I REF  such that the emitter current I E , obtained collectively from PMOS device  402  and PMOS array  404 , is maximized but not overdriven. Maximizing the emitter current I E , and hence the collector current I C , may provide the added benefit of reducing the effects of electromagnetic interference (EMI) on the temperature measurement, and maximizing the amount of filtering capacitance that may be coupled across the base emitter junction of BJT  104 . In addition, the larger the current in PMOS device  402  and/or the individual devices comprised in PMOS array  404 , the less current ratio error may be incurred as a result of PMOS device mismatch, which may also translate to a considerable reduction in temperature measurement errors. It should be noted that if too much current is drawn from PMOS device  406 , PMOS device  402  and/or any and/or all devices comprised in PMOS array  404  may operate in the ohmic region instead of remaining in saturation. This may compromise the accuracy of mirroring the current from PMOS device  406 , and lead to potentially sizeable temperature measurement errors as a result. For any given collector current I REF , the emitter current I E  will be a function of the β of BJT  104 , as defined in equation 5. Because β may be for the most part unknown to the user, and because it may change as a function of temperature, it may be desired to have an automated way to optimally set the low collector current I CL  of BJT  104  (in effect, by optimally setting I REF ) such that the emitter current I E  of BJT  104  is close to its optimum value.  
         [0034]     As shown in  FIG. 4 , circuit  432  may be configured with four NMOS arrays  422 ,  426 ,  428 , and  430 , and single NMOS device  424 . As before, each NMOS array may comprise a specified number of parallel-coupled NMOS devices, the specified number indicated by the respective value of ‘m’ in each case. For example, NMOS array  422  may comprise ten identical, parallel-coupled NMOS devices, NMOS array  426  may comprise two identical, parallel-coupled NMOS devices, and so on. By way of example, if PMOS device  402  and PMOS array  404  are determined to operate most accurately when the low collector current I CL  of BJT  104  is at 10 μA, then the initial value I EMAX  of the total drain current of NMOS array  422  may be set to 10 μA. In one embodiment, I EMAX  is set by setting I SUPPLY  to 5 μA, and applying that 5 μA current to the drain of NMOS array  430 . The drain current of NMOS array  430  may then be mirrored two to one by NMOS array  422 , effectively setting I EMAX  flowing into the drain of NMOS array  422  to 10 μA. The Enable signal may be asserted to initiate the β detection process, and the Clock signal may be used to time the logic sequence in Digital Control block  420 . Once the Enable signal has been asserted, switches  434 ,  436  and  438  may stepped in a binary sequence via control outputs sw 2 , sw 3  and sw 4 , respectively, where control output sw 2  may be configured as the least significant bit (LSB), and control output sw 4  may be configured as the most significant bit (MSB), thereby providing a way for incrementing I REF  from 1 μA to 7 μA in 1 μA steps. In other words, NMOS device  424  in conjunction with switch  434  and control output sw 2  may provide a 1 μA step, while NMOS array  426  in conjunction with switch  436  and control output sw 3  may provide a 2 μA step, and NMOS array  428  in conjunction with switch  438  and control output sw 4  may provide a 4 μA step.  
         [0035]     It should be noted again that the value of 10 μA is provided only as an example, and this value may be any specified value determined as the desired value of I E  corresponding to the low collector current I CL  of BJT  104 , and to be applied to the emitter of BJT  104 . Thus, the value of I SUPPLY  and the number of individual devices in each of the NMOS arrays (for example  422 ,  426 ,  428 ,  430 ) may be set in accordance with the desired step increment and the determined initial value I EMAX  for a corresponding desired I E  for BJT  104 .  
         [0036]     Still referring to  FIG. 4 , as I REF  is incremented, or stepped up, the current drawn from PMOS device  406  may increase, and may be mirrored, one to one, by PMOS device  418 . Meaning, again, that the current flowing from the drain of PMOS device  418  may be equivalent to the current flowing from the drain of PMOS device  406 . When the current in PMOS device  406 , and hence in PMOS device  428 , exceeds 10 μA, or the specified value, node  432  connected to the “Detect” port of digital control block  420  transitions (in this case from low voltage to high voltage), resulting in digital control block  420  no longer incrementing I REF , thereby halting the β detection process. Once this occurs, I REF  may be decremented by 1 μA, where it may remain for the duration of the temperature conversion. I REF  may therefore be set such that the emitter current I E  from the PMOS mirror comprising PMOS device  410  and PMOS array  404  is as close to its optimal value (in this case determined to be 10 μA) without exceeding it. Alternative ways of generating and incrementing I REF  are possible and while not shown, are contemplated.  
         [0037]     Thus, various embodiments of the systems and methods described above may facilitate the design of a temperature sensor system that uses a current control circuit to provide accurately ratioed currents to a low-β, transistor used to obtain temperature information, by controlling the collector current developed in the transistor.  
         [0038]     Although the embodiments above have been described in considerable detail, other versions are possible. Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications. Note the section headings used herein are for organizational purposes only and are not meant to limit the description provided herein or the claims attached hereto.