Programmable ideality factor compensation in temperature sensors

A temperature sensor circuit and system providing accurate readings using a temperature diode whose ideality factor may fall within a determined range. In one set of embodiments a change in diode junction voltage (ΔVBE) proportional to the temperature of the diode is captured and provided to an ADC, which may perform required signal conditioning functions on ΔVBE, and provide a numeric value output corresponding to the temperature of the diode. Errors in the measured temperature that might result from using diodes with ideality factors that differ from an expected ideality factor may be eliminated by programming the system to account for differing ideality factors. The gain of the temperature sensor may be matched to the ideality factor of the temperature diode by using an accurate, highly temperature stable reference voltage of the ADC to set the gain of the temperature measurement system. The reference voltage may have a trim capability to change the gain setting voltage by a digital address comprising a determined number of bits, with the programmable range for the reference voltage corresponding to a determined range of ideality factors.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to the field of integrated circuit design and, more particularly, to the design of temperature sensor and measurement devices.

2. Description of the Related Art

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't malfunction or break due to thermal problems.

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 (ΔVBE) in the base-emitter voltage (VBE) between the diodes is generally proportional to temperature. (It should be noted that while VBEgenerally 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, VBEis used herein to refer to the voltage developed across a PN-junction in general.) More specifically, a relationship between VBEand temperature is defined by the equation

VBE=η⁢kTq⁢ln⁢IIs(1)
where η is the ideality factor of the PN junction, k is Boltzman's constant, q is the charge of a single electron, T represents absolute temperature, ISrepresents saturation current and I represents the collector current. A more efficient and precise method of obtaining ΔVBEis to supply the PN junction of a single diode with two separate and different currents in a predetermined ratio. Consequently, ΔVBEmay be related to temperature by the equation

Δ⁢⁢VBE=η⁢kTq⁢ln⁡(N)(2)
where N is a constant representing a pre-selected ratio of the two separate currents that are supplied to the PN junction of the diode.

A typical dynamic range of ΔVBE, however, is small relative to dynamic ranges that are typical of analog-to-digital converters (ADCs). That is, ΔVBE, which is used to measure the PN junction temperature, generally has a small dynamic range, for example on the order of around 60 mV in some systems. Therefore it is generally required to further process ΔVBEin order to match the dynamic range of ADCs. Typically, in order to obtain the desired conversion values at various temperatures, ΔVBEis multiplied by a large gain, and then centered to zero, which can be accomplished by subtracting a fixed voltage.

In general, implementations today perform the temperature signal processing (TSP) in a separate temperature sensor circuit that generates a sufficiently large voltage signal, which is fed into a separate ADC that may have been designed using a number of different topologies. Temperature-to-digital converters (TDC) of such implementations usually contain complex circuits. The yield of these TDCs during the fabrication process may also be low as there are many components that need to be matched for a given process spread.

An example of a typical temperature measurement system, which includes an ADC, is illustrated inFIG. 1. A TSP circuit100is coupled to an ADC130. TSP100may comprise current sources104and106, where a current provided by104is an integer (N) multiple of a current provided by106, a diode102, an integration capacitor126, an offset capacitor122, a gain capacitor124, and an operational amplifier (OP-AMP)120, interconnected as illustrated inFIG. 1. P1110and P2112represent non-overlapping clocks that provide switching between two circuit configurations as shown. When P1110is closed, current source104powers TSP100and P2112is open. Similarly, when P2112is closed, current source106powers TSP100and P1110is open. Switching between current sources104and106, different currents are forced through the junction of diode102resulting in a change in diode-junction-voltage (ΔVBE). Although omitted inFIG. 1, it should be understood that when either P1110or P2112is open, the respective uncoupled current source may be shunted to ground. In the circuit configuration shown, voltage sampling occurs when P1110is closed, and charge transfer takes place when P2112is closed. In other words, during operation, switching from a configuration of P1110closed and P2112open to a configuration of P1110open and P2112closed, results in ΔVBEeffectively “pumping” charge to gain capacitor124, which in turn leads to integration capacitor126also receiving a charge. More specifically, opening P1110and closing P2112results in a value drop of diode-junction-voltage VBE, expressed as ΔVBE. Consequently, ΔVBEappears across the terminals of capacitor126, in case capacitor126is equal in value to capacitor124. If capacitor124is greater in value than capacitor126, then ΔVBEwill be amplified, or “gained up”, hence an amplified value Vtemp131will appear at the output of OP-AMP120. Voffset132is subtracted through offset capacitor122.

Voltage-temperature relationships characterizing TSP100may be described by the following equations:
Vtemp=CT/CI*ΔVBE(T)−CO/CI*Voffset, where
CT/CI=(ADCdynamic range)/(ΔVBE(Tmax)−ΔVBE(Tmin)), and
Voffset=(CT/CI*ΔVBE(Tmax)−(ADCdynamic range))*CI/CO.
Tmax and Tmin represent maximum and minimum diode temperatures, respectively. ADC dynamic range indicates a range of valid voltage values required for proper ADC operation.

Temperature measurement systems that employ diodes feature a variety of types of diodes. Some examples are the Prescott processor diode and the 2N3904 discrete diode. Each type of diode typically has its own ideality factor, which may lead to errors in measurements obtained using a temperature sensor in which a particular diode is configured. This problem generally occurs when temperature measurement systems are designed/trimmed for a single diode ideality factor. When using a diode different from one for which such a temperature measurement system was designed, a measurement error may be incurred. Any such error would have to be corrected if an accurate measurement was desired. A typical solution is to add a constant offset to the temperature sensor output. That is, a constant offset is added to an obtained temperature measurement value. This can generally be performed in the temperature sensor itself or in the external processor with which the sensor interfaces. However, this method of correction is error prone as a different ideality factor leads to a non-constant error across measured temperatures. This is due to the fact that the different ideality factor produces a gain error not an offset error. In some cases an offset register may be used to correct for this error.

Generally, as part of offering a solution, temperature sensor manufacturers typically design their devices to work optimally for a unique ideality factor (i.e. the Prescott processor ideality factor, ˜1.011) without any capability to adapt to different diodes without having to program in an offset value. Generally, ideality factors can change over process or can be incorrectly specified at the initial time of sensor design and may need to be corrected once the design is complete.

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

In one set of embodiments the invention comprises a system and method for performing temperature monitoring in a digital system by capturing a change in a diode junction voltage (ΔVBE), which is proportional to a temperature of the diode, and using an analog-to-digital converter (ADC) to perform on ΔVBErequired signal conditioning functions with the output of the ADC providing a numeric value corresponding to the temperature of the diode. Errors in the measured temperature that may result from using diodes with ideality factors that differ from the ideality factor for which the temperature measurement device has been primarily configured may be eliminated by programming the device to account for differing ideality factors. In one embodiment this is accomplished by matching the gain of the temperature sensor to the ideality factor of the diode that is used for performing the temperature monitoring.

In one embodiment, matching the gain of the temperature sensor to the ideality factor of the diode is performed by using an accurate, highly temperature stable reference voltage to set the gain of the temperature measurement system. The reference voltage may already have a trim capability to change the gain setting voltage by a digital address comprising a determined number of bits. Each least significant bit (LSB) of the trim word may change a 1.500V reference voltage by ˜1.1 mV. The reference voltage value (from 1.46488V to 1.5351V, for example) may be programmed through a coupled bus by an end user, using the trim bits. The programmable sensor may thus be used to accurately measure the temperature of diodes with ideality factors falling within a determined range that corresponds to the reference voltage range, for example within a range of 0.985 to 1.032. This range may be increased to accommodate a broader range of ideality factors, or may be decreased when accommodating a narrower range of ideality factors.

Thus, various embodiments of the invention may provide a means for performing temperature monitoring/measurement by applying a ΔVBEsignal to an ADC that performs signal-processing functions, including matching and centering the voltage range of ΔVBEto the dynamic range of the ADC, while accommodating a range of ideality factors for different diodes that may be used in the temperature monitoring/measurement system, thus obtaining measurements substantially free of errors that may occur due to the different ideality factors of the different diodes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As used herein, the word “alternately” is meant to imply passing back and forth from one state, action, or place to another state, action, or place, respectively. For example, “alternately applying a first current source and a second current source” would mean applying the first current source, then applying the second current source, then applying the first current source, then applying the second current source, and so on.

A “diode-junction-voltage” (VBE) refers to a voltage measured across the junction of a diode, or a difference in voltage between a voltage measured at the anode of the diode junction with respect to a common ground and a voltage measured at the cathode of the diode junction with respect to the common ground. A “change in diode-junction-voltage” (ΔVBE) refers to a change in diode-junction-voltage for a chosen diode, either in time or in different circuit configurations. For example, if in one circuit configuration VBE=700 mV for a diode, and in a different circuit configuration VBE=655 mV for the diode, then ΔVBE=45 mV for the diode when referencing to the two different circuit configurations. Similarly, for example, if at a time point t1VBE=650 mV for a diode, and at a time point t2VBE=702 mV for the diode, then ΔVBE=52 mV for the diode when referencing time points t1and t2. “Storing” a VBEor VBEvalue in an integrator generally refers to developing a charge corresponding to the VBEvalue within the integrator. “Adding” and/or “subtracting” a VBEor VBEvalue in the integrator generally refers to increasing and/or decreasing the developed charge within the integrator, correspondingly to the VBEvalue.

A diode is used as one way of accessing a PN-junction across which voltage measurements to obtain VBEmay be made. More generally, diode-junction may also mean PN-junction or NP-junction, which defines the physical attributes of the junction selected for obtaining temperature values through performing voltage measurements. Various embodiments of the circuit are described as utilizing a diode. However, in other embodiments, the operation performed by the diode may be achieved using other circuitry, such as a PN-junction (or NP-junction) present in devices other than a diode, for example bipolar junction transistors (BJTs). Therefore, the terms PN-junction, NP-junction, diode, and diode-junction are used interchangeably, and all respective terms associated therewith may be interpreted accordingly.

FIG. 2illustrates a block diagram of one embodiment of a temperature sensor circuit implemented in accordance with the present invention. In the embodiment shown, current sources I1210and I2212can be individually coupled to diode222via respective switches P3and P4. Diode222may be coupled to the inputs of ADC224as shown. For more detail on possible embodiments of ADC224and the coupling of diode222to ADC224, please refer to U.S. patent application Ser. No. 10/624,394 titled “Temperature-to-Digital Converter” invented by Troy L. Stockstad and filed on Jul. 22, 2003, which is hereby incorporated by reference in its entirety as though fully and completely set forth herein. One possible way of operating the circuit ofFIG. 2may be by alternately applying I1210and I2212to diode222, with the resulting ΔVBEacross diode222used by ADC224to provide a numeric (digital) value corresponding to the temperature of diode222. ADC224may 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 I1may be an N-multiple of the value of I2, as also illustrated in equation (2).

The ideality factor for diode222may be expressed in terms of equation (2) as follows:

η*T=Δ⁢⁢VBE*qk*ln⁡(N).(3)
Equation (3) indicates that as the ideality factor increases the temperature appears to be increasing. For example, assuming an ideality factor of 1.000 for diode222when the actual ideality factor of diode222was 1.008, the measured temperature of diode222would be increased by a factor of 0.008. In other words, the measured temperature of diode222would be higher by 0.008T.

In one embodiment, the variability of the temperature measurements due to varying ideality factors is addressed by providing a means for programming the ideality factor value required for diode222, in order to obtain an accurate temperature measurement. This programmability may be made available for all diodes, including diodes other than diode222shown, configured in circuits that are equivalent to the circuit shown inFIG. 2for obtaining temperature measurements. In one embodiment, the programming of the ideality factor is accomplished by changing the reference voltage for ADC224. An adjustment to the reference voltage Vrefof ADC224may be made based on the following relationship:
Vref—new=(ηdiode/ηADC)*Vref(4)
where ηdiodeis the ideality factor of diode222(for example, 1.008) and ηADCis the ideality factor for which ADC224has originally been set (for example, 1.000).

FIG. 3illustrates the temperature sensor circuit ofFIG. 2configured within a thermal management system300in which the temperature sensor circuit is coupled to a bus interface. InFIG. 3, temperature sensor circuit350comprises switching current302, which is provided to temperature diode304, which is itself coupled to delta-sigma ADC308. A bandgap reference voltage Vrefmay be provided to ADC308by bandgap reference circuit306. A set of registers may be configured to store the numeric value of the measured temperature as well as a variety of programmable parameters associated with temperature sensor circuit350, and the registers may be read and/or written through bus interface330. In one embodiment, the value of the measured temperature is stored in Temperature Register310, from which it is transmitted through SMDATA pin2via SMBus Interface330to a host device that may be coupled to thermal management system300. As shown inFIG. 3, an Ideality Factor Register316may be programmed via SMBus interface330to hold a desired ideality factor corresponding to Temperature Diode304configured in temperature sensor circuit350. Bandgap Reference Circuit306may be adjusted based on the value of the programmed ideality factor as illustrated by the coupling of Ideality Factor Register316to Bandgap Reference Circuit306, thus setting Vreffor ADC308. It will be apparent to those skilled in the art that a variety of other bus interfaces may be used in lieu of SMBus Interface330, and other configurations in which the ideality factor may be programmed to adjust the bandgap reference—Vref—are possible and may be contemplated. Furthermore, the circuit ofFIG. 2may serve as an embodiment of temperature circuit350ofFIG. 3, with delta-sigma ADC308being an equivalent of ADC224.

Referring again toFIG. 2, ADC224may be a delta-sigma ADC that performs required signal conditioning functions on ΔVBE, where setting Vrefmay result in a setting of the gain of ADC224as well as the offset voltage Voffsetof ADC224. The required ADC temperature gain in this embodiment may be expressed as:

Temp⁢⁢gain=VrefΔ⁢⁢VBE⁡(max)-Δ⁢⁢VBE⁡(min).(5)
As an example, the ratio ‘N’ between the current provided by current source I1210and the current provided by current source I2212may be selected to be 17 with a desire to obtain temperature measurements within a range of −64° C. to 191° C. In this case, equation (2) becomes:

Δ⁢⁢VBE⁡(max)=η*k*(273.15+191)q*ln⁡(17),(7)
where 273.15 is the conversion value required to convert from ° C. to ° K. Equation (7) then leads to the following ΔVBEvalue expressed in terms of the ideality factor of diode222:
ΔVBE(max)=η*113.32 mV.  (8)
Similarly, ΔVBEat the minimum temperature −64° C. may be expressed as:

Temp⁢⁢gain=15.00⁢⁢Vη*(113.32-51.06)⁢m⁢⁢V=24η.(11)
The required ADC voltage offset (Vos) gain may be expressed by:

VOS⁢⁢gain=TempGain*Δ⁢⁢VBE⁡(min)Vref.(12)
Substituting the results from equations (10) and (11), and the selected value of Vref(1.500V) into equation (12), the Vosgain becomes:

VOS⁢⁢gain=24η*η*(51.06⁢⁢m⁢⁢V)1.500⁢⁢V=0.81696.(13)
If, for example, ADC224is designed for η=1.000, then it follows from equations (11) and (12) that the Temp Gain=24 and the Vosgain=0.81696, respectively.

If a change in the ideality factor can lead to a change in Vrefwhile both the Temp gain and Vosgain remain the same, then programming for different ideality factors may be accomplished by trimming only Vref. More generally then, from equations (5) and (2), the temperature gain may be expressed in the following equation for a temperature measurement range of Tminto Tmaxand a current ratio of ‘N’:

Temp⁢⁢gain=Vrefη*kq*ln⁡(N)*(Tmax-Tmin).(14)
If ADC224is originally designed for an ideality factor η=1.000 with corresponding reference voltage value Vref(original) then equation (14) becomes:

It may become readily apparent that substituting η*Vref(original) for Vrefin equation (14) may lead to equation (15), and thus the value of the temperature gain as expressed in equation (15) becomes independent of η. In other words, if Vref(original) represents a reference voltage value originally assigned to ADC224corresponding to an initial ideality factor of ηi=1.000, then if η changes to a different value, changing the reference voltage value to a value of η*Vref(original) will result in the temperature gain not changing, as expressed in equation (15).

Similarly, having established that the temperature gain may not change under the conditions as described above, from equations (12) and (2) the voltage offset gain may be expressed generally as:

where Tminis expressed in terms of ° K. Having originally designed ADC224for an ideality factor η=1.000 with corresponding reference voltage value Vref(original), equation (16) may be written as:

Again, it is readily apparent that substituting η*Vref(original) for Vrefin equation (16) may lead to equation (17), and thus the value of the voltage offset gain as expressed in equation (17) becomes independent of η. In other words, if Vref(original) represents a reference voltage value originally assigned to ADC224corresponding to an initial ideality factor of ηi=1.000, then if η changes to a different value, changing the reference voltage value to a value of η*Vref(original) will result in the voltage offset gain not changing, as expressed in equation (17). Thus, programming for different ideality factors may be accomplished by trimming only Vref.

Turning again toFIG. 3, in one embodiment Ideality Factor Register316is a six-bit register used to provide a value to Bandgap Reference306to trim Vrefin 1.08 mV steps. In this embodiment, by way of example, the minimum value of Vrefmay be selected to be 1.46646V, and the corresponding maximum value of Vrefmay be selected to be 1.5344V, with Vref(original)=1.5V. Thus the minimum possible ideality factor for Temp Diode304may be 1.46646/1.5=0.9776, and the maximum possible ideality factor may be 1.5344/1.5=1.0229. In this embodiment, Vrefmay be trimmed by programming Ideality Factor Register316for ideality factors (of Temp Diode304) ranging from 0.9776 to 1.0229, thus making temperature sensor circuit350more versatile and not confined to operate with diodes of only one particular ideality factor.

Thus, various embodiments of the systems and methods described above may facilitate the design of temperature sensor circuit that uses a temperature diode to obtain temperature measurements, and which may operate accurately for a variety of diodes whose ideality factors fall within a programmable range. Different ideality factors may be user programmable through trimming the reference voltage of an ADC used in obtaining digital temperature measurements from the temperature diode, without changing either the temperature gain or the voltage offset gain.

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.