Low power tunable reference voltage generator

Abstract

A method and apparatus for generating an improved reference voltage for use, for example, in a system requiring accurate low power operation. In particular, our reference voltage generator is adapted to output VREF as a function of the voltage difference between V1 and V2. The reference voltage generator is further adapted to include our reference voltage tuner to compensate for predetermined sensitivities of the reference voltage VREF, and to adjust the absolute value of VREF. During manufacturing and system test, a driver may be used to drive a buffered or unbuffered version of VREF to off-chip test functionality. Also, a configuration memory may be used to store the trim settings during normal operation, and make such settings available to outside resources.

Claims

1. A tunable reference voltage generator comprising: a reference voltage generator adapted to develop a first reference voltage, said reference voltage generator comprising a sensitivity tuner adapted selectively to tune said first reference voltage to adjust for a predetermined sensitivity of said first reference voltage; and a variable gain amplifier, coupled to said reference voltage generator, adapted to receive said first reference voltage and to develop a second reference voltage as a predetermined function of said first reference voltage.

2. The tunable reference voltage generator of claim 1 wherein the sensitivity tuner is further characterized as comprising a programmable plurality of binary weighted transistors.

3. The tunable reference voltage generator of claim 1 wherein the reference voltage generator is further characterized as comprising: a first transistor, biased to operate in a subthreshold region; and a second transistor biased to operate in a subthreshold region, coupled in series with said first transistor so as to develop said first reference voltage on an electrode common to said first transistor and said second transistor.

4. The tunable reference voltage generator of claim 1 wherein the reference voltage generator is further characterized as comprising: a first transistor biased to operate in a subthreshold region; a second transistor biased to operate in a subthreshold region, coupled in series with said first resistor so as to develop a first reference voltage on an electrode common to said first transistor and said second transistor; a third transistor biased to operate in a subthreshold region; a fourth transistor biased to operate in a subthreshold region, coupled in series with said third resistor so as to develop a second reference voltage on an electrode common to said third resistor and said fourth resistor; and a difference amplifier adapted to develop the second reference voltage as a function of a voltage difference between the first reference voltage and the second reference voltage.

5. The tunable reference voltage generator of claim 1 wherein the reference voltage generator is further characterized as a bandgap voltage reference generator adapted to provide said first reference voltage.

6. The tunable reference voltage generator of claim 1 wherein the variable gain amplifier is further characterized as comprising: a difference amplifier adapted to: receive said first reference voltage and a function of said second reference voltage; and develop a difference voltage as a function of a voltage difference between said first reference voltage and a function of said second reference voltage; and a voltage divider adapted to: receive the difference voltage; and develop said second reference voltage as a function of the difference voltage.

7. The tunable reference voltage generator of claim 6 wherein the voltage divider is further characterized as comprising a variable divider coupled in series with a fixed divider, said variable divider adapted selectively to tune an absolute value of said second reference voltage.

8. The tunable reference voltage generator of claim 7 wherein the variable divider is further characterized as comprising a programmable plurality of transistors, each biased in the subthreshold region and coupled in series with each other in a diode stack topology.

9. A method for tuning a reference voltage, the method comprising the steps of: (1) developing a first reference voltage comprising: (1A) tuning said first reference voltage to adjust for a predetermined sensitivity of said first reference voltage; and (2) developing a second reference voltage as a predetermined function of said first reference voltage.

10. The method of claim 9 further characterized as: (1A1) programming a resistance, said resistance selected from a plurality of binary weighted increments; and (1A2) tuning said first reference voltage, as a function of said programmed resistance, to adjust for a predetermined sensitivity of said first reference voltage.

11. The method of claim 9 wherein step (2) is characterized as: (2A) developing a difference voltage as a function of a voltage difference between said first reference voltage and a function of said second reference voltage; and (2B) developing said second reference voltage as a function of the difference voltage.

12. The method of claim 9 further characterized as: (3) tuning said second reference voltage, as a function of a programmed resistance and a fixed resistance, to adjust for a predetermined sensitivity of said second reference voltage.

13. The method of claim 12 wherein said programmed resistance is further characterized as a function of a selected sum of forward biased diode voltage drops.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) Our invention may be more fully understood by a description of certain preferred embodiments in conjunction with the attached drawings in which:

(2) FIG. 1 illustrates, in block diagram form, a typical integrated system;

(3) FIG. 2 illustrates, in schematic diagram form, four prior art reference voltage generators;

(4) FIG. 3 illustrates, in block diagram form, a reference voltage generator constructed in accordance with our invention;

(5) FIG. 4 illustrates, in flow diagram form, the sequencing of the operations in the reference voltage generator of FIG. 3;

(6) FIG. 5 illustrates, in schematic diagram form, a reference voltage generator constructed in accordance with one embodiment of our invention;

(7) FIG. 6 illustrates, in schematic diagram form, a reference voltage generator constructed in accordance with another embodiment of our invention;

(8) FIG. 7 illustrates, in block diagram form, a reference voltage generator system constructed in accordance with yet another embodiment of our invention;

(9) FIG. 8 illustrates, in flow diagram form, the sequencing of the operations in the reference voltage generator system of FIG. 7;

(10) FIG. 9 illustrates, in schematic diagram form, a trimmable reference voltage generator with a first trimmer to compensate for predetermined sensitivities of the reference voltage, and a second trimmer to adjust the absolute value of the reference voltage.

(11) FIG. 10 illustrates, in flow diagram form, the sequencing of the operations of the trimmable reference voltage generator of FIG. 9; and

(12) FIG. 11 illustrates, in schematic diagram form, an example of a variable divider constructed in accordance with various embodiments of our invention.

(13) In the drawings, similar elements will be similarly numbered whenever possible. However, this practice is simply for convenience of reference and to avoid unnecessary proliferation of numbers, and is not intended to imply or suggest that our invention requires identity in either function or structure in the several embodiments.

DETAILED DESCRIPTION OF THE INVENTION

(14) Shown in FIG. 3, by way of example, is our reference voltage generator 12E, constructed in accordance with our invention; and in FIG. 4 is an exemplary method for the sequencing of the operations of reference voltage generator 12E. In general, for such an arrangement, series-coupled MOSFET transistors, M1 and M2, of a reference voltage generator 12F (VR 12F) are each biased in the subthreshold region (step 18) to develop a first output voltage (V.sub.1) on an electrode common to M1 and M2 (step 20). In general, a transistor biased in the subthreshold region is defined by the well known relationship, where the absolute value of the gate-to-source voltage (V.sub.GS) of the transistor is less than the absolute value of the threshold voltage (V.sub.th) of the transistor:
|V.sub.GS|<|V.sub.th|[Eq. 1]
Also, series-coupled MOSFET transistors, M3 and M4, of a reference voltage generator 12G (VR 12G) are each biased in the subthreshold region (step 24) to develop a second output voltage (V.sub.2) on an electrode common to M3 and M4 (step 26). A difference amplifier 22A develops an output reference voltage, V.sub.REF, as a function of the voltage difference between V.sub.1 and V.sub.2 (step 28). The electrical parameters of VR 12F and VR 12G may be tuned such that V.sub.REF is proportional-to-absolute temperature (PTAT), or complementary-to-absolute temperature (CTAT).

(15) In alternate embodiments, the series-coupled MOSFET transistor circuits of VR 12F and VR 12G may each be designed using various transistor types and gate, drain, source, and bulk electrode connections. In general, it is known that a variety of bulk electrode connections are possible, for example, when a transistor is biased in the subthreshold region. As examples, the bulk electrode may be coupled to the source electrode, the drain electrode, ground, a supply voltage, or a circuit node. Since typical integrated circuit design uses a common bulk electrode for same type transistors, we have chosen to simplify our disclosure by not further discussing the variety of known possible bulk electrode connections.

(16) In general, MOSFETs M1, M2, M3, and M4 may each be individually manufactured as an N-channel MOSFET or a P-channel MOSFET. Those skilled in the art will appreciate that gate electrode (G), drain electrode (D), and source electrode (S) for each transistor of reference voltage generator 12E may each be coupled, for example, to a bias voltage, a positive supply voltage, or ground, providing the respective transistor is biased in the subthreshold region. If desired, different V.sub.th values may be manufactured for M1, M2, M3, and M4 through various approaches, for example, by using a predetermined combination of doping implants, gate width sizes, gate length sizes, and bias voltages of the substrate and wells. Various alternate topologies that apply to VR 12F and VR 12G are discussed, for example, in the Related Application. Also, those skilled in the art of integrated circuits will recognize that difference amplifier 22A may be implemented using various hardware circuits and software algorithms. As discussed in our various embodiments, a difference amplifier is not restricted to any specific circuit structure.

(17) Shown in FIG. 5, by way of example, is our reference voltage generator 12H, constructed in accordance with one embodiment of our invention. In general, for such an arrangement, the series-coupled MOSFET transistors, P-channel M1 and N-channel M2, of VR 12I are each biased in the subthreshold region to develop a first output voltage, V.sub.1. Also, P-channel M3 and N-channel M4, of VR 12J, are each biased in the subthreshold region to develop a second output voltage, V.sub.2. For reference voltage generator 12H, the current I.sub.1 of VR 12I and the current I.sub.2 of VR 12J are constrained to be proportional to one another by coupling the gate electrodes of M1 and M3, respectively, to a selected bias voltage, V.sub.BIAS. Difference amplifier 22B develops V.sub.REF as a function of the voltage difference between V.sub.1 and V.sub.2. In this configuration, the electrical parameters of VR 12I and VR 12J may be selectively tuned such that V.sub.REF is substantially temperature insensitive, PTAT, or CTAT.

(18) By way of example, N-channel transistors M2 and M4 are each biased in the subthreshold region by coupling the gate electrode of each to the respective drain electrode. Also, P-channel transistors M1 and M3 are each biased in the subthreshold region by applying a predetermined V.sub.BIAS to the gate electrode. Using a current mirror principle, the current I.sub.1 of VR 12I and the current I.sub.2 of VR 12J are constrained to be proportional to one another. Note that the values of I.sub.1 and I.sub.2 are not constrained to equal one another, and can be skewed by a predetermined multiple. For example, I.sub.1 may be two times larger than I.sub.2. Typically, M2 and M4 will be manufactured with different V.sub.th values. As previously discussed, different V.sub.th values may be achieved for M2 and M4 using various manufacturing and transistor geometry techniques. Note that the use of different doping implants typically results in significant post-manufacturing variation since the doping levels of different implants will vary in an uncorrelated manner. This variation may be minimized by manufacturing the transistors with substantially identical implants. Finally, although V.sub.1 and V.sub.2 may each be used as independent reference voltages, using V.sub.REF as the reference voltage for the integrated system is generally preferred since, as we will explain later, V.sub.REF has improved stability when compared to either V.sub.1 or V.sub.2.

(19) As is well known, the current, I.sub.1, through M2 and the current, I.sub.2, through M4, when both are biased in the subthreshold region, and when the drain to source voltage (V.sub.DS) is greater than 3 times the thermal voltage (T), is given by equations 2 and 3, respectively:

(20) $\begin{array}{cc}{I}_1=K_2{e}^{\frac{V_1-V_{\mathrm{th},2}}{m_2{v}_T}}& \left[\mathrm{Eq}.2\right]\\ I_2=K_4{e}^{\frac{V_2-V_{\mathrm{th},4}}{m_4{v}_T}}& \left[\mathrm{Eq}.3\right]\end{array}$ where: Ki(i=1, 2, . . . )=a function of transistor dimensions, mobility, and temperature; Vth,i(i=1, 2, . . . )=the threshold voltage of device Mi(i=1, 2, . . . ); mi(i=1, 2, . . . )=the subthreshold slope factor for Mi(i=1, 2, . . . ); and vT=the thermal voltage, which is a function of temperature.

(21) If the associated P-channel MOSFETs M1 and M3 of VR 12I and VR 12J, are manufactured substantially similar, I.sub.1 and I.sub.2 may be set equal, as shown in Equation 4. As is known, I.sub.1 and I.sub.2 may also be multiples of one another for various sizes of M1 and M3. For example, if the width of M1 is 3 times larger than the width of M3, it follows that, in general, I.sub.1 will be 3 times larger than I.sub.2.

(22) $\begin{array}{cc}{K}_2{e}^{\frac{V_1-V_{\mathrm{th},2}}{m_2{v}_T}}=K_4{e}^{\frac{V_2-V_{\mathrm{th},4}}{m_4{v}_T}}& \left[\mathrm{Eq}.4\right]\end{array}$

(23) As is known, by taking the logarithm (ln) of both sides of equation 4, where: ln e.sup.x=x ln e, ln e=1, and ln xy=ln x+ln y, equation 6 may be derived from equation 5 as:

(24) $\begin{array}{cc}{m}_4{v}_T\ln \left(\frac{K_2}{K_4}\right)+\frac{m_4}{m_2}{V}_1-\frac{m_4}{m_2}{V}_{\mathrm{th},2}=V_2-V_{\mathrm{th},4}& \left[\mathrm{Eq}.5\right]\\ V_2-\frac{m_4}{m_2}{V}_1=V_{\mathrm{th},4}-\frac{m_4}{m_2}{V}_{\mathrm{th},2}+m_4{v}_T\ln \left(\frac{K_2}{K_4}\right)& \left[\mathrm{Eq}.6\right]\end{array}$

(25) Referring to equation 6, it can be seen that V.sub.REF is a function of

(26) $V_2-\frac{m_4}{m_2}{V}_1.$
If we further approximate that the subthreshold slope factors, m.sub.2 and m.sub.4, are equivalent, then the relationship simplifies to:

(27) $\begin{array}{cc}{V}_2-V_1=V_{\mathrm{th},4}-V_{\mathrm{th},2}+m_4{v}_T\ln \left(\frac{K_2}{K_4}\right)& \left[\mathrm{Eq}.7\right]\end{array}$
V.sub.REF, which is proportional to V.sub.2-V.sub.1, may be made substantially temperature-insensitive, PTAT, or CTAT by tuning parameters K.sub.2 and K.sub.4. Also, the temperature dependency of K.sub.2 and K.sub.4 are substantially the same and consequently these terms will cancel.

(28) As is known, in alternate embodiments, the series-coupled MOSFET transistor circuit of VR 12I and VR 12J may each be designed using various transistor types and gate, drain, source, and bulk electrode connections, resulting in slightly modified equations.

(29) The difference topology of reference voltage generator 12H is advantageous. For example, M2 and M4 may be designed using transistors with substantially the same doping implants. Such a topology achieves a significantly reduced sensitivity to PVT variations when compared to the circuit topologies 12A, 12B, and 12C of FIG. 2.

(30) Shown in FIG. 6, is our reference voltage generator 12K, constructed in accordance with another embodiment of our invention. In general, for such an arrangement, the series-coupled MOSFET transistors, N-channel M1 and N-channel M2, of VR 12L, are each biased in the subthreshold region to develop a first output voltage, V.sub.1. Also, N-channel M3 and N-channel M4 of VR 12M are each biased in the subthreshold region to develop a second output voltage, V.sub.2. For reference voltage generator 12K, the operational current of VR 12L, I.sub.1, is independent of the operational current of VR 12M, I.sub.2. Reference voltage generator 12K develops V.sub.REF as a function of the voltage difference between V.sub.1 and V.sub.2. The electrical parameters of VR 12L and VR 12M may be selectively tuned such that V.sub.REF is substantially temperature insensitive, PTAT, or CTAT.

(31) Referring again to reference voltage generator 12K, transistors M2 and M4 are each biased in the subthreshold region by coupling the gate electrode of each to the respective drain electrode. Also, transistors M1 and M3 are each biased in the subthreshold region by coupling the gate electrode of each to the respective source electrode. VR 12L and VR 12M operate such that V.sub.1 and V.sub.2 achieve a predetermined difference in voltage such that difference amplifier 22C may develop V.sub.REF as a function of the voltage difference between V.sub.1 and V.sub.2. Finally, although V.sub.1 and V.sub.2 may each be used as independent reference voltages, using V.sub.REF as the reference voltage for the integrated system 10 is generally preferred since, as we will explain later, V.sub.REF has improved stability when compared to either V.sub.1 or V.sub.2.

(32) By way of example, for reference voltage generator 12K, M1 and M2 are each biased in the subthreshold region. As is known, the current I.sub.1, associated with the series-coupled M1 and M2 is given by equations 8 and 9, respectively:

(33) $\begin{array}{cc}{I}_1=K_1{e}^{\frac{-V_{\mathrm{th},1}}{m_1{v}_T}}& \left[\mathrm{Eq}.8\right]\\ I_1=K_2{e}^{\frac{V_1-V_{\mathrm{th},2}}{m_2{v}_T}}& \left[\mathrm{Eq}.9\right]\end{array}$

(34) Current I.sub.1, as represented by the right side of each equation, may be set equal to each other:

(35) $\begin{array}{cc}{K}_1{e}^{\frac{-V_{\mathrm{th},1}}{m_1{v}_T}}=K_2{e}^{\frac{V_1-V_{\mathrm{th},2}}{m_2{v}_T}}& \left[\mathrm{Eq}.10\right]\end{array}$

(36) Once again, by taking the logarithm of both sides of equation 10, equation 13 may be derived as:

(37) $\begin{array}{cc}\ln \left(\frac{K_1}{K_2}\right)-\frac{V_{\mathrm{th},1}}{m_1{v}_T}=\frac{V_1-V_{\mathrm{th},2}}{m_2{v}_T}& \left[\mathrm{Eq}.11\right]\\ m_2{v}_T\ln \left(\frac{K_1}{K_2}\right)-m_2\frac{V_{\mathrm{th},1}}{m_1}=V_1-V_{\mathrm{th},2}& \left[\mathrm{Eq}.12\right]\\ V_1=V_{\mathrm{th},2}-\frac{m_2}{m_1}{V}_{\mathrm{th},1}+m_2{v}_T\ln \left(\frac{K_1}{K_2}\right)& \left[\mathrm{Eq}.13\right]\end{array}$

(38) Subsequently, for I.sub.2, the same approach may be used to derive V.sub.2:

(39) $\begin{array}{cc}{V}_2=V_{\mathrm{th},4}-\frac{m_4}{m_3}{V}_{\mathrm{th},3}+m_4{v}_T\ln \left(\frac{K_3}{K_4}\right)& \left[\mathrm{Eq}.14\right]\end{array}$

(40) It can be seen, that the voltage difference between V.sub.1 and V.sub.2, which is proportional to V.sub.REF, may be temperature-insensitive, PTAT, or CTAT by tuning parameters K.sub.1, K.sub.2, K.sub.3, and K.sub.4:

(41) 0$\begin{array}{cc}{V}_2-V_1=V_{\mathrm{th},4}-\frac{m_4}{m_3}{V}_{\mathrm{th},3}-V_{\mathrm{th},2}+\frac{m_2}{m_1}{V}_{\mathrm{th},1}+m_4{v}_T\ln \left(\frac{K_3}{K_4}\right)-m_2{v}_T\ln \left(\frac{K_1}{K_2}\right)& \left[\mathrm{Eq}.15\right]\end{array}$

(42) As is known, in alternate embodiments, the series-coupled MOSFET transistor circuit of VR 12L and VR 12M may each be designed using various transistor types and gate, drain, source, and bulk electrode connections, resulting in slightly modified equations.

(43) The difference topology of reference voltage generator 12K is advantageous, for example, where the same associated transistor types are manufactured for VR 12L and VR 12M. Such a topology achieves a significantly reduced sensitivity to PVT variations when compared to the circuit topologies of 12A, 12B, and 12C of FIG. 2.

(44) Shown in FIG. 7 is our reference voltage generator system 30, constructed in accordance with yet another embodiment of our invention; and in FIG. 8 is an exemplary method for the sequencing of the operations of reference voltage generator system 30. As preferred trim settings are determined for reference voltage generator 12, typically by off-chip test equipment, or, alternatively, by on-chip circuitry, the preferred trim settings may be subsequently provided to a configuration memory 38 for internal storage, (see, e.g. related co-application) and subsequently read by reference voltage generator 12 for operational use (step 32). Reference voltage generator 12 may develop V.sub.REF or I.sub.REF, based on trim settings, using for example, a selected one of the reference voltage generator topologies illustrated in FIG. 2, FIG. 3, FIG. 5, and FIG. 6 (step 36). Alternately, any number of known, or yet to be invented, reference voltage generator 12 topologies, may be used to develop V.sub.REF or I.sub.REF (see, for example, V.sub.REF, I.sub.1, and I.sub.2 of FIG. 3). Reference voltage generator 12 may include trimming capability, with a first control to compensate for predetermined sensitivities of the reference voltage V.sub.REF, and a second control to adjust the absolute value of V.sub.REF. Buffer 34 may receive the trim settings, and V.sub.REF or I.sub.REF, and subsequently develop a buffered xV.sub.REF or a buffered xI.sub.REF, based on the trim settings, for use by internal functions such as on-chip modules (step 40). Typically, to improve transient noise immunity, using the buffered V.sub.REF or the buffered I.sub.REF for internal functions is preferred.

(45) During manufacturing and system test, driver 42 may be used to receive the unbuffered V.sub.REF, the buffered xV.sub.REF, the unbuffered I.sub.REF, and the buffered xI.sub.REF, and may subsequently provide signals as a function of V.sub.REF and I.sub.REF for use by off-chip equipment (step 44). Providing V.sub.REF to external functions, such as test equipment, provides a way to measure and subsequently determine preferred trim settings to be used by internal trimmer circuitry of reference voltage generator 12 (see, e.g. related co-application).

(46) In one embodiment, buffer 34 may provide a buffered V.sub.REF using a unity gain amplifier.

(47) In another embodiment, a common source amplifier and a current mirror may be included with reference voltage generator 12 to output a reference current (I.sub.REF) for distribution throughout the integrated system.

(48) In yet another embodiment, driver 42 may include a multiplexing function so that a pad used to communicate a digital signal, an analog signal, or a power signal, during normal mode, may be used to measure the driven V.sub.REF or the driven I.sub.REF during a manufacturing or system test mode. In yet another embodiment, driver 42 may comprise a voltage-to-current converter or a current-to-voltage converter to accommodate test equipment which requires a current input, or a voltage input, respectively. For this example, V.sub.REF and I.sub.REF may be provided subsequent to a current-to-voltage conversion or a voltage-to-current conversion.

(49) In yet another embodiment, during manufacture or system test, V.sub.REF or I.sub.REF may be read externally and adjusted iteratively until V.sub.REF or I.sub.REF have been trimmed to a predetermined value.

(50) In still another embodiment, configuration memory 38 may comprise banks of registers, or alternate storage embodiments, comprising a plurality of configuration bits for storing the desired trim settings for the reference voltage generator 12. Configuration memory 38 may typically be implemented using non-volatile memory, although any known memory type, such as RAM, flash memory, and one-time programmable memory may be used.

(51) Shown in FIG. 9, by way of example, is a trimmable reference voltage generator 46 constructed in accordance with another embodiment of our invention; and in FIG. 10 is an exemplary method for the sequencing of the operations of our trimmable reference voltage generator 46. Reference voltage generator 12 includes a sensitivity trimmer 48 adapted selectively to trim a first reference voltage, V.sub.OUT, to adjust for predetermined circuit sensitivities, such as PVT sensitivities, of V.sub.OUT. Programmable selectable N-channel transistors M1, 2_M1, and 4_M1 (collectively, n_M1) are each coupled in parallel to one another, and are each biased to operate in a subthreshold region. As illustrated, V.sub.OUT is developed at the common electrode of parallel-connected transistors n_M1 and N-channel transistor M2.

(52) Also shown is an example topology of a variable gain amplifier 50 adapted selectively to adjust and trim an absolute value of a second reference voltage, V.sub.REF. An operational amplifier 52, coupled to receive V.sub.REF, develops a voltage difference, V.sub.DIFF, to drive the gate electrode of a P-channel transistor M3; a voltage divider, comprising a variable divider 54 and a fixed divider 56 develops V.sub.REF as a function of the resulting current through M3.

(53) In alternate embodiments, V.sub.REF may be developed, for example, from a selected voltage node of variable voltage divider 54, a selected voltage node of fixed voltage divider 56, or from the drain electrode of transistor M3. Also, the topology of variable voltage divider 54 and fixed voltage divider 56 are shown by way of example. According to the invention, variable voltage divider 54 and fixed voltage divider 56 may be swapped, interleaved, or implemented in any topology that develops V.sub.REF as a selected fraction of the voltage developed at the source electrode of transistor M3.

(54) During initialization (and, perhaps, at selected times thereafter), sensitivity trimmer 48 is adjusted to selectively trim V.sub.OUT to adjust for a predetermined sensitivity, such as a PVT sensitivity, of V.sub.OUT (step 58). Such a trim setting may be stored in configuration memory 38. The voltage regulator then develops a regulated voltage at the source electrode of transistor M3 as a function of a voltage difference between V.sub.OUT and V.sub.REF. In general, the circuit topology of variable voltage divider 54 and fixed voltage divider 56 provides a variable gain feedback network that develops V.sub.REF as a function of the voltage developed at the source of transistor M3, so that V.sub.REF is adjusted to a value which is a function of V.sub.OUT (step 60). Selective trimming of the adjusted absolute value of V.sub.REF may be accomplished by programming, for example, a transistor configuration of the variable divider 54 (step 62). The trim setting for variable divider 54 may be stored in configuration memory 38. Using known techniques, either or both of the trim settings may be made available to resources external to the integrated system.

(55) In alternate embodiments, V.sub.OUT may be provided using, for example, any selected one of the various reference voltage generator 12 topologies illustrated in FIG. 2, FIG. 3, FIG. 5, and FIG. 6. Alternately, any number of known, or yet to be invented, reference voltage generator topologies may be used to provide V.sub.OUT.

(56) In another embodiment, the one or more n_M1 transistors may use an alternate, non-binary weighting scheme.

(57) In other embodiments, alternate configurations of trimming topologies may be used to trim V.sub.REF, and to compensate for sensitivities.

(58) Shown in FIG. 11, by way of example, is a diode stack 64. For power management, variable divider 54 and fixed divider 56 of FIG. 9 may be implemented using a plurality of diode-connected transistors collectively comprising a diode stack 64, where each transistor is biased in the subthreshold region. Such a diode stack 64 may include a selection of programmable transistors coupled in series by a binary weighting scheme. In yet another embodiment, variable divider 54 and fixed divider 56 may include other topologies, for example, a predetermined combination of resistor, capacitor, and transistor structures.

(59) Thus it is apparent that we have provided an improved method and apparatus for our reference voltage generator, and, in particular, we submit that our method and apparatus provides optimized power consumption, resulting in extended battery life, reduced battery size, and reduced cost. In particular, we have provided an improved solution for low power supply requirements, while also providing an improved topology for trimming out PVT sensitivities. Also, we have provided for the characteristics of our reference voltage generator to be controllable and observable in a manufacturing and system test environment. Therefore, we intend that our invention encompass all such variations and modifications as fall within the scope of the appended claims.