CMOS constant voltage generator

This application claims priority from U.S. provisional application Ser. No. 60/539,051 filed on Jan. 23, 2004, incorporated herein by reference in its entirety.

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A portion of the material in this patent document is subject to copyright protection under the copyright laws of the United States and of other countries. The owner of the copyright rights has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the United States Patent and Trademark Office publicly available file or records, but otherwise reserves all copyright rights whatsoever. The copyright owner does not hereby waive any of its rights to have this patent document maintained in secrecy, including without limitation its rights pursuant to 37 C.F.R. § 1.14.

1. Field of the Invention

This invention pertains generally to integrated circuits, and more particularly to constant voltage generators.

2. Description of Related Art

A constant voltage generator (V ref generator) is widely used within many Integrated Circuit (IC) designs, such as voltage references, input buffers, voltage regulator circuits, and similar applications. Typically the most crucial requirement for a V ref generator is that of providing a constant output voltage regardless of operating voltage, ambient temperatures, operating temperatures and manufacturing process variations. Various BandGap References (BGR) which rely on the use of diodes or bipolar transistors (P-N junction potential) have been utilized for this purpose. Recently, however, CMOS V ref generators have drawn increasing attention because of their simple design, low power consumption, and their ability to be readily incorporated on-chip within a wealth of CMOS circuit designs.

FIG. 1 depicts a conventional CMOS V ref generator having a single input and output stage. The V ref output is generated through PMOS device MP 2 from drop R 1 in combination with active resistor MP 1 and R 2 forming a voltage divider with active circuits MN 1 and MN 2 . However, conventional CMOS V ref generators exhibit substantial operating voltage and temperature variations which makes them unsuitable for use in a number of important applications.

Accordingly, a need exists for a system and method of generating an accurate voltage reference from simple CMOS circuitry while overcoming the problems with process variations, supply voltage changes, and temperature drift. These needs and others are met within the present invention, which overcomes the deficiencies of previously developed voltage generator systems and methods.

Voltage reference generators according to the present invention utilize current mirror drivers with at least one compensation stage between the input and output stages of the device. An input stage comprises an input device, or preferably a stack of devices comprising at least one active device and one passive or active resistor comprising a load. For example, the input stage may comprise a self-biased transistor in series with at least one load resistor or at least one transistor load or transistor logic, the combination being coupled between drain and source supply voltages. A portion of the input stage is configured in a current-mirror relationship with a compensation stage which provides biasing to an output device, or stack of devices. Embodiments of the invention preferably utilize source degeneration resistors (i.e. passive or active) on the source or drain sides of one or more of the transistors to normalize current flow through the transistors over a wider V dd range, and optionally to provide temperature compensation with a proper selection of device temperature coefficient. Stacked elements may be used within the stages to reduce the effective resistance values. In addition, diode-coupled transistors can be incorporated, such as preferably in the output stage, to increase temperature compensation.

An embodiment of the present invention describes a constant voltage generator circuit, comprising: (a) a voltage source (i.e. current-mirror driver) having an input stage, at least one compensation stage, and at least one output stage; (b) means for establishing a first current-mirror relationship between the input stage and both the compensation stage and the output stage; (c) means for establishing a second current-mirror relationship, or biasing relationship, between the compensation stage and the output stage; and (d) at least one active resistor device within the output stage whose resistance is modulated in response to receiving a biasing signal from the second current-mirror to compensate a constant reference voltage output from the output stage.

In this voltage reference device each stage preferably comprises at least one transistor device, or a stack of transistors, or a combination of transistors and either active or passive resistors. The voltage reference circuit may also incorporate one or more source degeneration resistors within the compensation stage, and/or output stage. In a preferred embodiment the source degeneration resistors are configured with a positive temperature coefficient to provide additional temperature compensation within the circuit.

The voltage reference circuit may also incorporate one or more diode-connected transistors (NMOS or PMOS) in the output stage to aid in temperature compensation of the output voltage. The diode-connected transistors are preferably configured with a negative temperature coefficient. The means for establishing the first current-mirror relationship within the circuit preferably comprises self-biasing a transistor within the input stage and coupling that self-biasing signal from the input stage to bias a transistor in each of the compensation stage and output stage.

The means for establishing a first current-mirror preferably comprises an interconnection between NMOS transistors within the input, compensation and output stages. The means for establishing the second current-mirror relationship comprises self-biasing a transistor within the compensation stage and coupling that self-biasing signal from the compensation stage to bias a transistor in the output stage. Furthermore, the means for establishing a second current-mirror comprises an interconnection between PMOS transistors within the compensation and output stages. It will be appreciated that additional compensation stages may be added which bias active devices in the output stage to further increase the accuracy of regulation.

Another embodiment of the invention describes a constant voltage generator circuit, comprising: (a) a voltage source having an input stage, at least one compensation stage, and at least one output stage; (b) at least one first active device within the input stage is configured for receiving a self-biasing signal; (c) at least one second active device within the compensation stage is configured for receiving the self-biasing signal of the first active device to establish a first level of current mirroring on the compensation stage; (d) at least one third active device within the output stage is configured for receiving the self-biasing signal of the first active device according to the first level of current mirroring; (e) at least one fourth active device within the compensation stage is configured for receiving a self-biasing signal; (f) at least one fifth active device within the output stage is configured for receiving the self-biasing signal from the fourth active device to establish a second level of current-mirroring; (g) a voltage generator output connection is coupled within the output stage between the third active device and the fifth active device; and (h) at least one sixth active device within the output stage is configured for receiving the self-biasing signal from the fourth active device and having a resistance that varies in response to the biasing input toward compensating the voltage output from the voltage generator output.

The first current-mirror in this circuit is preferably established on the source supply voltage side of the respective circuit stages, while the second current-mirror is established on the drain supply voltage side of the respective circuit stages. The circuit/device comprises PMOS and NMOS transistors fabricated according to a CMOS process technology. The resistive characteristics of the transistors in the input stage, compensation stage, and the output stage, are configured by controlling their size, geometry, or both. In one embodiment, the size of the transistors is changed by open-circuiting (i.e. blowing) of electrical fuses within the circuit to select transistor sizing, or selecting a size within one or more mask steps, or both.

Another embodiment of the invention describes a method of generating a constant reference voltage, comprising: (a) forming a first current mirror relationship between an input transistor stage and at least one subsequent transistor stage; (b) forming a second current mirror relationship between a compensation stage and an output stage; and (c) wherein the biasing of the second current mirror relationship drives at least one active device in the output stage to modulate reference voltage output. The method can further comprise stabilizing the voltage reference output by adding degeneration resistances (passive or active resistors) in transistor stages which are coupled to the input transistor stage, and/or the use of diode-coupled transistors in the output stage.

Embodiments of the present invention can provide a number of beneficial aspects which can be implemented either separately or in any desired combination without departing from the present teachings.

An aspect of the invention is to provide increased voltage regulator output accuracy.

Another aspect of the invention is to decrease output voltage fluctuations which arise in response to fabrication process variations, changes in temperature, changes in operating voltage, and combinations thereof.

Another aspect of the invention is the use of diode-coupled transistors, having negative temperature coefficients, within the transistor stacks to reduce effective resistance.

Another aspect of the invention is the use of degeneration resistors for improving voltage compensation within the voltage generator.

Another aspect of the invention is that source degeneration resistors can be passive or active resistors.

Another aspect of the invention is that the resistance values of transistors can be controlled by changing their sizes (width and/or length), such as through blowing electrical fuses and/or using mask steps.

Another aspect of the invention is that transistors can be stacked and yet have the same input toward reducing effective resistance values.

Another aspect of the invention is the ability to incorporate the voltage generator into separate circuit devices (i.e. voltage references, regulator, etc.) or integration within other circuit elements.

A still further aspect of the invention is that improved voltage reference characteristics can be provided by the present circuit which can be fabricated according to generally conventional CMOS fabrication techniques.

Further aspects of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing preferred embodiments of the invention without placing limitations thereon.

The invention will be more fully understood by reference to the following drawings which are for illustrative purposes only:

FIG. 1 is a schematic of a conventional CMOS voltage reference generator, shown having an input and output stage.

FIG. 2 is a schematic of a CMOS voltage reference generator according to an embodiment of the present invention, shown with one compensation stage and the use of source degeneration resistors.

FIG. 3 is a schematic of a CMOS voltage reference generator according to an embodiment of the present invention, shown with one compensation stage and with active devices utilized as the degeneration resistors.

FIG. 4 is a schematic of a CMOS voltage reference generator according to an embodiment of the present invention, shown with two compensation stages.

Referring more specifically to the drawings for illustrative purposes, the present invention is embodied in the apparatus generally described in FIG. 2 through FIG. 4 . It will be appreciated that the apparatus may be adapted for a variety of applications, without departing from the basic concepts disclosed herein. The present invention is a new type of CMOS voltage reference (V ref ) generator which is directed toward achieving superior compensation performance (i.e., reduced sensitivity to supply voltage (V dd ) and temperature variations) in relation with conventional CMOS V ref generators. The apparatus and methods of the present invention can be implemented within separate circuit elements (i.e. voltage references, regulators, etc.) or integrated within other circuit elements, preferably those fabricated using CMOS processes (i.e. A/D converters, microcontrollers, comparator circuits and so forth).

FIG. 2 illustrates an example embodiment of a CMOS V ref generator according to the present invention which utilizes a voltage source configured with multiple current-mirror type drivers as a means of biasing the output stage. The input stage preferably comprises a simple bias circuit configured in a current mirror relationship with one or more other stages of the voltage generator. It should be appreciated that the input stage can be implemented as a simple combination of an active device with a passive or active transistor, although a more complex transistor stack or other topology may be utilized.

By way of example, the input stage comprises resistor R 1 in combination with transistor MN 1 and forms a bias circuit of a current mirror. Pairs of transistors are configured in a first current mirror relationship with MN 1 including in this case MN 2 and MN 4 . A second current mirror relationship is preferably established between MP 1 and MP 2 of the compensation and output stages, respectively. Optionally, source-degeneration resistors can be utilized, such as resistors R 2 , R 3 and R 4 , to improve operating voltage (V dd ) compensation characteristics. Diode-coupled transistors, such as MD 1 and MD 2 , in the output stage can comprise either NMOS or PMOS transistors. Transistor MN 3 in the output stage is shown comprising an active resistor having a resistance value controlled by the bias voltage generated by the compensation stage.

During operation, in response to increasing operating voltage the voltage at node A reaches approximately V tn1 , which is the threshold voltage of NMOS transistor MN 1 . Since transistors MN 1 and MN 2 form a current mirror, respective currents I 1 and I 2 are expected to be the same if the two transistors have the same size and structure. When the operating voltage goes up, the voltage at node A goes up since the voltage is divided by the resistance values of the two components R 1 and MN 1 . In response to a node A voltage increase, MN 2 is driven deeper into conduction and increased current flows through MN 2 . In addition, the drain voltage of MN 2 is determined by the resistance ratio of MP 1 and MN 2 . The two transistors MN 1 and MN 2 in this embodiment are configured with different characteristics.

To improve the operating voltage-dependent characteristics, a resistor is added at the source of MN 2 , called a source-degeneration resistor, which aids in maintaining a constant current flowing through transistor MN 2 in response to changes in supply voltage levels V dd . Since a voltage appears across R 2 , the gate-source voltage (V GS ) of MN 2 is smaller than that of MN 1 . When V dd is increasing, since a certain voltage still appears across the resistor, if the resistor is large enough so that a large portion of voltage appears across the resistor R 2 rather than MN 2 , the gate-source voltage (V GS ) and drain-source voltage (V DS ) of MN 2 can be accurately maintained to stabilize circuit response characteristics. Another advantage of adding R 2 is to maintain the node of PBIAS closer to a voltage less than V dd by a voltage amount V tp since the large voltage still appears across resistor R 2 . Source-degeneration resistor R 4 provides similar compensation benefits as provided by degeneration resistor R 2 .

The voltage of node PBIAS is expected to be lower than V dd by an amount V tp1 , which is a threshold voltage of PMOS transistor MP 1 . Due to the voltage divided across transistors MP 1 , MN 2 and resistor R 2 , the voltage of node PBIAS becomes slightly less than V tp . It will be appreciated that resistor R 2 helps node PBIAS maintain a closer voltage to V dd -V tp than in the absence of resistor R 2 . By adding a source-degeneration resistor at the source of MP 2 , the current flowing through MP 2 can be maintained more constantly over the operating voltage range. The use of a source degeneration resistor at the V dd side, especially at the source of the PMOS driver transistor in the current mirror structure, provides numerous benefits according to the present invention. The addition of source-degeneration resistor R 3 provides similar benefits.

Diode-coupled transistors can be optionally incorporated within the transistor stacks to provide temperature compensation, such as utilizing negative temperature coefficient diode-coupled transistors on the source side to achieve a stable temperature compensated output voltage level of V ref . It will be appreciated that the voltage drop across a given diode is reduced in response to temperature increases. Both NMOS and/or PMOS transistors can be utilized for creating the diode-coupled transistors within the stack.

Transistor MN 3 in the output stack is preferably configured with a positive temperature coefficient. As V dd increases, the voltage of node PBIAS increases sufficiently to bias transistor MN 3 into its linear region, wherein MN 3 acts like a linear active resistor for maintaining V ref output and providing temperature compensation in response to the increasing resistance value of MN 3 brought on by increasing temperature.

Transistor MN 4 is coupled to the input stage current mirror in a similar manner as transistor MN 2 within the compensation stage. Degeneration resistor R 3 (active or passive) operates in a similar manner as resistor R 2 to improve the current characteristics for V dd and reduce the operating current of the device.

FIG. 3 illustrates another embodiment of the invention in which the passive source degeneration resistors R 3 and R 4 of FIG. 2 have been replaced with active degeneration resistors MPS and MNS. These complementary active resistors are preferably biased, such as by the output reference voltage V ref to provide additional voltage and temperature compensation.

FIG. 4 illustrates another embodiment of the invention, in which an additional compensation stage is included. This example depicts the extra compensation stage as comprising a single transistor MP 3 and an active or passive degeneration resistor R 5 . The additional compensation stage further increases voltage compensation of the circuit. It should be appreciated that any desired number of compensation stages can be utilized according to the teachings of the present invention.

A number of example voltage generator embodiments have been shown by way of schematic and described herein. It should be appreciated, however, that the present invention can also be considered a novel method of providing output voltage regulation within a voltage source. An input stage, at least one compensation stage, and an output stage are coupled together within a voltage source. Each stage comprises at least one active device, or a stack of active devices, or active devices in combination with passive or active resistors. A first current mirror relationship is established between an input transistor stage and at least one subsequent transistor stage. A second current mirror relationship, or biasing relationship, is established between a compensation stage and an output stage. According to this method the biasing of the second current mirror relationship drives at least one active device in the output stage to stabilize the reference voltage output.

Although the description above contains many details, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. Therefore, it will be appreciated that the scope of the present invention fully encompasses other embodiments which may become obvious to those skilled in the art, and that the scope of the present invention is accordingly to be limited by nothing other than the appended claims, in which reference to an element in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather “one or more.” All structural and functional equivalents to the elements of the above-described preferred embodiment that are known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present claims. Moreover, it is not necessary for a device or method to address each and every problem sought to be solved by the present invention, for it to be encompassed by the present claims. Furthermore, no element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims. No claim element herein is to be construed under the provisions of 35 U.S.C. 112, sixth paragraph, unless the element is expressly recited using the phrase “means for.”