Patent Grant
7570035
The invention relates generally to voltage regulator circuits, and more specifically to the transient response of linear voltage regulator circuits.
The current required by an active electronic system is constantly in flux. Notwithstanding these current fluctuations, the power supply of an electronic system needs to provide a stable output voltage. Without a stable supply voltage digital circuitry will suffer from bit errors and analog circuitry performance will be degraded by bias point shifting. This requirement has been present in electronics circuits since their inception and is solved through the use of voltage regulators. A voltage regulator receives a supply voltage and outputs a regulated voltage to an electronic circuit. The regulated circuit is referred to as the load of the voltage regulator. Modern integrated circuits such as those found in computers and mobile phones commonly utilize linear voltage regulators. The use of linear voltage regulators is the favored method in integrated circuit applications because they provide a clean voltage supply, meaning that the regulated voltage is relatively free from noise.
There is a large body of prior art related to linear voltage regulators. A generalized diagram of a regulator circuit can be found in
The prevalence of portable electronics has placed increasingly restrictive demands on the performance of low power voltage regulators. In the interest of user convenience, portable electronics need to be designed for optimal power consumption to preserve battery life. One method of preserving power in an electronic system is to shift the system into different phases of power consumption depending upon the systems varying functionality requirements. This helps to preserve power due to the inverse relationship between functionality and power consumption in electronic circuits. The ratio of the variant phase currents, such as the operating current and the standby current, will increase as circuit performance is optimized. Low power consumption also requires a rapid transition between these phases. Fast transitions are desirable because a circuit in transition is dissipating more power than it does in a dormant state, but it is not yet accomplishing anything constructive for the circuit's operation.
The fast transition times required by modern circuits require rapid charge delivery in response to rapid changes in the regulated load. A bypass capacitor can deliver charge in response to very high frequency perturbations in the regulated load but is limited in the amount of charge it can supply. A linear voltage regulator can deliver charge in a sustainable and controlled manner. However, the speed of a linear voltage regulator is inherently limited by the stability requirement of its incorporated feedback loop. This is because without proper compensation a linear voltage regulator can suffer from instability and compensation limits the bandwidth of voltage regulators. The bandwidth must be limited because at high frequencies the characteristic of the loop will change from negative to positive feedback. If high frequency signals are amplified by positive feedback the circuit will become unstable and will be ineffective as a regulator. This property is one of the main drawbacks of linear voltage regulators. The most common substitute for linear voltage regulators are DC-DC switching regulators. This type of regulator also suffers from instability and tight speed constraints in that the switching speed needs to be around five times the bandwidth of the regulator. Circuits that combine the characteristics of DC-DC switching regulators and linear voltage regulators such as U.S. Pat. No. 5,309,082 to Pyane or U.S. Pat. No. 7,167,054 to Dening are not meant to alleviate the speed and power constraint but instead address the issues of power dissipation across the pass element and the high cost and poor regulation of DC-DC switching regulators.
A notable solution for the limited bandwidth problem of linear voltage regulators focuses on providing high frequency compensation that is not controlled by feedback. Such a circuit is described in U.S. Pat. No. 6,809,504 to Tang. An open loop system does not have frequency dependent stability constraints and therefore can operate at frequencies that exceed the requirements of linear voltage regulators. The implementation in the Tang circuit comprises pulse generators that input a set current to the load for a set time in response to a rapid change in the regulated current. The objective of such a circuit is for the predetermined current provided by the pulse to cancel the transient current that the slow linear regulator cannot track. The advantage of this approach is that the circuit can operate at very high frequency as there is no stability limitation on a system that does not have a feedback path. This approach carries a related disadvantage in that the open loop approach cannot measure and apply the exact current required. In some cases the predetermined compensation current may be so far from the desired current that the slow linear voltage regulator would have provided a better estimate on it own.
A method applied in the field of phase-locked loops to improve the transient response of a system loop utilizes both digital and analog closed loop filters. Such phase-locked loop architectures are called hybrids. An example of such architecture can be found in U.S. Pat. No. 5,978,425 to Takla. The general purpose of the phased-locked loop is to match up the clock scheme of the input signal to the clock utilized to receive the input signal. The approach utilizes a digital loop to provide a fast, though course, adjustment of the loop during a calibration phase. After calibration, a slower analog loop provides high resolution and accuracy. If this approach is not utilized the startup time for a phase locked loop could be extremely large. Control circuitry is necessary to determine when the digital loop has served its purpose and the analog loop can take over. Such a system utilizes the fast settling of a digital loop and the accuracy of an analog loop as complements to avoid the drawbacks and enhance the benefits of the respective loops.
The approach of utilizing analog and digital loops in tandem to improve the transient response of an electronic system can be applied to voltage regulators. An example of such a solution is developed further in the previously mentioned patent to Tang and is fully described in U.S. Pat. No. 6,975,494 also to Tang. This circuit incorporates a typical linear voltage regulator and augments its performance by adding independent discrete current sources controlled by voltage sensing circuitry on the load. The linear feedback circuit operates by voltage sensing changes in the regulated voltage through a feedback system. Likewise, the added circuitry functions by voltage sensing changes in the circuit and applying a set amount of current in response to the sensed voltage passing specific thresholds. The range in between these thresholds over which the discrete current boosting is not activated is called the dead zone. Unlike the open loop system this portion of the circuit can be subject to instability and oscillations due to its utilization of sensing and feedback.
An embodiment of the Tang approach is circuit 200 illustrated in
In one aspect of the invention, a circuit for regulating a voltage with minimal power and a rapid transient response is provided. A linear voltage regulator receives an unregulated input voltage node on one terminal of its pass element and provides a regulated voltage node. A feedback network and an amplifier adjust the current flowing through the pass element so that the regulated voltage node remains at a constant voltage. The constant voltage is determined by the feedback network and a reference voltage provided to the amplifier. A compare and control circuit is coupled to the feedback network and senses variations in the potential of at least one node in the network. This node could be the regulated voltage node or it could be a voltage that is coupled to the amplifier. Furthermore, this compare and control circuit adjusts the drive capability of the pass element in response to variations in the monitored nodes.
In another aspect of the invention, a method for regulating a voltage in response to rapid changes in the load is provided. First, the voltage is regulated using a linear voltage regulator in response to low frequency perturbations in the load. Concurrently, the voltage at certain sense nodes is monitored by compare and control circuitry to detect high frequency perturbations in the regulated load. In response to the detection of these high frequency perturbations, a control signal is generated to alter the drive capability of the linear voltage regulator's pass element. The drive capability is altered to produce a corresponding change in the regulated voltage node. The compare and control circuitry continues to monitor the selected node voltages of the regulated circuit to determine when the regulated voltage is within a certain range of the desired value. Once the voltage at the regulated voltage node is within such a tolerance the compare and control circuitry resets the drive capability of the pass element to its original value.
Reference now will be made in detail to embodiments of the disclosed invention, one or more examples of which are illustrated in the accompanying drawings. Each example is provided by way of explanation of the present technology, not as a limitation of the present technology. In fact, it will be apparent to those skilled in the art that modifications and variations can be made in the present technology without departing from the spirit and scope thereof. For instance, features illustrated or described as part of one embodiment may be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present subject matter covers such modifications and variations as come within the scope of the appended claims and their equivalents.
A need exists for a low power linear voltage regulator with a broad bandwidth control loop. The feedback control loop of the linear voltage regulator comprises the limiting factor in a broad bandwidth design. The loop must be augmented or altered in some fashion to alleviate the stability constrained power and speed relationship that is inherent in the application of negative feedback. The combination of a linear series regulator and a switching regulator in open or closed loop configuration such as in the Payne approach will not likely alleviate the speed constraint because switching regulators are usually more bandwidth limited than linear regulators. The Tang approach is also problematic because the addition of a separate digital feedback loop will likely require separate stabilization circuitry, independent current sources, additional power, and separate circuitry for generating Vref derivative comparison points.
The present invention utilizes a hybrid loop wherein the transconductance of the pass element is altered by a nonlinear feedback path.
When rapid variations in regulated load 311 occur, the linear regulator may not be able to keep pace and the regulated voltage will shift. In such cases, the linear loop is not able to keep up with a fast transient voltage swing on Vout and the voltages at Vr+Δ1 and Vr+Δ2 will swing in the same direction almost instantaneously since these nodes are DC coupled. For a large positive change in the current drawn by load 311, such that the voltage at nodes Vr+Δ1 sinks below Vref, comparator 308 will trip and send a signal to control circuitry 310 through node Vup. Likewise, if there is a large negative change in the current drawn by load 311 then the voltage at node Vr−Δ2 will rise above Vref which will trip comparator 309. Comparator 309 will then send a signal to control circuitry 310 through node Vdown. Control circuitry 310 outputs a digital code along bus Control to pass element 303 based on the signals it receives. The digital code sent along Control will adjust the transconductance of pass element 303 to provide for faster settling of the regulator.
The embodiment of the present invention illustrated in
There are several ways in which control circuitry 310 could alter the transconductance of pass element 303. One such method can be described with reference to
In the dual loop regulator architecture the independent current sources cannot be providing current to the load during the nonlinear loop's dead zone. As set forth in
Since the nonlinear circuit cannot source or sink current when the circuit is operating in its dead zone, a single current source can only act to regulate the load in an under-voltage or an over-voltage condition. If the nonlinear loop is designed to protect against both types of variation in the regulated loop, two current sources are required. A single current source coupled from Vout to Vin or from Vout to Vgnd will provide a unidirectional source or sink from Vout. A unidirectional current path can only affect the circuit by altering its magnitude. In accordance with the required dead zone behavior, such a current source must have a zero dead zone value. The result of these requirements is that a single current source will only be able to increase the current sourced to Vout or increase the current sunk from Vout but cannot do both. Therefore, a current source can only alleviate a single type of transient current swing in regulated load 209.
The present invention only requires a single current path for generating the regulated current. The reason only a single path is required is that the analog linear control loop is always in ultimate control of the current provided from pass element 303 to the regulated load regardless of the nonlinear loop control signal. The current sources for separate linear and nonlinear loops will both be very large because there are certain conditions in which either loop may have to provide the maximum current required by the regulated load. Substantial area savings are therefore realized through implementation of the present invention. In addition, this single current source will function to counter an over-voltage or an under-voltage condition thereby alleviating the need for two separate current sources for under-voltage and over-voltage conditions. This leads to further size savings since the two current source approach would likely consume more area than the one current source approach.
Circuit 300 is able to react to either type of rapid load transient because of its hybrid loop configuration. The circuit can react to either transient by rapidly decreasing or increasing the transconductance of pass element 303. With reference to the specific pass element configuration in circuit 400, control circuit 410 could be outputting a midpoint code in the nonlinear circuit's dead zone. The result would be that the effective transconductance of pass element 402 from the linear loop's perspective would comprise half of transistor array 407 and transistor 406. In a rapid transient over-voltage condition at Vout, the nonlinear circuit could quickly cut the current through transistor array 407 by outputting a zero code and deactivating all of the switches in array 405. In a rapid transient under-voltage condition at Vout, the nonlinear circuit can quickly increase the current through transistor array 407 by outputting its maximum code and activating all of the switches in array 405.
The described action of the nonlinear loop has a power savings advantage in addition to the area savings that accompany the single current source configuration. Current is wasted when using the multiple current source architecture as compared to the hybrid loop approach. With reference to circuit 200, current source 208 will activate during an over-voltage condition and sink excess current to ground. Contrarily, when an over-voltage condition occurs in circuit 400, linear pass element array 407 can be cut from the supply by deactivating the active switches of array 405. Succinctly, the independent current source shunts the additional current to ground while the present invention applies less current from the supply. This will amount to a small amount of current savings each time the over-voltage condition is encountered because, in keeping with the main objective of the circuit, the transient condition is very brief. However, modern circuits make such transitions many times so the integral charge savings can be substantial.
The dead zone behavior of control circuitry 410 is not limited to the discussed embodiment wherein the circuitry outputs a midpoint code. The dead zone output code could be at any point along the spread of possible codes. The selection of a dead zone output code will affect the design of the linear loop and can be selected to focus on a particular type of transient. If over-voltage conditions were an issue, the dead zone output current of control circuit 410 could be placed at a much higher level. This design would be able to strongly reduce the current supplied in order to correct for a rapid over-voltage transient. If under-voltage conditions were more of a concern for a particular load, the dead zone output code could be shifted towards the other end of the code range. In keeping with this approach, the transistors in array 405 and 407 do not have to be linearly sized. The midpoint code could still be output but the size of the devices controlled by each code could change in a nonlinear fashion dependent upon the particularized load transients of concern and the desired accuracy and settling time of the hybrid loop. In the extreme, control circuitry 410 could output a zero or maximum code while the nonlinear transconductance adjustment loop is in the dead zone. Such a circuit would only be able to rapidly respond to one type of voltage swing but would do so in an effective manner. An additional benefit of such a configuration would be that the nonlinear loop would not oscillate independently of the nonlinear loop. Several variations of this approach could easily be implemented since they would only require modifications to the digital logic in control circuitry 410.
The control and feedback detection circuitry can be implemented in several ways.
Another set of embodiments of the invention use different devices as linear pass elements.
A similar configuration is shown in circuit block 510 where the current source pass element devices are PNP BJTs 511 and 512. Switches 513 and 514 are configured in the same manner as for the PMOS source pass elements of circuit block 500. Error amplifier 301 would also have the same configuration as for the circuit block 500 in terms of the error amplifier's input terminal connections. Circuit block 520 is another implementation of the circuit wherein the current source pass element devices are NPN BJTs 521 and 522. In this case, error amplifier 301 would have the same configuration as for the NMOS current source implementation of
In another embodiment of the invention, broad bandwidth low power regulation is achieved in negative regulator configuration. Such a configuration 600 is illustrated in
The invention will function with any form of stabilization circuit for frequency compensation of the analog loop. It should be noted that one of the advantages of the present invention is that it alleviates the stability constraint placed on the circuit indirectly by increasing the transient response capabilities of the regulator. The commensurate increase in speed can be traded off to improve stability while maintaining the speed and power relationship of a circuit that does not utilize the present invention. Other techniques in the field of voltage regulators can be used together with the present invention to enhance its performance. Such techniques include the use of drive signals that are brought up to voltages in excess of the supply voltage or down to voltages below the ground voltage through the use of charge pumps.
In another embodiment of the invention, the transient response improvement scheme is applied to a cascade of regulators. In such an embodiment, the output of each regulator is applied as the input of a succeeding regulator. The resultant configuration provides for multiple power islands on a single chip. An illustration of two stages of such a configuration is circuit 700 in
Although embodiments of the invention have been discussed primarily with respect to specific embodiments thereof, other variations are possible. Various voltage regulator configurations may be used in place of, or in addition to, the circuit configurations presented herein. Functions may be performed by hardware or software, as desired. In general, any circuit diagrams presented are only intended to indicate one possible configuration, and many variations are possible. Those skilled in the art will also appreciate that methods and systems consistent with the present invention are suitable for use in a wide range of applications encompassing any that utilize feedback loops. While the specification has been described in detail with respect to specific embodiments of the invention, it will be appreciated that those skilled in the art, upon attaining an understanding of the foregoing, may readily conceive of alterations to, variations of, and equivalents to these embodiments. These and other modifications and variations to the present invention may be practiced by those skilled in the art, without departing from the spirit and scope of the present invention, which is more particularly set forth in the appended claims. Furthermore, those skilled in the art will appreciate that the foregoing description is by way of example only, and is not intended to limit the invention.
| Number | Name | Date | Kind |
|---|---|---|---|
| 5309082 | Payne | May 1994 | A |
| 5978425 | Takla | Nov 1999 | A |
| 6661210 | Kimball et al. | Dec 2003 | B2 |
| 6680837 | Buxton | Jan 2004 | B1 |
| 6703815 | Biagi | Mar 2004 | B2 |
| 6809504 | Tang et al. | Oct 2004 | B2 |
| 6965218 | Scoones et al. | Nov 2005 | B2 |
| 6975494 | Tang et al. | Dec 2005 | B2 |
| 6995474 | Tang et al. | Feb 2006 | B1 |
| 7141956 | Chapuis | Nov 2006 | B2 |
| 7167054 | Dening et al. | Jan 2007 | B1 |
| 7323853 | Tang et al. | Jan 2008 | B2 |
| 20090009147 | Jian | Jan 2009 | A1 |
| Number | Date | Country | |
|---|---|---|---|
| 20090033298 A1 | Feb 2009 | US |
