Abstract:
The present application describes a frequency compensation scheme for a linear voltage regulator circuit, or its special case, a low-drop out voltage regulator (LDO). According to one embodiment, the frequency compensation scheme includes two circuits, an inner loop compensation circuit ( 240 ), and a circuit ( 245 ) at the output in parallel with one of the resistors of the output voltage divider ( 235 ). These two compensation elements ( 240, 245 ) are not interdependent and may be adjusted separately to provide more optimal frequency compensation. Advantages include smaller compensation circuit elements, die or board area savings, better phase margin over process technology variations and operating conditions, and ease of design adjustment.

Description:
TECHNICAL FIELD  
       [0001]     The present application describes an improved frequency compensation scheme and specific embodiments of the scheme for linear and low dropout voltage regulators.  
       BACKGROUND  
       [0002]     Linear voltage regulator circuits are used to create a clean, well regulated output voltage from some higher, noisy voltage supply source. Such regulator circuits are needed in most electrical systems to provide clean voltage, such as for industrial/automotive circuit applications where the environment is particularly noisy, or such as for wireless applications where the battery power fluctuates and frame synchronization glitches would become very apparent in the audio band.  
         [0003]     High performance linear regulator circuits generally have very high gain and need to be frequency compensated in order to have stable performance over a very wide range of operating conditions. The higher the performance and wider the conditions, then the harder it is to provide simple compensation schemes to keep the regulator stable. Conditions include a large range of dropout voltages (difference between input supply voltage Vin and regulated output voltage Vout), a large range of load currents, and a large variety of off-chip capacitors. There is also temperature variation and technology process uncertainty especially for the pass transistor which switches Vin to Vout. Various kinds of frequency compensation schemes are used to provide stability. Examples include Miller compensation, nested Miller loops, and slow-rolloff compensation, along with additional off-chip or off-die load capacitor that may be part of the compensation. It&#39;s hard to find simple, small, frequency compensation schemes, which are desirable for cost and compactness reasons; this minimal size preference place further restrictions on the compensation scheme.  
         [0004]      FIG. 1A  illustrates a prior art typical linear voltage regulator with its frequency compensation element  140 , and C load,  150 . The goal of the circuit is to monitor the output voltage Vout via feedback and comparing it to some constant valued reference voltage Vref. When Vout is too high or too low, the circuit will self-adjust so that Vout returns to its nominal value, so that Vout remains essentially constant. There are three stages,  110 ,  120 ,  130 , partly for high gain (performance) purposes. There are several phase and gain shifts resulting from the various high impedance nodes and feedforward paths from the stages and the output objects. The compensation and load capacitors must be selected to avoid too much cumulative phase shift that would create positive feedback and make the circuit unstable. That is, the compensation must balance and locate the poles and zeroes at such frequencies so as to provide sufficient phase margin. High performance voltage regulators often require large or complicated compensation components to be stable. Furthermore, the traditional compensation elements interact with each other and are difficult to adjust independently, making it hard to provide optimal compensation.  
       SUMMARY  
       [0005]     This invention provides a frequency compensation technique that is particularly useful for high gain, high performance linear and/or low dropout voltage regulators which are inherently difficult to stabilize. According to one embodiment, the scheme includes two pieces, an inner loop compensation circuit and a circuit in parallel with one of the resistors in the output voltage divider. The advantages are smaller overall compensation elements, die area and cost savings, along with equal or improved phase margin and performance compared to regulators compensated by prior methods. Another key advantage of this new compensation technique is that design-wise it is simple to apply to get better results: unlike traditional methods like slow roll-off and nested Miller compensation, the new compensation elements are not inter-dependent; so they are easy to adjust independently and hence provide smaller and more efficient compensation. The new compensation for linear regulators allows the placing of poles and zeros strategically to avoid cumulative phase shift that would lead to positive feedback and instability.  
         [0006]     The foregoing is a summary and thus contains, by necessity, simplifications, generalizations and omissions of detail; consequently, those skilled in the art will appreciate that the summary is illustrative only and is not intended to be in any way limiting. As will also be apparent to one of skill in the art, the operations disclosed herein may be implemented in a number of ways, and such changes and modifications may be made without departing from this invention and its broader aspects. Other aspects, inventive features, and advantages of the present invention, as defined solely by the claims, will become apparent in the non-limiting detailed description set forth below.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:  
         [0008]      FIG. 1A  illustrates a conventional frequency compensation scheme for a voltage regulator circuit;  
         [0009]      FIG. 1B  illustrates various configurations for conventional frequency compensation schemes; and  
         [0010]      FIG. 2  illustrates an exemplary circuit for a voltage regulator with a frequency compensation scheme for placing independent pairs of poles and zeros.  
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0011]      FIG. 2  illustrates an exemplary circuit for a voltage regulator  200  with a frequency compensation scheme for placing independent pairs of poles and zeros. The voltage regulator  200  includes three circuit stages, input stage  201 , second stage  202 , and output stage  203 , along with voltage divider unit  204 . The input stage  201  includes an error amplifier unit  210 . The voltage divider unit  204  includes two resistors R A  and R B . The second stage  202  is usually to drive the large input capacitance of the output stage. The second stage usually also contains gain for the regulator to maintain high overall gain when the gain of the output stage becomes very low under light current load conditions. The output stage  203  includes a large pass device transistor  230 , usually a P-type or P-channel MOSFET, PMOS common source stage, or its equivalent P-type or PNP transistor for bipolar process technologies. For purposes of illustration, various elements of the voltage regulator  200  are shown and described; however, one skilled in the art will appreciate that the voltage regulator  200  can include additional interface components required for signal tuning for a given application. For example, the second stage may be a transimpedance amplifier containing a resistor as shown in prior art patent, U.S. Pat. No. 5,631,598. Furthermore, various elements of the voltage regulator  200  can be configured using discrete components such as resistors, capacitors, amplifiers and a pass device transistor. Or the various elements may all be inside the IC package or even on the IC die itself, such as the resistors R A  and R B . And this regulator may also be configured on large system ICs to regulate voltages on the large IC and supply current to other circuits on the same IC, or on multi-chip modules within the same package.  
         [0012]     The error amplifier  210  receives a reference signal Vref on an input terminal  205  and a feedback voltage from the output of the transistor  230  via a voltage divider  235  on an input terminal  206 . The error amplifier  210  generates an error signal representing the difference between the input voltages. The output of the error amplifier unit  210  is coupled to the second stage  220 . The second stage outputs a signal which is used to control the pass device transistor  230  to provide a regulated output voltage Vout. The second stage is also often designed to have some non-unity gain magnitude in order to increase the gain of the regulator, but it is typically designed with high bandwidth so that its frequency response has little effect on the overall regulator frequency response.  
         [0013]     The regulated output voltage Vout is generated to bias and be the supply for another circuit load, represented by the current load Iload. The output also contains a load capacitor  250  and its associated ESR, electric series resistance. This capacitor is used to aid frequency compensation of the voltage regulator  200 , and it is also used to damp any high frequency noise on the regulated voltage Vout so that the noise does not disturb any sensitive circuit loads. This capacitor however should not be so large as to delay intentional load transient responses, startup and shut down conditions, or be so large to take up much area. Therefore, since this load capacitor has a limited range of sizes, it is necessary to have other circuit elements to provide frequency response stability. A first compensation  240  may be used for frequency compensation purposes; it is connected between the output of the regulator, and to the input of the second stage  220 . A second compensation unit  245  is connected across the resistor R A  of the voltage divider  235  may be also used for frequency compensation. The second compensation unit  245  allows independent placement of a zero that can cancel an undesirable pole. The zero may also be located around the unity gain frequency of the regulator to lessen the negative phase shift, and thus improve the phase margin. The second compensation unit  245  is a capacitor in a preferred embodiment. The compensation unit  240  can include various configurations shown and described in  FIG. 1B , although using a capacitor or a capacitor with series resistor is desirable to minimize component sizes. Circuit units  240  and  245  together are adequate in many designs to provide good phase margin for the regulator  200 .  
         [0014]     A typical inner loop frequency compensation technique is shown in prior art  FIG. 1A  using the first circuit unit  240  with a configuration of  174 , a capacitor and resistor in series, known as Miller plus lead compensation. In this typical prior art case, the poles and zeros of the regulator are as follows. The dominant pole P dom  is created by the load capacitance  150  C load  and the output resistance of the output transistor  130 .  
               P   dom     ≈     1     2   ⁢   _   ⁢     (     Rds   130     )     *     C   load                 Equation   ⁢           ⁢     (   1   )               
 
         [0015]     The poles associated with the first stage unit  110  and second stage unit  120  are as follows. The G m &#39;s are the transconductances of the input transistors of the respective stages. C 1  and Z lead  (R 1 ) are shown in  174 . C 2nd  stage is the input capacitance of the 2 nd  stage. C_ 130  is the input capacitance of the pass device  130 .  
               P   InputStage     ≈       G   m1       2   ⁢     π   ⁡     (     C1   +     C     2   ⁢   nd   ⁢           ⁢   Stage         )                   Equation   ⁢           ⁢     (   2   )                   P   InverterStage     ≈       G   m2       2   ⁢   π   ⁢           ⁢     C   130                 Equation   ⁢           ⁢     (   3   )               
 
 Typically, to offset the effect of poles, the lead Z lead  compensation scheme introduces a zero at a frequency just above the unity gain frequency to improve the phase margin of the voltage regulator  100 . The zero introduced by the Miller-plus-Z lead  compensation is given by equation (4):  
               Z   LEAD     ≈     1       (       (     1     G   m3       )     -   R1     )     *   C1               Equation   ⁢           ⁢     (   4   )                   Z   ESR     ≈     1     2   ⁢   π   ⁢           ⁢     R   ESR     *     C   LOAD                 Equation   ⁢           ⁢     (   5   )               
 
 The zero associated with the ESR resistor of the load capacitor is given by equation (5), where Z ESR  is the impedance of the series resistor of the load capacitance  150 , C LOAD  is the load capacitance  150 , G m130  is the transconductance of the pass device transistor  130 . 
 
         [0016]     The diagram for this present application is given by  FIG. 2 . The regulator  100  mentioned previously is now itemized as regulator  200 ; the first circuit unit  140  is now  240  and so on with respect to labels. When the second compensation unit  245  is configured like in  FIG. 1B , as a capacitor C zero , an output zero-pole pair is created for the regulator  200 . The output zero Z 245  and pole P 245  values are given by Equations 6 and 7, where the terms R A  and R B . are the resistors of the voltage divider  235 .  
               Z   245     ≈     1     2   ⁢     π   ⁡     (       R   A     ⁢     C   ZERO       )                   Equation   ⁢           ⁢     (   6   )                   P   245     ≈     1     2   ⁢     π   ⁡     (       R   B     ⁢     C   ZERO       )                   Equation   ⁢           ⁢     (   7   )               
 
         [0017]     The terms of the pole-zero pairs introduced by the circuit units  240  and  245 , illustrated by Equations 6 and 7 do not coincide with the terms of poles and zeros illustrated by Equations 2-5 for the conventional compensation scheme. Furthermore, poles and zeros introduced by the circuit unit  245  do not depend on the intrinsic properties of the internal components of the regulator  200 , such as the transconductance of some transistor element. Thus, the frequency location of zero introduced by  245  can be adjusted quite independently of the regulator  200  and the circuit  240 , which is also used for compensation purposes. This allows design flexibility and ease. In many instances the zero from circuit  245  is best placed at approximately the unity gain frequency of the regulator in order to reduce the amount of phase shift leading to instability. There is also a corresponding pole created; it follows the zero in frequency location. Therefore, it would occur beyond unity gain frequency if the zero were located around unity; then the pole would not affect stability. Usually, the phase margin from applying both frequency compensation circuit units  240  and  245  is improved by up to about 10 degrees relative to using first compensation unit  240  by itself.  
         [0018]     For purposes of illustration, the voltage regulator  200  is configured using three stages; however, regulator  200  can be configured using any number of stages depending on the required gain-bandwidth needs and the operating conditions. Furthermore, both circuit units  240  and  245  can be configured using various combinations of passive elements as applicable for a given regulator  200 . In addition, the passive elements can be configured using variable elements. Also, the passive elements can consist of active elements; for example, the resistors can be configured using biased transistors.  
         [0019]     A few preferred embodiments have been described in detail herein. It is to be understood that the scope of the invention also comprehends embodiments different from those described, yet within the scope of the claims. Words of inclusion are to be interpreted as nonexhaustive in considering the scope of the invention. While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.  
         [0020]     The section headings in this application are provided for consistency with the parts of an application suggested under 37 CFR 1.77 or otherwise to provide organizational cues. These headings shall not limit or characterize the invention(s) set out in any patent claims that may issue from this application. Specifically and by way of example, although the headings refer to a “Field of the Invention,” the claims should not be limited by the language chosen under this heading to describe the so-called field of the invention. Further, a description of a technology in the “Description of Related Art” is not be construed as an admission that technology is prior art to the present application. Neither is the “Summary of the Invention” to be considered as a characterization of the invention(s) set forth in the claims to this application. Further, the reference in these headings to “Invention” in the singular should not be used to argue that there is a single point of novelty claimed in this application. Multiple inventions may be set forth according to the limitations of the multiple claims associated with this patent specification, and the claims accordingly define the invention(s) that are protected thereby. In all instances, the scope of the claims shall be considered on their own merits in light of the specification but should not be constrained by the headings included in this application.  
         [0021]     Realizations in accordance with the present invention have been described in the context of particular embodiments. These embodiments are meant to be illustrative and not limiting. Many variations, modifications, additions, and improvements are possible. Accordingly, plural instances may be provided for components described herein as a single instance. Boundaries between various components, operations and data stores are somewhat arbitrary, and particular operations are illustrated in the context of specific illustrative configurations. Other allocations of functionality are envisioned and may fall within the scope of claims that follow. Finally, structures and functionality presented as discrete components in the exemplary configurations may be implemented as a combined structure or component. These and other variations, modifications, additions, and improvements may fall within the scope of the invention as defined in the claims that follow.