Patent Publication Number: US-7907003-B2

Title: Method for improving power-supply rejection

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to the field of semiconductor circuit design, and more particularly to the design of improved power regulators. 
     2. Description of the Related Art 
     Many electronic power supplies feature voltage regulators, or regulator circuits, designed to automatically maintain a constant output voltage level to effectively provide a steady voltage to the electronic circuit to which power is being supplied, typically referred to as the load. More particularly, the object of a voltage regulator circuit is to maintain a steady output voltage regardless of current drawn by the load. Most present day voltage regulators operate by comparing the actual output voltage to a fixed—typically internal—reference voltage. The difference between the actual output voltage and reference voltage is amplified, and used for controlling a regulation element, to form a negative feedback servo control loop. The regulation element is typically configured to produce a higher voltage when the output voltage is too low, and in case of some regulators, to produce a lower voltage when the output voltage is too high. In many cases, the regulation element may be configured to simply stop sourcing current, and depend on the current drawn by the driven load to pull down the regulator output voltage. The control loop has to be carefully designed to produce the desired tradeoff between stability and speed of response. 
     The operation of power supplies is typically affected by variations on the input voltage (or power supply) line that provides the voltage based on which the regulated output voltage is generated. Any signal or noise (including transients, which may reach very high levels relative to the level of the desired output voltage) on the supply line may couple into, and may be amplified by the active circuitry, thereby degrading the performance of the power supply. Therefore, in addition to design considerations related to stability and speed of response, power supplies are also typically designed to achieve a desired power supply rejection ratio (PSRR), which is indicative of the amount of noise (on the supply line) that the power regulator is capable of rejecting. Various systems may specify different power supply rejection requirements. For example, an internal power regulator using a 25 pF output capacitor in an automotive environment may experience power supply variations that range from 5V to 26V and may include transient spikes as high as 40V. Thus, any power supply or regulator designed to properly function in such an environment would need to be designed to reject all such variations and transients. 
     Therefore, one measure of the effectiveness of a voltage regulator circuit is its ability to respond to system transients. For example, if the load coupled to a voltage regulator is an integrated circuit (IC) in which a large number of drivers may switch states simultaneously, the demand for current from the voltage regulator may change suddenly. An ideal voltage regulator is able to meet the demand for increased current while maintaining its designed output voltage V out . However, this may not always be practical for a given voltage regulator circuit and a given load. In practice, a load capacitance (coupled between the voltage output node and ground) is typically provided in order to meet the immediate demand for increased current. Typical solutions for increasing power supply rejection include use of a large load capacitor, and/or use of a pass transistor coupled at the output. 
     In addition, in some situations, a circuit used to implement a voltage regulator may be subject to short circuit or overload conditions for a significant amount of time. In such cases, the circuit may become damaged without protection against excessive currents that may result from such conditions. Similarly, other types of circuits (e.g., amplifiers) may also be susceptible to problems similar to those discussed above with regard to voltage regulators. Many other problems and disadvantages of the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein. 
     SUMMARY OF THE INVENTION 
     In one set of embodiments, a voltage regulator may comprise a regulator output configured to provide a regulated voltage, built around an error amplifier powered by a supply voltage and having a first input configured to receive a reference signal. The voltage regulator may include a pass transistor having a control terminal coupled to an output of the error amplifier, and a channel coupled between the supply voltage and the regulator output. A control loop may be formed by coupling the regulator output to a second input of the error amplifier, which may comprise an output stage configured to provide the output signal of the error amplifier. In one embodiment, the error amplifier may be configured to control its output stage to conduct current during a rising edge of the supply voltage to prevent the regulated output voltage from rising during the rising edge of the supply voltage. 
     The voltage regulator output may be configured with a voltage divider, which may include a first resistor coupled between the second input of the error amplifier and the regulator output, and a second resistor coupled between the regulator output and a voltage reference, which may be reference ground. In one set of embodiments, the error amplifier may comprise a first input transistor having a first channel terminal configured to draw a first portion of a first current generated from the supply voltage, and a control terminal configured as the first input of the error amplifier. The error amplifier may further have a second input transistor with a first channel terminal configured to draw a second portion of the first current, and a control terminal configured as the second input of the error amplifier. The first and second input transistors may constitute an input stage of the error amplifier, and may be coupled to the output stage of the error amplifier. 
     In one set of embodiments, the output stage of the error amplifier may include four output transistors, and a current mirror configured to provide current to the four transistors. The first output transistor may have a first channel terminal coupled to the regulator output and configured to draw a second current generated from the supply voltage, a second channel terminal coupled to a second channel terminal of the first input transistor, and a control terminal configured to receive a biasing signal. The second output transistor may have a first channel terminal configured to draw a third current generated from the supply voltage, a second channel terminal coupled to a second channel terminal of the second input transistor, and a control terminal configured to receive the biasing signal. The third output transistor may be configured with a first channel terminal coupled to the second channel terminal of the first output transistor, a second channel terminal coupled to a voltage reference (which may be reference ground), and a control terminal coupled to a control node. Finally, the fourth output transistor may have a first channel terminal coupled to the second channel terminal of the second output transistor, a second channel terminal coupled to the voltage reference, and a control terminal coupled to the control node. A capacitor may be coupled between the control node and the regulator output to achieve frequency compensation. In one embodiment, a capacitor may be configured between the supply voltage and the control terminals of the third and fourth transistors to effect additional current to flow through the respective channels of the third and fourth output transistors, to prevent the first and second output transistors from turning off during a rising edge of the supply voltage. 
     A method of operating an electronic circuit may include providing a supply voltage to the electronic circuit, providing a reference signal to the electronic circuit, generating an output signal based on the supply voltage, the reference signal, and an error signal, and generating a feedback signal based on the output signal, with an output stage of the electronic circuit generating the error signal based on the supply voltage, the reference signal, and the feedback signal. The output stage may be controlled from within the electronic circuit to have the output stage continue to conduct current during a rising edge of the supply voltage to prevent the output signal from rising to the level of the supply voltage during the rising edge of the supply voltage. In one embodiment, controlling the output stage may include preventing a pair of cascode transistors configured in the output stage from turning off during the rising edge of the supply voltage, by causing an additional current to flow through a pair of output transistors having their respective channels coupled between respective channel terminals of the pair of cascode transistors and a voltage reference. 
     Various embodiments of a regulator circuit may therefore provide improved power supply rejection for very large, fast steps on the voltage supply rail, without a need for any external components, and only requiring very few additional internal components (e.g. a 1 pF capacitor). The voltage regulators may be implemented with a topology that includes a PMOS pass device, while still providing very good rejection of power supply variations, and prevent loss of feedback control during large supply transients. 
     Other aspects of the present invention will become apparent with reference to the drawings and detailed description of the drawings that follow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing, as well as other objects, features, and advantages of this invention may be more completely understood by reference to the following detailed description when read together with the accompanying drawings in which: 
         FIG. 1  is a schematic diagram of one embodiment of a voltage regulator circuit according to prior art; 
         FIG. 2  is a transistor diagram of one embodiment of a voltage regulator circuit configured according to principles of the present invention; and 
         FIG. 3  is waveform diagram containing three voltage waveforms to illustrate the transient response of a power regulator to a step voltage of 30V magnitude. 
     
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. Note, the headings are for organizational purposes only and are not meant to be used to limit or interpret the description or claims. Furthermore, note that the word “may” is used throughout this application in a permissive sense (i.e., having the potential to, being able to), not a mandatory sense (i.e., must).” The term “include”, and derivations thereof, mean “including, but not limited to”. The term “connected” means “directly or indirectly connected”, and the term “coupled” means “directly or indirectly connected”. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a schematic diagram of one embodiment of a voltage regulator circuit  100 , according to prior art. In the embodiment shown, an input supply voltage Vdd is provided to operational amplifier  104 . The voltage regulator circuit provides an output voltage from the source terminal of transistor  106 . Current through transistor  106  and load  114  is controlled via a feedback path between the junction of resistors R 1  ( 108 ) and R 2  ( 110 ), which comprise a voltage divider circuit, and the inverting input of operational amplifier  104 . The operational amplifier is an error amplifier, used in the circuit to indicate an error between a reference voltage V ref    102 , which is provided to the non-inverting terminal of operational amplifier  104 , and the voltage present at the junction of R 1  ( 108 ) and R 2  ( 110 ), i.e., the feedback voltage, or V fb . Operational amplifier  104  is configured to provide an output signal that is proportional to the difference between the reference voltage V ref  and the feedback voltage V fb , which is used to drive the gate terminal of n-channel transistor  106 . This in turn controls the current passing through transistor  106 , and thus also controls the current passing through load  114 . 
     Using the circuit shown in  FIG. 1  as an example, a load having a suddenly increased demand for current may initially receive current from the load capacitance (which may be included in load  114 ). However, the load capacitance can only provide a finite amount of current, after which the voltage regulator circuit must provide current for both the load as well as for recharging the load capacitance. When this occurs, the feedback voltage may be pulled down somewhat (assuming discharge of the load capacitance), thereby causing the amplitude of the error signal produced by the error amplifier to increase. This in turn may result in an increased amount of current through transistor  106 . Eventually, the increased amount of current will cause both the output  112  and feedback V fb  voltages to be pulled up through the voltage divider network. 
     As mentioned above, power regulators are oftentimes required in an automotive environment, where an internal regulator may be configured with a 25 pF capacitor at the output, and may be required to reject power supply variations (on the power/voltage supply rail) that range from 5V to 26V, while also rejecting transient spikes, which may reach voltage levels as high as 40V. Referring to voltage regulator  100  in  FIG. 1 , when voltage regulator  100  is configured on an integrated circuit (IC), and is used to power up internal blocks configured on the IC, the IC may not have external pins available to use a large load capacitor to increase the power supply rejection. In addition, the threshold voltage variation with the current load at low power supplies (i.e. when the supply voltage is low) may make the use of an NMOS pass transistor (such as transistor  106 ) not possible, as there may be no headroom for the V GS  (gate-source voltage) of an NMOS pass transistor. 
       FIG. 2  shows one embodiment of a voltage regulator  200  that meets the requirements set forth above, without using an NMOS pass transistor and/or large load capacitor. In the embodiment shown in  FIG. 2 , error amplifier  201  may receive a reference voltage input V ref    208 , and may be coupled to resistors  230  and  232  and a load  236 , in a manner similar to error amplifier  104  in  FIG. 1 , to generate regulated voltage output V reg    234 . However, in regulator circuit  200 , NMOS pass transistor  106  from voltage regulator circuit  100  has been replaced with PMOS pass transistor  228 . The input stage of error amplifier  201  may be formed by PMOS transistors  210  and  214 . A current mirror formed using PMOS devices  212  and  216  may be configured to mirror a bias current (generated off Vdd in the channel of PMOS device  216 ) to the input stage of error amplifier  210 . The output stage of amplifier  201  may comprise a cascode stage (NMOS devices  222  and  226 ), coupled between NMOS devices  238  and  240 , and a current mirror (PMOS devices  220  and  224 ). A bias current generated through the channel of PMOS device  220  may be mirrored in the channel of PMOS device  224 . The output stage may generate the error signal provided at the control terminal (or gate) of PMOS pass device  228 . A signal (voltage) based on V reg    234  and established between resistors  230  and  232  may be fed back to the input stage, more specifically to the control (gate) terminal of PMOS device  214 , to create the feedback (control) loop. As seen in circuit  200 , a capacitor C CL    206  may be coupled between Vdd and the respective control (gate) terminals of NMOS transistors  204 ,  221 ,  238  and  240 , to increase current flowing through transistors  238  and  240  of the output stage of amplifier  201  during transients, or in general during rising edges of supply voltage Vdd. 
     Considering circuit  200  without capacitor  206 , a large and quick (on the order of ns or even a few μs) power supply step (transient) may cause cascode transistors  222  and  226  to begin turning off during the rising edge of the transition period (of the supply step), as output signal V reg    234  begins to rise. Beyond a certain point, PMOS transistor  214  of the input stage of amplifier  201  may turn off, and the gate-drain capacitance of PMOS device  214  may begin to dominate, causing the voltage at the drain of PMOS transistor device  214  (and consequently, at the source of NMOS transistor device  222 ) to rise, and effectively turn off cascode transistors  222  and  226 , causing regulated output V reg    234  no longer being controlled by the feedback loop, which may result in the regulated output V reg    234  rising to the supply voltage level Vdd. 
     In order to prevent the event described above, capacitor  206  may be coupled between power supply rail Vdd and the respective gate terminals of NMOS transistors  204 ,  221 ,  238 , and  240  as shown. In order to obtain adequate frequency compensation, capacitor  207 —having a value equivalent to the Miller capacitance associated with circuit  200 —may also be coupled between regulated output V reg    234  and the low impedance node formed at the source terminal of NMOS device  226  and drain terminal of NMOS device  240  coupled together within the output stage of amplifier  201 . It should be noted that circuit  200  may also include additional components configured to provide the bias voltage Vbnc for NMOS cascode device  218 . Those skilled in the art will appreciate that a variety of different biasing circuits are possible, and any one of the many possible biasing circuits may be configured in circuit  200  to provide the required bias voltage to NMOS device  218 . 
     Capacitor  206  coupled as shown in  FIG. 2  may operate to cause an additional current flowing through transistors  238  and  240 , (in other words, it may cause an increased current flow through NMOS devices  238  and  240 ), thereby causing the respective drains of transistors  238  and  240  to be pulled low during the rising edge of the supply voltage Vdd. This in turn may operate to prevent NMOS devices  222  and  226  from turning off, preventing output V reg    234  from rising during the rising edge of the supply voltage, thereby keeping V reg    234  from reaching the level of the supply voltage Vdd during transients and/or rising edges of Vdd. By preventing a drastic change on output V reg  during a power supply transient and/or during other power supply variations (supply transition, for short), the control loop of power/voltage regulator  200  may remain functional during the supply transition. In other words, capacitor  206  configured as shown may cause increased current to flow through NMOS devices  238  and  240 , keeping cascode NMOS transistors  222  and  226  turned on during the rising edge of the supply step to prevent output V reg  from rising. During the falling edge of the supply step, NMOS devices  238  and  240  may turn off, resulting in no current flowing through NMOS devices  238  and  240  when there is a very sharp falling edge on supply voltage Vdd. It should also be noted that in embodiments where the power supply line (Vdd) is configured with a large capacitor (for example in certain automotive applications), the falling edge on the supply rail Vdd may be slow enough to avoid transistors  238  and  240  from turning off. 
     Various embodiments of regulator circuit presented in  FIG. 2  may therefore provide improved power supply rejection for very large, fast steps on the voltage supply rail, without a need for any external components, and only requiring very few additional internal components (e.g. a 1 pF capacitor, such as capacitor  206  in power regulator circuit  200 ). In addition, circuit  200  may be implemented with a topology that includes a PMOS pass device (such as PMOS device  228  in  FIG. 2 ), while still providing very good rejection of power supply variations. Finally, various embodiments may prevent loss of feedback control during large supply transients, as also explained above. 
       FIG. 3  shows waveforms illustrating the transient response of (the output of) a power regulator, such as regulator  200 , without capacitor  206  (waveform  304 ) and with capacitor  206  (waveform  306 ) for a supply step of 30V (waveform  302 ), provided as a simulation of a transient that may occur on the supply line (Vdd) during regular operation of power regulator  200 . As seen in diagram  304 , the response without capacitor  206  results in a voltage pulse of 11V in magnitude. In contrast, as seen in diagram  306 , the response with capacitor  206  results in a voltage pulse of only 0.6V in magnitude. 
     Although the embodiments above have been described in considerable detail, other versions are possible. Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications. Note the section headings used herein are for organizational purposes only and are not meant to limit the description provided herein or the claims attached hereto.