Patent Publication Number: US-10761550-B2

Title: Current limitation for voltage regulator

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority under 35 U.S.C. § 119 of European patent application no. 18305859.3, filed Jul. 2, 2018, the contents of which are incorporated by reference herein. 
     FIELD 
     The described embodiments relate generally to methods and devices that provide for current limitation, and more particularly to methods and devices that provide for current limitation for voltage regulation. 
     BACKGROUND 
     Current limitation circuits can be used to protect voltage regulator against component overstress in case of output load current overloading. However, typical current limitation circuits can have instability and other issues. 
     Therefore, there are strong motivations for improving current limitation circuits, and mitigating instability and other issues. 
     SUMMARY 
     Typically, current limitation circuits can sense the output current and counter react on the main amplification stage to alter the characteristics of the amplifier of the voltage regulator. However, such method can have the disadvantage of modifying the loop response, and may cause instability issues. This specification discloses methods and systems for limiting output current of a voltage regulator, which can solve potential stability issues during regulator power up or current limitation use cases, as well as provide for other benefits. In some embodiments, this specification describes current limitation circuits that can limit the maximum output load current by acting on the reference input voltage of the regulator. Once the current limitation detects an over current load, the reference voltage is adjusted (or, for example, decreased) to limit the output load current. 
     Such embodiments can be used to manage the current limitation in case of output load current overloading to protect the regulator and to not overstress regulator components. These embodiments limit the current in case of output load current overloading by presenting a current limitation loop outside the regulator feedback loop. In turn, this current limitation loop can take over from the regulator feedback in case of output load current overloading. This can solve potential stability issues during regulator power up or current limitation use cases. It also simplifies the overall regulator stability analysis as the current limit loop is outside the regulator feedback loop. 
     The present invention provides for a device comprising: (a) a voltage regulator, the voltage regulator configured to output a regulated output voltage; (b) a reference voltage module, the reference voltage module configured to output a reference voltage to the voltage regulator; (c) a current sensor and comparator, the current sensor and comparator configured to sense an output current of the voltage regulator; (d) a controller for current limitation, (e) wherein the controller is configured to limit the output current of the voltage regulator in response to an output of the current sensor and comparator by adjusting the reference voltage from the reference voltage module. 
     In some embodiments, the current sensor and comparator is further configured to compare the output current of the voltage regulator with a reference current to activate the current limitation. 
     In some embodiments, the controller is further configured to limit the output current of the voltage regulator in response to the output of the current sensor and comparator indicating an overload by adjusting the reference voltage from the reference voltage module. 
     In some embodiments, the voltage regulator is comprising of: (a) an error amplifier, (b) a voltage conversion stage, (c) a feedback circuit, (d) wherein the error amplifier is configured to receive input from both the reference voltage module and the feedback circuit, and to provide output to the voltage conversion stage, (e) wherein the voltage conversion stage is configured to receive input from the error amplifier and to provide voltage regulator output voltage to the feedback circuit, ( 0  wherein the current sensor and comparator is further configured to generate a copy of the voltage regulator output current. 
     In some embodiments, the voltage conversion stage is comprising of: an output power stage driven by an output power stage controller. 
     In some embodiments, the output power stage controller is a linear controller or a switching controller. 
     In some embodiments, the feedback circuit is a voltage control circuit or a current control circuit. 
     In some embodiments, the reference voltage module further comprises a slew rate control, wherein the slew rate control is coupled to the voltage regulator, wherein the slew rate control limits an inrush current of the voltage regulator by controlling a slope of variation of an input to the voltage regulator. 
     In some embodiments, the controller is further configured to limit the output current of the voltage regulator by adjusting an output voltage of the slew rate control. 
     In some embodiments, the reference voltage module is able to concurrently provide a normal operational reference voltage that is not adjusted by the controller for current limitation to another voltage regulator. 
     The present invention also provides for a method for limiting output current of a voltage regulator, the method comprising: (a) a reference voltage module inputting a reference voltage to a voltage regulator; (b) the voltage regulator outputting a regulated output voltage and an output current, wherein the output voltage is based on the input reference voltage; (c) a current sensor and comparator sensing the output current of the voltage regulator; (d) a controller limiting the output current of the voltage regulator, (e) wherein the controller limits the output current of the voltage regulator in response to an output of the current sensor and comparator by adjusting the reference voltage from the reference voltage module. 
     In some embodiments, the method further comprising: the current sensor and comparator comparing the output current of the voltage regulator with a reference current to activate the output current limitation by the controller. 
     In some embodiments, the step of the voltage regulator outputting an output voltage and an output current is comprising of: (a) an error amplifier receiving input from both the reference voltage module and a feedback circuit, (b) the error amplifier providing output to a voltage conversion stage, (c) the voltage conversion stage providing voltage regulator output voltage to the feedback circuit, (d) the voltage conversion stage providing voltage regulator output current to the current sensor and comparator. 
     In some embodiments, the reference voltage module further comprises a slew rate control, wherein the slew rate control is coupled to the voltage regulator, wherein the slew rate control limits an inrush current of the voltage regulator by controlling a slope of variation of an input to the voltage regulator. 
     In some embodiments, the controller further limits the output current of the voltage regulator by adjusting an output voltage of the slew rate control. 
     The present invention provides for a computer program product comprising executable instructions encoded in a non-transitory computer readable medium which, when executed by a system, carry out or control the following method for limiting output current of a voltage regulator, the method comprising: (a) a reference voltage module inputting a reference voltage to a voltage regulator; (b) the voltage regulator outputting a regulated output voltage and an output current, wherein the output voltage is based on the input reference voltage; (c) a current sensor and comparator sensing the output current of the voltage regulator; (d) a controller limiting the output current of the voltage regulator, (e) wherein the controller limits the output current of the voltage regulator in response to an output of the current sensor and comparator by adjusting the reference voltage from the reference voltage module. 
     The above summary is not intended to represent every example embodiment within the scope of the current or future Claim sets. Additional example embodiments are discussed within the Figures and Detailed Description below. Other aspects and advantages of embodiments of the present invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a functional block diagram of a first device for limiting output current of a voltage regulator in accordance with some embodiments of the invention. 
         FIG. 2A  shows a circuit diagram of the  FIG. 1  device in accordance with some embodiments of the invention. 
         FIG. 2B  shows a circuit diagram of a reference voltage module (which is part of the  FIG. 2A  device) in accordance with some embodiments of the invention. 
         FIG. 2C  shows a circuit diagram of a voltage regulator (which is part of the  FIG. 2A  device) in accordance with some embodiments of the invention. 
         FIG. 2D  shows a circuit diagram of a current limitation controller and a current sensor and comparator (which are part of the  FIG. 2A  device) and an output load in accordance with some embodiments of the invention. 
         FIG. 3  shows a functional block diagram of a second device (which further includes a slew rate control) for limiting output current of a voltage regulator in accordance with some embodiments of the invention. 
         FIG. 4A  shows a circuit diagram of the  FIG. 3  device in accordance with some embodiments of the invention. 
         FIG. 4B  shows a circuit diagram of a reference voltage module (which includes a slew rate control and is part of the  FIG. 4A  device) in accordance with some embodiments of the invention. 
         FIG. 4C  shows a circuit diagram of a voltage regulator (which is part of the  FIG. 4A  device) in accordance with some embodiments of the invention. 
         FIG. 4D  shows a circuit diagram of a current limitation controller and a current sensor and comparator (which are part of the  FIG. 4A  device) and an output load in accordance with some embodiments of the invention. 
         FIG. 5  shows a method for limiting output current of a voltage regulator in accordance with some embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     It will be readily understood that the components of the embodiments as generally described herein and illustrated in the appended figures could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of various embodiments, as represented in the figures, is not intended to limit the scope of the present disclosure, but is merely representative of various embodiments. While the various aspects of the embodiments are presented in drawings, the drawings are not necessarily drawn to scale unless specifically indicated. 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by this detailed description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope. 
     Reference throughout this specification to features, advantages, or similar language does not imply that all of the features and advantages that may be realized with the present invention should be or are in any single embodiment of the invention. Rather, language referring to the features and advantages is understood to mean that a specific feature, advantage, or characteristic described in connection with an embodiment is included in at least one embodiment of the present invention. Thus, discussions of the features and advantages, and similar language, throughout this specification may, but do not necessarily, refer to the same embodiment. 
     Furthermore, the described features, advantages, and characteristics of the invention may be combined in any suitable manner in one or more embodiments. One skilled in the relevant art will recognize, in light of the description herein, that the invention can be practiced without one or more of the specific features or advantages of a particular embodiment. In other instances, additional features and advantages may be recognized in certain embodiments that may not be present in all embodiments of the invention. 
     This specification discloses methods and systems for limiting output current of a voltage regulator. In some embodiments, this specification describes current limitation circuits used for voltage regulator. Current limitation circuits are used to protect regulator against component overstress in case of output load current overloading. In some embodiments, current limitation circuits can limit the maximum output load current by acting on the reference input voltage of the regulator. Once the current limitation detects an over current load, the reference voltage is decreased to limit the output load current. Therefore, in some embodiments, a current limitation circuit acting on the reference voltage input of a voltage regulator can limit the maximum output load current of the voltage regulator. 
     In some embodiments, this specification describes methods and devices to manage the current limitation and limit the maximum output load current. This helps to protect the regulator, by acting on the reference input voltage instead of directly on the regulator output power stage. In case of output load current overloading, the current limitation decreases the reference input voltage of the regulator. This does not overstress the regulator components because the current limitation is achieved through the reference input voltage. This also does not impact the reference voltage module performance. As an example, there is no additional serial components on the reference voltage path and it is not loading current on the reference voltage during normal operation. Additionally, these methods and devices can be coupled easily with a slew rate control circuit to also limit the inrush current. 
     In some embodiments, this specification describes methods and devices that allow a simplest stability analysis because: 
     a. the current limitation takes over from the regulator feedback loop through the reference input voltage; 
     b. the current limitation loop is outside the regulator feedback loop; 
     c. the current limitation loop does not need a lot of gain to limit the maximum output load current of the regulator; 
     d. there is no conflict between the current limitation loop and the regulator feedback loop in term of stability. 
     Usually, there are issues in term of current limitation stability during regulator power up due to conflict with the regulator main loop. By placing the current limitation loop outside the regulator feedback loop, this problem is now overcome. 
     In some embodiments, this specification describes methods and devices that avoids overstress on regulator component during the current limitation as it will bias the whole regulator smoothly according the reference input voltage value. 
       FIG. 1  shows a functional block diagram of a first device  100  for limiting output current of a voltage regulator in accordance with some embodiments of the invention.  FIG. 1  shows the current limitation controller  170  having a feedback to the reference voltage module  110 . In case of output load current (Iout  124 ) overloading, the current limitation is activated by decreasing the reference voltage input of the voltage regulator  120 . In term of functionality, it can be noticed that the current limitation loop is outside the regulator feedback loop. In some embodiments, once an overload on the output load current (Iout  124 ) is detected by the current limitation, the reference voltage can be adjusted or decreased to limit the maximum output load current. 
     In  FIG. 1 , a first device  100  for limiting output current of a voltage regulator includes a reference voltage module  110 , a voltage regulator  120 , a current sensor and comparator  160 , and a current limitation controller  170 . The reference voltage module  110  is configured to input a reference voltage to the voltage regulator  120 . For example, the reference voltage module  110  can include a reference voltage, bandgap, buffer, etc. The voltage regulator  120  is configured to output a regulated output voltage. The voltage regulator outputs an output voltage (Vout  122 ) and an output current (Iout  124 ), wherein the output voltage is based on the input reference voltage. The current sensor and comparator  160  is configured to sense an output current (Iout  124 ) of the voltage regulator  120 . 
     The current limitation controller  170  is a controller for limiting the output current (Iout  124 ). In some embodiments, the current limitation controller  170  is a circuit which acts on the reference voltage input to limit the maximum output load current. The controller  170  is configured to limit the output current (Iout  124 ) of the voltage regulator  120  in response to an output of the current sensor and comparator  160  by adjusting the reference voltage from the reference voltage module  110 . In some embodiment, the current sensor and comparator  160  is further configured to compare the output current (Iout  124 ) of the voltage regulator  120  with a reference current to activate the current limitation. In some embodiments, the controller  170  is further configured to limit the output current (Iout  124 ) of the voltage regulator  120  in response to the output of the current sensor and comparator  160  indicating an overload by adjusting the reference voltage from the reference voltage module  110 . In some embodiments, the controller  170  is further configured to limit the output current (Iout  124 ) of the voltage regulator  120  in response to the output of the current sensor and comparator  160  indicating an overload by decreasing the reference voltage from the reference voltage module  110 . 
     The voltage regulator  120  includes an error amplifier  130 , a voltage conversion stage  140 , and a feedback circuit  150 . The error amplifier  130  is configured to receive input from both the reference voltage module  110  and the feedback circuit  150 , and to provide output to the voltage conversion stage  140 . The voltage conversion stage  140  is configured to receive input from the error amplifier  130  and to provide voltage regulator output voltage (Vout  122 ) to the feedback circuit  150 . The current sensor and comparator  170  is further configured to generate a copy of the voltage regulator output current (Iout  124 ). 
     In some embodiments, the voltage conversion stage  140  includes an output power stage driven by an output power stage controller. In some embodiments, the output power stage controller is a linear controller or a switching controller. In some embodiments, the feedback circuit is a voltage control circuit or a current control circuit. In some embodiments, the switching controller is a PWM (pulse width modulation) controller or a PFM (pulse frequency modulation) controller. 
       FIG. 2A  shows a circuit diagram of the  FIG. 1  device in accordance with some embodiments of the invention. In particular,  FIG. 2A  is showing one example embodiment of a circuit implementation of the  FIG. 1  device  100 . Therefore, one skilled in the relevant art will recognize, in light of the description of  FIG. 2A , that the invention can be practiced with other embodiments of a circuit implementation of the  FIG. 1  device  100 , and these other embodiments are not shown in this specification. 
     In  FIG. 2A , a circuit  200  for limiting output current of a voltage regulator includes a reference voltage module  210 , a voltage regulator  220 , a current sensor and comparator  260 , and a current limitation controller  270 . The reference voltage module  210  is configured to input a reference voltage to the voltage regulator  220 .  FIG. 2A  further shows that the voltage regulator  220  includes an error amplifier  230 , a voltage conversion stage  240 , and a feedback circuit  250 .  FIG. 2A  also shows an output load  290 . 
     As an example, the voltage conversion stage  240  shown in  FIG. 2A  is a linear controller, but, in other embodiments, other types of controller can be implemented. 
     Being an example implementation of device  100 , circuit  200  operates in a manner similar to device  100 . Therefore, for example,  FIG. 2A  shows the current limitation controller  270  having a feedback to the reference voltage module  210 . In case of output load current overloading, the current limitation is activated by decreasing the reference voltage input of the voltage regulator  220 . In term of functionality, it can be noticed that the current limitation loop is outside the regulator feedback loop. In some embodiments, once an overload on the output load current is detected by the current limitation, the reference voltage can be adjusted or decreased to limit the output load current. 
     Again, similarly, the reference voltage module  210  is configured to input a reference voltage to the voltage regulator  220 . The voltage regulator  220  is configured to output a regulated output voltage. The voltage regulator outputs an output voltage and an output current, wherein the output voltage is based on the input reference voltage. The current sensor and comparator  260  is configured to sense an output current of the voltage regulator  220 . The current limitation controller  270  is a controller for limiting the output current. In some embodiments, the current limitation controller  270  is a circuit which acts on the reference voltage input to limit the maximum output load current. The controller  270  is configured to limit the output current of the voltage regulator  220  in response to an output of the current sensor and comparator  260  by adjusting the reference voltage from the reference voltage module  210 . 
       FIG. 2A  shows an overview of circuit  200 , while  FIGS. 2B, 2C, and 2D  give the close-up views of various parts of circuit  200 . In particular,  FIG. 2B  is showing the close-up view of the reference voltage module  210  in accordance with some embodiments of the invention.  FIG. 2C  is showing the close-up view of the voltage regulator  220  in accordance with some embodiments of the invention.  FIG. 2D  is showing the close-up view of the current limitation controller  270 , the current sensor and comparator  260 , and the output load  290  in accordance with some embodiments of the invention. 
     As shown in  FIGS. 2A and 2C , the voltage regulator  220  is a classical LDO (low-dropout) regulator with feedback loop. The feedback voltage VFB is equal to VOUT*R2/(R2+R1). If VOUT is too low (VREF&gt;VFB), then VERROR, output of the error amplifier and gate voltage of M1, increases and IOUT increases. The amount of output current IOUT is proportional to the difference between VREF and VFB. In case VOUT remains low for a long time due to a high load current, output short circuit or at start-up where the output starts from GND, the output current can be very high. A copy of IOUT, IOUT_SENSE, is generated and compared with IREF. As shown in  FIGS. 2A and 2D , the difference in current, ICOMP=IOUT_SENSE−IREF, is fed to the current limitation controller  270 , as long as the Sensed Output Current IOUT_SENSE&gt;IREF (the current limitation controller  270  is built with NMOS which can generate a current in one direction only). As shown in  FIGS. 2A, 2B, 2C, and 2D : if the latter is true (IOUT_SENSE&gt;IREF) then Current Limitation Signal ILIM is generated by the current limitation controller  270  and fed to the node VREF. Due to the output impedance of the voltage buffer of the reference voltage module  210 , VREF drops as soon as ILIM is outputted. The higher ILIM (i.e. the higher IOUT), the lower VREF becomes. The drop of VREF decreases the difference between VREF and VFB thus limits IOUT. 
       FIG. 3  shows a functional block diagram of a second device  300  (which further includes a slew rate control  380 ) for limiting output current of a voltage regulator in accordance with some embodiments of the invention. In  FIG. 3 , the second device  300  for limiting output current of a voltage regulator includes a reference voltage module  310 , a voltage regulator  320 , a current sensor and comparator  360 , and a current limitation controller  370 .  FIG. 3  also shows that the voltage regulator  320  includes an error amplifier  330 , a voltage conversion stage  340 , and a feedback circuit  350 .  FIG. 3  shows that slew rate control  380  is included in reference voltage module  310 . But, in some embodiments, the slew rate control can be separate from the reference voltage module (Note: not shown in  FIG. 3 ). In some embodiments, the slew rate control  380  can be arranged to be positioned between the reference voltage module  310  and the voltage regulator  320  (Note: not shown in  FIG. 3 ). In some embodiments, the reference voltage module  310  can be arranged to be positioned between the slew rate control  380  and the voltage regulator  320  (Note: not shown in  FIG. 3 ). In some embodiments, the slew rate control  380  and the reference voltage module  310  can have a joint output into the voltage regulator  320 . In some embodiments, the slew rate control and the reference voltage module can have separate outputs into the voltage regulator. 
       FIG. 3  is very similar to  FIG. 1 , except the second device  300  further includes a slew rate control  380  for limiting output current of a voltage regulator. The addition of a slew rate control is to limit an inrush current of the voltage regulator by controlling a slope of variation of an input to the voltage regulator at start-up. Slew rate is defined as the change of voltage or current per unit of time. Therefore, the slew rate control  380  can limit an inrush current of the voltage regulator  320  by controlling a slope of variation of an input (such as voltage) to the voltage regulator. For example, in some embodiments, the slew rate control can limit an inrush current of the voltage regulator by reducing the slope of variation of an input voltage to the voltage regulator. 
     In some embodiments, the reference voltage module  310  further comprises a slew rate control  380 , wherein the slew rate control  380  is coupled to the voltage regulator  320 , wherein the slew rate control  380  limits an inrush current of the voltage regulator  320  by controlling a slope of variation of an input to the voltage regulator  320 . 
     In some embodiments, the controller  370  is further configured to limit the output current (Iout  324 ) of the voltage regulator  320  by adjusting an output voltage of the slew rate control. In some embodiments, the controller  370  can adjust an output voltage of the slew rate control, so that the voltage regulator  320  receives an “adjusted” reference voltage, and the output current (Iout  324 ) of the voltage regulator  320  is adjusted accordingly. 
     In some embodiments, the reference voltage module  310  is able to concurrently provide a normal operational reference voltage that is not adjusted by the current limitation controller  370  to another voltage regulator. This can mean that, in some embodiments, the reference voltage module  310  is coupled to both the voltage regulator  320  and another voltage regulator. Then the current limitation controller  370  can adjust an output voltage of the slew rate control, so that the voltage regulator  320  receives an “adjusted” reference voltage, while another voltage regulator receives an “unadjusted” reference voltage (which, in turn, can be considered to be a normal operational reference voltage that is not adjusted by the current limitation controller  370 ). 
       FIG. 4A  shows a circuit diagram of the  FIG. 3  device in accordance with some embodiments of the invention. In particular,  FIG. 4A  is showing one example embodiment of a circuit implementation of the  FIG. 3  device  300 . Therefore, one skilled in the relevant art will recognize, in light of the description of  FIG. 4A , that the invention can be practiced with other embodiments of a circuit implementation of the  FIG. 3  device  300 , and these other embodiments are not shown in this specification. 
     In  FIG. 4A , a circuit  400  for limiting output current of a voltage regulator includes a reference voltage module  410 , a voltage regulator  420 , a current sensor and comparator  460 , a current limitation controller  470 , and a slew rate control  480 . The reference voltage module  410  is configured to input a reference voltage to the voltage regulator  420 .  FIG. 4A  further shows that the voltage regulator  420  includes an error amplifier  430 , a voltage conversion stage  440 , and a feedback circuit  450 .  FIG. 4A  also shows an output load  490 . 
     Being an example implementation of device  300 , circuit  400  operates in a manner similar to device  300 . Therefore, for example,  FIG. 4A  shows the current limitation controller  470  having a feedback to the reference voltage module  410 . In case of output load current overloading, the current limitation is activated by decreasing the reference voltage input of the voltage regulator  420 . In term of functionality, it can be noticed that the current limitation loop is outside the regulator feedback loop. In some embodiments, once an overload on the output load current is detected by the current limitation, the reference voltage can be adjusted or decreased to limit the output load current. 
     Again, similarly, the reference voltage module  410  is configured to input a reference voltage to the voltage regulator  420 . The voltage regulator  420  is configured to output a regulated output voltage. The voltage regulator outputs an output voltage and an output current, wherein the output voltage is based on the input reference voltage. The current sensor and comparator  460  is configured to sense an output current of the voltage regulator  420 . The current limitation controller  470  is a controller for limiting the output current. In some embodiments, the current limitation controller  470  is a circuit which acts on the reference voltage input of the voltage regulator  420  to limit the maximum output load current. The current limitation controller  470  is configured to limit the output current of the voltage regulator  420  in response to an output of the current sensor and comparator  460  by adjusting the reference voltage from the reference voltage module  410 . 
       FIG. 4A  shows an overview of circuit  400 , while  FIGS. 4B, 4C, and 4D  give the close-up views of various parts of circuit  400 . In particular,  FIG. 4B  is showing the close-up view of the reference voltage module  410  and the slew rate control  480  in accordance with some embodiments of the invention.  FIG. 4C  is showing the close-up views of the voltage regulator  420  in accordance with some embodiments of the invention.  FIG. 4D  is showing the close-up views of the current limitation controller  470 , the current sensor and comparator  460 , and the output load  490  in accordance with some embodiments of the invention. 
     As shown in  FIGS. 4A, 4B, 4C, and 4D : the voltage regulator  420  and the current limitation controller  470  operate similarly to their counterparts in  FIGS. 2A, 2B, 2C , and  2 D. The difference is that the error amplifier  430  receives 2 voltages (NORMAL OPERATIONAL VREF_NORMAL, from the voltage source and REFERENCE VOLTAGE VREF from the slew rate control circuit), as shown in  FIGS. 4A, 4B, and 4C . MA and MB act as a minimum selector. This means that the lowest voltage from the gate voltages of MA (NORMAL OPERATIONAL VREF_NORMAL) and MB (REFERENCE VOLTAGE VREF) will regulate the output voltage VOUT. At the start-up of the voltage regulator, the NORMAL OPERATIONAL VREF_NORMAL may already be present or rises very quickly, this can cause a very high output current IOUT especially when VOUT has been discharged to GND and CLOAD has a high value. To limit IOUT, the slew rate control will generate a slowly increasing reference voltage REFERENCE VOLTAGE VREF matching in speed with VOUT increase rate. This is done by charging the capacitor CSLEW_RATE_CTRL with a current ISLEW_RATE_CTRL, as shown in  FIGS. 4A and 4B . The result is a linear voltage slope for REFERENCE VOLTAGE VREF (output of the slew rate control circuit). Such a slope is proportional to the current ISLEW_RATE_CTRL and inversely proportional to the capacitor CSLEW_RATE_CTRL. As long as REFERENCE VOLTAGE VREF is below NORMAL OPERATIONAL VREF_NORMAL, VOUT will follow REFERENCE VOLTAGE VREF. As soon as REFERENCE VOLTAGE VREF exceeds NORMAL OPERATIONAL VREF_NORMAL, the output voltage VOUT is then fixed by the NORMAL OPERATIONAL VREF_NORMAL (MA and MB act as a minimum selector) to the nominal desired voltage value. In case IOUT increases too much during normal operation (i.e. ISENSE_OUT exceeds IREF), ILIM is outputted and decreases REFERENCE VOLTAGE VREF. If REFERENCE VOLTAGE VREF drops below the NORMAL OPERATIONAL VREF_NORMAL, then it will take control of the voltage regulator (MA and MB act as a minimum selector) and limit the output current. 
       FIG. 5  shows a method  500  for limiting output current of a voltage regulator in accordance with some embodiments of the invention. As shown in  FIG. 5 , the method  500  begins at step  510 , where the method begins with a reference voltage module inputting a reference voltage to a voltage regulator. Then, the method proceeds to step  520 . In step  520 , the method continues with the voltage regulator outputting a regulated output voltage and an output current, wherein the output voltage is based on the input reference voltage. Next, at step  530 , the method goes on with a current sensor and comparator sensing the output current of the voltage regulator. Then, the method proceeds to step  540 . In step  540 , the method continues with a controller limiting the output current of the voltage regulator, wherein the controller limits the output current of the voltage regulator in response to an output of the current sensor and comparator by adjusting the reference voltage from the reference voltage module. 
     In this specification, example embodiments have been presented in terms of a selected set of details. However, a person of ordinary skill in the art would understand that many other example embodiments may be practiced which include a different selected set of these details. It is intended that the following claims cover all possible example embodiments. 
     Although the operations of the method(s) herein are shown and described in a particular order, the order of the operations of each method may be altered so that certain operations may be performed in an inverse order or so that certain operations may be performed, at least in part, concurrently with other operations. In another embodiment, instructions or sub-operations of distinct operations may be implemented in an intermittent and/or alternating manner. 
     It should also be noted that at least some of the operations for the methods may be implemented using software instructions stored on a computer useable storage medium for execution by a computer. As an example, an embodiment of a computer program product includes a computer useable storage medium to store a computer readable program that, when executed on a computer, causes the computer to perform operations, as described herein. 
     The computer-useable or computer-readable medium can be an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system (or apparatus or device), or a propagation medium. Examples of a computer-readable medium include a semiconductor or solid-state memory, magnetic tape, a removable computer diskette, a random access memory (RAM), a read-only memory (ROM), a rigid magnetic disc, and an optical disc. Examples of optical discs include a compact disc with read only memory (CD-ROM), a compact disc with read/write (CD-R/W), a digital video disc (DVD), and a Blu-ray disc. 
     The various aspects, embodiments, implementations or features of the described embodiments can be used separately or in any combination. Various aspects of the described embodiments can be implemented by software, hardware or a combination of hardware and software. 
     The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the described embodiments. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the described embodiments. Thus, the foregoing descriptions of specific embodiments are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the described embodiments to the precise forms disclosed. It will be apparent to one of ordinary skill in the art that many modifications and variations are possible in view of the above teachings.