Patent Publication Number: US-11378993-B2

Title: Voltage regulator circuit with current limiter stage

Description:
BACKGROUND 
     An electronic device may include an integrated circuit having an internal or “on-chip” voltage regulator that is used to provide power to an “off-chip” electrical load while regulating the voltage. 
     SUMMARY 
     Examples are disclosed herein that relate to automatically limiting an output current of a voltage regulator circuit responsive to detecting that the voltage regulator is in a current overload mode. In one example, a voltage regulator circuit includes an amplifier stage and a current limiter stage electrically connected to an output of the amplifier stage. The amplifier stage is configured to output a DC voltage based on a reference voltage and feedback from an output voltage. The current limiter stage is configured to operate in a quiescent mode and an overload mode. In the quiescent mode, the current limiter stage is configured to operate as a buffer stage that forms a closed feedback loop to an input of the amplifier stage. In the overload mode, the current limiter stage is configured to act as a current source that clamps an output current to a designated current. 
     This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Furthermore, the claimed subject matter is not limited to implementations that solve any or all disadvantages noted in any part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an example head-mounted display device (HMD) that includes an on-chip voltage regulator circuit. 
         FIG. 2  schematically shows a system block diagram of the HDM shown in  FIG. 1 . 
         FIG. 3A  shows a system block diagram of an example voltage regulator circuit operating in a quiescent mode. 
         FIG. 3B  shows a system block diagram of an example voltage regulator circuit operating in an overload mode. 
         FIG. 4A  shows a circuit diagram representing the voltage regulator circuit of  FIG. 3A . 
         FIG. 4B  shows a circuit diagram representing the voltage regulator circuit of  FIG. 3B . 
         FIG. 5  shows an example current limiter stage of a voltage regulator circuit that includes a current control stage operable to vary a designated current in an overload operating mode. 
         FIG. 6  is a graph showing an output voltage of an example voltage regulator circuit without inrush current limiting functionality and reliability operating in a quiescent mode. 
         FIG. 7  is a graph showing an output current of the voltage regulator circuit operating without inrush current limiting functionality and reliability in a quiescent mode. 
         FIG. 8  is a graph showing an output voltage of an example voltage regulator circuit with inrush current limiting functionality and reliability operating in an overload mode. 
         FIG. 9  is a graph showing an output current of an example voltage regulator circuit with inrush current limiting functionality and reliability operating in an overload mode. 
         FIG. 10  shows an example method for limiting current of a voltage regulator circuit. 
     
    
    
     DETAILED DESCRIPTION 
     Under certain operating conditions of an electronic device, a significant amount of current may be output from an “on-chip” voltage regulator integral to an integrated circuit to a discrete “off-chip” electronic component. Such high current can cause degradation of the integrated circuit, the electronic component, and/or intermediate electrical connections resulting in a reduced operational lifetime of the electronic device. As one example, an electronic device may include a discrete capacitor that is charged by an on-chip voltage regulator when the electronic device is turned on. Under certain operating conditions, a significant amount of current may be output from the on-chip voltage regulator of the integrated circuit to the discrete off-chip electronic component. Power-cycling of the integrated circuit can cause a significant amount of current to be output from the on-chip voltage regulator of the integrated circuit to the discrete off-chip electronic component. Such high current can cause degradation of the integrated circuit, the off-chip electronic component, and/or the intermediate electrical connections, which may result in a reduced operational lifetime of the electronic device. Additionally, high-current surges can also negatively affect other chips and/or other electrical components in the system via a brown-out event associated with the high current surges. 
     Accordingly, the present description is directed to a voltage regulator circuit including an amplifier stage and a current limiter stage electrically connected to an output of the amplifier stage. The amplifier stage is configured to output a DC voltage based on a reference voltage and feedback from an output voltage. The current limiter stage is configured to operate in a quiescent mode and an overload mode. In the quiescent mode, the current limiter stage is configured to operate as a buffer stage that forms a closed feedback loop to an input of the amplifier stage. In the overload mode, the current limiter stage is configured to act as a current source that clamps an output current to a designated current. The overload mode may be triggered based on an output current of the voltage regulator circuit being greater than a designated threshold current. As one example, such a condition may occur based on an output node of the voltage regulator circuit being shorted to ground. As another example, such a condition may occur in some instances during power cycling of an electronic device that includes the voltage regulator circuit. When the current overload condition that triggers operation in the overload mode is mitigated, the voltage regulator circuit is configured return to normal operation where the current limiter stage operates in the quiescent mode. 
     The disclosed example current limiter stages can be implemented on-chip at the transistor level without use of external off-chip electrical components or digital signal processing. Such a configuration allows for the current limiter stage to detect a current overload condition of the voltage regulator circuit more quickly than a configuration that relies on a digital signal processing block of an integrated circuit to detect a current overload condition. Moreover, such a current limiter stage implemented at the transistor level may be configured to switch the voltage regulator circuit back to normal operation in the quiescent operating mode once the current overload condition is cleared quicker than a configuration that uses, for example, a digital signal processing block of the integrated circuit to detect the current overload condition. Furthermore, since the current limiter stage is implemented at the transistor level on chip, the voltage regulator circuit may have a physical footprint that is smaller than a voltage regulator circuit that uses discrete, off-chip electrical components, such as a switcher that employs discrete inductors. 
     As an example use environment for a voltage regulator according to the present disclosure,  FIG. 1  shows an example head-mounted device (HMD)  100  worn by a user  102 . The HMD  100  comprises a see-through display  104  configured to present virtual imagery to provide the user  102  with an augmented reality experience.  FIG. 2  schematically shows a system block diagram of the HDM  100  shown in  FIG. 1 . The HMD  100  comprises a display processor integrated circuit (IC)  200  that is configured to control an image source  202 . The image source  202  is configured to visually present virtual imagery on the see-through display  104 . In some examples, the display processor integrated circuit  200  may take the form of a system on a chip (SoC). It will be appreciated that that display processor integrated circuit  200  may take any suitable form of integrated circuit also referred to as a “chip.” The display processor integrated circuit  200  comprises a voltage regulator circuit  204  configured to regulate a voltage of power provided to a load  206 . The load  206  may comprise a discrete, off-chip electronic component. In one example, the electronic component may comprise a capacitor that is used to power the see-through display  104 . It will be appreciated that the voltage regulator circuit  204  may be configured to regulate a voltage of any suitable electronic component of the HMD  100 . In some implementations, the display processor integrated circuit  200  may include a plurality of voltage regulator circuits to regulate voltages of different discrete electronic components electrically connected to the display processor integrated circuit  200 . The display processor integrated circuit  200  may include any suitable number of voltage regulator circuits. The HMD  100  is provided as a non-limiting example of an electronic device that comprises a voltage regulator circuit having current limiting functionality as described herein and the disclosed examples of voltage regulator circuits with such current limiting functionality may be implemented in any suitable type of electronic device. 
       FIGS. 3A and 3B  schematically show a system block diagram of the voltage regulator circuit  204  shown in  FIG. 2 . The voltage regulator circuit  204  is configured to operate in either one of two discrete operating modes. The first operating mode is a quiescent or steady state operating mode in which the voltage regulator circuit  204  operates during normal operating conditions as shown in  FIG. 3A . The second operating mode is an overload operating mode that is triggered based on an output current of the voltage regulator circuit  204  being greater than a threshold current of the voltage regulator circuit  204  as shown in  FIG. 3B . 
     The voltage regulator circuit  204  comprises an amplifier stage  300  and a current limiter stage  302 . The amplifier stage  300  comprises a negative input  304 , a positive input  306 , and an output  308 . The negative input  304  is configured to receive a reference voltage  310 . In one example, the reference voltage is set to a DC power supply voltage (e.g., VDD) of the voltage regulator circuit  204 . It will be appreciated that the reference voltage may be set to any suitable voltage to satisfy the design requirements of the electronic device in which the voltage regulator circuit is implemented. The positive input  306  is configured to receive feedback of an output voltage  312  of the voltage regulator circuit  204  via a feedback loop  314 . The amplifier stage  300  is configured to output a DC voltage  316  based on the reference voltage  310  and the feedback from the output voltage  312  of the voltage regulator circuit  204 . In the illustrated example, the amplifier stage  300  is configured as a differential amplifier stage that amplifies a difference between the feedback of the output voltage  312  and the reference voltage  310 , such that the DC voltage  316  output from the amplifier stage  300  is set to the reference voltage  310 . In other implementations, the amplifier stage  300  may be configured as a different type of amplifier stage. The current limiter stage  302  is electrically connected to the output  308  of the amplifier stage  300 . 
     As shown in  FIG. 3A , in the quiescent operating mode, the current limiter stage  302  is configured to operate as a buffer stage  320  that forms the closed feedback loop  314  that feeds the output voltage  312  of the voltage regulator circuit  204  back to the positive input  306  of the amplifier stage  300 . In the illustrated example, the buffer stage  320  is configured to operate as a unity gain buffer in the quiescent operating mode. In other implementations, the current limiter stage  302  may be configured to operate at a buffer stage that amplifies the DC voltage  316  output from the amplifier stage  300  with a designated gain that is not one and may be inverting in some instances. In implementations where the current limiter stage  302  is inverting, negative feedback may be provided to the input of the amplifier stage  300 . Such negative feedback may facilitate stable operation of the voltage regulator circuit  204  in the quiescent operating mode. 
     The voltage regulator circuit  204  may be configured to switch from the quiescent operating mode to the overload operating mode based on the output current  318  being greater than a threshold current of the voltage regulator circuit  204 . The threshold current of the voltage regulator circuit  204  may be set to any suitable threshold current. In some examples, the threshold current may be set based on the operating characteristics of the amplifier stage  300  (e.g., a threshold current of the op-amp). The switch from the quiescent operating mode to the overload operating mode may be triggered based on various operating conditions. As one example, the voltage regulator circuit  204  may switch from operation in the quiescent operating mode to operation in the overload operating mode responsive to a short circuit at an output node  324  of the voltage regulator circuit  204 . As another example, the voltage regulator circuit  204  may switch from operation in the quiescent operating mode to operation in the overload operating mode responsive to a non-short circuit, high current condition where the output current  318  is greater than the threshold current. For example, such a condition may occur during power cycling of the electronic device when a capacitor that receives power from the voltage regulator circuit is at least partially discharged. 
     As shown in  FIG. 3B , in the overload operating mode, the amplifier stage  300  clips the DC voltage  316  to zero volts based on the output current  318  fed back to the input  306  of the amplifier stage  300  being greater than the threshold current. When the current limiter stage  302  detects zero volts at the output  308  of the amplifier stage  300 , the current limiter stage  302  is configured to switch from operation in the quiescent operating mode to operation in the overload operating mode. In operation in the overload operating mode, the current limiter stage  302  is configured to act as a current source  322  that clamps the output current  318  of the voltage regulator circuit  204  to a designated current. The current limiter stage  302  acting as the current source  322  may set the designated current to any suitable current that protects the voltage regulator circuit  204  from degradation. 
     Furthermore, the voltage regulator circuit  204  may be configured to switch from the overload operating mode to the quiescent operating mode based on the output current  318  being less than the threshold current of the voltage regulator circuit  204 . In other words, when the overload current is removed, the voltage regulator circuit  204  may be configured to automatically return to normal operation in the quiescent stage. 
       FIGS. 4A and 4B  show circuit diagrams representing an example implementation of the voltage regulator circuit  204  of  FIGS. 3A and 3B , respectively. In particular,  FIGS. 4A and 4B  show an example current limiter stage  400  suitable for use in the voltage regulator circuit  204  at the transistor level. The current limiter stage  302  comprises a source follower field effect transistor (FET)  401 . The source follower FET  401  comprises a gate  402 , a drain,  404 , and a source  406 . The gate  402  of the source follower FET  401  is electrically connected to the output  308  of the amplifier stage  300 . 
     An output FET  408  (MOUT) comprises a gate  410 , a drain  412  and a source  414 . The gate  410  of the output FET  408  is electrically connected to the source  406  of the source follower FET  401 . The source  414  of the output FET  408  is electrically connected to a power supply node  416  (VDD). The power supply node  416  provides DC power to the source terminals of the various FETs in the current limiter stage  302 . The DC power may have any suitable voltage that complies with the design characteristics of the voltage regulator circuit  204 . The drain  412  of the output FET  408  is electrically connected to the output node  324  of the voltage regulator circuit  204 . 
     A current control stage  418  is electrically intermediate the power supply node  416  and the drain  404  of the source follower FET  401 . In the illustrated implementation, the current control stage  418  comprises a current control FET  420  (MB). In other implementations, the current control stage  418  may comprise one or more FETs and/or other electronic components that are configured to control the designated current of the voltage regulator circuitry  204  in the overload operating mode. The current control FET  420  comprises a gate  422 , a drain  424 , and a source  426 . The gate  422  of the current control FET  420  is tied to the drain  424  of the current control FET  420 . The source  426  of the current control FET  420  is electrically connected to the power supply node  416 . 
     A source follower replica FET  428  (MSF_REPLICA) comprises a gate  430 , a drain  432 , and a source  434 . The source  434  of the source follower replica FET  428  is electrically connected to the current control stage  418 , and specifically to the drain  424  of the current control FET  420 . The drain  432  of the source follower replica FET  428  is electrically connected to the drain  404  of the source follower FET  401 . 
     A first current source  436  is electrically connected to the power supply node  416  and electrically intermediate the power supply node  416  and the source  406  of the source follower FET  401  and the gate  410  of the output FET  408 . The first current source  436  is configured to output a current (I 1 ). A second current source  438  is electrically connected to the drain  404  of the source follower FET  401  and the drain  432  of the source follower replica FET  428 . The second current source  438  is configured to output a current (I 1 +ΔI). 
       FIG. 4A  shows the current limiter stage  302  operating in the quiescent operating mode. In the quiescent operating mode, the source follower FET  401  and the output FET  408  are configured to operate as the buffer stage  320  (shown in  FIG. 3A ) that forms the closed negative-feedback loop  314  to feedback the output voltage  318  of the voltage regulator circuit  204  to the input  306  of the amplifier stage  300 . The amplifier stage  300  outputs a DC signal having a voltage (referred to as the source follower gate voltage (SFGATE)) that is set based on the reference voltage  310  and the closed negative feedback loop  314 . The voltage SFGATE is equal to two transistor threshold voltage levels lower than the power supply voltage (VDD)—i.e., SFGATE equals VDD−2VGS. A gate voltage (PGATE) of the output FET  408  is equal to VDD−VGS. In this way, the source follower FET  401  act as a level shifter that increases the voltage between SFGATE and PGATE in the quiescent operating mode. Additionally, the source follower FET  401  buffers the SFGATE signal to PGATE of the output FET  408  with a gain of 1, such that the source follower FET  401  acts as a unity gain buffer. In the quiescent operating mode, the voltage regulator circuit  204  outputs a DC signal to a load  440  that is connected to the output node  324 . The DC signal has the output voltage  318  and the output current  318  (IOUT). 
       FIG. 4B  shows the current limiter stage  302  operating in the overload operating mode. In the overload operating mode, the output of the amplifier stage  300  is set to zero volts. In the illustrated example, the amplifier stage  300  clips to zero volts at the output  308  based on a short circuit at the output node  324 . It will be appreciated that other high current condition where the output current  318  is greater than the threshold current of the amplifier stage  300  may cause the amplifier state  300  to output zero volts. In the overload operating mode, when SFGATE goes to zero volts, the source follower FET  401  is configured to act as a triode switch that electrically connects the current control FET  420  of the current control stage  418  to the gate  410  of the output FET  408  such that the output current  318  of the voltage regulator circuit is clamped to the designated current that is controlled by the current control stage  418 . In particular, when the source follower FET  401  acts as a triode switch, the current I 1  from the first current source  436  flows through the source follower FET  401  to the drain  432  of the source follower replica FET  428 . The second current source  438  pulls the current I 1 +ΔI such that a current ΔI flows through the current control FET  420 . The source follower replica FET  401  is configured such that a drain-to-source voltage of the source follower replica FET  428  is equal to a drain-to-source voltage of the source follower FET  401  in the overload operating mode. As such, in the overload operating mode, such a configuration causes a current mirror to be formed between the current control FET  420  and the output FET  408 . Such a current mirror causes the output current  318  of the voltage regulator circuit  302  to clamp to the designated current. In particular, the designated current is equal to the current ΔI multiplied by a ratio of the widths of the current control FET  420  and the output FET  408  (i.e., IOUT=ΔI*width_MOUT/width_MB). In an example where the widths of the current control FET  420  and the output FET  408  are the same, the output current  318  is clamped to the current ΔI flowing across the current control FET  420 . In another example where the width of the current control FET  420  is much greater than a width of the output FET  408 , the output current  318  would be clamped to a significantly lower current than ΔI. The current control stage  418  may include any suitable configuration of electronic components to control the designated current of the voltage regulator circuit  204  in any suitable manner. 
     The current limiter stage  302  may be configured such that once the overload condition clears by the output current becoming less than the threshold operating current of the amplifier stage  300 , the voltage regulator circuit  204  automatically switches back to operation in the quiescent operating mode and the current limiter stages  302  acts as the buffer stage  322  that forms the closed negative feedback loop  314  that feeds the output voltage  312  of the voltage regulator circuit  204  back to the input  306  of the amplifier stage  300 . 
     In the illustrated example, the FETs of the current limiter stage  302  are depicted as a P-type metal oxide silicon field effect transistors (MOSFETs). In other implementations, different type(s) of FETs may be used in the current limiter stage, such as N-type FETs or J-type FETs. In some implementations, another type of transistor may be used in place of one or more of the P-FETs. In some implementations, such transistors may be symmetrical in order to provide the current clamping functionality in the overload operating condition. 
     In the implementation illustrated in  FIGS. 4A and 4B , the current control stage  418  comprises a single current control FET  420 . In other implementations, the voltage regulator circuit  204  may be configured to have a current control stage that includes other electronic component configurations that are configured to control the designated current differently.  FIG. 5  shows an example current limiter stage  500  suitable for use in the voltage regulator circuit  204  The current limiter stage  500  includes a current control stage  501 . The current control stage  501  may include any suitable electronic component configuration to set the designated current output by the voltage regulator circuit  204  in the overload operating mode. In some implementations, the current control stage  501  may comprise one or more current control FETS. In some implementations, the current control stage  501 A may comprise a plurality of current control FETS  502  in series as shown in box  504 . For example, four FETs may be electrically connected in series to provide a 4×1 width ratio of the current control FETs relative to the output FET in the output current equation (e.g., IOUT=ΔI*width_MOUT/width_MB*4). In some implementations, the current control stage  501 B may comprise a plurality of current control FETS  506  electrically connected in parallel as shown in box  508 . Further, in some implementations, the current control stage  501 C may comprise a plurality of current control FETS  510  connected via a plurality of switches  512  operable to vary the designated current as shown in box  514 . The switches  512  may allow for different FETs to be selectively turned on to dynamically set the designated current to a desired current. For example, in some instances, a first switch may be turned on and a second switch may be turned off to set a first designated current based on a single transistor. In other instances, the first switch and the second switch may be turned on to set a second designated current based on two transistors connected in series. Such a configuration may include any suitable number of FETs and any suitable number of switches arranged in any suitable manner to achieve any suitable granularity of programmability of the designated current. In still other implementations, another type of electronic component may be used to control the designated current. For example, in some implementations, a digital to analog converter may be used to control the designated current during the overload operating mode. Any suitable electronic component or configuration of multiple electronic components may be used to control the designated current to any suitable desired current during the overload operating mode. 
     In the illustrated implementation, the voltage regulator circuit  204  includes a first disable FET  516  and a second disable FET  518 . The first and second disable FETs  516 ,  518  may be turned on/off to selectively disable/enable the functionality of the current limiter stage  302  of the voltage regulator circuit  204 . In other implementations, such disable/enable functionality of the current limiter stage  302  may be selectively omitted. 
       FIGS. 6-7  show graphs illustrating example operation of a voltage regulator circuit without the inrush current limiting functionality and reliability describe herein.  FIG. 6  shows example voltage responses  600  (e.g.,  600 A,  600 B,  600 C) of a voltage regulator circuit without the inrush current limiting functionality. The different voltage responses  600 A,  600 B, and  600 C are representative of operation of the voltage regulator circuit under different simulated operating conditions (e.g., different temperature, loads).  FIG. 7  shows example current responses  700  (e.g.,  700 A,  700 B,  700 C) of the voltage regulator circuit without the inrush current limiting functionality. The different current response  700 A,  700 B, and  700 C are representative of operation of the voltage regulator circuit under different simulated operating conditions (e.g., different temperature, loads). In particular, the different current responses  700 A,  700 B, and  700 C illustrate the relatively high variability of operation of the voltage regulator circuit with different currents being output and different switching times occurring based on the different operating conditions. Under some such conditions, the output current of the voltage regulator circuit may be high enough to potentially cause degradation of the voltage regulator circuit. 
       FIGS. 8-9  show graphs illustrating example operation of a voltage regulator circuit with the inrush current limiting functionality describe herein.  FIG. 8  shows example voltage responses  800  (e.g.,  800 A,  800 B,  800 C) of a voltage regulator circuit with the inrush current limiting functionality. The different voltage responses  800 A,  800 B, and  800 C are representative of operation of the voltage regulator circuit under different simulated operating conditions (e.g., different temperature, loads) that correspond to the same operating conditions as the voltage responses  600 A,  600 B, and  600 C shown in  FIG. 6 . The example voltage responses  800 A,  800 B, and  800 C illustrate the automatic and seamless transition from an overload operating mode to a quiescent operating mode, in addition to providing a potential increased controlled response that is more reliable relative to the voltage responses  600 A,  600 B, and  600 C of the voltage regulator circuit without inrush current limiting functionality.  FIG. 9  shows example current responses  900  (e.g.,  900 A,  900 B,  900 C) of a voltage regulator circuit with the inrush current limiting functionality. The different current responses  900 A,  900 B, and  900 C are representative of operation of the voltage regulator circuit under different simulated operating conditions (e.g., different temperature, loads) that correspond to the same operating conditions as the current responses  700 A,  700 B, and  700 C shown in  FIG. 7 . The current responses  900 A,  900 B, and  900 C are potentially slower and more controlled than the current responses  700 A,  700 B, and  700 C shown in  FIG. 7 , but the more controlled current responses  900 A,  900 B, and  900 C exhibit a possible ten times reduction in current value over the current responses  700 A,  700 B, and  700 C shown in  FIG. 7 . The more controlled current responses  900 A,  900 B, and  900 C are a result of the inrush current limiting functionality of the voltage regulator circuit that automatically limits the output current of the voltage regulator circuit to the designated current in the overload operating condition. The reduced output current prevents high current from flowing through the voltage regulator circuit and circuits electronically connected to the voltage regulator circuit and thus prevents potential degradation of these electronic circuits. 
       FIG. 10  shows an example method  1000  of operating a voltage regulator circuit with an inrush current limiter. For example, the method  1000  may be performed by any of the voltage regulator circuits shown in  FIGS. 2-5  and described herein. At  1002 , an amplifier stage of the voltage regulator circuit responds to whether an output current of the voltage regulator circuit is greater than a threshold current. If the output current, is greater than the threshold current, then the voltage regulator circuit operates in an overload operating mode and the method  1000  moves to  1008 . Otherwise, the voltage regulator circuit operates in a quiescent operating mode and the method  1000  moves to  1004 . At  1004 , in the quiescent operating mode, the amplifier stage amplifies an output voltage of the voltage regulator circuit through a feedback loop, such that an output of the amplifier stage is set to a reference voltage. At  1006 , in the quiescent operating mode, a current limiter stage buffers the output of the amplifier stage to form the closed feedback loop that feeds back the output voltage of the voltage regulator circuit to the input of the amplifier stage and the method  1000  returns to  1002  to repeat the method  1000 . In some implementations, the current limiter stage may be inverting, such that negative feedback is provided to the input of the amplifier stage. At  1008 , in the overload operating mode, the amplifier stage amplifies an output voltage of the voltage regulator circuit, such that the output of the amplifier stage is set to zero volts. At  1010 , in the overload operating mode, the current limiter stage clamps the output current of the voltage regulator circuit to a designated current and returns to  1002  to repeat the method  1000 . 
     The method  1000  may be performed to automatically respond to a normal current condition of the voltage regulator circuit and operate in a quiescent operating mode to generate an output voltage based on a reference voltage and closed loop feedback of the output voltage of the voltage regulator circuit. Further, the method  1000  may be performed to automatically respond to a current overload condition of the voltage regulator circuit and quickly clamp the output current of the voltage regulator circuit to a designated current. Once the overload condition clears, the method  1000  may be performed to automatically switch back to normal operation in the quiescent operating mode. By performing such a method, brown out events related to high current surge conditions may be mitigated, and more generally operation of such a voltage regulator circuit may be more reliable relative to a voltage regulator circuit that lacks such automatic in rush current limiting functionality. 
     In an example, a voltage regulator circuit, comprises an amplifier stage configured to output a DC voltage based on a reference voltage and feedback from an output voltage of the voltage regulator circuit, and a current limiter stage electrically connected to an output of the amplifier stage, wherein the current limiter stage is configured to operate in a quiescent operating mode and an overload operating mode, such that in the quiescent operating mode, the current limiter stage is configured to operate as a buffer stage that forms a closed feedback loop that feeds back the output voltage of the voltage regulator circuit to an input of the amplifier stage, and in the overload operating mode, the current limiter stage is configured to act as a current source that clamps an output current of the voltage regulator circuit to a designated current. In this example and/or other examples, the current limiter stage optionally may include a source follower field effect transistor (FET), wherein a gate of the source follower FET may be electrically connected to the output of the amplifier stage, an output FET, wherein a gate of the output FET may be electrically connected to a source of the source follower FET, wherein a source of the output FET may be electrically connected to a power supply node, and wherein a drain of the output FET may be electrically connected to an output node of the voltage regulator circuit, and a current control stage electrically intermediate the power supply node and a drain of the source follower FET, wherein the current limiter stage may be configured to operate in the quiescent operating mode and the overload operating mode, such that in the quiescent operating mode, the source follower FET and the output FET may be configured to operate as the buffer stage that forms the closed feedback loop to feedback the output voltage of the voltage regulator circuit to the input of the amplifier stage, and in the overload operating mode, the source follower FET may be configured to act as a triode switch that electrically connects the current control stage to the gate of the output FET such that the output current of the voltage regulator circuit may be clamped to the designated current that may be controlled by the current control stage. In this example and/or other examples, the current limiter stage optionally may further include a source follower replica FET, wherein a source of the source follower replica FET may be electrically connected to the current control stage, wherein a drain of the source follower replica FET may be electrically connected to the drain of the source follower FET, and wherein the source follower replica FET may be configured such that a drain-to-source voltage of the source follower replica FET is equal to a drain-to-source voltage of the source follower FET in the overload operating mode. In this example and/or other examples, the current limiter stage optionally may further include a first current source electrically connected to the power supply node and electrically intermediate the power supply node and the source of the source follower FET and the gate of the output FET. In this example and/or other examples, the current limiter stage optionally may further include a second current source electrically connected to the drain of the source follower FET. In this example and/or other examples, the current control stage optionally may comprise one or more current control FETS. In this example and/or other examples, the current control stage optionally may comprise a plurality of current control FETS in series. In this example and/or other examples, the current control stage optionally may comprise a plurality of current control FETS connected via a plurality of switches operable to vary the designated current. In this example and/or other examples, the buffer stage optionally may comprise a unity gain buffer stage. In this example and/or other examples, the current limiter stage optionally may be configured to switch from operation in the quiescent operating mode to operation in the overload operating mode responsive to a short circuit at the output node. In this example and/or other examples, the current limiter stage optionally may be configured to switch from operation in the quiescent operating mode to operation in the overload operating mode responsive to the amplifier stage outputting zero volts. In this example and/or other examples, the current limiter stage optionally may be configured to switch from operation in the quiescent operating mode to operation in the overload operating mode responsive to the output current of the voltage regulator circuit being greater than a threshold current. 
     In another example, a voltage regulator circuit, comprises an amplifier stage configured to output a DC voltage based on a reference voltage and feedback of an output voltage of the voltage regulator circuit, and a current limiter stage including a source follower field effect transistor (FET), wherein a gate of the source follower FET is electrically connected to an output of the amplifier stage, an output FET, wherein a gate of the output FET is electrically connected to the source of the source follower FET, wherein the source of the output FET is electrically connected to a power supply node, and wherein the drain of the output FET is electrically connected to an output node, and a current control stage electrically intermediate the power supply node and the drain of the source follower FET, wherein the current limiter stage is configured to operate in a quiescent operating mode and an overload operating mode, such that in the quiescent operating mode, the source follower FET and the output FET are configured to operate as a buffer stage that forms a closed feedback loop to feedback the output voltage of the voltage regulator circuit to an input of the amplifier stage, and in the overload operating mode, the source follower FET is configured to act as a triode switch that electrically connects the current control stage to the gate of the output FET such that an output current of the voltage regulator circuit is clamped to a designated current controlled by the current control stage. In this example and/or other examples, the current limiter stage optionally may further include a source follower replica FET, wherein a source of the source follower replica FET may be electrically connected to the current control stage, wherein a drain of the source follower replica FET may be electrically connected to the drain of the source follower FET, and wherein the source follower replica FET may be configured such that a drain-to-source voltage of the source follower replica FET is equal to a drain-to-source voltage of the source follower FET in the overload operating mode. In this example and/or other examples, the current limiter stage optionally may further include a first current source electrically connected to the power supply node and electrically intermediate the power supply node and the source of the source follower FET and the gate of the output FET. In this example and/or other examples, the current limiter stage optionally may further include a second current source electrically connected to the drain of the source follower FET. In this example and/or other examples, the current control stage optionally may comprise one or more current control FETS. In this example and/or other examples, the current control stage optionally may comprise a plurality of current control FETS in series. In this example and/or other examples, the current control stage optionally may comprise a plurality of current control FETS connected via a plurality of switches operable to vary the designated current. 
     In yet another example, a method for limiting current in a voltage regulator circuit comprising an amplifier stage and a current limiter stage, the method comprises amplifying, via the amplifier stage, an output voltage of the voltage regulator circuit through a feedback loop, such that an output of the amplifier stage is set to a reference voltage based on an output current of the voltage regulator circuit being less than a threshold current, and such that the output of the amplifier stage is set to zero volts based on the output current of the voltage regulator circuit being greater than the threshold current, buffering, via the current limiter stage, the output of the amplifier stage based on the output of the amplifier stage being set to the reference voltage, and clamping, via the current limiter stage, the output current of the voltage regulator circuit to a designated current based on the output of the amplifier stage being zero volts. 
     It will be understood that the configurations and/or approaches described herein are exemplary in nature, and that these specific embodiments or examples are not to be considered in a limiting sense, because numerous variations are possible. The specific routines or methods described herein may represent one or more of any number of processing strategies. As such, various acts illustrated and/or described may be performed in the sequence illustrated and/or described, in other sequences, in parallel, or omitted. Likewise, the order of the above-described processes may be changed. 
     The subject matter of the present disclosure includes all novel and non-obvious combinations and sub-combinations of the various processes, systems and configurations, and other features, functions, acts, and/or properties disclosed herein, as well as any and all equivalents thereof.