Abstract:
Apparatus disclosed herein implement a fast transient precision current limiter such as may be included in an electronic voltage regulator. The current limiter includes two current sense element/current clamp control loops. A fast response time control loop first engages and clamps a current spike. A precision control loop then engages to more accurately clamp the output current to a programmed set point. The precision clamping loop includes an inner loop to linearize the precision current sense element. The inner loop forces the drain-to-source voltage (VDS) of the precision sense element to track the VDS of the regulator pass element. A more precise clamping operation results. Overall speed is not sacrificed as the fast response time clamping loop operates in parallel to protect circuitry while the precision clamping loop engages.

Description:
TECHNICAL FIELD 
     Structures described herein relate to electronic circuits, including current-limiting circuits and electronic power supply systems. 
     BACKGROUND INFORMATION 
     Some electronic power supply regulators may include over-current protection circuitry. Over-current protection may prevent catastrophic failure of regulator components, particularly power transistor output stages, in the event of a short circuit or a heavy loading condition at the regulator output. 
     Some power supply regulators may utilize a sense transistor to sense an overdrive condition and a current clamp to temporarily reduce the drive signal to the power output stage while the overdrive condition is present. However, sense transistor-based current clamps traditionally have poor direct current (DC) accuracy. Inaccuracies may be due to a combination of channel length modulation effects, sense element to pass element matching, and process, supply, and/or temperature (PVT) variations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an electronic current-limiting apparatus according to various example embodiments of the invention. 
         FIG. 2  is a detailed block diagram of an electronic current-limiting apparatus according to various example embodiments. 
         FIG. 3  is a circuit diagram of an electronic current-limiting apparatus according to various example embodiments. 
         FIG. 4  is a block diagram of a current-limited voltage regulator according to various example embodiments. 
     
    
    
     SUMMARY OF THE INVENTION 
     Apparatus disclosed herein implement a fast transient precision current limiter such as may be included in an electronic voltage regulator. The current limiter includes two current sense element/current clamp control loops. A fast response time control loop first engages and clamps a current spike. A precision control loop then engages to more accurately clamp the output current to a programmed set point. 
     Each of the control loops includes a current sense element coupled to a voltage regulation control input. Voltage regulation control circuitry senses the over-current condition as a voltage drop at the voltage regulator output terminals. The voltage regulation control circuitry responds by increasing the magnitude of a voltage regulation control drive signal to a pass element through which current from the voltage regulator flows to the load. Thus, large increases in the voltage regulation control drive signal are indicative of an over-current condition. The sense elements associated with the two current-limiting control loops sense large increases in the voltage regulation control drive signal as over-current conditions. 
     The precision clamping loop includes an inner loop to linearize the precision current sense element. The inner loop forces the drain-to-source voltage (VDS) of the precision sense element to track the VDS of the regulator pass element. The linearizing loop includes an element matching circuit which may, in some embodiments, be implemented as a signal comparator such as an operational transconductance amplifier (OTA). The comparator senses the regulator output voltage at a comparator input and generates an output signal which, when fed back to a second comparator input, causes the second comparator input voltage to track the regulator output voltage. 
     The linearization loop effectively causes the precision current sense element to follow the VDS of the regulator output pass element. The comparator generates a more accurate precision sense signal to be compared to a precision reference source representing the selected clamping point. A more precise clamping operation results. Overall speed is not sacrificed as the fast response time clamping loop operates in parallel to protect circuitry while the precision clamping loop engages. 
     DETAILED DESCRIPTION 
       FIGS. 1 and 2  are block diagrams of current-limiting apparatus  100  and  200  according to various example embodiments of the invention. The current-limiting apparatus  100  and  200  include a pass element  110 . The pass element  110  is embodied as a power MOSFET  215  in the current-limiting apparatus  200 . The pass element  110  receives a voltage regulation control input signal  114  on a voltage regulation control input path  115 . The pass element  110  allows a magnitude of current flow through a current channel of the pass element  110  that is proportional to the magnitude of the voltage regulation control input signal  114 . The current-limiting apparatus  100  and  200  also both include a fast transient feedback circuit  118  and a precision feedback circuit  124  operating in parallel. 
     The fast transient feedback circuit  118  is communicatively coupled to the voltage regulation control input path  115 . The fast transient feedback circuit  118  receives the voltage regulation control input signal  114  and decreases the magnitude of the voltage regulation control input signal  114  on the path  115  in an over-current condition. The fast transient feedback circuit  118  includes a fast transient current sense element  128  communicatively coupled to the voltage regulation control input path  115 . The fast transient current sense element  128  receives the voltage regulation control input signal  114  and generates a fast transient feedback sense current ISENSE_F. ISENSE_F is proportional to the magnitude of current flow through the pass element  110 . 
     At  FIG. 1 , the fast transient feedback circuit  118  of the current-limiting apparatus  100  also includes a first current comparator  135  communicatively coupled to the output of the fast transient current sense element  128 . The first current comparator  135  compares ISENSE_F to a first reference current I(REF_F) and generates a first current clamp control signal CC_F. 
     Some embodiments of the fast transient feedback circuit  118  may include a current mirror  210  as depicted in  FIG. 2  in conjunction with the current-limiting apparatus  200 . The current mirror  210  is communicatively coupled between the fast transient current sense element  128  and the first current comparator  135 . The current mirror  210  generates a first attenuated sense current of ratio 1/N(ISENSE_F) for comparison to the first reference current I(REF_F). 
     The fast transient feedback circuit  118  also includes a first over-current clamp  150  communicatively coupled to an output of the first current comparator  135 . The first over-current clamp  150  receives the first current clamp control signal CC_F. Responsive to CC_F, the first over-current clamp  150  conducts to clamp the voltage regulator control input path  115  and to decrease the magnitude of the voltage regulation control input signal  114 . The magnitude of current flow through the pass element  110  associated with the over-current condition decreases as a result. 
     The current-limiting apparatus  100  and  200  also include a precision feedback circuit  124 , as previously mentioned. The precision feedback circuit  124  is communicatively coupled to the voltage regulation control input path  115 . The precision feedback circuit  124  receives the voltage regulation control input signal  114  as representative of the magnitude of current flow through the pass element  110 /pass transistor  215 . The precision feedback circuit  124  operates in parallel with the fast transient feedback circuit  118  to decrease the magnitude of the voltage regulation control input signal  114  in an over-current condition. Although slower to react, the precision feedback circuit  124  may more accurately control the over-current condition than control provided by the fast transient feedback circuit  118 . The precision feedback circuit  124  effects more precise control by correcting for one or more component characteristic mismatches between the pass element  110 /pass transistor  215  and one or more components of the precision feedback circuit  124 . 
     The precision feedback circuit  124  includes a precision current sense element  155 . In some embodiments, the precision current sense element  155  may be implemented using a geometrical size that is an integer semiconductor finger fraction of the pass element  110 /pass transistor  215 . Doing so may enhance component characteristic matching between the precision current sense element  155  and the pass element  110 /pass transistor  215 . 
     The precision current sense element  155  is communicatively coupled to the voltage regulation control input path  115 . The precision current sense element  155  receives the voltage regulation control input signal  114  and generates a precision feedback current sense signal ISENSE_PS. ISENSE_PS is proportional to the magnitude of current flow through the pass element  110 /pass transistor  215 . 
     At  FIG. 1 , the precision feedback circuit  124  of the current-limiting apparatus  100  also includes an element matching circuit  158 . A first input  160  of the element matching circuit  158  is communicatively coupled to the output of the precision current sense element  155 . A second input  161  is communicatively coupled to a voltage regulation output path  162 . A negative feedback element  165  is communicatively coupled between an output  168  of the element matching circuit  158  and the first input  160 . The inner feedback loop  173  so formed causes a signal at the first input  160  to track the voltage at the regulation output path  162 . One or more component characteristic mismatches between the pass element  110  and the precision current sense element  155  are compensated via the inner feedback loop  173 . 
     Turning again to  FIG. 2 , the current-limiting apparatus  200  includes a component characteristic-matching inner loop  230  analogous to the loop  173  of the current-limiting apparatus  100 . The apparatus  200  includes a signal comparator  235  with a first input  240  communicatively coupled to an output of the precision current sense element  155 . A second input  241  is communicatively coupled to the voltage regulation output path  162 . The precision feedback circuit  124  also includes a negative feedback element  165  communicatively coupled between an output of the signal comparator  235  and the first input  240 . The inner loop  230  causes a signal at the first input  240  to track the voltage at the regulator output path  162 . Doing so compensates for component characteristic mismatches between the pass transistor  215  and the precision current sense element  155 . 
     The signal comparator  235  generates a precision feedback sense current ISENSE_P. Some embodiments of the precision feedback circuit  124  may include a current ratio apparatus  250  coupled to the output of the signal comparator  235 . The current ratio apparatus  250  divides ISENSE_P by a factor N to yield a second attenuated sense current 1/N(ISENSE_P). The current ratio apparatus  250  may be implemented using one or more current mirrors, geometrical finger size ratios between interconnected stages, and other methods as are well-known in the art. 
     The precision feedback circuit  124  also includes a second current comparator  180 . The second current comparator  180  is communicatively coupled to an output  168  of the element matching circuit  158  in the case of the current-limiting apparatus  100  of  FIG. 1 . The second current comparator  180  is communicatively coupled to an output of the signal comparator  235  or to the output of the current ratio apparatus  250  in the case of the current-limiting apparatus  200  of  FIG. 2 . Either the precision feedback sense current ISENSE_P or the second attenuated feedback sense current 1/N(ISENSE_P), as appropriate, is compared to a second reference current I(REF_P). A second current clamp control signal CC_P is generated at the output of the second current comparator  180  in the event of an over-current condition. 
     The precision feedback circuit  124  also includes a second over-current clamp  185  communicatively coupled to the second current comparator  180 . The second over-current clamp  185  receives the second current clamp control signal CC_P and decreases the magnitude of the voltage regulation control input signal  114 . The magnitude of over-current flow through the pass element  110  of the current-limiting apparatus  100  ( FIG. 1 ) and/or through the pass transistor  215  of the current-limiting apparatus  200  ( FIG. 2 ) is decreased as a result. 
       FIG. 3  is a circuit diagram of an electronic current-limiting apparatus  300  according to various example embodiments. The current-limiting apparatus  300  includes a pass transistor  215 . The pass transistor  215  receives a voltage regulation control input signal  114  at a gate terminal  310 . The pass transistor  215  allows a magnitude of current flow through a current channel associated with the pass transistor  215  that is proportional to the magnitude of the voltage regulation control input signal  114 . The current-limiting apparatus  300  also includes a fast transient feedback circuit  315  and a precision feedback circuit  318 . The fast transient feedback circuit  315  and the precision feedback circuit  318  operate in parallel to first control the fastest rise-time current transients and then to more precisely control over-current as the precision feedback circuit  318  engages. 
     The fast transient feedback circuit  315  is communicatively coupled to the voltage regulation control input path  115  to receive the voltage regulation control input signal  114 . The fast transient feedback circuit  315  interprets the magnitude of current flow through the pass transistor  215  from the magnitude of the voltage regulation control input signal  114 . The fast transient feedback circuit  315  decreases the magnitude of the voltage regulation control input signal  114  in an over-current condition, thereby decreasing drive to the pass transistor  215  and alleviating the over-current condition. 
     The fast transient feedback circuit  315  includes a fast transient current sense transistor  322  gate-coupled to the voltage regulation control input path  115 . The current sense transistor  322  receives the voltage regulation control input signal  114  and generates a fast transient sense current ISENSE_F. ISENSE_F is proportional to the magnitude of current flow through the pass transistor  215 . In some embodiments, the fast transient feedback circuit  315  also includes an N-ratio current mirror  325  communicatively coupled to the fast transient current sense transistor  322 . The N-ratio current mirror  325  receives ISENSE_F at a current mirror source transistor  328 . 
     The current mirror  325  generates a first attenuated sense current 1/N(ISENSE_F) through a current channel of a current mirror output transistor  333 . Either ISENSE_F, or 1/N(ISENSE_F) in the case of a current mirror  325  implementation, is compared to a first reference current I(REF_F). The first reference current I(REF_F) is generated by a first reference current source  337 . The first current reference source  337  in coupled in series with the current channel of the current mirror output transistor  333 . 
     The fast transient feedback circuit  315  further includes a first over-current clamp transistor  340 . The first over-current clamp transistor  340  is gate-coupled to a node  341  between the first reference current source  337  and the current channel of the current mirror output transistor  333 . In an over-current condition, the first attenuated sense current 1/N(ISENSE_F) exceeds the first reference current I(REF_F), causing the gate  343  of the clamp transistor  340  to be pulled low. The first over-current clamp transistor  340  consequently conducts and clamps the voltage regulation control input signal  114  on the input path  115 . The gate  310  of the pass transistor  215  is clamped such as to decrease the magnitude of current flow through the pass transistor  215  and to thereby alleviate the over-current condition. 
     The current-limiting apparatus  300  also includes a precision feedback circuit  318 , as previously mentioned. The precision feedback circuit  318  is communicatively coupled to the voltage regulation control input path  115  to receive the voltage regulation control input signal  114 . The precision feedback circuit  318  decreases the magnitude of the voltage regulation control input signal  114  in an over-current condition more accurately than control provided by the fast transient feedback circuit  315 . The precision feedback circuit  318  operates more accurately by compensating for one or more component characteristic mismatches between the pass transistor  215  and the precision feedback circuit  318 . 
     The precision feedback circuit  318  includes a precision current sense transistor  350  communicatively coupled to the voltage regulation control input path  115 . The precision current sense transistor  350  receives the voltage regulation control input signal  114  as representative of the magnitude of current flow through the pass transistor  215 . The precision current sense transistor  350  generates a precision feedback current sense signal ISENSE_PS proportional to the magnitude of current flow through the pass transistor  215 . 
     In some embodiments, the precision current sense transistor  350  may be fabricated to result in a geometrical size that is an integer semiconductor finger fraction of the pass transistor  215 . Doing so may further enhance characteristic matching between the two devices. 
     The precision feedback circuit  318  also includes an OTA  355 . A first input  358  of the OTA  355  is communicatively coupled to an output of the precision current sense transistor  350 . A second input  362  of the OTA  355  is communicatively coupled to a voltage regulation output path  162 . The OTA  355  generates a precision feedback sense current ISENSE_P on output  368 . 
     The precision feedback circuit  318  also includes a characteristic matching feedback transistor  365 . The feedback transistor  365  is gate-coupled to the output  368  of the OTA and is current channel coupled to the first OTA input  358 . The OTA  355  and the feedback transistor  365  operate as an inner component matching loop  372  within the precision feedback circuit  318 . The inner matching loop  372  causes a signal at the first OTA input  358  to track the regulator output voltage and to thereby compensate for one or more component characteristic mismatches between the pass transistor  215  and the precision current sense transistor  350 . 
     The precision feedback circuit  318  also includes a second reference current source  375 . The second reference current source  375  generates a second reference current I(REF_P). A precision loop transistor  380  is gate-coupled to the output  368  of the OTA  355  and is channel-coupled between a voltage rail  383  and the second reference current source  375 . The precision loop transistor  380  receives ISENSE_P at its gate  385  and generates a second attenuated sense current 1/N(ISENSE_P) for comparison to the second reference current I(REF_P). 
     The precision feedback circuit  318  further includes a second over-current clamp transistor  390 . The clamp transistor  390  is gate-coupled to a node  392  between the second reference current source  375  and the current channel of the precision loop transistor  380 . The clamp transistor  390  is driven to conduct during an over-current condition when the second attenuated sense current 1/N(ISENSE_P) is greater than the second reference current I(REF_P), causing the node  392  to be pulled low. Conduction of the clamp transistor  390  decreases the magnitude of the voltage regulation control input signal  114 . Excess current flow through the pass transistor  215  associated with the over-current condition decreases as a result. 
     It is noted that the current-limiting apparatus  300  as described and shown in  FIG. 3  is implemented with PMOS and NMOS transistors appropriate to the voltage rails V(IN)  395  and ground  383 . Analogous combinations of NMOS and PMOS transistors appropriate to other voltage rail pairs, and transistor types other than MOSFET (e.g., bipolar transistor types) are contemplated by the instant disclosure. 
       FIG. 4  is a block diagram of a current-limited voltage regulator  400  according to various example embodiments. The current-limited voltage regulator  400  includes the elements associated with the current-limiting apparatus  200 , arranged as described above with reference to  FIG. 2 . To avoid redundancy, the latter description is not repeated here. 
     The current-limited voltage regulator  400  also includes a voltage divider  410 . The voltage divider  410  is communicatively coupled between an output  162  of the voltage regulator  400  and a voltage rail  383  to provide an output voltage sense signal V(SENSE). The voltage regulator  400  also includes a voltage regulation error amplifier  420  communicatively coupled to the voltage divider  410 . The error amplifier  420  receives V(SENSE), compares V(SENSE) to a reference voltage input V(REF), and generates the voltage regulation control input signal  114 . 
     Apparatus described herein may be useful in applications other than current-limited voltage regulation apparatus. Examples of the electronic current-limiting apparatus  100 ,  200 , and  300  and of the current-limited voltage regulation apparatus  400  are intended to provide a general understanding of the structures of various embodiments. They are not intended to serve as complete descriptions of all elements and features of apparatus and systems that might make use of these structures. 
     The various embodiments may be incorporated into semiconductor analog and digital circuits for use in receptacle power converters, electronic circuitry used in computers, communication and signal processing circuitry, single-processor or multi-processor modules, single or multiple embedded processors, multi-core processors, data switches, and application-specific modules including multi-layer, multi-chip modules, among others. Such apparatus and systems may further be included as sub-components within a variety of electronic systems, such as televisions, cellular telephones, personal computers (e.g., laptop computers, desktop computers, handheld computers, tablet computers, etc.), workstations, radios, video players, audio players (e.g., MP3 (Motion Picture Experts Group, Audio Layer 3) players), vehicles, medical devices (e.g., heart monitor, blood pressure monitor, etc.), set top boxes, and others. 
     Apparatus disclosed herein implement a fast transient precision current limiter such as may be included in an electronic voltage regulator. The current limiter includes two current sense element/current clamp control loops. A fast response time control loop first engages and clamps a current spike. A precision control loop then engages to more accurately clamp the output current to a programmed set point by matching component electrical characteristics of a precision current sense element to those of a pass element, including a pass transistor. 
     By way of illustration and not of limitation, the accompanying figures show specific aspects in which the subject matter may be practiced. It is noted that arrows at one or both ends of connecting lines are intended to show the general direction of electrical current flow, data flow, logic flow, etc. Connector line arrows are not intended to limit such flows to a particular direction such as to preclude any flow in an opposite direction. The aspects illustrated are described in sufficient detail to enable those skilled in the art to practice the teachings disclosed herein. Other aspects may be used and derived therefrom, such that structural and logical substitutions and changes may be made without departing from the scope of this disclosure. This Detailed Description, therefore, is not to be taken in a limiting sense. The breadth of various aspects is defined by the appended claims and the full range of equivalents to which such claims are entitled. 
     Such aspects of the inventive subject matter may be referred to herein individually or collectively by the term “invention” merely for convenience and without intending to voluntarily limit this application to any single invention or inventive concept, if more than one is in fact disclosed. Thus, although specific aspects have been illustrated and described herein, any arrangement calculated to achieve the same purpose may be substituted for the specific aspects shown. This disclosure is intended to cover any and all adaptations or variations of various aspects. 
     The Abstract of the Disclosure is provided to comply with 37 C.F.R. §1.72(b) requiring an abstract that will allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. In the preceding Detailed Description, various features are grouped together in a single embodiment for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted to require more features than are expressly recited in each claim. Rather, inventive subject matter may be found in less than all features of a single disclosed embodiment. The following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separate embodiment.