Abstract:
Embodiments of the invention may provide for a load regulation tuner that reduces the load regulation effect. The load regulation tuner may include a load current controlled current source that is responsive to a load current from a power transistor of a linear regulator, where the load current controlled current source includes a sensing transistor that generates a fraction of the load current as a sensed partial load current. The load regulation tuner may also include a resistor in parallel with a load current controlled current source, and where the paralleled resistor and the load current controlled current source form at least a portion of a feedback block that adjusts an operation of the linear regulator to provide a substantially constant load voltage.

Description:
RELATED APPLICATION  
       [0001]     The present application claims priority to U.S. Provisional Application Ser. No. 60/829,562, filed Oct. 16, 2006, and entitled “Systems, Methods, And Apparatuses For Implementing A Load Regulation Tuner for Linear Regulation,” which is incorporated by reference in its entirety as if fully set forth herein. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention relates generally to a load regulation tuner for linear regulation, and more particularly to system, methods, and apparatuses for enhancing the performance of load regulation.  
       BACKGROUND OF THE INVENTION  
       [0003]     A voltage regulator is a circuit that provides a constant DC voltage between its output terminals in spite of changes in the load current drawn from the output terminals and/or changes in the DC power supply voltage that feeds the voltage regulator circuit.  FIG. 1A  describes a simplified DC model of a voltage regulator. As shown in  FIG. 1A , the equivalent circuit model of voltage regulators in DC domain can be described as an ideal voltage source V S  in series with an internal source resistor R S . The resistor R S  represents an equivalent series resistance calculated from non-ideal effects inside the voltage regulator.  FIG. 1B  illustrate a typical topology of linear regulators in accordance with the prior art.  
         [0004]     When non-ideal effects, such as input offset voltage, etc., are not dominant and ignored, the resistor R S  is basically equal to the output resistance of the regulator. As the load current I L  increases, there may be a non-ideal voltage drop ΔV LDR  (also referred to as the load regulation effect) across the source resistor R S  as shown below in equation (1):
 
Δ V   LDR   =R   S   ×ΔI   L   (1)
 
 As a result, the DC voltage drop ΔV LDR  over the desired regulator output voltage V S  is proportional to both the resistance R S  and the change in load current ΔI L .  FIG. 2A  illustrates the load regulation effect in the DC domain (Load regulation vs. I LOAD ), in accordance with the prior art. The load regulation effect in transient response in time domain is illustrated in  FIG. 2B . Load regulation effect is a dominant factor determining the best accuracy a regulator can achieve over process corners for products, especially for high load current and low-voltage applications. The load regulation effect is proportional to the resistance R S , which is approximately equal to the output resistance of the regulator, ΔV LDR /ΔI L . This means that the load regulation effect is minimized when the output resistance of the regulator decreases. Based on the typical linear regulator topology shown in  FIG. 1B , the closed-loop output resistance R O     —     REG , which is the actual output resistance of the regulator, can be described as.  
               R   O_REG     =       R   O_op       1   +   Aβ               (   2   )             
 
 R O     —     op  refers to the open loop output resistance, A is the total gain of the regulator, and β is the feedback factor of the regulator. The total gain of the regulator is inversely proportional to the square root of the load current, Thus, as can be seen from equation (2), Ro_reg increases as the load current increases resulting in high load regulation effect. Therefore, the focus of load regulation effect issues has been on the increasing of loop gain to reduce output resistance of the voltage regulator. It can be seen from equation (2) that as Aβ increases, R O     —     REG  decreases (i.e., R O     —     RED  approaches zero). 
 
         [0005]     In addition to reducing the load regulation effect, there is also a problem related to inter-connection voltage loss. Although inter-connection voltage loss is usually neglected by designers, the voltage loss due to resistors for inter-connection (including on-chip metal connection, off-chip bonding wire, metal connection, etc.) is another critical issue like the load regulation effect, which may cause significant effects in a heavy current load environment.  FIGS. 3A and 3B  illustrate typical connection resistance between a regulator and a load circuit where there is both an on-chip connection and an off-chip connection, in accordance with the prior art.  
       SUMMARY OF THE INVENTION  
       [0006]     Embodiments of the invention may provide for a load regulation tuner that reduces the load regulation effect. The load regulation tuner may include a sensing transistor mirroring a ratio of the load current from the power transistor inside the linear regulator, a feedback loop improving the accuracy of the ratio between the load current of the power transistor and the sensed current of the sensing transistor, and a current mirror mirroring a sensed partial load current flowing into the load current control current source. The load regulation tuner may also include a resistor in parallel with the load current controlled current source, and the paralleled resistor is contained in a feedback block of at least one linear regulator. According to an aspect of the invention, a delay resistor and a delay capacitor may also be inserted between the gates of the current mirror to add a time delay. In accordance with yet another aspect of the invention, the feedback loop includes a resistor ladder.  
         [0007]     According to another embodiment of the invention, there is a load regulation tuner comprising. The load regulation tuner may include a load current controlled current source that is responsive to a load current from a power transistor of a linear regulator, where the load current controlled current source includes a sensing transistor that generates a fraction of the load current as a sensed partial load current, and a current mirror connected to the sensing transistor and the power transistor for ensuring a substantially equal drain voltage for the sensing transistor and power transistor, thereby enhancing an accuracy of the sensing transistor in generating the fraction of the load current as the sensed partial load current. The load regulation tuner may also include a resistor in parallel with a load current controlled current source, and where the paralleled resistor and the load current controlled current source form at least a portion of a feedback block that adjusts an operation of the linear regulator to provide a substantially constant load voltage.  
         [0008]     According to yet another example embodiment of the invention, there is a method for providing a load regulation tuner. The method may include providing a current source that is responsive to a load current from a power transistor of a linear regulator, where the load current controlled current source includes a sensing transistor that generates a fraction of the load current as a sensed partial load current, and a current mirror connected to the sensing transistor and the power transistor, thereby ensuring an accuracy of the sensing transistor in generating the fraction of the load current as the sensed partial load current. The method may also include providing a resistor in parallel with the current source, where at least a portion of the sensed partial load current is provided to the paralleled resistor, and where the paralleled resistor and the current source form at least a portion of a feedback block that adjusts an operation of the linear regulator to provide a substantially constant load voltage.  
         [0009]     According to still another example embodiment of the invention, there is a system. The system may include a linear regulator having a first input port, a second input port, and an output port, where the first input port receives an input voltage reference, and where the output port provides a load voltage and a load current. The system may also include means for providing a feedback voltage signal to the second input port, where the means is connected in a feedback loop between the output port and second input port of the linear regulator, wherein the means includes at least an equivalent of a load current controlled current source and a resistor in parallel for adjusting the feedback voltage signal based upon a change in the load current to maintain the load voltage at a substantially constant level. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)  
       [0010]     Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:  
         [0011]      FIG. 1A  illustrates a simplified DC model of a voltage regulator in accordance with the prior art.  
         [0012]      FIG. 1B  illustrates a typical topology of linear regulators in accordance with the prior art.  
         [0013]      FIG. 2A  illustrates the load regulation effect in the DC domain road regulation effect VS. I LOAD ), in accordance with the prior art.  
         [0014]      FIG. 2B  illustrates the load regulation effect in the time domain, in accordance with the prior art.  
         [0015]      FIGS. 3A and 3B  illustrate typical connection resistance between a regulator and a load circuit where there is an on-chip connection and an off-chip connection, in accordance with the prior art.  
         [0016]      FIG. 4  illustrates a simple block diagram of the load regulation tuner, in accordance with an example embodiment of the invention.  
         [0017]      FIG. 5  illustrates a simple schematic diagram of the invention with a linear regulator in accordance with an example embodiment of the invention.  
         [0018]      FIG. 6  illustrates an example circuit with a linear regulation tuner with feedback factor β&lt;1, in accordance with an example embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]     Embodiments of the invention may provide for a stand-alone load regulation tuner, which is capable of accurately canceling the load regulation effect and inter-connection voltage loss due to an inter-connection resistance for any type of linear regulator without affecting the regulator&#39;s stability and Power Supply Rejection Ratio (PSRR) performance. Further, the load regulation tuner may reduce or cancel the load regulation effect by tuning a DC feedback factor to reduce or cancel the load regulation effect as well as the inter-connection resistance loss for different load current and output voltage levels.  
         [0020]     Embodiments of the invention will now be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the invention are shown. Indeed, these inventions may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout.  
         [0021]     A simple conceptual block diagram of a low drop-out voltage regulator with a load regulation tuner is shown in  FIG. 4 , according to an example embodiment of the invention. As shown in  FIG. 4 , the voltage regulator may include a voltage reference  12 , an amplifier such as an error-amplifier  16 , a pass device  18 , and an output load  14 . The voltage regulator may also include a load regulation tuner comprising a feedback block  22  and a load current sensing block  20 , according to an example embodiment of the invention.  
         [0022]     Still referring to  FIG. 4 , during operation of the voltage regulator, the error amplifier  16  may receive the reference voltage  12  as well as a feedback voltage from the feedback block  22 . Using the voltage reference  12  and the feedback voltage, the error amplifier  16  may determine an error signal as the difference between the reference voltage  12  and the feedback voltage, according to an example embodiment of the invention. The error amplifier  16  may control a gate voltage of the pass device  18  (e.g., power transistor) that outputs the constant output voltage. The constant output voltage is provided to both the output load  14  and the feedback block  22 . The feedback block  22  outputs a feedback voltage to the error amplifier  16  for use in canceling the load regulation effect. According to an example embodiment of the invention, the load current sensing block  20  may change a feedback factor of the feedback block  22  to cancel the load regulation effect to obtain a desired constant output voltage.  
         [0023]      FIG. 5  illustrates a more detailed schematic diagram of a load regulation tuner  402  utilized in a voltage regulator, in accordance with an example embodiment of the invention. As shown in  FIG. 5 , it will be appreciated that the load regulation effect may be based upon a DC voltage difference between the actual output voltage level and the desired output voltage level (i.e., reference voltage V REF    404 ), according to an example embodiment of the invention. Referring to the input nodes, the feedback voltage difference ΔV FB  may be equal to ΔV LDR *β, where ΔV LDR  is the voltage difference across the regulator and β is the feedback factor of the regulator. To fully cancel the load regulation effect, the load regulation (LDR) tuner  402  may need to compensate for the voltage difference ΔV FB  such that the output voltage V OUT    410  may be equal to the reference voltage V REF    404 .  
         [0024]     According to an example embodiment of the invention, the LDR tuner  402  may include a resistor  408  and a current controlled current source  406  to compensate for the voltage difference ΔV FB . In particular, the resistor  408  and current controlled current source  406  may be operative to provide a feedback voltage difference ΔV FB  of ΔV LDR *β. In other words, a load current controlled current source  406  with a resistor R LDR    408  (according to Thevenin&#39;s theorem, ΔV FB =I*R LDR =ΔV FB =ΔV LDR *β) may be inserted into the feedback loop to cancel the load regulation effect, so the output voltage V OUT    410  may be exactly equal to the reference voltage V REF    404 , as shown in  FIG. 5 , according to an example embodiment of the invention.  
         [0025]     Still referring to  FIG. 5 , to further reduce the inter-connection voltage loss due to inter-connection resistance, the LDR tuner  402  may also compensate for the inter-connection resistance. More specifically, the current controlled current source  406  (I F ) and/or the resistance R LDR    408  may be tuned so that ΔV FB /β=ΔV LDR +(R X *ΔI L ), where R X  represents the inter-connection resistance and ΔI L  is the change in load current. The LDR tuner  402  may also help minimize the variations of load regulation performance over process corners for products.  
         [0026]     Example embodiments of the load regulation tuner operating in conjunction with linear regulators are shown in  FIG. 6 . As shown in  FIG. 6 , capacitor C d    618  and resistor R d    614  may be inserted between the gates of the current mirror (transistors M n2    612  and M n3    608 ) for a time delay to make sure the response time of the load regulation tuner is slower than that of the regulator itself and further guarantee the stability of the regulator is not affected by the load regulation tuner.  
         [0027]     The load regulation tuner of  FIG. 6  may include a PMOS transistor M P1    602 , a PMOS transistor M P2    610 , a PMOS transistor M P3    606 , a NMOS transistor M N2    612 , a NMOS transistor M N3    608 , a NMOS transistor M N1    612 , a resistor R d    614  and a capacitor C d    618 , according to an example embodiment of the invention. The gate of the PMOS transistor M P1    602  may be connected the gate of the PMOS power transistor M p)    604 . The PMOS transistor M p1    608  may have its source connected to the supply voltage and a drain connected to the source of the PMOS transistor M p3    606 . The PMOS transistor M p3    606  may have a gate connected the gate of the PMOS transistor M p2    610  and a drain connected to a drain of the NMOS transistor M n3    608 . The NMOS transistor M p2    610  may have a source connected to a drain of the PMOS power transistor M p0    604 , and a gate connected to its drain and a drain of the NMOS transistor M n2    612 . The NMOS transistor M n2    612  may have a gate connected to a gate of the M n3    608  and a source connected to a ground. The NMOS transistor M n3    608  may have a gate connected to the gate of the NMOS transistor M n2    612  and a source connected to a ground. The resistor R d    614  may be connected between the gate of the transistor M n3    608  and a capacitor C d    618 . The top plate of the capacitor C d    618  may be connected to the resistor R d    614  and a gate of the transistor M n1    620 . The bottom plate of the capacitor C d    618  may be connected to a ground. The NMOS transistor M n1    620  may have a drain connected to a node V X    626 , which is a junction of the resistor R 2a    622  and R 2b    624 , and a source connected to a ground.  
         [0028]     As shown in  FIG. 6 , transistors Mp 1   602 , M P2    610 , M P3    606 , M N2    612 , M N3    608 , capacitor C d    618  and resistor R d    614  may construct a load current sensing block such as the load current sensing block  20  of  FIG. 4 , according to an example embodiment of the invention. The transistor M P1    602  may sense the load current of the power transistor M P0    604 . The size of the transistor M P1    602  may be much smaller than that of the power transistor M P0    604  so that only small fraction of the load current flows in the transistor M P1    602 , according to an example embodiment of the invention. The feedback composed with M P2    610 , M P3    606 , M N2    612 , M N3    608  may ensure that the current in both branches are equal or substantially equal, according to an example embodiment of the invention. It also improves the accuracy of the ratio between the load current of the transistor M P0    604  and the sensed current of the transistor M P1    602  because the feedback ensures the drain-source voltage of the transistors M P0    604  and M P1    602  are equal or substantially equal. The overall current consumption of the load regulation tuner may be very minimal. When load current changes, the current flow in the transistor M P1  may change as well as the gate-source voltage of the transistor M N3    608  causing the output resistance of the transistor M N1    620  to change. This leads the feedback factor to vary to cancel the load regulation effect so that the desired output voltage of the regulator is achieved.  
         [0029]     As shown in  FIG. 6 , the operation of this load regulation tuner can be controlled by adjusting the size of transistor M N1    620  and resistance R 2b    624  to suit different loading environments and applications. The load regulation tuner may tune the DC feedback factor of the voltage regulator to cancel the load regulation effect and the interconnection voltage loss due to the inter-connection resistance without affecting the frequency response and PSRR performance of the regulator.  
         [0030]     In the example embodiment of the invention shown in  FIG. 6 , the feedback circuit may include a resistor ladder composed of R 1a    622  and R 2b    624 . In alternative embodiments of the invention, the feedback circuit should be verified by checking whether the load regulation is fully cancelled in the regulator output. It will be appreciated that the load regulator of  FIG. 6  is operative to generate ΔV FB  o cancel the voltage difference (ΔV LDR ) between the desired output voltage and the actual output voltage with increased output current ΔI L . According to an example embodiment of the invention, ΔV FB  may be generated by R 1 , R 2a , R 2b  and Mn 1  with sensed load current, as illustrated in  FIG. 6 .  
         [0031]     Many modifications and other embodiments of the inventions set forth herein will come to mind to one skilled in the art to which these inventions pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the inventions are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.