Abstract:
A voltage regulator includes first and second transistors arranged in parallel and configured to regulate current flow to an output node, and a sensing circuit configured to sense a voltage level at the output node and provide a signal proportional thereto the regulator also includes a control circuit configured to receive the signal from the sensing circuit and provide control signals at control terminals of the first and second transistors such that voltage at the output node is maintained substantially at a selected level. The control circuit further configured to hold the second transistor in an off state while a demand for current at the output node remains below an output threshold. The second transistor is configured to control a large portion of load current above the output threshold. The regulator may also include a current bypass circuit configured to shunt leakage current of the second transistor to ground, away from the sensing circuit.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a voltage regulator circuit, and in particular, to a circuit having a low quiescent current, and high stability at high temperatures.  
         [0003]     2. Description of the Related Art  
         [0004]     Voltage regulator circuits are found in most electronic devices in use today. Such circuits are configured to receive, at an input, an unregulated voltage supply, and to provide, at an output, a regulated voltage at a selected voltage level, lower than the input. Such circuits are commonly used, for example, in devices that are powered by batteries, in order to maintain a steady voltage supply for the device, even as the output voltage of the battery gradually drops due to normal discharge of the battery. Voltage regulator circuits are also found in systems requiring a voltage supply at one voltage level but where power is available at a different voltage level.  
         [0005]     Voltage regulator circuits typically require some power to operate. For example, such circuits employ reference voltage generators, voltage sensing sub-circuits, and other sub-circuits that remain active while the regulator circuit is powered up, even when there is no load on the output. As a result, the regulator circuit will draw a current from the power supply, regardless of the load. This current is commonly referred to as the quiescent current.  
         [0006]     In a battery operated system such as that described, the quiescent current represents a constant drain on the battery, as long as the system is active. Accordingly, it would be desirable, especially in a battery powered system, to turn off the regulator when there is no load present. However, this is not always possible. In some applications, it is necessary to maintain a voltage level at the output even while there is minimal current draw. For example, some systems maintain a clock, a volatile memory, or some other circuit that has negligible power requirements, but must have a continuous voltage supply. Such circuits are found, for example, in automobiles, where various systems remain nominally active, perpetually, even while the automobile is not in operation.  
         [0007]     For example, a typical automobile audio system maintains a memory of radio settings, etc., which are stored in a volatile memory, such that if the power is disconnected the memory is erased. In addition, modern automobiles employ computers, which similarly must be kept powered to maintain data in memory. Each such system will employ a separate regulator circuit, such that the quiescent current draw on the battery may be multiplied many times. Some modern automobiles may include a dozen or more such systems.  
         [0008]     In view of the above, it is desirable to reduce the quiescent current of each voltage regulator circuit, in order to minimize the drain that the sum of the quiescent currents represents on the battery.  
       BRIEF SUMMARY OF THE INVENTION  
       [0009]     According to an embodiment of the invention, a voltage regulator is provided, including an output node configured to be coupled to a load circuit, a first power transistor having a first conduction terminal coupled to a voltage source and a second conduction terminal coupled to the output node, a second power transistor having a first conduction terminal coupled to the voltage source and a second conduction terminal coupled to the output node, and a control circuit configured to sense an output voltage at the output node and provide control signals to each of the power transistors. The control circuit is configured to control a conduction capacity of each of the first and second power transistors such that the output voltage remains approximately equal to a selected output voltage. The control circuit is further configured to hold the second transistor in an off state unless a load current drawn from the output node exceeds a threshold current.  
         [0010]     The control circuit comprises first and second biasing transistors coupled between a circuit ground and respective control terminals of the first and second power transistors and configured to regulate biasing currents of the respective power transistors first and second constant current sources are coupled between the voltage source and respective control terminals of the first and second power transistors.  
         [0011]     Additionally, a biasing resistor circuit is coupled between the voltage source and the control terminal of the second power transistor. The biasing resistor circuit, which includes the second constant current source, is configured to at least partially suppress a biasing current passing therethrough while the load current does not exceed the threshold current.  
         [0012]     According to one embodiment of the invention, the biasing resistor circuit includes a biasing resistance coupled between the voltage source and the control terminal of the second power transistor and parallel to the second constant current source. The biasing resistance is variable in inverse response to a level of current flowing therethrough.  
         [0013]     According to another embodiment of the invention, a voltage regulator is provided, including a first transistor formed on a semiconductor substrate and having first and second conduction terminals coupled to a first voltage source and an output node of the regulator, respectively, and a control circuit configured to monitor a voltage level at the output node and provide a control signal at a control terminal of the first transistor so as to maintain the voltage level at a selected value. The regulator further includes second, third, and fourth transistors.  
         [0014]     A first conduction terminal of the second transistor is coupled to the first voltage source, and, according to an embodiment of the invention, the second transistor is permanently biased in an off state. The third transistor is coupled in diode configuration between a second conduction terminal of the second transistor and a second voltage source—circuit ground, for example. The fourth transistor is coupled between the output node and the second voltage source, with a control terminal coupled to a control terminal of the third transistor such that the fourth transistor is configured to mirror current flow of the third transistor. The fourth transistor is configured to mirror the current of the third transistor at a rate such that current flowing in the fourth transistor is substantially equal to leakage current flowing in the first transistor.  
         [0015]     According to one embodiment of the invention, the second transistor is configured to leak current at a selected ratio, relative to the first transistor, across a selected range of temperatures. The ratio may be, for example, approximately 1:100. Additionally, the fourth transistor may be configured to mirror a current flowing in the third transistor at a ratio substantially reciprocal to the leakage current ratio of the second transistor relative to the first transistor. For example the current mirror ratio of the fourth transistor, relative to the third transistor, may be approximately 100:1.  
         [0016]     Alternatively, the current mirror ratio of the fourth transistor, relative to the third transistor, may be selected to result in a mirror current that exceeds the leakage current of the first transistor. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)  
       [0017]      FIG. 1  illustrates a voltage regulator according to an embodiment of the invention.  
         [0018]      FIG. 2  illustrates a voltage regulator according to another embodiment of the invention.  
         [0019]      FIG. 3  is a graph illustrating a relationship between current and resistance in a component of the embodiment of  FIG. 2 .  
         [0020]      FIG. 4  illustrates a simplified voltage regulator for descriptive purposes.  
         [0021]      FIG. 5  is a graph illustrating a relationship between temperature and output voltage of the circuit of  FIG. 4 .  
         [0022]      FIG. 6  illustrates a voltage regulator according to another embodiment of the invention.  
         [0023]      FIG. 7  illustrates a voltage regulator according to a further embodiment of the invention.  
         [0024]      FIG. 8A  is a graph illustrating a relationship between temperature and output voltage of the circuit of  FIG. 7 .  
         [0025]      FIG. 8B  is a graph comparing the plots of  FIGS. 5 and 8 A.  
         [0026]      FIG. 9  is a graph illustrating a relationship between temperature and resistance of a component of the circuit of  FIG. 7 .  
         [0027]      FIG. 10  illustrates a voltage regulator according to a further embodiment of the invention.  
         [0028]      FIG. 11  illustrates an embodiment in which a system employs a voltage regulator according another of the embodiments of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]     A voltage regulator  200  according to a first embodiment of the invention is shown in  FIG. 1 . The voltage regulator  200  of  FIG. 1  is a simplified diagram showing only those components necessary to describe and understand the function thereof.  
         [0030]     In the circuit of  FIG. 1 , a first voltage source V IN1  corresponds to the positive terminal of a battery, while a second voltage source V IN2  corresponds to the negative terminal of the battery, or the circuit ground. It will be recognized that this arrangement is only one of many possible configurations, illustrated here as an example, only.  
         [0031]     The voltage regulator  200  includes a power transistor  104  having a first conduction terminal  109  coupled to the first voltage source V IN1 , and a second conduction terminal  111  coupled to an output node  114 . A load circuit  116  is coupled to the output node  114  via output terminal  118 , and output voltage V OUT  at the node  114  is regulated by the power transistor  104 .  
         [0032]     First and second sense resistors  106 ,  108  are coupled in series between the output node  114  and the second voltage source V IN2 , with a feedback node  110  defined therebetween. A comparator  202  includes a non-inverting input  203  coupled to the feedback node  110  via feedback line  112 , an inverting input  205  coupled to a reference voltage source V REF . The comparator  202  also includes an inverting output  207 .  
         [0033]     The resistance values of the first and second resistors  106 ,  108  are selected such that, when the voltage level at the output node  114  is equal to the selected regulated output voltage V OUT  of the regulator  200 , a voltage level at the feedback node  110  is equal to the reference voltage V REF .  
         [0034]     For example, the voltage regulator  200  may be configured to provide a regulated voltage of around 5 volts at the output node  114 , and may employ a reference voltage of 1.26 volts. Accordingly, the values of the first and second resistors  106 ,  108  are selected such that, when the 5 volt regulated voltage is divided across the voltage divider formed by the first and second resistors  106 ,  108 , the voltage at the feedback node  110  is equal to the reference voltage, 1.26 volts. If resistor  106  is equal to 1.5 MΩ and resistor  108  is equal to 500 KΩ, such a condition is realized. Of course, it will be recognized that these are only exemplary values, and are not intended to represent a particular working circuit.  
         [0035]     Reference voltage sources suitable for use with a circuit of this type are well known in the art. For example, a band-gap reference voltage may be employed as the reference voltage source V REF .  
         [0036]     The inverted output  207  of the comparator  202  is connected to the control terminal of a first biasing transistor  210 , which is connected in series with the current source  214  between voltage sources V IN1  and V IN2 . Control node  213  is positioned between the control transistor  210  and the current source  214 . PNP bipolar transistor  204  is coupled between the first voltage source V IN1  and the output node  114  with the base thereof coupled to the control node  213 .  
         [0037]     The output  207  of the comparator  202  is also connected to the control terminal of a second biasing transistor  214 . The biasing transistor  214  is coupled in series with a biasing resistor circuit  216  between the first and second voltage sources V IN1 , V IN2 , with control node  215  located between the biasing resistor circuit  216  and the bias control transistor  214 . The control terminal of the power transistor  104  is coupled to the control node  215 .  
         [0038]     Comparator  202  is configured to provide an output voltage at output  207  that increases as the voltage potential at the non-inverting input  203  drops below that of the inverting input  205 . Conversely, when the voltage at the non-inverting input  203  is equal to, or greater than, the voltage potential at the inverting input  205 , the output of the comparator  202  is at a selected low voltage level. The low voltage level of the output  207  is selected such that the bias control transistors  210 ,  214  are each maintained at a conduction level sufficient to conduct the current provided by the constant current sources  211 ,  206 . Configuration of a comparator to provide such a low voltage level is within the abilities of one having ordinary skill in the art, and will not be discussed in detail herein.  
         [0039]     For the purposes of describing operation of the regulator circuit  200 , it will be assumed at the outset that the power transistors  104 ,  204  are in an off, or non-conducting state, and that output  207  of the comparator  202  is at its low voltage level. In this condition, all of the source voltage V IN1  is seen across the power transistors  104 ,  204  and the voltage potentials at the output node  114  and the feedback node  110  are both equal to the circuit ground. With the voltage at the non-inverting input  203  equal to ground, the higher reference voltage at the inverting input  205  will cause the inverted output  207  of the comparator  202  to move in a positive direction. As the voltage level at the control terminals of the bias control transistors  210 ,  214  rises, the conduction level of these transistors rises.  
         [0040]     Referring first to bias control transistor  210 , as bias current I 5  increases above the current level of constant current source  211 , the voltage at node  213  drops, which in turn causes PNP transistor  204  to begin to conduct through current path I 4 . A portion of this current is expressed as an emitter-base current and joins the bias current I 5  to provide the additional current flowing through bias control transistor  210 . The majority of the current flowing through power transistor  204  is transmitted to node  114  in accordance with the gain characteristics of transistor  204 . At this point the current is divided between load current I 1  flowing through the load  116 , and sense current I 2  flowing through the sense resistors  106 , 108 . The current in current paths I 1  and I 2  is divided according to known principles, and depends upon resistances in the respective current paths. As current I 2  flows through the sense resistors  106 ,  108 , the voltage at the feedback node  110  begins to rise. Provided the sense current I 2  is sufficient to create a voltage drop across sense resistor  108  substantially equal to the voltage level at the inverting input  205  of the comparator  202 , the circuit will reach equilibrium when the voltage drop across both sense resistors  106 , 108  rises to the selected output voltage. It may be seen that the power transistor  204  will begin to conduct current as soon as the conduction capacity of the bias control transistor  210  rises above the current level established by the constant current source  211 . Accordingly, the power transistor  204  responds very quickly to small imbalances in the circuit. The power transistor  204  may be configured to have a relatively low current capacity.  
         [0041]     In the example provided above, resistor  106  is equal to 1.5 MΩ and resistor  108  is equal to 500 KΩ, and the regulated voltage V OUT  is 5V. Given these values, the sense current I 2  will be 2.5 μA. Under no load conditions, in may be seen that a very low base current in power transistor  204  will be sufficient to provide an acceptable sense current I 2 . For example, in order to provide sufficient current to maintain the sense current I 2  at 2.5 μA, and given a gain factor of 100, transistor  204  will have a base current of 0.025 μA. Thus, the bias control transistor  210  only needs to increase conduction above the 1 μA of constant current source  211  by that amount.  
         [0042]     According to the embodiment of  FIG. 1 , the capacity of power transistor  204  is sufficient to provide the sense current I 2  and some additional load current I 1 . Under these conditions, the power transistor  104  is configured to remain in an off state, as will be described in detail below. Current I 2  flows continuously, regardless of the load on the regulator  200 , and contributes to the quiescent current of the circuit.  
         [0043]     Referring now to the bias control transistor  214 , this transistor is in series with the biasing resistor circuit  216 . When the output  207  of the comparator  202  is at its low voltage level, the conduction capacity of the transistor  214  is less than, or equal to, the current flowing in the constant current source  206 . As with the bias control transistor  210  and the constant current source  211 , the current source  206  provides a very low bias current I 6 , which generates a voltage drop across bias control transistor  214 , thereby maintaining a high voltage value at node  215 , which in turn holds the power transistor  104  in an off condition. As the voltage at the output  207  of the comparator  202  begins to rise, the current carrying capacity of the transistor  214  increases. When the current capacity of the transistor  214  exceeds the current flow of the constant current source  206 , current begins to flow in the resistor network formed by the resistor  208  and the variable resistor  212 . The variable resistor  212  is configured to vary in resistance in inverse relation to the current flowing therethrough. Accordingly, at very low current levels, the value of resistor  212  is very high.  
         [0044]     When the output  207  of the comparator  202  is at a low voltage level, the conduction capacity of the transistor  214  is equal to or less than the current value of the constant current source  206 . Accordingly, the voltage level at node  215  is very nearly equal to the voltage of the first voltage source V IN1 , and the resistance of the resistance circuit  216  is nearly zero, being dominated by the output impedance of the constant current source  206 , and all the voltage in the circuit is seen across the bias control transistor  214 . As soon as the current capacity of the bias control transistor  214  rises above the current level of the constant current source  206 , the resistance of the resistance circuit  216  rises sharply, thereby partially suppressing the increase in bias current I 6 . At this point, the majority of the voltage is still seen across the bias control transistor  214 , and the power transistor  104  remains in an off state.  
         [0045]     Inasmuch as the bias current I 6  contributes to the quiescent current of the regulator circuit  200 , the suppression of the increase thereof, at low output current levels, helps minimize the total quiescent current of the circuit.  
         [0046]     If the load current I 1  is minimal, the power transistor  104  does not turn on, and the regulator circuit stabilizes with the power transistor  204  providing the necessary current. However, if the load current I 1  is sufficiently high, voltage at the feedback node  110  remains below the reference voltage, voltage at the output  207  of the comparator  202  continues to rise, and the current capacity of the bias control transistor  214  also continues to rise.  
         [0047]     As the current capacity of the bias control transistor  214  continues to rise, the current through the variable resistor  212  increases, and the resistive value of this resistor decreases. This serves to reduce the rate of change of voltage at the node  215 , and to delay turn-on of power transistor  104 . Thus, for low current requirements, power transistor  104  remains in an off condition while power transistor  204  provides the necessary current. At the same time, bias current I 6  is held at a low value by the initially high resistance of the resistance circuit  216 .  
         [0048]     Eventually, as current I 6  continues to increase, the variable resistor  212  reaches a negligible resistance value and the voltage difference between first and second voltage sources V IN1  and V IN2  is substantially divided between resistor  208  and bias control transistor  214 . Thereafter, as current capacity of the bias control transistor  214  continues to increase, the voltage at node  215  drops in a linear fashion, and power transistor  104  begins to conduct current I 3 .  
         [0049]     When a load  116  is connected to the output terminal  118 , current path I 1  conducts, drawing off a portion of the current I 4  from the current path I 2 , causing the voltage across the first and second resistors  106 ,  108  to begin to drop. As the voltage at the feedback node  110  begins to drop below the reference voltage V REF , the output  107  of comparator  202  begins to rise, inducing the transistor  204  to increase conduction until the balance between the voltage at the feedback node  110  and the reference voltage is restored.  
         [0050]     If the load current I 1  rises to near the capacity of transistor  204 , sense current I 2  is drawn down, the voltage at output  207  of comparator  202  rises, increasing conduction of bias control transistor  214 , pulling down voltage at node  215 , and power transistor  104  begins to conduct current I 3  as described above, and current output I 1  of the voltage regulator  200  increases until equilibrium is restored. In this way, the voltage regulator  200  maintains a substantially steady output voltage V OUT , regardless of the size of the load  116 , up to the capacities of the power transistors  204  and  104 , and the voltage source V IN1 . This is accomplished while maintaining a very low quiescent current level, especially under low-load conditions.  
         [0051]     The threshold at which power transistor  104  begins to conduct is a design consideration controlled by factors such as the capacity and gain factor of transistor  204 , turn-on voltage of transistor  104 , and the response parameters of transistor  218 , as well as many other variable that one of ordinary skill will recognize. The threshold may be expressed in reference to various parameters, including the output current I 1 , the output voltage V OUT , voltage at the feedback node  110 , the bias current I 6 , or the voltage at comparator output  207 .  
         [0052]     Referring now to  FIG. 2 , a voltage regulator  201  is shown incorporating many of the features of the voltage regulator  200  of  FIG. 1 , and providing increased detail with respect to the circuitry of the comparator  202  and the biasing circuit  216 .  
         [0053]     Referring, in particular, to the biasing resistor circuit  216 , it may be seen that the current control resistor  212  is represented by an NMOS transistor  218  having a control terminal tied to the first voltage source V IN1 . In this configuration, the transistor  218  will function substantially as a diode connected transistor. While the conduction capacity of the bias control transistor  214  remains at less than, or equal to, the current value of the constant current source  206 , virtually all of the voltage of the network will be seen across the bias control transistor  214 , such that the voltage potential at the control terminal of the power transistor  104  will be maintained at a voltage level very nearly equal to the voltage at the first voltage source V IN1 . Consequently, the power transistor  104  will be in a full off state. As the current capacity of the bias control transistor  214  increases, current will begin to flow through the resistor  208  and transistor  218 , and the voltage level at the node  215  will begin to rise. However, as described with reference to the current controlled resistor  212  of  FIG. 1 , as the transistor  218  begins to conduct current, the resistance across this transistor will drop, partially offsetting the drop of resistance across the bias control transistor  214 , which will in turn delay a significant drop of voltage at the node  215 , thereby delaying turn-on of the power transistor  104 . During this delay, power transistor  204  will begin to conduct, as described previously. Once transistor  218  is in a full on condition, the voltage at node  215  will drop in a linear fashion with respect to the rise in current I 6 , as more and more of the voltage will be seen across transistor  208 .  
         [0054]     According to an embodiment of the invention, a zener diode  221  is provided between the control and output terminals of transistor  218 .  
         [0055]     Referring now to  FIG. 3 , a chart plotting the resistance seen across the resistor series  219  comprising resistor  208  and transistor  218  in relation to the current flowing in current path I 6  is shown. It may be seen that, when the current flowing in I 6  exceeds the value of the constant current source  206  of 1 μA, the resistance of resistor series  219  jumps from around 70 KΩ to around 800 KΩ. As I 6  continues to increase, R 216 drops until the value of R 216 is substantially equal to the 35 KΩ of the resistor  208 .  
         [0056]     An advantage of the embodiments described with reference to  FIGS. 1 and 2  is the extremely low quiescent current when there is little or no load on the circuit. For example, according to one embodiment of the invention, each of the constant current sources  206 ,  211 , is configured to generate a current of about 1 μA each. Additionally, the biasing resistor circuit  216  serves to hold the bias current I 6  at a low level under low-load conditions. Given sense resistors  106 ,  108  of 1.5 MΩ and 500 KΩ, respectively, and a V OUT  of around 5 volts, the sense current I 2  is around 2.5 μA. The reference voltage source V REF  and the comparator  202  will each draw a current as well. In total, the quiescent current is around 6-8 μA.  
         [0057]     Referring now to  FIG. 4 , a simplified voltage regulator circuit  100  is illustrated for the purpose of explaining complications that may arise in some applications of low quiescent current circuits such as those described with reference to  FIGS. 1 and 2 , in order to facilitate an understanding of another embodiment of the invention. It will be recognized that the voltage regulator  100  functions in a manner similar to that described with reference to the voltage regulators  200  and  201  of  FIGS. 1 and 2 . The regulator  100  includes a control circuit  101  comprising a differentiator  102  having an inverting input  105  receiving a reference voltage V REF , a non-inverting input  103  coupled to a feedback node  110  between sense resistors  106 ,  108 , and an output  107  coupled to the control terminal of the power transistor  104 . In the simplified circuit of  FIG. 100 , the low capacity power transistor  204  is not included, inasmuch as the features described make reference to the power transistor  104 , and circuitry analogous to the biasing circuitry of  FIGS. 1 and 2  is considered to be comprised by the comparator  102 .  
         [0058]     It has been considered that, by providing high resistance values in the first and second resistors  106 ,  108 , the sensing current I 2  required to establish the appropriate voltages across these resistors may be minimized. For example, by establishing the resistance values of the first and second resistors  106 ,  108  at 1.5 MΩ and 0.5 MΩ, respectively, the sensing current I 2  is around 2.5 μA.  
         [0059]     In general, such a solution works well in a circuit of the type shown in  FIG. 1 . However, under certain conditions, simply increasing the value of the voltage divider resistors can create other problems in the circuit. It is known that, under high temperature conditions, transistors such as the power transistor  104  are subject to leakage current, and that the leakage current rises sharply at some threshold temperature. Under normal conditions, the leakage current of the power transistor  104  is well below the level of the sensing current, even at the reduced level indicated above. However, when the transistor  104  is heated to a temperature exceeding a threshold value of, for example, around 150° C., the leakage current of the transistor  104  increases sharply. While the regulator circuit  100  is under load, that is, while there is an additional current I 1 , the leakage current is compensated for by the control circuitry  101 , which merely reduces the level of conduction of the transistor  104  by a value equal to the leakage current.  
         [0060]     However, under a no load condition, the transistor  104  is maintained very nearly in a full off condition, already. The sensing current I 2  is the only current flowing in the circuit, and is equal to I 3 . In response to the additional leakage current, the control circuit  101  attempts to completely shut off the transistor  104 . However, when the level of the leakage current rises to such a point that it exceeds the sensing current, the voltage levels at the output node  114  and the feedback node  110  rise above their rated levels. Because the control circuit  101  is already in a fully off condition, the transistor  104  cannot be further shut down. Furthermore, the resistance of resistors such as those commonly used for sense resistors  106 ,  108  tends to rise as the temperature rises, which further increases the voltage seen across these resistors. Under these conditions, the voltage level at the output node  114  may rise significantly.  
         [0061]      FIG. 5  is a graph showing the output voltage V OUT-A  of a test circuit configured as described above, with a supply voltage of around 12 volts and an output voltage of around 5.04 volts. The graph of  FIG. 5  shows the actual output voltage V OUT  of such a circuit under no load conditions, in relation to the temperature of the transistor  104 . It may be seen that, as the temperature rises above a threshold voltage around 155° C., the output voltage rises sharply.  
         [0062]     As was previously described, regulator circuits of the kind described above are commonly used in systems that require a constant voltage supply, even under nominal off conditions of the system. An example provided was that of various automobile systems. In an automobile computer, for example, the memory must be supplied with a constant voltage in order to maintain data in the memory. When the automobile is not operating, most of the functions of the associated computer are also inactive, and very little current is drawn. However, a voltage supply is provided to maintain the memory intact. Because of the scale of integration practiced in modern computers of this type, such computers are very sensitive to fluctuations in input voltage. If such a system were subjected to input voltages rising as high as two to four volts above the rated output voltage, such as shown in  FIG. 5 , the system would be damaged or destroyed.  
         [0063]     The temperature conditions described above are not unusual in such circuits, inasmuch as the normal operating temperatures of high capacity power transistors like transistor  104  of  FIG. 4  fall easily within the range of around 150° C., under normal to heavy load conditions. During operation, such temperatures are acceptable, and leakage current is compensated for as previously described. However, when the load is suddenly removed, as when the automobile is turned off, there is a time lag between the time when the load is removed and when the temperature of the circuit drops to a safe level. During this time lag, there is a significant danger of damage to the system, due to excessive output voltage.  
         [0064]      FIG. 6  illustrates a low quiescent current circuit  120  according to one embodiment of the invention. The features described with reference to the voltage regulator circuit  100  of  FIG. 4  that are also found in the voltage regulator circuit  120  of  FIG. 6  are indicated with the same reference numerals.  
         [0065]     In addition to components previously described, the regulator circuit  120  further includes a second transistor  122  having a first conduction terminal  123  coupled to the input voltage V IN1  and a second conduction terminal  125  coupled to a conduction terminal  127  of a third transistor  124 . The second transistor  122  has a control terminal  121  coupled to its first conduction terminal  123 . It may be seen that the second transistor  122  is configured so as to remain in a permanently off, or non-conducting condition. The third transistor  124  has a second conduction terminal  137  coupled to the circuit ground V IN2 , and a control terminal  135  coupled to its first conduction terminal  127 . A fourth transistor  126  includes a control terminal  133  coupled to the control terminal  135  of the third transistor  124  in a current mirror configuration, with a first conduction terminal  129  coupled to the output node  114  and a second conduction terminal  131  coupled to the circuit ground V IN2 .  
         [0066]     According to an embodiment of the invention, the second transistor  122  is configured and scaled, relative to the first transistor  104 , so as to admit a leakage current at a ratio of approximately 1:100, relative to the leakage current of the power transistor  104 . In turn, the fourth transistor  126  is configured and scaled, relative to the third transistor  124 , so as to mirror the current of the third transistor  124  at a rate of approximately 100:1.  
         [0067]     As shown in the embodiment of  FIG. 6 , the second transistor  122  is a PMOS transistor with its gate terminal coupled to its source terminal. Accordingly, during normal operation of the circuit, the second transistor  122  remains in an off, or non-conducting state. With no current flowing in the current path I 7 , the diode connected third transistor  124 , and the mirror connected fourth transistor  126  are also, therefore, in an off state. Accordingly, there is also no current flowing in the current path I 8 .  
         [0068]     When the temperature of the circuit  120  reaches a point that the power transistor  104  begins to conduct leakage current in path I 3 , the second transistor  122  also begins to conduct leakage current in path I 7 . Because of the scaling difference between the first and second transistors  104 ,  122 , the second transistor  122  will leak current at a 1:100 ratio, relative to the leakage current of the first transistor  104 . Thus, if the leakage current of the first transistor  104  is equal to 5 μA, the leakage current of the second transistor  122  will be equal to approximately 0.05 μA. When leakage current begins to flow in the second transistor  122 , the third transistor  124  turns on to conduct current I 7  to ground. In response, the fourth transistor  126  turns on and begins conducting a mirror current I 8 . Because of the relative scaling of the third and fourth transistors  124 ,  126 , the current I 8  flows at a ratio of 100:1 with respect to the current I 7 . Thus, if the current I 7  is equal to 0.05 μA, the current in current path I 8  will be equal to approximately 5 μA. In this way, the 5 μA leakage current of the power transistor  104  is shunted from the output node  114  through the fourth transistor  126  to ground. Accordingly, the first and second resistors  106 ,  108  are not subjected to the leakage current, and the voltage at the output node  114  is maintained at the rated voltage.  
         [0069]     According to one embodiment of the invention, the third transistor  124  is scaled much smaller, perhaps an order of magnitude smaller, than the second transistor  122 , such that leakage current of its own does not interfere with operation of the system.  
         [0070]     Additionally, according to another embodiment of the invention, the fourth transistor  126  is scaled such that, during operation, current I 8  is greater than the leakage current flowing in the power transistor  104 . In this way, minor variations in the operating characteristics of the transistors of the circuit, arising as a result of normal production manufacturing techniques, do not result in a circuit in which the current I 8  is insufficient to shunt all of the leakage current from current I 3 . A slightly greater current I 8  will merely prompt the control circuit  101  to increase conductivity of the power transistor  104  to a very small degree in response.  
         [0071]     The second, third, and fourth transistors may be referred to as leakage current control transistors.  
         [0072]     Referring now to  FIG. 7 , a voltage regulator circuit is illustrated in which features of the embodiments illustrated in  FIGS. 2 and 6  are combined.  
         [0073]     Referring now to  FIG. 8A , a graph is illustrated showing the output voltage V OUT-B  of a circuit such as that shown in  FIG. 7 , in which the voltage V OUT-B  is shown in relation to the temperature of the circuit. It may be seen that, as the temperature rises, the output voltage V OUT-B  remains between 5.16 volts and around 5.18 volts. When the temperature exceeds 155 degrees, the output voltage begins to rise, reaching around 5.2 volts at 170 degrees. Referring again to  FIG. 5 , it may be seen that this rise corresponds to the rise of the voltage V OUT-A , in which the voltage begins to rise at the same point, but rises to around 9.5 volts at 170 degrees.  
         [0074]     Referring to  FIG. 8B , the plots of output voltages V OUT-A  and V OUT-B  are shown on a common chart for easier comparison. It may be seen that, over the range of temperature from 155 to 170 degrees, voltage V OUT-A  rises more than 4 volts, while across the same range of temperature, V OUT-B  rises less than 0.04 volts.  
         [0075]      FIG. 9 , illustrates a plot showing the current I 2  flowing through the sensing resistors  106 ,  108  is shown in relation to temperature in the circuit. It will be recalled that the resistance of the sensing resistors  106 ,  108  tends to rise with temperature. As a consequence, the current level necessary to maintain a proper sensing voltage at feedback node  110  drops accordingly.  
         [0076]     Referring now to  FIG. 10 , a voltage regulator circuit  400  is illustrated, according to an embodiment of the invention, in which the features described with reference to previous embodiments are incorporated.  
         [0077]      FIG. 11  shows a vehicle system  130 . The system  130  includes an engine  132  and a system battery  134 . An alternator  136  and voltage regulation and charging components  138  draw energy from the engine during operation to recharge the battery  134 . The system  130  includes various electronic components that must have a continuous voltage supply, even while the rest of the system  130  is not in operation. For example, an onboard computer  140  includes a memory  172  in which are stored various data, including engine performance data and error and malfunction codes. The memory  172  requires a constant regulated voltage source to retain the data in the memory. The system  130  also includes an audio system  144  and a clock  146 . Each comprises a volatile memory that depends on a constant regulated voltage source. Accordingly, each component  140 ,  144 ,  146  is provided with a voltage regulator  500  employing principles described with reference to disclosed embodiments of the invention.  
         [0078]     It will be recognized that each of the voltage regulators  500  of  FIG. 4  may be integrated with the respective component  140 ,  144 ,  146 , or may be provided as a discrete component. Alternatively, a single regulator  500  may be provided to supply a regulated voltage supply to a plurality of system components.  
         [0079]     While the system  130  is shown in  FIG. 11  as an automobile, the system  130  may be any device that includes components that require an uninterrupted voltage supply, even while other components of the system are inactive, especially systems that employ batteries for primary or auxiliary power. For example, such alternate systems may include other vehicles such as a boat or airplane, smaller devices such as notebook computers, PDA&#39;s, handheld games, solar powered monitoring systems, communications equipment, etc.  
         [0080]     One having ordinary skill in the art will recognize many variations and modifications of the embodiments described herein. For example, the gain factors and relative operating ratios of the various transistors, and the output and reference voltage levels, may be adjusted according to design considerations of particular circuits and particular requirements. While the transistors described with reference to various embodiments are shown as being of particular configurations and conductivity types, it is well within the abilities of one having ordinary skill in the art to design a circuit that is functionally similar to the voltage regulator circuit  120 , using other types of active devices, and devices having different conductivity characteristics. Some regulator circuits may require additional power transistors to supply a required current load. All such variations and modifications are considered to fall within the scope of the invention.  
         [0081]     Values of particular parameters such as turn-on thresholds of the power transistors, current suppression threshold of the biasing resistor circuit, biasing levels, current capacities, etc, are dictated by requirements of particular applications, and may be established without undue experimentation.  
         [0082]     All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, are incorporated herein by reference, in their entirety.  
         [0083]     From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.