Abstract:
A voltage regulator controls a regulated output voltage (Vout) by feeding it back to a differential input stage ( 13 ) receiving a reference voltage (Vref) and applying an output ( 3 ) to a control electrode of a follower transistor (M 4 ) that is coupled to an output stage ( 15 ) which generates the output voltage (Vout). The output stage operates pull-up (M 7 B) and pull-down (M 5 B) transistors in response to a signal ( 6 A) produced by the follower transistor (M 4 ) during normal regulation operation, and provides fast settling of the output voltage by turning on a transient pull-up transistor (M 7 A) or transient pull-down transistor (M 5 A) in response to the signal ( 6 A) produced by the follower transistor (M 4 ) during a fast increasing or decreasing transition, respectively, of the load current (I L ). A filtering resistor (R FLT ) is coupled between the output voltage and a common electrode of the transient pull-up and pull down transistors.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates generally to low-dropout voltage regulators (LDO voltage regulators), and more particularly to improvements which provide low noise at medium and high frequencies, fast settling of the regulated output voltage, and low power consumption. 
         [0002]    Current consumption (and hence power consumption) of various digital logic circuits and especially other integrated circuits such as analog front end circuits, that are manufactured using various modern integrated circuit manufacturing processes can instantly, i.e., within a few picoseconds to a few nanoseconds, vary between zero and a large maximum value, e.g. 5 to 150 milliamperes. At the same time, very precise regulation is required for the supply voltages provided by voltage regulators to such digital logic circuits and other integrated circuitry such as analog front end circuits. Very low levels of noise at the signal frequency are required in the regulated supply voltages for some applications (e.g. radios and capacitive sensors), because such noise may become mixed with the main signals. Unfortunately, the circuitry needed to reduce the high noise levels may also reduce the accuracy of the circuits or systems to which the regulated supply voltages are applied. Low noise at high frequencies may be achieved with filtering by means of a load capacitor. To achieve the advantage of such filtering, the voltage regulator which provides the supply voltage must be slow, with bandwidth significantly below the signal band, e.g. 10 to 100 times lower than the signal band. At the same time, the regulated supply voltages for the above mentioned applications should be able to settle very rapidly to the required supply voltage value during recovery from large, very rapid changes in the amount of load current demanded by such applications. 
         [0003]    Fast settling of an LDO voltage regulator output signal requires use of a fast voltage feedback loop. Unfortunately, this is opposed to the above-mentioned noise filtering, and requires that the LDO voltage regulator have a large current-supplying capability e.g., roughly 10 to 100 times the nominal or quiescent current of the LDO voltage regulator, in order to quickly charge and/or discharge the capacitance of the user application or load to which the regulated voltage is applied. 
         [0004]    Known fast LDO voltage regulators including 2 feedback loops can achieve load settling times which are limited mainly by the maximum output current capability of the LDO voltage regulators. Multiple current gain boost paths are provided in a single gain stage in such known fast LDO voltage regulators. Prior Art  FIG. 1  shows such an LDO voltage regulator  10 - 1  which can provide very fast-response load voltage regulation, with fast reaction times in response to step changes in the amount of current demanded by a load (e.g., a load such as integrated digital logic circuitry or a capacitive touch sensor to which the regulated voltage is applied) without substantially increasing the power consumption of the LDO voltage regulator, and without the need to use a large external load bypass capacitor. The fast LDO voltage regulator of Prior Art  FIG. 1  is similar to the fast LDO voltage regulator shown in  FIG. 2  of commonly assigned U.S. Pat. No. 7,633,280 entitled “Low Drop Voltage Regulator with Instant Load Regulation and Method” issued to Ivanov et al. Dec. 15, 2009.  
         [0005]    Prior Art  FIG. 1  shows a high-speed, low-power LDO voltage regulator  10 - 1  including an input stage having differentially coupled N-channel input transistors M 0  and M 1 , P-channel active load transistors M 2  and M 3 , and tail current source  4 . LDO voltage regulator  10 - 1  also includes an output stage including P-channel pass transistor M 7 , N-channel pull-down transistor M 5 , N-channel cascode transistor M 6 , and a voltage source  11  which produces a constant bias voltage V 0  on the gate of cascode transistor M 6 . A gain stage is coupled between the differential input stage and the output stage, and includes P-channel source follower transistor M 4  and resistor R 5 . 
         [0006]    The sources of input transistors M 0  and M 1  are connected to tail current source  4 . The gate of input transistor M 0  is connected to reference voltage Vref, and the gate of input transistor M 1  is connected by conductor  5  to the junction between resistors R 1  and R 2 . The sources of load transistors M 2  and M 3  and source follower transistor M 4  are connected to regulated output voltage conductor  6  on which the regulated output voltage Vout is produced. The drains of input transistors M 0  and M 1  are connected by conductors  2  and  3  to the drains of active load transistors M 2  and M 3 , respectively. The gates of load transistors M 2  and M 3  are connected to conductor  2  and their sources are connected to output conductor  8 . Resistor R 1  is connected between output conductor  8  and conductor  5 , which is connected to the gate of input transistor M 1 , and resistor R 2  is connected between conductor  5  and V SS  so that resistors R 1  and R 2  form a voltage divider. The gate of source follower transistor M 4  is connected by conductor  3  to the drains of input transistor M 1  and active load transistor M 3 , and also is connected to one terminal of compensation capacitor C 0 . The source of source follower transistor M 4  is connected to Vout by output conductor  8 . 
         [0007]    Output conductor  8  also is connected to the drain of pass transistor M 7 , the source of which is connected to V DD . Output conductor  8  also is connected to the drain of pull-down transistor M 5 , the source of which is connected to V SS . A load circuit  16  modeled as a parallel connection of a load capacitor C L  and a load current source I L  are connected to output conductor  8 . A load circuit  16  may demand a load current that undergoes very fast, large-magnitude transitions. The gate of pass transistor M 7  is connected by conductor  9  to the drain of cascode transistor M 6  and to one terminal of a pull-up resistor R 7 , the other terminal of which is connected to V DD . The source of cascode transistor M 6  is connected by conductor  6  to the gate of pull-down transistor M 5 . The gate of cascode transistor M 6  is connected by conductor  18  to receive a bias voltage V 0  on the (+) terminal of voltage source  11 , the (−) terminal of which is connected to V SS . The drain of source follower transistor M 4  is connected by conductor  6  to one terminal of resistor R 5 , the other terminal of which is connected to V SS . 
         [0008]    LDO voltage regulator  10 - 1  of Prior Art  FIG. 1  includes three feedback loops. A first “accuracy” feedback loop includes input transistors M 0  and M 1  and source follower transistor M 4 . A second feedback loop includes common-gate source follower transistor M 4 , pull-down transistor M 5 , cascode transistor M 6 , and pass transistor M 7 . A third feedback loop includes source follower transistor M 4 , resistor R 5 , cascode transistor M 6 , and pass transistor M 7 . Internal capacitor C 0  provides compensation for the first feedback loop. Capacitor C 0  also decreases the bandwidth of the “accuracy” feedback loop including transistors M 0  and M 1  and source follower transistor M 4 , thereby decreasing the overall peak-to-peak noise at the output of voltage regulator  10 - 1 . 
         [0009]    The constant bias voltage V 0  on conductor  18  causes the current in cascode transistor M 6  to be substantially increased as the voltage on conductor  8  is decreased enough to turn pull-down transistor M 5  off, so as to maintain a minimum current in pull-down transistor M 5 . In LDO voltage regulator  10 - 1 , accuracy is determined by the “slow” loop including transistors M 0 , M 1  and M 4 . The bandwidth gm 0 /C 0  (where “gm 0 ” is the transconductance of transistor M 0 ) and high-frequency settling is determined by the two “fast” loops including transistors M 4  and M 5  and M 4 , M 6 , and M 7 , respectively. 
         [0010]    During a large step increase of the current demanded by the load  16 , a large amount of current must be supplied by pass transistor M 7 . That requires the gate voltage of pass transistor M 7  to rapidly decrease. But pass transistor M 7  is very large and has a large gate capacitance, so a large amount of current must be rapidly drawn out of the large gate capacitance of pass transistor M 7  so it can supply the large step increase in current demanded by load  16 . 
         [0011]    The large step increase in demanded load current causes the regulated output voltage Vout to rapidly decrease, and that decreases the current through source follower transistor MP 4  and resistor R 5 . The decreased current through resistor R 5  lowers the source voltage of cascode transistor M 6  and causes it to turn on harder, thereby increasing its drain current and rapidly discharging the large gate capacitance of pass transistor M 7  so as to rapidly turn it on and supply the step increase in the demanded load current. 
         [0012]    If the load current demanded by load  16  undergoes a large step decrease, this causes Vout to rapidly increase because load  16  suddenly is not sinking the large current being supplied by pass transistor M 7 . Consequently, the source voltage of source follower transistor M 4  increases, causing it to turn on harder. That causes the gate voltage of pull-down transistor M 5  to rapidly increase, so pull-down transistor M 5  immediately sinks the available charge from capacitance associated with output conductor  8 , thereby allowing sufficient time for pass transistor M 7  to decrease its drain current. The rate at which the amplified drain current produced by pass transistor M 7  decreases is determined by its gate capacitance and the resistance of pull-up resistor R 7 . This is how voltage regulator  10 - 1  of Prior Art  FIG. 1  responds very rapidly to a step decrease in the demanded load current from a large value to a small value. 
         [0013]    However, regulator  10 - 1  does not have adequate high-frequency noise filtering based on load capacitance C L , and therefore it requires high current (and hence high power) to adequately lower the noise. The high-frequency filtering using the load capacitor is not efficient because of the high speed of the fast loop including source follower transistor M 4  and the gate capacitance of pull-down transistor M 5 . There are several reasons for that filtering inefficiency. In order to achieve the high-frequency noise filtering (without benefit of the present invention), one technique that can be used is to make the load capacitor C L  very large, but this expedient makes it necessary that the size of pull-down transistor M 5  also be very large. However, larger the size of pull-down transistor M 5 , the more high-frequency noise is likely to be injected into regulator  10 - 1  through its power supplies. This is likely to substantially increase the cost of the integrated circuit chip. The resistance of resistor R 5  may be reduced in order increase the current through source follower transistor M 4 . It would be necessary to reduce the current through pull-up resistor R 7  and make both pull-up transistor M 7  and pull-down transistor M 5  very large. (If the resistance of resistor R 5  is reduced, then more current flows through source follower transistor M 4  and cascode transistor M 6 . The current that flows through source follower transistor M 4  comes from pull-up transistor M 7 . The current from pull-up transistor M 7  is split between source follower transistor M 4  and pull-down transistor M 5  (apart from transistors M 2  and M 3 , of course). Therefore, increasing current in source follower transistor M 4  means less current flows through cascode transistor M 5 . This increases the “on” resistance of pull-down transistor M 5 , which is undesirable. Since the current in cascode transistor M 6  increases when the resistance R 5  increases, that means the gate voltage of pull-up transistor M 7  increases. Since it is a P channel transistor, the VGS voltage (gate-to-source voltage) of pull-up transistor M 7  decreases, which means less current flows through pull-up transistor M 7 . Therefore, a reduction in the resistance R 5  must be accompanied by a reduction in the resistance R 7  to allow for more current through pull-down transistor M 5 .) 
         [0014]    Unfortunately, due to the high speed of the fast loop (with bandwidth gm 4 /Cg 5 , where gm 4  is the transconductance of source follower transistor M 4  and Cg 5  is the gate capacitance of pull-down transistor M 5 ), the high-frequency filtering involving the load capacitor C L  is not efficient, and high quiescent current is required in transistor M 4  in order to adequately decrease high-frequency noise. 
         [0015]    Thus, there is an unmet need for a voltage regulator and method which provide the combination of low noise at medium and high frequencies, very fast settling of the regulated output voltage, and low power consumption. 
         [0016]    There also is an unmet need for a low-cost voltage regulator and method which provide the combination of low noise at medium and high frequencies, very fast settling of the regulated output voltage, and low power consumption. 
         [0017]    There also is an unmet need for a voltage regulator and method which provide the combination of low noise at medium and high frequencies, very fast settling of the regulated output voltage, and low power consumption without use of an external resistor to accomplish filtering of high frequency noise from the regulated output voltage. 
       SUMMARY OF THE INVENTION 
       [0018]    It is an object of the invention to provide a voltage regulator and method which provide the combination of low noise at medium and high frequencies, very fast settling of the regulated output voltage, and low power consumption. 
         [0019]    It is another object of the invention to provide a low-cost voltage regulator and method which provide the combination of low noise at medium and high frequencies, very fast settling of the regulated output voltage, and low power consumption. 
         [0020]    It is another object of the invention to provide a voltage regulator and method which provide the combination of low noise at medium and high frequencies, very fast settling of the regulated output voltage, and lo form a w power consumption without use of an external resistor to accomplish filtering of high frequency noise from the regulated output voltage. 
         [0021]    Briefly described, and in accordance with one embodiment, the present invention provides a voltage regulator that controls a regulated output voltage (Vout) by feeding it back to a differential input stage ( 13 ) receiving a reference voltage (Vref) and applying an output ( 3 ) to a control electrode of a follower transistor (M 4 ) that is coupled to an output stage ( 15 ) which generates the output voltage (Vout). The output stage operates pull-up (M 7 B) and pull-down (M 5 B) transistors in response to a signal ( 6 A) produced by the follower transistor (M 4 ) during normal regulation operation, and provides fast settling of the output voltage by turning on a transient pull-up transistor (M 7 A) or transient pull-down transistor (M 5 A) in response to the signal ( 6 A) produced by the follower transistor (M 4 ) during a fast increasing or decreasing transition, respectively, of the load current (I L ). A filtering resistor (R FLT ) is coupled between the output voltage and a common electrode of the transient pull-up and pull down transistors. 
         [0022]    In one embodiment, the invention provides Voltage regulator circuitry ( 10 - 2 ) including a differential input stage ( 13 ) having a first input ( 1 ) coupled to receive a reference voltage (Vref), a second input ( 5 ) coupled to a regulated output conductor ( 8 ) of the voltage regulator circuitry ( 10 - 2 ), and an output ( 3 ). An output stage ( 15 ) for producing a regulated output voltage (Vout) on the regulated output conductor ( 8 ) includes a transient pull-up transistor (M 7 A) having a first electrode coupled to a first supply voltage (V DD ) and a second electrode coupled to the regulated output conductor ( 8 ) and a transient pull-down transistor (M 5 A) having a first electrode coupled to a second supply voltage (V SS ) and a second electrode coupled to the regulated output conductor ( 8 ). The output stage also includes a primary pull-up transistor (M 7 B) having a first electrode coupled to the first supply voltage (V DD ) and a second electrode coupled to the regulated output conductor ( 8 ) and a primary pull-down transistor (M 5 B) having a first electrode coupled to the second supply voltage (V SS ) and a second electrode coupled to the regulated output conductor ( 8 ). In one embodiment, the output stage also includes a filtering resistor (R FLT ) coupled between the regulated output conductor ( 8 ) and the second electrodes ( 14 ) of the primary pull-up transistor (M 7 B) and the primary pull-down transistor (M 5 B). A gain stage (M 4 ,M 6 ,R 5 A,R 5 B) includes a cascode transistor (M 6 ) having a first electrode coupled to control electrodes of the primary pull-down transistor (M 5 B) and the transient pull-down transistor (M 5 A) and a second electrode coupled to the control electrodes of the primary pull-up transistor (M 7 B) and the transient pull-up transistor (M 7 A). The gain stage also includes a follower transistor (M 4 ) having a first electrode coupled to the regulated output conductor ( 8 ), a control electrode coupled to the output ( 3 ) of the differential input stage ( 13 ), and a second electrode coupled to the first electrode of the cascode transistor (M 6 ) and the control electrodes of the primary pull-down transistor (M 5 B) and the transient pull-down transistor (M 5 A). 
         [0023]    In one embodiment, the filtering resistor (R FLT ) operates in conjunction with a load capacitance (C L ) to filter noise from the regulated output voltage (Vout). 
         [0024]    In one embodiment, the differential input stage ( 13 ), output stage ( 15 ), and gain stage (M 4 ,M 6 ,R 5 A,R 5 B) operate to regulate the output voltage (Vout) to a predetermined value having a predetermined relationship to the regulated output voltage (Vout) by controlling the primary pull-up transistor (M 7 B) and the primary pull-down transistor (M 7 A) during normal voltage regulating operation, and also operate to turn on the transient pull-up transistor (M 7 A) or the transient pull-down transistor (N 5 A) in response to a sufficiently large, fast transition of a load current (I L ) flowing in the regulated output conductor ( 8 ), depending on the direction of the transition of the load current (I L ), so as to cause rapid settling of the regulated output voltage (Vout) back to the predetermined value. 
         [0025]    In one embodiment, the transistors are MOS (metal-oxide-semiconductor) transistors, the first electrodes are sources, the second electrodes are drains, and the control electrodes are gates. In one embodiment, the transient pull-up transistor (M 7 A), the primary pull-up transistor (M 7 B) and the source follower transistor (M 4 ) are P-channel transistors, and the transient pull-down transistor (M 5 A), the primary pull-down transistor (M 5 B) and the cascode transistor (M 6 ) are N-channel transistors. 
         [0026]    In one embodiment, the input stage ( 13 ) includes a first input transistor (M 0 ) having a source connected to a tail current source ( 4 ), a gate coupled to receive the reference voltage (Vref), and a drain coupled to a gate and a drain of a first load transistor (M 2 ) and a gate of a second load transistor (M 3 ), and also includes a second input transistor (M 1 ) having a source coupled to the tail current source ( 4 ), a gate coupled to the regulated output voltage conductor ( 8 ), and a drain coupled to a drain of the second load transistor (M 3 ) and to the output ( 3 ) of the differential input stage ( 13 ). 
         [0027]    In one embodiment, the gate of the second input transistor (M 1 ) is coupled to the regulated output conductor ( 8 ) by means of a resistive divider circuit (R 1 ,R 2 ). 
         [0028]    In one embodiment, sources of the first (M 2 ) and second (M 3 ) load transistors are coupled to the regulated output voltage (Vout). 
         [0029]    In one embodiment, the filtering resistor (R FLT ) is integrated on an integrated circuit chip with the voltage regulator circuitry ( 10 - 2 ). 
         [0030]    In one embodiment, a channel resistance of the transient pull-up transistor (M 7 A) during a first-direction transition of the load current (I L ) is lower than a channel resistance of the primary pull-up transistor (M 7 B) and a channel resistance of the transient pull-down transistor (M 5 A) during a second-direction transition of the load current (I L ) is lower than a channel resistance of the primary pull-down transistor (M 5 B). 
         [0031]    In one embodiment, the predetermined relationship is represented by a predetermined ratio of the output voltage (Vout) to the reference voltage (Vref). 
         [0032]    In one embodiment, the gain stage (M 4 ,M 6 ,R 5 A,R 5 B) includes a voltage source ( 11 ) for producing a constant bias voltage (V 0 ) on a control electrode of the cascode transistor (M 6 ). 
         [0033]    In one embodiment, a compensation capacitor (CO) is coupled between the output ( 3 ) of the differential input stage ( 13 ) and the second supply voltage (V SS ). 
         [0034]    In one embodiment, the load capacitance (C L ) is integrated on an integrated circuit chip with the voltage regulator circuitry ( 10 - 2 ) and is coupled to the regulated output conductor ( 8 ) and wherein a load ( 16 ) coupled to the regulated output conductor ( 8 ) demands a step change in current supplied by the voltage regulator circuitry ( 10 - 2 ) to the load. 
         [0035]    In one embodiment, the invention provides a method for producing fast settling of a regulated output voltage (Vout), the method including controlling the accuracy of the regulated output voltage (Vout) produced by a voltage regulator ( 10 - 2 ) by feeding back the regulated output voltage (Vout) to an input ( 5 ) of a differential input stage ( 13 ) having a reference voltage (Vref) applied to a reference input ( 1 ) of the differential input stage ( 13 ) and applying an output ( 3 ) of the differential input stage ( 13 ) to a control electrode of a follower transistor (M 4 ) having a first electrode coupled to an output stage ( 15 ) which generates the regulated output voltage (Vout) on a regulated output conductor ( 8 ), and controlling a primary pull-up transistor (M 7 B) and a primary pull-down transistor (M 5 B) of the output stage ( 15 ), each coupled to the regulated output conductor ( 8 ), in response to a signal ( 6 A) produced on a second electrode of the follower transistor (M 4 ) during normal regulation operation of the voltage regulator ( 10 - 2 ) to maintain a predetermined level of the regulated output voltage (Vout); and providing fast settling of the regulated output voltage (Vout) by turning on a transient pull-up transistor (M 7 A) disposed in the output stage ( 15 ) and coupled to the regulated output conductor ( 8 ), in response to a signal ( 6 A) produced on the second electrode of the follower transistor (M 4 ) during a fast increasing transition of the load current (I L ), and turning on a transient pull-down transistor (M 5 A) disposed in the output stage ( 15 ) and coupled to the regulated output conductor ( 8 ), in response to a signal ( 6 A) produced on the second electrode of the follower transistor (M 4 ) during a fast decreasing transition of the load current (I L ). 
         [0036]    In one embodiment, the method includes providing the transistors as MOS (metal-oxide-semiconductor) transistors, wherein the first electrodes are sources, the second electrodes are drains, and the control electrodes are gates, the method including coupling a first terminal of a filtering resistor (R FLT ) to drains of the primary pull-up transistor (M 7 B) and primary pull-down transistor (M 5 B) and coupling a second terminal of the filtering resistor (R FLT ) to drains of the transient pull-up transistor (M 7 A) and transient pull-down transistor (M 5 A) to filter noise from the regulated output voltage (Vout). 
         [0037]    In one embodiment, the method includes supplying a rapidly increased amount of load current (I L ) during a fast transition to an increased amount of load current (I L ) demanded by the load ( 16 ) by applying the regulated output voltage (Vout) to the source of the follower transistor (M 4 ) to cause the drain of the follower transistor to rapidly turn a cascode transistor (M 6 ) on harder, and causing a drain of the cascode transistor (M 6 ) to turn on the transient pull-up transistor (M 7 A) in response to an increasing drain current produced by the cascode transistor (M 6 ). 
         [0038]    In one embodiment, the method includes sinking a rapidly increased amount of load current (I L ) during a fast transition to a decreased amount of load current (I L ) demanded by the load ( 16 ) by applying the regulated output voltage (Vout) to the source of the follower transistor (M 4 ) to cause the drain of the follower transistor to rapidly turn on the transient pull-down transistor (M 5 A). 
         [0039]    In one embodiment, the invention provides a voltage regulator ( 10 - 2 ) for producing a fast settling regulated output voltage (Vout), including circuitry for controlling the accuracy of the regulated output voltage (Vout) produced by a voltage regulator ( 10 - 2 ), including means (R 1 ,R 2 ) for feeding back the regulated output voltage (Vout) to an input ( 5 ) of a differential input stage ( 13 ) having a reference voltage (Vref) applied to a reference input ( 1 ) of the differential input stage ( 13 ), means ( 3 ) for applying an output of the differential input stage ( 13 ) to a control electrode of a follower transistor (M 4 ) having a first electrode coupled to an output stage ( 15 ) which generates the regulated output voltage (Vout) on a regulated output conductor ( 8 ), and means (M 6 ,R 7 B,R 5 B) for controlling a primary pull-up transistor (M 7 B) and a primary pull-down transistor (M 5 B) of the output stage ( 15 ), each coupled to the regulated output conductor ( 8 ), in response to a signal ( 6 A) produced on a second electrode of the follower transistor (M 4 ) during normal regulation operation of the voltage regulator ( 10 - 2 ) to maintain a predetermined level of the regulated output voltage (Vout); and circuitry for providing fast settling of the regulated output voltage (Vout), including means (M 6 ,R 7 B,R 7 A) for turning on a transient pull-up transistor (M 7 A) disposed in the output stage ( 15 ) and coupled to the regulated output conductor ( 8 ), in response to a signal ( 6 A) produced on the second electrode of the follower transistor (M 4 ) during a fast increasing transition of the load current (I L ), and means (M 6 ,R 5 B,R 5 A) for turning on a transient pull-down transistor (M 5 A) disposed in the output stage ( 15 ) and coupled to the regulated output conductor ( 8 ), in response to a signal ( 6 A) produced on the second electrode of the follower transistor (M 4 ) during a fast decreasing transition of the load current (I L ). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0040]      FIG. 1  is a schematic diagram of a prior art fast-settling LDO voltage regulator. 
           [0041]      FIG. 2  is a schematic diagram of a low noise, fast settling, low power LDO voltage regulator which is a preferred embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0042]    High-speed, low-power LDO voltage regulator  10 - 2  in  FIG. 2  includes an input stage  13  including differentially coupled N-channel input transistors M 0  and M 1 , P-channel active load transistors M 2  and M 3 , and a tail current source  4 . LDO voltage regulator  10 - 1  also includes an output stage  15  including a P-channel “transient pull-up” transistor M 7 A, a N-channel “transient pull-down” transistor M 5 A, a P-channel “transient pull-up” transistor M 7 B, a “primary pull-down” transistor M 5 B, pull-up resistors R 7 A and R 7 B, and a filtering resistor R FLT . A gain stage is coupled between input stage  13  and output stage  15 , and includes a P-channel source follower transistor M 4 , pull-down resistors R 5 A and R 5 B, a cascode transistor M 6  and a voltage reference  11 . 
         [0043]    The sources of input transistors M 0  and M 1  are connected to tail current source  4 . The gate of input transistor M 0  is connected to an input or reference voltage Vref. Resistors R 1  and R 2  are connected in series between Vout and V SS . The gate of input transistor M 1  is connected by conductor  5  to the junction between resistors R 1  and R 2 , which form a voltage divider that operates to feed back a predetermined proportion of Vout to the gate of input transistor M 1 . (Of course, Vout could be coupled directly to the gate of input transistor M 1 .) The sources of active load transistors M 2  and M 3  and source follower transistor M 4  are connected to Vout on conductor  8 . The drains of input transistors M 0  and M 1  are connected to the drains of active load transistors M 2  and M 3 , respectively. The drain of input transistor M 1  also is connected by conductor  3  to a compensation capacitor C 0 . The gates of load transistors M 2  and M 3  are connected to the drain of input transistor M 0 . (However, the sources of load transistors M 2  and M 3  could be connected to V DD  instead of output conductor  8 . Also, there are numerous other implementations of input stage  13  which could provide satisfactory performance.) 
         [0044]    Output conductor  8  is connected to the drain of transient pull-up transistor M 7 A, the source of which is connected to V DD . Output conductor  8  also is connected to the drain of transient pull-down transistor M 5 A, the source of which is connected to V SS . Output conductor  8  also is connected to a load circuit  16  which is modeled as a parallel connection of a load capacitor C L  and a load current source I L . Load capacitor C L  may be integrated on the same integrated circuit die along with LDO voltage regulator  10 - 2 . The current demanded by load  16  may undergo very fast (e.g., between 0 and tens of microseconds), large-magnitude (e.g., 0 to 1-2 amperes) transitions. 
         [0045]    The gate of transient pull-up transistor M 7 A is connected by conductor  9 B to one terminal of pull-up resistor R 7 B. the other terminal of pull-up resistor R 7 B M 6 . Pull-up resistor R 7 A is coupled between conductor  9 B and V DD . The gate of cascode transistor M 6  is connected by conductor  18  to receive the bias voltage V 0  on the (+) terminal of voltage source  11 , the (−) terminal of which is connected to V SS . 
         [0046]    The source of primary pull-up transistor M 7 B is connected to V DD . The drain of primary pull-up transistor M 7 B is connected by conductor  14  to the source of source follower transistor M 4 . The source electrode of source follower transistor M 4  is connected by conductor  14  to the drains of primary pull-up transistor M 7 B and primary pull-down transistor M 5 B and to one terminal of filtering resistor R FLT , the other terminal of which is connected by Vout conductor  8  to the drains of transient pull-up transistor M 7 A and transient pull-down transistor M 5 A. The gate of source follower transistor M 4  is connected to conductor  3 . 
         [0047]    The gate of primary pull-down transistor M 5 B is connected by conductor  6 A to the source of cascode transistor M 6 , the drain of source follower transistor M 4 , and one terminal of resistor R 5 B. The other terminal of pull-down resistor R 5 B is connected by conductor  6 B to the gate of transient pull-down transistor M 5 A and one terminal of resistor R 5 A, the other terminal of which is connected to V SS . 
         [0048]    In operation, if load  16  suddenly demands a large increase in load current I L , that pulls Vout lower. The resistive divider R 1 ,R 2  therefore lowers the gate voltage of input transistor Ml. Therefore, less of the tail current I 0  flows through input transistor M 1 , and more flows through input transistor M 0  and current mirror input (active load) transistor M 2  and therefore is mirrored through current mirror output (active load) transistor M 3 . This increase in the current through active load transistor M 3  increases the gate voltage of source follower transistor M 4 . This in turn reduces the current through source follower transistor M 4 , thereby decreasing the voltage developed across pull-down resistors R 5 B and R 5 A and therefore decreases the voltage on conductor  6 A and the source of cascode transistor M 6 . This causes cascode transistor M 6  to rapidly increase the amount of current through pull-up resistors R 7 A and R 7 B, to a sufficiently high level to turn transient pull-up transistor M 7 A on hard. This causes transient pull-up transistor M 7 A to rapidly supply the sharply increased amount of current being demanded by load  16 , thereby providing fast settling of regulator  10 - 2  back to a level close to the proportion of Vref determined by the voltage divider R 1 ,R 2 . (The decreased voltage on conductor  6 A also results in reduced current through primary pull-down transistor M 5 B and keeps transient pull-down transistor M 5 A off.) At that point, the feedback through input stage  13  and the gain stage (including source follower transistor M 4  and cascode transistor M 6 ) causes regulator  10 - 2  to resume normal regulation of Vout by means of primary pull-up transistor M 7 B, primary pull-down transistor M 5 B, and filtering resistor R FLT . 
         [0049]    Essentially the opposite operation occurs if the current demanded by load  16  suddenly decreases. That is, if the amount of load current I L  demanded by load  16  suddenly decreases by a large amount, the amount of current being supplied to load  16  by primary pull-up transistor M 7 B causes Vout to rapidly increase. This causes resistive divider R 1 ,R 2  to increase the gate voltage of input transistor M 1 . Therefore, more of tail current I 0  flows through input transistor M 1 , and therefore less tail current flows through input transistor M 0 , current mirror input (active load) transistor M 2 , and current mirror output (active load) transistor M 3 . The decrease in the current through active load transistor M 3  decreases the gate voltage of source follower transistor M 4 , which in turn increases the current through source follower transistor M 4 . That increases the voltage developed across pull-down resistors R 5 B and R 5 A and increases the voltages on conductors  6 A and  6 B. The resulting increased voltage on the gate of transient pull-down transistor M 5 A causes it to turn on hard and rapidly pull Vout back to a level close to the proportion of Vref determined by voltage divider R 1 ,R 2 . (The resulting increased voltage on conductor  6 A also causes cascode transistor M 6  to sharply decrease the amount of current through pull-up resistors R 7 A and R 7 B to thereby turn transient pull-up transistor M 7 A off and reduce the amount of current through primary pull-up transistor M 7 B.) At that point, the feedback through input stage  13  and the gain stage (including source follower transistor M 4  and cascode transistor M 6 ) causes regulator  10 - 2  to resume normal regulation of Vout by means of primary pull-up transistor M 7 B, primary pull-down transistor M 5 B, and filtering resistor R FLT . 
         [0050]    Low-resistance (e.g., 5 to 100 ohms) filtering resistor R FLT  is connected to the previously mentioned slow loop including primary pull-up and pull down transistors M 7 B and M 5 B, between the slow loop and the fast loop including transistors M 7 A and M 5 A. This substantially improves the high-frequency filtering that is based on load capacitance C L  and reduces the high-frequency noise in the regulated output voltage Vout. Also, providing the resistance of filtering resistor R FLT  helps stabilize the main loop from input to output (including transistors M 0 , M 1  and M 4 ), and also the above-mentioned fast loop including transistors M 7 A and M 5 A, in addition to reducing the high-frequency noise. Furthermore, providing filtering resistor R FLT  in the main loop from input to output (including transistors M 0 , M 1  and M 4 ) and integrated on the same die as LDO voltage regulator  10 - 2  avoids the need for the user to provide a costly external resistor to reduce the high-frequency noise that would be required by the fast-settling voltage regulator  10 - 1  of Prior Art  FIG. 1 . 
         [0051]    Transient pull-up transistor M 7 A and transient pull-down transistor M 5 A are off during normal operation and therefore do not affect Vout during that time. During transient operation, transient pull-up transistor M 7 A and transient pull-down transistor M 5 A act as resistors M 5 B or M 7 B because they are trioding at these moments due to the larger current through R FLT . Transient pull-up transistor M 7 A and transient pull-down transistor M 5 A are actually slightly larger than primary pull-up transistor M 7 B and primary pull-down transistor M 5 B, respectively, so transient pull-up transistor M 7 A and transient pull-down transistor M 5 A actually are turned on somewhat harder, and they essentially overcome or overpower the high channel resistances of primary transient pull-up transistor M 7 B and primary transient pull-down transistor M 5 B, respectively. 
         [0052]    LDO voltage regulator  10 - 2  of  FIG. 2  dramatically improves the response time and settling of large, extremely fast loop current transients while also providing low levels of high-frequency noise in Vout without incurring substantial additional cost. This is accomplished by the combination of multiple feedback loops with different speeds and nonlinear transitions from one loop to another and by use of nonlinear signal filtering in the multiloop system in order to achieve both the fast settling and the low mid-and high frequency noise levels in the regulated output voltage signal. Furthermore, the very fast regulated output voltage settling times and low noise levels are achieved at very low current and power consumption levels. 
         [0053]    Transient pull-down transistor M 5 A and transient pull-up transistor M 7 A are OFF at small load current levels, and load  16  is “separated” from the “fast” loops (the loop including transistor M 4 , resistor R 5 B, and transistor M 5 A and the loop including transistors M 4  and M 6 , resistor R 7 B, and transistor M 7 A) by resistor R FLT . High-frequency noise is filtered by filtering circuitry having the time constant R FLT C L . The voltage drop across filtering resistor R FLT  is small, e.g., below approximately 50-100 millivolts. R FLT  helps filter out noise at medium and high frequencies, and should be included inside the above-mentioned fast loops because then it can be integrated inside the integrated circuit die. (Filtering resistor R FLT  will cause a DC shift in the output voltage as compare to the ideal expected output voltage (the 50-100 millivolts mentioned in the previous sentence), so if the resistance R FLT  is too large, then the proper DC regulation is not maintained.) In this circuit, low-frequency accuracy is defined by the “slow” main loop including transistors M 0 , M 1 , and M 4  that accomplishes the DC regulation. The loop current, when the dominant operation occurring in output stage  15  is transferred from primary pull-down transistor M 5 B to transient pull-down transistor M 5 A or from primary pull-up transistor M 7 B to transient pull-up transistor M 7 A, is determined by voltage drop across primary pull-down resistor R 5 B or primary pull-up resistor R 7 B, respectively. Also, filtering resistor R FLT  has the advantage of improving the stability of the fast loop including transistor M 4 , resistor R 5 B, and transistor M 5 A and the loop including transistors M 4  and M 6 , resistor R 7 B, and transistor M 7 A. 
         [0054]    While the invention has been described with reference to several particular embodiments thereof, those skilled in the art will be able to make various modifications to the described embodiments of the invention without departing from its true spirit and scope. It is intended that all elements or steps which are insubstantially different from those recited in the claims but perform substantially the same functions, respectively, in substantially the same way to achieve the same result as what is claimed are within the scope of the invention. For example, some of the components could be replaced with bipolar transistors. A somewhat different input stage could be used, such as a current mirror operational amplifier or a folded cascode operational amplifier. Also, one or more of resistors R 5 A, R 5 B, R 7 A and R 7 B could be replaced by current sources. A combination of current sources and R 5 A, R 5 B, R 7 A and R 7 B might also be employed to find a desired balance between speed and stability. The gate of cascode transistor M 6  could be biased by something other than a voltage source.