Patent Publication Number: US-8531172-B2

Title: Family of current/power-efficient high voltage linear regulator circuit architectures

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 13/163,930, titled “FAMILY OF CURRENT/POWER-EFFICIENT HIGH VOLTAGE LINEAR REGULATOR CIRCUIT ARCHITECTURES,” filed Jun. 20, 2011, which is a continuation of U.S. application Ser. No. 12/050,874, titled “FAMILY OF CURRENT/POWER-EFFICIENT HIGH VOLTAGE LINEAR REGULATOR CIRCUIT ARCHITECTURES,” filed Mar. 18, 2008, each of which are hereby expressly incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The field of the invention relates to microelectromechanical systems (MEMS). More specifically, the invention relates to voltage regulators for MEMS devices having a display with periods of low current consumption. One particular application can be found in MEMS display devices. The invention also relates to optical MEMS devices, in general, and bi-stable displays in particular. 
     2. Description of the Related Technology 
     Microelectromechanical systems (MEMS) include micro mechanical elements, actuators, and electronics. Micromechanical elements may be created using deposition, etching, and or other micromachining processes that etch away parts of substrates and/or deposited material layers or that add layers to form electrical and electromechanical devices. MEMS technology is used, for example, in bi-stable display devices. One type of MEMS bi-stable display device is called an interferometric modulator. As used herein, the term interferometric modulator or interferometric light modulator refers to a device that selectively absorbs and/or reflects light using the principles of optical interference. In certain embodiments, an interferometric modulator may have a pair of conductive plates, one or both of which may be transparent and/or reflective in whole or part and capable of relative motion upon application of an appropriate electrical signal. In this type of device, one plate may be a stationary layer deposited on a substrate and the other plate may be a metallic membrane separated from the stationary layer by an air gap. The position of one plate in relation to another can change the optical interference of light incident on the interferometric modulator. 
     Because of the bi-stable characteristic of the display, the current load of the display varies greatly. The current load is largest while the display is being driven to change the image, when some or all of the bi-stable elements change states. Between the image update or refresh periods, the current load of the display is near zero. Under extremely low load conditions, the power consumption of conventional power supply regulator circuits dominates the total power consumption of the driver IC. A power supply configured to efficiently source current at a regulated voltage over widely varying current load is needed. 
     SUMMARY OF CERTAIN EMBODIMENTS 
     The system, method, and devices of the invention each have several aspects, no single one of which is solely responsible for its desirable attributes. Without limiting the scope of this invention, its more prominent features will now be discussed briefly. After considering this discussion, and particularly after reading the section entitled “Detailed Description of Certain Embodiments” one will understand how the features of this invention provide advantages over other display devices. 
     One aspect is a voltage regulator circuit, including an input stage having an input bias current, and an output stage having an output bias current, the output stage being configured to supply an output current at a regulated output voltage, where at least one of the input bias current and the output bias current is dependent at least in part on the output current. 
     Another aspect is a method of controlling a bias current in an output stage of voltage regulator circuit, the circuit configured to provide current substantially at a regulated output voltage. The method includes sensing a difference between a voltage based on the output voltage and a reference voltage, and generating a bias current based on the difference. 
     Another aspect is a voltage regulator circuit, including an input stage, and an output stage having an output bias current, the output stage being selectively connectable to a fixed current source and to a variable current source. 
     Another aspect is a voltage regulator circuit, including an input stage having an input bias current, and an output stage having an output bias current, the output stage being configured to supply an output current at a regulated output voltage, where at least one of the input bias current and the output bias current is based at least in part on the difference between a voltage based on the output voltage and a reference voltage. 
     Another aspect is a display including a plurality of bi-stable display elements, and a voltage regulator circuit, the voltage regulator circuit including an input stage having an input bias current, and an output stage having an output bias current, the output stage being configured to supply an output current at a regulated output voltage, where at least one of the input bias current and the output bias current is based at least in part on the output current. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an isometric view depicting a portion of one embodiment of a bi-stable display, which is an interferometric modulator display in which a movable reflective layer of a first interferometric modulator is in a relaxed position and a movable reflective layer of a second interferometric modulator is in an actuated position. 
         FIG. 2  is a diagram of movable mirror position versus applied voltage for one embodiment of the bi-stable display of  FIG. 1 . 
         FIGS. 3A and 3B  are system block diagrams illustrating an embodiment of a visual display device comprising a bi-stable display. 
         FIG. 4  is a block diagram of a particularly efficient power supply regulator. 
         FIG. 5A  is a schematic diagram of one embodiment of an input stage which can be used in a power supply regulator such as that shown in  FIG. 4 . 
         FIG. 5B  is a schematic diagram of another embodiment of an input stage which can be used in a power supply regulator such as that shown in  FIG. 4 . 
         FIG. 6A  is a schematic diagram of an embodiment of an output stage which can be used in a power supply regulator such as that shown in  FIG. 4 . 
         FIG. 6B  is a schematic diagram of another embodiment of an output stage which can be used in a power supply regulator such as that shown in  FIG. 4 . 
         FIG. 6C  is a schematic diagram of yet another embodiment of an output stage which can be used in a power supply regulator such as that shown in  FIG. 4 . 
         FIG. 7  is a schematic diagram of an embodiment of a power supply regulator configured to generate both an input bias current and an output bias current based at least in part on the current output of the regulator. 
         FIG. 8  is a schematic diagram of an embodiment of a power supply regulator configured to generate both an input bias current and an output bias current based at least in part on the current output of the regulator. 
     
    
    
     DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS 
     The following detailed description is directed to certain specific embodiments of the invention. However, the invention can be embodied in a multitude of different ways. In this description, reference is made to the drawings wherein like parts are designated with like numerals throughout. As will be apparent from the following description, the embodiments may be implemented in any device that is configured to display an image, whether in motion (e.g., video) or stationary (e.g., still image), and whether textual or pictorial. More particularly, it is contemplated that the embodiments may be implemented in or associated with a variety of electronic devices such as, but not limited to, mobile telephones, wireless devices, personal data assistants (PDAs), hand-held or portable computers, GPS receivers/navigators, cameras, MP3 players, camcorders, game consoles, wrist watches, clocks, calculators, television monitors, flat panel displays, computer monitors, auto displays (e.g., odometer display, etc.), cockpit controls and/or displays, display of camera views (e.g., display of a rear view camera in a vehicle), electronic photographs, electronic billboards or signs, projectors, architectural structures, packaging, and aesthetic structures (e.g., display of images on a piece of jewelry). MEMS devices of similar structure to those described herein can also be used in non-display applications such as in electronic switching devices. 
     Embodiments of the invention more particularly relate to displays which present widely varying current load to their voltage supplies. These embodiments for such displays are particularly power efficient because they are configured to modify their overhead current according to the current load. This is particularly advantageous for use in display devices which have periods of extremely low current load. Such displays include bi-stable displays, such as interferometric modulation displays, LCD displays, and DMD displays. Other displays, such as those with elements having three or more stable states can also benefit from increased power efficiency when using a power supply configured to modify its overhead current according to the current load. 
     An example of a display element which, when used in a display, results in widely varying current load on the voltage supplies is shown in  FIG. 1 , which illustrates a bi-stable display embodiment comprising an interferometric MEMS display element. In these devices, the pixels are in either a bright or dark state. In the bright (“on” or “open”) state, the display element reflects a large portion of incident visible light to a user. When in the dark (“off” or “closed”) state, the display element reflects little incident visible light to the user. Depending on the embodiment, the light reflectance properties of the “on” and “off” states may be reversed. MEMS pixels can be configured to reflect predominantly at selected colors, allowing for a color display in addition to black and white. 
       FIG. 1  is an isometric view depicting two adjacent pixels in a series of pixels of a visual display, wherein each pixel comprises a MEMS interferometric modulator. In one embodiment, one of the reflective layers may be moved between two positions. In the first position, referred to herein as the relaxed position, the movable reflective layer is positioned at a relatively large distance from a fixed partially reflective layer. In the second position, referred to herein as the actuated position, the movable reflective layer is positioned more closely adjacent to the partially reflective layer. Incident light that reflects from the two layers interferes constructively or destructively depending on the position of the movable reflective layer, producing either an overall reflective or non-reflective state for each pixel. 
     The depicted portion of the pixel array in  FIG. 1  includes two adjacent pixels  12   a  and  12   b . In the pixel  12   a  on the left, a movable reflective layer  14   a  is illustrated in a relaxed position at a predetermined distance from an optical stack  16   a , which includes a partially reflective layer. In the pixel  12   b  on the right, the movable reflective layer  14   b  is illustrated in an actuated position adjacent to the optical stack  16   b.    
     With no applied voltage, the cavity  19  remains between the movable reflective layer  14   a  and optical stack  16   a , with the movable reflective layer  14   a  in a mechanically relaxed state, as illustrated by the pixel  12   a . However, when a potential difference is applied to a selected row and column, the capacitor formed at the intersection of the row and column electrodes at the corresponding pixel becomes charged, and electrostatic forces pull the electrodes together. If the voltage is high enough, the movable reflective layer  14  is deformed and is forced against the optical stack  16 . A dielectric layer (not illustrated in this Figure) within the optical stack  16  may prevent shorting and control the separation distance between layers  14  and  16 , as illustrated by pixel  12   b  on the right in  FIG. 1 . The behavior is similar regardless of the polarity of the applied potential difference. Because the load presented to the power supply by the pixels  12   a  and  12   b  is capacitive, the current from the power supply is largest when the pixels  12   a  and  12   b  are being driven so as to charge and discharge, and is minimal when the pixels  12   a  and  12   b  are being held in either of the two stable states. 
       FIG. 2  illustrates one process for using an array of interferometric modulators in a bi-stable display. 
     For MEMS interferometric modulators, the row/column actuation protocol may take advantage of a hysteresis property of these devices illustrated in  FIG. 2 . It may require, for example, a 10 volt potential difference to cause a movable layer to deform from the relaxed state to the actuated state. However, when the voltage is reduced from that value, the movable layer maintains its state as the voltage drops back below 10 volts. In the embodiment of  FIG. 2 , the movable layer does not relax completely until the voltage drops below 2 volts. There is thus a range of voltage, about 3 to 7 V in the example illustrated in  FIG. 2 , where there exists a window of applied voltage within which the device is stable in either the relaxed or actuated state. This is referred to herein as the “hysteresis window” or “stability window.” For a display array having the hysteresis characteristics of  FIG. 2 , the row/column actuation protocol can be designed such that during row strobing, pixels in the strobed row that are to be actuated are exposed to a voltage difference of about 10 volts, and pixels that are to be relaxed are exposed to a voltage difference of close to zero volts. After the strobe, the pixels are exposed to a steady state voltage difference of about 5 volts such that they remain in whatever state the row strobe put them in. After being written, each pixel sees a potential difference within the “stability window” of 3-7 volts in this example. This feature makes the pixel design illustrated in  FIG. 1  stable under the same applied voltage conditions in either an actuated or relaxed pre-existing state. Since each pixel of the interferometric modulator, whether in the actuated or relaxed state, is essentially a capacitor formed by the fixed and moving reflective layers, this stable state can be held at a voltage within the hysteresis window with almost no power dissipation. Essentially no current flows into the pixel if the applied potential is fixed. For this reason the display dissipates most of the power during data write and/or refresh periods. 
       FIGS. 3A and 3B  are system block diagrams illustrating an embodiment of a power efficient display device  40 , in which bi-stable display elements, such as pixels  12   a  and  12   b  of  FIG. 1  may be used with a power supply configured to modify its overhead current according to the current load. The display device  40  can be, for example, a cellular or mobile telephone. However, the same components of display device  40  or variations thereof are also illustrative of various types of display devices such as televisions and portable media players. 
     The display device  40  includes a housing  41 , a display  30 , an antenna  43 , a speaker  44 , an input device  48 , and a microphone  46 . The housing  41  is generally formed from any of a variety of manufacturing processes as are well known to those of skill in the art, including injection molding, and vacuum forming. In addition, the housing  41  may be made from any of a variety of materials, including but not limited to plastic, metal, glass, rubber, and ceramic, or a combination thereof. In one embodiment the housing  41  includes removable portions (not shown) that may be interchanged with other removable portions of different color, or containing different logos, pictures, or symbols. 
     The display  30  of exemplary display device  40  may be any of a variety of displays, including a bi-stable display, as described herein. In other embodiments, the display  30  includes a flat-panel display, such as plasma, EL, OLED, STN LCD, or TFT LCD as described above, or a non-flat-panel display, such as a CRT or other tube device, as is well known to those of skill in the art. However, for purposes of describing the present embodiment, the display  30  includes an interferometric modulator display, as described herein. 
     The components of one embodiment of exemplary display device  40  are schematically illustrated in  FIG. 3B . The illustrated exemplary display device  40  includes a housing  41  and can include additional components at least partially enclosed therein. For example, in one embodiment, the exemplary display device  40  includes a network interface  27  that includes an antenna  43  which is coupled to a transceiver  47 . The transceiver  47  is connected to a processor  21 , which is connected to conditioning hardware  52 . The conditioning hardware  52  may be configured to condition a signal (e.g. filter a signal). The conditioning hardware  52  is connected to a speaker  45  and a microphone  46 . The processor  21  is also connected to an input device  48  and a driver controller  29 . The driver controller  29  is coupled to a frame buffer  28 , and to an array driver  22 , which in turn is coupled to a display array  30 . A power supply  50  provides power to all components as required by the particular exemplary display device  40  design. 
     The network interface  27  includes the antenna  43  and the transceiver  47  so that the exemplary display device  40  can communicate with one or more devices over a network. In one embodiment the network interface  27  may also have some processing capabilities to relieve requirements of the processor  21 . The antenna  43  is any antenna known to those of skill in the art for transmitting and receiving signals. In one embodiment, the antenna transmits and receives RF signals according to the IEEE 802.11 standard, including IEEE 802.11(a), (b), or (g). In another embodiment, the antenna transmits and receives RF signals according to the BLUETOOTH standard. In the case of a cellular telephone, the antenna is designed to receive CDMA, GSM, AMPS or other known signals that are used to communicate within a wireless cell phone network. The transceiver  47  pre-processes the signals received from the antenna  43  so that they may be received by and further manipulated by the processor  21 . The transceiver  47  also processes signals received from the processor  21  so that they may be transmitted from the exemplary display device  40  via the antenna  43 . 
     In an alternative embodiment, the transceiver  47  can be replaced by a receiver. In yet another alternative embodiment, network interface  27  can be replaced by an image source, which can store or generate image data to be sent to the processor  21 . For example, the image source can be a digital video disc (DVD) or a hard-disc drive that contains image data, or a software module that generates image data. 
     Processor  21  generally controls the overall operation of the exemplary display device  40 . The processor  21  receives data, such as compressed image data from the network interface  27  or an image source, and processes the data into raw image data or into a format that is readily processed into raw image data. The processor  21  then sends the processed data to the driver controller  29  or to frame buffer  28  for storage. Raw data typically refers to the information that identifies the image characteristics at each location within an image. For example, such image characteristics can include color, saturation, and gray-scale level. 
     In one embodiment, the processor  21  includes a microcontroller, CPU, or logic unit to control operation of the exemplary display device  40 . Conditioning hardware  52  generally includes amplifiers and filters for transmitting signals to the speaker  45 , and for receiving signals from the microphone  46 . Conditioning hardware  52  may be discrete components within the exemplary display device  40 , or may be incorporated within the processor  21  or other components. 
     The driver controller  29  takes the raw image data generated by the processor  21  either directly from the processor  21  or from the frame buffer  28  and reformats the raw image data appropriately for high speed transmission to the array driver  22 . Specifically, the driver controller  29  reformats the raw image data into a data flow having a raster-like format, such that it has a time order suitable for scanning across the display array  30 . Then the driver controller  29  sends the formatted information to the array driver  22 . Although a driver controller  29 , such as a LCD controller, is often associated with the system processor  21  as a stand-alone Integrated Circuit (IC), such controllers may be implemented in many ways. They may be embedded in the processor  21  as hardware, embedded in the processor  21  as software, or fully integrated in hardware with the array driver  22 . 
     Typically, the array driver  22  receives the formatted information from the driver controller  29  and reformats the video data into a parallel set of waveforms that are applied many times per second to the hundreds and sometimes thousands of leads coming from the display&#39;s x-y matrix of pixels. 
     In one embodiment, the driver controller  29 , array driver  22 , and display array  30  are appropriate for any of the types of displays described herein. For example, in one embodiment, driver controller  29  is a conventional display controller or a bi-stable display controller (e.g., an interferometric modulator controller). In another embodiment, array driver  22  is a conventional driver or a bi-stable display driver (e.g., an interferometric modulator display). In one embodiment, a driver controller  29  is integrated with the array driver  22 . Such an embodiment is common in highly integrated systems such as cellular phones, watches, and other small area displays. In yet another embodiment, display array  30  is a typical display array or a bi-stable display array (e.g., a display including an array of interferometric modulators). In some embodiments, display array  30  is another display type. 
     The input device  48  allows a user to control the operation of the exemplary display device  40 . In one embodiment, input device  48  includes a keypad, such as a QWERTY keyboard or a telephone keypad, a button, a switch, a touch-sensitive screen, a pressure- or heat-sensitive membrane. In one embodiment, the microphone  46  is an input device for the exemplary display device  40 . When the microphone  46  is used to input data to the device, voice commands may be provided by a user for controlling operations of the exemplary display device  40 . 
     In some implementations control programmability resides, as described above, in a driver controller which can be located in several places in the electronic display system. In some cases control programmability resides in the array driver  22 . 
     Power supply  50  can include a variety of energy storage devices as are well known in the art. For example, in one embodiment, power supply  50  is a rechargeable battery, such as a nickel-cadmium battery or a lithium ion battery. In another embodiment, power supply  50  is a renewable energy source, a capacitor, or a solar cell, including a plastic solar cell, and solar-cell paint. In another embodiment, power supply  50  is configured to receive power from a wall outlet. The power supply  50  may also have a power supply regulator configured to supply current for driving the display at a substantially constant voltage. In some embodiments, the constant voltage is based at least in part on a reference voltage, where the constant voltage may be fixed at a voltage greater than or less than the reference voltage. For a passive matrix bi-stable display, two or more power supply regulators outputting different voltage levels are usually present. For example, the display may require a common node, a +5V supply relative to common, and a −5V supply relative to common. Each regulator will be connected to the battery or other energy source and be configured to output a desired regulated voltage relative to the common node. The array driver  22  receives the different voltage levels and switches them to the rows and columns with the appropriate timing according to the display write process being used. When a given row or column of the array is switched from one voltage level to another during a data write operation, capacitances are charged and discharged and the power regulators deliver current to the display array  30 . In between data write operations, no switching is being performed, and the capacitors maintain their existing charge levels. The only current being supplied at these times is due to leakage through dielectric layers, which is very low. 
     Those of skill in the art will recognize that the above-described architecture may be implemented in any number of hardware and/or software components and in various configurations. For example, in some embodiments, the power supply regulator is external to power supply  50 . 
       FIG. 4  is a block diagram of a particularly efficient power supply regulator  100  configured to supply current for driving the display. The overhead current of power supply regulator  100  is dependent on its current output. The power supply regulator  100  has an input stage  115  which receives an input bias current from input bias current generator  110 , and an output stage  125  which receives an output bias current from output bias current generator  120 . The input stage  115  is configured to drive the output stage  125 , and the output stage  125  is configured to provide the load  130  with a sufficient current Iout at voltage Vout based on a reference voltage Vref. In some embodiments, the output voltage Vout is substantially equal to the reference voltage Vref. In some embodiments, the output voltage Vout is less than or greater than Vref. In the embodiment shown in  FIG. 4 , power supply regulator  100  is configured to source current Iout to load  130  at voltage Vout, where Vout is substantially equal to Vref. One advantageous aspect of this architecture is that it allows for the input and output stages to be powered from different power supplies. This allows for separate optimization of power for each stage. 
     The input stage  115  is configured to provide a signal to the output stage  125  based on the difference of the voltage Vout and the reference voltage Vref. The output stage  125  is configured to provide a current Iout to load  130  based on the signal received from the input stage  115 . 
     In the embodiment shown in  FIG. 4 , at least one of the input bias current generator  110  and the output bias current generator  120  is configured to generate a bias current based at least in part on the output current Iout. This feature is particularly advantageous because the power supply regulator  100  is configured to dynamically determine the bias current for either or both of the input stage and the output stage. One or more of the bias currents may be determined based at least in part on the current output Iout. At most, only a portion of the input and output bias current is provided to the load. Therefore, any bias current not provided to the load decreases efficiency. Dynamic determination of either or both of the bias currents based on the output current Iout provides for a particularly efficient voltage supply because large bias currents are generated only when large bias currents are needed. In some embodiments, the dynamic determination aspect may be selectably turned on or off. For example if the current load becomes less than a certain amount, the bias current can be supplied by a fixed source supplying small, but sufficient ibias. 
     In some embodiments, if insufficient current is available for the load, the voltage output Vout drops. In response, either or both of the input bias current generator  110  and the output bias current generator  120  modify the corresponding bias current based on difference between output voltage Vout and reference voltage Vref. 
     A relatively large difference between the output voltage Vout and the reference voltage Vref indicates that a larger bias current is necessary in at least one of the input stage  115  and the output stage  125 . Accordingly, when a relatively large difference between the output voltage Vout and the reference voltage Vref exists, either or both of the input bias current generator  110  and the output bias current generator  120  is configured to increase the bias current provided. Once either or both of the input bias current generator  110  and the output bias current generator  120  receives the increased bias current, they cooperatively provide an increased output current Iout. In response, the difference between the output voltage Vout and the reference voltage Vref will decrease. Once the difference between the output voltage Vout and the reference voltage Vref is sufficiently small, the at least one of the input stage  115  and the output stage  125  stops increasing its bias current and maintains its bias current at only slightly more than is sufficient to supply the load  130  with current sufficient to generate the acceptable output voltage Vout. 
     Similarly, a relatively small difference between the output voltage Vout and the reference voltage Vref indicates that a smaller bias current is sufficient in at least one of the input stage  115  and the output stage  125 . Accordingly, when a relatively small difference between the output voltage Vout and the reference voltage Vref exists, either or both of the input bias current generator  110  and the output bias current generator  120  is configured to decrease the bias current provided. Once either or both of the input bias current generator  110  and the output bias current generator  120  receives the decreased bias current, they cooperatively provide decreased output current Iout. In response, the difference between the output voltage Vout and the reference voltage Vref will increase. Once the difference between the output voltage Vout and the reference voltage Vref is sufficiently large, the at least one of the input stage  115  and the output stage  125  stops decreasing its bias current and maintains its bias current at only slightly more than is sufficient to supply the load  130  with current sufficient to generate the acceptable output voltage Vout. 
       FIG. 5A  shows one embodiment of input stage  150  which can be used in a power supply regulator such as that shown in  FIG. 4 . Input stage  150  has a differential amplifier  160  connected to buffer stage  170 . The buffer stage  170  produces an output signal which can be used as an input for an output stage, such as output stage  125  of  FIG. 4 . 
     Differential amplifier  160  is configured to receive a reference voltage Vref and a feedback voltage Vfb. In some systems, the feedback voltage Vfb may be generated based on the output voltage of the voltage supply regulator. The difference between the reference voltage Vref and the feedback voltage Vfb is amplified by differential amplifier  160 , which drives p-follower  152 . The output of p-follower  152  is the input signal for the output stage, and is also used to generate bias current ibias_buf, which is the bias current for the p-follower  152 . Bias current ibias_buf is generated by mirror transistor  154 , which mirrors the current in load transistor  156 . Diode connected load transistor  156  acts as a load for active transistor  158 . Accordingly, the differential amplifier  160  drives p-follower  152  with a voltage based on the difference between the voltage Vref and the feedback voltage Vfb. The p-follower  152  produces the input signal for the output stage, where the input signal also drives active transistor  158 , inducing a current therein. The induced current is sourced by load device  156 , and is mirrored by mirror transistor  154 . The mirrored current is the bias current ibias_buf for the p-follower  152 . Accordingly, when the input signal for the output stage is higher, the bias current for the p-follower  152  is higher. Similarly, when the input signal for the output stage is lower, the bias current for the p-follower  152  is lower. 
     In some embodiments, an additional current source (not shown) may also provide bias current for the p-follower  152 . The additional current source may provide an amount of bias current which depends on the output current of the regulator in a different way than the current of mirror  154 . In some embodiments, the additional current source provides current which is substantially independent of the output current of the regulator. For example, the additional current source may provide a substantially fixed current so that even if the current based on output current is very low, the bias current is at least equal to the current from the fixed additional current source. 
     Input stage  150  may be used to generate a signal Vo for an output stage, where the output stage is configured to generate an output voltage Vout based on the signal generated by the input stage  150 . Because the bias current of the p-follower device  152  is generated based at least in part on the difference between the reference voltage Vref and the feedback voltage Vfb, and because the feedback voltage Vfb is generated based on the output voltage Vout (which is based on the current output), the bias current of the p-follower device  152  is dependent on the current output of the supply voltage regulator. 
       FIG. 5B  shows another embodiment of an input stage  200  which can be used in a power supply regulator such as that shown in  FIG. 4 . Input stage  200  includes a differential pair formed by transistors XDPN and XDPP, a dynamic tail current generator formed by transistors XB 1  and XB 2 , diode connected load transistors XLN and XLP, mirror transistors XNM 1  and XNM 2 , positive current subtractor formed by transistors XPS 1 -XPS 3 , negative current subtractor formed by transistors XNS 1 -XNS 3 , and mirror transistors XNSM 1  and XNSM 2 . 
     The bias tail current generator dynamically generates a current for the differential pair. The total current of the tail current generator is provided to the differential pair transistors XDPN and XDPP, and is conducted by the transistors XDPN and XDPP to the load transistors XLN and XLP. Because the transistors XDPN and XDPP are connected as a differential pair, the current in each of the transistors XDPN and XDPP depends on the difference in the gate voltages Vfb and Vref of the transistors XDPN and XDPP, respectively. For example, if Vfb is lower than Vref, more current will go through XDPN than goes through XDPP. As will be seen, the dynamic bias tail current generation is based on the difference in the differential pair currents. When the difference in the differential pair currents is small, a minimum bias tail current is provided, and when the difference is larger, a larger bias tail current is provided. 
     Input stage  200  has a positive current subtractor formed by transistors XPS 1 -XPS 3 , which provides a bias voltage for bias tail current transistor XB 1 . Transistor XB 1  will provide a bias current to the differential pair which is mirrored from transistor XPS 3  of the positive current subtractor. Transistor XPS 3  sources an amount of current to transistor XPS 1  which depends on the difference in currents of XPS 1  and XPS 2 , according to the equation I XPS3 =I XPS1 −I XPS2 . The current in XPS 1  is mirrored from load transistor XLP, and is, therefore, dependent on the current in transistor XDPP of the differential pair. The current in XPS 2  is mirrored from load transistor XLN through mirror transistors XNM 2  and XNM 1 , and is, therefore, dependent on the current in transistor XDPN of the differential pair. The current in XPS 3  is, therefore, based on the difference between the currents in the differential pair, where if the current in XDPP is greater than the current in XDPN, the current in XPS 3  is a positive amount based on the magnitude of the difference. Accordingly, bias tail current transistor XB 1  provides a current to the differential pair based on the magnitude of the difference between the currents in the differential pair. Because XPS 3  cannot source a negative current, if the current in XDPP is less than the current in XDPN, XPS 3  sources zero current to transistor XPS 1 , and bias tail current transistor XB 1 , likewise sources zero current to the differential pair. 
     Input stage  200  has a negative current subtractor formed by transistors XNS 1 -XNS 3 , which provides a bias voltage for bias tail current transistor XB 2 . Transistor XB 2  will provide a bias current to the differential pair which is mirrored from transistor XNS 3  of the negative current subtractor through mirror transistors XNSM 1  and XNSM 2 . Transistor XNS 3  sinks an amount of current from transistor XNS 2  which depends on the difference in currents of XNS 2  and XNS 1 , according to the equation I XNS3 =I XNS2 −I XNS1 . The current in XNS 1  is mirrored from load transistor XLP, and is, therefore, dependent on the current in transistor XDPP of the differential pair. The current in XNS 2  is mirrored from load transistor XLN through mirror transistors XNM 2  and XNM 1 , and is, therefore, dependent on the current in transistor XDPN of the differential pair. The current in XNS 3  is, therefore, based on the difference between the currents in the differential pair, where if the current in XDPN is greater than the current in XDPP, the current in XNS 3  is a positive amount based on the magnitude of the difference. Accordingly, bias tail current transistor XB 3  provides a current to the differential pair based on the magnitude of the difference between the currents in the differential pair. Because XNS 3  cannot sink a negative current, if the current in XDPN is less than the current in XDPP, XNS 3  sinks zero current from transistor XNS 2 , and bias tail current transistor XB 3 , likewise sources zero current to the differential pair. 
     In some embodiments, an additional current source XB 0  may also provide bias current for the differential pair. The additional current source XB 0  may provide an amount of bias current which depends on the output current of the regulator in a different way than the current of bias tail current transistors XB 1  and XB 2 . In some embodiments, the additional current source XB 0  provides current which is substantially independent of the output current of the regulator. For example, the additional current source XB 0  may provide a substantially fixed current so that even if the current based on output current is very low, the bias current is at least equal to the current from the additional current source XB 0 . 
     Input stage  200  may be used to generate a differential signal (Vop−Von) for an output stage, where the output stage is configured to generate an output voltage Vout based on the signal generated by the input stage  200 . Because the bias tail current of the differential pair is generated based at least in part on the difference between the reference voltage Vref and the feedback voltage Vfb, and because the feedback voltage Vfb is generated based on the output voltage Vout (which is based on the current output), the bias tail current of the differential pair is dependent on the current output of the supply voltage regulator. 
       FIG. 6A  shows an embodiment of an output stage  250  which can be used in a power supply regulator such as that shown in  FIG. 4 . Output stage  250  includes signal transistor XS, bias transistor XB, mirror transistor XM, and an operational transconductance amplifier OTA. 
     The signal transistor XS receives an input signal (from, for example, the input stage of  FIG. 4 ) and sinks a current according to the received signal. When the output stage  250  is used in a power supply regulator such as that shown in  FIG. 4 , the bias transistor XB sources a bias current for the signal transistor XS and for an output current for the load, where the output current is the current sourced by the bias transistor XB minus the current sunk by the signal transistor XS. The power supply regulator operates by modifying the input signal such that if more current is needed for the load, the signal transistor sinks less current, leaving more for the load. Similarly, if less current is needed for the load, the input signal is modified such that the signal transistor sinks more current, leaving less for the load. 
     The bias transistor XB sources the bias current based on a reference current mirrored from the OTA through mirror transistor XM. In this embodiment, the OTA generates a current based on the difference between a reference voltage Vref and a feedback voltage Vfb. Because Vfb is generated based on the voltage output of the power supply regulator, the difference between the reference voltage Vref and the feedback voltage is related to the current output of the power supply regulator. Accordingly, the bias current of the output stage  250  is based at least in part on the current output of the power supply regulator. The adjustment of the current allows for the bias transistor XB to provide large amounts of current when needed, and to provide less current when less is sufficient. In addition, because of the dynamic control of the bias current, the transistor XB can be smaller than what would otherwise be required to provide the large currents. The smaller size results in better power and area efficiency of the circuit. 
     In some embodiments, the output of the regulator is targeted to be the dominant pole. Accordingly, the poles associated with the bias current control must lie at relatively high frequencies to achieve good phase margin. This may be achieved, for example, by using current mode control so that all nodes associated with the bias control have relatively low impedance. Following this principle, the OTA of  FIG. 6A  produces an output current which is proportional to the difference between the regulator output and the target regulation level. In some embodiments, the OTA operates at a low voltage supply to reduce power consumption. 
     In some embodiments, an additional current source (not shown) may also provide bias current for the signal transistor XS and for the output current for the load. The additional current source may provide an amount of bias current which depends on the output current of the regulator in a different way than the current of bias transistor XB. In some embodiments, the additional current source provides current which is substantially independent of the output current of the regulator. For example, the additional current source may provide a substantially fixed current so that even if the current based on output current is very low, the bias current is at least equal to the current from the fixed additional current source. 
       FIG. 6B  shows another embodiment of an output stage  300  which can be used in a power supply regulator such as that shown in  FIG. 4 . Output stage  300  includes signal transistor XS, bias input transistor XBIN, mirror transistor XM, and bias transistor XB. 
     The signal transistor XS receives an input signal (from, for example, the input stage of  FIG. 4 ) and sinks a current according to the received signal. When the output stage  300  is used in a power supply regulator such as that shown in  FIG. 4 , the bias transistor XB sources a bias current for the signal transistor XS and for an output current for the load, where the output current is the current sourced by the bias transistor XB minus the current sunk by the signal transistor XS. The power supply regulator operates by modifying the input signal such that if more current is needed for the load, the signal transistor XS sinks less current, leaving more for the load. Similarly, if less current is needed for the load, the input signal is modified such that the signal transistor XS sinks more current, leaving less for the load. 
     The bias transistor XB sources the bias current based on a reference current mirrored from the bias input transistor XBIN through mirror transistor XM. In some embodiments, the input for the bias input transistor XBIN is generated by the power source regulator based on the current sourced to the load. For example, in some embodiments, the input for the bias input transistor XBIN is based on the difference between a voltage based on an output voltage of the regulator and a reference voltage. Because the input for the bias input transistor XBIN is generated based on the current output of the power supply regulator, the bias current of the output stage  300  is based at least in part on the current output of the power supply regulator. 
     In some embodiments, an additional current source (not shown) may also provide bias current for the signal transistor XS and for the output current for the load. The additional current source may provide an amount of bias current which depends on the output current of the regulator in a different way than the current of bias transistor XB. In some embodiments, the additional current source provides current which is substantially independent of the output current of the regulator. For example, the additional current source may provide a substantially fixed current so that even if the current based on output current is very low, the bias current is at least equal to the current from the fixed additional current source. 
       FIG. 6C  shows yet another embodiment of an output stage  350  which can be used in a power supply regulator such as that shown in  FIG. 4 . Output stage  350  includes signal transistor XS, bias input transistor XBIN, bias reference transistor XB 0 , mirror transistors XM 1  and XM 2 , and bias transistor XB. 
     The signal transistor XS receives an input signal and sinks a current according to the received signal. When the output stage  350  is used in a power supply regulator such as that shown in  FIG. 4 , the bias transistor XB sources a bias current for the signal transistor XS and for an output current for the load, where the output current is the current sourced by the bias transistor XB minus the current sunk by the signal transistor XS. The power supply regulator operates by modifying the input signal such that if more current is needed for the load, the signal transistor XS sinks less current, leaving more for the load. Similarly, if less current is needed for the load, the input signal is modified such that the signal transistor XS sinks more current, leaving less for the load. 
     The bias transistor XB sources the bias current based on a reference current mirrored from the bias reference transistor XB 0  through mirror transistors XM 1  and XM 2 . The current in the bias reference transistor XB 0  is equal to the current sourced by current reference IREF which is not sunk by the bias input transistor XBIN. In this embodiment, the input for the bias input transistor XBIN is the same as the input for the signal transistor XS, and is generated by the power source regulator based on the current sourced to the load. For example, in some embodiments, the input for the bias input transistor XBIN and for the signal transistor XS is based on the difference between a voltage based on an output voltage of the regulator and a reference voltage. Because the input for the bias input transistor XBIN is generated based on the current output of the power supply regulator, the bias current of the output stage  350  is based at least in part on the current output of the power supply regulator. 
     In some embodiments, an additional current source (not shown) may also provide bias current for the signal transistor XS and for the output current for the load. The additional current source may provide an amount of bias current which depends on the output current of the regulator in a different way than the current of bias transistor XB. In some embodiments, the additional current source provides current which is substantially independent of the output current of the regulator. For example, the additional current source may provide a substantially fixed current so that even if the current based on output current is very low, the bias current is at least equal to the current from the fixed additional current source. 
       FIG. 7  shows an embodiment of a power supply regulator  400  configured to source a supply current for the load, and to generate both an input bias current and an output bias current based at least in part on the current output of the regulator. Power supply regulator  400  has an input stage  410 , an output stage  420  and a feedback stage  430 . Input stage  410  is similar to input stage  200  of  FIG. 5B , output stage  420  is similar to output stage  300  of  FIG. 6B . 
     In this embodiment, the output stage  420  is supplied by power supply voltage VPHV and the input stage  410  is supplied by power supply voltage VDDA. Because in some embodiments the input stage  410  can operate at a lower supply voltage, VDDA may be less than VPHV. This allows the input stage  410  to operate with lower power consumption. In some embodiments, the output stage also operates at a lower supply voltage. In some embodiments, the output stage can be configured to selectably operate with VPHV when the current output of the regulator is high and to operate with VDDA when the current output of the regulator is below a threshold. 
     Feedback stage  430  is a switched capacitor divider circuit which is configured to be programmed with a division factor. In this embodiment, feedback stage  430  takes the voltage output of the power supply regulator  420  and divides it according to its programming. With this configuration, the output voltage will be substantially equal to the division factor times the reference voltage Vref. 
       FIG. 8  shows an embodiment of a power supply regulator  350  configured to source a supply current for the load, and to generate both an input bias current and an output bias current based at least in part on the current output of the regulator. Power supply regulator  350  has an input stage  360 , an output stage  370  and a feedback stage  380 . Input stage  360  is similar to input stage  150  of  FIG. 5A , and output stage  370  is similar to output stage  250  of  FIG. 6A , and feedback stage  380  is similar to feedback stage  430  of  FIG. 7 . 
     Although shown as separate devices in this schematic, some embodiments integrate one or more portions of power supply regulator  350  with different architectures. For example, the OTA of the output stage  370  may be integrated with the amplifier of the input stage  360  to achieve better performance matching between the two amplifiers. 
     As shown, the amplifier  355  drives an N-type pull-down device  359  of the output stage  370  through a P source follower  357 . Since the amplifier is driving an N pull-down device  359 , its output can swing over a limited range. This allows for a lower supply voltage for the amplifier, resulting in lower power consumption. 
     The P-type source follower  357  serves at least two purposes. First, it provides a buffer to the output of the amplifier and thus enables the use of a high gain amplifier without introducing a low frequency pole. Second, it level-shifts up the output of the error amplifier, thus providing additional overdrive to the N pull-down device  359 . In the embodiment shown in  FIG. 8 , the amount of the level-shift is a function of the pull-down current by feeding back current into the source follower through P device  361 . Thus, the level-shift is larger when the regulator sinking current is larger. This helps reduce the required size of the N pull-down device. 
     While the above detailed description has shown, described, and pointed out novel features as applied to various embodiments, it will be understood that various omissions, substitutions, and changes in the form and details of the device or process illustrated may be made by those skilled in the art without departing from the spirit of the invention. As will be recognized, the present invention may be embodied within a form that does not provide all of the features and benefits set forth herein, as some features may be used or practiced separately from others.