Abstract:
The invention specifies a circuit arrangement for voltage regulation in which, in addition to a control loop having a comparator ( 4 ), an output stage ( 3 ) and a feedback path, an auxiliary regulator ( 11–14 ) is provided which limits the voltage drop across the output stage ( 3 ) and, for this purpose, comprises a control element ( 11 ) and a further comparator ( 13 ). Hence, the output stage ( 3 ) of the voltage regulator may advantageously have a withstand voltage which is lower than the supply voltage which can be supplied at the input ( 1 ). On account of its good supply voltage suppression, the voltage regulator described is particularly well suited to supplying on-chip VCOs.

Description:
REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of PCT/DE03/00860, which was not published in English, filed on Mar. 17, 2003, that claims the benefit of the priority date of German Patent Application No. DE 102 15 084.5, filed on Apr. 5, 2002, the contents of which are both herein incorporated by reference in their entirety. 

   FIELD OF THE INVENTION 
   The present invention relates to a circuit arrangement for voltage regulation. 
   BACKGROUND OF THE INVENTION 
   Ever larger scales of integration in integrated circuits are normally accompanied by a constant decrease in the supply voltage for the integrated circuits. In this context, certain integration technologies may use semiconductor components with different supply voltages. By way of example, CMOS (Complementary Metal Oxide Semiconductor) production techniques use transistors for designing analog circuits, particularly for forming interfaces for the integrated circuit, with a comparatively high withstand voltage in addition to transistors which are suitable for designing digital circuits and have a significantly lower withstand voltage. 
   In order to supply integrated circuits, which need various supply voltages internally, with just one external supply voltage, there is normally an “on-chip” voltage regulator, which is usually in the form of a continuously operating linear regulator. In this case, such voltage regulators should be able to manage without external inductances or capacitances. 
   Particularly when actuating resonant circuits, for example in order to generate radio-frequency carrier signals, a voltage regulator whose output voltage has good supply voltage suppression (Power Supply Rejection Ratio, PSRR) and at the same time has low inherent noise is desired in order to supply the voltage controlled oscillators which are normally provided in that case, so as not to impair the phase noise in the oscillator which is to be powered. 
   The document V. R. von KAENEL, A high-speed, low-power clock generator for a microprocessor application, IEEE Journal of Solid-State Circuits, Vol. 33, No. 11, November 1998 specifies a phase locked loop in a clock generator, which phase locked loop shows a generalized illustration of a voltage regulator using a circuit diagram, cf. FIG. 2 therein. 
   The paper G. W. Den Besten, B. Nauta, Embedded 5 V-to-3.3 V Voltage Regulator for Supplying Digital ICs in 3.3 Volt CMOS Technology, IEEE Journal of Solid-State Circuits, Vol. 33, No. 7, July 1998 specifies a voltage regulator for converting an input voltage of 5 volts into an output voltage of 3.3 volts in CMOS circuitry. FIGS. 2 a  and 2 b  show ordinary voltage regulators which use either a P-channel MOS transistor, cf. FIG. 2 a , or an N-channel MOS transistor, cf. FIG. 2 b , as regulating transistor. The gate connection of the regulating transistor is respectively actuated by a difference amplifier, to which firstly a reference voltage, which is provided by a bandgap circuit, for example, and secondly a signal derived from the regulator&#39;s output voltage are supplied. Although the circuit variant with the PMOS transistor provides a large voltage control range, it has the drawback of inadequate supply voltage suppression at frequencies above the amplifier bandwidth. Although the regulator circuit with the NMOS transistor exhibits good PSRR properties, it has a relatively low achievable output voltage. 
   If the regulating transistor in a circuit in line with FIG. 2 a  or 2 b  in the latter document is equipped with a withstand voltage which is lower than the input voltage of the voltage regulator, then, particularly in the case of a resistive-capacitive load mixture, turning on the voltage regulator may result in a voltage drop across the regulating transistor which is larger than its admissible voltage. If the difference amplifier used, which actuates the regulating transistor, is a bandgap voltage source, whose voltage first needs to build up starting at 0 volt, then even the full input voltage is applied across the regulating transistor at the instant-of turning on. 
   SUMMARY OF THE INVENTION 
   The following presents a simplified summary in order to provide a basic understanding of one or more aspects of the invention. This summary is not an extensive overview of the invention, and is neither intended to identify key or critical elements of the invention, nor to delineate the scope thereof. Rather, the primary purpose of the summary is to present one or more concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later. 
   The present invention provides a circuit arrangement for voltage regulation which is integratable and in which a regulating transistor used may be a transistor whose withstand voltage is lower than the input voltage which powers the voltage regulator. 
   A circuit arrangement for voltage regulation in accordance with an aspect of the present invention includes:
         an input connection for supplying a supply voltage,   an output connection for tapping off an output voltage,   an output stage which comprises a control input and a controlled path having a first and a second load connection, the first load connection being coupled to the input connection of the circuit arrangement, and the second load connection being connected to the output connection of the circuit arrangement and being coupled to a reference potential connection via an electrical load,   a reference generator which provides a reference potential at its output,   a comparator which comprises a first input, which is connected to the reference generator, and which comprises a second input, which is coupled to the output connection of the circuit arrangement, for regulating the output voltage, and,   an auxiliary regulator for limiting the voltage drop across the output stage, comprising a control element which is connected firstly to the input connection and secondly to the controlled path of the output stage at a circuit node and comprising a further comparator having a nominal value input, which is coupled to the control input of the output stage, and having an actual value input, which is coupled to the circuit node.       

   The auxiliary regulator is used, particularly at the moment at which the voltage regulator is turned on, to limit the voltage drop across the output stage to an admissible level. It is thus possible for the output stage to be advantageously produced with semiconductor components whose withstand voltage is lower than the supply voltage which can be supplied at the input connection. 
   This is based on the present principle of the auxiliary regulator limiting the voltage between the circuit node at one of the load connections of the controlled path of the output stage and the control input of the output stage to a maximum voltage magnitude, for example 0.5 volt. However, as soon as the voltage at the control input of the output stage exceeds a particular value, the control element forms a closed switch between the input connection and the circuit node on the controlled path of the output stage, so that the output stage&#39;s control range is not reduced during normal operation. 
   To actuate the control element in the auxiliary regulator using the further comparator, there is a voltage source, such as a “floating battery”, which is connected between one of the inputs of the further comparator in the auxiliary regulator and the control input of the output stage in the circuit arrangement. 
   This thus causes the potential on the nominal value input of the further comparator in the auxiliary regulator to be higher than the potential on the control input of the output stage by a definable voltage magnitude. This sets the voltage on the voltage node of the circuit arrangement such that it is fundamentally the same as the sum of the voltage on the control input of the output stage and the floating battery voltage. In this context, in line with the present principle, the restriction applies that the auxiliary regulator is automatically used to limit the voltage on the circuit node to the supply voltage as soon as the voltage on the control input of the output stage exceeds that voltage value which is obtained from the difference between the supply voltage and the fixed voltage magnitude provided by the floating battery. In this case, the auxiliary regulator represents a short circuit, that is to say a closed switch, for its output stage. 
   In line with another aspect of the present invention, an output stage and/or a control element in the auxiliary regulator are respectively in the form of MOS transistors. 
   In this case, the MOS transistor in the output stage is preferably provided as a MOS transistor which is designed for a low withstand voltage and a regular threshold voltage. The MOS transistor in the auxiliary regulator&#39;s control element is preferably in the form of a transistor in which the conductivity type of the channel is complementary to that of the output stage, but has a higher withstand voltage than the transistor in the output stage. In this context, the gate connection respectively represents the control input of the control element or output stage, while the source and drain connections of the MOS transistors respectively represent the connections of the controlled paths. 
   To form the feedback signal which is derived from the output voltage and is supplied to the comparator which actuates the output stage, a suitable/circuit component, such as a voltage divider, is employed. The design of this is dependent firstly on the desired output voltage and secondly on the voltage which the reference generator delivers at its output. In the case of bandgap reference sources produced in silicon technology, this bandgap voltage is normally 1.2 volts. 
   The circuit arrangement&#39;s comparator, which actuates the output stage of the regulator, and the further comparator in the auxiliary regulator are respectively in the form of differential amplifier or operational amplifier, which respectively comprise an inverting input and a noninverting input. 
   In this case, that differential amplifier which actuates the output stage is advantageously preferably designed such that its output signal can be controlled up to the positive supply voltage. 
   A further improvement in the suppression of disturbances on the supply voltage can be achieved by developing the circuit arrangement with a further control loop which supplies the reference generator. 
   This involves forming a further control loop which comprises a control element, a comparator and a feedback path from the control element to the comparator via a voltage divider. In this case, an output on the control element is coupled to a supply connection on the reference generator. Advantageously, this additional auxiliary voltage can also supply the floating battery, and also the comparator which actuates the output stage and the further comparator, which is provided in the auxiliary regulator. 
   In this case, there is a switch which can change over the voltage to be supplied to the reference generator for the purpose of powering it between the actual supply voltage for the regulating circuit and the auxiliary voltage generated. 
   To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative aspects and implementations of the invention. These are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is explained in more detail below using a plurality of exemplary embodiments with reference to the drawing, in which: 
       FIG. 1  shows a simplified circuit diagram in accordance with an aspect of the present invention. 
       FIG. 2  uses a circuit diagram to show an exemplary embodiment of a fixed-value voltage source in line with  FIG. 1  in accordance with an aspect of the present invention. 
       FIG. 3  shows a development of the voltage regulator from  FIG. 1  with a further control loop in accordance with an aspect of the present invention. 
       FIG. 4  shows an exemplary diagram of the operational amplifier shown in  FIGS. 1 and 3  in CMOS circuitry in accordance with an aspect of the present invention. 
       FIG. 5  uses a graphical representation to show the voltage profiles of selected node voltages in the circuit from  FIG. 1  as a function of the supply voltage in accordance with an aspect of the present invention. 
       FIG. 6  shows the turn-on behaviour of the circuit from  FIG. 1  with a disturbance overlaid on the supply voltage in accordance with an aspect of the present invention. 
       FIG. 7  shows an enlarged detailed illustration of the graphical representation from  FIG. 6  in accordance with an aspect of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention will now be described with respect to the accompanying drawings in which like numbered elements represent like parts. The figures provided herewith and the accompanying description of the figures are merely provided for illustrative purposes. One of ordinary skill in the art should realize, based on the instant description, other implementations and methods for fabricating the devices and structures illustrated in the figures and in the following description. 
     FIG. 1  shows a circuit arrangement for voltage regulation with an input connection  1  for supplying a supply voltage of 2.5 volts and an output connection  2  for tapping off a regulated output voltage of 1.5 volts in accordance with an aspect of the present invention. The output stage provided is an N-channel MOS field effect transistor  3  having a gate connection, a source connection and a drain connection. The source connection of the output stage  3  forms the output connection  2  of the circuit. The gate connection is connected to the output of a differential amplifier  4 , which operates as a comparator and has an inverting input and a noninverting input. The noninverting input of the comparator  4  is connected to the output of a bandgap reference generator  5  which provides a bandgap voltage of 1.2 volts. The output  2  of the circuit is connected to the inverting input of the comparator  4  via a voltage divider  6 ,  7 , comprising a series circuit made up of a 300 ohm resistor  6  and a 1.2 kilo ohm resistor  7 , and is also coupled to a reference potential connection  8  to which the bandgap generator  5  is also connected. Connected between the output connection  2  and the reference potential connection  8  of the regulating circuit shown in  FIG. 1  there are also a resistor  9  and, in parallel therewith, a capacitance  10 , which represent a resistive-capacitive load. 
   In line with the present principle, to protect the output stage  3  from the relatively high input voltage or supply voltage when the regulator is turned on, a further transistor  11  is provided which is in the form of a P-channel MOS field effect transistor and whose drain connection is connected to the drain connection of the output stage  3  at a circuit node  12  in the regulator. The source connection of the transistor  11  is connected to the input connection  1  of the regulator circuit. To form the auxiliary regulator, a further differential amplifier  13  is also provided, whose output is connected to the gate connection of the transistor  11  operating as a control element. The noninverting input of the further comparator  13  is connected to the circuit node  12 , while the inverting input of the further comparator  13  operating as an operational amplifier is connected to the output of the comparator  4  via a floating battery  14 . The connections of the floating battery have been provided with the reference symbols  15  and  16 . 
   The floating battery raises the potential on the gate of the output stage  3  by 0.5 volt and supplies this voltage of increased potential to the inverting input of the comparator  13 . While the withstand strength of the NMOS output transistor  3  is merely 1.5 volts, the PMOS transistor  11  has a withstand voltage of 2.5 volts. 
   The regulating transistor  3  operates as a source follower, where the source voltage follows the gate voltage. The auxiliary regulator, whose control element  11  is connected to the drain path of the regulating transistor  3 , causes the drain connection  12  of the regulating transistor  3  to be no more than 0.5 volt above its gate voltage. This is done through the feedback actuation of the amplifier  13  and of the floating battery voltage of approximately 0.5 volt. The voltage on the circuit node  12  is set by means of the differential amplifier  13  such that it is essentially equal to the sum of the voltage on the gate connection of the transistor  3  and the floating battery voltage of 0.5 volt. In this case, however, the voltage on the circuit node  12  is automatically limited to 2.5 volts, namely to the supply voltage, as soon as the voltage on the gate of the transistor  3  exceeds the value 2 volts. This is because the transistor  11  represents a closed switch in this case. 
   The linear regulator described provides a significantly improved PSSR (Power Supply Rejection Ratio). While the regulator is turning on, that is to say while the regulating voltage is running up, the additional auxiliary regulator  11 ,  12 ,  13 ,  14  protects the output transistor  3 , which has a withstand voltage of only 1.5 volts, from an overvoltage, which would otherwise be present immediately between its drain connection and its gate connection when it turns on. 
   In line with the principle described, the positive supply voltage for the NMOS regulating transistor  3  with a withstand voltage of 1.5 volts is held, during the turn-on operation, at a value which is no more than 0.5 volts above its gate voltage. This effectively prevents breakdown in the regulating transistor  3 . In this case, although the relatively thin gate oxide layer and the relatively short channel of the transistor  3  result in a low withstand capability for its gate-source voltage of just 1.5 volts, they permit the desired, good PSSR, which, in particular, permits highly sensitive, voltage controlled oscillators to be supplied with voltage, such as are needed in resonance circuits, particularly in mobile radios. 
   Whereas the transistor  3  is a transistor having a conventional threshold voltage, the transistor  11  is designed for analog circuitry and has a corresponding threshold voltage. 
   The voltage source  14  may alternatively also be in the form of a level shifter circuit. 
     FIG. 2  shows the floating battery  14  from  FIG. 1 , whose output connection  15  provides a voltage which is always 0.5 volt above the voltage applied to its input  16  in accordance with another aspect of the invention. In this case, the output voltage from the voltage source  14  is precisely the magnitude of the threshold voltage of the PMOS transistor  17  above the input voltage on the node  16 . In this arrangement, the gate connection of the transistor  17  is connected to the input  16 , and its controlled path connects a reference potential connection  18  to the output connection  15 . The transistor  17  is connected as a source follower and is powered by a BIAS current source  19 , which is connected to the reference potential connection  18 , via a current mirror  20 . The current mirror  20  comprises two further PMOS transistors, whose gates are connected to one another and which are connected to the supply potential connection  21  of the voltage source  14  by a respective connection on their controlled paths. The input transistor of the current mirror  20  is connected to the diode in this arrangement. 
   In the present aspect, the transistor  17  has a threshold voltage of 0.5 volt. Threshold voltages for PMOS transistors in the range between 0.5 and 0.7 volt are usual. 
     FIG. 3  shows a development of the voltage regulator arrangement from  FIG. 1  in accordance with yet another aspect of the present invention. This voltage regulator arrangement brings about a further improvement in the scatter of interference by virtue of an additional control loop which provides an additional, regulated supply voltage for the reference generator  5  and for the differential amplifiers  4 ,  13 . 
   The design and advantageous action of the circuit shown in  FIG. 3  largely correspond to those of the circuit shown in  FIG. 1  and will therefore not be repeated at this juncture. The text below merely outlines the added components, their interconnection and the additional functionality with its advantages. 
   In line with  FIG. 3 , there is a further control loop with a control element, in the form of a transistor  22  of the P-channel type, having a control input and a controlled path, the control input, that is to say the gate connection of the transistor  22 , being connected to the output of a comparator  23  which is in the form of a differential amplifier. The comparator  23  is connected to the input connection  1  in order to be supplied with voltage. The source connection of the control element transistor  22  is likewise connected to the input connection  1 , while the drain connection forms the output  24  of the further control loop. At this output, a regulated voltage of 2.25 volts is provided. Connected between the output  24  of the further control loop and reference potential connection  8 , to form a voltage divider, is a series circuit comprising a resistor  25  of 1.05 kilo ohms and a resistor  26  of 1.2 kilo ohms whose tap point is connected to the inverting input of the differential amplifier  23  for the purpose of tapping off a divided voltage. The noninverting input of the differential amplifier  23  is connected to the output connection  15  of the bandgap reference generator  5  in order to supply the bandgap voltage at the level of 1.2 volts constant. The output  24  of the further control loop for supplying the reference generator  5  with voltage is connected to a respective connection for supplying a supply voltage for the floating battery  14 , for the differential amplifier  13  and for the differential amplifier  4 . In addition, a changeover switch  27  is used to set up a connection to the input connection  16  of the reference generator  5 . A further input on the changeover switch  27  is connected to the input connection  1  of the circuit for voltage regulation. The changeover switch  27  has, for the purpose of supplying a changeover command, a control input to which the output of a comparator  28  is connected. The comparator  28  has two inputs which are connected firstly to the output  24  of the further control loop and secondly to the output  15  of the reference generator  5 . 
   The PMOS regulating transistor  22  is used to provide a regulated voltage having the highest possible voltage level. The comparator  28  in conjunction with the voltage changeover switch  27  make it possible for turning on the supply voltage on the connection  1  to involve this voltage first supplying the reference generator  5  and later, when the auxiliary voltage which can be tapped off at the output  24  has run up, being changed over to said auxiliary voltage. This means that the scatter of interference, particularly on the reference generator  5  and the amplifier  4 , can be reduced further, so that the quality of the voltage which can be tapped off at the output connection  2 , and which is regulated, is improved further. 
     FIG. 4  shows an exemplary two-stage operational amplifier designed using CMOS circuitry that can be employed in the arrangements shown in  FIGS. 1 and 3  in accordance with an aspect of the present invention. In  FIG. 1 , the operational amplifiers  4  and  13 , and, in  FIG. 3 , additionally, the operational amplifier  23 , are in the form of two-stage operational amplifiers, as shown in  FIG. 4 . 
   The operational amplifier shown in  FIG. 4  has an inverting input  30 , a noninverting input  31  and an output  32 . In addition, auxiliary inputs  33 ,  34  are provided. The operational amplifier is connected between a supply potential connection  35  and a reference potential connection  36 . While a changeover command for putting the operational amplifier into a quiescent state (power down) can be supplied at the auxiliary input  33 , a quiescent or biasing current (BIAS) can be supplied at the connection  34 . The operational amplifier is designed as a differential amplifier and its output  32  provides a signal which is dependent on the voltage difference between the signals applied to the inputs  30 ,  31 . The operational amplifier  34  is a two-stage design and is equipped with a Miller compensation element for stabilizing the frequency response. 
   Of significance for the operational amplifier shown in  FIG. 4  when used in a circuit for voltage regulation as shown in  FIGS. 1 and 3  is the fact that the signal which can be tapped off at the output  32  can be controlled almost up to the positive supply voltage which is supplied at the supply connection  35 . In the two-stage operational amplifier, this is achieved by virtue of the two complementary output transistors  37 ,  38  in the output stage of the operational amplifier both being actuated using a signal which is dependent on the input difference voltage. 
     FIG. 5  uses a graphical representation to show a turn-on operation in the voltage regulator from  FIG. 1 . In this case, various voltage levels A, B, C, D are shown as a function of the bandgap voltage provided by the reference generator  5 . This bandgap voltage has been run up from 0 volt to its rated value of 1.2 volts in the graphical representation in  FIG. 5  for simulation purposes. The fundamentally constant voltage difference of approximately 0.5 volt between the level A at the input of the output stage and the level B at the circuit node  12  is clearly visible only when the supply voltage no longer allows this voltage difference does the signal level B remain constant, while the level C (which describes the voltage value at the control input of the control element  11  in the auxiliary regulator) changes to 0 volt. The full supply voltage is thus applied to the gate of the transistor. The curve D describes the regulated voltage at the output  2  of the circuit. The graphical representation shown in  FIG. 5  accordingly makes use of the effective limitation of the voltage drop across the output stage  3 . 
     FIG. 6  shows the profile of the supply voltage from 0 volt up to 2.5 volts and back again over the time axis t for the circuit arrangement in  FIG. 1 . In this case, the supply voltage has interference overlaid on it with an amplitude of 100 mV. It can be seen that, for the regulated output voltage D, the value of this interference has been reduced to 1 mV. The graphical representation in  FIG. 6  thus shows the good PSRR properties, that is to say the good suppression of interference on the supply voltage, which is brought about by the present principle of voltage regulation. 
   This is further illustrated from the illustration in  FIG. 7 , which shows an enlargement of a detail from the graphical representation in  FIG. 6  with higher resolution on the time axis. 
   Although the invention has been shown and described with respect to a certain aspect or various aspects, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described components (assemblies, devices, circuits, etc.), the terms (including a reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component which performs the specified function of the described component (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiments of the invention. In addition, while a particular feature of the invention may have been disclosed with respect to only one of several aspects of the invention, such feature may be combined with one or more other features of the other aspects as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the term “includes” is used in either the detailed description or the claims, such term is intended to be inclusive in a manner similar to the term “comprising.” 
   LIST OF REFERENCE SYMBOLS 
   
       
         1  Input connection 
         2  Output connection 
         3  Output stage 
         4  Comparator 
         5  Reference generator 
         6  Resistor 
         7  Resistor 
         9  Resistive load 
         10  Capacitive load 
         11  Control element 
         12  Circuit nodes 
         13  Further comparator 
         14  Fixed-value voltage source 
         15  Output 
         16  Input 
         17  Transistor 
         18  Reference potential connection