Patent Publication Number: US-6992472-B2

Title: Circuit and method for setting the operation point of a BGR circuit

Description:
PRIORITY 
   This application is a continuation of pending international application PCT/DE03/02147, filed on Jun. 27, 2003, which claims the benefit of priority to German Patent Application DE 102 37 122.9, filed Aug. 13, 2002, both which are herein incorporated by reference in their entirety. 

   FIELD 
   The present application relates to a circuit and a method by means of which the operating point of a BGR circuit can be set. 
   BACKGROUND 
   Circuits that generate a constant output voltage independent of fluctuations in temperature and supply voltage are required in multifarious ways in semiconductor circuit engineering. They are used across the board in analog, digital and mixed analog/digital circuits. A frequently used type of such circuits are the so-called BGR (Bandgap Reference) circuits. 
   The basic principle of a BGR circuit is to add two partial signals (voltages or currents) that exhibit an opposite temperature characteristic. Whereas one of the two partial signals drops with increasing temperature, the other partial signal rises with increasing temperature. An output voltage that is temperature-constant over a certain range is then derived from the sum of the two partial signals. The output voltage of a BGR circuit is also denoted as reference voltage below in accordance with customary usage. 
   A stable operating point of a BGR circuit is situated at a Bandgap voltage of 1.211 V. This reference voltage can be converted into yet other voltages by means of a voltage divider. A BGR circuit can have a further stable operating point at 0 V depending on the offset of an operational amplifier used for the BGR circuit and on leakage current. Situated between the two stable operating points is an unstable operating point. This unstable operating point is in the vicinity of 0 V in the case of small leakage currents and small offset voltages. When starting the BGR circuit, the BGR circuit must be brought from the stable operating point at 0 V to the higher stable operating point that is derived from the Bandgap voltage of 1.211 V. An additional start-up circuit is generally used for this purpose. 
   In order to set the higher operating point in the BGR circuit, an external setting current is frequently fed into the BGR circuit. This setting current must be switched off completely during normal operation of the BGR circuit. 
   During the introduction of new technologies, which are not stable at high volumes, the unstable operating point can be displaced by several 100 mV toward positive voltages because of impaired offset and leakage current properties. If the switch-off point of the external setting current is subjected to high fluctuations because of a strong dependence on process and matching, the switch-off points must be selected to be so low when developing the BGR circuit that the BGR circuit is not influenced by the setting current during normal operation. However, a low switch-off point can lead to problems in the BGR circuit, since it may be that the unstable operating point is reached instead of the higher stable operating point. 
   Therefore, when setting the higher stable operating point the starting performance of the BGR circuit is monitored so that the switch-off point of the setting current can be determined as accurately as possible. Two modes of procedure are known for this purpose. Firstly, the output voltage of the BGR circuit can be monitored. Secondly, the current in a BGR cell can be measured. 
   The determination of the current through the BGR cell has proved to be the better of the two modes of procedure, since the switch-off point can be set to 1/100, 1/10 or ½ of the operating current of the BGR cell. The switch-off point is set to ¼ of the operating current of the BGR cell in order to design as robustly as possible a circuit that serves for setting the operating point of the BGR circuit and for subsequently switching off the setting current. 
   When connecting a resistive load to the BGR circuit, it is to be ensured that a large portion of the output current flows into the load and not through the BGR cell. Consequently, the output current of the BGR circuit is not suitable in this case for determining the current in the BGR cell. 
   A BGR circuit with a setting circuit for setting the operating point of the BGR circuit is described in European patent application EP 1 063 578 A1. For this purpose, the reference voltage generated by the BGR circuit is compared with a voltage that is situated in a voltage range between the desired operating point and a metastable operating point. Other BGR circuits with associated setting circuits for setting the operating point of the BGR circuit are to be found in US patents U.S. Pat. No. 5,087,830 A, U.S. Pat. No. 6,346,848 B1 and U.S. Pat. No. 5,867,013 A. 
     FIG. 1  illustrates a known BGR circuit  1  and setting circuit  2 . The BGR circuit  1  has an operational amplifier OP 1 , resistors R 1 , R 2 , R 3  and R 4 , and diodes D 1  and D 2 . Here, resistors R 1 , R 2  and R 3  as well as the diodes D 1  and D 2  are assigned inside the BGR circuit  1  to a BGR cell  3 . 
   The resistors R 2  and R 1  as well as the diode D 2  are arranged serially in the specified sequence. One end of this series circuit is connected to the output of the operational amplifier OP 1 , and the other end is connected to ground VSS. In the same way, the resistor R 3  and the diode D 1  are connected in series and connected to the output of the operational amplifier OP 1  and to ground VSS. 
   The connecting line between the resistors R 1  and R 2  is connected to the inverted input of the operational amplifier OP 1 . The connecting line between the resistor R 3  and the diode D 1  is connected to the non-inverted input of the operational amplifier OP 1  via a further connecting line. An additional current Iein can be coupled into this further connecting line. A resistor R 4  is also connected between the output of the operational amplifier OP 1  and ground VSS. 
   The output of the operational amplifier OP 1  also constitutes the output of the BGR circuit  1 . A temperature-stabilized reference voltage can be tapped at the output of the BGR circuit  1  during its normal operation. The temperature stability of the reference voltage is based on the opposite nature of the temperature dependencies of the two voltages that drop across the resistor R 3  and across the diode D 1 , respectively. The diode D 1  and the diode D 2  can be constructed in each case, for example, from a bipolar transistor whose base terminal is connected to its collector terminal. The base/emitter voltage of the diode D 1  then has, for example, a temperature coefficient of −2 mV/K. The temperature dependence of the voltage dropping across the resistor R 3  is a function of the dimensioning of the resistors R 1 , R 2  and R 3 , and of the temperature coefficients of the thermal voltage VT of the diode D 2 . The voltage dropping across the resistor R 3  has a temperature coefficient of +2 mV/K, owing to a suitable selection of these components and because of the design of the BGR circuit  1  in terms of circuit engineering. This results overall in a reference voltage that is stable over a certain temperature range. 
   The setting circuit  2  is connected downstream of the BGR circuit  1 . The setting circuit  2  comprises transistors N 1 , N 2 , P 1 , P 2 , P 3  and P 4 , as well as a constant current source I 1 . The transistors N 1 , N 2 , P 1 , P 2 , P 3  and P 4  are MOSFETs. The respective doping of their channels is specified by the letters N and P respectively, in their reference symbols. This nomenclature also applies to transistors mentioned below. 
   The transistors N 1  and N 2  are connected in a current mirror circuit downstream of the input of the setting circuit  2 . Flowing in this case through the transistor N 1  is the input current of the setting circuit  2 , which is at the same time the output current of the BGR circuit  1 . The mirrored input current flows through the transistor N 2  into the transistor P 1 , which is connected, in turn, to the transistor P 2  in a current mirror circuit. The transistor P 2  is also included in a differential amplifier stage that also comprises the transistor P 3  and the constant current source I 1 . Here, the constant current source I 1  is connected to the drain/source paths of the transistors P 2  and P 3 . The transistors P 3  and P 4  form a further current mirror. The transistor P 4  generates the current Iein that is coupled into the BGR circuit  1  from the setting circuit  2 . 
   The function of the circuit arrangement as shown in  FIG. 1  is as follows. The setting circuit  2  may be used to replicate in the transistor N 1  the current flowing through the resistor R 3  and the diode D 1 . For this purpose, the transistors N 1  and N 2  are set via their W/L ratio such that their steepness gm corresponds to the resistor R 3 . However, the resistor R 3  and the steepness gm do not match because of fluctuations in the production process and different temperature coefficients. By contrast, the diode D 1  has a similar temperature response and current response to those of the thermal voltage VT of the transistors N 1  and N 2 . The arrangement shown in  FIG. 1  thus yields only an inaccurate replication of the current flowing in the BGR cell  3  through the resistor R 3  and the diode D 1 . 
   The current flowing through the transistor N 1  is mirrored into the differential amplifier stage by means of the current mirror circuits constructed from the transistors N 1  and N 2  and, respectively P 1  and P 2 . The current generated in the differential amplifier stage by the constant current source I 1  is the minimum current that must flow through the transistor N 1 . If the current flowing through the transistor N 1  is smaller than this minimum current, the differential amplifier stage causes the differential current of these two currents to flow through the drain/source path of the transistor P 3 . The current Iein is yielded as mirror image of the differential current by means of the current mirror constructed from the transistors P 3  and P 4 . 
   The current Iein is coupled into the BGR circuit  1  at the non-inverting input of the operational amplifier OP 1  and flows away there to ground VSS via the diode D 1 . As a result, the current Iein generates via the diode D 1  a voltage drop that results, in turn, in a positive potential difference between the inputs of the operational amplifier OP 1 . The operational amplifier OP 1  increases its output voltage because of the positive potential difference at its inputs. 
   The setting circuit  2  is designed such that the current Iein is switched off as soon as there is enough current flowing in the BGR cell  3  that it is possible to reach only the stable operating point of the BGR circuit  1 . The current generated by the constant current source I 1  in this case prescribes when the current Iein is switched off. The constant current source I 1  can be constructed, for example from a resistor and a diode, or from a PTAT (Proportional to Absolute Temperature) generator. 
   BRIEF SUMMARY 
   Accordingly, a circuit for setting the operating point of a BGR circuit is provided that has a high precision and a simple topology. A corresponding method is also provided. In addition to the BGR circuit, which can be used to generate a temperature-stabilized reference voltage, the circuit has a setting circuit. 
   By way of introduction only, in one embodiment the BGR circuit includes an operational amplifier from whose output voltage the reference voltage is to be derived, and a BGR circuit branch with two components. The temperature dependencies of the two components are opposed during operation of the BGR circuit. These can be, in particular, the temperature dependencies of the voltages respectively dropping across the components. One input of the operational amplifier is connected to the BGR circuit branch via a connecting line. The output voltage that can be tapped at the output of the operational amplifier drops across the BGR circuit branch. 
   The setting circuit in one embodiment includes a voltage comparator, an auxiliary circuit branch, a first current source and a second current source. The auxiliary circuit branch has the same components in the same arrangement as the BGR circuit branch. The first current source feeds the auxiliary circuit branch. The voltage comparator compares the output voltage of the operational amplifier with the voltage that drops across the auxiliary circuit branch. The second current source generates a setting current as a function of this comparison, and thereby feeds the connecting line. 
   The circuit enables the setting of the operating point of the BGR circuit by coupling in the setting current. The setting current is generated using the voltage comparison. 
   During the voltage comparison, the voltage dropping across the BGR circuit branch is compared with the voltage dropping across the auxiliary circuit branch. The voltage dropping across the auxiliary circuit branch is produced by the current generated by the first current source in the auxiliary circuit branch. Since the auxiliary circuit branch is an exact simulation of the BGR circuit branch, the voltage comparison also constitutes a comparison of the current flowing through the BGR circuit branch with the current generated by the first current source. The result of the comparison determines the magnitude of the setting current. The setting current generates a voltage difference at the inputs of the operational amplifier, and thereby causes the operational amplifier to change its output voltage accordingly. 
   Moreover, the circuit also permits the setting current to be switched off. If the voltage comparison delivers a specific result, it can be provided that the switch-off point is reached, and that the setting current is accordingly switched off. This may be the case when the output voltage of the operational amplifier is as large as or larger than the voltage dropping across the auxiliary circuit branch. This means that the switch-off point is determined by the magnitude of the current generated by the first current source. 
   The BGR circuit branch in one embodiment has a resistor and a downstream diode. The diode may be constructed in particular from a transistor whose base terminal or gate terminal is connected to its collector/emitter path or to its drain/source path. The connecting line between the BGR circuit branch and the input of the operational amplifier is arranged between the resistor and the diode. In accordance with the design of the circuit, the auxiliary circuit branch likewise has a resistor and a series-connected diode. 
   The connecting line may be coupled on the side of the operational amplifier to its non-inverting input. Since ideally no current flows through the inputs of an operational amplifier, the setting current flows off via the BGR circuit branch and, in particular, via the diode. 
   In one embodiment, the voltage comparator is a differential amplifier with a third current source and first and second transistors. The output voltage of the operational amplifier is present at the first transistor, and the voltage dropping across the auxiliary circuit branch is present at the second transistor. The differential amplifier constitutes a simple and cost-effective voltage comparator. 
   In one embodiment, the differential amplifier is dimensioned such that if the output voltage of the operational amplifier is lower than the voltage dropping across the auxiliary circuit branch, the current generated by the third current source flows substantially through the first transistor. A first current mirror may be connected downstream of the first transistor. 
   A current generated by a fourth current source can be coupled between the first transistor and the first current mirror. In one embodiment, the current generated by the fourth current source has half the value of the current generated by the third current source. This permits the setting current to be switched off even more abruptly. 
   Alternatively, a second current mirror may be provided. In this case, the second current mirror is fed on the input side from the second transistor and is connected on the output side to the gate or base terminals of the first current mirror. This likewise permits the setting current to be switched off as abruptly as possible. 
   The second current source may include at least one third current mirror, whose input current comes from the comparison carried out by the voltage comparator, and whose output current is the setting current. The first current source can, for example, be constructed from a resistor and a diode, or from a PTAT (Proportional to Absolute Temperature) generator. 
   The above circuit can be used when starting the BGR circuit, for example from the switched-off state. 
   The method in one embodiment serves for setting the operating point of a BGR circuit which generates a temperature-stabilized reference voltage. The BGR circuit has an operational amplifier and a BGR circuit branch. The BGR circuit branch comprises two components whose temperature dependencies are opposed during operation of the BGR circuit. These temperature dependencies can be, in particular, the temperature dependencies of the voltages respectively dropping across the components. One input of the operational amplifier is connected to the BGR circuit branch via a connecting line. The output voltage that can be tapped at the output of the operational amplifier drops across the BGR circuit branch. In normal operation of the BGR circuit, the aim is for the reference voltage to be obtained from the output voltage of the operational amplifier. 
   An auxiliary voltage is first generated. The auxiliary voltage drops across an auxiliary circuit branch that resembles the BGR circuit branch in its arrangements and dimensions. The output voltage is compared with the auxiliary voltage. A setting current is generated as a function of the result of the comparator and the setting current is fed into the connecting line. The setting current may be generated only when the output voltage of the operational amplifier is lower than the auxiliary voltage. 
   The method can be used to set the operating point of the BGR circuit with high precision and with a very low outlay. The method also permits the setting current to be shut down again when normal operation of the BGR circuit is stopped. 
   The foregoing summary has been provided only by way of introduction. Nothing in this section should be taken as a limitation on the following claims, which define the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a circuit diagram of a BGR circuit and a setting circuit from the prior art; 
       FIG. 2  shows a circuit diagram of a first embodiment of the circuit according to the invention; 
       FIG. 3  shows a circuit diagram of a second embodiment of the circuit according to the invention; 
       FIG. 4  shows a circuit diagram of a third embodiment of the circuit according to the invention; and 
       FIG. 5  shows a circuit diagram of the BGR circuit with a further setting circuit. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
     FIG. 2  illustrates a first embodiment including the BGR circuit  1 , already shown in  FIG. 1 , and a setting circuit  4 . The BGR circuits  1  of  FIGS. 1  and  2  are identical. Consequently, identical components in  FIGS. 1 and 2  have the same reference symbols. 
   The setting circuit  4  has a resistor R 5 , a diode D 3 , transistors N 3 , N 4 , P 5 , P 6 , P 7  and P 8 , as well as constant current sources I 2  and I 3 . The input of the setting circuit  4  is connected to the output of the BGR circuit  1 . Connected downstream of the input of the setting circuit  4  is a differential amplifier stage that comprises the constant current source I 3  and the transistors P 5  and P 6 . Connected downstream of the drain/source path of the transistor P 5  is a current mirror circuit with the transistors N 3  and N 4 . The drain/source path of the transistor N 4  is a current mirror circuit constructed from the transistors P 7  and P 8 . This current mirror circuit generates in the drain/source path for the transistor P 8  the setting current Iein that, like the circuit arrangement shown in  FIG. 1 , is fed into the BGR circuit  1  at the non-inverting input of the operational amplifier OP 1 . 
   The resistor R 5  and the diode D 3  are connected in series. This series circuit is fed on the side of the resistor R 5  from the constant current source I 2 , and the series circuit is connected to ground VSS on the side of the diode D 3 . The connection between the resistor R 5  and the constant current source I 2  is connected to the gate terminal of the transistor P 6 . 
   The resistor R 5  and the diode D 3  of the setting circuit  4  are respectively of the same design as the resistor R 3  and the diode D 1 . Consequently, the series circuit constructed from the resistor R 5  and the diode D 3  has the same design as the right-hand circuit branch of the BGR cell  3 . A current generated by the constant current source I 2  flows through the series circuit constructed from the resistor R 5  and the diode D 3 . This current flow generates a voltage drop across the series circuit. The voltage dropping across the corresponding series circuit in BGR circuit  1  is equal to the output voltage of the operational amplifier OP 1 . Since this voltage is simultaneously the output voltage of the BGR circuit  1 , the voltage dropping across the resistor R 3  and the diode D 1  can be compared by means of the differential amplifier stage with the voltage dropping across the resistor R 5  and the diode D 3 . 
   A current flows through the transistors P 5  or P 6  as a function of the comparison described above. If the voltage present at the output of the BGR circuit  1  is lower than the voltage dropping across the resistor R 5  and the diode D 3 , the current denoted by the constant current source I 3  flows through the drain/source path of the transistor P 5 . By means of the current mirror circuits constructed from the transistors N 3  and N 4  or, respectively, P 7  and P 8 , this current generates the current Iein. The current Iein acts in the BGR circuit  1  as has already been explained in the description relating to  FIG. 1 . 
   If the voltage present at the output of the BGR circuit  1  is higher than the voltage dropping across the resistor R 5  and the diode D 3 , the current generated by the constant current source I 3  flows away through the drain/source path of the transistor P 6  to ground VSS. In this case, no current flows through the transistor P 5 , and the current Iein is switched off. 
   One advantage of the setting circuit  4  shown in  FIG. 2  over the setting circuit  2  shown in  FIG. 1  is that a true simulation of the right-hand circuit branch of the BGR cell  3  is used in the setting circuit  4 . The simulation in the setting circuit  4  renders it possible to set the switch-off point of the current Iein precisely when setting the operating point of the BGR circuit  1 . The switch-off point thus accurately defined permits the current Iein generated by the setting circuit  4  to be switched off at substantially higher current values than the current Iein generated by the setting circuit  1 . This guarantees that the higher stable operating point of the BGR circuit  1  is reached, and that the current Iein does not disturb the normal operation of the BGR circuit  1 . 
   Shown in  FIGS. 3–5  are second, third, and fourth embodiments of the setting circuits  5 ,  6  and  7 . 
   In contrast to the setting circuit  4 , the setting circuit  5  in  FIG. 3  includes an additional constant current source I 4 . The current generated by the constant current source I 4  is coupled into one branch of the differential amplifier stage between the transistors P 5  and N 3 . In this embodiment, the current generated by the constant current source I 4  has half the value of the current generated by the constant current source I 3 . The coupling in of the additional current permits the current Iein to thereby be switched off even more abruptly in comparison to the setting circuit  4 . 
   The setting circuit  6  shown in  FIG. 4  includes an additional current mirror circuit constructed from transistors N 5  and N 6 . In this case, the transistor N 6  is connected as a diode and is fed from the transistor P 6 . The drain/source path of the transistor N 5  is connected to the gate terminals of the transistors N 3  and N 4 . 
   The setting circuit  7  shown in  FIG. 5 , in contrast to the setting circuit  2 , contains an operational amplifier OP 2 , transistors P 9  and P 10 , and a resistor R 6  and a diode D 4  connected downstream of the input of the setting circuit  7 . The non-inverting input of the operational amplifier OP 2  is coupled to the output of the BGR circuit  1 . The inverting input of the operational amplifier OP 2  is connected to a terminal of the resistor R 6 . Connected to the other terminal of the resistor R 6  is the diode D 4  which, in turn, is connected to ground VSS with its second terminal. 
   Like the resistor R 5  and the diode D 3  from  FIGS. 2 to 4 , the resistor R 6  and the diode D 4  constitute exact simulations of the resistor R 3  and the diode D 1 . 
   The gate terminals of the transistors P 9  and P 10  are connected to the output of the operational amplifier OP 2 . The drain/source path of the transistor P 9  or P 10  feeds the resistor R 6  or the transistor N 1 , respectively. 
   In the setting circuit  2 , because of the resistive connection of the output of the BGR circuit  1 , the current in the BGR cell  3  may not be measured exactly using the setting circuit  2 . The setting circuit  5  avoids this by using the operational amplifier OP 2  as voltage/current converter. In this case, the operational amplifier OP 2  compares the voltages present at its inputs, and sets its output voltage correspondingly. On the basis of the downstream transistors P 9  and P 10 , the output voltage generates two currents of which one feeds the simulation of the right-hand circuit branch of the BGR cell  3 , and the other feeds the transistor N 1 . Because of this circuit arrangement, the current flowing through the resistor R 6  and the diode D 4  has the same current value as the current flowing through the right-hand circuit branch of the BGR cell  3 . The same also holds for the current flow through the transistor N 1 . The circuit arrangement connected downstream of the transistor N 1  is identical to the circuit arrangement of the setting circuit  2 . 
   As shown and described, in various embodiments, the switch-off point of the current Iein is determined either by means of a comparison in which the output voltage of the BGR circuit  1  is compared with the voltage generated by the constant current source I 2 , across the simulation of the right-hand circuit branch of the BGR cell  3 , or by replicating the current through the BGR cell  3  and defining the switch-off point with the aid of the replicated current. 
   It is therefore intended that the foregoing detailed description be regarded as illustrative rather than limiting, and that it be understood that it is the following claims, including all equivalents, that are intended to define the spirit and scope of this invention. Other variations may be readily substituted and combined to achieve particular design goals or accommodate particular materials or manufacturing processes.