Patent Publication Number: US-7911195-B2

Title: Electronic circuits and methods for starting up a bandgap reference circuit

Description:
This application claims priority to German Patent Application 10 2006 031 549.9, which was filed Jul. 7, 2006, and is incorporated herein by reference. 
     BACKGROUND 
     The invention relates to electric circuits comprising a bandgap reference circuit and a start-up circuit and to methods for starting up a bandgap reference circuit. 
     Bandgap reference circuits are, for instance, required as voltage or current reference sources in integrated circuits, and normally need a start-up circuit in order to work reliably. Otherwise there may be the risk that the bandgap reference circuits may work at an incorrect operating point. Bandgap reference circuits, for instance, are disclosed in published German application for patent No. 10 2004 004 305 A1. 
     The principle of bandgap reference circuits is the following: The voltage difference between two diodes is used to generate a proportional to absolute temperature (PTAT) current in a first resistor. This current is used to generate a voltage across a second resistor. The voltage across the second resistor is added to the voltage of one of the two diodes of the bandgap reference circuit or to a further diode. 
       FIG. 4  shows an example of a bandgap reference circuit  1  and a conventional start-up circuit  42 . This example is provided as an illustration of the general problems associated with start-up circuits for bandgap reference circuits. 
     In this example, the bandgap reference circuit  1  comprises an operational amplifier A 1  having an inverting input  3 , a non-inverting input  4  and an output  5 . The operational amplifier A 1  in this example is not an ideal operational amplifier but what is known as an OTA. An OTA is a voltage-controlled current source. The output  5  of the operational amplifier A 1  supplies a voltage that is applied to the gate terminals of a first PMOS transistor P 1  and a second PMOS transistor P 2  to form a closed control loop. A supply voltage VDD is applied to the PMOS transistors P 1 , P 2 . 
     The first PMOS transistor P 1  is connected to the inverting input  3  of the operational amplifier A 1 , to a first diode D 1  and to a first resistor R 1 . The terminals of the first diode D 1  and of the first resistor R 1  that are on the opposite side from the first PMOS transistor P 1  are connected to ground. The node resulting from the connection of the first PMOS transistor P 1  to the first resistor R 1  and the first diode D 1  is denoted by B 1 . 
     The second PMOS transistor P 2  is connected to the non-inverting input  4  of the operational amplifier A 1 , to a second resistor R 2  and to a third resistor R 3 . The terminal of the third resistor R 3  on the opposite side from the second PMOS transistor P 2  is connected to a first terminal of a second diode D 2 , whose second terminal is connected to ground. The terminal of the second resistor R 2  on the opposite side from the second PMOS transistor P 2  is also connected to ground. The node resulting from the connection of the second PMOS transistor P 2  to the second and third resistors R 2 , R 3  is denoted by B 2 . 
     The bandgap reference circuit  1  also comprises an output transistor P 3 , to which the output voltage Vout of the bandgap reference circuit  1  is applied at an output node BGout of the bandgap reference circuit  1  and across an output resistor Rout connected to ground and the output node BGout. 
     The gate terminal of the output transistor P 3  is also connected to the gate terminals of the two PMOS transistors P 1 , P 2 . This connection forms a node BIAP. 
     In many cases, the operational amplifier A 1  draws its bias current from the bandgap reference circuit  1  itself, for instance, by means of an additional current mirror, so that the operational amplifier A 1  is also not fully functional until the bandgap reference circuit  1  has started up. The required bias current can also be generated independently of the bandgap reference circuit  1 , and has a reasonably well known magnitude. 
     The conventional start-up circuit  42  for the bandgap reference circuit  1 , provided for illustrating the general problem, comprises a PMOS transistor P 4 , a first NMOS transistor N 1  and a second NMOS transistor N 2 . 
     The conventional start-up circuit  42  for the bandgap reference circuit  1  works as follows. 
     If the output voltage Vout of the bandgap reference circuit  1  has not yet reached a certain level, i.e., the bandgap reference circuit  1  has not yet started up, then an auxiliary circuit comprising the PMOS transistor P 4  of the start-up circuit  42  and the first NMOS transistor N 1  switches on the second NMOS transistor N 2 . The second NMOS transistor N 2  pulls the node BIAP downwards so that an electric current begins to flow in the heart of the bandgap circuit, i.e., inside the bandgap reference circuit  1 . The operational amplifier A 1  should then assume full control of the bandgap reference circuit  1  at this point in time. Without this start-up assistance, the two inputs  3 ,  4  of the operational amplifier A 1  could sit at ground potential, and the operational amplifier A 1  would have no reason to change its state. 
     If there is sufficient electrical current flow in the heart or core of the bandgap circuit and hence the output voltage Vout at the output node BGout of the bandgap reference circuit  1  is sufficiently high, then the second NMOS transistor N 2  of the start-up circuit  42  can be turned off again, so that the bandgap reference circuit  1  is brought automatically into its correct operating point by the operational amplifier A 1 . 
     Assuming that the operational amplifier A 1  has a non-negligible offset voltage in the negative direction, i.e., the non-inverting input  4  of the operational amplifier A 1  must be taken in the negative direction in order to bring its output  5  into the center position, and assuming that the start-up circuit  42  is just being operated at a preliminary operating point, then a “moderate” electrical current flows in the bandgap reference circuit  1 . Then the two inputs  3 ,  4  of the operational amplifier A 1  are also taken to a “moderate” start-up state. In this case, it may happen that the general conditions are inadequate for sensible operation of the operational amplifier A 1 . 
     If, nonetheless, sensible operation of the operational amplifier A 1  of the bandgap reference circuit  1  is possible, then it is conceivable that the operational amplifier A 1  is controlling in the wrong direction: if the electrical current within the bandgap reference circuit  1  is not large enough, and hence the electrical voltages across the resistors R 1 , R 2 , R 3  are not large enough for a non-negligible electrical current to flow through the two diodes D 1 , D 2 , then the inputs  3 ,  4  of the operational amplifier A 1  are also driven at a negligible level. Assuming the aforementioned offset voltage of the operational amplifier A 1 , the operational amplifier then controls in the wrong direction, i.e., the operational amplifier A 1  of the bandgap reference circuit  1  tries to reduce the electrical current inside the bandgap reference circuit  1 . If, however, the start-up circuit  42  has already reached a start-up state at which it would like to turn off, it is evident that the bandgap reference circuit  1  may never reach its required operating point. In fact to reach this operating point it would require a sufficient electrical current to flow through the two diodes D 1 , D 2 , so that the operational amplifier A 1  is driven beyond its own offset voltage. Only then will the automatic control work satisfactorily. 
     The turn-off point of the conventional start-up circuit  42  is hence relatively critical. In particular, for relatively low supply voltages and output voltages and relatively low temperatures, the conditions described above may be so unfavorable that it becomes impossible to design the start-up circuit  42  using sensible component values. 
     SUMMARY OF THE INVENTION 
     In one aspect of the invention, an electronic circuit comprises a bandgap reference circuit which comprises at least one diode path comprising a semiconductor diode, wherein the diode path comprises a resistor connected in series with the semiconductor diode, and the voltage across the resistor is proportional to the absolute temperature of the semiconductor diode, and a start-up circuit for starting up the bandgap reference circuit, which assists the start-up of the bandgap reference circuit until the voltage across the resistor reaches a preset threshold voltage and turns off automatically when the voltage across the resistor has reached the threshold voltage. 
     Bandgap reference circuits are generally known, for instance, from P. E. Allen, D. R. Holberg, “CMOS Analog Circuit Design,” 2nd edition, Oxford University Press, New York, USA 2002, page 157; J. H. Huijsing et al., (Editor), “Analog Circuit Designs,” Kluwer Academic Press, 1996, pages 269-350; A. Annema, “Low-Power Bandgap Reference Featuring DTMOSTs,” IEEE Journal of Solid-State Circuits, Vol. 34, No. 7, July 1999, pages 949-952, R. J. Widlar, “New Developments in IC Voltage Regulators,” IEEE Journal of Solid-State Circuits, Vol. SC-6, No. 1, February 1971, page 2 et seq; Tsividis, “A CMOS Voltage Reference,” IEEE Journal of Solid-State Circuits, Vol. SC-13, No. 6, December 1978, page 774 et seq; and Doyle, “A CMOS Subbandgap Reference Circuit With 1-V Power Supply Voltage,”IEEE Journal of Solid-State Circuits, Vol. 39, No. 1, January 2004, page 252 et seq. They comprise, for instance, two diode paths, each of which comprises a semiconductor diode. Conventional diodes, for instance, can be used as semiconductor diodes, as is the case in the bandgap reference circuit  1  described above. The term semiconductor diode, however, is used here not only for conventional diodes but generally for semiconductors having diode properties, such as transistors. Transistors useful for bandgap reference circuits may be vertical bipolar transistors, for instance, or MOSFETs operated in the sub-threshold region for example. 
     Bandgap reference circuits can supply a reference voltage at their outputs, as is the case for the bandgap reference circuit  1  described in the introduction. Bandgap reference circuits can also supply a reference current. 
     According to an embodiment of the invention electric circuit, the criterion for turning off the start-up circuit is the voltage across the resistor, with the resistor being connected in series with the semiconductor diode of the bandgap reference circuit. The voltage across the resistor is proportional to the absolute temperature of the semiconductor diode. Once the bandgap reference circuit has started up, a sufficiently large electrical current flows within the bandgap reference circuit for the voltage across this resistor to reach the threshold voltage. 
     In another aspect of the invention, an electronic circuit comprises a bandgap reference circuit and a start-up circuit for starting up the bandgap reference circuit, which assists the start-up of the bandgap reference circuit until a difference voltage between a potential of a node within the bandgap reference circuit and another node of the bandgap reference circuit, which is at a potential proportional to an output voltage or an output current of the bandgap reference circuit, reaches a preset threshold voltage and turns off automatically when the difference voltage has reached the threshold voltage. 
     The turn-off criterion for this embodiment of the inventive electronic circuit is the attainment of a preset difference voltage, for instance, generated by the differential amplifier of the start-up circuit. The difference voltage is obtained from the potential of the node within the bandgap reference circuit and a potential of the other node of the bandgap reference circuit, which is at a potential proportional to the output voltage or the output current of the bandgap reference circuit. 
     The node within the bandgap reference circuit is in particular a node within a diode path of the bandgap reference circuit, wherein the diode path comprises a semiconductor diode. If the bandgap reference circuit comprises two diode paths, each comprising a diode semiconductor, then the potential at the node within the bandgap reference circuit may also be a mean value of the potentials of two nodes within the bandgap reference circuit. 
     In another aspect of the invention, a method for starting up a bandgap reference circuit comprises assisting the start-up of a bandgap reference circuit by means of a start-up circuit, wherein the bandgap reference circuit comprises at least one diode path comprising a semiconductor diode and a resistor connected in series with the semiconductor diode. The voltage across the resistor is proportional to the absolute temperature of the semiconductor diode and the start-up circuit assists the start-up of the bandgap reference circuit while the voltage across the resistor within the bandgap reference circuit is less than a preset threshold voltage. The method further comprises automatically turning off the start-up circuit when the voltage across the resistor has reached the threshold voltage. 
     In a further aspect of the invention, a method for starting up a bandgap reference circuit comprises assisting the start-up of a bandgap reference circuit by means of a start-up circuit, wherein the bandgap reference circuit comprises a diode path comprising a semiconductor diodes. The start-up circuit assists the start-up of the bandgap reference circuit while a difference voltage between a potential of a node within the diode path and another node of the bandgap reference circuit, which is at a potential proportional to an output voltage or an output current of the bandgap reference circuit, is less than a preset threshold voltage. The method further comprises automatically turning off the start-up circuit when the difference voltage has reached the threshold voltage. 
     The difference voltage can, for instance, be monitored or generated by a differential amplifier. If the bandgap reference circuit comprises two diode paths, each comprising one semiconductor diode, then the potential at the node within the bandgap reference circuit may also be a mean value of the potentials of two nodes within the bandgap reference circuit. 
     The invention also provides a method for operating a start-up circuit for a bandgap reference circuit, wherein the criterion for turning off the start-up circuit is the attainment of a preset voltage across a resistor connected in series with a semiconductor diode of the bandgap reference circuit, wherein the voltage across the resistor is proportional to the absolute temperature of the semiconductor diode, or the criterion for turning off the start-up circuit is the attainment of a preset difference voltage, which is between a potential of a node within the bandgap reference circuit and another node, which is at a potential proportional to the output voltage or proportional to the output current of the bandgap reference circuit. 
     The semiconductor diode of the bandgap reference circuit may be a conventional diode, for instance, as is the case in the bandgap reference circuit  1  described above. The term semiconductor diode, however, is used here not only for conventional diodes but generally for semiconductors having diode properties, such as, for instance, transistors. Transistors useful for a bandgap reference circuit may be vertical bipolar transistors or MOSFETs operated in the sub-threshold region, for example. 
     Bandgap reference circuits can supply a reference voltage at their output, as is the case in the bandgap reference circuit  1  described in the background. Bandgap reference circuits may also supply a reference current, however. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a bandgap reference circuit including a first exemplary embodiment of a start-up circuit; 
         FIG. 2  is a bandgap reference circuit including a second exemplary embodiment of a start-up circuit; 
         FIG. 3  is a graph illustrating how the start-up circuit of  FIG. 2  works; and 
         FIG. 4  is a bandgap reference circuit including a conventional start-up circuit. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
       FIG. 4  has been discussed in the background section. 
       FIG. 1  shows the bandgap reference circuit  1  already described in the background, and a first exemplary embodiment of a start-up circuit  2  for the bandgap reference circuit  1 . 
     In this exemplary embodiment, the start-up circuit  2  comprises an operational amplifier A 2  comprising a non-inverting input  6 , an inverting input  7  and an output  8 , plus a resistor R 4  and a MOS diode  9 . The operational amplifier A 2  in this exemplary embodiment is again in this case a non-ideal operational amplifier, and once again is an OTA. An OTA is a voltage-controlled current source. 
     The resistor R 4  of the start-up circuit  2  is connected by one of its terminals to the output  8  of the operational amplifier A 2  and to one of the terminals of the MOS diode  9 . The other terminal of the resistor R 4  is connected to ground, and the second terminal of the MOS diode  9  is connected to the node BIAP. 
     The two inputs  6 ,  7  of the operational amplifier A 2  of the start-up circuit  2  are connected to the respective two terminals of the third resistor R 3  of the bandgap reference circuit  1 , so that the voltage across the third resistor R 3  is applied to the inputs  6 ,  7  of the operational amplifier A 2  of the start-up circuit  2 . The voltage across the third resistor R 3  is proportional to the absolute temperature of the second diode D 2 . 
     If the bandgap reference circuit  1  has still not reached a sufficient start-up state, then a relatively low electrical current flows through the third resistor R 3 . This means that the operational amplifier A 2  is driven at a relatively low level, so that the operational amplifier A 1  is also driven at a relatively low level, because, assuming the electrical currents through the two diodes D 1 , D 2  are negligible, the two nodes B 1 , B 2  are at the same potential. Thus the two operational amplifiers A 1 , A 2  supply only relatively small output currents, so that the node BIAP is enabled via the resistor R 4  of the start-up circuit  2  and the MOS diode  9 . 
     In order to turn off the MOS diode  9 , a non-negligible voltage drop across the third resistor R 3  of the bandgap reference circuit  1  is required; in the present exemplary embodiment a voltage drop of 10 mV is required. Only once this voltage drop across the third resistor R 3  is reached does the operational amplifier A 2  of the start-up circuit  2  supply a sufficiently large electrical current for the electrical potential at the node S 1 , which is formed by the connection of the output  8  of the operational amplifier A 2 , the MOS diode  9  and the resistor R 4  of the start-up circuit  2 , to be at such a level that the MOS diode  9  is reverse biased, and the start-up circuit  2  thereby turns itself off automatically. 
     The bandgap reference circuit  1  can consequently be designed using component values lying within a far larger range than is possible with the conventional start-up circuit  42  shown in  FIG. 4 . 
     The closed loop via the operational amplifier A 1  of the bandgap reference circuit  1  works correctly when the offset voltage at the inputs  3 ,  4  of the operational amplifier A 1  is surmounted. This is typically the case for a relatively low mV level. It is desirable if the start-up circuit  2  turns off reliably when the bandgap reference circuit  1  has reached its final operating point. For the exemplary embodiment, this is the case for several tens of millivolts, for instance, 50 mV. Thus a relatively wide range is obtained within which the turn-off threshold of the start-up circuit  2  can lie. 
       FIG. 2  shows another embodiment of a circuit  22  for the bandgap reference circuit  1 . The start-up circuit  22  comprises a differential amplifier A 3  comprising two inverting inputs  27   a ,  27   b , two non-inverting inputs  26   a ,  26   b  connected together, and an output  28 , plus a resistor R 5 , which is connected to ground by its one terminal and to the output  28  of the differential amplifier A 3  by its other terminal, and a MOS diode  29 , which is connected on one side to the output  28  of the differential amplifier A 3  and on the other side to the node BIAP. 
     The first inverting input  27   a  is connected to the node B 1 , and the second inverting input  27   b  is connected to the node B 2 . The two non-inverting inputs  26   a ,  26   b  are connected to a node B 0 . The node B 0  is part of a potential divider comprising a resistor R 6  and a resistor R 7 . The two resistors R 6 , R 7  form the output resistance Rout ( FIG. 1 ), so that at the node B 0  there is a voltage proportional to the output voltage Vout of the bandgap reference circuit  1 . 
     The differential amplifier A 3  of the start-up circuit  22  thereby compares the voltage drop across the first and second resistors R 1 , R 2  with the voltage at the node B 0 , i.e. with a scaled version of the output voltage Vout of the bandgap reference circuit  1 . 
     A graph shown in  FIG. 3  is used to illustrate how the start-up circuit  22  works. The graph of  FIG. 3  shows the voltage curve  31  at the nodes B 1 , B 2  and the voltage curve  32  at the node B 0  plotted against an electrical current supplied to the two PMOS transistors P 1 , P 2 . The voltage curve  32  at the node B 0  is linear. The voltage curve at the nodes B 1 , B 2  is initially approximately linear, before the voltage  31  flattens off as the two diodes D 1 , D 2  conduct. Consequently, by monitoring the intersection point  33  of the two voltages  31 ,  32 , one can establish extremely well whether the diode paths comprising the two diodes D 1 , D 2  are already passing a sufficiently large electrical current. The differential amplifier A 3  now monitors precisely this criterion and switches off the start-up circuit  22  when it is clearly exceeded. 
     The differential amplifier A 3  having its two inverting inputs  27   a ,  27   b  and its two non-inverting inputs  26   a ,  26   b  respectively is intended to suggest an averaging process in each case. In the present exemplary embodiment, this is implemented in circuitry by two transistors being connected in parallel with a differential input stage in each case. The reason for this lies in the relatively equal loading of the nodes B 1 , B 2 , for example by gate leakage currents. Alternatively, a conventional amplifier solution can also be chosen, for instance, by comparison of the voltages at the nodes B 1 , B 0 . 
     The start-up circuit  22  shown in  FIG. 2  can also supply a start-up current without the resistor R 5 . In this case, however, it must be taken into account that when the bandgap reference circuit  1  is completely turned off, the differential amplifier A 3  would supply no current, and hence a dedicated start-up circuit for this point would be required. 
     The claimed start-up circuit for a bandgap reference circuit  1  and the claimed method for starting up a bandgap reference circuit, and the start-up circuits  2 ,  22  shown in  FIGS. 1 and 2 , respectively, are not restricted to the bandgap reference circuit  1  shown. In particular, instead of the two diodes D 1 , D 2 , transistors can also be used, for instance, vertical bipolar transistors or MOSFETs, for instance, in the sub-threshold region. In addition, the first and second resistors R 1 , R 2  are not absolutely necessary. 
     In particular, it is also possible that the mean value of the potentials at the nodes B 1  and B 2  is not used for the difference voltage of the electronic circuit shown in  FIG. 2 , but just one of the potentials of the nodes B 1  or B 2 . In addition, the potential at the terminal of the resistor R 3  connected to the diode D 2  can be used instead of the potential at the node B 2 . 
     Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventor to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of his contribution to the art.