Patent Publication Number: US-2011068767-A1

Title: Sub-volt bandgap voltage reference with buffered ctat bias

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 12/350,899, (the &#39;899 application), filed Jan. 8, 2009, which claims the benefit of U.S. provisional patent application No. 61/020,133, (the &#39;133 application) filed Jan. 9, 2008, which is hereby incorporated by reference. 
    
    
     BACKGROUND 
     Bandgap voltage references are one of the main building blocks used in electronic circuits. Bandgap voltage references may be used in a myriad of applications, including cell phones, MP3 players, personal digital assistants, cameras, video recorders, and others. 
     Simply stated, a bandgap voltage reference receives a power supply and generates an output voltage. The bandgap voltage reference may be designed to provide an output voltage that is stable over temperature, or it may be designed to provide an output voltage that varies over temperature, for example to compensate for a change caused by temperature in another circuit or circuit element. 
     The output of the reference voltage may be used for a number of purposes. For example, a reference voltage output that is stable over temperature, that is, has a low temperature coefficient, can be placed across an external resistor to generate a current that is stable over temperature. Also, a reference voltage output can be used along with a regulator circuit to provide a regulated power supply. 
     Conventional bandgap circuits provide output voltages on the order of the bandgap of silicon or higher, that is, they provide output voltages that are at or exceed approximately 1.26 volts, though this value depends on the specific processing technology used. However, many modern circuits require a voltage less than the bandgap of silicon. For example, many newer technologies provide devices that have excessive leakage when their drain voltages are higher than approximately 1 volt. Also, lower voltages are often used where it is particularly desirable to save power. Another drawback of conventional circuits is that their temperature characteristics cannot be adjusted without changing their output voltage. 
     Thus, what is needed are circuits, methods, and apparatus that provide bandgap voltage references having output voltages less than the bandgap of silicon. It is also desirable that the output voltage and temperature coefficient be independently adjustable. 
     SUMMARY 
     Accordingly, embodiments of the present invention provide circuits, methods, and apparatus that provide voltage references having a temperature independent output voltage that is 10 less than the bandgap of silicon. The temperature coefficient and absolute voltage of the voltage reference output can be independently adjusted. 
     A specific embodiment of the present invention generates two voltage sources, one of which is proportional-to-absolute temperature (PTAT), the other of which is complementary-to-absolute temperature (CTAT). These voltages are placed across a first resistor. The first resistor is further connected to a second resistor to form a resistor divider. The resistor divider provides a reduced voltage that is below that bandgap of silicon. 
     In this specific embodiment of the present invention, the temperature coefficient of the reference voltage provided by the resistor divider can be set by adjusting the first resistor. The absolute voltage provided can be set by adjusting the second resistor. 
     Various embodiments of the present invention may incorporate one or more of these and the other features described herein. A better understanding of the nature and advantages of the present invention may be gained by reference to the following detailed description and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a symbolic representation of a bandgap voltage reference that is improved by the incorporation of embodiments of the present invention; 
         FIG. 2  is a block diagram of an electronic system that may be improved by the incorporation of an embodiment of the present invention; 
         FIG. 3  is a simplified schematic of a bandgap voltage reference according to an embodiment of the present invention; 
         FIG. 4  is a flowchart of a method of generating a bandgap voltage reference according to an embodiment of the present invention; 
         FIG. 5  is a schematic of a bandgap voltage reference according to an embodiment of the present invention; and 
         FIG. 6  is a flowchart illustrating another method of generating a bandgap voltage according to an embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       FIG. 1  is a symbolic representation of a bandgap voltage reference that is improved by the incorporation of embodiments of the present invention. The bandgap voltage reference receives a power supply and generates an output voltage Vref. The power supply may be a positive voltage, shown here as VDD, and ground. Alternately, ground and a negative voltage may be provided. In still other embodiments of the present invention, positive and negative voltages, or positive, negative voltages along with ground may be received. As a result, the output voltage provided may be above, or below, ground. 
       FIG. 2  is a block diagram of an electronic system that maybe improved by the incorporation of an embodiment of the present invention. This figure includes a bandgap voltage reference, amplifier A 1 , power transistor M 1 , and a load circuit. 
     The bandgap voltage reference provides an output Vref, which is received by an inverting input of the amplifier A 1 . The output of amplifier A 1  drives power transistor M 1 . Power transistor M 1  provides current to the load circuit. The resulting regulated power supply of Vout at the load circuit is fed back to amplifier A 1 , where it is compared to the reference voltage Vref. Differences between these two voltages drive the output of amplifier A 1  such that these two voltages are equalized. 
     For example, if Vout is higher than desired, the output of amplifier A 1  increases. This, in turn reduces the current provided by M 1 , thus lowering the regulated output voltage Vout. Similarly, if Vout is lower than desired, the output of amplifier A 1  decreases, turning M 1  on harder, thereby increasing its current. This results in an increase in the voltage Vout. 
     It is often desirable that the regulated voltage Vout be stable over temperature. That is, it is desirable that the regulated voltage Vout has a low temperature coefficient. In some circuits, it is also desirable that the regulated voltage Vout be less than the bandgap of silicon. Accordingly, embodiments of the present invention provide a bandgap voltage reference that provides a reference voltage output that is less than the bandgap of silicon and has a low temperature coefficient. In other embodiments of the present invention, the temperature coefficient may be set to compensate for temperature effects seen elsewhere. For example, it may be desirable that at high-temperature the load circuit receives a higher regulated voltage. Alternately, it may be desirable that at high temperatures the load circuit receives a lower regulated voltage. Accordingly, the bandgap voltage reference temperature coefficient provided by a bandgap voltage reference according to an embodiment of the present invention can be adjusted. 
       FIG. 3  is a simplified schematic of an embodiment of the present invention. This figure includes two current sources to provide currents that are proportional-to-absolute temperature. Also included are resistors R 2  and R 3 , and diode D 1 . 
     Applying the principles of superposition and removing R 2 , the current sources generate a voltage Vref across R 3  that is proportional-to-absolute temperature, and a voltage V 1  across diode D 1 . The voltage V 1  across diode D 1  decreases as the temperature increases. Accordingly, the voltage V 1  across diode D 1  is complementary-to-absolute temperature. 
     With R 2  included, a voltage that is the difference between a first voltage that is complementary-to-absolute temperature and a second voltage that is proportional-to-absolute temperature is placed across resistor R 2 . This in turn generates a current that strongly decreases as temperature increases. This is shown in the included graphs. 
     The proportional-to-absolute temperature current IPTATI is combined with the current in R 2 . The magnitude of the resistor R 2 , and thus the resulting current through R 2 , can be adjusted such that Vref has a low temperature coefficient. Moreover, the output voltage Vref can be adjusted by changing the value of R 3 . In a specific embodiment of the present invention, R 3  is a series of resistors, the series of resistors having switches at a number of intermediate nodes, where the output Vref is coupled to an intermediate node between two of the series of the resistors by one of the switches. 
       FIG. 4  is a flowchart of a method of generating a bandgap voltage reference according to an embodiment of the present invention. Specifically, in act  410 , a current that is proportional-to-absolute temperature is generated. This current is mirrored and provided to a diode to generate a voltage that is complementary-to-absolute temperature in act  420 . 
     In act  430 , the proportional-to-absolute temperature current is mirrored again and provided to a first terminal of a first resistor and a first terminal of a second resistor. In act  440 , the complementary-to-absolute temperature voltage is applied to a second terminal of the second resistor. A bandgap reference voltage is then available at the first terminal of the first resistor. The second resistor may be scaled to provide the desired temperature coefficient for the output voltage, while the first resistor may be scaled to adjust the absolute voltage of the bandgap reference voltage. 
       FIG. 5  is a schematic of a bandgap voltage reference according to an embodiment of the present invention. This figure includes proportional-to-absolute temperature current generating circuit including diodes D 1  and D 2 , resistor R 1 , and amplifier OA 2 . 
     Amplifier OA 2  generates a current through transistor M 5 , which is mirrored through transistors M 2 , M 3 , and M 4 . Transistors M 2 , M 3 , M 4 , and M 5  may each be the same size, or they may have different sizes. In this example, they are p-channel devices, though in other embodiments they may be bipolar PNP transistors, multiple p-channel devices, or other devices. The current mirrored by M 2  provides current for the output stage of amplifier OA 1 , which may thus have an open drain output stage. The current mirrored by transistor M 3  is provided to diode D 1 , resulting in a voltage V 1 . Similarly, current in transistor M 4  is provided to resistor R 1  and diode D 2 , resulting in a voltage V 2 . Amplifier OA 2  compares voltages V 1  and V 2  and adjusts the current in M 5 , and thereby the currents in transistors M 3  and M 4 , such that voltages V 1  and V 2  are equal. 
     Diode D 2  is a multiple of diode D 1 . As shown here, diode D 2  is “N” times the size of diode D 1 . Typically, this is achieved by replicating a diode the size of diode D 1  N number of times. For example, diode D 2  may be made up of eight diodes, each the size of diode D 1 . In a specific embodiment of the present invention, the diodes are implemented using substrate PNPs, though in other embodiments of the present invention they may be other P-N junctions. Resistors R 1 , R 2 , and R 3  may be polysilicon or other type of resistor. 
     As before, the resulting voltage V 1  is complementary-to-absolute temperature. The voltage V 1  is buffered by amplifier OA 1  and provided to the resistor R 2 . In this example, amplifier OA 1  acts as a voltage follower to prevent R 2  from bleeding current from the diode D 1 . 
     Again, ignoring resistor R 2 , the voltage across resistor R 3  is proportional-to-absolute temperature. This means the voltage across R 3  would have a large temperature coefficient temperature coefficient. Accordingly, R 2  is inserted and connected to the complementary-to-absolute temperature voltage provided by amplifier OA 1 . As before, this voltage has a large negative temperature coefficient. By adjusting R 2 , these temperature coefficients are canceled, resulting in an output voltage Vref having a low temperature coefficient. Moreover, resistor R 3  may be adjusted to provide a desirable output voltage Vref. 
     Care should be taken in the design of bandgap voltage reference circuits to ensure that they properly start up when their power supply is turned on. For example, in the present circuit, if the current in transistor M 5  is zero, the voltages V 1  and V 2  will both be zero and thus be equal. Though undesirable, this is a stable state. Accordingly, this specific embodiment of the 10 present invention employs a start-up circuit that provides an initial current in transistor M 5  such that this undesirable state does not occur. 
       FIG. 6  is a flowchart illustrating another method of generating a bandgap voltage according to an embodiment of the present invention. In act  610 , a first current is generated. In act  620 , the first current is mirrored and provided to a first diode to generate a first voltage. The first current is mirrored and provided to a second diode that is in series with a resistor to generate a second voltage in act  630 . 
     In act  640 , the first current is adjusted such that the first voltage and the second voltages are equal. The first voltage is then provided to a first terminal of a second resistor in act  650 . The first current is then mirrored and provided to a second terminal of the second resistor and a first terminal of the third resistor. The output voltage is then available at the first terminal of the third resistor. 
     The above description of exemplary embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form described, and many modifications and variations are possible in light of the teaching above. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated.