Abstract:
A bandgap voltage reference circuit and related method characterized in having a first current source for generating a first current having a positive temperature coefficient, a second current source for generating a second current having a negative temperature coefficient, and a resistive element to receive both the first and second current to develop a reference voltage. By configuring the circuit such that the magnitudes of the positive and negative temperature coefficients are substantially the same, the reference voltage becomes substantially invariant with changes in temperature. Another circuit is provided in conjunction with the voltage reference circuit to substantially equalize the drain-to-source voltage of the transistors used in the voltage reference circuit.

Description:
FIELD  
         [0001]    This invention relates generally to bandgap voltage reference circuits, and in particular, to bandgap voltage reference circuits and related methods that add two currents having respectively opposite polarity temperature coefficients to generate a substantially temperature-invariant reference voltage.  
         GENERAL BACKGROUND  
         [0002]    A bandgap voltage reference circuit is typically used to provide a voltage reference for other circuits to use in performing their intended operations. Generally, it is desired that the reference voltage generated by a bandgap circuit is substantially invariant. This is so even if there are substantial variations in the environment temperature. Thus, many, if not all, bandgap circuits incorporate temperature compensating circuitry in order to generate a substantially temperature-invariant reference voltage.  
           [0003]    [0003]FIG. 1 illustrates a schematic diagram of a prior art bandgap voltage reference circuit  100 . The bandgap circuit  100  consists of PMOS transistors Q 11 , Q 12 , and Q 13 , and NMOS transistors Q 14  and Q 15  configured as current mirrors to generate substantially equal currents I 11  , I 12 , and I 13 . The bandgap circuit  100  further consists of resistor R 11  and diode D 11  coupled in series with PMOS transistor Q 11  and NMOS transistor Q 14  to receive current I 11 , a diode D 12  coupled in series with PMOS transistor Q 12  and NMOS transistor Q 15  to receive current I 12 , and resistor R 12  and diode D 13  coupled in series with PMOS transistor Q 13  to receive current I 13 . The diodes D 11 , D 12 , and D 13  are forward biased with their cathode coupled to ground terminal. The output reference voltage of the bandgap circuit  100  is generated at the node between the PMOS transistor Q 13  and resistor R 12 .  
           [0004]    The temperature compensation of the output reference voltage of the bandgap circuit  100  operates as follows. The current I 12  generates a voltage V 13  across the diode D 12 . The voltage V 13  has a negative temperature coefficient −TαV 13 . The current I 11  generates a voltage V 12  across the diode D 11 . The voltage V 12  also has a negative temperature coefficient −TαV 12  that is more negative than the temperature coefficient −Tα 13  of voltage V 13  (i.e. −TαV 12 &lt;−TαV 13 ). The current mirror causes the voltage V 11  on the node between transistor Q 14  and resistor R 11  to be substantially equal to the voltage V 13 . Thus, the voltage VR 11  across the resistor R 11  (VR 11 =V 11 −V 12 ) has a positive temperature coefficient +TαR 11  due to −TαV 12  being more negative than −TαV 13 . Since the current I 11  through resistor R 11  is proportional to the voltage VR 11  across the resistor R 11 , the current I 11  likewise has a positive temperature coefficient +TαI 11 .  
           [0005]    The current mirror causes the current I 13  to be substantially equal to the current I 11 . Therefore, the current I 13  also has a positive temperature coefficient +TαI 13 . It follows then that the voltage VR 12  across resistor R 12  has a positive temperature coefficient +TαV 12  since VR 12  is proportional to the current I 13 . Additionally, the current I 13  generates a voltage V 14  across the diode D 13  that has a negative temperature coefficient −TαV 14 . The reference voltage VREF is the sum of voltages VR 12  and V 14 , both of which have opposite polarity temperature coefficients. Thus, by proper design of the bandgap circuit  100 , the reference voltage VREF can be made substantially temperature invariant across a particular temperature range.  
           [0006]    [0006]FIG. 2 illustrates a schematic diagram of another prior art bandgap circuit  200 . The bandgap circuit  200  operates similar to bandgap circuit  100 . Briefly, the voltage V 22  across the diode D 22  has a negative temperature coefficient −TαV 22  and the voltage V 21  across the diode D 21  also has a negative temperature coefficient −TαV 21  that is more negative than −TαV 22 . The operational amplifier U 21  causes the voltage V 23  at the positive terminal of the operational amplifier U 21  to be substantially the same as voltage V 22  across diode D 22 , which also has a similar negative temperature coefficient −TαV 23 . Since −TαV 21  is more negative than −TαV 23 , the voltage VR 21  across resistor R 21  has a positive temperature coefficient +TαVR 21 , and accordingly the current I 21  through resistor R 21  also has a positive temperature coefficient +TαI 21 . The current I 21 , as well as current I 22  through resistor R 22 , are derived from the current I 20  through PMOS transistor Q 21 . Thus, they all have a positive temperature coefficient. The reference voltage VREF is thus the addition of the voltage V 22  and the voltage drop across resistor R 22 , both of which have opposite polarity temperature coefficients which can be made to cancel out.  
           [0007]    A drawback of the prior art bandgap circuits  100  and  200  stems from the reference voltage VREF being a combination of two voltage drops in series. In bandgap circuit  100 , the reference voltage VREF is a combination of V 14  across the diode D 13  and VR 14  across the resistor R 12 . In bandgap circuit  200 , the reference voltage VREF is a combination of V 22  across the diode D 22  and VR 22  across the resistor R 22 . Because of this, the power supply voltage VDD needs enough headroom to accommodate both voltages that form the reference voltage VREF in addition to the source-drain voltages of transistor Q 13  or Q 21 . The reference voltage VREF typically requires about 1.2V and the source-drain voltage of transistor Q 13  or Q 21  requires at least 0.2V. Thus, the minimum power supply voltage VDD required is about 1.4V, which makes the prior bandgap circuits  100  and  200  not compatible with emerging technologies that use VDD at significantly lower voltage than 1.4V, such as 1V. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 illustrates a schematic diagram of a prior art bandgap voltage reference circuit;  
         [0009]    [0009]FIG. 2 illustrates a schematic diagram of another prior art bandgap voltage reference circuit;  
         [0010]    [0010]FIG. 3 illustrates a schematic diagram of an exemplary bandgap voltage reference circuit in accordance with an embodiment of the invention;  
         [0011]    [0011]FIG. 4 illustrates a schematic diagram of an exemplary bandgap voltage reference circuit in accordance with another embodiment of the invention; and  
         [0012]    [0012]FIG. 5 illustrates a block diagram of an exemplary integrated circuit in accordance with another embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0013]    [0013]FIG. 3 illustrates a schematic diagram of an exemplary bandgap voltage reference circuit  300  in accordance with an embodiment of the invention. The bandgap circuit  300  comprises a +Tα current source  302  that generates a current I 31  that has a positive temperature coefficient +TαI 31 , a −Tα current source  304  that generates a current I 32  that has a negative temperature coefficient −TαI 32 , and a resistor R 30  having one end coupled to the outputs of the current sources  302  and  304  and the other end coupled to ground. The currents I 31  and I 32  combine to form current I 30  flowing through resistor R 30  to generate the reference voltage VREF for the bandgap circuit  300 . Since reference voltage VREF varies proportional to the current I 30 , which is formed of currents I 31  and I 32  having opposite temperature coefficients +TαI 31  and −TαI 32 , the reference voltage VREF can be made to be substantially temperature invariant by proper design of the +Tαcurrent source  302  and the −Tα current source  304 .  
         [0014]    [0014]FIG. 4 illustrates a schematic diagram of an exemplary bandgap voltage reference circuit  400  in accordance with a more specific embodiment of the invention. The bandgap circuit  400  comprises a +Tα current source section  402 , a −Tα current source section  404 , an optional transistor source-to-drain voltage matching circuit  406 , and a resistor R 43  to generate the reference voltage VREF across thereof. The +Tα current source section  402 , in turn, comprises PMOS transistors Q 41 , Q 42 , Q 43 , operational amplifier U 41 , resistor R 41 , and diodes D 41  and D 42 . The −Tα current source section  404 , in turn, comprises an operational amplifier U 42 , PMOS transistors Q 44  and Q 45 , and resistor R 42 . And, the optional transistor source-to-drain voltage matching circuit  406 , in turn, comprises an operational amplifier U 43  and PMOS transistor Q 46 .  
         [0015]    The +Tα current source section  402  operates as follows. The PMOS transistors Q 41 , Q 42 , and Q 43  are configured as a current mirror to generate substantially equal currents I 41 , I 42 , and I 43 . More specifically, the PMOS transistors Q 41 , Q 42 , and Q 43  have sources coupled to the power supply rail VDD and gates coupled together. The diode D 42  is configured to receive the current I 42  in a forward bias manner to develop across it a voltage V 42  that has a negative temperature coefficient −TαV 42 . The diode D 41  is configured to receive the current I 41  in a forward bias manner to develop across it a voltage V 41  that has a negative temperature coefficient −TαV 41  that is more negative than −TαV 42 .  
         [0016]    The operational amplifier U 41 , having the voltage V 42  applied to its negative terminal, generates a gate voltage for the PMOS transistors Q 41 , Q 42 , and Q 43  that causes a voltage V 40  to appear at the positive terminal of the operational amplifier U 41  that is substantially the same as voltage V 42 , along with substantially the same temperature coefficient (−TαV 40 =−TαV 42 ). Since the temperature coefficient −TαV 41  of voltage V 41  is more negative than the temperature coefficient −TαV 40  of voltage V 40 , the voltage VR 41  across the resistor R 41  exhibits a positive temperature coefficient +TαVR 41 . Therefore, the current I 41 , being proportional to the voltage VR 41 , also exhibits a positive temperature coefficient +TαI 41 . The current mirror mirrors the current I 41  to the current I 43  which as a result, has a positive temperature coefficient +TαV 43 . The current I 43  serves as the positive temperature coefficient current that forms the reference voltage VREF of the bandgap circuit  400 .  
         [0017]    The −Tα current source section  404  operates as follows. The voltage V 42  is applied to the negative input of the operational amplifier U 42 . The operational amplifier U 42  having its output drive the gate of PMOS transistor Q 44  causes a voltage V 39  to be generated at the positive input of the operational amplifier U 42  that is substantially the same as voltage V 42 , along with substantially the same temperature coefficient (−TαV 39 =−TαV 42 ). The positive input of the operational amplifier U 42  is connected to the drain of the PMOS transistor Q 44  and to resistor R 42 . As a result, a drain current I 44  is generated that is proportional to the voltage V 39 . Since the voltage V 39  has a negative temperature coefficient −TαV 39 , the current I 44  also has a negative temperature coefficient −TαI 44 . The PMOS transistors Q 44  and Q 45  having their gates connected together mirror the current I 44  to current I 45  flowing through transistor Q 45 . The current I 45  thus has a negative temperature coefficient −TαI 45 . The current I 45  serves as the negative temperature coefficient current that forms the reference voltage VREF of the bandgap circuit  400 .  
         [0018]    The positive temperature coefficient current I 43  and the negative temperature coefficient current I 45  add to form current I 46  which flows through the resistor R 43  to form across it the reference voltage VREF. The reference voltage VREF can be made substantially temperature invariant by proper design of resistors R 41  and R 42  and diodes D 41  and D 42 .  
         [0019]    The optional transistor drain-to-source voltage matching circuit  406  is provided to substantially equalize the source-to-drain voltages of the transistors Q 41 , Q 42 , Q 43 , Q 44  and Q 45 . The source-to-drain voltages for transistors Q 41 , Q 42  and Q 44  are already set to VDD-V 42 . The operational amplifier U 43  is configured as a voltage follower to produce a voltage V 46  (substantially equal to voltage V 42 ) at the drains of transistors Q 43  and Q 45 . Thus, the optional transistor source-to-drain voltage matching circuit  406  also causes the source-to-drain voltage of transistors Q 43  and Q 45  to be at approximately Vdd−V 42 . This reduces errors that would result from different voltages across the finite output resistances of transistors Q 41 , Q 42 , Q 43 , Q 44 , and Q 45 .  
         [0020]    An advantage of the bandgap reference voltage circuits  300  and  400  over the prior art bandgap circuits  100  and  200  stems from the generating of the positive and negative temperature coefficient currents at different circuit sections and then combining them to form the reference voltage VREF. This uses less VDD voltage to implement, allowing VDD to be smaller so that the circuits  300  and  400  can be used on technologies requiring relatively low VDD.  
         [0021]    [0021]FIG. 5 illustrates a block diagram of an exemplary integrated circuit  500  in accordance with another embodiment of the invention. Generally, the bandgap reference voltage circuits  300  and  400  are used as part of an integrated circuit. Accordingly, integrated circuit  500  comprises a bandgap voltage reference circuit  502  such as bandgap circuit  300  or  400 , and one or more circuits, such as illustrated first, second, and third circuits  504 ,  506  and  508 , that use the reference voltage VREF generated by the bandgap circuit  502  in performing their intended operations. Although the bandgap circuit  502  is illustrated as part of integrated circuit  500 , it shall be understood that the bandgap voltage reference circuit  502  could also be implemented as discrete components. In addition, the bandgap circuit  502  can also be implemented with NMOS, CMOS, bipolar, and other transistor technology.  
         [0022]    In the foregoing specification, the invention has been described with reference to specific embodiments thereof It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.