Abstract:
A system and method is disclosed for providing a bandgap reference voltage generator that can successfully operate with a low operating voltage. Three current sources are controlled to provide same amount of current through three paths. The first current source is used to enable a first negative temperature coefficient module, while the second and third current sources are used to enable a first positive temperature coefficient module. The three current sources together are used to enable a reference voltage output module, which is connected to a current summing module for producing a bandgap reference voltage independent of temperature variations.

Description:
BACKGROUND 
   The present disclosure relates generally to electronic circuits, and more particularly to bandgap reference circuits. Still more particularly, the present disclosure relates to bandgap reference circuits that can operate at a low voltage. 
   Reference circuitries generate reference voltages and currents that are used in a variety of semiconductor applications, including flash memories, Dynamic Random Access Memories (DRAMs) and analog devices. These circuitries are required to be stabilized despite process and temperature variations, and must be implemented without modification of its fabrication process. A reference voltage that exhibits little dependence on temperature is essential in many analog circuits. If a voltage reference is temperature independent, it is usually process independent as well, since variations in most process parameters affect voltage reference through variations in temperature. 
   A conventional bandgap reference generator is one of the more popular reference voltage generators that can stabilize reference voltage despite process and temperature variations. Bandgap is the energy gap in a semiconductor that separates the valence band, where electrons cannot conduct, and the conduction band, where electrons can conduct. A bandgap reference generator typically operates by creating a device that has a nominally zero temperature coefficient. One method of achieving the nominally zero temperature coefficient is to use a positive temperature coefficient of one part of the device to cancel out a negative temperature coefficient of the other part of the device. 
   Bipolar transistors may be used for forming the bandgap reference circuits. The base-emitter voltage of a bipolar transistor typically exhibits a negative temperature coefficient. The difference between the base-emitter voltages of two bipolar transistors with unequal current densities operating together exhibit a positive temperature coefficient. Therefore, a bandgap voltage generator may be designed by connecting two bipolar transistors in parallel with unequal current densities and ensuring that the positive and negative temperature coefficients cancel each other out. 
   Typically, the minimum operating voltage to drive a reference voltage generator must exceed 1.25 volts, or the bandgap voltage of silicon, because the common-collector structure of a bipolar transistor and the input common-mode voltage of an amplifier require at least that much voltage to drive any bandgap reference voltage generator. 
   However, with the spread of battery-operated, portable applications such as cellular phones and wearable computing devices, device designs increasingly demand low-power and low-voltage circuitries due to power supply limitations. In addition, advanced deep sub-micron Complementary Metal-Oxide-Semiconductor (CMOS) technologies require low power supply voltage. Therefore, it is understood that in the near future, the operating voltage of most devices will be below 1 volt. 
   Desirable in the art of bandgap reference voltage generator designs are additional designs and methods with which bandgap reference circuitries can successfully operate with a low operating voltage such as one below one volt. 
   SUMMARY 
   In view of the foregoing, this disclosure provides a system and method for providing a bandgap reference voltage generator that can successfully operate with a low operating voltage. Three current sources are controlled to provide the same amount of current through three paths. The first current source is used to enable a first negative temperature coefficient module, while the second and third current sources are used to enable a first positive temperature coefficient module. The three current sources together are used to enable a reference voltage output module, which is connected to a current summing module for producing a bandgap reference voltage independent of temperature variations. 
   In one example, a bandgap reference circuit comprises first, second and third current sources CS 1 , CS 2 , and CS 3  adjusted to have the same current, the first current source feeding into a first BJT device module Q 1 , the second current source feeding into a second BJT device module Q 2  through a first resister R 1 , and the third current source connecting to a grounding voltage supply through a second resister R 2 . Other components of the circuit include a first voltage passing unit connecting an output of CS 1  as its input and connecting its output to a first end of a third resister R 3  and a first output of a current summing circuit; a second voltage passing unit connecting an output of CS 3  as its input and feeding its output to a first end of a fourth resistor R 4  and a second output of the current summing circuit; and a fifth resister R 5  connecting to a third output of the current summing circuit on a first end and the grounding voltage supply on a second end thereof. In such a circuit, a first current through R 5  bears a linear relationship with a summation of a second current through R 3  and a third current through R 4 , and the outputs of the first and second voltage passing units track their respective inputs, and predetermined values for R 1 , R 2 , R 3 , R 4 , and R 5  are selected in conjunction with selections Q 1  and Q 2  so that a reference voltage of the circuit across R 5  is independent of temperature variations. 
   Various aspects and advantages will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating the principles of the disclosure by way of examples. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a schematic of a bandgap reference voltage generator in accordance with one example of the present disclosure. 
       FIG. 2  illustrates a sample schematic of a current summing circuit used for the bandgap reference voltage generator of  FIG. 1 . 
   

   DESCRIPTION 
   In the present disclosure, a bandgap reference voltage generator and a method to operate the same are disclosed. In  FIG. 1 , a bandgap reference voltage generator  100  is presented. The bandgap reference voltage generator  100  includes three current sources  102 ,  104 , and  106 , whose current outputs are I 1 , I 2  and I 3 , respectively. The current outputs of current sources  102  and  104  are connected, respectively via nodes  108  and  110 , to the positive and negative input terminals, respectively, of an operational amplifier  112 . The output of the operational amplifier  112  is designed and fed to current sources  102 , 104  and  106  in such a way that I 1 , I 2  and I 3  are all equal to one and another. The operational amplifier  112  is further designed in such a way that the voltage at node  108 , or V 108 , is equal to the voltage at node  110 , or V 110 , since amplifier&#39;s output feedbacks, through nodes  108  and  110 , into the positive and negative input terminals of the amplifier. The output of current source  102  is further connected, via node  108 , to the emitter of a pnp bipolar junction transistor (BJT) Q 1 . The output of current source  104  is further connected, via node  110 , to a resistor R 1 , which is further connected to a pnp BJT Q 2 . Q 2  is designed so that it has a larger base emitter area than Q 1  (or, having several BJTs connect in parallel). For example, the base emitter area of Q 2  may be eight times the base emitter area of Q 1 . The bases and collectors of BJTs Q 1  and Q 2  are connected to VSS. It is typical that VSS is connected to ground. As such, VSS may be referred to as a grounding voltage supply for the purpose of this disclosure. 
   The output of current source  106  is connected, via a node  114 , to a resistor R 2 , which is further connected to VSS. The output of current source  106  is also connected, via node  114 , to the positive input terminal of a unit-gain operational amplifier  116 , whose output terminal is fed back to its negative input terminal. Similarly, the output of current source  102  is also connected, via node  108 , to the positive input terminal of a unit-gain operational amplifier  118 , whose output terminal is fed back to its negative input terminal. The output terminal of operational amplifier  118  is connected to a node  120 , which is further connected to a current summing module  122  and one end of a resistor R 3 , whose other end is connected to VSS. Since operational amplifier  118  is a unit-gain amplifier, the voltage at node  108  is carried to node  120 . The output terminal of operational amplifier  116  is connected to a node  124 , which is further connected to current summing module  122  and one end of a resistor R 4 , whose other end is connected to VSS. Since operational amplifier  116  is a unit-gain amplifier, the voltage at node  114  is carried to node  124 . The current summing module  122  is also connected to a node  126 , whose voltage, or V REF , is the reference voltage of the bandgap reference voltage generator  100 . Node  126  is further connected to one end of a resistor R 5 , whose other end is connected to VSS. The combination of unit-gain amplifiers  116  and  118 , as well as resistors R 3 , R 4  and R 5  can be seen as a reference voltage output module  128 , which generates the output voltage V REF . The combination of current source  102  and BJT Q 1  can be seen as a negative temperature coefficient module  130 , while the combination of current sources  102  and  104 , resistor R 1 , and BJTs Q 1  and Q 2  can be seen as a positive temperature coefficient module  132 . 
   The currents going through nodes  120 ,  126  and  124  are respectively I 4 , I 5  and I 6 . The current summing module  122  operates in such a way that I 5  is equal to the sum of I 4  and I 6 . To summarize, the bandgap reference voltage generator  100  has two main properties:
 
 I   1   =I   2   =I   3   (Equation 1A)
 
 I   5   =A ×( I   4   +I   6 )  (Equation 1B)
 
where A is a factor to show that I 5  bear a linear relation with the summation of I 4  and I 6  (or is proportional to the summation of I 4  and I 6 ). For the illustration below, A is deemed to be “1” for simplification. Furthermore, the base-emitter voltage of BJT Q 1 , or V be1 , is equal to V 108 :
 
V 108 =V be1   (Equation 2)
 
and the base-emitter voltage of BJT Q 2 , or V be2 , is equal to V 110  minus the voltage drop across resistor R 1 , which is I 2 *R 1 . Since operational amplifier  112  forces V 108  and V 110  to equate, the following relationship is true:
 
 V   be1   =V   be2   +I   2   *R 1  (Equation 3)
 
After rearranging Equation 3, the following is derived:
 
 I   2 =( V   be1   −V   be2 )/ R 1  (Equation 4)
 
Voltage at node  114 , or V 114 , is equal to the voltage drop across resistor R 2 :
 
 V   114   =I   3   *R 2  (Equation 5)
 
Since according to Equation 1A, I 3  is equal to I 2 , Equation 5 can be rewritten as:
 
 V   114   =I   2   *R 2  (Equation 6)
 
Substituting Equation 4 into Equation 6, the following is true:
 
 V   114 =( V   be1   −V   be2 )*( R 2/ R 1)  (Equation 7)
 
The voltage at node  120 , or V 120 , and the voltage at node  124 , or V 124 , are as follows:
 
 V   120   =I   4   *R 3  (Equation 8); and
 
 V   124   =I   6   *R 4  (Equation 9)
 
Since it is established earlier that V 108  is equivalent to V 120 , and that V 114  is equivalent to V 124 , Equations 8 and 9 can be rewritten into Equations 10 and 11, respectively, as follows:
 
 I   4   =V   108   /R 3  (Equation 10); and
 
 I   6   =V   114   /R 4  (Equation 11)
 
Substituting Equation 2 into Equation 10, the following is true:
 
 I   4   =V   be1   /R 3  (Equation 12)
 
Then, substituting Equation 7 into Equation 11, the following is true:
 
 I   6 =( V   be1   −V   be2 )*( R   2/(   R 1* R 4))  (Equation 13)
 
   Substituting Equations  12  and  13  into Equation 1B. the following is derived:
 
 I   5   =V   be1   /R 3+( V   be1   −V   be2 )*( R 2/( R 1 * R 4))  (Equation 14)
 
The output voltage or the voltage at node  126  (i.e., V REF ) is:
 
V REF   =I   5   *R 5  (Equation 15)
 
Substituting Equation 14 into Equation 15, the following is derived:
 
 V   REF   =V   be1 *( R 5/ R 3)+( V   be1   −V   be2 )*(( R 2* R 5)/( R 1* R 4))  (Equation 16)
 
Taking the consideration of temperature dependence, the change in V REF , or dV REF , with respect to change in temperature, or dT, is as follows:
 
 dV   REF   /dT =( R 5/ R 3)* dV   be1   /dT +(( R 2* R 5)/( R 1* R 4))* d ( V   be1   −V   be2 )/ dT   (Equation 17)
 
If the change in reference voltage with respect to the change in temperature is zero, reference voltage is no longer dependent on a change in temperature. Therefore, if dV REF /dT=0, the following is true:
 
 dV   be1   /d ( V   be1   −V   be2 )=−(( R 3* R 2)/( R 1* R 4))  (Equation 18)
 
Therefore, by choosing the right values for R 1 , R 2 , R 3  and R 4  with respect to dV be1 /d(V be1 −V be2 ), thereby rendering dV REF /dT=0, a bandgap reference voltage that is independent of temperature variations can be generated.
 
     FIG. 2  illustrates a sample schematic of a current summing module  122  used for the bandgap reference voltage generator of  FIG. 1 . The current summing module can vary in many different ways as long as the three current paths bear the linear relationship as described above. 
   Since the highest voltage in the bandgap reference voltage generator  100  is V be1 , which is typically less than 1 volt, or V REF , the operating voltage for this design can be lower than 1 volt. As an example, and depending upon the size of BJTs Q 1  and Q 2 , the bandgap reference voltage generator  100  can operate with an operating voltage such as 500–700 mV and as low as V 120  plus 50 mV. Since the rest of the circuit is independent of the level of the operating voltage, an operating voltage below 1 volt is sufficient to drive the bandgap reference voltage generator  100 , thereby generating a reference voltage independent of temperature variations in accordance with this disclosure. 
   The above disclosure provides many different embodiments, or examples, for implementing different features of the disclosure. Specific examples of components, and processes are described to help clarify the disclosure. These are, of course, merely examples and are not intended to limit the disclosure from that described in the claims. 
   Although illustrative embodiments of the disclosure have been shown and described, other modifications, changes, and substitutions are intended in the foregoing disclosure. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the disclosure, as set forth in the following claims.