Abstract:
According to the present invention, a circuit, utilizing a minimum number of bipolar devices and current mirror scaling devices, generates a bandgap reference voltage. The bandgap voltage generated by the bandgap reference circuit is a function of a plurality of sized current mirror devices, the ratio of a first resistor to a second resistor, and the number and relative sizing of bipolar junction transistors used. The bandgap reference circuit generates a bandgap reference voltage which is suitable for use in a variety of integrated circuit devices, such as a zero power static random access memory (SRAM). If used in a zero power SRAM application, the bandgap reference voltage may be utilized to determine when the primary power source of the zero power SRAM has fallen below a predetermined voltage level and a secondary power source must be substituted for the primary power source.

Description:
This is a Continuation, of application Ser. No.: 08/235,362, filed Apr. 29, 1994 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to integrated circuits, and more specifically to a MOS bandgap reference circuit. 
     Bandgap reference circuits are used in a variety of integrated circuit devices as a means for sensing changes in the voltage or current level of a device, so that appropriate changes in the operation of the integrated circuit device may be made. In many memory integrated circuit devices, it is desirable to ensure that data stored in the memory is retained and not lost or corrupted upon a loss of power to the device. For example, static random access (SRAM) memory devices referred to as “zero power” devices must be able to sense and respond to changes in the supply voltage. In a zero power SRAM, the data content of the SRAM is protected when the power supply voltage supplied to the SRAM drops below some predetermined voltage level. Typically, the data content of the zero power SRAM is protected by switching from a primary power source to a secondary power source when the power supply voltage to the zero power SRAM falls below the predetermined voltage level. 
     In order to switch from the primary power source to a secondary power source, it is necessary to be able to sense the voltage level of the primary power source and automatically switch to the secondary power source when appropriate. A bandgap reference circuit is one effective means to determine when it is necessary to switch from the primary power source to the secondary power source of a zero power SRAM. U.S. Pat. No. 4,451,742 issued May 29, 1984 to Aswell describes switching from a primary to a secondary power source and is herein incorporated by reference. However, typical bandgap reference circuits require a large number of bipolar devices which of course consume a large portion of the integrated circuit area of the SRAM. Therefore, because bandgap reference circuits may be effectively used in a zero power SRAM to determine the switching point, it would be desirable to be able to use an improved bandgap reference circuit which has fewer bipolar devices and thus consumes less area and power than bandgap reference circuits currently available. 
     SUMMARY OF THE INVENTION 
     It would be advantageous in the art to utilize a bandgap reference circuit which has fewer bipolar junction transistors than the prior art bandgap reference circuit. 
     It would further be advantageous to the art to utilize a bandgap reference circuit which provides scaling of current through bipolar junction transistors. 
     Therefore, according to the present invention, a bandgap reference circuit which utilizes a minimum number of bipolar devices and current mirror scaling devices generates a bangap reference voltage. The bandgap voltage generated by the bandgap reference circuit is a function of a plurality of sized current mirror devices, the ratio of a first resistor to a second resistor, and the number and relative sizing of bipolar junction transistors used. The bandgap reference circuit generates a bandgap reference voltage which is suitable for use in a variety of integrated circuit devices, such as a zero power static random access memory (SRAM). If used in a zero power SRAM application, the bandgap reference voltage may be utilized to determine when the primary power source of the zero power SRAM has fallen below a predetermined voltage level and a secondary power source must be substituted for the primary power source. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, and further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a schematic diagram of a bandgap reference circuit, according to the prior art; 
     FIG. 2 is a schematic diagram of a bandgap reference circuit, using p-well technology, according to a first preferred embodiment of the present invention; 
     FIG. 3 is a schematic diagram of a bandgap reference circuit, using n-well technology, according to a second preferred embodiment of the present invention; 
     FIG. 4 is a schematic diagram of a bandgap reference circuit, using p-well technology, according to a third preferred embodiment of the present invention; 
     FIG. 4a is a schematic diagram of a cascode connected transistor bandgap reference circuit, using p-well technology, according to a fourth preferred embodiment of the present invention; 
     FIG. 5 is a schematic diagram of a bandgap reference circuit which may be used to achieve a lower trip point, according to the present invention; 
     FIG. 6 is a schematic diagram of a circuit which could be used to add hysteresis to the bandgap reference circuit, according to the present invention; and 
     FIG. 7 is a schematic diagram of a secondary power supply which may be used to power the bandgap reference circuits of FIGS. 2,  3 ,  4 ,  4 a and  5  when the primary source of these bandgap reference circuits has fallen below a predetermined voltage level. 
    
    
     DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, a schematic diagram of a bandgap reference circuit  1 , according to the prior art is shown. Bandgap reference circuit  1  has three bipolar current legs and is comprised of bipolar junction transistors T 1 , T 2 , and T 3 , p-channel transistors P 1  and P 2 , n-channel transistors N 1 , N 2  and N 3 , and resistors R 1  and R 2 . P-channel transistors P 1  and P 2 , and N-channel transistors N 1 , N 2 , and N 3  are sized transistors and are selected such that P-channel transistors P 1  and P 2  have a size ratio of 4:1 to N-channel transistors N 1 , N 2 , and N 3 . This sizing is reflected in FIG. 1 by the encircled numbers by the transistors. 
     The current mirror devices N 1 , N 2 , and N 3  in each of the three bipolar current legs are each set to the same current. Therefore, the current densities of bipolar junction transistors T 1  and T 3  are approximately ten times that of bipolar junction transistor  72 , since the area of bipolar junction transistor T 2  is ten times that of T 1  and T 3 , to produce a in( 10 ) multiplier on the transistor voltage Vt across resistor R 1 . Additionally, with the same current in each bipolar current leg, the length of resistor R 2  must be longer than the length of resistor R 1 , by a factor of approximately ten times. The resistance of resistors R 1  and R 2  is a function of their lengths, as is known in the art. 
     The present invention employs a improved bandgap reference circuit which generates an actual band gap reference voltage which may be used by zero power circuitry, such as resistive divider and a comparator, to determine the trip point by matching a fraction of VCC to the bandgap reference voltage. The improved bandgap reference circuit of the present invention offers several advantages over the prior art bandgap reference circuit, including a reduced number of bipolar junction transistors and thus a reduced number of bipolar current legs, and scaled current through the bipolar junction transistors through the use of current mirror devices. 
     FIG. 2 shows a schematic diagram of a bandgap reference circuit, using p-well technology, according to a first preferred embodiment of the present invention. Bandgap reference circuit  10  is a simple form of a bandgap reference circuit, according to the present invention, having only two bipolar current legs, and is comprised of bipolar junction transistors T 1  and T 2 , p-channel transistors P 1  and P 2 , n-channel transistors N 1 , N 2 , and N 3 , and resistors R 1  and R 2 . The number of bipolar current legs of FIG. 2 over FIG. 1 is reduced by connecting resistor R 2  to the bases of bipolar junction transistors T 1  and T 2 . 
     Bipolar junction transistors T 1  and T 2  provide Vbe voltage drops at different current densities and as indicated by FIG. 2; bipolar junction transistor T 2  has M emitters whereas bipolar transistor T 1  has a single emitter. P-channel transistors P 1  and P 2  have the same current density and the same source voltages as node a and node b. Therefore, the bipolar current density voltage, v, which appears across resistor R 1  is equal to equation 1 which provides:              v   =         kT   q                   ln        I3     I1   /   M         =       kT   q                   ln        MI3   I1                 (   1   )                                
     where k is Boltzman&#39;s constant, T is the temperature in degrees Kelvin, q is the electronic charge,  13 I 3 is the current through p-channel transistor P 1 ,  11 I 1 is the current through resistor R 1 , and M is the number of emitters of bipolar junction transistor T 2 . N-channel transistors N 1 , N 2 , and N 3  function as current mirrors that help set the current ratios of I 2  and I 3  to I 1 . Therefore, if n-channel transistor N 1  has a width w1, n-channel transistor N 2  has a width w2, and n-channel transistor N 3  has a width w3, then current I 2  and current I 3  are defined as shown below in equations 2 and 3:              t2   =       W2   W3        t1             (   2   )               t3   =       W1   W3        t1             (   3   )                                
     Thus the current I 1  through resistor R 1  is equal to:              t1   =       v   R1     =         kT   qR1                   ln        M3   t1       =       kT   qR1                   ln        MW1   W3                   (   4   )                                
     The voltage at node c is equal to VCC- 1 I2R2, which when referenced to positive voltage supply VCC is equal to:              t2R2   =       W2   W3                     kT   q                     R2   R1                   ln        MW1   W3               (   5   )                                
     Referenced to VCC, the bandgap reference voltage equation is then:              Vout   =     Vt   =       W2   W3                     kT   q                     R2   R1                   ln        MW1   W3                 (   6   )                                
     Typical values used for the widths w1, w2, and w3, M, R 1 , and R 2  are such that:                MW3   W3     =       20                 and                   W2   W3                     R2   R1       =   8             (   7   )                                
     Given these typical values, bandgap reference circuit  10  generates a voltage below VCC of about 1.3 volts. 
     As is evident from the above equation and FIG. 2, the present invention offers several important advantages over the prior art bandgap reference circuit  1  of FIG.  1 . The current through bipolar junction transistors T 1  and T 2  is scaled using current mirror devices n-channel transistors N 1 , N 2 , and N 3 , such that a desired current density ratio may be achieved with a lower bipolar junction transistor ratio than was possible with the prior art bandgap reference circuit  1  of FIG.  1 . An additional advantage is that the scaled current I 2  through resistor R 2  permits resistor R 2  to be a lower resistor value. 
     FIG. 3 is a schematic diagram of a bandgap reference circuit, using n-well technology, according to a second preferred embodiment of the present invention. Referring to FIG. 3, bandgap reference circuit  20  generates a voltage above VCC  Vss of about 2.6 volts, roughly double the magnitude generated by FIG. 2, because 2 bipolar junction transistors rather than one bipolar junction transistor are used in each bipolar current leg. “ As discussed above, the p-well bandgap reference circuit  10  of FIG. 2 generates a voltage below Vcc of approximately 1.3 volts. Since the n-well bandgap reference circuit  20  of FIG. 3 uses two rather than one bipolar junction transistor, a voltage equal to approximately 2.6 volts above Vss, rather than approximately 1.3 volts below Vcc, is generated by bandgap reference circuit  20 ” . 
     Bandgap reference circuit  20  is comprised of p-channel transistors P 1 , P 2 , P 3 , P 4 , and P 5  which act as current mirrors, n-channel transistors N 1  and N 2 , source follower bipolar junction transistors T 1 , T 2 , T 3 , and T 4 , and resistors R 1  and R 2 . Bipolar junction transistors T 1 , T 2 , T 3 , and T 4  are sized transistors and are selected such that bipolar junction transistors T 1  and T 2  have a size ratio of 1:4 with respect to bipolar junction transistors T 3  and T 4 . The sizing is reflected in FIG. 3 by the encircled  1 &#39;s and  4 &#39;s. Similarly, p-channel transistors are sized transistors and are selected such that p-channel transistors P 4  and P 5  have a size ratio of 1:5 with respect to p-channel transistors P 1  and P 2 . Thus, for bandgap reference circuitry  20 , the current density ratio is  20:1. “ P-channel transistor P3 is a sized transistor having a size denoted by β, as shown in FIG. 3.”    
     Therefore, based on the sized transistors, VOUT is defined as follows:              VOUT   =       2      Vbe     +       R2   R1                     ln        (   20   )            kT   q                 (   8   )                                
     A typical value of VOUT is approximately 2.5. Bandgap reference circuit  20  may be made to have better operating characteristics by adding cascode transistors and thereby increase the output impedances of the MOS devices of bandgap reference circuit  20 . Such cascode transistors would improve the match between the source follower transistors T 1 , T 2 , T 3 , and T 4  and the p-channel transistors which acts as current mirrors. 
     Referring to FIG. 4, a schematic diagram of a bandgap reference circuit, using p-well technology, according to a third preferred embodiment of the present invention is shown. Bandgap reference circuitry  30  comprises bipolar junction transistors T 1 , T 2 , T 3 , and T 4 , p-channel transistors P 1  and P 2 , n-channel transistors N 1 , N 2 , N 3 , N 4 , and N 5 , and resistors R 1  and R 2 . Bandgap reference circuitry  30  is analogous to circuitry  20  of FIG. 3, except that bandgap reference circuitry  30  is applicable to p-well technology rather than n-well technology. Here, current source n-channel transistors N 1 , N 2 , N 3 , N 4 , and N 5  are still ratioed 1:5 as shown, but are connected to VSS or ground voltage potential. Likewise, bipolar junction transistors T 1 , T 2 , T 3 , and T 4  are sized such that the ratio of 1:4 is maintained. Current source n-channel transistor N 3  is a sized transistor having a size denoted by β, as shown in FIG.  4 . The equation for VOUT is analogous to equation (8), except that VOUT is referenced to VCC rather than VSS. If higher drain impedance is desired to provide the current matching required, cascode current mirror circuit, well known in the art, may be added to FIG.  4 . 
     If higher drain impedance is required to provide the current matching required, a bandgap reference circuit having cascode current mirror circuitry may be used. Referring to FIG. 4 a , a schematic diagram of a cascode connected transistor bandgap reference circuit  40 , using p-well technology, according to a fourth preferred embodiment of the present invention. Cascode connected transistor bandgap reference circuit  40  achieves a similar function to the circuitry of FIG. 4, but with greater accuracy. 
     Cascode connected transistor bandgap reference circuit  40  is comprised of bipolar junction transistors T 1 , T 2 , T 3 , and T 4 , p-channel transistors P 1 , P 2 , P 3 , P 4 , P 5 , and P 6 , n-channel transistors N 1 , N 2 , N 3 , N 4 , N 5 , N 6 , N 7 , N 8 , N 9 , N 10 , N 11 , N 12 , N 13  and N 14 , and resistors R 1  and R 2 . The transistors of FIG. 4 a  are sized as indicated by the encircle  encircled numbers. For instance n-channel transistors N 4  and N 11  are sized transistors having a size denoted by β, as shown in FIG. 4 a . As in FIG. 4, FIG. 4 a  utilizes p-well technology. 
     Added cascode current mirror transistors N 2 , N 3 , N 4 , N 5 , and N 6  provide better current matching capabilities while cascode source-follower transistors P 3 , P 4  provide better voltage matching capabilities than the configuration shown in FIG.  4 . The bias level at point a may be set to keep the cascode current mirror transistors N 2 , N 3 , N 4 , N 5 , and N 6 , and current mirror transistors N 9 , N 10 , N 11 , N 12 , and N 13  in saturation. Similarly, the bias level at point b may be set to keep cascode source-follower transistors P 3  and P 4 , as well as source-follower transistors P 1  and P 2 , in saturation. A start-up circuit may be required to establish initial current flow at start-up. 
     The functionality of the bandgap reference circuits shown in FIGS. 3,  4  and  4   a  will be adversely impacted as the supply voltage of the circuits is reduced. For example, Vout for the bandgap reference circuit  20  of FIG. 3 is approximately 2.5 volts. For operation of the circuit up to 4.5 volts, the Vout to Vcc difference is 2 volts, which permits an ample voltage range for the n-channel source follower above Vout. Since this n-channel source follower in bandgap reference circuit  20  or its p-channel equivalent in circuit  30  of FIG. 4 will always be in the bulk, rather than a well, it will have a 2.5 volt back gate bias. Thus, the transistor voltage Vt will be approximately 1.1 volts to 1.5 volts, thereby providing adequate voltage for the current sources to operate. 
     If, however, a lower trip point, such as 4.0 volts, is desired, with operation of the bandgap reference circuit down to 3.5 volts to 3.7 volts, there would not be adequate voltage. Referring to FIG. 5, a schematic diagram of a bandgap reference circuit  50  which may be used to achieve a lower trip point is shown. Bandgap reference circuit  50  is comprises of p-channel transistors P 1 , P 2 , and P 3 , n-channel transistors N 1  and N 2 , bipolar junction transistors T 1  and T 2 , and resistors R 1  and R 2 . The transistors are sized as indicated by the encircled numerals in FIG. 5, such that p-channel transistors P 1  and P 3  have a size ratio of 5:1; and bipolar junction transistors T 1  and T 2  have a size ratio of  1:4. “ P-channel transistor P2 is a sized transistor having a size denoted by β, as shown in FIG. 5.”    
     The Vout level would be approximately 1.25 volts and the Vt of the transistors would range from 0.9 volts to 1.2 volts. Thus, a supply voltage of Vcc =3.5 volts, the operating voltage for the current source transistors would be equal to 1.05 volts (3.5 volts −1.25 volts −1.2 volts). Thus, the bandgap reference circuit  50  of FIG. 5 may be used to obtain a considerably lower trip point. 
     Additionally, the bandgap reference circuits described above may be enhanced by adding hysteresis capability. Referring to FIG. 6, a schematic diagram of a circuit  60  which could be used to add hysteresis to a bandgap reference circuit is shown. Circuit  60  is comprised of resistors R 1 , R 2 , R 3 , and R 4 , transistors T 1  and T 2 , inverters I 1  and I 2 , comparator C, and the Bandgap Reference Circuit shown in the block. Hysteresis can be added by bypassing resistance in the resistor divider with a MOS switch which is controlled by a signal derived from the comparator output C. A positive state at node a indicated that node b is below the trip point. Transistor T 2  is turned on and transistor T 1  is turned off in order to move node b, thereby producing hysteresis. 
     In FIGS. 2,  3 ,  4 ,  4   a  and  5 , the operation of the bandgap reference circuit is a function of the ratio of resistors R 1  and R 2 , and the ratios of the current mirror and source follower portions of the circuit—not the precise values given resistors R 1  and R 2 . Thus, by carefully choosing the resistor and transistor ratios, a bandgap reference circuit may be realized using few bipolar transistors. 
     The bandgap reference voltage generated by the present invention will typically be used to determine if the bandgap reference voltage is below a predetermined voltage level. If it is below the predetermined voltage level, then a high gain comparator will flip at the predetermined trip point causing the zero power SRAM to be powered by a secondary power source rather than a primary power source. Referring to FIG. 7, a secondary power source which may power the bandgap reference circuits of FIGS. 2,  3 ,  4 ,  4   a  and  5  when the primary power source of these bandgap reference circuits has fallen below a predetermined voltage level is shown in addition to the primary power source. The secondary power source Vee may be Vcc like the primary power source, as shown in FIG. 7, or any other desired value. U.S. Pat. No. 4,451,742 issued May 29, 1984 to Aswell describes switching from a primary to a secondary power source and is herein incorporated by reference. 
     There are several advantages of the present invention over the prior art bandgap voltage circuit. A reduced number of bipolar junction transistors are used according to the present invention, and thus less area is used. Operation of the bandgap reference circuit is dependent on the ratios achieved through careful selection of the values of resistors R 1  and R 2 , as well as the sizes of the transistors, and not on the absolute values of these components. The current mirror devices are scaled such that current going through the bipolar current legs is scaled. Also, multiple trip points can be set by multiplexing multiple taps on the divider. These multiple trip points are chosen to meet customer demands; typical values might be 5%, 10%, 20%, etc. of the value of VCC. Additionally, the present invention allows for VOUT to be brought to an output pin of the zero power SRAM and thus easily measured. 
     While the invention has been particularly shown and described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.