Abstract:
A circuit and method for initiating operation of a bandgap reference circuit. A start pulse circuit provides a start pulse when the bandgap circuit is powered up. A transistor receives the pulse as an input, and applies the pulse to a regenerative bandgap reference circuit. The bandgap reference circuit output voltage is forced above a normal output voltage, producing a feedback current through the bandgap reference circuit, providing a current level which exceeds the normal stable operating level and output voltage level range. When the pulse ceases, the regenerative bandgap reference circuit output voltage decreases to its normal stable value, and the regenerative bandgap reference circuit is placed in its normal stable operating state.

Description:
DESCRIPTIVE TITLE OF THE INVENTION 
     The present invention relates to bandgap circuits which generate a substantially temperature insensitive voltage reference signal. Specifically, a bandgap circuit is provided which avoids any metastable operation. 
     Bandgap circuits are used in bipolar and BiCMOS circuit designs for producing a stable reference voltage. The stable reference voltage is used to control other voltage levels within a chip, and to provide a bias current that is proportional to absolute temperature. The voltage regulation and bias current applications are used extensively in analog circuits such as cellular telephone applications. Bandgap circuits must not only provide the required voltage and current functions, but must be power efficient since the cellular telephone circuits are powered by batteries. The bandgap circuit is integral to the operation of the circuit, and its reliability is essential to avoid catastrophic circuit failure. 
     The bandgap circuits are known to have three operating states. The first operating state provides a normal operation which produces the required regulated voltage, or bias current. The second state is a zero current state, which means that the circuit is not operable at all. The third operating state is the metastable state representing a circuit failure. One of the more common problems with bandgap circuits is the failure to enter the normal operational state from the zero state. If the bandgap circuit enters the metastable state, the output voltage does not attain a final reference value, and the circuit may remain in the metastable state, with the result that the entire cellular telephone circuit may fail. 
     The solution to the problem is to provide additional start-up circuitry which forces the bandgap circuit into its normal operational state. However, additional start-up circuitry adds overhead to the power budget for the circuit device placing additional burden upon the battery power supply. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a start-up circuit which unconditionally places a bandgap circuit in its normal stable operational state independent of manufacturing process variations, temperature variations and power voltage supply variations. The pulse start-up circuit does not interfere with normal bandgap operations, and draws no additional current from the power supply once the bandgap circuit is in the normal, stable operational mode. 
     In accordance with the invention, a start pulse circuit generates a pulse when the power supply voltage is applied to the bandgap circuit. The pulse is applied through a transistor to the regenerative bandgap reference circuit, and forces the output voltage of the regenerative bandgap reference circuit to a voltage higher than the normal output voltage. At the end of the pulse, the regenerative bandgap reference circuit output voltage decreases to a stable normal voltage reference value, avoiding the metastable state. 
     In accordance with one embodiment of the invention, a momentary start pulse is produced from a logic gate and delay circuit. The enable voltage for the bandgap reference circuit is applied to the delay circuit and a second input of the logic gate. A pulse is formed from the logic gate which is used to drive the regenerative bandgap reference circuit into an overvoltage output state. Once the pulse has ended, the delay circuit and logic gate are effectively decoupled from the bandgap circuit and no additional power burden occurs on the battery power supply. 
     Still other objects and advantages of the present invention will become readily apparent by those skilled in the art from the following detailed description, wherein it is shown and described preferred embodiments of the invention, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, without departing from the invention. Accordingly, the description is to be regarded as illustrative in nature and not as restrictive. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates bandgap circuit operation state. 
     FIG. 2 illustrates the bandgap output voltage V BG  over different power supply voltages and temperatures. 
     FIG. 3 is a schematic drawing of a preferred embodiment in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The bandgap reference voltage circuit produces a bandgap voltage V BG  which remains essentially constant over changes in voltage supply as well as temperature. FIG. 1 illustrates a steady state bandgap performance when the bandgap circuit is operating in its normal, stable mode of operation. V BG  remains essentially the same with variations in V CC , the supply voltage over a temperature range of −50° C. to +150° C. 
     The circuit has two stable states, 1) the zero state where no current is conducted through the bandgap circuit, and 2) the normal stable where the final reference output voltage is derived, shown in FIG.  2 . The circuit may operate in a metastable state, illustrated in FIG. 2, which is unstable and between the zero state and normal state. Metastable state operation may last, for a brief period of time, with the circuit then assuming one or the other of the stable states to FIG.  1 . 
     The bandgap reference voltage circuit  12  comprises two bipolar transistors  13  and  14 , having emitter area ratios of N, which receive identical currents I from the current mirror circuit  29 . When MOSFET  18  is enabled, two current values of I are produced from MOSFETS  16 ,  16  to the collectors of transistors  13  and  14 . The emitters of transistors  13  and  14  are connected to resistor  19  and resistor  20 . The output voltage V BG  for the bandgap circuit is essentially the base voltage which has been produced from bipolar transistors  13  and  14 . 
     The bandgap voltage, which can be demonstrated for the embodiment of FIG. 3, to be substantially independent from temperature and power supply voltage variations, is a function of the values of resistors  19 , and  20 . Assuming I to be equal currents flowing through the collectors of transistors  13  and  14  from the current mirror comprising MOFSET  15 ,  16 , the bandgap voltage V BG  may be expressed as follows: 
     
       
         2IR 3 +IR 6 +VBE 14 =VBG  (1) 
       
     
     
       
         2IR 3 +VBE 13 =VBG  (2) 
       
     
     where R 3  is the value of resistor  20 , and R 6  is the value of resistor  19 . 
     The base emitter voltage for each of the transistors  19  and  20  can be expressed as follows: 
     
       
         VBE 14 −VBE 13 =IR 6   (3) 
       
     
     Each of the currents through the collectors of transistors  13  and  14 , may be represented as follows: 
     
       
         
           
             
               
                 
                   
                     I 
                     13 
                   
                   = 
                   
                     I 
                     = 
                     
                       n 
                        
                       
                           
                       
                        
                       
                         I 
                         s 
                       
                        
                       
                         e 
                          
                         
                           ( 
                           
                             
                               V 
                               
                                 BE 
                                 13 
                               
                             
                             VT 
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
             
               
                 
                   
                     I 
                     14 
                   
                   = 
                   
                     I 
                     = 
                     
                         
                     
                      
                     
                       
                         I 
                         s 
                       
                        
                       
                         e 
                          
                         
                           ( 
                           
                             
                               V 
                               
                                 BE 
                                 14 
                               
                             
                             VT 
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
                 
         
             
         
      
     
     where I s  is the saturation current for each of the transistors  13  and  14 . N is the ratio of the emitter areas of the transistors  13  and  14 , and can be solved from the foregoing equations 4 and 5 by dividing equations 4 and 5 to derive the value of n:                e        :                     (         V     BE   14       -     V     BE   13         VT     )       =   n           (   6   )                                
     The above representation may be represented as 
     
       
         VBE 14 −VBE 13 =V T Inn  (7) 
       
     
     From equations, a value of current I can be derived as follows: 
     
       
         V T Inn=IR 6   (8) 
       
     
     
       
         
           
             
               
                 
                   I 
                   = 
                   
                     
                       
                         V 
                         T 
                       
                       
                         R 
                         6 
                       
                     
                      
                     ln 
                      
                     
                         
                     
                      
                     n 
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
                 
         
             
         
      
     
     From equation 7 and 1, a value of the bandgap voltage may be derived as follows:                  V                 B                   E             13         +     V                 T                   (     ln                   (   n   )       )                     (     1   +       2        R   3         R   6         )         ,           (   10   )                                
     where VBE 13  has a negative temperature coefficient, and VT, which equals        KT   Q                          
     has a positive temperature coefficient. 
     The present invention avoids the metastable state by applying a pulse of limited duration to force the regenerative bandgap circuit to produce an output voltage higher than the stable state reference value. A pulse circuit for providing the pulse is shown as  10  in FIG.  3 . Turning now to FIG. 3, an input voltage level is applied to  9  which renders the bandgap circuit operable. An inverter  20  enable MOSFET  18  in response to the voltage level applied at  9  to provide current from the battery power supply V CC  to the bandgap circuit. 
     The enable voltage applied to 9 is used to initiate a start pulse from the start pulse circuit  10 . The start pulse circuit  10  includes a NAND gate  33  having first and second inputs. A first input is connected to a delay circuit comprising series resistor  36  and capacitor  37 . The enable voltage applied at  9  is applied to the second input of NAND gate  33 , and to an inverter  34  which supplies an inverted enable voltage to the delay circuit. The result is a pulse from NAND gate  33  having a duration defined by the delay time of the delay circuit which is inverted by NAND gate  39 . 
     The inverted start pulse is used to render a MOSFET  40  conductive, which forces the output voltage V BG  of the bandgap circuit  12  to a higher voltage than the steady state reference voltage produced in the stable state. These offsetting temperature coefficients result in a steady voltage V bandgap V BG . 
     The foregoing stable bandgap voltage V BG  is forced by the pulse from MOSFET  40 . MOSFET  40  drives the collector of bipolar transistor  13  high, and applies a gate voltage on MOSFET  31  through resistor  29 . The result is that MOSFET  31  conducts current, driving the emitter of bipolar transistor  30  high. The emitter of transistor  30  is connected to the base of each of transistors  26 ,  21  and the two bipolar transistors  13  and  14  of the bandgap reference circuit. The result is that the bandgap output voltage V BG  rises, thereby forcing transistors  13  and  14  into higher conduction levels, raising the output voltage V BG  above the stable operating voltage. As the pulse produced by pulse circuit  10  ends, the output voltage V BG  decreases to the level of the second stable output voltage V BG . Following the ceasation of the start-up pulse MOSFET  40  and MOSFET  31  are rendered off. As a collector of transistor  13  is driven lower in voltage, the MOSFET  31  will be held into conduction even though the start pulse has been removed. Voltage is fed back from the output node V BG  to the base of bipolar transistors  13  and  14 , maintaining the voltage at its stable state. Transistors  30  and  26  provide current gain for driving a load impedance which may be connected to the output reference node V BG . 
     Because the bandgap voltage V BG  has been driven to over shoot its stable state by the start pulse, reliable starting of the circuit results. The values for resistor  36  and capacitor  37  are selected so that the bandgap voltage V BG  will sufficiently overshoot the reference bandgap voltage, avoiding any possibility of the bandgap circuit getting stuck in the metastable state. 
     Following the starting of the bandgap circuit, the pulse start circuit  10  ceases operation avoiding any unnecessary power consumption. 
     When the enable voltage  9  is returned to 0, indicating that power is being removed from the bandgap circuitry, transistor  18  is turned off and the circuit returns to a zero current stable state. The foregoing description of the invention illustrates and describes the present invention. Additionally, the disclosure shows and describes only the preferred embodiments of the invention but, as mentioned above, it is to be understood that the invention is capable of use in various other combinations, modifications, and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein, commensurate with the above teachings and/or the skill or knowledge of the relevant art. The embodiments described hereinabove are further intended to explain best modes known of practicing the invention and to enable others skilled in the art to utilize the invention in such, or other, embodiments and with the various modifications required by the particular applications or uses of the invention. Accordingly, the description is not intended to limit the invention to the form disclosed herein. Also, it is intended that the appended claims be construed to include alternative embodiments.