Abstract:
A startup circuit for activating a bandgap circuit is provided, including a switching circuit, an activating circuit, and a controlling circuit. The controlling circuit is used for monitoring and comparing two voltages to determine whether the switching circuit should be turned on so as to activate the bandgap circuit. One of the two voltages that are monitored is a zero temperature coefficient voltage, and the other of the two voltages that are monitored is a negative temperature coefficient voltage.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/596,874, which was filed on Oct. 27, 2005 and is included herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a startup circuit, and more particularly to a startup circuit applied in a bandgap voltage generator. 
   2. Description of the Prior Art 
   Conventionally, a bandgap voltage generator is utilized for generating a precise voltage and reference voltage, where the voltage should be a fixed voltage that is unaffected by the environment temperature. A startup circuit is coupled to the bandgap voltage generator for activating the bandgap voltage generator. After the bandgap voltage is generated, the startup circuit will be turned off automatically in order to reduce power consumption. 
   Please refer to  FIG. 1 .  FIG. 1  is a diagram illustrating a prior art startup circuit  110 . The startup circuit  110  is utilized in a bandgap voltage generator  100 . If an error has occurred in the turn on time and the turn off time in the startup circuit  110 , the bandgap voltage generator  100  will not operate properly. For example, if transistor M 1  of the startup circuit  110  is turned off (i.e. the voltage at terminal C is smaller than the threshold voltage V th  of the transistor M 1 ), but the BJT transistor Q 1  of the bandgap voltage generator  100  is not turned on yet (i.e. the voltage V in  at the terminal A is smaller than the base-emitter voltage V be  of the transistor Q 1 ), then misjudging of the bandgap voltage generator  100  will occurred. On the other hand, if transistors Q 1  and Q 2  of the bandgap voltage generator  100  are turned on (i.e. the voltages Vin, Vip at the terminals A, B are larger than the base-emitter V be  of the transistors Q 1  and Q 2 , respectively), but the transistor M 1  of the startup circuit  110  is not turned off (i.e. the voltage at the terminal C is larger than the threshold voltage V th  of the transistor M 1 ), the startup circuit  110  will affect the biasing condition of the bandgap voltage generator  100 , in which an error bandgap voltage is generated. Therefore, in order to avoid the above-mentioned problem, the startup circuit  110  should satisfy the following two equations: 
   
     
       
         
           
             
               
                 
                   
                     
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   According to the equations (1) and (2), the resistor R 1  and the current I M3  of the startup circuit  110  should be kept within a predetermined range to guarantee the normal operation of the bandgap voltage generator  100 . Therefore, the startup circuit  110  should be well designed to conform to the variation of the bandgap voltage generator  100 . 
   SUMMARY OF THE INVENTION 
   One of the objectives of the present invention is to provide a startup circuit, a bandgap voltage generator utilizing the startup circuit, and a startup method of the bandgap voltage generator to solve the above-mentioned problem. 
   According to an embodiment of the present invention, a startup circuit is disclosed. The startup circuit is utilized for activating a bandgap voltage generator, wherein the bandgap voltage generator comprises a first terminal for providing a first voltage level and a second terminal for providing a second voltage level. The startup circuit comprises a switching circuit, an activating circuit, and a controlling circuit. The switching circuit is coupled to the bandgap voltage generator; the activating circuit is coupled to the switching circuit for conducting the switching circuit to activate the bandgap voltage generator; and the controlling circuit is coupled to the switching circuit for monitoring the variation of the first voltage level and the second voltage level to control the conductivity of the switching circuit. 
   According to an embodiment of the present invention, a bandgap voltage generating circuit is disclosed. The bandgap voltage generating circuit comprises a bandgap voltage generator and a startup circuit. The bandgap voltage generator has a first terminal for providing a first voltage level and a second terminal for providing a second voltage level. The startup circuit is utilized for activating the bandgap voltage generator, and the startup circuit comprises: a switching circuit, an activating circuit, and a controlling circuit. The switching circuit is coupled to the bandgap voltage generator; the activating circuit is coupled to the switching circuit for conducting the switching circuit to activate the bandgap voltage generator; and the controlling circuit is coupled to the switching circuit for monitoring the variation of the first voltage level and the second voltage level to control the conductivity of the switching circuit. 
   According to an embodiment of the present invention, a startup method is disclosed. The startup method is utilized in a bandgap voltage generator, wherein the bandgap voltage generator comprises a first terminal for providing a first voltage level and a second terminal for providing a second voltage level, the startup method comprising: providing a switching circuit, coupled to the bandgap voltage generator; receiving an operating voltage level for conducting the switching circuit to activate the bandgap voltage generator; and monitoring the variation of the first voltage level and the second voltage level to control the conductivity of the switching circuit. 
   These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram illustrating a prior art startup circuit. 
       FIG. 2  is a schematic diagram illustrating the startup circuit of an embodiment of the present invention. 
       FIG. 3  is an operating flowchart of the startup circuit in  FIG. 2 . 
   

   DETAILED DESCRIPTION 
   Please refer to  FIG. 2 .  FIG. 2  is a schematic diagram illustrating a startup circuit  210  according to an embodiment of the present invention. The startup circuit  210  comprises a switching circuit  220 , an activating circuit  230 , a controlling circuit  240 , and a referent circuit  250 . The controlling circuit  240  comprises a differential circuit  242  and a current mirror module  244 , wherein the switching circuit  220  comprises a transistor M 1 ; the activating circuit  230  comprises a resistor R 1 ; the differential circuit  242  comprises transistors M 10 ˜M 12 ; the current mirror module  244  comprises transistors M 2 ˜M 4 , M 8 , M 13  and M 14 ; and the referent circuit  250  comprises transistor M 9  and resistor R 6 . Please note that a bandgap voltage generator  200  in  FIG. 2  can be implemented by any circuit configuration that is able to generate the bandgap voltage, and both theory and operation of the bandgap voltage generator are prior art, and therefore omitted here for brevity. According to this embodiment of the present invention, the transistors M 5 ˜M 7  of the bandgap voltage generator  200  are the same as the transistors M 9  and M 10 ; and the resistors R 2 , R 4 , and R 6  have the same resistance level. Furthermore, the transistor M 11  is the same as the transistor M 12 ; the transistors M 3 , M 4 , M 13 , M 14  have the same specification; and the aspect ratio of the transistor M 8  is 1.5 times the aspect ratio of the transistor M 2 . 
   When the startup circuit  210  begins to operate, the resistor R 1  in the activating circuit  230  adjusts the voltage at terminal C to approach an operating voltage level V DD  according to the operating voltage level V DD , and then turns on the transistor M 1 . When the transistor M 1  is turned on, the drain voltage of the transistor M 1  will turn on the transistors M 5 , M 6 , M 7 , M 9 , and M 10  to form a current source circuit. Accordingly, all of the transistors in the controlling circuit  240  can be turned on to form a push-pull comparator. In  FIG. 2 , before the transistors Q 1  and Q 2  in the bandgap voltage generator  200  are turned on, the voltages V in , V ip , and V x  at the terminals A, B, and D respectively are the same (because I M9 =I M5 =I M6 ), where the voltage V x  at the terminal D that is generated by the referent circuit  250  can be a referent voltage, in which the value of the referent voltage is equal to the voltages at terminals A and B of the bandgap voltage generator  200 . Furthermore, due to the current mirroring relationship between the current I M8  and the current I M2 , the current I M8  is 1.5 times the current I M3 . Accordingly, the voltage at the terminal C is kept near the operating voltage level V DD  to keep the transistor M 1  of the switching circuit  220  in an on condition, i.e. the current I M8  is utilized for increasing the voltage level of the control terminal of the transistor M 1 . The current supply of the bandgap voltage generator  200  continues to supply current to make the voltage V in  at the terminal A be higher than the different voltage V be  between the base and emitter of the transistor Q 1 , for turning on the transistor Q 1 ; then the current I M5  that originally passed through the resistor R 2  will be divided so a part of the current flows to the transistor Q 1  (BJT). Accordingly, the voltage V in  at the terminal A is lower than the voltage V x  at the terminal D. In other words, the voltage V x  at terminal D that is generated by the referent circuit  250  corresponding to the voltage V ip  at the terminal B of the bandgap voltage generator  200  (i.e. the voltage on resistor R  3  in the bandgap voltage generator  200  is a positive temperature coefficient voltage device), the voltage V x  at terminal D is a substantially zero temperature coefficient voltage of the bandgap voltage generator  200 , and the voltage V in  at terminal A is the negative temperature coefficient voltage of the bandgap voltage generator  200 . Therefore, the transistors M 10 ˜M 12  of the differential circuit  242  vary the currents that pass through the transistor M 13  and M 14  and this is caused by both the above-mentioned positive and negative temperature coefficient voltages. In this embodiment, the current I M13  that passes through the transistor M 13  is represented by the following equation: 
   
     
       
         
           
             
               
                 
                   
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   and the current I M14  that passes through the transistor M 14  is represented by the following equation: 
   
     
       
         
           
             
               
                 
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   In the current mirror module  244 , the transistors M 13  and M 4  form a current mirror; the transistors M 14  and M 3  form a current mirror; and the transistors M 2  and M 8  form a current mirror. Therefore, the current I M13  that flows through the transistor M 13  is equal to the current I M4  that flows through the transistor M 4  (i.e. I M13 =I M4 ); and the current I M14  that flows through the transistor M 14  is equal to the current I M3  that flows through the transistor M 3  (i.e. I M3 =I M3 ). Furthermore, because the aspect ratio of the transistor M 8  is 1.5 times the aspect ratio of the transistor M 2 , the current I M8  that flows through the transistor M 8  is 1.5 times the current of the transistor M 2  (i.e. I M8 =1.5*I M2 ). Accordingly, when the current I M3  of the transistor M 3  is larger than the current I M8  of the transistor M 8 , the voltage at the terminal C will be pulled down into the ground voltage, and then turn off the transistor M 1  of the switching circuit  220 ; in other words, the current I M3  is utilized for decreasing the voltage level of the control terminal of the transistor M 1 . Accordingly, the condition to turn off the transistor M 1  is shown as below:
 
 I   M3   +gm ( M 11, M 12)( V   x   −V   in )&gt;1.5 I   M3   −gm ( M 11, M 12)( V   x   −V   in )  (5)
 
   When the transistor M 1  is turned off, the negative feedback loop formed by the operating amplifier A 1  of the bandgap voltage generator  200  can sustain the bandgap voltage generator  200  to operate under an appropriate circumstance. In the embodiment of the present invention, the resistor R 1  and the current IM 3  can be designed to a lager value according to requirements of the bandgap voltage generator  200  for overcoming the process variation. 
   Please refer to  FIG. 3 .  FIG. 3  is an operating flowchart of the startup circuit  210  in  FIG. 2 . Please note that, provided that substantially the same result is achieved, the steps of the flowchart shown in  FIG. 3  need not be in the exact order shown and need not be contiguous, that is, can include other intermediate steps. The steps of operating the startup circuit  210  are briefly listed as follows: 
   Step  300 : Activating circuit  230  turns on the switching circuit  220  to activate the bandgap voltage generator  200 ; 
   Step  302 : The differential circuit  242  compares the substantially zero and the negative temperature coefficient voltages of the bandgap voltage generator  200  to generate the current I M13  and the current I M14 ; 
   Step  304 : The current mirror module  244  determines the conductivity of the switching circuit  220  according to the different current between the current I M13  and the current I M14 ; if the different current between the current I M13  and the current I M14  is larger than a predetermined value, go to step  306 ; otherwise, go to step  302 ; 
   Step  306 : The current mirror module  244  turns off the switching circuit  220 . 
   Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.