Patent Publication Number: US-6906581-B2

Title: Fast start-up low-voltage bandgap voltage reference circuit

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to a bandgap voltage reference circuit, more particularly, to a fast start-up low-voltage bandgap voltage reference circuit. 
   2. Description of the Prior Art 
   In general, reference voltage can be generated by voltage-dividing of resistors or by the self-bias of a transistor. However, such reference voltage is not independent of working voltage and temperature, as well as the variation in the manufacturing. In order to solve the problems, a bandgap voltage reference circuit is provided. 
   The principle of the bandgap voltage reference circuit is to implement components having characteristics of positive temperature coefficient and negative temperature coefficient respectively. And then add the voltages or the currents of these components in a predetermined proper proportion to generate a value independent of temperature, and such value can be output as a reference. 
     FIG. 1  is a diagram showing the bandgap voltage reference circuit in such kind. As shown, the transistors M 1 , M 2 , Q 1 , Q 2 , the resistor R 1  and the amplifier OP 1  form a self-bias circuit generating a current in positive proportion to 
             V   T     ⁢   ln   ⁢           ⁢   N     R1     ,         
wherein N is the Emitter Area Ratio of the transistors Q 1  and Q 2 . Therefore, the bandgap voltage Vbg is: 
             Vbg   =       V   BE2     +       R   2     ⁢         V   T     ⁢   ln   ⁢           ⁢   N     R1                 (   1   )               
   wherein V BE2  has a negative temperature coefficient −2.2 mV/° C., and V T  has a positive temperature coefficient +0.85mV/° C. Aspect Ratio of M 1 , M 2  and M 3  are all equal, the Formula (1) can be rewritten as: 
           V   BG     ⁡     (   T   )       =       (       V   BE0     -     2.2   ×       10     -   3       ·   Δ     ⁢           ⁢   T       )     +           (       V   T0     +     0.085   ×       10     -   3       ·   Δ     ⁢           ⁢   T       )     ⁢   ln   ⁢           ⁢   N     R1     ·   R2           
 
   wherein ΔT=T−300 ° K (i.e. the difference of working temperature and the room temperature), V BE0  is V BE  under room temperature and the value is around 0.6V, V T0  is V T  under room temperature and the value is around 0.026V. In order to make the temperature coefficient of V BG  equal to “0”, make 
             ∂     V   BG         ∂   T       =   0     ,       then   ⁢           -     2.2   ×     10     -   3       ⁢   T     +             (     0.085   ×     10     -   3       ⁢   T     )     ·   ln     ⁢           ⁢   N     R1     ·   R2       =   0.         
 
   So, 
               ln   ⁢           ⁢   N     R1     ·   R2     =   25.88     ,             V   T0     ⁢   ln   ⁢           ⁢   N     R1     ·   R2     =       25.88   ×   0.026     =   0.67       ,       
 
then make ΔT=0, and the Formula (1) will become: 
         V   BG     =         V   BE0     +           V   T0     ⁢   ln   ⁢           ⁢   N     R1     ·   R2       =     0.6   +   0.67   +   1.27           
 
   In general, V BG  is around 1.27V, and the value varies depending on different manufacturing processes (for example, V BE0  may vary between 0.5V˜0.7V). Even the bandgap voltage Vbg independent of temperature can be obtained, however, it should be around 1.2V to offset the positive/negative temperature coefficient, which means this circuit will not work when the working voltage VCC is lower than 1.2 V. 
     FIG. 2  shows a low-voltage bandgap voltage reference circuit pretty common in prior art, in which the circuit will work under low VCC. As shown in  FIG. 2 , the resistor R 2  is connected parallel to the resistors R 3  and R 4  having voltages Va and Vb respectively, which is a modification of the circuit shown in  FIG. 1  in which the resistor R 2  is connected serial to the voltage Vb. Assuming R 3 =R 4  and the transistors M 1 , M 2 , Q 1 , Q 2 , the resistor R 1  and the amplifier OP 1  form a self-bias circuit. When the self-bias circuit is steady, the corresponding currents will be: 
               I   R3     =         V   a     R3     =       V   BE1     R3               (   2   )                 I   Q1     =         V   T     ⁢   ln   ⁢           ⁢   N     R1             (   3   )                 I   M1     =       I   M3     =         I   R3     +     I   Q1       =         V   BE1     R3     +         V   T     ⁢   ln   ⁢           ⁢   N     R1                   (   4   )               
   Therefore, changing the proportion between the R 1  and R 3  will generate a current independent of temperature. With R 5 , the current can be transformed to the bandgap voltage Vbg as follows, 
             Vbg   =     R5   ⁡     (         V   ⁢           ⁢   BE1     R3     +       V   ⁢           ⁢   T   ⁢           ⁢   ln   ⁢           ⁢   N     R1       )               (   5   )             
 
   Since the circuit in  FIG. 2  is achieved by the addition of currents (I R3 +I Q1 ), it will not be limited by the condition that the working voltage should be around 1.2V (as the prior art illustrated in  FIG. 1 ) and will work below 1V. However, when starting, the currents on the transistors Q 1  and Q 2  are much lower than that on the resistors R 3  and R 4 , and also R 3 =R 4 , so the voltage Va is almost equivalent to Vb. In such circumstances, the amplifier OP 1  will not pull up the self-bias voltage to a steady stage. Therefore, when starting, the self-bias circuit needs to be set up to a steady stage with an external reset signal. For example, As shown in  FIG. 2 , the starting unit  21  provides a reset signal to turn on an auxiliary transistor Mx when the self-bias circuit is not in the steady stage. And then the starting unit  21  has to monitor the current Ix on the transistor M 0  to turn off the auxiliary transistor Mx when the current Ix reaches to a threshold value (i.e. when the self-bias circuit reaches the steady stage). In one embodiment, the starting unit  21  comprises a power-on reset circuit. 
   SUMMARY OF THE INVENTION 
   The primary object of the present invention is to provide a fast start-up low-voltage bandgap voltage reference circuit which can fast start up and work under low voltage. 
   The fast start-up low-voltage bandgap voltage reference circuit of the present invention comprises: a first current generator, which is implemented by a self-bias unit and a current mirror for generating a first reference current with positive temperature coefficient; a second current generator, which is connected to a node with negative temperature coefficient in the first current generator to generate a second reference current with negative temperature coefficient; and an output resister for converting the first reference current and the second reference current into a low-voltage bandgap voltage independent of temperature. 
   Wherein the self-bias circuit further comprises a first pair of transistors M 1  and M 2  with the gates connected to each other; a first amplifier whose output end is connected to the gates of the transistors M 1  and M 2  and whose input ends are connected to the drains of the transistors M 1  and M 2  respectively; a third transistor Q 1  whose emitter is connected to one input end of the first amplifier; a first resistor; and a fourth transistor Q 2  whose emitter is connected to another input end of the first amplifier through the first resistor. 
   Since the bandgap voltage of the bandgap voltage reference circuit of the present invention is generated by using the output resistor to transform the current obtained from adding the first reference current which has a positive temperature coefficient and the second reference current which has a negative temperature coefficient, therefore the bandgap voltage reference circuit of the present invention will work normally when the working voltage VCC is lower than 1.2 V. Moreover, the circuit of the first and the second transistors are not connected parallel with the resistor, so the first amplifier will obtain a bigger voltage difference between the two input ends when starting, which enables the first pair of transistors M 1  and M 2  to become steady rapidly. 
   Other and further features, advantages and benefits of the invention will become apparent in the following description taken in conjunction with the following drawings. It is to be understood that the foregoing general description and following detailed description are exemplary and explanatory but are not to be restrictive of the invention. The accompanying drawings are incorporated in and constitute a part of this application and, together with the description, serve to explain the principles of the invention in general terms. Like numerals refer to like parts throughout the disclosure. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The objects, spirits and advantages of the preferred embodiments of the present invention will be readily understood by the accompanying drawings and detailed descriptions, wherein: 
       FIG. 1  shows the diagram of a bandgap voltage reference circuit in prior art. 
       FIG. 2  shows the diagram of a low-voltage bandgap voltage reference circuit in prior art. 
       FIG. 3  shows the diagram of an embodiment of the fast start-up low-voltage bandgap voltage reference circuit in accordance with the present invention. 
       FIG. 4  shows the diagram of another embodiment of the fast start-up low-voltage bandgap voltage reference circuit in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE PRESENT INVENTION 
   The following embodiments will illustrate the fast start-up low-voltage bandgap voltage reference circuit of the present invention in detail. 
     FIG. 3  is showing the diagram of an embodiment of the fast start-up low-voltage bandgap voltage reference circuit of the present invention. As shown in  FIG. 3 , the fast start-up low-voltage bandgap voltage reference circuit  30  of the present invention comprises two current generators, namely the first current generator  31  and the second current generator  32 . The first current generator  31  is substantially the same with the conventional bandgap voltage reference circuit shown in FIG.  1 . The first current generator  31  shown in  FIG. 3  is used to generate a first reference current I 1  with positive temperature coefficient, while the second current generator  32  is used to generate a second reference current  12  with negative temperature coefficient. 
   As shown in  FIG. 3 , the output end of the first amplifier OP 1  is connected to both gates of the first and the second transistors M 1 , M 2 . The first input end (e.g. the negative input end) and the second input end (e.g., the positive input end) of the first amplifier OP 1  are connected to the drains of the first and the second transistors M 1 , M 2  respectively. The third transistor Q 1  includes a third emitter which is connected to the first input end of the first amplifier OP 1 . The first resistor R 1  is connected to the second input end of the first amplifier OP 1 . The fourth transistor Q 2  has a fourth emitter thereof being connected to the first resister R 1 . 
   The first transistor M 1 , the second transistor M 2 , the third transistor Q 1 , the fourth transistor Q 2 , the first resistor R 1  and the first amplifier OP 1  form a self-bias circuit for generating a current as follows, 
             I1   =       I   M1     =         V   T     ⁢   ln   ⁢           ⁢   N     R1               (   6   )             
 
   since V T  has a characteristic of positive temperature coefficient, I 1  can be expressed as the function of the positive temperature coefficient. 
   The first current generator  31  further includes a fifth transistor M 3  and a node located between the first resistor R 1  and the emitter of the fourth transistor Q 2  for outputting a node voltage. The node voltage has a characteristic of negative temperature coefficient. The fifth transistor M 3  has a gate thereof being connected to both gates of the first and the second transistors M 1 , M 2 . The drain of the fifth transistor M 3  outputs the first reference current I 1 . 
   Next, the second current generator  32  comprises a voltage-controlled current source and a current mirror. The voltage-controlled current source comprises a second amplifier OP 2 , a sixth transistor M 4  and a second resistor R 7 . One input end (for example, a negative input end) of the second amplifier OP 2  is connected to the emitter of the fourth transistor Q 2  for accepting the node voltage, while another input end (positive input end) thereof is connected to the working voltage VSS through the second resistor R 7 . The output end of the second amplifier OP 2  is connected to the gate of the sixth transistor M 4 . Therefore, the current on the R 7  is V BE2 /R 7 . The sixth and seventh transistors M 4  and M 5  establish a current mirror, and its Aspect Ratio can be 1 to 1. The seventh transistor M 5  includes a seventh gate and a seventh drain. The seventh gate of the seventh transistor M 5  is connected to the sixth gate of the sixth transistor M 4 . The seventh drain of the seventh transistor M 5  outputs the second reference current. The current on the drain of the seventh transistor M 5  is: 
             I2   =       V   BE2     R7             (   7   )             
 
   since V BE2  has a characteristic of negative temperature coefficient, I 2  can be expressed as the function of the negative temperature coefficient. 
   Because the I 1  and I 2  are connected in parallel, the current on the output resistor R 8  is I 1 +I 2 . Thereby the bandgap voltage Vbg is: 
             Vbg   =       R8   ⁡     (     I1   +   I2     )       =     R8   ⁡     (           V   T     ⁢   ln   ⁢           ⁢   N     R1     +       V   BE2     R7       )                 (   8   )             
 
   Of course, a starting circuit can be added to the bandgap voltage reference circuit of the present invention so as to increase the steadiness when starting. As shown in  FIG. 3 , the bandgap voltage reference circuit  30  further comprises an eighth transistor M 0 , an auxiliary transistor Mx (also referred as the ninth transistor) and a starting circuit  33 . The starting circuit  33  is used to check the current Ix on the eighth transistor M 0  to control the auxiliary transistor Mx. If the current Ix on the eighth transistor M 0  is 0 (zero), the auxiliary transistor Mx will be turned on, and if the current Ix is not zero, the auxiliary transistor Mx will be turned off. Since the starting circuit  33  controls the auxiliary transistor Mx only depending on the fact that if the current Ix is equal to 0, the circuit is really easy to design and implement. 
     FIG. 4  is showing the diagram of another embodiment of the present invention. Basically, the circuit shown in  FIG. 4  is similar to the circuit shown in  FIG. 3 , which also comprises the first current generator  41  and the second current generator  42 . The only difference is that, in the embodiment shown in  FIG. 4 , the negative input end of the second amplifier OP 2  is connected to the positive input end of the first amplifier OP 1  for accepting the node voltage. That means, the node voltage is the voltage Vb of the first current generator  41 . Therefore, the current on the seventh transistor M 5  is: 
             I2   =       V   BE1     R7             (   9   )               
   since V BE1  has a characteristic of negative temperature coefficient, I 2  can be expressed as the function of the negative temperature coefficient. 
   Because the I 1  and I 2  are connected in parallel, the current on the output resistor R 8  is I 1 +I 2 . Thereby the bandgap voltage Vbg is: 
             Vbg   =       R8   ⁡     (     I1   +   I2     )       =     R8   ⁡     (           V   T     ⁢   ln   ⁢           ⁢   N     R1     +       V   BE1     R7       )                 (   10   )             
 
   Since the bandgap voltage Vbg of the bandgap voltage reference circuit of the present invention is generated by using the output resistor to transform the current obtained from adding the first reference current which has a positive temperature coefficient and the second reference current which has a negative temperature coefficient, therefore the bandgap voltage reference circuit of the present invention will work normally when the working voltage VCC is lower than 1.2 V. 
   The voltage difference between the two input ends of the first amplifier OP 1  is V a −V b =V T  ln N−I R1 R 1 . When the circuit is starting and the I R1  is not big enough, the voltage difference V a −V b  will be lager than 0 (zero), and will consequently cause the output of the first amplifier OP 1  to go down. Therefore, the current on the transistors M 1  and M 2  will increase so as to cause the voltage difference V a −V b  to keep going down till the self-bias circuit becomes steady. Since the transistors Q 1  and Q 2  are not connected parallel with the resistor, a bigger voltage difference V a −V b  will be obtained when starting, which will cause the transistors M 1  and M 2  to become steady rapidly. As a result, the bandgap voltage reference circuit of the present invention will not need an external reset signal for prompt start up. 
   Of course, a starting circuit can be added to the bandgap voltage reference circuit of the present invention so as to increase the steadiness when starting. As shown in  FIG. 4 , the bandgap voltage reference circuit  40  further comprises an eighth transistor M 0 , an auxiliary transistor Mx (also referred as the ninth transistor) and a starting circuit  43 . The starting circuit  43  is used to check the current Ix on the eighth transistor M 0  to control the auxiliary transistor Mx. If the current Ix on the eighth transistor M 0  is 0 (zero), the auxiliary transistor Mx will be turned on, and if the current Ix is not zero, the auxiliary transistor Mx will be turned off. Since the starting circuit  43  controls the auxiliary transistor Mx only depending on the fact that if the current Ix is equal to 0, the circuit is really easy to design and implement. 
   While the present invention has been shown and described with reference to a preferred embodiment thereof, and in terms of the illustrative drawings, it should be not considered as limited thereby. Various possible modification, omission, and alterations could be conceived of by one skilled in the art to the form and the content of any particular embodiment, without departing from the scope and the sprit of the present invention.